Evaluation of Treatment of 

Effects of Debris in Coolant on 

ECCS and CSS Performance in 

Pressurized Water Reactors and 

Boiling Water Reactors

Office of Nuclear Regulatory Research 

NUREG/CR-7011

PNNL-17972
Evaluation of Treatment of 

Effects of Debris in Coolant on 

ECCS and CSS Performance in

Pressurized Water Reactors and

Boiling Water Reactors

Manuscript Completed: April 2010 

Date Published: May 2010 

Prepared by

J.M. Cuta, B.E. Wells, C.H. Delegard and B.F. Saffell

Pacific Norhwest National Laboratory

902 Battelle Boulevard

Richland, WA 99352

J. Burke, NRC Project Manager

NRC Job Code N6636

Office of Nuclear Regulatory Research 

NUREG/CR-7011

PNNL-17972
iii

ABSTRACT

This report identifies significant differences in the industry and regulatory guidance for the 

treatment of suction strainer performance in pressurized water reactors (PWRs) compared to that 

in boiling water reactors (BWRs) during operation of their respective emergency core cooling 

systems, containment spray systems, and residual heat-removal systems in postulated loss-ofcoolant-accident events. These differences are evaluated for their technical significance, 

conservatism, and consistency. Recommendations are provided for appropriate guidance on

technical requirements to address the issues in a consistent and conservative (or prototypic)

manner for both reactor types.

v

FOREWORD

The objective of this report is to identify any significant disparities in the regulatory treatment of 

emergency core cooling system (ECCS) suction strainer performance for boiling-water reactors 

(BWRs) and pressurized-water reactors (PWRs), evaluate the technical significance of the 

differences, and provide recommendations on the path forward for handling the disparities. 

Differences are assessed in terms of their significance with regard to maintaining long-term core 

cooling in postulated design-basis accident conditions. Recommendations are provided to 

establish a conservative and appropriately consistent regulatory framework for both reactor 

types.

In the evolution of the technical knowledge base used to verify the adequate ECCS suction 

strainer performance for the currently licensed and operating nuclear reactors in the United 

States, differences have developed in the regulatory treatment of PWRs and BWRs. Some of the 

differences in the guidance for suction strainer performance analyses are due to technical 

differences in the basic design of the two reactor types. In many cases, however, the differences 

arose because of additional knowledge and changes in the regulatory focus in the time periods 

that the issues were addressed for each reactor type. 

The disparities were identified by reviewing various reports from analytical and experimental 

studies, technical reports from industry groups such as the Boiling Water Reactor Owners Group 

(BWROG) and the Pressurized Water Reactor Owners Group (PWROG), associated NRC staff 

safety evaluations, and Regulatory Guide 1.82 revision 3. These differences were then grouped 

into five categories: debris characteristics, debris generation, debris transport, head loss on the 

suction strainers, and downstream effects of debris in the recirculation coolant. 

Recommendations are provided on changing the guidance for defining the technical 

requirements to address this issue in a consistent and conservative manner for both reactor types. 

The NRC staff concurs with the majority of the recommendations for BWRs in this report such 

as the need to conduct plant-specific walkdowns to determine debris types, conduct chemical 

effects testing, evaluate downstream effects, and assess existing head loss test data for the 

suction strainers. The BWROG has begun to address these issues and has discussed its 

resolution strategies and associated schedules with the NRC staff in a series of public meetings 

starting in June 2008. However, the NRC staff disagrees with the following recommendations in 

this report that also apply to the current method of evaluating licensee responses to Generic 

Letter 2004-02 for PWRs: 

 

The report recommends that free-jet expansion of the zone-of-influence (ZOI) in a loss-ofcoolant accident (LOCA) event be determined using an experimentally validated model 

applicable to both reactor types. This issue is concerned with the shape and extent of the 

high-energy jet ZOI in a LOCA (i.e., the ANSI/ANS-58.2 method). Current guidance in the 

Safety Evaluation for NEI-04-07 uses a spherical ZOI to encompass the effects of jet 

expansion resulting from impingement on structures and components. The spherical zone is 

a practical convenience that accounts for multiple jet reflections and mutual interference of 

jets from opposing sides of a guillotine break as well as pipe whip. It is important to note 
vi

that when the spherical volume is computed using an acceptable approximation for 

unimpeded free-jet expansion, the actual energy loss involved in multiple reflections is 

conservatively neglected to maximize the size of the ZOI. The staff believes that this 

produces conservative results without an unnecessarily complex evaluation. 

Portions of two recommendations are concerned with the quantity of debris generated outside 

the ZOI from unqualified coatings in PWRs. Specifically, the report recommends that 

guidance for coating thickness and failure characteristics be developed. The current staff 

guidance for PWRs is that plant-specific values regarding the unqualified coating thicknesses 

and failure characteristics should be used. The NRC staff considers this approach adequate. 

Unqualified coatings consist of many different types of materials and are produced by many 

different manufacturers, so the staff believes generic guidance for failure characteristic is 

infeasable.

Another recommendation is that the methods used for evaluation of downstream ex-vessel 

effects (i.e., erosion and abrasion of components) in PWRs be validated with more extensive 

testing. The staff considers the industry and staff guidance contained in WCAP 16406-P-A 

Rev 1 and the associated safety evaluation (SE) is adequate. As discussed in the SE, 

sufficient testing was done by NRC and industry to support the WCAP conclusions. 

Two recommendations are related to downstream in-vessel effects for PWRs. The 

recommendations are that prototypical testing be performed at post-LOCA temperatures, 

pressures, and flow conditions to evaluate blockage and debris deposition on the fuel. These 

issues are being addressed in WCAP 16793-NP-Rev 1, which is currently under review by 

the NRC staff. The PWROG has performed extensive testing and analyses for this topical 

report, and an SE is being prepared. The staff considers the test methodology used by the 

owners group at ambient temperatures to be conservative because it does not take advantage 

of the fluid viscosity change or higher flow turbulence that would exist at elevated 

temperatures. In addition, the methodology assumes all chemical precipitates formed in the 

containment pool pass through the strainer and into the reactor core. The LOCADM analysis 

also assumes all dissolved chemicals and suspended debris contained in the coolant are 

deposited as the coolant boils off. 

NRC also notes the statement in the Executive Summary: ―The debris generation methodology 

must be conservative if the overall analysis is to be conservative.‖ The NRC position is that the 

overall evaluation of ECCS strainer performance should be conservative. An expectation of 

conservatism in each aspect of that evaluation could result in an overly conservative overall 

evaluation. Therefore, NRC disagrees with the referenced statement in the Executive Summary.

NRC has prepared a draft revision to RG 1.82 (DG 1234) that adopts the recommendations 

discussed in this report with the exceptions discussed above. This revision to the regulatory 

guide incorporates lessons learned from resolution of Generic Letter 2004-02 (GL 04-02) and 

from staff guidance contained in safety evaluations prepared for industry topical reports for GL 

04-02. Upon publication of this revision, the regulatory guidance will be consistent for BWRs 

and PWRs except for guidance that is unique to each specific reactor type.
vii

CONTENTS

ABSTRACT...................................................................................................................................III

FOREWORD..................................................................................................................................V

CONTENTS.................................................................................................................................VII

EXECUTIVE SUMMARY ........................................................................................................XIII

ACKNOWLEDGMENTS ......................................................................................................... XIX

ACRONYMS AND ABBREVIATIONS.................................................................................. XXI

1. INTRODUCTION...................................................................................................................1

1.1 Background............................................................................................................... 1

1.2 Differences in Guidance for PWRs and BWRs........................................................ 6

2. DEBRIS CHARACTERISTICS .............................................................................................9

2.1 Physical Debris......................................................................................................... 10

2.1.1 Physical Debris in BWRs.............................................................................. 10

2.1.2 Physical Debris in PWRs.............................................................................. 13

2.1.3 Evaluation of BWR and PWR Guidance on Physical Debris....................... 19

2.1.4 Recommendations for BWR and PWR Guidance on Physical Debris......... 21

2.2 Chemical Debris....................................................................................................... 22

2.2.1 Chemical Debris in PWRs ............................................................................ 23

2.2.2 Chemical Debris in BWRs............................................................................ 28

2.2.3 Evaluation of Chemical Debris in PWRs and BWRs................................... 31

2.2.4 Recommendations for Guidance on Chemical Debris in PWRs

and BWRs ................................................................................................... 32

3. INSULATION AND COATINGS DEBRIS GENERATION..............................................35

3.1 Insulation Debris Generation.................................................................................... 35

3.1.1 BWR Guidance and NRC Staff Evaluation on ZOI for Insulation Debris... 35

3.1.2 PWR Guidance and NRC Staff Evaluation on ZOI for Insulation Debris.... 37

3.2 Coatings Debris Generation ..................................................................................... 39

3.2.1 Summary of BWR Guidance and NRC Staff Evaluation for 

Coatings Debris............................................................................................ 39

3.2.2 PWR Guidance and NRC Staff Evaluation for Coatings Debris 

Generation .................................................................................................... 40

3.3 Comparison and Evaluation of Guidance for Insulation and Coating Debris 

Generation............................................................................................................... 42

3.3.1 ZOI Determination........................................................................................ 42
viii

3.3.2 Debris Generation Within the ZOI................................................................ 44

3.3.3 Comparison of Debris Generation Outside of the ZOI................................. 51

4. EFFECTS OF DEBRIS IN RECIRCULATION LOOP COOLANT..................................53

4.1 Debris Captured on Suction Strainers ...................................................................... 53

4.1.1 Guidance from BWROG for Suction Strainer Head Loss Calculations....... 54

4.1.2 Industry Guidance for Suction Strainer Head Loss Calculations 

in PWRs........................................................................................................ 59

4.1.3 Regulatory Guidance on Head Loss across Suction Strainers...................... 64

4.1.4 Comparison of Regulatory Guidance for BWRs and PWRs ........................ 68

4.1.5 Recommendations for Guidance on Head Loss Calculations....................... 69

4.2 Debris Carried Through Sump or Suppression Pool Suction Strainers.................... 70

4.2.1 Guidance from the BWROG for Debris Transport through Suction 

Strainers and Effects on Downstream Components..................................... 72

4.2.2 Industry Guidance for PWRs on Debris Transport through Suction 

Strainers and Effects on Downstream Components..................................... 73

4.2.3 Regulatory Guidance on Debris Transport through Suction Strainers 

and Effects on Downstream Components.................................................... 75

4.2.4 Comparison of Regulatory Guidance for BWRs and PWRs ........................ 76

4.2.5 Recommendations for Guidance on Debris Transport through Suction 

Strainers and Effects on Downstream Components..................................... 77

4.3 Effects of Debris in Reactor Vessel and Core .......................................................... 77

4.3.1 Guidance from BWROG for Debris Effects in Reactor Vessel and Core .... 82

4.3.2 Industry Guidance for PWRs on Debris Effects in Reactor Vessel 

and Core ....................................................................................................... 83

4.3.3 Regulatory Guidance for Debris Effects in Reactor Vessel and Core .......... 84

4.3.4 Recommendations on Determining Debris Effects in Reactor Vessel 

and Core ....................................................................................................... 85

5. DEBRIS TRANSPORT IN SUPPRESSION POOL AND CONTAINMENT SUMP.........87

5.1 BWR Guidance for Debris Transport in Suppression Pool...................................... 87

5.2 PWR Guidance for Debris Transport in Containment Sump ................................... 88

5.3 Evaluation of Guidance for Debris Transport in Suppression Pool and 

Containment Sump.................................................................................................. 89

5.4 Recommendations on Guidance for Debris Transport in Suppression Pool and 

Containment Sump.................................................................................................. 89

6. RECOMMENDATIONS ......................................................................................................91

7. REFERENCES......................................................................................................................95

APPENDIX A REGULATORY GUIDE 1.82, REV. 3, NOVEMBER 2003, PWR-BWR 

COMPARISON...................................................................................................................A.1
ix

APPENDIX B TIME-LINE OF EVOLUTION OF POST-LOCA ECCS SAFETY 

ISSUES IN COMMERCIAL LWRS ..................................................................................B.1

xi

FIGURES

Figure 3.1. Comparison of the Volume of a Jet With Pressure Greater than or Equal to Pj (the 

material failure pressure) from Saturated Water and Steam Breaks (Figure 2 of DRF 

A74-00004 Appendix)27.............................................................................................47

TABLES

Table S.1.Summary of recommendations for developing conservative and consistent guidance 

for analysis of post-LOCA recirculation cooling in PWRs and BWRs. ........................... xvi

Table 1.1. Summary of differences in guidance for PWRs and BWRs................................... 7

Table 2.1. Non-insulation drywell debris sources and quantities in BWRs. ............................ 12

Table 2.2. Physical debris evaluated in PWRs and BWRs....................................................... 19

Table 2.3. Comparison of BWR and PWR guidance on physical debris. ................................ 20

Table 2.4. Contributors to chemical debris in PWRs and BWRs. ............................................ 23

Table 2.5. Test conditions for ICET experiments..................................................................... 24

Table 2.6. Results for ICET Experiments................................................................................. 25

Table 2.7. ICAN experimental matrix. ..................................................................................... 28

Table 3.1. BWR values of coating debris, Table 3 of NEDO-32686, Rev. 0........................... 40

Table 3.2. ZOI as a function of reactor type and debris material. ............................................ 44

Table 3.3. Debris generation as a function of reactor type and debris material. ...................... 45

Table 3.4. Material Failure Pressure Correction Factors (NEDO-32686, Rev. 0, from DRF 

A74-00004)............................................................................................................. 46

Table 3.5. Insulation failure pressures. ..................................................................................... 48

Table 3.6. Coating thicknesses for BWR and PWR. ................................................................ 49

Table 4.1. Typical downstream components for ECCS and CSS in LWRs. ............................ 72

Table 4.2. Summary of BWR ECCS components that draw from suppression pool. .............. 79
xii

Table 4.3. Summary of PWR ECCS components that draw from water storage tank or 

sump........................................................................................................................ 80

Table 6.1. Summary of recommendations for developing conservative and consistent 

guidance for analysis of LOCA and post-LOCA recirculation cooling in

PWRs and BWRs.................................................................................................... 91
xiii

EXECUTIVE SUMMARY

The U.S. Nuclear Regulatory Commission (NRC) licenses and regulates commercial nuclear 

power plants in the United States with the goal of ensuring the protection of public health and 

safety and of the environment. The specific regulatory positions relevant to the issues of sumps 

and suppression pools performing the functions of water sources for emergency core cooling, 

containment heat removal, and containment atmosphere cleanup are documented in Regulatory 

Guide 1.82, Revision 3, Water Sources for Long-Term Recirculation Cooling Following a Lossof-Coolant Accident. Guidelines for evaluating the adequacy of the sump and suppression pool 

for long-term recirculation cooling following a loss-of-coolant accident (LOCA) are also 

provided. The regulatory positions for the two types of reactors are similar, except for minor 

divergences due to differences in the basic designs. However, in the evolution of the regulatory 

process to verify the safety of the current licensed and operating Pressurized Water Reactors 

(PWRs) and Boiling Water Reactors (BWRs), differences have developed in the regulatory 

treatment of their respective emergency core cooling systems (ECCSs), containment spray 

systems (CSSs), and residual heat-removal systems (RHRSs) in postulated LOCA events. The 

differences arose mainly for historical reasons or because of the state of knowledge and 

regulatory focus at the time of the relevant reviews, and are not generally due to technical 

differences in the basic design of the two reactor types. 

In a LOCA that could occur in a BWR or PWR, the postulated pipe break results in the rapid 

escape of high-energy liquid water or steam from the primary system. High-volume jets of 

coolant from the break can strike piping, walls, and equipment, stripping away insulation and 

surface coatings (e.g., paint, epoxy) and damaging or destroying other material (e.g., equipment 

labels, tags, tape) in the path of the jet. The energy of the jet generated by a postulated LOCA 

event is sufficient to reduce much of this material to fibers or fine particulate debris, which can 

be readily transported by the escaping coolant to the suppression pool (in BWRs) or the 

containment sump (in PWRs). In addition, chemicals in the coolant can interact with other 

materials (e.g., insulation, metal components) present in the containment environment to form 

chemical reaction by-products (e.g., corrosion products, solid precipitates) that constitute 

chemical debris. As emergency cooling water is drawn from the suppression pool or sump by 

the ECCS, the debris will be carried to the suction strainers, forming a debris bed that will result 

in an increased pressure drop (head loss) across the strainer or screen. This increase in pressure 

drop could potentially have adverse effects on short-term emergency cooling and long-term 

cooling of the reactor core in post-LOCA conditions, including loss of net positive suction head 

(NPSH) at the ECCS pumps, structural failure of strainers and supports, and flow blockages or

equipment damage due to debris in the coolant. 

Closure of the issues raised in the mid-1990s related to debris clogging of BWR suction strainers 

occurred in 2001 with the issuance October 18, 2001 of NRC Memorandum Completion of Staff 

Reviews of NRC Bulletin 96-03 – Potential Plugging of Emergency Core Cooling Suction 

Strainers by Debris in Boiling Water Reactors, and NRC Bulletin 95-02 – Unexpected 

Clogging of a Residual Heat Removal (RHR) Pump Strainer while Operating in Suppression 

Pool Cooling Mode, (ML0129702290). NRC staff concluded that BWR licensees were 
xiv

cognizant of the need to minimize latent debris sources by regular cleaning of the suppression 

pool and by developing and maintaining pro-active housekeeping procedures and Foreign 

Material Exclusion (FME) programs. NRC staff further concluded that installing large-capacity 

passive strainers provides assurance that NPSH can be maintained when operating the ECCS in 

recirculation mode during a LOCA.

The ECCS strainer performance has entered the ―implementation phase‖ for PWRs with the 

dissemination of Regulatory positions provided in NRC Staff Review Guidance Regarding 

Generic Letter 2004-02 Closure in the Area of Strainer Head Loss and Vortexing

(ML080230038), issued in March 2008. This document contains detailed guidance in the areas 

of test scaling, debris near-field settlement simulation, surrogate debris similitude requirements, 

testing procedures, post-test data processing, and extrapolation to conditions beyond the tested 

database. The specific issue of downstream effects on components, however, is still under 

review, with the submittal in April 2009 of Revision 1 of WCAP-16793-NP, Evaluation of LongTerm Cooling Considering Particulate, Fibrous and Chemical Debris in the Recirculating Fluid, 

by the PWR Owners Group (PWROG). The SE on this report is expected to be completed in 

2010.

The objective of the work documented here is to evaluate the significance of disparities in the 

treatment of suction strainer performance for BWRs and PWRs. This report presents a technical 

evaluation of three main issues in which guidance is significantly different for BWRs and PWRs, 

and provides recommendations for appropriate treatment that is consistent for the two reactor 

types. The main issues are as follows;

debris generation

- definition of the zone of influence (ZOI) of the destructive effects of the high-energy 

water or steam jet due to the pipe break characerizing a particular LOCA scenario

- definition of source terms due to chemical interactions in the coolant and with 

containment materials exposed to coolant during the LOCA

- definition of source terms due to physical damage to containment materials, 

structures, and system components during the LOCA

debris bed formation on suction strainers

debris transport during the LOCA and post-LOCA long-term cooling of the core

- in the suppression pool or sump 

- in the primary system downstream of the suction strainers

The general conclusion of this evaluation is that the two types of reactors should have the same 

or similar technical bases for ensuring adequate long-term cooling of the reactor core following a 

LOCA. The overall analysis for either reactor type should include the following elements:

1. a prototypic or conservative estimate of how much debris would be generated in a given 

LOCA event

2. a prototypic or conservative methodology for determining how much debris would reach the 

suction strainer(s) 

3. a prototypic or conservative methodology for estimating how the debris bed forms, including 

effects of chemical debris 
xv

4. a prototypic or conservative methodology for estimating the head loss across the suction 

strainer(s) for all possible debris bed characteristics for a given plant

5. a prototypic or conservative methodology for estimating the amount of debris that could pass 

through the screens or strainers

6. a prototypic or conservative methodology for estimating the effects of debris in the coolant 

on downstream components, including the vessel and core (e.g., increased pressure loss, 

erosion, plugging)

The conservatism of the overall approach for determining NPSH margin depends primarily on 

developing a sound technical basis for defining the ZOI and the models for debris generation 

within the ZOI for a given LOCA event. This definition should be equivalent for both types of 

reactors (although dependent on jet specifics), and should be formulated to yield conservative 

estimates of the amount and specific types of debris generated. The debris generation 

methodology must be conservative if the overall analysis is to be conservative. 

The debris transport methodology should be consistent between PWR and BWR systems, since 

the same type of hydrodynamic forces are involved. Differences should relate only to 

differences in details of containment configuration, sump or pool dimensions, and structural 

design of the strainers. Similarly, the methodologies for determining debris bed formation and 

head loss across the debris bed for the suction strainers should reflect similar hydrodynamic 

behavior, with differences reflecting differences in strainer geometries and support structures. 

Table S.1 lists the specific recommendations developed to provide consistent guidance for 

analysis of the effects of debris on system performance during a LOCA and subsequently during 

post-LOCA recirculation cooling in PWRs and BWRs. The technical basis for these 

recommendations and the existing guidance on the related issues are discussed in detail in the 

body of this report.
xvi

Table S.1. Summary of recommendations for developing conservative and consistent guidance 

for analysis of post-LOCA recirculation cooling in PWRs and BWRs.

Index

number Recommendation

Debris Characteristics (see Section 2)

2.1 Plant-specific determination of the types, quantities, and distributions of physical 

debris, similar to the individual plant walkdowns for PWRs, is recommended for all 

commercial light water reactors, including BWRs. A sampling methodology, such 

as the guidelines offered through the SE to NEI 04-07, should be implemented 

across all plants to determine the relative quantity of fibrous debris. Methods to 

estimate the quantities and types of insulation debris, the largest contributor to the 

post-LOCA debris inventory, should be unified across BWRs and PWRs.

2.2 A determination of the effects of coolant, solutes, and insulation on the creation of 

chemical debris and the influence of the debris on head loss and downstream effect, 

along the lines of the ICET program and Westinghouse studies conducted for PWRs,

is recommended for BWRs.

Debris Generation (see Section 3)

3.1 The zone of influence (ZOI) of the high-energy jet of steam or saturated liquid water 

released in a LOCA should be determined using an experimentally validated free-jet 

expansion model that is applicable to both BWR and PWR conditions.

3.2

A validated basis that is consistent as applicable between reactor types for 

insulation material failure pressures should be developed for the range of 

thermodynamic conditions encountered in LOCA scenarios.

3.3

A validated basis consistent as applicable between reactor types for qualified and 

unqualified coatings thickness should be developed.

3.4

Reducing potential debris quantity by means of the definition of a specific ZOI 

extent, debris location, and contribution to subsequent head loss should only be 

considered after validated and consistent approaches for free-jet expansion, debris 

material failure pressure, and debris quantity are established.

3.5 A validated approach consistent as applicable between reactor types for the failure 

of insulation and coating systems outside of the ZOI is recommended.

Debris Bed Formation on Strainers (see Section 4)

4.1

Evaluate the specific strainer designs currently installed in BWR plants, on a plantby-plant basis (if necessary), with regard to test scaling, debris near field settlement 

simulation, surrogate debris similitude requirements, range of independent 

variables tested, and testing procedures, to determine if the tests and evaluations 

can be considered prototypic or conservative with respect to these parameters.

4.2

Apply the same standards and guidance to evaluations of submittals from BWR 

licensees regarding suction strainer head loss calculations, including the potential 

for thin bed effects, as are applied to submittals from PWR licensees.
xvii

Table S.1. Summary of recommendations for developing conservative and consistent guidance 

for analysis of post-LOCA recirculation cooling in PWRs and BWRs.

Index

number Recommendation

Downstream Effects of Debris in Recirculating Coolant (see Section 4)

4.3

Require validation of debris ingestion models with experimental data obtained for 

conditions where the maximum amount of debris is able to pass through the suction 

strainers. This should include the evaluation of conditions where an incomplete 

debris bed might form, and generally corresponds to conditions where the effect of 

debris on strainer head loss may be relatively low.

4.4

Require validation of abrasion and erosion wear models for specific particulate 

materials and ranges of particle sizes postulated for debris generated in BWR and 

PWR LOCA scenarios.

4.5

Apply the same standards and guidance to evaluations of submittals from BWR 

licensees regarding effect of debris in the recirculation coolant on downstream 

components as are applied to submittals from PWR licensees.

4.6

Require prototypic testing of debris mixtures in core flow at pressures and 

temperatures corresponding to post-LOCA conditions to determine the effect of 

local blockages on local fuel rod cladding temperatures for postulated for BWR and 

PWR LOCA scenarios. Include testing to show the effects of debris left behind by 

core boil-off.

4.7

For PWRs, require testing to determine the effects on local fuel rod cladding 

temperatures of chemical plate-out (with and without trapped debris) for forced flow 

and core boil-off conditions in postulated for LOCA scenarios.

4.8

Apply similar standards and guidance to evaluations of submittals from BWR 

licensees regarding effects of debris in the reactor vessel and core as are applied to 

submittals from PWR licensees.

Debris Transport in Sump or Suppression Pool (see Section 5)

5.1

Unless an assumption of 100% transport is employed, the approach used for flow 

field modeling in the sump and suppression pool should be validated and consistent 

in the basic approach and the degree of conservatism of assumptions.

5.2

Settling behavior of debris in the sump and suppression pool, if credited, should be 

based on the properties of the specific debris material, considering particle density,

geometry, and size distribution.

xix

ACKNOWLEDGMENTS

The work described in this report was sponsored by the Office of Nuclear Regulatory Research 

of the U.S. Nuclear Regulatory Commission. The authors would like to express their gratitude to 

the NRC Project Manager John Burke for his guidance and to the Office of Nuclear Regulatory 

Research and Office of Nuclear Reactor Regulation reviewers for their support and comments in 

performing this study. The authors wish to acknowledge the contribution of Mark Morgan in the 

early phase of this work and the excellent editorial contributions of Wayne Cosby. Finally, we 

wish to thank Loni Peurrung and Al Ankrum for their review of the final report.

xxi

ACRONYMS AND ABBREVIATIONS

ACRS Advisory Committee on Reactor Safeguards

ANS American Nuclear Society

ANSI American National Standards Institute

ASTM American Society for Testing and Materials

BWR boiling water reactor

BWROG BWR Owners Group

CFD computational fluid dynamics

CSHL clean screen head loss

CSS containment spray system

DBA design basis accident

DBLOCA design-basis loss-of-coolant accident

ECCS emergency core cooling system

EOP Emergency Operations Procedure

EPRI Electric Power Research Institute

FME Foreign Material Exclusion (program)

HEPA high-efficiency particulate air (filter)

HVAC heating, ventilation and air conditioning

ICET integrated chemical effects testing

IOZ inorganic zinc

LANL Los Alamos National Laboratory

lbm pounds, mass

LOCA loss-of-coolant accident

M molar

m micrometer

NDE non-destructive evaluation

NEDO Nuclear Energy Development Organization

NEI Nuclear Energy Institute

NPSH net positive suction head

NRC U.S. Nuclear Regulatory Commission
xxii

PCI Performance Contracting, Inc. (developer of stacked-disk suction strainer 

design for BWRs)

ppm parts per million

psi pounds per square inch

PWR pressurized water reactor

PWROG PWR Owners Group

QA quality assurance

QC quality control

RHR residual heat removal

RHRS residual heat removal system 

RMI reflective metal insulation

SE safety evaluation

SLC standby liquid control

SPB sodium pentaborate

SSE safe shutdown earthquake

TBE thin bed effect

TSP trisodium phosphate

wt% weight percent

ZOI zone of influence
1

1. INTRODUCTION

In the evolution of the technical knowledge base used to verify the adequate suction strainer 

performance for the currently licensed and operating nuclear reactors in the United States, 

differences have developed in the regulatory treatment of pressurized water reactors (PWRs) and 

boiling water reactors (BWRs). Some of the differences in the guidance for suction strainer 

performance analysis are due to technical differences in the basic design of the two reactor types, 

but in many cases, the differences arose for historical reasons or because of the state of 

knowledge and regulatory focus at the time of the relevant reviews. 

The objective of this report is to identify any significant disparities in the treatment of suction 

strainer performance for BWRs and PWRs, and evaluate the technical significance of the 

differences, to provide a basis for evaluating the potential impact on safety. This work considers 

only currently licensed reactors and does not address the relevance of this issue to new reactor 

designs or advanced passive reactors. Differences are assessed in terms of their effects on 

maintaining long-term core cooling in postulated design-basis accident conditions. 

Recommendations are provided on appropriate guidance for defining the technical requirements 

to address this issue in a consistent and conservative manner for both reactor types.

Section 1.1 summarizes the current regulatory position on this issue and provides a brief 

historical timeline of relevant developments. Section 1.2 briefly describes the areas where 

treatment of the reactor types differs. 

1.1 Background

The U.S. Nuclear Regulatory Commission (NRC) licenses and regulates commercial nuclear 

power plants with the goal of ensuring the protection of public health and safety and of the 

environment. The specific regulatory positions relevant to the issue of suction strainer 

performance are documented in Regulatory Guide 1.82, Revision 3, Water Sources for LongTerm Recirculation Cooling Following a Loss-of-Coolant Accident. The issue is addressed in 

three main parts: 

strainer blockage, expressed primarily as the potential for loss of required net positive suction 

head (NPSH) at the inlet to the ECCS pumps

availability of long-term recirculation cooling capability

debris sources and generation

Appendix A provides a side-by-side listing of the relevant positions in Regulatory Guide 1.82, 

Revision 3 for BWRs and PWRs, in a table containing the full text of the guidance provided. 

The table also includes notes comparing each Regulatory Position for the two reactor types, 

pointing out differences and similarities. 

The comparison shows that there are no significant differences in the regulatory positions for the 

two types of reactors, other than minor divergences due to differences in the basic designs. 

Based on Regulatory Guide 1.82 (Revision 3), it is clearly the intent of the NRC that there 
2

should be no significant differences in the requirements imposed on BWRs and PWRs in regard 

to demonstrating appropriate suction strainer performance in a LOCA event. However, this issue 

has been significantly re-evaluated three times in the past 30 years, and attention has been drawn 

to different aspects of the issue at different times. As a result, the issue has recently been 

examined independently for BWRs (in response to strainer plugging events at BWRs in the 

1990s) and for PWRs (in the ongoing effort to resolve Generic Safety Issue 191). These factors 

all tend to result in different treatment between the two reactor types. A timeline of significant 

events related to strainer blockage is included in Appendix B. The timeline is intended to 

present an overall prespective on the evolution of this complex issue, but is not meant to be 

inclusive of every industry and regulatory report or action. The following discussion 

summarizes the major highlights of this history. 

The initial regulatory guidance published in Regulatory Guide 1.82 in 1974 on ECCS intake 

blockage was relatively simple; the head loss across the suction strainer was required to preserve 

an adequate NPSH with the strainer surface area 50% blocked. This was considered a 

conservative assumption at that time. However, there were subsequent concerns about verifying 

adequate recirculation flow for the ECCS from the sumps in PWRs. These concerns were 

centered on problems with air entrainment, vortexing, and sump blockage due to fibrous debris, 

and resulted in the development of Unresolved Safety Issue A-43, Containment Emergency 

Sump Performance, published in January 1979. The principle focus of USI A-43 was on PWR 

designs, although analysis and experiments for BWRs were also conducted under this program.

The matter was considered resolved in 1985 by the publication of NUREG-0896, USI A-43 

Regulatory Analysis, and NUREG-0897, Containment Emergency Sump Performance. 

Section 6.2.2 of the Standard Review Plan was revised, and Revision 1 was developed for 

Regulatory Guide 1.82 (published November 1985). The guidance specifying the assumption of 

a 50% flow area blockage was revised to recommend that a deterministic analysis of potential 

blockage of strainers be performed. In December 1985, NRC issued Generic Letter 85-22, 

Potential for Loss of Post-LOCA Recirculation Capability Due to Insulation Debris Blockage, 

notifying licensees of the USI A-43 evaluation results. This generic letter stated that, while a 

backfit would not be imposed on operating reactor licensees, licensees performing plant 

modifications such as insulation changeouts should conduct a strainer performance analysis as 

specified in Revision 1 of Regulatory Guide 1.82.

Attention later shifted to suction strainers in BWRs with the occurrence of strainer blockage 

incidents at four domestic power plants and one foreign BWR-type reactor:

1988, 1989; strainer blocking events at Grand Gulf Nuclear station (BWR/6, Mark III) during 

testing of residual heat removal (RHR) pump suction strainers(a)

May 1992, March 1993; strainer blockage events(b)at Perry Nuclear station (BWR/6, 

Mark III)

 

(a) The RHR loop has separate suction strainers from those seen by the ECCS pumps, but of the same design.

(b) Strainers were deformed due to excessive head loss in one incident at the Perry plant.
3

July 1992; strainer blockage incident at the Barsebäck Unit 2 BWR plant(c)in Sweden, 

resulting from fibrous debris blown into containment from damaged insulation due to 

opening of a relief valve 

January 1994; cloth-like material partially blocked the ECCS suction strainers at Browns 

Ferry Unit 2 (BWR/4, Mark I) (discovered by divers assessing conditions in suppression 

pool)

September 1995; strainer blockage incident at Limerick Unit 1 (BWR/4, Mark II); pump 

cavitation in ECCS loop indicated 

In response to these incidents, NRC began a re-examination of this issue for BWR systems. 

While taking interim steps to address the problem, NRC also initiated the following detailed 

studies of debris generation in postulated LOCA events, which defines the limiting case for 

debris in the suppression pool:

In May 1993, issued Bulletin 93-02, Debris Plugging of Emergency Core Cooling Suction 

Strainers, to all nuclear power plant licensees, requesting interim actions to remediate 

potential problems related to debris in the suppression pool 

In February 1994, issued Supplement 1 to Bulletin 93-02, requesting further interim actions 

of licensees 

In October 1995, issued Bulletin 95-02, Unexpected Clogging of a Residual Heat Removal 

Pump Strainer While Operating in Suppression Pool Cooling Mode, to all operating BWR 

licensees, requesting that they take action to ensure that unacceptable build-up of debris in 

the suppression pool would not occur during normal operation

In May 1996, issued Revision 2 of Regulatory Guide 1.82, altering the debris blocking 

evaluation guidance for BWRs, since it had become apparent that Revision 1 guidance was 

not comprehensive enough

In May 1996, issued Bulletin 96-03, Potential Plugging of Emergency Core Cooling Suction 

Strainers by Debris in Boiling Water Reactors, to all holders of operating licenses, requiring 

that they implement specific measures, as appropriate to plant-specific conditions, to ensure 

ECCS performance following a LOCA

In October 1997, issued Generic Letter 97-04, Assurance of Sufficient Net Positive Suction 

Head for Emergency Core Cooling and Containment Heat Removal Pumps, to all nuclear 

power plant licensees, requesting current information on NPSH analyses 

In July 1998, issued Generic Letter 98-04, Potential for Degradation of the Emergency Core 

Cooling System and Containment Spray System after Loss-of-Coolant Accident Because of 

Construction and Protective Coatings Deficiencies and Foreign Material in Containment, to 

all licensees of operating nuclear power plants (BWR and PWR), requesting information on 

licensees‘ programs for ensuring that protective coatings do not detach from substrate during 

a design basis loss-of-coolant accident (DBLOCA).

 

(c) Plant configuration at Barsebäck is similar to a BWR/4 with Mark II containment.
4

In November 1996, the BWROG submitted Utility Resolution Guidance for NRC review and 

approval in NEDO-32686, Rev. 0, Utility Resolution Guidance for ECCS Suction Strainer 

Blockage. In August 1998, NRC published the Safety Evaluation (SE) on this document, 

granting approval of the guidance, but with some important reservations and limitations. Shortly 

thereafter, in October 1998, the BWROG published the approved guidance as NEDO-32686-A, 

Utility Resolution Guidance for ECCS Suction Strainer Blockage. This document consists of 

four volumes and contains the original(d) NEDO-32686, Revision 0 (including all references 

from subcontracted research), the SE document issued by NRC, and a summary of the specific 

elements NRC did not accept or approve.

Through continuing evaluation of operational events, various analytical and experimental 

studies, and other research work, the NRC developed a comprehensive technical basis for 

resolving the BWR strainer blockage issue. The NRC guidance, and the technical basis, was 

summarized in LA-UR-01-1595, BWR ECCS Strainer Blockage Issue: Summary of Research 

and Resolution Actions, in March 2001. 

In 1996, during the investigation of the BWR strainer blockage issue, NRC initiated a study of 

Generic Safety Issue 191, Assessment of Debris Accumulation on PWR Sump Performance, to 

determine if debris in a PWR containment could impede ECCS or CSS operation. This work led 

to the development of the technical basis for the NRC position that sump blockage is a credible 

concern for PWRs. The NRC implemented the resolution stage of this issue in September 2001 

and documented this position with the following actions:

In September 2001, published NUREG/CR-6762, Volume 1, GSI-191 Technical Assessment: 

Parametric Evaluations for Pressurized Water Reactor Recirculation Sump Performance

In August 2002, published three additional volumes of NUREG/CR-6762:

o Volume 2, Summary and Analysis of US Pressurized Water Reactor Industry Survey 

Responses and Responses to GL 97-04

o Volume 3, Development of Debris Generation Quantities in Support of the Parametric 

Evaluation

o Volume 4, Development of Debris Transport Fractions in Support of the Parametric 

Evaluation

In November 2003, issued Revision 3 of Regulatory Guide 1.82, expanding and clarifying 

the guidance for both BWRs and PWRs related to maintaining adequate recirculation coolant 

flow in a LOCA and post-LOCA environment

In June 2003, issued Bulletin 2003-01, Potential Impact of Debris Blockage on Emergency 

Sump Recirculation at Pressurized-Water Reactors, to all PWR licensees, requesting that 

they implement interim measures to mitigate the potential impacts of post-LOCA debris on 

strainer performance.

In September 2004, issued Generic Letter 2004-02, Potential Impact of Debris Blockage on 

Emergency Recirculation During Design Basis Accidents at Pressurized Water Reactors, to 

 

(d) The BWROG did not revise the original document to address the NRC concerns and specific limitations. The 

guidance on these issues consists of advising the BWR licensees that they must justify the approach used for 

such issues in their individual submittals.
5

all PWR licensees, requesting that they perform an analysis of their strainer performance 

using a mechanistic methodology and perform any necessary plant modifications (e.g., 

strainer replacement) that the analysis demonstrated to be necessary.

To develop an approved methodology for PWR licensees to use in responding to Generic Letter 

2004-02, the PWR Owners Group (PWROG) sponsored guidance documents developed through 

the Nuclear Energy Institute and Westinghouse Electric Corporation. Documents submitted to 

the NRC for review and approval included:

In April 2002, the PWR Owners Group submitted NEI 02-01, Condition Assessment 

Guidelines: Debris Sources Inside PWR Containments.

o In September 2002, this submittal was superseded by Revision 1 to NEI 02-01

(ML030420318), in response to comments from member utilities and NRC staff, and to 

incorporate new information on latent debris.

o In meetings with NEI in 2002 and 2003, NRC provided comments on NEI 02-01, 

indicating that the document provided reasonable overall guidance, but also 

recommended additional work.

In May 2004, the PWROG submitted NEI 04-07, Pressurized Water Reactor Sump 

Performance Evaluation Methodology.

o In December 2004, NRC issued the SE on NEI 04-07, approving the Baseline 

Methodology for sump performance issues, with certain limitations and modified 

positions. By direct reference, substantial portions of NEI 02-01 are included in the SE 

on NEI 04-07.

In August 2006, the PWROG submitted technical report WCAP-16406-P, Rev. 1, Evaluation 

of Downstream Sump Debris Effects in Support of GSI-191, for review and approval.

o In December 2007, NRC issued the SE of WCAP-16406-P, approving the guidance 

provided, but subject to limitations and conditions.

In September 2007, the PWROG completed submittal of technical report WCAP-16530-NP, 

Evaluation of Post-Accident Chemical Effects in Containment Sump Fluids to Support GSI191, for review and approval.

o In December 2007, NRC issued the SE of WCAP-16530-NP, approving the guidance 

provided, but subject to limitations and conditions.

In June 2007, the PWROG submitted technical report WCAP-16793-NP, Evaluation of 

Long-Term Cooling Considering Particulate, Fibrous and Chemical Debris in the 

Recirculating Fluid, for review and approval.

o In February 2008, the NRC developed a draft SE of WCAP-16793-NP, with the intention 

of releasing the final SE by the end of March 2008.

o On March 7, 2008, the NRC transmitted the Revision 0 draft of the SE of WCAP-16793-

NP, requesting comment on any factual errors or clarity concerns from the PWROG.

o On July 24, 2008, the NRC issued a recision of the draft SE for WCAP-16793-NP, 

having determined that additional information is needed on certain subject areas in the 

report, which the PWROG agreed to address by submitting Revision 1 of TR WCAP16793-NP.
6

o In April 2009, Revision 1 of TR WCAP-16793-NP was submitted for review.

As can be seen from the above timeline summaries, the issue of suction strainer clogging 

followed independent paths for BWRs and PWRs. This is a major reason for differences 

between the BWROG and PWROG guidance, and in the regulatory guidance. However, there 

are no significant technical reasons for treating the two types of reactors differently in regard to 

the general issue of ensuring adequate long-term cooling of the reactor core following a LOCA 

event. The following section summarizes the specific differences between the guidance provided 

for BWRs and PWRs with regard to this issue.

1.2 Differences in Guidance for PWRs and BWRs

The detailed comparison in Appendix A shows that there are no significant differences in the 

treatment of PWRs and BWRs in Regulatory Guidance 1.82, as of Revision 3. In both designs, 

a LOCA event is expected to generate debris in containment due to the high-energy jet of coolant 

escaping at the break location. The great majority of this debris has essentially the same 

characteristics in a PWR or BWR containment, with only a few exceptions specific to each 

design. In both designs, the escaping coolant, along with drainage from sprays and 

condensation, is expected to transport LOCA-generated debris and latent debris into reservoirs 

that supply long-term cooling water to the ECCS, CSS, and RHRS. In both BWRs and PWRs, 

the emergency coolant is drawn from the reservoir through suction strainers designed to filter the

debris from the coolant. Both types of plants can experience blockage of the suction strainers

due to debris build-up, and both can draw fine-scale debris through the suction strainers, 

introducing it into the recirculation loop, containment spray system, RHR system, and the reactor 

vessel and core.

Table 1.1 summarizes 12 technical issues where the guidance from industry groups and from 

NRC staff (as provided in SEs for specific submittals) for analyzing BWRs and PWRs has been 

identified as significantly different. This summary is based on the BWROG and PWROG 

guidance documents and the NRC SEs of these documents. Table 1.1 also notes similarities or 

differences in the relevant physical phenomena for BWRs and PWRs. These specific areas fall 

into four categories: debris characteristics, debris generation, debris bed formation on the 

strainers, and downstream effects of debris in the recirculating coolant. As shown in Table 1.1,

in nearly all cases there are no significant differences in the relevant phenomena between BWRs 

and PWRs. They could, therefore, be dealt with in a consistent manner in analyses for the two 

types of reactors.

These issues are discussed in greater detail in the following sections, in terms of the BWR and 

PWR industry guidance and the NRC SEs of the formal documentation of this guidance. In each 

of these sections, specific recommendations are developed regarding appropriate treatment of the 

issues. Section 2 discusses issues related to debris characteristics with particular attention to 

debris generated by chemical reactions. Section 3 discusses debris generation in terms of the 

definition of the zone of influence (ZOI) of the jet from the break, and the mechanical response 

of containment material impacted by the jet. Section 4 discusses appropriate characterization of 

the head loss across the debris bed on the strainer and evaluating the effects of debris 

downstream of the sump screens or suction strainers. Section 5 addresses the issue of debris 

settling within the sump or suppression pool. Section 6 presents a detailed summary of the 
7

evaluations and recommendations developed for defining consistent guidance for analyzing 

PWRs and BWRs with respect to these technical issues.

Table 1.1. Summary of differences in guidance for PWRs and BWRs.

 Technical Issue

Treatment in 

Guidance

Similar or Different Effects

in BWRs and PWRs?

Debris Characteristics (see Section 2)

Debris generation 

source terms include 

calcium silicate, MinK, Microtherm

insulation.

BWR: limited

PWR: detailed

Similar. Debris generation from insulation (including 

calcium silicate) is due to mechanical and thermal 

effects of jet impact; physical effects are same for 

LOCA conditions in PWRs and BWRs; differences are 

only in pressure and temperature values, and in some 

cases, jet medium.

Debris generation 

source terms include 

latent debris.

BWR: limited

PWR: detailed

Similar. Except for suppression pool sludge in BWRs, 

sources and types of latent debris are essentially the 

same in PWRs and BWRs.

Debris generation 

source terms include 

assessment of 

coatings.

BWR: limited

PWR: more 

detailed than 

for BWRs, but 

still limited

Similar. Coatings are essentially the same materials in 

PWRs and BWRs.

Debris generation 

source terms include 

chemical effects.

BWR: not 

considered

PWR: limited

Different in degree, similar in kind. For all PWRs, 

chemical additions are required for normal and 

emergency operations. Only some BWRs add

chemicals to coolant, and in limited quantities; should 

be assessed on a plant-specific basis.

Debris Generation (see Section 3)

Analysis includes 

ZOI adjustment for 

air jet testing, a 

separate ZOI for 

protective coatings, 

and includes option 

for a spherical ZOI 

approach

BWR: yes

PWR: yes, but 

with different 

limitations and 

restrictions 

than for BWRs

Similar. In both BWRs and PWRs, LOCA break 

results in high-energy steam jet or saturated water jet 

that rapidly turns to steam during blowdown. 

Insulation and protective coatings are subjected to 

mechanical impact at high temperatures and pressures; 

the physical extent of conditions severe enough to 

destroy or damage these materials is a critical 

component in determining the total debris generation.

Debris Bed Formation on Suction Strainers (see Section 4)
8

Table 1.1. Summary of differences in guidance for PWRs and BWRs.

 Technical Issue

Treatment in 

Guidance

Similar or Different Effects

in BWRs and PWRs?

Analysis considers 

head loss predictions 

for debris bed on 

suction strainers.

BWR: yes

PWR: yes, but 

with more 

limitations and 

restrictions on 

experimental 

databases

Similar. In both BWRs and PWRs, the hydrodynamics 

of flow through the debris bed are essentially identical; 

only the geometries differ in different BWR and PWR 

plants.

Downstream Effects of Debris in Recirculating Coolant (see Section 4)

Analysis considers 

erosion due to debris 

transport in the 

ECCS, CSS, and 

RHR system.

BWR: no

PWR: yes

Similar. Except for suppression pool sludge, erosive 

debris sources are essentially the same for BWRs and 

PWRs.

Analysis considers 

effects of debris (e.g., 

wear and flow 

blockage) on 

downstream 

components.

BWR: no

PWR: yes

Similar. BWR and PWR systems include pumps, 

valves, nozzles, orifices, and heat exchangers of similar 

size and for similar functions.

Analysis considers 

effects of debris (e.g., 

wear and flow 

blockage) on reactor 

vessel and fuel 

coolability.

BWR: no, 

except for 

active strainer 

designs

PWR: limited

Similar. BWR and PWR fuel assemblies have similar 

geometry, with spacer grids, inlet orifice plates, etc., 

with similar ability to capture debris from flow field. 

Both designs utilize core boil-off for long-term core 

cooling in design basis LOCA events, in which debris 

in the coolant could be left behind in the core. 

Debris Transport in Sump or Suppression Pool (see Section 5)

Analysis considers 

effect of debris 

settling in the sump 

or suppression pool.

BWR: optional

PWR: optional

Similar. Settling rates of particulate should be similar

in PWR sumps and BWR suppression pools, since 

debris constituents are similar.
9

2. DEBRIS CHARACTERISTICS

In the event of a LOCA in a BWR or PWR, the postulated pipe break results in the rapid escape 

of liquid water and/or steam of high energy from the primary system. The ensuing pressure 

wave and high-volume flow of coolant outside its intended channel will impinge on installed 

materials, destroying or altering their original forms and displacing them from their original 

locations as debris. Such installed materials include pipe insulation, coatings (e.g., paint, epoxy), 

and other installed materials (e.g., equipment labels, tags, tape). Some of this material can be 

damaged to such a degree by the energy of the LOCA that it will be reduced to fibers or fine 

particulate, which is readily transported by the blowdown from the pipe rupture. The escaping 

coolant and other flowing water (e.g., from containment sprays) will carry the material fibers and 

particles to the suppression pool (in BWRs) or the containment sump (in PWRs). As emergency 

cooling water is drawn from the suppression pool or sump by the ECCS, the debris will be 

carried to the suction strainers. 

Dirt, dust, and materials inadvertently left behind during outages and maintenance (e.g., rags, 

tools, HEPA filters, paper and plastic sheeting) are not installed (i.e., intentionally part of the 

BWR or PWR systems) but also exist within the region affected by post-LOCA coolant flows. 

Such materials, called transient or latent debris, also may be carried to the suction strainers or 

sump screens. These sources of debris are common to both BWRs and PWRs, although their 

quantities and distributions vary according to plant design (even within BWR or PWR groups), 

the applications of the materials, and differences in plant cleanliness and housekeeping 

procedures. The transient debris and the installed material will also be subject to damage and 

erosion both during the blowdown and during the long-term cooling period.

Other sources of debris exist that are unique to each reactor type. BWRs contain suppression 

pool sludge as a debris source that is not found in PWRs, which do not have a suppression pool. 

The suppression pool present in each BWR is constructed of mild carbon steel but may be lined 

with stainless steel or be coated on the wetted surfaces as a corrosion barrier. In addition to the 

materials collected in the suppression pools (dirt, debris), the sludge in the mild steel lined 

suppression pools contains steel corrosion products. PWRs produce debris under post-LOCA 

conditions from chemical effects (or reactions) that may occur to a lesser extent for BWRs. The 

difference lies in the fact that BWRs are cooled with essentially neutral demineralized water 

while PWRs are cooled with a dilute boric acid solution. During normal operations, the reactor 

coolant system (RCS) in PWRs has decreasing boron concentrations over the operating cycle. In 

the event of a LOCA, the refueling water storage tank (RWST) injects cool borated water into 

the RCS system. Typically, ~2800 parts of boron per million parts of water, or ~0.26 moles 

boron per liter (0.26 M), are present in the post-LOCA RCS, including the contribution from the 

RWST. The boric acid acts as a soluble neutron poison (―chemical shim‖), to ensure that the 

reactor remains shutdown following a LOCA. 

The chemical composition of the PWR boric acid coolant also is changed through the addition of 

chemicals to adjust to higher pH (i.e., neutral or greater) under post-LOCA conditions, primarily 
10

to aid in radioiodine control. The pH adjustment is accomplished in different ways in different 

plants, including the addition of

sodium hydroxide (NaOH; to about 0.2 M)

trisodium phosphate (TSP, Na3PO4·12H2O; to about 0.01 M)

sodium tetraborate (STB, also known as borax, Na2B4O7·10H2O; to about 0.028 M, or 0.11

M boron). 

These dissolved chemicals can interact with other materials present in the post-LOCA PWR 

containment environment to form chemical reaction by-products. 

Although the BWR cooling water does not contain dissolved boric acid during normal operation 

and therefore is immune from debris formation related to buffer additions/chemical reaction byproducts, chemical reactions including metal and insulation corrosion and interactions of 

dissolved materials with each other and with solids are possible in post-LOCA BWR waters. 

Some BWRs also manually add a solution of sodium pentaborate (SPB; Na2B10O16·10H2O) to 

the post-LOCA coolant by way of the standby liquid control (SLC) system as a back-up 

reactivity control or for radioiodine control. The typical quantity(e)of SPB added to the postLOCA coolant is equivalent to 600 lbm (pounds, mass) of boron (~3000 to 4000 gallons of 8 to 

10 wt% SPB) and is sufficient to give a final boron concentration of ~1100 ppm or ~0.1 M.

The debris generated by the physical destruction of materials or the mobilization of latent and

transient materials in BWRs and PWRs in the post-LOCA environment within containment is 

considered in Section 2.1. The debris generated by chemical reactions in BWRs and PWRs in 

the post-LOCA coolant and its interactions with the wetted materials, both other debris and fixed 

hardware, are considered in Section 2.2. These sections also include assessment of the 

comparative scope of knowledge of debris sources and generation in both PWRs and BWRs.

2.1 Physical Debris

The primary source of physical debris in both BWRs and PWRs is thermal insulation on piping, 

vessels, and other components of the system. Other significant contributors are transient debris 

(i.e., foreign materials), fixed debris (e.g., coatings), and latent debris (e.g., dirt, dust, and [for 

BWRs only] suppression pool sludge). The characteristics and sources of BWR physical debris 

are considered in Section 2.1.1. Physical debris sources and characteristics in PWRs are 

discussed in Section 2.1.2.

2.1.1 Physical Debris in BWRs

Debris for BWRs is categorized according to its genesis within the drywell or the wetwell (or 

Mark III containment), including the suppression pool. The primary source of drywell debris is 

thermal insulation materials. The primary source of wetwell debris is the suppression pool 

sludge. Section 2.1.1.1 describes pipe insulation sources, and Section 2.1.1.2 describes other 

drywell debris. Section 2.1.1.3 describes wetwell debris.

 

(e) For specific examples, see Columbia Generating Station, Calculation, ―Dose Calculation Database.‖ 

Calculation Number NE-02-04-1, Revision 2. Energy Northwest. Richland, WA. 2004. ADAMS accession 

number ML042930379, or Nuclear Engineering Calculation EC-059-1041, Rev. 02, ―Suppression Pool pH Post 

LOCA.‖ Susquehanna Steam Electric Station, Berwick, PA. 2006. ADAMS accession number ML063060122.
11

2.1.1.1 BWR Pipe Insulation

The BWROG guidance1offers four methods for determining the quantity of pipe insulation 

subject to destruction in a LOCA in a ZOI of the pipe break. (The definition of the ZOI 

obviously has a very important influence on the amount of debris generated. This issue is 

discussed in detail in Section 3. In Section 2, the topic is confined to a discussion of the 

characteristics of the debris only.) In all four methods, the fraction of the insulation located 

within the ZOI assumed to be destroyed into ―fines‖ is determined. A methodology for 

determining the actual degree of damage to various insulation materials within the ZOI (e.g., 

from complete destruction as fines to partially shredded into larger mats) is specified in Table 4 

of the guidance. 

2.1.1.2 Other BWR Drywell Debris

Transient debris is non-permanent plant material (tools, rags, temporary filters, dirt, and dust) 

brought into the drywell, often during an outage. Routine housekeeping and Foreign Material 

Exclusion (FME) programs are used to control transient debris. Fixed debris is non-insulation 

material that becomes debris by being dislodged during a LOCA. Paints and coatings or 

concrete fragments displaced by direct jet impingement are considered to be fixed debris. Other 

fixed debris that can become dislodged by a LOCA stream flow includes tape and wire ties. 

Fixed debris also arises after long exposure to LOCA conditions and might originate from 

coatings not qualified to withstand the environmental conditions (e.g., temperature, humidity, 

radiation) associated with design basis accidents (DBAs). Drywell debris sources and bounding 

quantities for some of these sources defined in the BWROG guidance are summarized in 

Table 2.1.
12

Table 2.1. Non-insulation drywell debris sources and quantities in BWRs.

Transient Fixed Latent

Dirt/dust (incl. ablated 

concrete)—150 lbm

Paint/coatings—47 lbm inorganic zinc (IOZ), 

85 lbm IOZ top-coated with epoxy, 71 lbm 

epoxy

Unqualified 

coatings*

Other—not quantified (e.g., 

tools, rags, temporary filters) Concrete – included with transient dust/dirt

Degraded 

qualified 

coatings*

Fabric equipment covers (e.g., for fire hose reel) Adhesive backed 

tags or labels

Permanent tags/stickers

Cloth equipment bags

Fire hoses

Ropes

Ventilation system filters

Cloth

Non-piping thermal insulation

Tape

Wire ties

Paper (signs, postings, diagrams)

Plastic laminate and sheeting

Rust from unpainted steel—50 lbm

Other material stored in the drywell

* Note that the Safety Evaluation of the BWROG guidance does not accept the URG position that 

unqualified coatings will fail later in the LOCA event after the pressure is reduced. The SE implies that 

the unqualified coatings should be assumed to fail early in the event.

2.1.1.3 BWR Wetwell Debris

Wetwell debris consists largely of suppression pool sludge, but also includes LOCA-generated 

debris (dirt and dust in the wetwell that is above the normal suppression pool level, corrosion 

products from unpainted steel, and unqualified paint), and any transient debris that had been 

dropped or introduced into the suppression pool such that, at LOCA initiation, it was already 

present.

The transient debris sources identified in the BWROG guidance document for the wetwell are 

somewhat more extensive than the transient debris defined in the drywell. Besides tools, rags, 

and temporary filters, the wetwell debris includes ropes, fiber or paper mats placed over gratings 

during outages, plastic sheeting, cloth-based duct tape, and anti-contamination clothing. Active 

FME and housekeeping programs, including surveillance and cleaning of the suppression pool, 

are designed to limit the amounts of transient debris present at the time of the LOCA. The 

transient debris is categorized as fibrous and non-fibrous. Some non-fibrous transient debris, 

including plastic sheeting, plastic clothing, plastic tags and step-off pads, and rubber gasket 

materials, pose another hazard to strainer clogging because of their potential to block large crosssectional areas. Two other types of transient debris pose little risk to strainer clogging: very 
13

heavy or very light materials. Tools and metallic hardware constitute non-fibrous transient 

debris that is dense, sinks rapidly in the suppression pool, and is not readily moved by stream 

currents. Transient debris that can float on the pool surface (e.g., foam insulation) cannot reach 

the submerged inlet strainers unless it degrades to lose its buoyancy or the suppression pool 

stream flows become sufficiently turbulent to entrain the debris.

Suppression pool sludge is primarily carbon steel corrosion products and dirt/dust. The balance 

between the sludge generation rate and the pool cleaning thoroughness and frequency controls 

the quantity of suppression pool sludge. A survey of 12 BWRs of various ages and all three 

containment types (Mark I, II, and III) showed a median value of dry sludge generation rate at 

88 lbm per year (Section 3.2.4.3.2 of Reference 1). The amount of sludge present at a given time 

in the suppression pool is greater than the annual sludge generation rate, however. The largest 

load was reported for the Duane Arnold plant in 1985, conservatively estimated as ~1350 lbm of 

sludge. The sludge load in the 15 other plants (excluding the Duane Arnold measurement) 

averaged ~300 lbm, with a maximum of about 510 lbm (Appendix J of the SE of Reference 1).

The BWROG guidance suggests that an accumulation rate of 150 lbm per year could be assumed 

to be conservative, but each plant should estimate its individual rate based on experience. Then, 

some margin should be added to account for uncertainties in operation variation and sampling 

and analysis. The BWROG guidance also recommends measurement considerations for 

determining the sludge generation rate. The guidance recommends a bounding value of 300-lbm 

annual sludge generation rate if plant-specific measurements are not available.

The sludge particle size distribution in the BWROG guidance was determined in two separate 

surveys (Section 3.2.4.3.1 of Reference 1). The particle size distributions were in three particle 

size bins – 0 to 5 m, 5 to 10 m, and 10 to 75 m. The combined results of the first (five plant) 

and second (an additional nine plant) surveys, which included all three containment types and a 

range of plant ages, closely matched the results of the first testing set alone. Based on the 

similarity of the results for the sampled plants, the BWROG guidance recommends assuming 

that the measured distribution can be applied to all plants. 

The size distribution on a particle number basis was 83% in the range 0 to 5 m (2.5 m 

average), 11% in the 5- to 10- m range (7.5 m average), and 6% in the 10- to 75- m range 

(42.5- m average). The initial (five plant) survey showed 81% in the 0 to 5 m range, 14% in 

the 5- to 10- m range, and 5% in the 10- to 75- m range. The air-dried weight of this debris 

was estimated as 0.385 times the water-saturated sludge weight. Further details of sludge 

characteristics (phases, chemical composition) are not specified in the guidance document. Its 

origin as a steel corrosion product and dirt/dust led to the presumption that the sludge is 

composed of iron oxides/hydroxides (rust), silicate minerals (dirt), and cellulose (dust from paper 

and fabrics). Confirmation of the sludge material composition is needed to predict, more 

accurately, the potential for the material to be transported to the strainers and, at the strainers, 

model their impact in contributing to blockage.

2.1.2 Physical Debris in PWRs

Guidance on physical debris sources in PWRs is provided in NEI 02-012and NEI 04-073

guidelines and in the NRC SE of NEI 04-07.4The debris sources within PWRs are similar to 
14

many of those identified for BWRs, except that PWRs do not have a wetwell and therefore do 

not accumulate the associated sludge debris found in BWRs. However, the guidance identifies 

sources of latent debris that include dirt, dust, and fibers in the dry containment. LOCAgenerated debris in PWRs is defined as broken or dislodged materials (such as insulation, 

coatings, tape, and dust) due to the action of fluid released from a postulated break of a highenergy water line inside containment.

The debris sources in operating PWRs have been assessed in each plant by facility walkdowns 

(see Section 5.1 of NEI 02-01).2The facility walkdowns were performed to determine the 

locations and amounts of insulation materials, unqualified coatings, and foreign materials present 

within containment. The extents of such walkdowns were plant specific due to plant design and 

to the extents of prior and ongoing individual assessment programs.

The walkdowns typically were conducted by personnel familiar with the equipment installation 

and by the ECCS systems engineer. The guidance recommends that they be conducted in the 

brief interval after the containment building has been cleaned following a refueling outage but 

prior to restart. This is suggested primarily to provide a good assessment of transient materials. 

Insulation and coatings could presumably be assessed earlier during the outage, as these plant 

features are unlikely to change as a result of outage activities.

2.1.2.1 NEI 02-01 Guidance for PWRs

The purpose of the guidance in NEI 02-012is to provide the plant operators with a consistent and 

systematic approach to gather information on the sources, types, and locations of potential debris 

that could be transported to the strainer in the event of a LOCA. Appendix A of NEI 02-01 

provides summary descriptions of the nature of information that should be collected in PWR 

plant walkdown surveys, the types of personnel necessary, preparations needed for walkdowns, 

and other considerations. Results that should be obtained are specified to include the types and 

quantities of the various insulation, insulation samples, piping layout drawings, cable tray layout 

drawings, assessments of plant housekeeping, and FME. The walkdowns may also serve as 

confirmation of the plant design configuration, and can help ensure that as-built drawings are 

available to assess GSI-191 issues. Details of the guidance with respect to pipe insulation, 

coatings, and foreign materials are discussed in Sections 2.1.2.1.1, 2.1.2.1.2, and 2.1.2.1.3, 

respectively.

2.1.2.1.1 PWR Pipe Insulation

The guidance notes that investigating the insulation, particularly the fibrous insulation, is a key 

part of the walkdown. One objective of the walkdown is to update as-built knowledge of the 

insulation distribution (i.e., identifying and documenting any insulation that may have been 

replaced during plant operations because of piping changes or insulation removal and 

replacement in the course of weld inspections). Other fibrous materials, such as filter media, fire 

barrier materials, and fibrous cable insulation, should be catalogued since these materials have 

been identified as potential sump screen blockage sources. The guidance lists 12 types of 

insulation (including Nukon , calcium silicate [Cal-Sil], and Min-K) that are routinely used in 

PWR containments. Although the guidance emphasizes fibrous insulation over other types of 

insulation, it is known that relatively small amounts of particulate or microporous insulations can 
15

have significant effect on sump strainer head loss. These other types of insulation must also be 

carefully identified and quantified. 

The guidance recommends that the type and distribution of the insulation, the thickness and size, 

the physical condition, the types of fastening, and other details should be mapped, beginning 

with the primary system and extending to other piping, equipment, temporary equipment, and 

structures. Penetrations within the crane wall and bioshield wall, which could potentially be 

influenced by a high-energy line break, are to be surveyed as well.

During the survey, other sources of fibrous materials, such as insulated equipment, penetration 

insulations, fire barriers, heating, ventilation and air conditioning (HVAC) air cleaning filter 

media, electrical cable trays, and electrical cables within containment should be logged. In 

addition, piping insulation that might be eroded and transport due to the impingement from 

containment spray should be considered. Because the size and location of regions affected by a 

LOCA can vary greatly over the large range of postulated events, the guidance recommends that 

the entire inventory of insulation should be surveyed.

2.1.2.1.2 PWR Coatings

The quantities and distributions of coatings within containment also were surveyed by walkdown 

during PWRs‘ refueling outages. The guidance recommended that walkdowns to survey 

coatings be performed by a coatings specialist or other personnel familiar with the application 

and maintenance of coatings. Consistent with the NRC position, the guidance assumes that all 

coatings within the ZOI of a given LOCA, whether DBA-qualified or unqualified, are expected 

to fail. The coatings outside of the ZOI that are DBA-qualified are not expected to contribute to 

the coatings debris unless current information from plant walkdowns (as discussed in Section 

2.1.2.1 above) shows them to be degraded. The unqualified coatings also may be assumed to fail 

even if outside the ZOI. 

The DBA-qualified coatings commonly used in PWR containments are listed in Section 5.2.2.3 

of NEI 02-01 according to their application to concrete or steel substrates. Surfaces that may 

have unqualified coatings are listed in Section 5.2.2.4 of NEI 02-01 and generally include 

installed equipment (e.g., accumulator tanks, valves, manipulator crane, electrical cabinets, 

instrumentation, pump motors) as opposed to structural elements.

The objective of the coatings walkdown is to map the locations and class (DBA-qualified or 

unqualified) of all coatings. The approximate areas and thicknesses of the coatings also are to be 

mapped. Documentation of the coatings surveys is part of the program.

2.1.2.1.3 PWR Foreign Materials

Walkdowns are also recommended in the NEI guidance for assessing foreign materials in the 

plant. Foreign materials include tape, equipment labels, construction and maintenance debris, 

temporary equipment, dirt, dust, and lint that may be transported to the strainers after an accident 

or block water flow into the sump. By their nature, the surveys for dirt, dust, and lint should be 

more regionally directed such that areas where this type of material builds up (e.g., in cable trays, 

corners, floor recesses, ledges) are identified as well as the general distribution of the material. 

The guidance recommends that foreign material should be minimized by FME and housekeeping 
16

practices. The nature and quantity of buildups of dust and dirt are to be recorded or logged in 

some manner, and, if possible, samples of the dust and dirt should be collected to assess particle 

size and density.

2.1.2.2 NEI 04-07 Guidance for PWRs

The guidance in NEI 04-073recommends determining debris quantities based on the results of 

the containment walkdown and information on the debris properties. The physical properties of 

many of the debris materials are provided in NEI 04-07. The debris characteristics for PWRs are 

described in Section 3.4.3 of NEI 04-07 for insulation and coatings, and in Section 3.5 for latent 

debris. The associated NRC SE4provides further guidance on the debris characteristics.

Relevant debris characteristics include the size distribution, particle shape, micro-density 

(particle density), and macro-density (as-fabricated bulk density) of the material. The PWR 

guidance relies on the BWROG destruction testing done as described in the BWR guidance 

document.1 However, that destruction testing did not generate all data needed for either BWR or 

PWR conditions and variety of materials. Therefore, the industry guidance for PWRs 

recommends adopting a two-regime size distribution for materials within the ZOI; small fines 

(which are smaller than 4 inches by 4 inches and can pass through gratings, trash racks, or 

radiological fences), and large pieces (which consist of all fragments larger than 4 inches by 4 

inches.) The portion of fibrous or particulate (e.g., calcium silicate) debris classified as fines is 

assumed to exist as individual fibers or particles as a result of break flow impact. While the fiber 

in the fines cannot be further reduced by water-flow erosion, the particulate fines still are 

susceptible to erosive comminution. 

In the baseline methodology, the large pieces of fibrous debris are assumed not to be subject to 

further size reduction and therefore are not transported to the sump. For purpose of performing a 

refined debris transport analysis that credits debris settlement, the NRC SE4 indicated further 

refinement was necessary. Although the SE found that the transport guidance for small debris 

fines was acceptable, the guidance for the large pieces of debris was not acceptable because of 

the unrealistic assumption that large pieces of debris could not be transported particularly for 

plants whose configurations lead to fast pool velocities. The SE also found that the method 

recommended for determining the quantity of fine debris trapped in inactive pools based on the 

volume ratio of inactive pools to the total pools was unrealistic for plants with large inactive 

pools. Therefore the SE recommended that licensees limit the maximum fraction of fine debris 

being trapped in inactive pools to 15 percent to avoid non-conservative results. The NRC 

evaluation recommends a further subdivision of small fines into suspended fines (which largely 

remain suspended in the sump pool flows) and small pieces (which will transport along the floor 

in the water-stream flow). Section VI.3.2.1, Fibrous Insulation Debris-Size Categorization, of 

Appendix VI of the SE recommends a path forward to determine the physical qualities and 

distributions of the suspended fines and small pieces with respect to the other two classifications 

of fibrous debris, large pieces and intact insulation.

In the same manner as for insulation, coatings within the ZOI are considered to be fines of the 

dimensions of the original pigments. The coatings particle dimensions were conservatively 

assumed to be 10 m.
17

The guidance recommends assuming that all jacketed insulation outside of the ZOI remains 

intact and is not eroded or disintegrated by any impinging flow (i.e., containment spray or flow 

from the break.) The behavior can be inferred by the lack of further destruction of damaged 

jacketed material from within the ZOI during transport to the sump. Testing has confirmed this 

behavior with Nukon blankets (Section 3.4.3.3.4 of Reference 3). The testing also showed that 

as much as 52% of the mass of Nukon within the ZOI was reduced to fragments that could be 

classified as small fines (Section 3.4.3.2 of Reference 3). Based on these results, the guidance 

recommends assuming, as a measure of conservatism, that the fibrous insulation within the ZOI 

breaks up into 60% small fines. The NRC SE4took issue with the methodology used to arrive at 

the 60% figure, but accepted the recommendation as conservative. The guidance recommends 

assuming similar size-distribution values as those obtained with Nukon for many other materials, 

based on BWROG testing. For fibrous materials that were not tested, the recommendation is to 

assume 100% reduction to small fines as a conservatism.

The guidance recommends assuming all reflective metal insulation (RMI) within the ZOI fails as 

75% small fines and 25% large pieces. This is based on BWROG testing of a single RMI type 

and observation of larger destructive pressures for other RMI types.

For particulate insulation and other containment materials for which destruction test results were 

limited or unavailable, the guidance recommends assuming 100% destruction to small fines 

within the ZOI. Such materials include Cal-Sil, Microtherm, Min-K, Koolphen, fire barrier, and 

lead wool. Materials outside of the ZOI are by definition not affected by the break jet, but may 

be damaged by water flows from the containment spray or water draining to the containment 

sump. Jacketed materials (insulations, fire barriers, lead wool) can be expected to remain 

undamaged under such conditions. Unjacketed materials are conservatively considered to fail 

completely to small fines. 

The DBA-qualified coatings are expected to remain intact, but the non-qualified, indeterminate, 

or unacceptable coatings are assumed to fail completely outside of the ZOI.

In Section 3.4.3.5 of the NEI 04-07 3guidance, a sample calculation is provided to determine the 

quantity of debris materials (insulation and coatings) generated in the ZOI and the contribution 

by the total quantity of unqualified coatings outside of the ZOI. NRC 4takes issue with the 

estimate of the quantity of insulation based on the testing which was limited to one insulation 

type (NUKON) and particularly coatings debris. The NRC recommendation is that assessments 

be made based on plant-specific characterization and not default coatings thicknesses.

2.1.2.2.1 Insulation and Coatings Debris Characteristics

Section 3.4.3.6 of NEI 04-07 3provides tabulated information on debris characteristics of 15 

types of insulation (fibrous and particulate) and 5 types of coatings. The associated SE report4

discussion takes issue with some of the values and ranges of values presented in the guidance. 

Where large ranges are given, the SE4recommends that as-installed plant-specific values be 

used. Of greater concern for the SE4is the use of characteristic sizes (fiber or particle diameters) 

rather than specific surface areas for head loss correlations (this objection may be moot, 

however, because, in practice, essentially all PWRs perform testing to validate the strainer 

designs and do not rely on correlations). The SE agrees with the recommendations for assumed 
18

size of the coatings debris. Overall, the SE4recommends that insulation qualities be assessed in 

each plant and that specific surface areas be properly determined for use in head loss 

calculations.

2.1.2.2.2 Latent Debris Quantities and Characteristics

The latent debris inside containment is characterized and quantified by 

estimating the horizontal and vertical surfaces areas

evaluating the rate of debris buildup in those areas

defining the observed debris characteristics

determining the fraction of the surface area that is susceptible to debris buildup.

Based on these prior estimates, the quantity and composition of latent debris can be calculated. 

The guidance provided in Section 3.5.2 of NEI 04-073details the specific steps to be taken to 

perform these evaluations. The associated SE4offers additional suggestions on how to perform 

these evaluations. Among the suggestions is the use of lint-free Masolin cloths or highefficiency particulate air (HEPA) vacuuming with mild brushing as a means to assess debris 

deposits on vertical surfaces (both provide readily quantifiable debris mass measurements).

The guidance in Section 3.5.2.3 of NEI 04-07 describes two alternative methods for defining 

latent debris characteristics. The first method is a strict analysis of collected latent debris 

samples to determine compositions and physical properties. The second method is to assume 

compositional and physical properties of the debris based on conservative values. The latter 

method is recommended due to the probable prohibitive cost of exhaustive characterization 

testing, the impracticality of separating particles from fibers, and the likelihood that the bulk 

densities of fibrous debris would be altered in the process of collecting and handling. To pursue 

the latter method, a reasonable estimate of the relative mix of particles to fibers must be 

obtained. 

Material density and particle size also are important physical characteristics for debris. The bulk 

fiber density is assumed to be that of water to make the fiber particulate neutrally buoyant. The 

particle bulk density is assumed to be 100 lbm per cubic foot, slightly above that for dry sand or 

packed earth (95 lbm/ft3). Finally, the particle diameter is assumed to be 10 m.

The associated SE4recommends that, in the absence of measurements to the contrary (i.e., the 

first method or observation of an inordinate amount of paint chips), fiber would constitute 15% 

of the latent debris mass. The SE further states that fiber bulk density is not the value of merit 

for transportability to the sump screen, but it is conservative to assume that all fiber is 

transported to the sump screen. The assumed particle density of the fiber should be 1.5 g/cm3. 

Similarly, the particle density of the particulate material (―dirt‖), 2.7 g/cm3, is the value of merit, 

not the bulk density. Finally, the SE4accepts the assumed particulate particle diameter of 10 m 

as being conservative, but perhaps overly so, as much of the particulate latent debris mass could 

be due to hardware and larger paint chips and sand grains. The SE provides alternative and more 

refined means to estimate the hydraulic properties of the particulate debris.4
19

The NRC SE4points out two additional values that are not addressed in the NEI guidelines. 

These are the dry bed (i.e., in the absence of associated water) accumulation of latent fibers and 

the fiber-specific surface area, both of which are needed for head loss calculations. NRC 

recommends either experimental measurement of these values or using the dry-bed values for 

fiberglass as being conservatively bounding. 

2.1.3 Evaluation of BWR and PWR Guidance on Physical Debris

The sources of many types of physical debris at BWRs and PWRs are similar or identical, 

including primarily insulation materials and coatings, but also installed materials (e.g., labels, 

tags, tape) and non-installed transient materials such as dirt, dust (both particulate and fibrous), 

and maintenance debris (e.g., rags, tools, filters). BWRs also have the suppression pool sludge 

(steel corrosion products plus dirt and dust), which is not found in PWRs. Table 2.2 summarizes 

the types of physical debris found in BWRs and PWRs as reflected in the guidance documents 

showing the expected numerous similarities and differences, largely in the presence and 

characteristics of suppression pool sludge in BWRs and in the emphases placed on the materials.

Table 2.2. Physical debris evaluated in PWRs and BWRs.

PWR BWR

Primary Insulation on Piping and Vessels:

Fiber blanket, reflective metal, particulate, 

microporous

Fiber blanket, reflective metal, particulate, 

microporous particulate

Installed Materials:

Filters, fire barrier materials, fibrous cable 

insulation, insulation in wall penetrations, 

coatings (paint), labels, tags, tape

Coatings (paint), fabric, permanent tags, ropes,

filters, tape, wire ties

Latent Materials:

Unqualified coatings, degraded qualified 

coatings, dirt, dust, lint, tape, labels

Unqualified coatings, degraded qualified coatings, 

tags, labels, dirt and dust in wetwell, fibers, 

suppression pool sludge (carbon steel corrosion 

products)

Transient Materials:

Construction and maintenance debris, 

temporary equipment, dirt, dust, lint

Tools, rags, filters, ablated concrete, dirt and dust 

in drywell; ropes, fiber, paper mats, plastic 

sheeting, rubber gasket materials in wetwell;

paper, plastic sheets in both

The BWR determinations of physical debris types generally pre-date those of the PWRs. The 

types and quantities of certain types of debris for BWR plants was based on values derived from 

surveys of a number of plants at different times and over various plant operating cycles. The 

recommended assumed quantities for debris types other than insulation identified in the BWROG 

guidance include the following;

from the drywell:

o 150 lbm of dirt and dust 

o 47 lbm of inorganic zinc coating 
20

o 85 lbm of inorganic zinc with epoxy top coat 

o 71 lbm of epoxy 

o 50 lbm of rust from unpainted steel 

in the suppression pool:

o 150 lbm per year sludge generation rate.

The insulation contributions were determined by ZOI calculations. The above recommendations 

are assumed in the URG to be bounding values, but the guidance advises that licensees should 

verify that these values are bounding for their particular plant. The guidance document 

recommends that licensees develop plant-specific estimates for the items in the drywell. The 

guidance also recommends that plant-specific sludge generation rates should be based on 

experience, or determined with conservative sludge estimation techniques. Table 2.3 compares 

the PWR and BWR approaches in summary form, showing the similarities and differences.

Table 2.3. Comparison of BWR and PWR guidance on physical debris.

PWR BWR

Determine inventory of debris by quantities 

and types based on walkdowns of plant; update 

records such that ―as built‖ documentation 

reflects actual plant conditions; systematic 

guidance provided for walkdown procedures, 

which defines the expertise required, the 

methodologies, and the approaches used to 

survey debris sources and distributions

1.) Use generic/representative values 

developed by BWROG based on plant 

surveys and recommended as applicable to 

all plants

2.) Alternative recommendation; determine 

quantities and types of debris based on 

plant-specific conditions (procedure to be 

devised and defended by licensees) for 

insulation debris defined by ZOI

Material properties (geometry, size, density) of 

debris determined from direct measurement of 

sampled species or from recommended values 

from guidance documents

Material properties of debris determined from 

direct measurement of sampled species, or 

from recommended values from guidance 

documents

ZOI spherical ZOI spherical, except below as used for 

coatings

Within ZOI, RMI fails as 75% small fines 

(fines and pieces <1-inch), 25% larger pieces Fraction of the insulation located within the 

ZOI assumed to be destroyed into ―fines‖ is 

determined. Methodology for determining the 

actual degree of damage to various insulation 

materials within the ZOI (e.g., from complete 

destruction as fines to partially shredded into 

larger mats) is specified in Table 4 of the 

guidance.

Within ZOI, tested fibrous insulation breaks up 

with 60% small fines, the remainder of larger 

fragments; assume 100% small fines for 

untested materials. 

Within ZOI, particulate insulation and other 

materials with limited or no test results (CalSil, Microtherm, fire barriers, etc.) fail 100% 

as small fines 
21

Within ZOI, all coatings (qualified and 

unqualified) assumed to fail as 10- m particles

Within ZOI, all coatings assumed to fail on a 

projected surface (base of expanding cone) 

located 10D from a nominal-sized break 

(bounding value of 20 ft for 24-inch pipe 

break), doubled to account for intervening 

structures; generic to all BWRs.

Outside of the ZOI, all jacketed insulation 

assumed to remain intact

Not specified, but presumed to remain intact.

Outside of the ZOI, all non-qualified coatings 

assumed to fail as fine particulate; qualified 

coatings remain intact, unless current 

walkdown evaluations show that they have 

become degraded

Outside of the ZOI, non-qualified coatings 

must be evaluated for detachment or their 

susceptibility to detach after prolonged 

exposure to post-LOCA environment within 

containment. Form of failed coatings not 

specified.

1.) Latent debris inside containment estimated 

by detailed measurement and assessment 

methodologies to determine density and 

particle size (some plants use default <200 

lbm; 15% fibrous)

2.) Assume bounding values of 62 lbm/ft3

bulk density (water density) for fiber 

debris; 100 lbm/ft3and 10 m particle size 

for particulate debris 

1.) Assume suppression pool sludge 

accumulates at rate of 150 lbm/year (must 

justify this as bounding for specific plant)

2.) Assume suppression pool sludge 

accumulates at rate of 300 lbm/year 

(bounding; no plant-specific justification 

needed)

3.) Sludge particle-size distribution 

determined from test sampling reported in 

BWROG document; assumed applicable 

to all plants

4.) Latent debris; e.g., dirt and dust in the 

wetwell that is above the normal 

suppression pool level, corrosion products 

from unpainted steel, and unqualified paint 

and any transient debris that had been 

dropped or introduced into the suppression 

pool such that, at LOCA initiation, it was 

already present.

2.1.4 Recommendations for BWR and PWR Guidance on Physical Debris

The PWR guidance for estimating the quantities and properties of physical debris is provided by 

the Nuclear Energy Institute,2in NEI-04-07,3and the associated NRC Safety Evaluation.4 For 

PWRs, individual facility walkdowns are directed to obtain and confirm information on debris 

types and quantities. For BWRs, estimates of quantities based on generalizations derived from 

plant surveys were done for certain types of debris. Information on physical properties of debris 

for PWRs can be based on plant-specific observations or may be based on generalized data 

obtained as consensus values. Such consensus values are provided by the guidance documents 

and associated NRC staff evaluation. For example, unless evidence to the contrary is observed 

during plant walkdowns, fiber is assumed to constitute 15% of the latent debris, the fiber particle 

density is 1.5 g/cm3, and the particulate matter density is 2.7 g/cm3 with an assumed particle 
22

diameter of 10 m noted by the SE to be conservative. Information on insulation and coatings 

properties likewise are provided.

Systematic direction on performance of the PWR plant walkdowns also is provided in the 

guidance document. This direction is prescriptive, thus guiding the various PWR operators to 

apply similar expertise, methodologies, and approaches to survey physical debris sources and 

distributions.

Recommendation 2-1: Plant-specific determination of the types, quantities, and distributions of 

physical debris, similar to the individual plant walkdowns for PWRs, is 

recommended for all commercial light water reactors, including BWRs. A 

sampling methodology, such as the guidelines offered through the SE to 

NEI 04-07, should be implemented across all plants to determine the 

relative quantity of fibrous debris. Methods to estimate the quantities and 

types of insulation debris, the largest contributor to the post-LOCA debris 

inventory, should be unified across BWRs and PWRs. 

2.2 Chemical Debris

Solutes in post-LOCA PWR coolant can react with materials exposed to the coolant under postLOCA conditions to create precipitates. Solutes in post-LOCA PWR coolant include boric acid 

(used in the reactor coolant system during normal plant operation as well as in the water injected 

by the ECCS), and chemical buffers (trisodium phosphate, sodium hydroxide, or sodium 

tetraborate) added to adjust pH to a neutral value or greater to inhibit iodine volatility and limit 

corrosion under post-LOCA operations. Solutes also can arise from partial dissolution of solids 

(e.g., aluminum, calcium silicate insulation, concrete) into the coolant. The reactions of the 

PWR solutes with each other and with the post-LOCA debris are referred to as chemical effects, 

and the solid precipitates arising from these reactions are termed chemical debris in the present 

discussion. The reactions include interactions of the solutes with other materials in the 

circulating post-LOCA coolant (e.g., insulation), interactions of the solutes with other fixed 

materials (e.g., metals to form corrosion products), or interactions of the solutes derived from 

fixed materials with themselves (e.g., from dissolution of calcium silicate insulation to form 

other precipitates).

Although all BWRs operate without solutes in their normal coolant (i.e., the coolant is essentially 

pure water), some BWRs inject the SPB in the standby liquid control system in the event of a 

LOCA. Like the PWRs, solutes also can arise for BWRs by the partial dissolution of solids, such 

as metals (including aluminum RMI), other insulation, or concrete, in the released coolant. 

Thus, the interactions of the post-LOCA debris, the fixed materials, and the neutral water or 

water plus the SPB solute may also create chemical debris or precipitates in the BWR postLOCA system. Table 2.4 summarizes the chemical additions that can lead to chemical debris 

generation in PWRs and BWRs.
23

Table 2.4. Contributors to chemical debris in PWRs and BWRs.

PWR BWR

Primary Coolant:

Boric acid (varies from nominally 2800 ppm to near 

0 ppm boron depending on time in cycle)

Water

Secondary Sources of Water:

RWST boric acid Suppression pool

Chemical Additions to Post-LOCA Coolant:

One of the following:

sodium hydroxide (NaOH) up to ~0.2 M

TSP (Na3PO4·12H2O) up to ~0.011 M as 

phosphate

STB (Na2B4O7·10H2O) up to 0.11 M as boron 

Generally none; some plants add SPB; 

Na2B10O16·10H2O) up to ~0.2 M as boron

Dissolved Solids:

Insulation, concrete, metals Insulation, concrete, metals

The chemical effects discussed herein are based on the outcomes of chemical effects testing and 

other work done for PWRs in prior extensive testing programs. Similar considerations or testing 

programs on chemical effects in generating post-LOCA debris in BWRs have been done on only 

a limited basis. Chemical debris in PWRs is considered in Section 2.2.1; chemical debris in 

BWRs is discussed in Section 2.2.2. The different treatments of chemical debris for BWRs and 

PWRs are evaluated in Section 2.2.3. Based on this assessment, recommendations for uniformly 

addressing chemical debris for commercial light water reactors are provided in Section 2.2.4.

2.2.1 Chemical Debris in PWRs

In the research activities associated with the resolution of GSI-191, the NRC Advisory 

Committee on Reactor Safeguards (ACRS) raised a concern that chemical interactions between 

the ECCS containment spray water and exposed material surfaces (such as metals, paint, and 

insulation debris) could impede water recirculation in a post-LOCA PWR. As part of the 

response to this concern, the Integrated Chemical Effects Testing (ICET) program5 was 

performed at the University of New Mexico under the direction of Los Alamos National 

Laboratory (LANL). The objectives of the ICET program were to determine, characterize, and 

quantify chemical reaction products that might arise in a post-LOCA PWR environment and to 

determine and quantify gelatinous materials that might arise during post-LOCA recirculation. 

The concerns about chemical effects arise because of the ~2800 ppm boric acid in the postLOCA sump fluid and the post-LOCA upward pH adjustment to decrease radioiodine volatility, 

which is accomplished by injecting sodium hydroxide (NaOH), TSP (Na3PO4·12H2O), or STB 

(sodium tetraborate; Na2B4O7·10H2O). These chemicals in the coolant can interact with materials 

(e.g., insulation, metal, concrete, and coatings) present in containment. 

Five sets of post-LOCA coolant and insulation compositions were investigated under the ICET 

program, as shown in Table 2.5. Tests 1 through 4 comprise a 2×2 matrix that varied the 

insulation materials (fiberglass only or a fiberglass/Cal-Sil combination), and the buffer (NaOH 
24

or TSP). The fifth test simulated the conditions of an ―ice condenser‖ PWR, which uses STB as 

the additive for the upward pH adjustment, and fiberglass insulation.

Table 2.5. Test conditions for ICET experiments.

Test [H3BO3], 

mg/L

[Buffer], mg/L [Btotal], 

mg/L pH Insulation, mg/L

Na2B4O7·10H2O NaOH Na3PO4·12H2O Fiberglass Cal-Sil

1 16000 0 7677 0 2800 9.3–9.5 5270 0

2 16000 0 0 4000 2800 7.1–7.4 5270 0

3 16000 0 0 4000 2800 7.3–8.1 1050 20800

4 16000 0 9600 0 2800 9.5–9.9 1050 20800

5 6850 10580 0 0 2400 8.2–8.5 5270 0

Adjustments of pH in Tests 1 and 4 represent PWRs using NaOH spray injection, Tests 2 and 3 represent PWRs using dry 

Na3PO4·12H2O for pH adjustment, and Test 5 represents PWRs using Na2B4O7·10H2O within melting ice for pH adjustment.

All tests also contained 90 mg/L pulverized concrete plus ―dirt‖ debris in a 1:3 weight ratio, 43 to 100 mg/L HCl, and 0.3 to 

0.7 mg/L LiOH.

Each test conducted at 60°C for 30 days with 59 aluminum 3003 alloy coupons, 134 galvanized steel coupons, 100 copper 

coupons, 3 uncoated mild steel coupons, 77 mild steel coupons coated with inorganic zinc, and 1 concrete coupon.

As shown in Table 2.5, the cooling water used in PWRs contains up to 2400 to 2800 ppm (~0.22 

to 0.26 M) total boron. The pH of the PWR boric acid coolant is adjusted upward in post-LOCA 

conditions. This is accomplished by adding NaOH to about 0.2 M in sodium, or TSP to about 

0.011 M in phosphorus and 0.032 M in sodium, or STB to about 0.056 M in sodium (see 

Table 2.5).

The ICET experimental findings are presented comprehensively in the original NUREG report5

and summarized concisely with additional interpretation in a subsequent journal article.6 A 

general overview of the observations made in the five ICET experiments is presented in 

Table 2.6. These general observations and the more detailed data descriptions and 

interpretations based on the ICET experiments suggest that significant chemical interactions 

would be expected in some post-LOCA PWR environments, with the interactions varying 

according to the insulation type and the buffering system.

The results summarized in Table 2.6 show that the primary impact of the chemicals in the postLOCA coolant is the possible formation of chemical precipitates that may increase head loss 

more than other types of particulates. A large amount of sediment was present in ICET 3 and 

ICET 4 due to the large quantity of calcium silicate insulation (Cal-Sil) used in these tests. Some 

of this debris may remain unaltered but a significant amount of the Cal-Sil dissolves in water and 

then re-precipitates to form much finer solids that, with fibrous debris, pose risk to strainer flow. 

Thus, the relative change in solids quantity from the Cal-Sil reactions likely is small but the 

character of the resulting solids may change significantly. The sediment quantities are much 

lower for Tests 1, 2, and 5, which have no Cal-Sil. In these cases, the sediment material is 

composed of fiberglass and ―dirt‖ (i.e., a mixture of soil and crushed concrete). Table 2.6 shows 

that Cal-Sil is not only the predominant source of debris solids, but is also the origin of 

significant dissolved calcium and silicon (silicate). Test 4, the case with the higher pH of the 

two tests with Cal-Sil, also has the higher silicate concentration.
25

Table 2.6. Results for ICET Experiments.

Test Insulation Buffer

Al 

Corr., 

g

Steel 

Corr., 

g

Sediment

Precipitates

Particulate 

Deposits on 

Fiberglass

Final Conc., mg/L

Type Wt., g Al Ca Si pH

1 Fiber-glass NaOH -98.6 -23.3

Fiberglass, 

―dirt‖

292 Al, B as 

Al(OH)3

Al corrosion 

product deposits 350 12 14 9.3–9.5

2 Fiber-glass TSP -0.9 1.4

Fiberglass, 

―dirt‖

256 None

Small amount of 

phosphate 

deposits

<0.5 8 89 7.1–

7.4

3

Fiber-glass 

& Cal-Sil TSP 0.6 -1.1 CalSil 78,000 Ca, P as Ca phosphates

TSP reacted with 

Ca from Cal-Sil 

to form Ca 

phosphates; CalSil deposits

<0.5 105 86 7.3–

8.1

4

Fiber-glass 

& Cal-Sil NaOH 0.0 0.2 CalSil 86,000 None Cal-Sil deposits <0.5 46 180 9.5–9.9

5 Fiber-glass STB -11.2 0.0

Fiberglass, 

―dirt‖

89 Al, B as 

Al(OH)3

Low amounts of 

chemical 

deposits

50 32 8

8.2–

8.5

The tests with Cal-Sil or TSP or both (Tests 2, 3, and 4) have the lowest aluminum corrosion 

rates. This is probably because both phosphate and silicate are known to provide corrosion 

protection to aluminum. Subsequent analyses of the aluminum coupons exposed to Cal-Sil 

showed a silicate passivation layer composed of Al2OSiO4.

7

 Of the tests without Cal-Sil or 

phosphate (Tests 1 and 5), aluminum corrosion is higher for Test 1 than for Test 5, probably 

because Test 1 has the higher (more alkaline) pH, which favors aluminum corrosion. The 

amorphous aluminum hydroxide corrosion product was assumed to be a form of Al(OH)3 but 

also contained significant associated boron. The aluminum hydroxide solids were very fine and 

were largely observed upon cooling the 60°C fluids to room temperature at the completion of 

testing. Another source of fine precipitate was calcium phosphate, which arose from the 

interaction of the added TSP with the calcium dissolved from Cal-Sil in Test 3. No separate 

precipitates were observed in Tests 2 and 4.

The loading of water-borne solids onto fibrous mats is of great concern in sump screen clogging. 

The observations of particulate deposits onto fiberglass (present in all ICET experiments) are 

summarized in Table 2.6. Not surprisingly, the deposit quantities are highest for Tests 3 and 4, 

which contained Cal-Sil. Aluminum corrosion products were found in Test 1, which also had the 

greatest amount of aluminum corrosion. Although Test 3 was the only experiment that 

combined phosphate with Cal-Sil, and thus produced observable calcium phosphate precipitates, 

small amounts of phosphate-bearing solids were found embedded in the fiberglass in Test 2, 

which contained added TSP. The calcium source, presuming the solids contained calcium, could 

have arisen from the concrete debris added to this (and every) test or from the fiberglass itself. 

Separate brief (30, 60, and 90-minute contact time) dissolution studies of individual solids in 

PWR coolant solutions show much greater calcium dissolution from concrete than from 

fiberglass at all temperatures (~88 and 129°C) and pHs (~4.1, 8, and 12) studied7.

The susceptibility or resistance of fiberglass fibers to corrosion is also influenced by the solution 

composition. The primary agent of fiberglass corrosion is high pH, while the presence of 
26

dissolved aluminum likely helps the glass fibers resist corrosion. 9, 10 The effect of dissolved 

aluminum on fiberglass leaching is demonstrated by the measured silicon concentrations in the 

ICET 1 and ICET 2 test fluid. Although the significantly higher pH in ICET 1 should have 

resulted in much greater dissolution of fibers, the Si concentration is approximately 6-fold 

greater in ICET 2.

Subsequent studies were conducted by Westinghouse Electric Company LLC to supplement the 

ICET program. The testing had the following objectives: 7

identify quantities of containment material classes (structural materials such as aluminum, 

carbon steel, concrete, and zinc, and insulations such as aluminum silicate, calcium silicate, 

E-glass, amorphous silica, Interam E class, and mineral wool) and the potential of each to 

cause chemical effects 

perform dissolution testing of 11 specific materials (aluminum sheet, carbon steel, galvanized 

steel, powdered concrete, Cal-Sil, Nukon fiberglass, high density fiberglass, mineral wool, 

Min-K, E-glass, Interam foil-backed insulation, and FiberFrax fire retardant material), based 

on further consideration of the material classes, for a total of 66 tests, at the following 

conditions:

o at three pH levels (4.1, 8, and 12) 

o two temperatures (190 and 265°F, ~88 and ~129°C) characteristic of the early 

post-LOCA thermal excursion 

o with material to coolant ratios scaled based on industry survey

perform precipitation testing for a total of 60 tests, consisting of

o the high temperature (265°F) tests during cooling of the solutions (33 tests)

o the pH 4.1 tests for all 11 materials adjusted to pH 8 with TSP (11 tests) 

o the pH 4.1 tests for all 11 materials adjusted to pH 8 with STB (11 tests) 

o five dissolution mixtures; 

 pH 4.1 Interam with pH 12 aluminum 

 pH 4.1 CalSil with pH 12 aluminum

 pH 4.1 concrete with pH 4.1 galvanized steel

 pH 4.1 concrete with pH 12 carbon steel

 pH 4.1 CalSil with pH 12 high density fiberglass

develop chemical models for the aluminum, calcium, and silicon solution concentrations and 

masses of the three observed precipitate types (aluminum oxyhydroxide, aluminum silicates 

such as sodium aluminum silicate, and calcium phosphate, for plants using TSP for pH 

control)

testing and demonstration of particulate generators to prepare representative precipitates for 

use in sump screen head testing.

The dissolution tests were sampled after 30, 60, and 90 minutes of contact and analyzed for 

aluminum, calcium, silicon magnesium, phosphorus, sulfur, iron, zinc, and titanium 

concentrations. Dissolved mass values were negligible for phosphorus, magnesium, and titanium 
27

for the ten material classes tested; aluminum, calcium, and silicon gave the highest 

concentrations and thus were most likely to form precipitates. Dissolution rates were determined 

based on the three sample analyses obtained for each of the 66 test solution and temperature 

combinations. Based on these observations, the model assumed that all of the aluminum and 

calcium, when calcium was present with phosphate, would precipitate. The model also assumed 

that dissolved aluminum, sodium, and silicate would precipitate as NaAlSi3O8, with the amount 

limited by the silicate. As a simplifying but unproven assumption, any excess aluminum was 

assumed to precipitate as AlOOH, the net effect being that all of the aluminum would precipitate 

as a fine particulate posing similar risks to strainer blockage.

Of the 33 precipitation tests from the high temperature material dissolutions, 10 formed 

precipitates. These included all three of the aluminum tests (all three pH levels tested) to give 

hydrated AlOOH, the fiberglass test at pH 12 to give NaAlSi3O8 (albite), the concrete tests at pH 

4.1 and 8 (calcium aluminum silicates), mineral wool at pH 4.1 (hydrated AlOOH), FiberFrax at 

pH 4.1 (hydrated AlOOH) and 12 (NaAlSi3O8), and galvanized steel at pH 12 (the zinc silicate, 

Zn2SiO4). Two of the TSP tests, with CalSil and with concrete, gave precipitates, each with 

calcium phosphate and with accompanying silicate and AlOOH, respectively. None of the tests 

with added pH 8 STB yielded a precipitate. For the five dissolution test mixtures, only the last 

one with pH 4.1 CalSil and pH 12 fiberglass gave solids (a sodium calcium aluminum silicate).

Of the 33 precipitation tests from high temperature dissolutions, the solids quantities were 

greatest, in decreasing order, for the pH 12 aluminum test (AlOOH), the pH 12 FiberFrax test 

(NaAlSi3O8), the pH 8 concrete test (calcium aluminum silicate), and pH 12 high-density 

fiberglass test (NaAlSi3O8). The solids amounts in the mixtures with TSP and the solution 

mixtures were not measured.

These precipitation studies showed that the solids of most concern are AlOOH, various calcium 

or sodium aluminosilicates, and calcium phosphate. Recipes to prepare such solids for strainer 

testing were developed and the product filtration qualities determined. Overall, the results of the 

Westinghouse testing7support and broaden the findings of the ICET experimentation. 

Researchers in the Japanese Nuclear Energy Safety (JNES) organization11 recently performed 

tests simulating the containment vessel of a PWR and a single test under simulating BWR 

conditions. These ~800-hour (~33 day) integrated chemical assessment tests, dubbed ICAN, 

were performed to examine flow rate pressure losses (net pump suction head, NPSH) and 

dissolved element concentrations in recirculating 60°C coolant accompanied by spray flow in gas 

spaces. Other test parameters such as scale (1,000 liters) and types, quantities, and placements of 

material surfaces were patterned on the ICET experiments. Two types of insulation materials 

were tested under ICAN – calcium silicate and rock wool. Eight ICAN experiments have been 

performed. The experiments are outlined in Table 2.7.
28

Table 2.7. ICAN experimental matrix.

Test, 

ICAN Insulation Coolant Buffer pH Wt. Lost, g Comments (a) (b) Al Fe

-1 Rock wool H3BO3 ? ? ? ? ? Dry condenser

-2

Rock wool, 

calcium silicate H3BO3 ? ? ? ? ?

-3

Rock wool, 

calcium silicate H3BO3 ? ? ? ? ? With added heating and cooling

-4 Rock wool H3BO3 Na2B4O7 8.3 8.4 -0.05 6.60 Ice condenser

-5 Rock wool H3BO3

N2H4, 

NaOH 7.5 7.0 0.67 18.53

-6 Rock wool none none 3.2 5.9 0.67 59.0 Like BWR; galvanized steel also tested

-7 Rock wool H3BO3, NaOH 9.9 9.9 0.44 0.63 Dry condenser to repeat ICAN 1

-8 Rock wool H3BO3

N2H4, 

NaOH 7.5 7.3 0.02 9.50

Like ICAN 5 but galvanized steel 

added in place of some of the carbon 

steel

(a) At the end of spray cycle.

(b) At end of 33 days of testing.

Details on the configurations of the prior ICAN experiments numbered 1 though 3 were sparse 

and no references were provided in JNES-SS-080411 to describe this earlier work. The dissolved 

concentrations of aluminum, silicon, iron, and copper were found to roughly match the 

solubilities of the corresponding oxides and hydroxides observed in the testing [i.e., gibbsite, 

Al(OH)3, and amorphous Al(OH)3, quartz, SiO2, and amorphous silica, SiO2 (am), hematite, 

Fe2O3, goethite, FeOOH, cupric oxide, CuO, and zinc oxide, ZnO]. The changes in the pressure 

losses with time were complex for these tests and the report provided observations but little 

overall interpretation of the pressure loss testing.

A third set of integrated tests under PWR conditions was performed by Framatome in 

Germany12. The testing was performed in a loop tank with 50°C (122°F) solution containing 

2200 ppm boron as (unbuffered) boric acid and mineral wool insulation for 140 hours (almost 6 

days). Neither the use of buffering agents (e.g., STB) nor pH monitoring was mentioned in the 

article. Some neutralization of the boric acid would occur by interaction with the mineral wool 

insulation. The testing showed dissolution of zinc from galvanized surfaces. No zinc oxide 

particle erosion was noted in areas of low velocity recirculation but zinc oxide erosion was seen 

in high velocity regions. Pressure loss at the strainer caused by accumulation of zinc oxide and 

iron corrosion products on the insulation was observed to commence after about 10 hours. It was 

shown that the pressure loss could be avoided if the pump flow rates were restricted to minimum 

rates in the first 10 hours after the LOCA.

2.2.2 Chemical Debris in BWRs

The chemical effects arising from interactions of solutes in the post-LOCA coolant with other 

materials in BWRs have to date not been a subject of study in the United States. Unlike the 

PWR coolant and the contained boric acid chemical shim, the BWR coolant is essentially pure 

water and does not contain solutes. However, some BWRs add SPB, sodium pentaborate, to the 

post-LOCA coolant and the SPB additions may lead to chemical effects similar to those observed 

in the PWRs. Although the BWR coolant is chemically simpler than that of the PWRs, chemical 

reactions in BWRs are still possible in the interaction of the post-LOCA coolant (water) with 
29

various materials contacted by that coolant. As in the case with the PWRs, the temperature, pH 

profiles, and plant-specific BWR materials will impact the chemical effects that may occur in 

post-LOCA situations. 

A recent Japanese study has investigated the corrosion of rock wool (Thermboard 1080, Nippon 

Rockwool Corporation) and calcium silicate (Nippon Keical Limited) insulation materials in 

PWR and BWR coolant compositions.11 The rock wool and Keical calcium silicate insulation 

materials are used in Japan. The two insulation materials were both contacted with sodium 

tetraborate solution (2312 ppm in boron), a hydrazine-bearing boric acid solution (2800 ppm 

boron), and a dilute (0.002 M; pH ~3) hydrochloric acid (HCl) solution. The last was stated by 

the researchers to model BWR coolant with the HCl arising from decomposition of cable 

insulation. The leaching tests were run at 60°C (140°F) and samples drawn at 3, 6, 24, 120, and 

480 hours (20 days). Dissolution weight losses also were measured for each material/solution 

combination as a function of time.

The most evident instance of chemical reaction in the BWR post-LOCA coolant is the interaction 

of water with calcium silicate insulation. Phases such as tobermorite [Ca4(Si6O15(OH)2)(H2O)5], 

and various carbonates (e.g., CaCO3; NaHCO3) are the primary constituents in Cal-Sil, with 

sodium, iron, and magnesium also present (Dallman et al. 2006; Volume 5).5 Upon contact with 

water, the Cal-Sil will partially dissolve to add calcium, silicate, sodium, carbonate, and other 

solutes to the post-LOCA coolant. This is shown by the JNES tests of Keical (the calcium 

silicate insulation).11 Dissolution of the Keical in the dilute HCl solution (pH ~3) increased the 

pH to 9.21 after 3 hours and the pH crept to 9.66 after 480 hours. Calcium concentrations 

reached ~26 ppm after 3 hours and were ~36 ppm (~0.001 M) after 480 hours, while silicon was 

about 19 ppm at 3 hours, rising to ~64 ppm (0.0023 M) after 480 hours.

With the pH controlled to lower values (~8.4) by sodium tetraborate, the dissolved calcium 

concentration rose from 63 to 138 ppm over the 3 to 480-hour test interval for the Keical calcium 

silicate product. The silicon concentration likewise was higher in the pH ~8.4 sodium tetraborate 

solution than in the unbuffered HCl solution, rising from ~12 to 83 ppm. With the pH ~7.6 boric 

acid solution, both calcium and silicon showed concentrations that were higher yet (86 to 227 

ppm calcium as contact time increased from 3 to 480 hours and ~20 to 117 ppm silicon as time 

increased from 3 to 480 hours). Overall, the concentrations of the calcium and silicon decreased 

as pH increased from about 7.6 to 9.7. It is also clear from these tests that the calcium silicate 

insulation, in the absence of significant buffering, will drive the pH to fairly alkaline levels (pH 

~9.7) as would be expected by its likely complement of contained sodium and calcium 

carbonates.

The pH values observed for the dissolution tests with the rock wool were about 4.2 in the 

unbuffered HCl solution (much lower than the pH ~9.7 observed with Keical), 7.6 in the boric 

acid solution (similar to that observed with Keical), and 8.4 in the sodium tetraborate solution 

(again similar to Keical). The most prominent solutes from the rock wool dissolution again were 

calcium and silicon. The calcium and silicon concentrations were lower for the rock wool than 

for the calcium silicate (Keical).
30

The solutes derived from Cal-Sil or the Japanese equivalent Keical can react with each other and 

also interact with components within the containment. For example, the dissolving Cal-Sil 

provides significant alkalinity, which can act to increase aluminum metal corrosion. In an 

opposite effect, however, the silicate present from dissolving Cal-Sil in a simulated post-LOCA 

coolant in the ICET experimentation conducted at the University of New Mexico has been 

credited with decreasing the corrosion of aluminum in post-LOCA PWR chemical systems.5, 13

The corrosion of aluminum present in the containments of BWRs is further complicated for those 

BWRs having dissolved SPB added to the post-LOCA coolant by means of the standby liquid 

control (SLC) system. The boron is added to provide nuclear reactivity control, and some SPB 

formulations are enriched in 10B. The SPB addition also provides pH buffering to decrease 

iodine fission product release. About 600 lbm of boron is added as SPB (Na2B10O16·10H2O) to 

the post-LOCA coolant via the SLC system. The SPB addition, in the form of about 3000 to 

4000 gallons of solution that is 8 to 10 wt% in SPB, is sufficient to give a final boron 

concentration of ~1100 ppm or ~0.1 M to the circulating post-LOCA coolant. The initial 

suppression pool pH is 5.3 (i.e., in equilibrium with atmospheric carbon dioxide), and the pH 

after injection of the SPB is greater than 7 (Table 15 of Gallagher 2004).14

In the absence of SPB, the BWR coolant has negligible buffering capacity. Therefore, the 

coolant is expected to assume the pH imposed by its interactions with the water-exposed solids 

surfaces (insulation, concrete, metal, and coatings). As shown by the Keical calcium silicate 

testing, exposure of the nominal BWR coolant (0.002 M HCl) to this insulation caused the initial 

pH of ~3 (the measured pH in the ICAN-6 test was 3.2) to rise to about 9.7 while exposure of the 

same coolant to rock wool insulation raised the pH only to about 4.2.

Dissolved borate significantly increases aluminum corrosion,13 probably because of the 

formation of stable aluminate-borate complexes.15 Thus, the rate of aluminum corrosion at pH 

10 increases by a factor in the range of 25 to 64 in the presence of ~0.25 M borate. This 

increases the corrosion rate from 0.019 g/m2·hour in water16 to values in the range 0.459 to 1.22 

g/m2·hour in 0.236 to 0.259 M borate.17 Although the studied borate concentrations are typical 

of PWRs, similar aluminum corrosion rates are observed in 0.1 M borate solution that would be 

typical in BWRs that use SPB addition.13

In a survey of 69 PWRs, all plants reported aluminum surfaces to be present in the containment. 

Of these 69, the maximum reported ratio of aluminum surface area to coolant volume is 5.42 ft2

of aluminum surface area per ft3of coolant (~177 cm2/liter). The prevalence of aluminum at 

some BWRs that have installed aluminum-based reflective metal insulation is probably greater 

than in PWRs because the hydrogen generation associated with aluminum does not impact the 

inert gas filled BWR containments (i.e., Mark I and Mark II designs). The corroded aluminum 

largely would precipitate to form aluminum hydroxides [e.g., Al(OH)3 or, at higher temperatures, 

AlOOH as seen in the Westinghouse studies]. Because of the larger exposed aluminum surface 

area in some BWRs with aluminum RMI as compared to PWRs, greater quantities of corroded 

aluminum could form for these BWRs if post-LOCA manual SLC injection is used (i.e., 

dissolved borate is present in the coolant).
31

For BWRs that do not inject SPB during a LOCA (i.e., in the absence of dissolved borate), 

aluminum corrosion could be high, depending on the quantity of HCl formed by cable insulation 

decomposition. Weight losses of immersed aluminum coupons were measured in the ICAN 

tests. The test results (Table 2.7) show aluminum weight losses as great in the acidic simulated 

BWR system (ICAN-6) as in any of the simulated PWR systems (all other ICAN tests) with 

dissolved borate.

In Test 5 of the ICET experimental program executed to study PWR chemical effects, the 

conditions investigated were akin to the conditions that could be obtained in BWRs using SPB in 

the SLC system. Test 5 of the ICET series for PWRs studied the chemical effects of a system 

that contained shredded fiberglass insulation, concrete, metal coupons (mild steel, steel coated 

with inorganic zinc paint, galvanized steel, copper, and aluminum), concrete powder, and soil. 

These materials are also present in BWRs. In this test, the total boron concentration was 

2400 ppm, or 0.22 M, which is about double the concentration available in the post-LOCA 

coolant of a BWR using SPB. The pH of Test 5 ranged from 8.2 to 8.5; the pH of the postLOCA BWR coolant using SPB is expected to be greater than 7.7

Test 5 of the ICET series thus 

provides useful information for understanding chemical effects in BWRs. 

As already noted, the test ICAN-6 (Table 2.7) was designed to emulate the pure water BWR 

coolant amended by the hydrochloric acid generated by cable deterioration. Accordingly, the 

test system containing 7.8×10-4 M HCl, with calculated pH of 3.1, had a measured pH of 3.2. It 

is seen that the low pH (probably abetted by chloride) led to significantly greater carbon steel 

(Fe) corrosion compared with the parallel ICAN-4, -5, -7, and -8 tests buffered by borates to 

much higher pH (ranging from 7.5 to 9.9). If such high initial and unbuffered HCl 

concentrations do, indeed, exist in post-LOCA BWRs, high carbon steel corrosion should be 

expected. The depletion of acid strength exhibited over the duration of the ICAN-6 testing, and 

likely to occur over the course of the post-LOCA period, will lead to formation of flocculent iron 

hydroxide precipitates and significant solids loading. If, however, the HCl is neutralized as soon 

as it forms by interaction with calcium silicate insulation (a condition not tested), much less 

carbon steel corrosion would be anticipated.

2.2.3 Evaluation of Chemical Debris in PWRs and BWRs

Chemical debris concerns are more complicated for PWRs than for BWRs, given the varied 

chemical constituents and their diverse combinations found across PWR plants. Experimentation 

under the ICET program was performed by LANL and the University of New Mexico to 

understand interactions of PWR coolants with construction and insulation materials. Five 

different sets of coolant composition, all based on borate but with three different buffers, and 

insulation (two combinations, with and without added Cal-Sil) were required to address the 

varied systems available. The ICET experimentation indicated influences on chemical debris 

quantity and quality arose from 

1. simple dissolution and fragmentation of the Cal-Sil, 

2. the effects of borate, phosphate, and silicate on aluminum corrosion, 

3. the nature of the aluminum hydroxide product from aluminum corrosion (and the interaction 

of the corrosion product with borate), 

4. the interaction of calcium with phosphate to produce a fine precipitate, 
32

5. the effects of the solution composition on fiberglass corrosion, and 

6. the collection of the various chemical debris solids onto fiberglass.

Of the five ICET experiments, only one experiment (ICET-5) provided information that would 

be useful for BWRs. The usefulness, however, would be limited to BWRs using SLC systems to 

inject borate into the post-LOCA coolant and having fiberglass but no calcium silicate insulation.

Westinghouse performed subsequent studies of the influences of pH, time, and temperature on 

the dissolution in PWR coolant of 11 different containment construction and insulation materials 

(and the ensuing precipitation).7 The precipitating solids found to be most prominent were 

AlOOH, various calcium or sodium aluminosilicates, and calcium phosphate. The rates of 

precipitation were modeled, the product filtration qualities were determined, and recipes to 

prepare these solids for strainer testing were developed. The Westinghouse studies were 

designed for PWR application but also would be useful for those BWRs having borate in the 

post-LOCA coolant (i.e., SLC systems inject during a LOCA).

A set of testing analogous to the ICET experiments was conducted by the Japanese Nuclear 

Energy Safety organization.11 Eight so-called ICAN experiments were performed with one of 

the ICAN tests being done under nominal BWR conditions (i.e., borate-free coolant) in the 

presence of rock wool insulation and various materials of construction (e.g., mild steel, 

aluminum, copper, concrete). In the same report, the JNES also described dissolution kinetics 

testing of rock wool and calcium silicate insulation materials with two borate solutions 

(modeling post-LOCA PWR coolant) and a dilute hydrochloric acid solution (modeling postLOCA BWR coolant).

Overall, there is limited test data that is relevant to BWR post-LOCA chemical debris effects. 

Although the chemical system for BWRs is simpler than for PWRs, at least in the number of 

solutes, the JNES study and related insights gained from ICET experiment 5, applicable to 

BWRs using SLC systems during a LOCA, show that the formation of chemical debris in postLOCA BWR coolants cannot be ignored. The ICAN tests and the ICET experimentation, 

particularly Test 5, suggest the scope of chemical interactions that might be anticipated. 

However, the potential post-LOCA chemical debris-forming situations studied for BWRs have 

hardly been exhausted. In particular, integrated testing of BWR post-LOCA conditions with 

calcium silicate insulation is recommended. 

2.2.4 Recommendations for Guidance on Chemical Debris in PWRs and BWRs

In light of the test matrix conceived and executed for the PWRs under the ICET program, similar 

experimental studies for BWRs could be proposed following survey of the BWR plants and the 

identification of materials of concern. For example, and paralleling the design of the ICET 

program, a 2×2 test matrix could be advanced for an ICET-type experimental program for 

BWRs. The testing would examine the effects of the presence and absence of SPB in the 

presence and absence of Cal-Sil insulation with background fiberglass insulation, representative 

or bounding aluminum RMI, and representative quantities of other materials of construction. 

The post-LOCA coolant composition in BWRs also should be determined. The ICAN testing 

posited a nominal pH ~3 hydrochloric acid solution but the evolution of this degree of acidity 

immediately after the LOCA may be unduly conservative. Directed single component insulation 
33

and construction material dissolution studies, along the lines the testing reported for PWR 

coolants by Westinghouse,7and solution mixing tests also are recommended for BWR coolant 

compositions. It is believed that the outcomes of the proposed studies would raise the 

understanding of chemical debris generation effects in the post-LOCA BWR system to be 

equivalent to the PWR understanding. Efforts similar to those conducted for PWRs into head 

loss studies also are recommended.

Recommendation 2-2: A determination of the effects of coolant, solutes, and insulation on the 

creation of chemical debris and the influence of the debris on head loss 

and downstream effect, along the lines of the ICET program and 

Westinghouse studies conducted for PWRs, is recommended for BWRs.

35

3. INSULATION AND COATINGS DEBRIS GENERATION

Debris is generated inside the reactor drywell/containment during a LOCA as high-energy fluid 

is released from a pipe rupture. Damge to containment materials occurs as the result of the 

impinging steam jet or saturated liquid jet that becomes two-phase because of rapid 

depressurization to containment ambient. The high-energy fluid can damage adjacent equipment 

and material, particularly insulation and coatings, creating debris that can be transported to the 

suppression pool or containment sump. In addition, debris may be generated in regions not 

directly impacted by the high-energy fluid due to the harsh post-LOCA containment 

environmental conditions (e.g., temperature, pressure, humidity, radiation).

The basic methodology for determining the amount of debris generated in a given LOCA event 

consists primarily of determining the zone of influence (ZOI) of the high-energy jet resulting 

from the pipe break. The ZOI is by definition the volume within which the jet is expected to 

generate debris from the insulation, coatings, and other materials typically present on reactor 

system equipment or containment walls. In some approaches recommended in the guidance 

documents, the ZOI is defined one way for determining insulation debris generation and in a 

different manner for coatings debris generation. The ZOI for various materials is different based 

on the ability of the specific material to withstand a LOCA jet.

Regulatory Guide 1.82, Rev. 3 18 provides the NRC staff regulatory positions for insulation and 

coatings debris generation. The full text of the relevant sections of the Regulatory Guide is 

included in Appendix A. 

The BWROG and PWROG guidance on specific issues of insulation debris generation during a 

LOCA event is summarized in Section 3.1. Section 3.2 contains similar summaries for coatings 

debris generation. These sections also include summaries of the NRC staff Safety Evaluations of 

the industry guidance documents, and additional NRC staff review guidance. The guidance for 

BWRs and PWRs on these issues is compared and evaluated in Section 3.3. This subsection also 

includes recommendations to clarify and reconcile the guidance provided determining debris 

generation for BWRs and PWRs.

3.1 Insulation Debris Generation

Insulation within the reactor containments is manufactured from materials that may fail due to a 

LOCA event, either as fibrous or particulate debris, depending on insulation type. The amount 

of insulation debris generated in a given LOCA event depends on the amount and type of 

insulation material within the ZOI defined for the particular pipe break of the LOCA event. The 

BWROG guidance on defining the ZOI for generation of insulation debris is summarized in 

Section 3.1.1. The PWROG guidance on this issue is summarized in Section 3.1.2. In both 

sections, the NRC staff evaluations of the respective guidance are also summarized.

3.1.1 BWR Guidance and NRC Staff Evaluation on ZOI for Insulation Debris

This section summarizes the BWR guidance and NRC staff evaluation for insulation debris 

ZOIs. No interpretation of the guidance or the evaluation has been made here. Section 3.3 

contains comparisons and evaluations of the guidance provided.
36

In BWRs, the coolant lost from the primary system in a LOCA event can be a single-phase steam 

jet or a saturated liquid water jet that expands very rapidly to a two-phase jet because of the large 

difference between the primary system operating pressure and ambient pressure in the 

containment. The approach recommended in the BWR guidance for defining the ZOI for 

insulation debris generation is to determine the volume of the region where the pressure of the jet 

exceeds the material failure pressure of the insulation. This region is assumed to be equal in 

volume to the region within the dynamic pressure surface of a freely expanding steam jet where 

the dynamic pressure is equal to the material failure pressure of the insulation.1 As an analytical 

simplification, the volume of the region defined by the material failure pressure is assumed to be 

a sphere centered on the break location, rather than a pair of truncated cones (typical of a doubleended pipe break) or a thickening disk (typical of a simple pipe separation).

The material failure pressure for a given insulation is determined empirically. The BWR 

guidance document provides tables of the free space expansion of a jet as a function of break 

geometry, based on the Continuum Dynamics Report 96-01 Rev. 3.19 (f) Insulation material 

failure pressures for types of insulation found in BWR containments are also provided in the 

guidance document, based on the Continuum Dynamics Report 96-06 Rev. A.20 (g) For LOCA 

events where the jet is saturated water/two-phase (rather than a pure steam jet), correction factors

are provided in NEDO-32686,1to adjust the material failure pressure for a steam jet to the failure 

pressure for a saturated water jet. These empirical correction factors are based on the General 

Electric report DRF A74-00004.27 (h)Thus, for the BWR approach, insulation ZOI volumes are 

functions of the material failure pressure of the insulation, the break configuration, and the jet 

medium (i.e., a steam jet or a saturated water/two-phase jet).

In the SE of NEDO-32686, Rev. 0 (which is included in the BWR guidance document NEDO32686-A

1

), NRC staff noted concerns about scaling the results of air jet testing for damage or 

failure of insulation to BWR drywells. NRC staff also had concerns about relating the measured 

pressures to the pressure distribution in the free space expansion of a steam or saturated-liquid 

water jet. However, the staff found the jet medium correction factors acceptable.

The BWR guidance document1provides four methods for determining the ZOI. These methods, 

in decreasing level of conservatism, can be summarized as:

Method 1, the entire drywell constitutes the ZOI, and all insulation materials therein fail.

Method 2, target-based analysis using limiting (i.e. largest ZOI) double-ended guillotine 

break ZOIs in which individual insulation debris volumes are determined by the lowest 

insulation failure pressure and largest break diameter (using the approach summarized 

above).

Method 3, break-specific analysis using break-dependent ZOIs; similar in approach to 

Method 2, but break-specific ZOI shape and insulation material quantities within the ZOI 

from a specific break location are considered.

 

(f) Reference 4 of NEDO-32686, Rev. 0; this report appears in Volume 3 of the guidance document.

(g) Reference 6 of NEDO-32686, Rev. 0; this report appears in Volume 3 of the guidance document.

(h) Reference 30 of NEDO-32686, Rev. 0; this report appears in Volume 4 of the guidance document.
37

Method 4, direct scaling from computational fluid dynamics (CFD) modeling of the 

measured data, as presented in Continuum Dynamics Report, Rev. 3.19 The calculated 

dynamic pressures from the jet expansion data are used to determine debris quantities; the 

analysis then continues as per Method 3.

All of these methods include consideration of the location of the insulation materials relative to 

the lowest elevation of gratings in the drywell. This is significant, because it affects the 

transportability of the failed insulation.

The NRC staff concluded in the SE of the BWROG guidance that Method 1 is clearly a 

bounding and conservative method. Regarding the spherical ZOI of Methods 2 and 3, the NRC 

staff concluded that the volume of the ZOI would be ―...sufficiently large to envelop the entire 

zone over which destruction would actually occur.‖1 Further, the SE notes that Methods 2 and 3 

are ―...sufficiently conservative to compensate for...‖ concerns about the air jet scaling issues and 

jet pressures noted above and are considered acceptable for use with insulations with low 

dynamic pressures (i.e., low failure pressures). For insulations with noted high dynamic 

pressures, the staff recommended that licensees consider the concerns related to the jet pressures 

on a plant-specific basis.

The NRC staff did not consider Method 4 acceptable without further detailed justification, citing: 

lack of ―...the details of the analysis and how the code would be benchmarked...‖ 

―...BWROG has not yet demonstrated...that a CFD code can accurately predict the specific 

ZOI for a pipe break...,‖ and 

the ―...BWROG has not yet provided sufficient detail for the staff to reach any specific 

conclusions relative to the adequacy of using a CFD model for the purpose of determining 

the ZOI for a pipe break....‖

Regarding insulation materials beyond the jet impingement, the BWR guidance states that ―...it 

has been determined that additional transportable debris would not be generated as a result of 

bulk flow velocities in the drywell...for the materials evaluated.‖1 No NRC staff response is 

identified from NEDO-32686-A.1

3.1.2 PWR Guidance and NRC Staff Evaluation on ZOI for Insulation Debris

This section summarizes the PWR guidance and NRC staff evaluation for insulation debris ZOIs. 

No interpretation of the guidance of the evaluation has been made here. Section 3.3 contains 

comparisons and evaluations of the guidance provided.

The PWR guidance for determining the insulation debris ZOI is provided in NEI 04-07.3 For the 

baseline calculation, the guidance document recommends defining the ZOI as a sphere with the 

center at the location of the break. The debris generation region is determined through the 

described analytical calculations via ANSI/ANS-58.2-1988.22 The radius of the spherical ZOI is 

defined such that its volume is twice the volume around the break in which the escaping fluid in 

the form of a freely expanding jet has sufficient energy to generate debris. The factor of two is 

included to account for a double-ended guillotine break (i.e., assuming a jet issues from each end 
38

of the break). The spherical ZOI radius is a function of the insulation failure pressure of interest, 

and is expressed in terms of the break diameter.

The baseline ZOI is selected based on the location of insulation with the lowest destruction 

pressure inside the containment that could potentially be affected by the LOCA. Walls and 

robust boundaries that could deflect the jet may be accounted for by assuming that insulation 

behind such barriers will be free from damage. The ZOI determination may be simplified for 

some breaks by assuming that the entire subcompartment becomes the ZOI.

The NRC staff concluded that the baseline ZOI calculation was acceptable but provided 

comments regarding the methodology that should be considered and implemented.4 These 

details included determination of the mass flux from the break, irreversible losses, equivalent 

insulation damage pressures, and conditions of jet expansion. The NRC staff compared the jet 

expansion results of NEI 04-07 4to those of the NEDO-32686, Rev. 0 1(i.e., Continuum 

Dynamics Report19) and concluded that the Continuum Dynamics Report19 approach is ―a more 

capable method of modeling steam jets than the ANSI (ANSI/ANS-58.2-1988 22) model.‖ 

Further, the NRC staff accepted the baseline calculation approach of defining the ZOI based on 

the insulation type with the lowest destruction pressure provided that 

1. no other potential debris generation sources with a lower destruction pressure exist

2. defensible destruction pressure values are available for all materials of concern.

Given the NRC staff concerns related to the ZOI model and insulation failure pressures, the staff 

recommended that, for all material types characterized by air jet testing, the destruction pressures 

should be reduced by 40%. This reduction is imposed to ―...account for potentially enhanced 

debris generation in a two-phase PWR jet.‖ 4

Two refinements to the baseline spherical ZOI determination are provided in the PWROG 

guidance document. The first method uses debris-specific spherical ZOIs. That is, the failure or 

destruction pressure of the specific insulations is considered such that there are multiple ZOIs for 

a single break. For the second method, the use of the freely expanding jet models, which were 

employed for the spherical ZOI determination, is recommended. In each method, fixed 

boundaries are accounted for as in the baseline calculation.

The NRC staff agreed that both the first and second ZOI refinement methods are appropriate4

with the inclusion of the 40% reduction in destruction pressures noted above. The staff provided 

three additional refinements related to the application of worst-case thermal hydraulic conditions 

to every break location, the equivalent mass flux application to both ends of a guillotine break 

(i.e., the factor of two for the equivalent volume), and reduction of the effective total pressure at 

the break due to friction losses in lines leading to the break.

Covered (jacketed) insulation material outside of the ZOI is considered undamaged insulation, 

and will not generate transportable debris. As a conservatism, all unjacketed insulation outside 

of the ZOI is presumed to fail (NEI 04-07).3 Per SE NEI 04-07,3the NRC staff agreed that

covered insulations will not form significant debris outside of the ZOI.
39

3.2 Coatings Debris Generation

Primary coatings typically found in BWR containments include untopcoated inorganic zinc, 

inorganic zinc with epoxy topcoat, epoxy primer/topcoat for steel and concrete, and epoxy 

surfacer systems for concrete (Bechtel Report 22754094.12A).23 The coatings in PWR 

containments are system-specific combinations of epoxy surfacers, inorganic zinc primers, epoxy 

and phenolic primers, and epoxy and epoxy phenolic topcoats.3

The coatings are designated as either as ―qualified‖ or ―non-qualified‖ (unqualified) for BWRs.23 

For PWRs, the equivalent terms are ―DBA-qualified and acceptable‖ or ―DBA-unqualified and 

unacceptable.‖3 The coatings types will subsequently be referred to herein as qualified or 

unqualified regardless of reactor type. For both reactor types, the qualified coatings are 

differentiated as such by acceptable performance at DBA conditions and in radiation tolerance 

testing. Qualification is also dependent on factors such as surface preparation, coating 

preparation and application methods, and curing conditions as well as quality assurance/quality 

control (QA/QC) documentation thereof, in addition to coating type (ASTM D5144-00).25

Summaries of the BWR and PWR guidance for coating material debris generation are provided 

in Sections 3.2.1 and 3.2.2, respectively. These sections also summarize the NRC staff 

evaluations of this guidance.

3.2.1 Summary of BWR Guidance and NRC Staff Evaluation for Coatings Debris

This section summarizes the BWR guidance and NRC staff evaluation for coatings debris. No 

interpretation of the guidance or the evaluation has been made here. Section 3.3 contains 

comparisons and evaluations of the guidance provided.

The BWR guidance for coating material debris generation is provided in NEDO-32686, Rev. 0.1 

Paint/coatings on walls and equipment are classified as fixed debris, i.e., material that is part of 

the permanent plant that becomes a debris source only after exposure to the effects of a LOCA. 

Concrete floor coatings are assumed not to be a debris source because thay are located in the 

bottom of the drywell and should not be subjected to jet impingement (Bechtel report 

22754094.12A).22

The guidance document states that ―Where a LOCA jet directly impacts a coated surface it is 

conservatively assumed the jet will strip off all the applied coating in the affected area without 

regard to the coating qualification.‖ The jet impingement area is defined in Bechtel Report 

22754094.12A, 23 referencing ANSI/ANS-58.2-1988,22 as bounded by a 24-inch unrestrained 

pipe break with an impingement range to the drywell wall of 10 pipe diameters, or 20 feet. The 

diameter of the jet at 20 feet is 19.6 feet, which is then doubled to account for pipe hangers, 

structural steel, valves, or other coated items in the jet path.

Generic values, intended to be bounding, for the maximum amount of particulate debris from 

different coatings are provided in the guidance document, based on the impingement area and 

coating thickness, as shown in Table 3.1. In the SE of this document, the NRC staff did not 

identify any concerns relative to the information shown in Table 3.1.
40

Table 3.1. BWR values of coating debris, Table 3 of NEDO-32686, Rev. 0. 1

Coating Thickness

Max Debris 

Volume Max Debris Weight

Inorganic Zinc (IOZ) 0.005 inches-- 0.2516 ft3___ 47 lb.

IOZ Top Coated with Epoxy 0.008 inches--

(epoxy topcoat) 0.6500 ft3

___ 85 lb.

100% Epoxy Coating 0.015 inches__ 0.7550 ft3___ 71 lb.

Latent drywell debris sources are considered as ―...debris which would not be present until later 

in the LOCA event and includes unqualified coatings as well as other material which may 

become debris after exposure to a LOCA environment.‖ Qualified coatings that are not subject 

to the direct jet impact are not considered as a possible latent debris source. The Bechtel Report 

22754094.12A23 states that ―...the properly applied qualified coatings...can all be expected to 

survive a LOCA intact beyond the jet impingement zone.‖

Regarding unqualified/indeterminate paint/coatings beyond the jet impingement, the BWR 

guidance advises licensees to determine if unqualified coatings are present in the drywell and to 

consider whether a qualified coating may have degraded such that its qualification is in doubt. 

The guidance states further, 

―If indeterminate or unqualified coatings are present, an evaluation should be 

conducted to establish the quantity of this latent particulate debris assumed to be 

available for transport from the drywell to the wetwell. Dependent on several 

plant-specific factors, it may be possible to show that the failure of 

indeterminate/unqualified coatings would not occur until late enough in the 

LOCA progression that there would is no transport mechanism available...The 

Bechtel report provides helpful information for evaluating this situation.‖

The NRC staff position regarding the BWR guidance for unqualified/indeterminate coatings 

beyond the jet impingement is that the guidance is ―...incomplete and unsupported.‖1 Licensees 

are cautioned by the NRC staff to carefully evaluate the potential impact of unqualified and 

indeterminate coatings on ECCS suction strainer head loss and are encouraged to support their 

evaluations with test data.1

3.2.2 PWR Guidance and NRC Staff Evaluation for Coatings Debris Generation

This section summarizes the PWR guidance and NRC staff evaluation for coatings debris. No 

interpretation of the guidance or the evaluation has been made here. Section 3.3 contains 

comparisons and evaluations of the guidance provided.

PWR guidance for coating material debris generation is provided in NEI 04-07.3 DBA-qualified 

and acceptable protective coatings have a recommended ZOI radius of one break diameter (i.e. 

1D) based on an assigned destruction pressure and pressure isobars obtained using the spherical 

ZOI approach (based on ANSI/ANS-58.2-1988 22) described in Section 3.1.2.
41

In SE NEI 04-07,4the NRC staff supports the position that all coatings, regardless of their 

qualification, fail within the ZOI. However, the NRC staff position in SE NEI 04-07 regarding 

the ZOI is that ―...licensees should use a coatings ZOI spherical equivalent determined by plant 

specific analysis, or 10D...,‖ where D is equal to the break diameter. As reported in NRC Staff 

Review Guidance 08,26 the staff positions for licensees who use the reduced ZOI value rather 

than the default 10D are that ZOIs of 4D or greater should be used for qualified epoxy coatings 

and 5D or greater for qualified untopcoated inorganic zinc coatings.

(i)

Material outside of the ZOI can be subjected to containment spray and/or be immersed in the 

post-DBA pool. The PWR guidance is to assume that DBA-qualified and acceptable coatings 

outside of the ZOI do not fail. However, all exposed DBA-unqualified and 

unacceptable coatings (e.g., coatings on piping that is not shielded by undamaged insulation) are 

assumed to fail. 

The NRC staff agreed with the assumption that DBA-qualified and acceptable coatings outside 

of the ZOI do not fail.4 The staff noted, however, that periodic assessments of the DBAqualified and acceptable coatings must be conducted to ensure that degradation has not occurred 

such that those coatings are no longer qualified or acceptable. All DBA-qualified and 

acceptable coatings that have degraded are to be treated per the guidance for unqualified 

coatings. Per NRC Staff Review Guidance 08,26 licensees should not reduce the unqualified 

coatings failure percentage below 100%. It is noted in Review Guidance 0826 that if licensees 

are able to specifically determine their unqualified coatings types and align those types with 

specific tests, the licensees may be able to credit a reduction in failure of those coatings types.

To determine the quantity of coating particulate generated, NEI 04-07 specifies that plantspecific information should be used to estimate the thickness of the coatings. If insufficient 

information is available, guidelines for both in and out of the ZOI are provided. Within the ZOI, 

the thickness is specified by the coating system: 

0.003 inches, inorganic zinc primer 

0.006 inches epoxy/epoxy phenolic topcoat. 

Outside of the ZOI, the DBA-unqualified and unacceptable coatings thickness is specified as 

―...the worst case of 3 mils (0.003 inch) inorganic zinc primer.‖ 

The staff concluded that ―...the baseline alternatives to plant-specific data for the determination 

of the coating thickness may not be conservative and are not acceptable without plant specific 

justification.‖4 In addition, the NRC staff concluded that the DBA-unqualified and 

unacceptable coating equivalent thickness of 0.003 inches is ―...not acceptable without plantspecific justification‖.

 

(i) Recent work has suggested that ZOI for inorganic zinc coatings may need to be revised. This is documented in 

―Interim Report of the Evaluation of a Deviation Pursuant to 10CFR21.21(a)(2)‖ dated 2/12/10. 

ADAMS ML100480138. As a result, NRC staff guidance on this issue is being revised.
42

3.3 Comparison and Evaluation of Guidance for Insulation and Coating Debris 

Generation

The BWR and PWR guidance and NRC staff evaluations summarized above are compared in 

this section. The basic approach in both guidance documents is to determine the extent of the 

effect of the high-energy fluid expelled from a LOCA break (i.e., the ZOI) for a specific 

insulation or coating, and then quantify the amount of debris generated within and outside of the 

ZOI. The high-energy fluid expelled from a LOCA break acts as a jet. The jet behavior in the 

containment environment establishes isobars in the containment, which are dependent on the jet 

medium. ZOIs for specific insulations and coatings are established from the jet isobars, 

dependent on the material type. The quantity of debris can thus be estimated, based on the 

volume of material within the ZOI.

The guidance methodologies of ZOI determination are compared in Section 3.3.1 with respect to 

the jet pressure field and jet medium. In Section 3.3.2, the guidance on determining debris 

generation inside the ZOI is compared via insulation failure pressure, and coatings qualification 

and type. The guidance on the effect of the extent of the ZOI is also considered. Approaches to 

considering insulation and coatings debris generation outside of the ZOI are compared in 

Section 3.3.3.

3.3.1 ZOI Determination

Industry has established methodologies for determining ZOIs for insulation and coating 

materials. The influence of the basis of the jet pressure field on the ZOI is discussed in Section 

3.3.1.1, and a recommendation is provided in Section 3.3.1.2.

3.3.1.1 Jet Pressure Field

The BWR and PWR guidance documents both use the concept of a freely expanding jet to define 

the ZOIs and thereby debris generation, which is the fundamental issue of post-LOCA coolant 

flow. The methodology for determining the region of influence for the freely expanding jet is 

therefore critical.

The methodologies recommended in the PWR and BWR guidance documents establish jet 

pressure-field ZOIs for insulation and coatings debris generation. Table 3.2 summarizes these 

methodologies and the NRC staff responses for both types of ZOI definitions, as discussed in 

Sections 3.1 and 3.2.

There are basic differences for determination of the ZOI volume within the BWR guidance and 

between the BWR and PWR guidance. The BWR guidance specifies three possible methods that 

use of the results of anaysis with the CFD code NPARC (as provided in the Continuum 

Dynamics Report 96-01 Rev. 3 19) to determine the insulation ZOI. The coatings ZOI, however, 

is defined based on a conical jet, as per ANSI/ANS-58.2-1988 22. The PWR guidance uses 

ANSI/ANS-58.2-1988 for both the insulation and coatings ZOIs.

In SE NEI 04-07,4the NRC staff compared the jet expansion results of ANSI/ANS-58.2-1988 in 

the PWR guidance (as applied in NEI 04-07) to the results in the BWR guidance (i.e., from the 

Continuum Dynamics Report 96-01 Rev. 3 19 (NEDO-32686, Rev. 0 1)) and concluded that the 

approach in the BWR guidance was ―a more capable method of modeling steam jets than the 
43

ANSI (ANSI/ANS-58.2-1988 22) model.‖ However, the NRC staff did not fully endorse the 

BWR guidance for determining the ZOI based on the Continuum Dynamics Report 96-01, Rev. 

3. Methods 2 and 3 (as the basis for the insulation ZOI) were generally accepted, but the NRC 

staff did not accept Method 4, in which the ZOI is based directly on Continuum Dynamics 

Report 96-01, Rev. 3. Method 4 would require further justification, particularly in regard to the 

uncertainty of the CFD model (see Section 3.1.1).

The BWR coatings ZOI definition references Appendix C of ANSI/ANS-58.2-1988,22 while the 

PWR insulation ZOI and coatings ZOI definitions reference Appendices B, C, and D of this 

standard. These appendices each have the disclaimer ―This appendix is not part of American 

National Standard Design Basis for Protection of Light Water Reactor Nuclear Power Plants 

Against Effects of Postulated Pipe Rupture, ANSI/ANS-58.2-1988, but is included for 

information only.‖

NRC staff discussions regarding acceptance/rejection of the ZOI definitions are essentially

focused on the accuracy of the free jet expansion model, the application thereof either directly or 

via a spherical equivalent, and the effect of plant specifics on the ZOI (e.g., break configuration, 

drywell geometry, impingements, debris location and quantity, etc.). Concerns about the PWR 

approach using ANSI/ANS-58.2-1988, which focused on the accuracy of the methodology, were 

addressed via the methodology for application of the results. Specifically, the SE4states ―The 

staff‘s position is that the overall approach to determining ZOI is sufficiently conservative (by 

conserving the volume of a freely expanding jet to isobars of demonstrated destruction pressure) 

to allow use of the ANSI/ANS standard for determining ZOI.‖

3.3.1.2 Recommendations for ZOI Determination

As described above, ZOI determination is based on the approach for modeling the free jet 

expansion in both the BWR and PWR guidance. The treatment of free jet expansion should be 

similar in both approaches because the physical behavior of a steam or saturated water jet is not 

dependent on the reactor type.

The BWR and PWR insulation and coatings ZOI bases are not consistent. As described 

previously and summarized in Table 3.2, the basis accepted by the NRC staff for BWR 

insulation ZOIs using Methods 2 and 3 is Continuum Dynamics Report 96-01 Rev. 3 19, but the 

staff specified Method 4 as unacceptable without futher justification because it is based directly 

on Continuum Dynamics Report 96-01 Rev. 3 19. The NRC staff accepted the BWR use of 

ANSI/ANS-58.2-1988 20 for the coatings ZOI. For the PWRs, the NRC staff used Continuum 

Dynamics Report 96-01 Rev. 3 19 to help establish the acceptability of ANSI/ANS-58.2-1988.

20

 

As noted, the NRC staff discussions regarding acceptance/rejection of the ZOI definitions are 

essentially focused on the accuracy of the free jet expansion model, and the application thereof 

either directly or via a spherical equivalent. However, the NRC staff judged the acceptability of 

the free jet expansion models by considering the conservatism of the overall approach to 

determining the ZOI. Determination of whether one approach for modeling the free jet 

expansion for the ZOI is more technically valid than another is not possible based on the 

available analyses.
44

Table 3.2. ZOI as a function of reactor type and debris material.

Reactor Type Debris Material ZOI (Basis Reference) NRC Staff Position

BWR Insulation

Method 1, Entire Drywell Accepted

Method 2, Spherical with 

Bounding Radius (Continuum 

Dynamics Report 96-01 Rev. 

3)18

Accepted with 

Clarification

Method 3, Spherical with Break 

and Insulation Specific Radii 

(Continuum Dynamics Report 

96-01 Rev. 3)

Accepted with 

Clarification

Method 4, Non-Spherical Free 

Jet Expansion, Direct CFD 

Application (Continuum 

Dynamics Report 96-01 Rev. 3)

Not Accepted without 

Further Justification

PWR Insulation

Baseline, Spherical with 

Bounding Radius (ANSI/ANS58.2-1988)20

Accepted with 

Modification(a)

Refinement 1, Spherical with 

Insulation Specific Radii 

(ANSI/ANS-58.2-1988)

Accepted with 

Modification(a)

Refinement 2, Non-Spherical 

Free Jet Expansion 

(ANSI/ANS-58.2-1988)

Accepted with 

Modification(a)

BWR Coating

Generic value

(Bechtel Report 22754094.12A, 

ANSI/ANS-58.2-1988)

Accepted

PWR Coating Spherical 

(ANSI/ANS-58.2-1988)

Accepted with 

Modification(b)

(a) Reduction of insulation failure pressures by 40% for all material types characterized with air jet 

testing.

(b) Expansion of ZOI to default value of 10D or depending on coating type.

Recommendation 3.1 The zone of influence (ZOI) of the high-energy jet of steam or 

saturated liquid water released in a LOCA should be determined 

using an experimentally validated free-jet expansion model that is 

applicable to both BWR and PWR conditions. 

3.3.2 Debris Generation Within the ZOI

The determination of debris quantity with respect to the region of the ZOI and the effect of 

debris type are discussed in Section 3.3.2.1. Direct comparisons of the BWR and PWR industry 

approaches to insulation and coatings debris generation are made in Sections 3.3.2.1.1 and 

3.3.2.1.2, respectively. The insulation comparison addresses failure pressure, and the coatings 

comparison considers the guidance for qualified and unqualified coatings, as well as coating 
45

types. Industry guidance for the effect of the ZOI extent on debris generation is discussed in 

Section 3.3.2.2.

3.3.2.1 Quantity of Debris Generation

Table 3.3 provides the BWR and PWR guidance for the extent of insulation and coating debris 

generation within the ZOIs, which are defined as summarized in Sections 3.1 and 3.2. The 

differences within the BWR guidance and between the BWR and PWR guidance for the region 

of ZOI effect and quantity of debris are apparent.

The BWR guidance for determining insulation debris generation requires specific licensee 

evaluation, depending on the applied method and the specific LOCA scenario (e.g., break 

location, break type, insulation type and quantity, etc.). In contrast, fixed generic quantities are 

defined for the coatings debris. The insulation debris generation is a direct function of the 

insulation material‘s failure pressure (excepting Method 1, in which all insulation is assumed to 

fail), while the coating debris generation is not. This approach is inconsistent and complicates 

the separate determination of debris generation within and outside of a ZOI. In addition, as 

noted in Section 3.3.1.1, the free jet expansion models for determining the insulation and 

coatings debris quantities are different.

Table 3.3. Debris generation as a function of reactor type and debris material.

Reactor 

Type

Region of ZOI Effect Quantity of Debris

Insulation Coatings Insulation Coatings

BWR Isobar Volume(a, b) Generic Surface 

Area(c)

Volume within ZOI Fixed Generic

Volume

PWR Isobar Volume(d) Isobar Volume(e) Volume within ZOI Volume within ZOI

(a) Method 1 includes entire drywell as ZOI.

(b) Methods 2 through 4 use insulation specific failure pressure; break specific jet.

(c) Specified as area of a 24-inch-diameter jet at 20 feet with 10-degree half angle.

(d) Baseline ZOI based on insulation inside containment with minimum destruction pressure. Refinements 

1 and 2 use insulation specific failure pressures.

(e) Coating specific or default ZOI.

The guidance from the PWROG for determining the insulation and coatings debris generation 

relies on the material failure pressure. However, the baseline insulation ZOI is determined from 

the insulation inside containment with lowest destruction pressure.

3.3.2.1.1 Insulation Failure Pressure

Except for the BWR coatings ZOI, all ZOIs in Table 3.2 are based on the pressure at which the 

insulation or coating materials fail. The approaches to determining the pressure at which the 

materials fail are functions of the jet medium (i.e., steam or saturated water). As noted above in 

Section 3.1, the NRC staff cited concerns with the approaches to determining insulation failure 

pressures for both the BWR and PWR industry guidance.

The staff accepted the jet medium insulation failure correction factors provided by the BWR 

guidance. As noted in Section 3.1.1, the correction factors for the jet medium are taken from 
46

DRF A74-00004.27 Table 3.4 lists the BWR correction factors, and Figure 3.1 is also taken 

directly from DRF A74-00004.

The jet volume differences of Figure 3.1 are computed via the ANSI/ANS-58.2-1988 model.22 

These results are applied to the BWR insulation ZOIs, which are based on the Continuum 

Dynamics Report 96-01, Rev. 3 as discussed in Section 3.3.1.1. Comparing the correction 

factors from Table 3.4 and Figure 3.1 clearly indicates that the Table 3.4 values are conservative 

with respect to Figure 3.1 (however, evaluation of the conservatism or accuracy of Figure 3.1 is 

neither indicated nor implied by this comparison). The saturated water ZOI determined using the 

correction factors in Table 3.4 will be larger than indicated by Figure 3.1. The conservatism of

Table 3.4 relative to Figure 3.1 may be very significant for materials with failure pressures 

greater than 60 psi, assuming the trend of decreased correction factor with increased failure 

pressure shown in Figure 3.1 is extrapolated beyond the conditions evaluated, i.e., above 60 psi.

The NRC staff did not accept the PWR guidance insulation ZOIs, and reduced the insulation 

damage pressure by 40%, thereby increasing the ZOIs to account for the two-phase PWR jet. As 

reported in Section 3.1.2, SE NEI 04-07 stated that the failure pressure reduction is to ―...account 

for potentially enhanced debris generation in a two-phase PWR jet.‖ This approach is in 

disagreement with the BRW guidance, which has equivalent or decreased ZOIs for saturated 

water jets (i.e., see Table 3.4). Additionally, large-scale jet impact testing with insulation has 

demonstrated that saturated water jets are less destructive than steam jets (NEA/CSNI/R 

(95)11).28 It is reasonable to expect that the same debris materials may have different failure 

pressures, depending on the jet medium.

Table 3.4. Material Failure Pressure Correction Factors 

(NEDO-32686, Rev. 0, from DRF A74-00004 27)

Material Failure Pressure (psi) Correction Factor

>60 0.4

50-60 0.5

40-50 0.7

30-40 0.8

20-30 0.9

0-20 1.0
47

Figure 3.1. Comparison of the Volume of a Jet With Pressure Greater than or Equal to Pj (the 

material failure pressure) from Saturated Water and Steam Breaks (Figure 2 of DRF 

A74-00004 Appendix)27

The insulation failure pressures provided by the PWR and BWR industry guidance are compared 

in Table 3.5 for the same insulation types. The BWR guidance for steam jets is taken from 

NEDO-32686, Rev. 0 and and the PWR guidance is from NEI 04-07. The failure pressures for 

saturated water jets are determined via the BWR guidance for BWRs and the NRC staff position 

on the PWR guidance for PWRs, Section 3.3.1.2. 

The failure pressures due to steam jets have the same or similar values in the BWR and the PWR 

guidance (with the notable exception of the value for calcium silicate [CaSil], which is from twophase testing in the PWR guidance). However, the values for failure pressures due to a saturated 

water jet are significantly different, with the PWR failure pressures significantly lower than those 

for BWRs. For example, the saturated water jet failure pressure for PWR reactors for NUKON®, 

a fiber insulation, is 40% to 80% lower than the BWR values, and for Calcium Silicate, which 

will fail as particulate, up to 95% lower. This difference is due in large part to differences in 

regulatory guidance provided for the different reactor types. As referenced in Section 3.3.1.2 in 

the summary of guidance for PWRs, the NRC staff reduced the experimentally determined 

insulation damage pressure by 40% to ―...account for potentially enhanced debris generation in a 

two-phase PWR jet.‖ This is in direct disagreement with the BWR guidance, which has 

equivalent or decreased ZOIs for saturated water jets (see Table 3.4), thereby effectively 

increasing the insulation damage pressure (Table 3.5). 

While increasing the debris generation (by defining a decreased failure pressure relative to 

measured values) is obviously a more conservative approach, it is not necessarily more 

technically valid than relying directly on measured values. This is particularly the case when test 

results are not consistent with the imposed conservatism, and the conservatism is not uniformly 
48

applied to both reactor types. The same types of insulation are used for containment components 

in BWRs and PWRs. Thus, insulation material failure pressure is not a direct function of reactor 

type, even though it may be affected differently by different thermodynamic conditions during a 

LOCA blowdown. However, the differences in the PWR and BWR guidance can be expected to 

result in significant differences in the quantity and type of debris predicted to be generated for 

the same or similar conditions in a LOCA scenario.

Recommendation 3.2 A validated basis that is consistent as applicable between reactor types

for insulation material failure pressures should be developed for the 

range of thermodynamic conditions encountered in LOCA scenarios.

Table 3.5. Insulation failure pressures.

Material(a)

BWR Failure Pressure

(psi)

PWR Failure Pressure

(psi)

Steam 

Jet(b)

Saturated Water 

Jet(c)

NEI 04-07(d) Saturated 

Water Jet

SE NEI 04-07(e)

Darchem DARMET® 190 246, 363, 458 190 114

Transco RMI 190 246, 363, 458 190 114

Jacketed NUKON® with modified 

―Sure-Hold‖ Bands, Camloc®

Strikers and Latches(f)

190 246, 363, 458 150 90

Diamond Power MIRROR® with 

modified ―Sure-Hold‖ Bands, 

Camloc® Strikers and Latches(f)

190 246, 363, 458 150 90

Calcium Silicate with Aluminum 

Jacketing(g, h) 160 234, 355, 446 24 24

K-wool 40 78, 115, 144 40 24

Temp-MatTM with Stainless Steel 

Wire Retainer 17 17 17 10.2

Knaupf® 10 10 10 6

Jacketed NUKON® with 

standard bands 10 10 10 6

Unjacketed NUKON® 10 10 10 6

Koolphen-K® 6 6 6 3.6

Diamond Power MIRROR® with 

standard bands 4 4 4 2.4

Min-K 4 4 4 2.4

(a) Only the same or similar materials listed from each source.

(b) NEDO-32686, Rev. 0, Table 2.

(c) The three values are minimum, median, and maximum failure pressures as functions of radial offset and axial 

separation, determined from NEDO-32686, Rev. 0, Table 1 data. Failure pressures determined by adjusting 

NEDO-32686, Rev. 0, Table 1 ―A‖ values by the correction factor (Table 3.3, Section 3.3.1.2) and linearly 

interpolating to the pressure corresponding to the ―corrected A‖ value.

(d) NEI 04-07, Table 3-1.

(e) SE NEI 04-07, Table 3-2. See Section 3.3.1.2. Failure pressure for saturated water jet.

(f) PWR listing does not specify ―modified‖ ―Sure-Hold‖ bands nor Camloc® Strikers and Latches.

(g) PWR listing includes stainless steel bands.

(h) PWR failure pressure for saturated water jet.
49

3.3.2.1.2 Coatings Qualification and Type

Comparison of the BWR and PWR industry approaches to coatings debris generation is made 

with regard to the guidance for qualified and unqualified coatings as well as coating types.

3.3.2.1.2.1 Effect of Coating Qualification

Both the BWR and PWR industry guidance specify that all coating material within the zone of 

direct LOCA influence fails, regardless of its qualification. The NRC staff supports this 

position. This approach is conservative, and has a justifiable technical basis.

3.3.2.1.2.2 Coatings Types

The quantity of coating debris generated is a function of the coating thickness. The BWR 

guidance provides generic values for coatings debris quantities (as shown in Table 3.1; see 

Section 3.2.1), based on the ZOI area and the coating thickness as described in Bechtel Report 

22754094.12A.23 The specific coating thicknesses from Bechtel Report 22754094.12A are 

provided in Table 3.6, in comparison with values defined in the PWR guidance. Comments, as 

discussed below, are also included in Table 3.6.

Table 3.6. Coating thicknesses for BWR and PWR.

Coating

BWR(a) PWR

Coating 

Thickness 

(inch) Notes

Coating 

Thickness 

(inch) Notes

Inorganic Zinc 

(IOZ) 0.005 - 0.003 Default value

Epoxy 0.013

0.005 inch IOZ, plus 

0.008 inch epoxy 

topcoat

0.006 Default value; epoxy or 

epoxy pheloic topcoat

100% Epoxy 0.015 Up to 0.024 inches 

possible - No information provided

Other(b)-

Up to 0.006 inches 

possible, depending 

on coating type

- No information provided

Unqualified -

No information 

provided 0.003 inorganic zinc primer

(a) BWR reference source is Bechtel Report 22754094.12A.

(b) Other coatings include alkyd, vinyl, silicone, silicone alkyd, and silicone acrylic.

Bechtel Report 22754094.12A indicates that some metal surfaces have a three-coat epoxy 

system, where each coat may be 0.004 to 0.008 inches thick. Thus, the Table 3.6 thickness for 

100% epoxy coating is indicated to be low by possibly 0.009 inches. Further, Bechtel Report 

22754094.12A indicates that additional coatings (alkyd, vinyl, silicone, silicone alkyd, and 

silicone acrylic) may be present and have a thickness of up to 0.006 inches, depending on coating 

type. These coatings are unqualified.24 No information (i.e., typical maximum thickness) is 

provided directly to enable the contribution to the debris loading of unqualified coatings to be 

determined.
50

The PWR guidance provides default values that are recommended for use, in lieu of plant 

specific information. The recommended coating thicknesses provided are listed in Table 3.6, for 

direct comparison to the BWR recommended values. The NRC staff does not accept the PWR 

guidance provided in NEI 04-07, and recommends that licensees should perform plant-specific 

evaluations of existing coatings and their current condition.

The NRC staff also recommended increases in the PWR coatings ZOI, as summarized in 

Section 3.2.2. These recommendations were made due to the different temperature responses of 

different coatings systems, and because the coatings testing referenced in NEI 04-07 was not 

performed at conditions replicating the effects of LOCA jet pressures and temperatures. Pressure 

by itself is not as detrimental to coatings as the synergistic effect of pressure and temperature, as 

noted in NEDO-32686, Rev. 0.1

BWR and PWR coatings types and coatings requirement/standards are specified in Bechtel 

Report 22754094.12A23 and ASTM D5144-00 25 (per NEI 04-07), respectively. Comparison of 

these two documents provides no indication that coating variations may be expected between 

reactor types. For example, an inorganic zinc coating in a BWR would be expected to have the 

same thickness as in a PWR, although, as addressed in the NRC staff recommendation for plant 

specific evaluations, variations in the actual application both in and between the reactor types are 

expected.

The greater default thickness of the BWR coatings may result in a more conservative coatings 

debris quantity, but the ―fixed bounding volume‖ may be nonconservative. No guidance for the 

determination of the thickness of unqualified coatings is provided for BWRs.

Recommendation 3.3 A validated basis consistent as applicable between reactor types 

for qualified and unqualified coatings thickness should be 

developed.

3.3.2.3 Effect of ZOI Extent

The BWR guidance (specifically, Methods 2, 3, and 4 for defining the ZOI) and the PWR 

guidance (specifically, the Baseline ZOI, including Refinements 1 and 2, and the coatings ZOI) 

result in the potential exclusion of debris sources, depending on the break location, drywell 

component configuration, and the extent of the ZOI. This issue is expected to be of particular 

significance for those approaches that rely on a spherical equivalent ZOI (see Table 3.2).

The simplification of defining a spherical ZOI of equivalent volume to the destructive volume of 

a free jet is assumed to be conservative in the BWR and PWR guidance documents and is 

accepted as such in the NRC staff evaluations of this approach. This assumption is based on 

qualitative arguments on the possible dissipation of the energy of the jet due to reflections and 

deflections from drywell equipment, possible interference from opposing jets, etc. These 

arguments ignore the long reach of the jet cone, which could carry pressures above the material 

destruction pressures much farther from the break than would be reached by the spherical ZOI, 

and the possibility that deflections of the jet could also extend the destruction pressures beyond 
51

the reach of the spherical ZOI. Without careful evaluation of specific plant layouts and break 

locations, it is not clearly demonstrable that the spherical ZOI would in all cases be conservative, 

compared to a more physically realistic determination of the actual ZOI of the jet(s) that could be 

generated in a given LOCA.

3.3.2.4 Recommendations for Determining ZOI Extent

Approaches that make use of simplifications (e.g., impingement with barriers, extent of ZOI with 

regards to debris location, significance of debris source to subsequent head loss, etc.) intended as 

conservatisms may not in all cases result in conservative estimates of debris quantities. The 

recomendations in the preceeding sections address the free jet expansion modelling for the ZOI 

(Recommendation 3.1), the failure pressure of the insulation material (Recommendation 3.2), 

and the quantity of coating debris in terms of coating thickness (Recommendation 3.3). These 

recommendations address aspects of debris generation that may significantly alter the quantity of 

debris generated by a LOCA. 

The simplification of relating the extent of the ZOI to the location of debris-generating material

is specifically affected by Recommendations 3.1 and 3.2. If the flow field of the LOCA break jet 

and the material failure pressure(s) are not technically valid, it is not possible to determine that 

the estimated volume of the zone of influence of the jet is conservative. If this volume is then 

approximated by a spherical region, there is no assurance of conservatism in the assumption that 

material outside of the artificially defined spherical ZOI can be excluded as debris sources.

Establishing 1) consistent and validated fundamentals of free jet expansion, 2) the effect of the 

jet medium, 3) the pressures at which debris-generating materials fail, and 4) the quantity of 

debris generated (as addressed in Recommendations 3.1 through 3.3) would allow for increased 

confidence in the conservatism of potentially reducing the quantity of debris generation by 

means of defining a specific ZOI extent relative to the location of debris-generating material. 

Recommendation 3.4 Reducing potential debris quantity by means of the definition of a 

specific ZOI extent, debris location, and contribution to subsequent 

head loss should only be considered after validated and consistent 

approaches for free-jet expansion, debris material failure pressure, 

and debris quantity are established.

3.3.3 Comparison of Debris Generation Outside of the ZOI

The BWR and PWR guidance for insulation and coatings debris generation inside of the ZOI is 

summarized in Sections 3.1 and 3.2 above. Comparison of the guidance for debris generation 

outside of the ZOI is made for insulation material in Section 3.3.3.1 and coatings in 

Section 3.3.3.2.

3.3.3.1 Insulation Debris Generation

The BWR industry position for insulation debris generation outside of a ZOI is that no additional 

transportable debris would be generated. The PWR guidance specifies that jacketed insulation 

that is undamaged will not generate transportable debris, but that all unjacketed insulation is 

presumed to fail. Thus, depending on the ZOI extent and plant-specific quantity of unjacketed 
52

insulation, there may be significant additional potential debris sources accounted for in the PWR 

guidance. 

3.3.3.2 Coatings Debris Generation Outside of the ZOI

The BWR industry guidance for coatings debris generation outside of a ZOI is that all 

unqualified or sufficiently degraded coatings may be available for transport (i.e., will fail and 

become debris) but allows for licensees to negate its contribution due to a lack of transport 

mechanism. The NRC staff does not support the latter approach.

The PWR guidance for coating debris generation outside of a ZOI specifies that all exposed 

DBA-unqualified and unacceptable coatings fail. The NRC staff agrees with this position, but 

specifies that licensees must periodically assess the DBA-qualified and acceptable coatings to 

ensure that degradation has not occurred to render them a debris source.

The general BWR and PWR guidance is similar, but the lack of specific details for unqualified 

coatings (see Section 3.3.2.1.2.2) may allow for differences in debris generation per unit area.

3.3.3.3 Recommendations for Debris Generation Outside of the ZOI

Failure of insulation material and coating systems outside of the ZOI are functions of the postLOCA environment within containment and the material‘s properties. Differences in debris 

generation outside the ZOI for BWRs and PWRs should be due only to differences in the postLOCA environment.

The PWR position for insulation debris outside of the ZOI, which is to assume that all 

unjacketed insulation will fail, is clearly more conservative than that of the BWR guidance, 

which specifies that no additional transportable insulation debris will be generated. However, 

the difference is impossible to quantify given the disparate ZOI development and debris 

generation parameters.

The lack of specific details for coatings debris generation (e.g., no BWR guidance is provided for 

unqualified coatings thickness, so the quantity is indeterminant), confounded in the same manner 

as with the insulation debris by the disparate ZOI development and debris generation parameters, 

makes it impossible to determine the conservatism of one approach for coatings debris 

generation with regards to the other.

The quantity and type of debris generated by a LOCA is impacted by debris generation outside 

of the ZOI.

Recommendation 3.5 A validated approach consistent as applicable between reactor 

types for the failure of insulation and coating systems outside of 

the ZOI is recommended.
53

4. EFFECTS OF DEBRIS IN RECIRCULATION 

LOOP COOLANT

This section discusses guidance for determining the effects of debris in the recirculation loop 

coolant. Guidance for BWRs, from the BWROG, is documented in NEDO-32686-A,1 Utility 

Resolution Guidance for ECCS Suction Strainer Blockage, Volumes 1, 2, 3, and 4, prepared by 

the BWROG (NEDO-32686-A). Guidance for PWRs, from the Nuclear Energy Institute, is 

documented in NEI 04-07,3Pressurized Water Reactor Sump Performance Evaluation 

Methodology3and WCAP-16406-P (Revision 1), Evaluation of Downstream Sump Debris 

Effects in Support of GSI-191. NRC guidance is documented in the SE reports on the BWROG 

document and on the documents presenting industry guidance for PWRs. Additional guidance is 

also provided in evaluations of responses to GL 2004-02, NRC Bulletin 95-02, and NRC Bulletin 

96-03.

The general structure of this section presents industry guidance, then regulatory guidance, then 

provides an evaluation and recommendations to reconcile guidance for the two reactor types. 

First, the relevant points of industry guidance from the listed documents are summarized, with 

separate subsections for BWRs and PWRs. In these subsections, specific guidance that was not 

acceptable to NRC is noted. These subsections are followed by a summary of the relevent 

guidance from NRC, including guidance related to issues not accepted in the industry guidance. 

Differences in the guidance provided for BWRs and PWRs are then summarized, and finally, 

specific recommendations are presented to unify and reconcile the guidance provided for both 

types of reactors. The specific topics discussed are listed below.

1) Section 4.1 discusses guidance for determining the amount of debris captured on 

the suction strainers. 

2) Section 4.2 discusses guidance for determining the amount of debris that passes 

through the suction strainers. (This section also discusses guidance for 

determining effects on downstream components.)

3) Section 4.3 discusses guidance for evaluating the effects of debris in the reactor 

vessel and core. 

4.1 Debris Captured on Suction Strainers

This subsection is concerned with guidance provided for examining what happens when the 

suction strainers capture debris, as they are designed to do. The pressure drop (also expressed as 

head loss) across the strainer will be greater than it would be if the water were clean (or if all 

debris were fine enough to simply pass through the screen.) Regulatory Positions 1.1 (PWRs) 

and 2.1 (BWRs) from Regulatory Guidance 1.82, Revision 3, specify that net positive suction 

head (NPSH) must be maintained, to ensure the availability of the sump (PWRs) or the 

suppression pool (BWRs) water supply for long-term cooling of the reactor core in post-LOCA 

conditions.

Determining the pressure drop across a suction strainer is a relatively straightforward problem in 

hydraulics when the coolant does not contain debris. The manufacturer generally has performed 
54

experimental measurements of pressure loss across the strainer for a range of flow velocities at 

the conditions of interest, so that the ―clean‖ loss coefficient for a particular plant‘s suction 

strainer design is known from plant-specific analyses and design requirements. It can be 

presumed that the suction strainers are designed such that this loss coefficient will yield an 

acceptably low pressure drop for clean fluid. However, the purpose of the suction strainers is to 

prevent debris from entering the recirculation loops, containment spray loops, and residual heat 

removal loops, so it must be expected that they could capture a significant amount of debris 

material in the course of a LOCA event. This will result in an increased pressure drop (or head 

loss) across the suction strainer, if the ECCS pumps are to maintain the required flow rate of 

cooling water from the sump or suppression pool.

Determining what the increased head loss will be as a function of debris loading is an issue that 

must be satisfactorily resolved to show that NPSH can be maintained throughout the LOCA and 

post-LOCA recirculation cooling. Section 4.1.1 describes the approach recommended by the 

BWROG for suppression pool suction strainers. Section 4.1.2 describes the approach 

recommended by the Nuclear Energy Institute (NEI) and Westinghouse Electric for PWR 

systems. Section 4.1.3 describes the guidance provided by NRC for resolution of this issue. The 

differences between the guidance developed for BWRs and PWRs are summarized in Section 

4.1.4. Recommendations for developing consistent guidance for the two systems are provided in 

Section 4.1.5.

4.1.1 Guidance from BWROG for Suction Strainer Head Loss Calculations

Guidance for calculating the head loss due to a debris bed on the ECCS suction strainers in a 

BWR is provided in NEDO-32686-A,1in Section 3.2.6.2.2 Total Strainer Debris Loading.(j) The 

general approach is as follows:

1. Determine the debris source terms for each of the debris species in the suppression pool 

(including latent debris and debris generated by the LOCA).

2. Assume that the debris present in the suppression pool accumulates on the functioning 

strainers in amounts proportional to the flow rate through each strainer, and determine the 

limiting quantity of debris for each strainer.

3. Calculate the head loss for each functioning strainer, following the methodology and 

calculational procedures documented in NEDO-32686-A,1using plant-specific debris source 

terms as input. (The methodology is based on evaluation of strainer head loss data obtained 

at the EPRI/NDE facility in Charlotte, North Carolina, and is documented in Appendices A 

and B of C.D.I. Report No. 95-09,

(k)

Testing of Alternate Strainers with Insulation Fiber and 

Other Debris, Revision 4, prepared by Continuum Dynamics, Inc,. Princeton, New Jersey,29 

This report is contained in the BWROG guidance document.)

Appendix A of C.D.I. Report No. 95-09 in the BWROG guidance document provides guidance 

for determining strainer head loss due to fibrous debris in combination with sludge and other 

miscellaneous debris. The guidance document notes that this evaluation must be conducted for 

 

(j) This is part of Subsection 3.2.6 Verification of Adequate ECCS Pump NPSH, in Section 3.2 Methodology for 

Sizing Passive ECCS Suction Strainers.

(k) The C.D.I report is Reference 3 of NEDO-32686-A and appears in Volume 2 of the Utility Resolution Guidance 

document.
55

every plant, since it is assumed that some quantity of fiber will in every case be present in the 

suppression pool post-LOCA, even for plants that have primarily reflective metal insulation. 

Appendix B of C.D.I. Report No. 95-09 in the BWROG guidance document provides guidance 

for determining strainer head loss due to RMI debris. For plants with both types of insulation, 

the guidance document recommends that the higher of the limiting strainer head loss values 

determined from these two approaches should be used as the input for determining ECCS pump 

NPSH. (The SE issued by NRC on this document does not endorse this recommendation. See 

Section 4.1.3.)

The experimental data used to develop the BWROG guidance for determining strainer head loss 

consists of testing with five(l)full-size passive strainers of different designs. These included a 

truncated cone, a 20-point star, two stacked-disk designs (prototypes #1 and #2 from 

Performance Contracting, Inc.), and a 60-point star design. Of these alternative strainer designs, 

only the stacked-disk configuration has been developed and installed in BWR plant suppression 

pools as part of the response to the strainer clogging issue. (It should be noted, however, that 

none of the specific designs tested are directly prototypic of new strainers installed in operating 

plants.)

The passive strainer designs were tested with varying quantities of debris consisting of fibrous 

insulation, RMI, corrosion products, and miscellaneous debris, over a range of flow rates. For 

plants with debris species that were not included in the testing (as documented in Reference 3 of 

NEDO-32686-A), the guidance document recommends that the licensee develop the necessary 

―estimated adjustments‖ to the model developed from the test data, to calculate the head loss due 

to the untested debris species. Precisely how this could be done in a physically correct manner is 

not specified.

The test results are presented in the BWROG guidance document as non-dimensional head loss 

curves correlated to the ratio of the fiber bed thickness and the strainer diameter, which is 

defined as the diameter of the (uniform) stacked discs. The maximum fiber bed thickness-todiameter ratio obtained in the testing is assumed bounding. Without presenting any dimensional 

analysis to justify this ratio as a physically meaningful scaled relationship, the BWROG 

guidance document asserts that a lower fiber bed thickness-to-diameter ratio ―assures 

applicability of the existing non-dimensional head loss correlation.‖ For strainers with higher 

fiber loadings, however, the document notes that strainer-specific testing may be required. The 

guidance document also notes that it is the responsibility of the licensee to develop 

documentation supporting any adjustments in the head loss calculation.

If the head loss calculation shows that adequate NPSH is available to the ECCS pump, then the 

strainer design is assumed to be acceptable (provided all other strainer design requirements not 

related to NPSH are also satisfied). If the analysis shows that adequate NPSH is not available for 

the worst-case conditions, further guidance is provided on ways to reduce the calculated strainer 

head loss. The following subsections describe the detailed guidance provided for each step in the 

analysis.

 

(l) The document describes seven strainer designs tested, but one was an ―active‖ strainer, and another was only a 

2/3rd segment of a particular design. So in effect, the testing included only five prototypic passive suction 

strainer types.
56

4.1.1.1 Passive Strainer Head Loss with Fibrous Debris and Pool Sludge

In Appendix A of C.D.I. Report No. 95-09 in NEDO-32686-A, the BWROG guidance document 

presents a step-by-step procedure for calculating passive strainer head loss across a fibrous 

debris bed with corrosion products and other debris. The strainer geometry must scale uniformly 

to the geometry of one of the tested strainers (i.e., all dimensions match the tested strainer 

dimensions if multiplied by a single constant). This approach uses a relationship fitted to the test 

data by least-squares approximation to express the head loss across the debris bed on a given 

strainer as a function of 

the Reynolds number of flow through the bed (defined using the inter-fiber spacing as the 

characteristic length)

the ratio of the mass of corrosion products to the mass of fiber debris

the approach velocity to the strainer

the nominal thickness of the fiber bed

the inter-fiber spacing 

the viscosity and density of the water in the pool

the acceleration of gravity

empirical coefficients from the data-fitting process. 

The BWROG guidance document notes that in applying this procedure, the licensee must 

determine the appropriate plant-specific input values for the mass of corrosion products in the 

suppression pool and the volume and mass of fiber debris generated in a given LOCA. The 

approach velocity must also be calculated based on required ECCS flow rates. The inter-fiber 

spacing of the intact insulation, which defines an upper bound on the size of fiber debris, must be 

determined for the plant-specific fibrous insulation. (Appendix E of C.D.I. Report No. 95-09 in 

the BWROG guidance document provides values for Nukon®, Kaowool®, and Tempmat®.)

For plants with no other debris sources than fibrous insulation and the sludge in the suppression 

pool, the BWROG guidance document asserts that this is all that needs to be done to determine 

the strainer head loss. For most plants, however, it is expected that there will be other types of 

debris, such as RMI particulate or other miscellaneous debris, and the analysis must continue to 

follow the procedural steps for determining the possibly greater head loss due to the presence of 

these materials. 

4.1.1.2 Passive Strainer Head Loss with Fiber, Pool Sludge, and Miscellaneous Debris 

In this context, ―miscellaneous‖ debris refers specifically to paint chips, rust flakes, cement dust, 

sand, zinc, and calcium silicate. These are the materials that were tested in the experiments 

reported in C.D.I. Report No. 95-09 in the guidance document (NEDO-32686-A),1and have 

empirical coefficients listed for use in the head loss calculation. However, the testing to obtain 

information on the head loss effects of this material was not performed in the same manner as the 

testing with fibrous debris and corrosion products (pool sludge) only. 

In the tests with fibrous debris and pool sludge, a pumped loop drove the required flow rate 

through the strainer, simulating the effect of flow drawn into the ECCS recirculation loop. For 

the miscellaneous debris, the testing was performed in a ―gravity head‖ apparatus. This 
57

consisted of a vertical pipe containing a static column of water supported from below, with the 

strainer being tested inserted near the bottom of the column. A predetermined load of 

miscellaneous debris was placed on the strainer or suspended in the column of water, and at the 

appropriate time, a trap door was opened suddenly at the base of the pipe. The head loss was 

determined by measuring the fall in the water level in the pipe, and the time required for the level 

to travel a specified distance.

The effect of this debris on the overall head loss for the strainer is accounted for with a factor, 

Kbu, termed the ―bump up‖ factor, which is multiplied by the head loss due to fiber and pool 

sludge only. The recommended procedure is to define Kbu as the ratio of the head loss calculated 

for all debris sources over the head loss for the fiber and pool sludge only (calculated as 

described in Section 4.1.1.1 above.) That is, 

fiber sludge

alldebris

bu H

H

K

The ΔH terms in the above ratio are simplified to linear functions of the approach velocity, 

ΔH = a+bU, and the coefficients a and b are defined from the test data for the debris material 

tested. The coefficients consist of empirically determined weighted summations of the mass 

fractions of the various components present in the debris. The strainer head loss is calculated as 

the value obtained for the fiber and sludge alone (as described in Section 4.1.1.1), times the 

―bump up‖ factor.

4.1.1.3 Passive Strainer Head Loss with RMI Debris

In Appendix B of C.D.I. Report No. 95-09 in NEDO-32686-A, the guidance document presents a 

step-by-step procedure for calculating passive strainer head loss across a debris bed that includes 

RMI fragments. The basis for the recommended approach is the full-scale testing at the Electric 

Power Research Institute (EPRI) Non-Destructive Evaluation (NDE) center (documented in 

C.D.I. Report No. 95-09 in NEDO-32686-A) and blast-test RMI samples obtained in another test 

program. Only three of the five strainer designs were tested with RMI debris in the testing at the 

EPRI facility: the truncated cone, the 20-point star, and the 60-point star designs. Test 

measurements were obtained for the three strainer types with debris from 1.5-mil aluminum, 6.0-

mil aluminum, and 2.5-mil stainless steel RMI. The stacked disk prototype #1 and #2 strainers, 

which are the only designs in this test series that resemble advanced strainers installed in BWRs, 

were not tested with RMI debris. The guidance document asserts that the available data can be 

extrapolated for the material of interest. 

The procedure recommended in the BWROG guidance document gives the head loss across the 

RMI debris bed as a function of the approach velocity, the projected bed thickness, and empirical 

coefficients derived from the test data. The guidance document notes that this approach is valid 

only if the head loss across the bed is less than 10 ft of water (4.33 psi), which is the maximum 

height of the water column in the gravity head testing reported in C.D.I. Report No. 95-09 in 

NEDO-32686-A. The BWROG guidance document does not provide any recommendations for 

alternative approaches if this calculation produces a head loss greater than 10 ft of water.
58

Even for plants using only reflective metal insulation, the BWROG guidance document assumes 

that there will be some quantity of fibrous debris present, and there will always be sludge 

(corrosion products) in the suppression pool. Therefore, the recommended approach for 

determining the total strainer head loss is to calculate a value for the fiber and sludge alone 

(using the Appendix A methodology, as described in Section 4.1.1.1), and a value for RMI debris 

(using the Appendix B methodology), and take the higher of the two for the NPSH margin 

calculation. Similarly, if the plant also contains a significant quantity of miscellaneous debris, 

the total strainer head loss is taken as the higher of the value due to RMI debris and the value 

obtained for fiber, sludge, and miscellaneous debris (using the Appendix A methodology, as 

described in Section 4.1.1.2). (The SE issued by NRC on this document does not endorse this 

recommendation. See Section 4.1.3.)

4.1.1.4 Thin Bed Head Loss on Passive Strainers

The BWROG guidance document acknowledges that for flat plate strainers (typical of those 

installed at many BWRs prior to remediation in response to NRCB 95-02 and NRCB 96-03), the 

head loss can be high with only a small amount of fiber in the coolant. The BWROG guidance 

document defines this as an amount required to cover the strainer to a depth of about 1/8 inch. 

(Subsequently, it has been found that filtering beds can occur at even lower fibrous debris loads, 

as noted in the SE for the BWROG guidance document. See Section 4.1.3.) The BWROG 

guidance document notes that this thin bed effect (TBE) ―may not be applicable to strainers with 

low approach velocities, such as a strainer with large surface area,‖ and that for alternate 

geometry strainers, such as the star and stacked disk strainers, the TBE does not occur.

The recommendation in the BWROG guidance document for dealing with the TBE is two-fold. 

For plant-specific evaluations where the strainer design corresponds to any of the alternate 

geometries tested to develop the methodology from Appendix A of C.D.I. Report No. 95-09 in 

NEDO-32686-A, the BWROG guidance document indicates that it is not necessary to consider 

the TBE. The guidance document asserts that these strainer designs do not develop thin debris 

beds, and therefore it is necessary to consider only the maximum quantity of fibrous debris 

available for deposition on the strainer. For flat plate designs, such as a truncated cone strainer, 

the evaluation of head loss should consider both the maximum fibrous debris source term and a 

source term that is sufficient to cover the strainer flow area to a depth of only 1/8 inch. In such 

cases, the higher of the two values should be used to calculate the NPSH margin for the ECCS 

pumps.

4.1.1.5 Active Clearing of Debris from Suction Strainers

The BWROG evaluated two possible methods for actively mitigating the build-up of debris on 

the suction strainers; back-flushing during ECCS operation, and a ―self-cleaning‖ strainer design 

in which a mechanical system would remove debris from the strainer. The guidance document 

specifically recommends against the use of strainer backflush as the primary means of verifying 

that debris clogging will not compromise NPSH margin. The main reason for this position is the 

time frame and frequency of backflushing required for it to be effective. However, the guidance 

document suggests that use of backflush for ―defense in depth‖ might be a viable option to 

consider for plants with existing capability to perform such an operation.
59

The BWROG evaluation of the effectiveness of ―self-cleaning‖ strainers included testing the 

performance of a single active strainer as part of the experimental program at the EPRI/NDE 

facility for alternative passive strainer designs. The ―self-cleaning‖ strainer includes a rotating 

plow and brush assembly driven by a water-powered turbine. Water passing through the strainer 

provides the motive force so that the device removes debris from the flat front face of the 

strainer. The experimental results showed that the design is effective for the debris types and 

loadings tested.

The guidance document recommends installing a self-cleaning strainer design only if it proves 

impossible to achieve the required system performance with passive strainers. Although the 

BWROG considers self-cleaning strainers as a potentially effective option for mitigating the 

problems associated with debris in the suppression pool coolant, there are significant issues that 

must be addressed. These include qualifying the design as safety-grade equipment, performing 

adequate testing to qualify the design for the full range of operating conditions, and developing 

appropriate maintenance, surveillance, and instrumentation requirements for the active strainers. 

In addition, the evaluation must consider the effect of debris on downstream components.

4.1.2 Industry Guidance for Suction Strainer Head Loss Calculations in PWRs

The recommended ―baseline methodology‖ for calculating the head loss from a debris bed that 

could form on the ECCS suction strainers(m)in a PWR is documented in Section 3.7 of NEI 04-

07.3 Sample problems for insulation debris are provided in the industry guidance document to 

illustrate the methodology, which consists of three basic steps:

1. Determine the clean suction strainer head loss.

2. Determine the head loss due to the debris bed on the suction strainer.

3. Add the results of the first two steps and compare to the plant-specific NPSH margin.

If the total head loss is below the NPSH limit for the ―worst case‖ debris bed, no further analysis 

is needed to show that sufficient NPSH margin exists even when the strainers are fouled with 

debris. If the predicted total head loss across the suction strainer and debris bed is too high to 

maintain NPSH margin, further guidance is provided in Section 4 of NEI 04-07.3 The following 

subsections describe the detailed guidance provided for each step in the analysis.

4.1.2.1 Clean Suction Strainer Head Loss

The clean screen head loss (CSHL) for an unfouled suction strainer is dependent on plantspecific screen design, sump geometry, and thermal-hydraulic conditions in the sump during the 

particular LOCA being evaluated. The CSHL depends mainly on the size of the openings in the 

strainer (mesh for wire-grids or hole diameter and pitch for perforated plates), the flow rate 

through the strainer, the water temperature and pressure, and the depth of water in the sump 

(which may not be sufficient to fully submerge the strainers during portions of some LOCA 

scenarios.)

 

(m) In the NEI document, the suction strainers are referred to as ―sump screens,‖ using the terminology for PWRs 

current at that time. The term ―suction strainer‖ has since been adopted as applicable to both PWRs and BWRs, 

and is used throughout this section. 
60

The guidance document does not provide detailed directions on how to calculate the clean 

strainer head loss, since this is simply an exercise in the standard methods of analysis in fluid 

mechanics. However, it is helpfully noted that some of this information is typically available 

from the manufacturer of the strainer material, and in some cases, the CSHL has been 

documented in other plant licensing calculations. The guidance document also notes that it may 

be necessary to include losses due to plant-specific features of the sump screen in the overall 

CSHL calculation, such as the support structures, bracing (for mechanical loads), and other sump 

structures, such as vortex suppressors.

4.1.2.2 Debris Bed Head Loss

The methodology recommended in the industry guidance document provides an approach for 

determining the debris bed head loss for a given total quantity and type of debris over a specified 

surface area at a given ECCS pump flow rate. The approach is based on the following scenario:

1. The suction strainer is initially clean, but the LOCA is filling the floor pool with coolant 

containing a homogenous mixture of debris (latent and LOCA-generated).

2. At switchover of ECCS pump suction from the water storage tank to the recirculation sump, 

debris-laden coolant begins to move through the suction strainer.

3. Debris smaller than the screen mesh (or hole size) passes through the strainer, but larger 

debris begins to form a debris bed on the surface of the strainer. It is assumed that the debris 

bed covers the strainer surface uniformly and continues to build until all available debris has 

been deposited on the strainer.

a. For fibrous debris, or a mixture of fiber and particulate debris, the fibers form a fibrous 

mat that will also trap particulate once the mat has formed.

b. For debris consisting only of RMI particles, the debris bed forms mainly because of 

particles larger than the strainer holes building up on the surface.

c. For mixed debris beds consisting of fiber, particulate, and RMI particles, the debris bed is 

assumed to form in much the same way as with mixed fiber and particulate debris, except 

that the fiber bed traps RMI fragments as well as other particulate debris.

d. For particulate debris (which could also include RMI particles, but not fiber) or a mixture 

of particulate and fibrous debris, the debris bed forms because of particles larger than the 

strainer holes building up on the screen surface.

4. The debris bed is assumed to cover the entire strainer and build up with a uniform thickness. 

As the debris bed builds, the head loss across the bed increases until a steady-state value is 

reached with the debris bed at its thickest condition.

The approach in the guidance document does not provide a method for directly analyzing the 

process described in the above scenario. Instead, it recommends specific head loss correlations 

for determining the final steady-state head loss across the fully formed debris bed, as described 

in step 4 of the basic scenario. However, the SE issued by NRC on this document does not 

accept the use of correlations, and recommends more comprehensive treatment of debris bed 

formation, including thin bed effect. This is discussed in Section 4.1.3.
61

4.1.2.2.1 Fiber and Particulate Debris Bed

For a debris bed consisting of fibers and particulate material, the industry guidance document 

recommends the head loss correlation presented in NUREG/CR-6224, Parametric Study of the 

Potential for BWR ECCS Strainer Blockage Due to LOCA Generated Debris.

30

 The correlation, 

which is generally referred to as ―the NUREG/CR-6224 correlation,‖ was developed from test 

data(n) modeling debris accumulation on suction strainers in a BWR suppression pool. The 

issues of debris blockage are similar for BWRs and PWRs, and it is the position of the guidance 

document from NEI that the validated range of the correlation makes it equally applicable to 

PWR systems.

The general form of the correlation reflects a semi-empirical simplification of the momentum 

equation for flow through a particle bed. It has two additive components, the first one linear 

with velocity (for laminar flow) and the second one with a second-order velocity term (for 

turbulent flow). This relationship gives pressure drop as a function of the bed porosity, the 

superficial velocity of the fluid (i.e., volumetric flow divided by the total cross-sectional area of 

the bed perpendicular to the flow direction), and various geometric and empirical coefficients. 

From this basic model, the NUREG/CR-6224 correlation was developed to give head loss across 

a debris bed as a function of the following parameters:

1. the fluid approach velocity (i.e., the volumetric flow rate through the strainer, divided by the 

effective surface area)

2. the density and viscosity of the water

3. the mixed debris bed ―solidity‖ (i.e., one minus the porosity of the bed)

4. the surface-to-volume ratio of the debris forming the bed

5. the actual debris bed thickness.

For conservatism, the industry guidance document recommends using the highest flow rate 

through the strainer, based on the maximum pump flows ―as identified in current NPSH 

calculations‖ (see p. 3-55 of NEI 04-07).3 The density and viscosity of the water are properties 

determined primarily by the water temperature, and as a conservative simplification, the 

guidance document suggests that the lowest expected temperature during ECCS operation may 

be used for the head loss analysis. If a more realistic estimate is needed, it may be necessary to 

examine thermal-hydraulic conditions in the sump calculated at multiple times during the LOCA 

transient, since it is not obvious which temperature and flow rate would be limiting. 

The remaining three parameters that must be quantified to apply the NUREG/CR-6224 

correlation depend on the physical properties of the debris, including density, geometric shape, 

and size distribution. Drawing on the work in NUREG/CR-6224 and NUREG/CR-6371,31 the 

guidance document presents relationships for the solidity, the surface-to-volume ratio of the 

debris, and the bed thickness, and describes a computational procedure for applying this 

 

(n) The debris bed in this testing consisted of Nukon® fiber and iron oxide particulate, but in the NEI document, 

the correlation is assumed to apply to any debris bed consisting of only fiber and particulate. This approach was 

not accepted by the NRC. See Section 4.1.3.
62

empirical model. The procedure yields a converged estimate of the head loss and thickness for 

the debris bed.

The industry guidance document also contains repeated reminders within the discussion of 

individual components of the correlation that this is an empirical model. When applied in plantspecific analysis, it is necessary to determine that the validation data appropriately represent the 

debris type(s), mixture, and concentrations as well as the thermal-hydraulic conditions of the 

plant. If the plant-specific materials or combinations of materials are not included in the 

database, then head loss testing of the material(s) may be required to validate the NUREG/CR6224 correlation for the plant-specific conditions.

4.1.2.2.2 RMI Debris Bed

For a debris bed consisting only of RMI fragments, the industry guidance document recommends 

essentially the same approach as described above for a fiber-and-particle bed, but with a different 

head loss correlation. For an RMI debris bed, the guidance document recommends the head loss 

correlation presented in NUREG/CR-6808,32 Knowledge Base for Effect of Debris on 

Pressurized Water Reactor Emergency Core Cooling Sump Performance (2003). This 

correlation is based on testing with RMI debris and is an empirical function of the approach 

velocity (squared), geometry factors, the interfoil gap thickness, and an empirical coefficient. 

The interfoil gap thickness is an empirical parameter determined from testing documented in 

NUREG/CR-6808.32

The industry guidance document shows an example of a bounding formula for 2.5-mil stainless 

steel RMI debris. However, the document does not include guidance on how to determine 

suitably conservative empirical parameters for plant-specific conditions that are not bounded by 

the validation database of the correlation presented in NUREG/CR-6808.32

4.1.2.2.3 Mixed Debris Bed (fiber, particulates, and RMI)

For a mixed debris bed consisting of fiber, particulates, and RMI particles, the industry guidance 

document recommends developing a conservative estimate of head loss by superposition. The 

NUREG/CR-6224 30 correlation, described in Section 4.1.2.2.1 above, is used to estimate the 

head loss, assuming that the bed is composed only of fiber and ordinary particulate. The 

correlation from NUREG/CR-6808 32 is then used to estimate the head loss, assuming the debris 

bed is composed only of RMI fragments. The total head loss of the debris bed is then estimated 

as the sum of the two independent models. 

The guidance document considers this approach automatically conservative, and notes that there 

is no need to consider the actual form of the mixed debris bed. The document further notes that 

this approach may give ―overly conservative‖ results for conditions where there are only ―trace 

amounts‖ of fibrous material in the total debris load. More realistic methods may be required for 

analysis of plants where there are only trace amounts of either RMI or fiber in the mixed debris 

load. 
63

4.1.2.2.4 Particulate Debris and Mixed Particulate and Fiber Debris Bed

The industry guidance document recommends that particulate debris from calcium silicate (CalSil)(o)

alone or microporous insulation alone can be treated simply as particulate debris using the 

correlation from NUREG/CR-6808,32 as described in Section 4.1.2.2.2 above for RMI-only 

debris beds. Only limited data are available for debris beds composed of these materials, and if 

plant-specific measurements are required, the testing parameters must include appropriate 

particle sizes, size distributions, and surface-to-volume ratios. For a mixed bed of microporous 

and Cal-Sil particulate, the limited available data show relatively high head loss, and 

NUREG/CR-6224 30 does not appear to be applicable.

For beds composed of fibrous debris mixed with particulate material (Cal-Sil or microporous 

material), the head loss for the bed can be treated as purely fibrous debris using the 

NUREG/CR-6224 head loss correlation, if the particulate material is less than about 20% of the 

debris by mass. At concentrations above 20%, this correlation tends to be non-conservative. 

The industry guidance document notes that there is not sufficient data currently to develop a 

general correlation for this type of debris and recommends removing such insulation from the 

plant, if possible, or reducing it to less than 20% of potential debris. If that is not an option, it 

may be necessary to perform experiments for plant-specific conditions.

4.1.2.3 Thin Bed Head Loss

The industry guidance document for PWRs (NEI 04-07) recommends that as part of the 

sensitivity studies to determine the most limiting debris loading configuration, the analysis 

should include the case of sufficient fiber to form only a thin bed 1/8-inch thick. In this case, the 

particulate material in the plant-specific mixture of debris will form a particulate layer of debris 

on top of the fiber bed, which may result in a higher head loss than would be obtained with a 

thicker bed with more uniformly distributed particles trapped within it. This is referred to as the 

thin bed effect (TBE).

(p)

 The recommended approaches for the head loss calculations are as 

described above for fiber and particulate beds (see Section 4.1.2.2.1), using a formulation of the 

NUREG/CR-6224 correlation optimized for debris loading that could create a thin bed on the 

strainer.

4.1.2.4 Partially Submerged Strainer

The guidance document recommends that the same head loss computation methods described 

above assuming a fully submerged strainer are applicable to a partially submerged strainer. The 

only significant variation is in accounting for the reduced effective strainer area. It is also 

necessary to account for buoyant debris that would tend to congregate around the strainers 

because of the net flow toward the sump. If the debris transport analysis shows that only a 

negligible amount of buoyant debris can reach the suction strainers because of upstream trash 

racks and gates, this factor can be neglected in the head loss analysis. 

 

(o) Cal-Sil is used in many PWRs and has a wide range of composition materials, including diatomaceous earths, 

perlite, and asbestos fibers. Plant-specific characterization (using scanning electron microscopy, at a minimum) 

is warranted to fully characterize the material.

(p) This approach was based on limited information related to the thin bed effect, and was not accepted in the SE of 

this document. See Section 4.1.3. The general definition of the  ̳thin bed effect‘ has been expanded to include a 

dense particulate layer that may form in or on a fiber bed. 
64

However, rather than using total head loss in comparison to NPSH, the guidance document 

suggests that as an alternative, the bed thickness can be used as the sole criterion for evaluating 

suction strainer performance. The document states that numerical simulations(q)―confirm‖ that a 

debris bed height of approximately one-half of the pool height will prevent adequate water flow 

to the recirculation loop. Therefore, if the total calculated head loss (in feet of water) across the 

submerged portion of the strainer (i.e., debris bed head loss plus the ―clean‖ head loss) is less 

than one-half the pool height, the partially submerged strainer can be expected to operate 

properly.

4.1.2.5 Alternative Methods for Head Loss Calculations

The Baseline Methodology recommended for suction strainer head loss calculations, summarized 

in the preceding sections, relies primarily on the NUREG/CR-6224 correlation, developed and 

validated with data from flat, horizontal screens. Section 4.2.5 of NEI 04-07 offers guidance for 

refinements that could make the analysis somewhat more realistic or better fitted to plantspecific conditions. The document specifically recommends that analysis should include a 

transient model that can investigate a range of the following features:

amount and type(s) of debris reaching the strainers and the rate of transport at any given time 

during the LOCA and post-LOCA recirculation cooling

size distribution and type of debris reaching the strainer

filtration efficiency of the fibrous bed (i.e., how well it traps particulate material)

ECCS flow rate (approach velocity)

recirculation pool temperature

plant-specific geometry for sump dimensions, strainer configuration, number and 

arrangement of strainers, and flood height.

The industry guidance document discusses a number of areas where the simplifications and 

conservatisms in the Baseline Methodology could be refined to yield a potentially more realistic 

estimate of head loss across the debris-laden strainer. These are discussed only as suggestions 

and in general invite the licensee to undertake a considerable body of work to develop and 

validate an alternative approach. The guidance document also notes that new advanced suction 

strainer designs, particularly self-cleaning or ―active‖ strainers, will require developing new head

loss correlations, since these strainers will not generally fall within the databases of existing 

correlations such as the NUREG/CR-6224 correlation. These correlations ―should be developed 

by the designer and/or vendor of the new sump screen.‖

4.1.3 Regulatory Guidance on Head Loss across Suction Strainers

NRC issued SE reports on the BWROG guidance document (NEDO-32686) and the main 

guidance document for PWRs (NEI-04-07) in 1998 and 2004, respectively. In general, the NRC 

evaluations concluded that there is sufficient overall conservatism in the methodologies 

presented in the guidance documents, but certain portions of these documents were specifically 

not accepted. The unacceptable portions generally involved concerns related to the conservatism 

 

(q) The guidance document does not provide a specific reference for these calculations, nor does it identify the code 

used.
65

or completeness of selected parameters, or insufficient technical justification of an approach. 

Additional work would be required in plant-specific submittals utilizing these aspects of the user 

group guidance document. 

In addition to the SE reports on the guidance documents, NRC developed guidance for responses 

to GL 2004-02 and NRC Bulletins 96-03 and 95-02. Section 4.1.3.1 summarized the Regulatory 

guidance for BWRs. Section 4.1.3.2 summarizes the Regulatory guidance for PWRs.

4.1.3.1 Regulatory Guidance for BWRs

In the SE issued in 1998 for the BWROG guidance document, the methodology for determining 

strainer head loss with debris in the coolant was judged insufficient and potentially nonconservative for some debris configurations. NRC staff evaluations also recommended using 

head loss correlations developed by the vendors, based on performance testing with prototypic or 

conservative debris loading, rather than the correlations developed in the BWROG document. 

Furthermore, in other work related to the strainer clogging issue, including staff evaluations that 

led to the issuance of NRC Bulletins 95-02 and 96-03, NRC staff concluded that the thenexisting BWR strainer designs required modification to ensure adequate functionality in postLOCA conditions. Installation of alternative passive strainers with larger surface area was 

suggested as a possible resolution option.(r) 

Closure of the multi-plant actions (MPAs) related to NRC Bulletins 95-02 and 96-03 is 

documented in NRC Memorandum(s) Completion of Staff Reviews of NRC Bulletin 96-03 –

Potential Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling Water 

Reactors, and NRC Bulletin 95-02 – Unexpected Clogging of a Residual Heat Removal (RHR) 

Pump Strainer while Operating in Suppression Pool Cooling Mode, issued October 18, 2001. 

In evaluating responses to NRCB 95-02, NRC staff concluded that BWR licensees are cognizant 

of the need to minimize latent debris sources by regular cleaning of the suppression pool and by 

developing and maintaining pro-active housekeeping procedures and FME programs. In 

evaluating responses to NRCB 96-03, NRC staff concluded that the installation of large-capacity 

passive strainers provides conservative assurance that NPSH can be maintained when operating 

the ECCS in recirculation mode during a LOCA.

In essence, the regulatory guidance to BWR licensees on this issue consists of two main 

positions. First, licensees should minimize debris source terms through regular cleaning of the 

suppression pool, effective housekeeping practices, and a pro-active FME program. Second, the 

licensees should install alternative strainers with designs that have been adequately tested and 

validated by the manufacturer or vendor, including testing to determine the effect of debris 

loading on strainer head loss. The SE of the BWROG document does not, however, include 

detailed guidance on the type and extent of testing that must be performed to demonstrate the 

conservatism of a particular strainer design. Acceptability of a given methodology for validation 

 

(r) Other resolution options presented in the BWROG document include installation of self-cleaning strainers, or 

developing operational procedures for a backflush system. In the SE for this document, NRC staff specifically 

discouraged the use of self-cleaning strainers, and recommended backflushing only as a ―defense-in-depth‖ 

measure. All BWR licensees chose to implement the option of installing large capacity passive strainers.

(s) ADAMS accession number ML012970229.
66

of a strainer design is determined in evaluations of topical reports submitted by vendors or 

licensees. 

4.1.3.2 Regulatory Guidance for PWRs

In the SE of the NEI guidance document for PWRs, issued in 2004, the basic approach of using 

the NUREG/CR-6224 correlation or variations of this correlation to determine head loss across a 

debris-laden suction strainer was rejected as unsupportable with the existing database of the 

correlation. PWR licensees using this correlation were required to provide appropriate 

experimental data demonstrating that this correlation could accurately predict head loss across 

their specific suction strainer design with the worst-case debris bed that could be generated in 

their particular plant. Alternatively, licensees could demonstrate the conservatism of their 

suction strainer design by obtaining appropriate experimental data showing that the head loss 

would remain low enough to maintain NPSH for the most adverse potential debris loading during 

a LOCA.

In effect, these two alternatives require essentially the same type of work, and PWR licensees 

opted to conduct prototypical head loss testing to qualify designs of plant-specific replacement 

suction strainers. NRC staff developed detailed review guidance to establish appropriate 

evaluation criteria for review of plant-specific submittals. This work included guidance for 

plant-specific chemical effect evaluations and coatings evaluations (which are discussed in 

Sections 2 and 3 of this report), as well as guidance for strainer head loss evaluations.

The documentation of regulatory positions regarding strainer head loss for PWRs is provided in 

NRC Staff Review Guidance Regarding Generic Letter 2004-02 Closure in the Area of Strainer 

Head Loss and Vortexing, issued(t) March 2008. This document contains detailed guidance in 

the areas of test scaling, debris near-field settlement simulation, surrogate debris similitude 

requirements, testing procedures, post-test data processing, and extrapolation to conditions 

beyond the tested database. Specific items covered in this document were taken directly from 

the Content Guide for Generic Letter 2004-02 Supplemental Responses (ML073110278), and are 

briefly summarized here.

1. Item #1: provide a summary of the methodology, assumptions, and results of prototypical 

head loss testing for the strainer (including chemical effects), including

a. basis for scaling debris amounts and flow rates

b. preparation of surrogate debris, and how it compares to actual debris

c. paint chips or particulate paint debris should be shown to be conservative or prototypical 

with regard to head loss

d. discussion of how near-field settlement was treated in testing

e. discussion how debris is prepared and introduced into the fluid

f. complete definition of basis for test termination, to assure that testing captured the 

maximum head loss

g. discussion of method for modeling of representative geometries for sump pit and other 

hardware that could affect coolant flow path

 

(t) This document has ADAMS accession number ML080230038 and is available from the NRC web site.
67

h. discussion of thin bed and circumscribed beds, as appropriate 

i. explanation of how chemical effects were accounted for during testing

j. addition of relevant vendor test reports, calculations, and specifications in the submittal

2. Item #2: show that the design can accommodate the maximum volume of debris that is 

predicted to arrive at the strainer;

a. explain what the maximum debris load is expected to be and how the strainer design 

accommodates it

b. for testing, the maximum debris load should include 100% (scaled) of the debris from the 

break being tested

c. if significantly different debris mixtures can result from different postulated breaks, each 

should be tested or evaluated

3. Item #3: show that the strainer design prevents formation of a thin bed, or can accommodate 

partial thin bed formation;

a. testing should include attempts to form a thin bed by incremental addition of fiber (in the 

form of easily suspended fines)

b. Determine the most problematic debris loads and describe how they were implemented in 

the testing

4. Item #4: provide a summary of the methodology, assumptions, and results for the clean 

strainer head loss calculation (vendor reports are acceptable for this purpose)

5. Item #5: provide a summary of the methodology, assumptions, and results for the debris head 

loss analysis

6. Item #6: determine if the suction strainers are partially submerged or vented for any accident 

scenario. Describe any other failure criteria (in addition to loss of NPSH) that are used to 

address flow blockage at the strainer

a. If not fully submerged, a strainer is assumed to fail if the head loss across the strainer 

exceeds half the height of the pool (unless the licensee can show otherwise)

7. Item #7: if near-field settling is credited in the head-loss testing, it must be justified as 

prototypic of plant conditions.

a. near-field settling in the testing should not be allowed to occur, unless the licensee can 

show that it would actually occurs in the plant; even in such case, the flow and turbulence 

near the test strainer must be prototypic, or demonstrably conservative

b. In plants with complex or widely varying flow parameters in the post-LOCA sump, the 

testing must include the full range of conditions; average flow rates are unlikely to be 

sufficient

8. Item #8: If temperature/viscosity relationships are used to scale test results to plant 

conditions, provide the basis for determining that in the testing, ―boreholes or other 

differential-pressure induced effects did not affect the morphology of the test debris bed.‖

9. Item #9: describe the role of containment accident pressure in evaluating whether or not 

flashing would occur across the strainer surface
68

In addition to the detailed discussion of the items summarized in the above list, Appendix A of 

the regulatory guidance document contains more in-depth discussions on the approach to strainer 

head loss testing and the application of the test results to plant head loss evaluations. 

Specifically, NRC staff have observed testing for some PWR replacement strainer designs, and 

have noted problems with testing practices that could affect the prototypicality or conservatism 

of testing. These issues include (but may not be limited to)

1. Test strainer not scaled appropriately to the plant strainer design

2. Debris simulants not prototypic of plant materials

3. Debris transport in the test flume or test tank/sump not prototypic to the plant

4. Duration of testing not long enough to determine peak head losses

5. Post-test scaling of test data to plant conditions not technically correct

6. Thin bed testing is inadequate

7. Insufficient consideration of sequence of debris components in the testing.

The regulatory guidance document contains detailed discussions of each of these issues, and also 

includes NRC positions of the specifics of test facility design, testing procedures, and treatment 

of test data. Applicants are directed to consider conservative plant hydraulic conditions, and 

worst-case failure assumptions. Conservative assumptions for analytical modeling to determine 

―inputs‖ to the head loss testing must consider conservative plant hydraulic conditions, and 

conservative debris loading on the strainer. This includes

All ECCS and CSS pumps are in operation for an extended period (up to the 30-day 

maximum mission time), resulting in maximum flow rate determined for ―worst-case single 

failure assumption‖

Sump pool subcooling is assumed to be at minimum at the start of recirculation phase

Sump pool is operating at minimum level

Eroded fine fiber debris is assumed present at the strainer at the beginning of recirculation 

phase

Agglomeration and/or settling out of debris in the sump pool is (usually) not taken into 

account

Debris generation rate, amount, type, sequence of arrival in sump, etc., is determined from 

―worst-case single failure assumption‖

If the inputs to the head loss testing are conservative, the test facility is scaled properly, and the 

testing procedures are conservative, the NRC staff considers that the measured head loss is also 

conservative.

4.1.4 Comparison of Regulatory Guidance for BWRs and PWRs

For both PWR and BWR analysis, the basis of the guidance provided by the respective industry 

representatives is experimental testing of various strainer designs under various conditions of 

debris loading. The industry guidance for PWRs relies on empirical correlations derived and 

validated for a range of strainer types and debris constituents. The BWROG guidance relies on 

empirical models developed from generic testing with seven alternative strainer designs, only 
69

two of which were similar in configuration to advanced strainer designs installed by licensees. 

For plant-specific conditions (i.e., strainer configurations, or debris characteristics) that are 

outside the ranges of the relevant experimental databases, both guidance documents warn 

licensees that they must justify extrapolation of the models, and this may require additional 

testing.

Although NRC approved both the BWROG guidance document (NEDO-32686-A) and the 

guidance document for PWRs (NEI-04-07), NRC staff did not accept the approach used in either 

document for determining debris laden strainer head loss. For PWRs and BWRs, NRC staff 

have recommended testing of suction strainer designs. The main difference is in the level of 

specificity of the guidance provided.

For BWR licensees, regulatory guidance is to install alternative strainers with designs that have 

been adequately tested and validated, including testing to determine the effect of debris loading 

on strainer head loss. The guidance does not, however, include details on the type and extent of 

testing that must be performed to demonstrate the conservatism of a strainer design. 

Documentation

33 of testing of large-area stacked disk strainer designs by the two main vendors 

(General Electric and Performance Contracting, Inc., which have provided approximately 80% of 

alternative strainers in BWR plants) shows that the majority of testing was generic (i.e., not 

plant-specific), and performed in the same facility and in the same manner as the testing 

documented in the BWROG guidance document. Some additional testing has been performed at 

specific plants and for specific strainer designs, but the documentation of this work is scattered 

through a number of different submittals, and has not been systematically compared for 

consistency and completeness.

For PWR licensees, regulatory guidance is to perform extensive testing of plant-specific strainer 

designs. As discussed in Section 4.1.3.2, guidance has been specified in exhaustive detail, 

defining the recommended approach to testing in the areas of test scaling, debris near field 

settlement simulation, surrogate debris similitude requirements, testing procedures, post-test data 

processing, and extrapolation to conditions beyond the tested database.

4.1.5 Recommendations for Guidance on Head Loss Calculations

The basic concept of relying on experimental evidence in developing a methodology for 

evaluating head loss due to debris bed formation is a valid engineering approach to the problem. 

However, the proper execution of the approach is vital to obtaining reliable results, and this is the 

basis for the additional guidance provided by NRC staff to both the PWR and BWR industries on 

this issue. 

The detailed guidance developed by NRC staff for PWR licensees is based on first-hand 

experience with the manifold difficulties of obtaining appropriate experimental data to support 

an analysis methodology. The BWR licensees face the same difficulties and have the same 

requirement to show that their approach yields test data that is prototypic or conservative, 

compared to specific plant conditions. Therefore, the testing underlying the work presented by 

the BWR licensees should be held to the same standards as those required from PWR licensees. 

This suggests the following recommendations.
70

Recommendation 4.1 Evaluate the specific strainer designs currently installed in BWR 

plants, on a plant-by-plant basis (if necessary), with regard to test 

scaling, debris near field settlement simulation, surrogate debris 

similitude requirements, range of independent variables tested, 

and testing procedures, to determine if the tests and evaluations 

can be considered prototypic or conservative with respect to these 

parameters.

Recommendation 4.2 Apply the same standards and guidance to evaluations of 

submittals from BWR licensees regarding suction strainer head 

loss calculations, including the potential for thin bed effects, as are 

applied to submittals from PWR licensees.

4.2 Debris Carried Through Sump or Suppression Pool Suction Strainers

This subsection discusses guidance provided for determining the amount and type of debris that 

could be expected to pass through a suction strainer during a LOCA event and post-LOCA 

recirculation cooling. This subsection also discusses guidance provided for determining the 

amount and type of damage that such debris could cause in downstream components of the ECC, 

CS, and RHR systems.

Suction strainers are designed to severely limit the debris that can enter the ECCS, CSS, and 

RHR loops, but it is not possible to completely exclude all debris without incurring 

unacceptable head losses across the strainers. Regulatory Positions 1.1.1.12 (PWRs) and 2.1.2.2 

(BWRs) from Regulatory Guidance 1.82, Revision 3, specify that the possibility of debrisclogging at flow restrictions downstream of the suction strainers should be assessed to ensure 

adequate long term cooling following a LOCA event. 

For BWR systems, the original design criterion for determining the size of the openings in the 

suction strainers depended on the plant design. For BWR/2, /3, /4, and /5 designs, the strainer 

hole size was determined by the throat diameter for the containment spray nozzles or the core 

spray nozzles. For the BWR/6 design, the hole size was determined by the size of the cyclone 

separator orifices in the flushing subsystem for the ECCS pump seals. Suction strainer hole sizes 

prior to installation of new designs in response to strainer clogging issues, is reported in the 

BWROG guidance document as ranging from 0.06 inch to 0.6 inch, based on sampling from 16 

plants (47% of all operating BWRs). Of the sampled plants, 50% reported hole sizes of 0.125 

inch (1/8 inch) and approximately 38% reported hole sizes of 0.094 inch (3/32 inch). For PWR 

systems, the original design criterion for strainer openings was defined by the containment spray 

nozzle throat size. Typically, this dictated an upper limit of 1/8 inch (0.125 inch) for the size of 

the openings. New replacement strainers installed in response to GSI-191 issues resolution 

typically have openings 0.094 inch (3/32 inch), and in some designs are only 0.0625 inch (1/16

inch) or smaller.

However, even the smallest strainer openings are large compared to the expected size ranges of 

fibrous and particulate debris, which have mean values on the order of 0.01 to 0.001 inch. Paint 

chips and some types of particulate debris have typical sizes in the micron and sub-micron range. 

It is therefore inevitable that some amount of debris would be carried through the strainers and 
71

subsequently reach the downstream components, including the reactor pressure vessel and core, 

and it is necessary to determine the quantity and characteristics of debris material that could get 

through. 

The two main concerns with the presence of debris in the coolant being pumped through the 

ECCS and CSS systems and the reactor vessel are the possibility of plugging at flow restrictions, 

and excessive wear that could lead to failure of components within these systems. Both of these 

concerns could result in loss of recirculation cooling. The plant piping for these systems and the 

primary system are unlikely to be at risk since the pipes are relatively large in diameter and are 

thick-walled stainless steel with a high resistance to abrasive wear. The components of interest 

in evaluating the effect of debris in the coolant are pumps, valves, orifices, heat exchangers, 

areas within the reactor and core, and instrumentation tubing. Table 4.1 summarizes the types of 

these components that are found in PWR and BWR plants, and potential problems due to debris 

that could compromise ECCS, CSS, or RHR system performance.

The common causes of potential damage due to debris for all of the components listed in 

Table 4.1 are flow blockage or excessive wear due to abrasion. Flow blockage could shut down 

the recirculation loop for emergency cooling, and abrasion could lead to a secondary failure in 

the loop, which would also shut down emergency cooling. It is therefore advisable to determine 

where and how such problems could occur, and assess the severity of the consequences. 

Section 4.2.1 describes the approach that is recommended by the BWROG for BWR systems. 

Section 4.2.2 describes the industry guidance for PWR systems. Regulatory guidance on this 

issue is summarized in Section 4.2.3, and the treatment of BWR and PWR systems is compared 

in Section 4.2.4. Recommendations for appropriate development of consistent guidance for the 

two systems are provided in Section 4.2.5.
72

Table 4.1. Typical downstream components for ECCS and CSS in LWRs.

Component Potential problems due to debris

Pumps:

Centrifugal (single- and

multi-stage)

Wear on bearing surfaces, seals, impeller, causing

increased pressure drop

decreased flow rate at given speed

increased vibration

leaking at shaft seals

loss of pressure boundary integrity

Valves:

Needle valves

Manual globe valves (with and 

without diaphragm seals)

Check valves

lift type

piston type

swing type

tilting disc type

diaphragm valves

gate valves (manual, airoperated, motor-operated)

globe valves (automated)

butterfly valves

General hazards of debris in flow:

Wear on seals

Sticking open (when valve should be shut)

Sticking shut (when valve should be open)

Plugging hazard for small valves:

needle valves with labyrinthine flow paths

globe valves with small-diameter holes in cage

sealed globe valves (limited clearance between seal and 

bonnet)

Orifices:

Spray nozzles (typically 3/8 in.) Erosion due to abrasion

Plugging due to accumulation of debris

Heat Exchangers:

Primary side tubing Debris accumulation in U-bend

Scale build-up on tube inner wall

Erosion of tube wall, potential for leakage of primary coolant

Instrumentation lines and tubing:

In vessel, recirculation loop, 

sump or suppression pool

Plugging due to debris entering tubing; settled debris covering 

taps

4.2.1 Guidance from the BWROG for Debris Transport through Suction Strainers and 

Effects on Downstream Components

The BWROG guidance document does not provide recommendations for methods of 

determining the amount of debris that could be carried through the strainers. It is assumed that 

passive strainers will allow essentially no particulate to pass through because of the fibrous 

debris bed that very quickly develops on the strainer. Significant amounts of debris are assumed 

to pass through the strainers only if the fiber bed fails to develop, or if the plant has installed selfcleaning strainers. Even if that were the case in a specific plant, the BWROG guidance 

maintains the position that there is no need to consider effects on downstream components.
73

This guidance is based on a General Electric study of the effects of debris on components 

downstream of the strainers, GE-NE-T23-00700-15-21 (Rev. 1), Evaluation of the Effects of 

Debris on ECCS Performance (Reference 11 of NEDO-32686-A).21 Based on the General 

Electric evaluation, the guidance document concludes that there is ―no safety concern for the 

potential failure of the ECCS pumps, inadequate cooling capacity from the RHR heat 

exchangers, plugging of the core spray header nozzles, plugging of containment spray nozzles, 

corrosion or chemical reaction with other reactor materials, or fuel bundle flow blockage‖ due to 

debris in the recirculating coolant. The guidance document considers the issue essentially 

closed, based on the work reported in GE-NE-T23-00700-15-21 (Rev. 1), and does not include 

any suggestions, recommendations, or methodology for determining effects of debris on 

components downstream of the suction strainers. It also neglects any effects of suppression pool 

sludge, which may reach the suction strainers well before incoming material from the drywell 

can establish a debris bed.

4.2.2 Industry Guidance for PWRs on Debris Transport through Suction Strainers and 

Effects on Downstream Components

The industry guidance document containing recommendations related to this issue is WCAP16406-P, Revision 1, Evaluation of Downstream Sump Debris Effects in Support of GSI-191. 

This document was developed to supplement NEI 04-07,3in response to the NRC staff‘s finding 

that the guidance in NEI 04-07 did not fully address the potential safety impact of LOCAgenerated debris on downstream components. The guidance provided in WCAP-16406-P is 

comprehensive and detailed, and includes sample calculations illustrating applications to 

hypothetical plant conditions. Due to the complexity of this document, the PWROG stated the 

intention of providing training in proper application of the overall methodology for members, if 

it was accepted by NRC. 

To address the specific question of how much debris can get through the strainers, WCAP16406-P describes the development and application of a ―Debris Ingestion Model‖ that licensees 

can apply to plant-specific conditions. The shape and size distributions of the debris are the 

main constraints on the amount of debris that can get through the strainers. The model ignores 

the effect of filtering due to the debris bed and assumes instead that any particulate material in 

the coolant that is small enough to pass through the strainer will do so. The guidance document 

also suggests that in plant-specific analysis, additional conservatism can be introduced by 

assuming that debris particulate considerably larger than the strainer hole size can still pass 

through and contribute to the debris load. 

The Debris Ingestion Model assumes that there is no settling of debris within the floor pool of 

the sump. In addition, the guidance document suggests that in plant-specific analysis, the 

licensee could apply the extremely conservative assumption that the debris concentration 

remains constant in the ECCS throughout the post-LOCA recirculation period. Alternatively, the 

guidance document develops a methodology for calculating the reduction in debris concentration 

due to settling within the reactor vessel and elsewhere in the system. In general, this approach is 

based on simple one-dimensional modeling of the system, assuming velocity dependence for 

settling rates. Large heavy particles are expected to settle out in the lower plenum of the reactor 

vessel, but small light particles are assumed to carry through back to the sump and into the 

recirculation loop repeatedly without settling out.
74

The guidance document also contains a methodology for calculating the reduction in fibrous 

debris in the recirculating fluid due to capture on the strainers, using a model from LANL 

report(u)LA-UR-04-5416.34 This model is recommended for plant-specific analysis where a 

―more realistic but still conservative‖ approach is required to appropriately characterize debris 

transport through the suction strainers.

In general, the perspective of the industry guidance document for PWRs is that debris effects on 

downstream components will not be a problem for long-term operation in post-LOCA 

conditions. However, this is not treated as a blanket assumption for all PWRs, and the document 

provides recommendations for specific analyses that should be done to evaluate this issue for 

plant-specific conditions.

The industry guidance document for PWRs describes the development of two empirical models 

to represent wear due to debris in the coolant; one based on abrasive wear, the other on erosive 

wear. Abrasive wear is defined as the removal of material due to the presence of hard or sharp 

particles between two moving surfaces in close proximity. Examples of affected surfaces in 

pumps are wear rings, impeller hubs, bushings, and diffuser rings. Erosive wear is defined as the 

removal of material due to particles in the flowing fluid impinging on a component surface or 

edge. Examples of surfaces that might be affected by erosive wear are valve internal flow paths, 

spray nozzle orifices, and heat exchanger tubing, particularly in the vicinity of sharp bends.

The industry guidance document presents detailed examples of evaluation methods applying the 

abrasive wear model to pumps used in the ECCS, CSS, and RHRS. Using plant-specific data, 

the licensee can obtain estimates of wear rates and evaluate the consequences of such wear for 

the specific components of the plant. Similarly, the guidance document presents examples for 

the erosive wear model, which applies to pumps, valves, orifices, and heat exchangers. The 

document specifically recommends evaluating both hot-leg and cold-leg break scenarios to 

determine the worst-case conditions of debris loading for potential wear damage to the system 

components. However, the document fails to note that these may not be the same conditions that 

lead to the worst case for head loss across the strainers due to the formation of the debris bed. 

The worse case break location for debris load on the strainer may not be the same worse case 

break location for debris downstream of the strainer. For suction strainer performance, the worst 

case probably would include a high percentage of fiber debris. For effects on downstream 

components, debris loading that is high in particulate, especially small sharp-edged particles that 

have high hardness values, is likely to be the most adverse. 

To evaluate potential effects of debris on instruments that have sensing lines connected to the 

recirculation flow path and must function to support Emergency Operations Procedures (EOPs), 

the guidance document recommends specific methods to evaluate the potential for abrasive wear 

or erosion, or the possibility of plugging of such lines. The guidance document concludes that 

such analyses can show that instrumentation lines will not be subjected to abrasive wear or 

erosion and that debris blocking of instrument lines is not a viable failure mechanism.

 

(u) Subsequently issued by NRC as NUREG/CR-6885 in October 2005.
75

The guidance document considers flow blockage due to plugging of pumps, orifices, nozzles, 

valves, or heat exchanger tubing an unlikely mode of failure for the recirculation loop. This is 

based on analyses using conservatively bounding assumptions on the size of particles that can 

pass through the suction strainer openings. The design-basis for the size of these openings is the 

smallest flow path that the recirculating fluid is expected to encounter. 

Based on these assumptions, the industry guidance document expects that licensees will be able 

to show in plant-specific analyses that debris particulate (both particles and fiber) will be too 

small to plug even the narrowest flow paths in the loop. The flow velocity in the narrow regions 

is expected to be high enough to preclude settling, and particulate debris will simply be swept 

through the system. However, the guidance document strongly reminds licensees that the 

recommendations provided were developed assuming passive strainers. For active strainers, the 

licensee must determine the size of particulate material that can pass through the holes, the debris 

concentration, and the resulting wear and plugging potential of this debris, which may be quite 

different from that of debris passed through passive strainers.

4.2.3 Regulatory Guidance on Debris Transport through Suction Strainers and Effects on 

Downstream Components

In the SE for the BWROG guidance document, there is no discussion of the BWROG position 

that there is ―no safety concern‖ due to potential effects of debris on downstream components. 

This issue is not discussed in the memorandum on completion of NRC staff reviews of NRC 

Bulletin 96-03 and NRC Bulletin 95-02 in October, 2001 (ML0129702290). This suggests that 

as of 2001, NRC staff accepted the BWROG position on this issue.

In the SE for the industry guidance document for PWRs (NEI 04-07), issued in 2004, NRC staff 

found the guidance in NEI 04-07 insufficient in that it did not fully address the potential safety 

impact of LOCA-generated debris on components downstream of the containment sump. The 

SE offered specific guidance on what should be considered to address this issue. The major 

positions are summarized as follows:

1. evaluations for resolution of GSI-191should include the effects of debris on pumps and 

rotating equipment, piping and valves, and heat exchangers downstream of the containment 

sump related to ECCS and CSS. In particular, any throttling valves installed in the ECCS for 

flow balancing should be evaluated for blockage potential

2. evaluations should consider, on a plant-specific basis, equipment used for both long-term and 

short-term system operation lineups, conditions of operation, and mission times, at the 

maximum flow rates expected during operation

a. for pumps and rotating equipment, consideration should be given to wear and abrasion of 

surfaces (e.g., running surfaces, bushings, wear rings); tight clearance components, or 

components where process water is used to either lubricate or cool should be identified 

and evaluated.

b. component rotor dynamics changes and long term effects of vibrations caused by 

potential wear should be evaluated in the context of pump and rotating equipment 

operability and reliability, including potential impact on pump internal loads, to address 

such concerns as rotor and shaft cracking
76

c. for system piping, containment spray nozzles, and instrumentation tubing, consider how 

settling of debris and fines in low fluid velocity areas could impact system operating 

characteristics; evaluations should include tubing connections such as provided for 

differential pressure from flow orifices, elbow taps, venturi nozzles, and reactor 

vessel/RCS leg connections for reactor vessel level

d. for valves and heat exchangers, wetted materials should be evaluated for susceptibility to 

wear, surface abrasion, and plugging.

3. evaluations should consider the effect of possible decreased heat exchanger performance 

resulting from plugging, blocking, plating out of slurry materials, or tube degradation with 

respect to overall system required hydraulic and heat removal capability

4. an overall ECC or CS system evaluation integrating limiting or worst-case pump, valve, 

piping, and heat exchanger conditions should be performed and include the potential for 

reduced pump/system capacity resulting from internal bypass leakage or external leakage

5. the potential for leakage past seals and rings to areas outside containment caused by wear 

from debris fines should be evaluated with respect to fluid inventory, overall accident 

scenario design, and licensing bases environmental and dose consequences

In the SE for WCAP-16406-P, Revision 1,35 which was developed by the PWROG in response to 

the guidance from the SE of NEI 04-07, NRC staff found the approach for performing 

assessments of the impact of debris on various equipment required by the ECCS, CSS and NSSS 

acceptable, subject to certain conditions and limitations. These conditions and limitations are 

specified in detail in Section 4 of the SE, but can be summarized as three main concepts:

1. licensees must use plant-specific information in performing the analyses

2. licensees must verify that models and/or data are applicable to plant-specific conditions

3. licensees must show that they have considered all equipment that could see debris-laden 

coolant, and analyzed the ―worst case‖ conditions in all particulars.

4.2.4 Comparison of Regulatory Guidance for BWRs and PWRs

NRC staff appear to have accepted the BWROG position that there is no safety concern related 

to effects of debris on downstream components, and it is not necessary to perform plant-specific 

analyses to address this issue. No additional guidance is offered to BWR licensees on this issue. 

In direct contrast, NRC staff treats this issue as a significant concern in the SE for the industry 

guidance document for PWRs, and developed detailed and specific guidance on how the issue 

should be addressed.

The difference in the regulatory positions for BWRs and PWRs is due to the evolving nature of 

debris clogging concerns in nuclear power plants, and the earlier development of guidance for 

the BWRs, compared to PWRs. The actual nature of the technical issues involved is essentially 

the same for the two reactor types. There is nothing unique to PWRs that make them more 

susceptible to problems due to debris in downstream ECCS and CSS components, compared to 

BWRs, except possibly for the greater likelihood of chemical interaction problems in PWRs. 

Rather, the reverse might be considered more likely, at least in terms of potential damage due to 

material debris, as BWR systems have the suppression pool and its latent debris to deal with 
77

immediately upon activation of the ECCS, while PWRs would draw clean emergency cooling 

water from storage tanks for approximately the first 30 minutes of an event.

4.2.5 Recommendations for Guidance on Debris Transport through Suction Strainers 

and Effects on Downstream Components

The BWROG guidance is over-generalized from limited data and extremely liberal assumptions 

regarding the amount of debris that can be transported through the strainers. The industry 

guidance for PWRs, as expanded in WCAP-16406-P, Revision 1 and the additional regulatory 

guidance from NRC staff included in the SE for that document, defines a sound engineering 

approach to this issue. However, it requires appropriate experimental validation to verify overall 

conservatism in the methodology. Guidance on this issue should also be cognizant of the fact 

that for debris ingestion models, a ―conservative‖ estimate of debris passing through the strainer 

is not the same as a ―conservative‖ estimate of the amount of debris trapped on the strainer. In 

some plants, the bounding case for each analysis may not be the same postulated LOCA event. 

These observations suggest the following recommendations.

 

Recommendation 4.3 Require validation of debris ingestion models with experimental 

data obtained for conditions where the maximum amount of debris 

is able to pass through the suction strainers. This should include 

the evaluation of conditions where an incomplete debris bed might 

form, and generally corresponds to conditions where the effect of 

debris on strainer head loss may be relatively low.

Recommendation 4.4 Require validation of abrasion and erosion wear models for 

specific particulate materials and ranges of particle sizes 

postulated for debris generated in BWR and PWR LOCA 

scenarios.

Recommendation 4.5 Apply the same standards and guidance to evaluations of 

submittals from BWR licensees regarding effect of debris in the 

recirculation coolant on downstream components as are applied to 

submittals from PWR licensees.

4.3 Effects of Debris in Reactor Vessel and Core

This subsection discusses guidance provided for evaluating the effect on flow in the vessel and 

core as a result of debris that passes through the sump screen or suction strainer during a LOCA 

event and post-LOCA recirculation cooling. As noted in Sections 4.2, the issue of debris in the 

emergency cooling water is addressed by Regulatory Positions 1.1.1.12 (PWRs) and 2.1.2.2 

(BWRs) from Regulatory Guidance 1.82, Revision 3. Both of these Regulatory Positions 

specifically require consideration of the build up of debris in the core fuel assemblies and fuel 

assembly inlet debris screens when assessing long-term cooling following a LOCA event.

The main concern with the presence of debris in the coolant being pumped into the reactor vessel 

is the possibility of flow blockage, resulting in loss of adequate cooling of the fuel rods, leading 

to high fuel cladding temperatures that could cause fuel damage. The time frame of greatest 
78

interest is long-term post-LOCA cooling. This is mainly because it will take time for sufficient 

debris to build up to cause problems, but also because during the initial stages of the LOCA 

event, coolant is leaving the core and vessel, generally at an extremely rapid rate, and debris 

blockage is essentially impossible. However, the main function of the ECCS is to get coolant to 

the core as quickly as possible in a LOCA. In a relatively short time, debris-laden water will 

enter the core. 

For PWRs, there will be a delay of approximately 20-30 minutes duration, while the storage tank 

empties and before ECCS pumps start drawing from the sump. For BWRs, ECCS pumps 

drawing from the suppression pool are activated very early in the LOCA scenario. For both 

systems, a significant amount of debris will be present as soon as the ECCS pumps begin to draw 

cooling water from the sump or suppression pool. The amount of debris will tend to increase for 

some time interval, as debris is washed into the sump or suppression pool from containment. 

The BWR ECCS components that can draw water from the suppression pool vary with plant 

design, as summarized in Table 4.2. All BWR designs have two or three ECCS components that 

can inject suppression pool water into the vessel. (The exception is the BWR/2 design, which 

has only the core spray system.) These components create two main paths for debris to reach the 

core. The core spray systems (both high- and low-pressure) spray water containing debris 

directly over the top of the core, or directly into the top of the core bypass region (BWR/5 and 

BWR/6). The coolant injection systems, when drawing from the suppression pool rather than the 

condensate storage tank, inject water containing debris into one of the vessel feedwater lines or 

recirculation lines. From the injection point, water containing debris can flow into the 

downcomer, through the jet pumps, into the lower plenum, and upward into the core.

The PWR ECCS components that draw water from the sump are essentially the same for all 

plants, although with significant variation in design details. The basic systems are summarized 

in Table 4.3. The location at which the ECCS water is injected can be the hot leg or the cold leg, 

depending on the LOCA scenario. In some Westinghouse plants, ECCS water can be injected 

directly into the vessel upper plenum or upper head. As with the BWR systems, this creates two 

main paths for debris to reach the core. Cold-leg injection sends sump water into the vessel 

downcomer where it can flow into the lower plenum and from the lower plenum up through the 

core. Hot-leg injection (and upper plenum or upper head injection) sends sump water into the 

vessel above the core, and debris-laden coolant enters the core from the top.
79

Table 4.3. Summary of BWR ECCS components that draw from suppression pool.

ECCS component Action Plant Type(s)

Core Spray System

sprays water on top of core through nozzles on 

2 independent sparger rings within core shroud 

above the fuel assemblies

2 low-pressure loops (activated at <285 psig)

draws water from suppression pool

BWR/2

BWR/3

BWR/4

High Pressure Core 

Spray System

provides high-pressure core cooling for small, 

intermediate, and large line breaks

single-loop system, with motor-driven pump

draws water from the condensate storage tank

alternatively, draws water from suppression pool

pumps water to sparger on upper core shroud

BWR/5

BWR/6

Low Pressure Core 

Spray System

single loop system with motor-driven pump

draws water from suppression pool

discharges water through core spray sparger 

directly into core bypass region inside the core 

shroud

BWR/5

BWR/6

Low Pressure 

Coolant Injection 

System

can be part of Residual Heat Removal System, or a 

separate system

2 recirculation loops

injects water into reactor recirculation system 

discharge lines

draws water from suppression pool

BWR/3

BWR/4

BWR/5

BWR/6

High Pressure 

Coolant Injection 

System

turbine-driven; needs no external power

pumps water into vessel feedwater piping

draws water from condensate storage tank

alternatively, draws water from suppression pool

for core cooling during small and intermediate 

break LOCAs

BWR/3

BWR/4
80

Table 4.3. Summary of PWR ECCS components that draw from water storage tank or sump.

ECCS 

component Action Plant Type(s)

Cold Leg 

Accumulators

(Core Flood 

Tank System)

(Safety Injection 

Tanks)

passive system consisting of a pressurized tank 

filled with borated water on each cold leg of the 

reactor vessel

activated by drop in reactor coolant system 

pressure below 600 psig 

injects coolant directly into reactor vessel to 

rapidly reflood core following a LOCA

Westinghouse

Combustion

 Engineering

Babcock & Wilcox

High Head 

(Pressure) 

Injection System

provides high-pressure core cooling for small to 

intermediate size LOCAs

two-loop system, with centrifugal charging pumps

draws water from the borated water storage tank 

during injection phase

draws water from boron injection tank to maintain 

shutdown margin following steamline break 

accident

(optionally) can be used during recirculation 

phase following a LOCA

Westinghouse

Combustion

 Engineering

Babcock & Wilcox

Intermediate 

Head (Pressure) 

Injection System

provides intermediate-pressure core cooling for 

small- or intermediate-size break loss of coolant 

accidents 

2-loop system with 2 multi-stage centrifugal 

pumps

draws water from the borated water storage tank 

during injection phase 

draws water from the containment sump during 

recirculation phase

normal alignment injects directly into cold leg; can 

be manually aligned to inject into hot leg

Westinghouse

Low Head 

(Pressure) 

Injection System

injection portion of Residual Heat Removal 

System; provides low-pressure core cooling for 

large break loss of coolant accidents

two-loop system, with single-stage centrifugal 

pumps

draws water from the borated water storage tank 

during injection phase

draws water from the containment recirculation 

sump during recirculation phase

normal alignment injects directly into cold leg; can 

be manually aligned to inject into hot leg

(optionally) can supply coolant to the intermediate 

and high pressure injection systems

Westinghouse

Combustion

 Engineering

Babcock & Wilcox
81

Table 4.3. Summary of PWR ECCS components that draw from water storage tank or sump.

ECCS 

component Action Plant Type(s)

Containment 

Spray System

reduces reactor building pressure following a loss 

of coolant accident or steam line break

redundant 2-loop system, each consisting of spray 

pump, shutdown cooling heat exchanger, and 

spray nozzles

spray nozzles are mounted in concentric circles on 

headers near top of containment dome

initially draws water from the borated water 

storage tank, then switches to containment sump at 

beginning of recirculation phase 

sodium hydroxide added to spray to capture 

radioactive iodine

provides cooling of hot sump water during 

recirculation phase of LOCA

Westinghouse

Combustion

 Engineering

Babcock & Wilcox

Residual (Decay) 

Heat Removal 

System

aligned with High Head Injection System during 

the injection phase following a LOCA when 

coolant is drawn from the storage tank

aligned with Low Head Injection System during 

recirculation phase following a LOCA, when 

coolant is drawn from the containment sump

Westinghouse 

Combustion

 Engineering

Babcock & Wilcox

Upper Head 

Injection System

passive subsystem of ECCS to provide additional 

core cooling during system blowdown during a 

LOCA

activates at 1250 psig; shuts down when pressure 

drops below 1185 psig

injects borated water from accumulator tank into 

vessel upper head 

Westinghouse
82

For both PWR and BWR primary systems, the design basis for long-term core cooling in postLOCA conditions postulates a stable two-phase flow configuration in the core for some break 

locations. The inlet flow rate is just sufficient to match a boil-off rate in the partially submerged 

core, and this has been shown analytically to maintain fuel rod temperatures within 

acceptable limits. Because the coolant leaves the core as steam, any debris in the recirculating 

flow will be left behind in the core. This is another source of potential blockage in the fuel 

assemblies, in addition to the potential plugging of inlet orifices and other flow paths for cooling 

water entering at the bottom or top of the core. 

The approach for evaluating the effect of debris in the vessel and core recommended by the 

BWROG for BWR systems is described in Section 4.3.1. Section 4.3.2 describes the industryrecommended approach for PWR systems. Regulatory guidance for BWRs and PWRs on this 

issue is summarized and compared in Section 4.3.3. Recommendations for appropriate 

development of consistent guidance for the two systems are provided in Section 4.3.4.

4.3.1 Guidance from BWROG for Debris Effects in Reactor Vessel and Core

The BWROG guidance on evaluating debris effects in the reactor vessel and core is based on the 

same General Electric21 study in which the effects of debris on ECCS components are evaluated 

(see Section 4.2.1). It is assumed that debris will be transported to the reactor vessel only if the 

plant is equipped with self-cleaning strainers. The guidance document asserts that the General 

Electric study demonstrates that debris in the coolant will not adversely affect core cooling. This 

is based on the assumption that because flow velocities in the lower plenum will be quite low, 

―much of the debris‖ suspended in the coolant from the suppression pool will settle out in the 

lower plenum and will never reach the core inlet. Because most of the debris will not remain 

suspended in the flowing fluid, very little will be available to be caught on the lower tie plate, 

inlet debris screen, or other narrow flow paths at the core inlet.

If some local blockage occurs, the guidance document assumes it will be innocuous since very 

little material will remain in suspension after the coolant passes through the lower plenum. (The 

possibility of creating a flow blockage due to the build up of debris in the lower plenum is 

dismissed as ―not credible‖ in the General Electric study.) In addition, the guidance document 

asserts that because the core flow rate is relatively low in the latter stages of the transient, even if 

some local blockage might occur due to debris, it is unlikely to cause problems, as the flow rate 

has only to remain high enough to balance the core boil-off rate. The guidance document does 

not present any recommendations for considering the potential effect of debris left behind in the 

fuel assemblies as a result of the boil-off, due to local blockages or degraded heat transfer from 

the fuel rods.

The guidance document dismisses the potential for fuel bundle flow blockage and fuel damage 

on the strength of ―General Electric‘s judgment that, on a best-estimate basis,‖ it would not 

adversely affect core cooling, ―even in the highly unlikely situation of a blocked bundle inlet.‖ 

This argument is based on a SE of the GE11 and GE13 fuel (General Electric Report).(v) This 

report shows that adequate core cooling would be maintained, even with complete flow blockage 

 

(v) See Section 4.5, page 10 of General Electric Report, 10 CFR 50.59 Safety Evaluation of the GE11 and GE13 

Fuel Bundle Debris Filter, prepared by J.L. Embley, dated September 7, 1995. (GE Class III Proprietary 

Information.) This document is Reference 12 of Reference 11 of NEDO-32686-A.
83

of the lower tie plate debris filter for a single bundle. Core spray cooling would deposit enough 

water from the top to keep the core below the 2200oF (~1200oC) peak cladding temperature limit. 

The guidance document does not consider the potential effect of debris in the coolant sprayed 

into the core from the top, which would be left behind in the fuel assemblies as a result of the 

boil-off.

The guidance provided consists only of the suggestion that ―licensees should review their plantspecific conditions to assure they are bounded by the GE evaluation and address any unresolved 

issues.‖ 

4.3.2 Industry Guidance for PWRs on Debris Effects in Reactor Vessel and Core

The industry guidance document for PWRs (WCAP-16406-P) was evaluated by the NRC staff as 

incomplete in the treatment of debris effects in the reactor vessel and core (SE WCAP-16406-P). 

A second document was submitted for review (WCAP-16793-NP, Revision 0),36 as a supplement 

to WCAP-16406-P, providing more specific and detailed guidance on assessing 

the impact on long-term core cooling of debris in the ECCS

the effects of debris that could form blockages in the fuel bundles or adhere to the cladding 

surface

the effects of chemical precipitates that could plate out on fuel cladding surfaces.

Revision 1 of WCAP-16793-NP was accepted for review by NRC in April 2009, and the SE on 

this document is expected to be completed by early summer 2010. Because of this extended time 

frame, the industry guidance described in this section is based only on WCAP-16406-P, Revision 

1 and its corresponding SE. 

In general, the industry guidance document for PWRs (WCAP-16406-P) expects that debris 

effects on core flow will not be a problem for long-term operation in post-LOCA conditions. 

However, this is not treated as a blanket assumption for all PWRs in all accident conditions. The 

guidance document provides recommendations for specific analyses that should be done to 

evaluate this for plant-specific conditions.

As in the case of the BWROG guidance, the industry guidance for PWRs asserts that collection 

of a large volume of fibrous debris in the lower plenum (or upper plenum) sufficient to 

completely block flow to the core is ―not considered credible.‖ However, the effect of debris 

carried to the core should be evaluated based on plant-specific debris loading (as determined in 

responses to GL 2004-02 provided in NEI 04-07).3

Because fibrous debris has the capability of collecting on any structure in the reactor vessel, the 

guidance document recommends that plant-specific analyses should be performed to determine 

the effect of fibrous, mixed fibrous-particulate, and particulate debris on flow through the fuel 

assemblies. In cold-leg recirculation mode (which can be used for both hot-leg and cold-leg 

postulated breaks), ECCS water is injected into the cold leg and follows the normal flow path 

through the reactor; i.e., through the downcomer, the lower plenum, and on up through the core. 
84

For a cold-leg break, long-term core cooling is achieved by relatively low velocity flow 

(typically about 0.2 ft/sec) from the lower plenum driven by a matching boil-off of liquid 

inventory in the core. For a hot-leg break, core flow is driven directly by the recirculation loop 

and can be up to an order of magnitude higher (i.e., up to about 2 ft/sec). Boiling may occur in 

the core, depending on the specific break scenario. The guidance document offers 

recommendations for determining the rate of accumulation of debris in the lower plenum, due 

mainly to settling of particulate, but generally assumes that fiber will not settle out even at low 

flow velocities because of its low density. The tight clearances in the lower core plate support 

structure and between the rods and spacer grids is expected to be very effective at trapping 

debris, and the guidance document outlines general steps for determining the flow reduction due 

to local blockages, based on geometry and hydraulics modeling.

In hot-leg recirculation mode, the flow path through the vessel is the reverse of normal. ECCS 

water is injected into the hot leg, flows into the upper plenum and then down through the core. 

In some break scenarios, the ECCS flow rate is balanced with the core boil-off rate to achieve 

adequate core cooling. In such cases, the flow regime in the two-phase region of the core will be 

counter-current, with steam flowing upward (carrying some entrained liquid droplets) and 

saturated liquid water flowing downward. As a result, the velocities are even lower in the lower 

plenum, compared to cold-leg injection. The guidance document offers general 

recommendations for determining the rate of accumulation of debris in the lower plenum, due 

mainly to settling of particulate, and models for determining fibrous debris build up on fuel rods 

and spacer grids.

The guidance document suggests options for remedial actions that might be taken if the plantspecific analysis shows problems with reduced core flow and elevated core temperatures due to 

the capture of debris within the fuel assemblies or core inlet structures. These suggestions 

include

remove all fibrous insulation from containment

install ―pre-conditioned‖ suction strainers or intermediate debris interceptors to trap a larger 

amount of debris before it enters the ECCS loop(s)

switch to hot-leg recirculation to back-flush the core (as per current EOPs for hot-leg 

switchover, but with additional justification), if the problem occurs in cold-leg recirculation.

The guidance document notes that this list is not exhaustive. Plant-specific features should be

evaluated to determine additional strategies to mitigate debris collection in the core during ECCS 

recirculation.

4.3.3 Regulatory Guidance for Debris Effects in Reactor Vessel and Core

As noted in the introduction to this section, Regulatory Positions 1.1.1.12 (PWRs) and 2.1.2.2 

(BWRs) from Regulatory Guidance 1.82, Revision 3 specifically require consideration of the 

build up of debris in the core fuel assemblies and fuel assembly inlet debris screens when 

assessing long-term cooling following a LOCA event. In the SE for the BWROG guidance 

document (NEDO-32686-A), issued in 1998, NRC staff did not reject the BWROG position that 

there is no safety concern related to effects of debris on downstream components, including the 
85

reactor vessel and core, nor did the SE offer any guidance on plant-specific analyses to address 

this issue. 

In direct contrast, NRC staff treated this issue as a significant concern in the SE for the industry 

guidance documents for PWRs, and developed detailed and specific guidance on how the issue 

should be addressed. In the SE for WCAP-16406-P, Revision 1, NRC staff found the treatment 

of debris effects in the reactor vessel and core incomplete. The SE states that ―NRC staff has 

reached no conclusions regarding the information presented in TR WCAP-16406-P, Section 9 

[which addresses reactor internal and fuel blockage evaluations.]‖ The SE further states that 

―Licensees should refer to TR WCAP-16793-NP and the NRC staff‘s SE of the TR WCAP16793-NP in performing their reactor internal and fuel blockage evaluations.‖

In the SE of WCAP-16406-P, NRC staff identified seven specific issues regarding the evaluation 

of reactor internal components and fuel. These are summarized below.

1. evaluation methodology should account for differences in PWR RCS and ECCS designs that 

could affect core conditions such as boiling time

2. evaluation methodology should consider that hot spots could be produced from debris 

trapped by swelled and/or ruptured cladding

3. long-term core boiling effects on debris and chemical concentrations in the core should be 

accounted for

4. evaluation methodology should consider debris and chemicals that might be trapped behind 

spacer grids and could potentially affect heat transfer from the fuel rods

5. consideration should be included for plating out of debris and/or chemicals on the fuel rods 

during long-term boiling

6. evaluations should address effect of high concentrations of debris and chemicals in the (core 

due to long-term boiling) on the natural circulation elevation head that brings coolant into the 

core

7. if hot spots are found to occur, evaluations should address cladding embrittlement and 

demonstrate that a coolable geometry is maintained

The methodology presented in WCAP-16793-NP addresses these seven issues. The SE for 

WCAP-16793-NP will present the NRC staff‘s assessment of the methodology.

4.3.4 Recommendations on Determining Debris Effects in Reactor Vessel and Core

The BWROG guidance is inadequate in that it over-generalizes from limited data and does not 

consider the wide variation of plant-specific conditions. The industry guidance for PWRs uses a 

sound approach, but any approach must be validated with appropriate experimental data and its 

applicability verified for specific plant conditions. Given the current state of knowledge about 

debris blockage in fuel assemblies and core inlet structures, it is very difficult to define 

―conservative‖ assumptions with confidence. Testing in prototypic geometries is needed to

explore effects of such factors as the amount and type of debris and the debris mixture. The 

effects of debris left behind by core boil-off should also be investigated. The limited studies that 

have been performed have dealt only with debris deposited by forced flow through such 

structures as the bundle inlet plate, debris screen, and spacer grids.
86

Recommendation 4.6 Require prototypic testing of debris mixtures in core flow at 

pressures and temperatures corresponding to post-LOCA 

conditions to determine the effect of local blockages on local fuel 

rod cladding temperatures for postulated for BWR and PWR 

LOCA scenarios. Include testing to show the effects of debris left 

behind by core boil-off.

Recommendation 4.7 For PWRs, require testing to determine the effects on local fuel rod 

cladding temperatures of chemical plate-out (with and without 

trapped debris) for forced flow and core boil-off conditions in

postulated for LOCA scenarios.

Recommendation 4.8 Apply similar standards and guidance to evaluations of submittals 

from BWR licensees regarding effects of debris in the reactor 

vessel and core as are applied to submittals from PWR licensees.
87

5. DEBRIS TRANSPORT IN SUPPRESSION POOL AND 

CONTAINMENT SUMP

In addition to the quantity of LOCA generated or pre-existing debris, the quantity of debris that 

physically reaches the suction strainers in the BWR suppression pool or PWR containment sump 

can significantly impact the head loss and downstream debris effects. This section considers the 

BWR and PWR industry guidance and NRC staff position on reducing potential debris quantity 

because of settling in the suppression pool or containment sump.

The NRC regulatory positions for debris transport in the suppression pool and containment sump 

are provided in Regulatory Guide 1.82, Rev. 3,18 Sections 1.3.3.1 through 1.3.3.6 for PWRs, and 

in Sections 2.3.2.4 and 2.3.2.5 for BWRs. The position specified for the PWRs (see Section 

1.3.3.4), suggests that credit may be taken for settling of debris, provided the approach used is 

shown to be appropriately validated and conservative. For BWRs, Section 2.3.2.4 specifically 

prohibits considering debris settling ―until LOCA-induced turbulence in the suppression pool has 

ceased.‖ The analogous time frame in a PWR (i.e. the injection phase of the LOCA) has flow 

from the break and spray drainage; no recirculation flow is occurring. (The full text of this 

regulatory guidance, with a summary of the differences in requirements, is included in Appendix 

A.)

The BWR industry guidance on debris transport in the suppression pool during a LOCA event is 

summarized in Section 5.1. The PWR industry guidance on debris transport in the containment 

sump(s) during a LOCA event is summarized in Section 5.2. Both sections include a discussion 

of the NRC staff evaluations of the respective industry guidance. The BWR and PWR guidance 

and NRC staff evaluations are compared and evaluated in Section 5.3. Section 5.4 summarizes 

recommendations for consistent treatment of debris transport in both BWRs and PWRs.

5.1 BWR Guidance for Debris Transport in Suppression Pool

This section summarizes the BWR industry guidance and correspondingNRC staff evaluation. 

No interpretation of the guidance or the evaluation has been made here. Industry guidance and 

NRC staff evaluations for this issue are discussed in Section 5.3.

The BWR guidance (NEDO-32686-A)1for debris transport in the suppression pool conforms to 

Regulatory Position 2.3.2.4, specifying the ―conservative assumption‖ that ―No credit should be 

taken for the settling of fibrous debris, sludge, and other light material during the high energy 

phase...‖ of a LOCA. Further, the ―conservative and simplifying assumption‖ is recommended 

that all modes of recirculation within the suppression pool will preclude the settling of fibrous 

debris, and it will always be available for transport to the strainers. Equivalent conservative and 

simplifying assumptions are made for all modes of recirculation within the suppression pool, 

precluding the settling of sludge and any ―relatively light debris.‖

An alternative approach suggested in the BWR guidance allows for settling of fibrous debris, 

sludge, and ―relatively light debris.‖ For each debris type, this approach requires establishing the 

expected flow velocities in the pool during and subsequent to the high energy phase of the 

postulated LOCA. The settling behavior of the specific debris species must also be determined, 
88

as well as the effect of the specific LOCA failure condition on flow velocities and system 

alignments/modes during the event. Appendices B and E of NUREG/CR-6224 30 and 

NUREG/CR-6368 37 are identified as sources of relevant information on the settling velocity of 

specific debris.

The NRC staff accepted the BWR industry guidance in their SE of the guidance document 

(NEDO-32686-A),1summarizing the approach as follows: 

High energy phase: no settling in the pool (all suppression pool debris will be suspended or 

re-suspended)

Low energy phase: 

o Option 1: no settling (all suppression pool debris remain suspended)

o Option 2: settling accounted for using appropriate models.

The NRC staff notes that Appendix B to NUREG/CR-6224 30 provides the suppression pool 

settling data only for specific debris types. Licensees using the NUREG/CR-6224 suppression 

pool transport methods are cautioned about extrapolating the experimental data and models to 

untested debris species. Such extrapolation should be justifiable and validated.

5.2 PWR Guidance for Debris Transport in Containment Sump

This section summarizes the PWR industry guidance and corresponding NRC staff evaluation. 

No interpretation of the guidance of the evaluation has been made here. Industry guidance and 

NRC staff evaluations for this issue are discussed in Section 5.3.

The baseline PWR guidance for debris transport assumes the transportation of 100% of the small 

fines in the active volumes of the pool during recirculation, but no transport of the large pieces 

(NEI 04-07).3 Thus, any small fines debris in the active pool volumes is assumed to reach the 

suction strainers. However, analytical refinement options are suggested for reducing the 

conservatism in assumptions underpinning the baseline model. For debris transport in general,

the PWR guidance suggests using approaches such as developing models of flow in the active 

sump, such as nodal network models or three-dimensional CFD models. No specific guidance is 

provided for using the nodal network approach to determine rates of debris transport within the 

sump, but the CFD approach is noted as specifically applicable to determining appropriate 

settling rates in the sump. The guidance document notes that if the settling velocities of debris 

species are known, in the area of the sump where fluid velocities are higher than the settling 

velocity of a given species, ―it may be conservatively assumed that debris in this area (of the 

given type and size being analyzed) will be transported to the sump screen.‖ The logical 

converse of this is that in regions where the fluid velocity is lower than the settling velocity of a 

given species, credit may be taken for debris in the sump that would settle out before reaching 

the screen.

The NRC staff does not comment on the baseline PWR guidance with respect to debris settling 

in the sump (SE NEI 04-07).4 The NRC staff accepted the nodal network method as an 

alternative method for determining debris transport to the sump screens, but only if licensees use 

experimental data to ensure that their use of the approach is conservative with respect to debris 
89

type and quantity. The staff accepted the CFD method, but provided specific comments and 

guidance on how it should be implemented (Appendices III and IV, SE NEI 04-07).

5.3 Evaluation of Guidance for Debris Transport in Suppression Pool and 

Containment Sump

This section provides a comparison of the PWR and BWR industry guidance for suppression 

pool and containment sump debris transport summarized in Sections 5.1 and 5.2. The 

approaches for determining the flow field and debris material settling characteristics are 

compared. 

The ―baseline‖ guidance from industry for both BWRs and PWRs with respect to debris settling 

within the suppression pool or sump is that no credit should be taken for settling. Each industry, 

however, provides alternative debris transport methods wherein credit may be taken for settling 

of debris before reaching the strainer. Although the BWR guidance states that it will be 

necessary to establish the expected flow velocities in the pool during and subsequent to the high 

energy phase of the postulated LOCA, no guidance for determining the flow field is provided. 

Thus, BWR licensees could employ different methodologies to analysis of similar containment 

environments.

The PWR guidance is more specific with regard to the methodology for determining the flow 

field, but a specific modeling approach is not identified. As with the BWR guidance, different 

PWR licensees may thus employ different methodologies. 

The relevant Regulatory Positions provide specific guidance defining conditions when debris 

settling cannot be considered, and NRC staff provides additional detail and requirements for the 

approach. However, the SEs by NRC staff allow individual licensees to develop alternative 

methods for predicting debris transport, which could include settling.

5.4 Recommendations on Guidance for Debris Transport in Suppression Pool 

and Containment Sump

Although the geometry can differ significantly between the suppression pool for BWRs and the 

containment sumps for PWRs, the basic characteristics of their respective flow fields can be 

expected to be quite similar during a postulated LOCA event. For both systems, water is flowing 

into the pool or sump from containment and at the same time, water is being rapidly drawn out to 

meet the performance requirements of the ECCS and CSS. To ensure that debris transport 

through these coolant reservoirs is treated with comparable conservatism in the analysis for each 

type of reactor, similar analytical approaches should be required for both systems. The analytical 

approach to determining the flow field should not be a function of reactor type.

Recommendation 5.1 Unless an assumption of 100% transport is employed, the approach 

used for flow field modeling in the sump and suppression pool should 

be validated and consistent in the basic approach and the degree of 

conservatism of assumptions.

In guidance provided by the Owners Groups and NRC, licensees are advised to determine debris 

material settling characteristics on a plant-specific basis or via NUREG/CR-6224 or 
90

NUREG/CR-6772,38 consistent with PWR Regulatory Position 1.3.3.4 of Regulatory Guide 1.82, 

Rev. 3 18). Settling velocities from the NUREG references for NUKONTM insulation may differ 

by up to three orders of magnitude, depending on the assumed size of the fiber particles. 

Differences in the settling characteristics of the same debris material are thus possible for BWR 

and PWR licensees. Debris settling characteristics of the material are physical properties of the 

material, not functions of reactor type.

Recommendation 5.2 Settling behavior of debris in the sump and suppression pool, if 

credited, should be based on the properties of the specific debris 

material, considering particle density, geometry, and size distribution.
91

6. RECOMMENDATIONS

This section summarizes and prioritizes the recommendations developed in the preceding 

sections for developing Regulatory guidance on appropriate treatment of the technical issues 

related to effects of debris on system performance during postulated design basis LOCA events. 

The recommendations are intended to provide a basis for conservative treatment of these issues 

in analyses for both reactor types, and for consistent treatment of the same phenomena for both 

reactor types. Table 6.1 lists the specific recommendations from Sections 2 through 5, following 

the organization of Table 1.1, which summarizes the differences in guidance for PWR and BWR 

analysis of post-LOCA cooling of the reactor core.

Table 6.1. Summary of recommendations for developing conservative and consistent guidance 

for analysis of LOCA and post-LOCA recirculation cooling in PWRs and BWRs.

Index

number Recommendation

Debris characteristics (see Section 2)

2.1

Plant-specific determination of the types, quantities, and distributions of physical 

debris, similar to the individual plant walkdowns for PWRs, is recommended for all 

commercial light water reactors, including BWRs. A sampling methodology, such 

as the guidelines offered through the SE to NEI 04-07, should be implemented 

across all plants to determine the relative quantity of fibrous debris. Methods to 

estimate the quantities and types of insulation debris, the largest contributor to the 

post-LOCA debris inventory, should be unified across BWRs and PWRs.

2.2

A determination of the effects of coolant, solutes, and insulation on the creation of 

chemical debris and the influence of the debris on head loss and downstream effect, 

along the lines of the ICET program and Westinghouse studies conducted for PWRs, 

is recommended for BWRs.

Debris generation (see Section 3)

3.1

The zone of influence (ZOI) of the high-energy jet of steam or saturated liquid water 

released in a LOCA should be determined using an experimentally validated free-jet 

expansion model that is applicable to both BWR and PWR conditions.

3.2

A validated basis that is consistent as applicable between reactor types for 

insulation material failure pressures should be developed for the range of 

thermodynamic conditions encountered in LOCA scenarios.
92

Table 6.1 (contd)

Index

number Recommendation

3.3

A validated basis consistent as applicable between reactor types for qualified and 

unqualified coatings thickness should be developed.

3.4

Reducing potential debris quantity by means of the definition of a specific ZOI 

extent, debris location, and contribution to subsequent head loss should only be 

considered after validated and consistent approaches for free-jet expansion, debris 

material failure pressure, and debris quantity are established.

3.5 A validated approach consistent as applicable between reactor types for the failure 

of insulation and coating systems outside of the ZOI is recommended.

Debris bed formation on screens or strainers (see Section 4)

4.1

Evaluate the specific strainer designs currently installed in BWR plants, on a plantby-plant basis (if necessary), with regard to test scaling, debris near field settlement 

simulation, surrogate debris similitude requirements, range of independent 

variables tested, and testing procedures, to determine if the tests and evaluations 

can be considered prototypic or conservative with respect to these parameters.

4.2

Apply the same standards and guidance to evaluations of submittals from BWR 

licensees regarding suction strainer head loss calculations, including the potential 

for thin bed effects, as are applied to submittals from PWR licensees.

Downstream effects of debris in recirculating coolant (see Section 4)

4.3

Require validation of debris ingestion models with experimental data obtained for 

conditions where the maximum amount of debris is able to pass through the suction 

strainers. This should include the evaluation of conditions where an incomplete 

debris bed might form, and generally corresponds to conditions where the effect of 

debris on strainer head loss may be relatively low.

4.4

Require validation of abrasion and erosion wear models for specific particulate 

materials and ranges of particle sizes postulated for debris generated in BWR and 

PWR LOCA scenarios.

4.5

Apply the same standards and guidance to evaluations of submittals from BWR 

licensees regarding effect of debris in the recirculation coolant on downstream 

components as are applied to submittals from PWR licensees.

4.6

Require prototypic testing of debris mixtures in core flow at pressures and 

temperatures corresponding to post-LOCA conditions to determine the effect of 

local blockages on local fuel rod cladding temperatures for postulated for BWR and 

PWR LOCA scenarios. Include testing to show the effects of debris left behind by 

core boil-off.

4.7

For PWRs, require testing to determine the effects on local fuel rod cladding 

temperatures of chemical plate-out (with and without trapped debris) for forced flow 

and core boil-off conditions in postulated for LOCA scenarios.

4.8

Apply similar standards and guidance to evaluations of submittals from BWR 

licensees regarding effects of debris in the reactor vessel and core as are applied to 

submittals from PWR licensees.

Debris transport in sump or suppression pool (see Section 5)
93

Table 6.1 (contd)

Index

number Recommendation

5.1

Unless an assumption of 100% transport is employed, the approach used for flow 

field modeling in the sump and suppression pool should be validated and consistent 

in the basic approach and the degree of conservatism of assumptions.

5.2

Settling behavior of debris in the sump and suppression pool, if credited, should be 

based on the properties of the specific debris material, considering particle density,

geometry, and size distribution.

The overall methodology must be an integrated approach, even when it is conducted in segments. 

It must also have consistent modeling between segments of the analysis. Assumptions must be 

conservative for all parts of the analysis to produce an overall conservative result. This means 

that assumptions may need to shift when performing one part of the analysis, compared to those 

used for another part. The merit of a multi-conservatism approach is the relatively high level of 

confidence that may be ascribed to the result. The guidance to licensees from the NRC SEs of 

the BWROG guidance, the PWROG guidance, Regulatory Guide 1.82, and other Regulatory 

guidance on specific issues should provide a consistent overall methodology that captures the 

appropriate conservatisms in all elements of the model.

95

7. REFERENCES

1. NEDO-32686-A, Utility Resolution Guidance for ECSS Suction Strainer Blockage, Volumes 1, 2, 3, 

and 4, prepared by the Boiling Water Reactor Owners Group, 1998 (ADAMS Accession No. 

ML092530482, ML092530500, ML092530505, ML092530507).

2. NEI 02-01, Revision 1, Condition Assessment Guidelines: Debris Sources Inside PWR Containments, 

Nuclear Energy Institute, Washington, D.C., 2002 (ADAMS Accession No. ML030420318).

3. NEI 04-07, Pressurized Water Reactor Sump Performance Evaluation Methodology (Developed by 

Westinghouse and Alion Science and Technology under sponsorship of the Westinghouse and B&W 

Owners Groups and under the technical guidance of the NEI PWR Sump Performance Task Force

and the Electric Power Research Institute (EPRI), 2004 (ADAMS Accession No. ML050550138).

4. SE NEI 04-07, Pressurized Water Reactor Sump Performance Evaluation Methodology, Revision 1, 

U.S. Nuclear Regulatory Commission, Washington, D.C., 2004 (ADAMS Accession No. 

ML043280641).

5. Dallman J., Letellier B., Garcia, J., Madrid, J., Roesch, W., Chen, D., Howe, K., Archuleta, L., 

Sciacca, F., and Jain, B. P., Integrated Chemical Effects Test Project: Consolidated Data Report and 

Test #1 - #5 Data Reports. NUREG/CR-6914, Volumes 1-6, U.S. Nuclear Regulatory Commission, 

Washington, D.C., 2006. (ADAMS Accession No. ML062560105. ML062560111, ML062560119, 

ML062560122, ML062560129, ML062560133).

6. Chen, D., Howe, K. J., Dallman, J., Letellier, B. C., Klasky, M., Leavitt, J., and Jain, B., 

Experimental Analysis of the Aqueous Chemical Environment Following a Loss-of-Coolant 

Accident, Nuclear Engineering and Design, Volume 237, Issue 20-21, pp. 2126-2136, November 

2007.

7. Klasky, M., Zhang, J., Ding, M., Letellier, B., Chen, D., and Howe, K, Aluminum Chemistry in a 

Prototypical Post-Loss-of-Coolant-Accident, Pressurized-Water-Reactor Containment Environment, 

NUREG/CR-6915, The US Nuclear Regulatory Commission, Washington, D.C., 2006. (ADAMS 

Accession No. ML070160448).

8. Lane, A. E., Andreychek, T. S., Byers, W. A., Jacko, R. J., Lahoda, E. J. and Reid, R. D., Evaluation 

of Post-Accident Chemical Effects in Containment Sump Fluids to Support GSI-191, WCAP-16530-

NP-A, Westinghouse Electric LLC, Pittsburgh, PA, 2008 (ADAMS Accession No. ML081150379).

9. Oelkers, E. H., and Gislason, S. R., The Mechanism, Rates and Consequences of Basaltic Glass 

Dissolution: I. An Experimental Study of the Dissolution Rates of Basaltic Glass as a Function of 

Aqueous Al, Si and Oxalic Acid Concentration at 25°C and pH = 3 and 11, Geochimica et 

Cosmochimica Acta, Volume 65, Issue 21, pp. 3671–3681, November 2001.

10. Buckwalter, C. Q., and Pederson, L.R., Inhibition of Nuclear Waste Glass Leaching by 

Chemisorption, Journal of the American Ceramic Society, Volume 65, Issue 9, pp. 431-436, 1982.
96

11. Safety Standard Division, Fiscal 2007 PWR Sump Screen Chemical Effect Test, JNES-SS Report, 

JNES-SS-0804, Japan Nuclear Energy Safety Organization, Tokyo, Japan, 2008 (ADAMS Accession 

No. ML082350152, ML082350156, ML082350157, ML082350158, ML082350159).

12. Ludwig, H., and Roth, F., 2006. Einfluss von Korrosionsvorgängen auf die gesicherte 

Sumpfansaugung nach KMV-Störfällen (Influence of Corrosion Processes on the Protected Sump 

Intake after Coolant Loss Accidents), Framatome ANP, GmbH, Jahrestagung Kenrtechnik 2006 

(Nuclear Technology Annual Convention), 2006 (ADAMS Accession No. ML083510156).

13. Chen, D., Howe, K. J., Dallman, J., and Letellier, B. C., Corrosion of Aluminium in the Aqueous 

Chemical Environment of a Loss-of-Coolant Accident at a Nuclear Power Plant, Corrosion Science, 

Volume 50, pp. 1046-1057, 2008.

14. Gallagher, M. P., Request for License Amendments Related to Application of Alternative Source 

Term, Limerick Generating Station, Units 1 & 2, Exelon Nuclear, Kennett Square, PA. 2004

(ADAMS Accession No.ML040980153).

15. Tagirov B., Schott, J., Harrichoury, J.-C., and Escalier, J., Experimental Study of the Stability of 

Aluminate-Borate Complexes in Hydrothermal Solutions, Geochimica et Cosmochimica Acta, 

Volume 68, Issue 6, pp. 1333–1345, 2004.

16. Tabrizi, M. R., Lyon, S. B., Thompson, G. E., and Ferguson, J. M., The Long-Term Corrosion of 

Aluminium in Alkaline Media, Corrosion Science, 32, pp. 733-742, 1991.

17. Jain, V., He, X., and Pan, Y. M., Corrosion Rate Measurements and Chemical Speciation of 

Corrosion, Products Using Thermodynamic Modeling of Debris Components to Support GSI-191, 

NUREG/CR-6873, US Nuclear Regulatory Commission, Washington, D.C., 2005 (ADAMS 

Accession No. ML051610123).

18. Regulatory Guidance 1.82, Revision 3, Water Sources for Long-Term Recirculation Cooling 

following a Loss-of-Coolant Accident, U.S. Nuclear Regulatory Commission, Washington, D.C., 

2003 (ADAMS Accession No. ML033140347).

19. Continuum Dynamics Report 96-01 Rev. 3, Zone of Influence as Defined by Computational Fluid 

Dynamics, 1996 (NEDO-32686-A, Volume 3, ADAMS Accession No. ML092530505).

20. Continuum Dynamics Report 96-06 Rev. A, Air Jet Impact Testing of Fibrous and Reflective 

Metallic Insulation, 1996 (NEDO-32686-A, Volume 3, ADAMS Accession No. ML092530505).

21. GE-NE-T23-00700-15-21, rev. 1, Evaluation of the Effects of Debris on ECCS Performance, General 

Electric Company, 1996 (NEDO-32686-A, Volume 4, ADAMS Accession No. ML092530507). 

22. ANSI/ANS-58.2-1988, Design Basis for Protection of Light Water Nuclear Power Plants Against the 

Effects of Postulated Pipe Rupture. American National Standards Institute/American Nuclear Society, 

1988.

23. Bechtel Report 22754094.12A, Performance of Containment Coatings During a Loss of Coolant 

Accident, Bechtel Power Corporation, 1994 (NEDO-32686-A, Volume 4, ADAMS Accession No. 

ML092530507).
97

24. EPRI-1009750, Analysis of Pressurized Water Reactor Unqualified Original Equipment 

Manufacturer Coatings, Electric Power Research Institute, Palo Alto, CA, 2005.

25. ASTM D5144-00, Standard Guide for Use of Protective Coating Standards in Nuclear Power Plants, 

ASTM International, West Conshohocken PA.

26. NRC Staff Review Guidance Regarding Generic Letter 2004-02 Closure in the Area of Coatings 

Evaluation, U.S. Nuclear Regulatory Commission, Washington, D.C., 2008 (ADAMS Accession No. 

ML080230462).

27. DRF A74-00004, Total Pressure Topography and Zone of Destruction for Steam and Mixer 

Discharge from Ruptured Pipes, GE Nuclear Energy, General Electric Company, 1996 (NEDO32686-A, Volume 4, ADAMS Accession No. ML092530507).

28. NEA/CSNI/R (95)11, Knowledge Base for Emergency Core Cooling System Recirculation 

Reliability, Nuclear Energy Agency, Committee on the Safety of Nuclear Installations, 1996.

29. C.D.I. Report No. 95-09, Testing of Alternate Strainers with Insulation Fiber and Other Debris, 

Revision 4, prepared by Continuum Dynamics, Inc., Princeton, New Jersey, 1996 (NEDO-32686-A, 

Volume 2, ADAMS Accession No. ML092530500).

30. NUREG/CR-6224, Parametric Study of the Potential for BWR ECCS Strainer Blockage Due to 

LOCA Generated Debris, U.S. Nuclear Regulatory Commission, Washington, D.C., 1995 (ADAMS 

Accession No. ML083290498).

31. NUREG/CR-6371, SEA96-3104-A:4, C.J. Shaffer, W. Bernahl, J. Brideau, and D.V. Rao, 

BLOCKAGE 2.5 Reference Manual, U.S. Nuclear Regulatory Commission, Washington, D.C., 

December 1996.

32. NUREG/CR-6808, Knowledge Base for Effect of Debris on Pressurized Water Reactor Emergency 

Core Cooling Sump Performance, U. S. Nuclear Regulatory Commission, Washington, D.C., 2003.

33. PCI-97, Summary Report on Performance of Performance Contracting, Inc.‘s Sure-FlowTM Suction 

Strainer with Various Mixes of Simulate Post-LOCA Debris, September 1997.

34. Letellier B. C., Dale, C. B., Maji, A., Howe, K., and Carles, F., Screen Penetration Test Report, LAUR-04-5416, Los Alamos National Laboratory, 2005 (ADAMS Accession No. ML051020162).

35. SE WCAP-16406-P (Revision 1), Evaluation of Downstream Sump Debris Effects in Support of GSI191, U.S. Nuclear Regulatory Commission, Washington, D.C., 2007 (ADAMS Accession No. 

ML073520295).

36. WCAP-16793-NP, Revision 0, Evaluation of Long-Term Cooling Considering Particulate, Fibrous 

and Chemical Debris in the Recirculating Fluid, Westinghouse Electric Company, Pittsburgh, PA, 

2007.

37. NUREG/CR-6368, Experimental Investigation of LOCA-Generated Fibrous Debris and Sludge in 

Suppression Pools, U.S. Nuclear Regulatory Commission, Washington, D.C., 1995.
98

38. NUREG/CR-6772. GSI-191: Separate-Effects Characterization of Debris Transport in Water, U.S. 

Nuclear Regulatory Commission, Washington, D.C., 2002.
APPENDIX A

REGULATORY GUIDE 1.82, REV. 3, NOVEMBER 2003, 

PWR-BWR COMPARISON
A.1

Regulatory Guide 1.82, Rev. 3, November 2003, 

PWR-BWR Comparison

PWR BWR Difference(s)

1.1 Features Needed To Minimize the 

Potential for Loss of NPSH

The ECC sumps, which are the source of 

water for such functions as ECC and 

containment heat removal following a 

LOCA, should contain an appropriate 

combination of the following features and 

capabilities to ensure the availability of the 

ECC sumps for long-term cooling. The 

adequacy of the combinations of the 

features and capabilities should be 

evaluated using the criteria and 

assumptions in Regulatory Position 1.3.

2.1 Features Needed To Minimize the 

Potential for Loss of NPSH

The suppression pool is the source of 

water for such functions as ECC and 

containment heat removal following a 

LOCA in conjunction with the vents and 

downcomers between the drywell and the 

wetwell. It should combine the following 

features and capabilities to ensure the 

availability of the suppression pool for 

long-term cooling. The adequacy of the 

combinations of the features and 

capabilities should be evaluated using the 

criteria and assumptions in Regulatory 

Position 2.3.

Guidance essentially the 

same for PWR and 

BWR.

1.1.1 ECC Sumps, Debris Interceptors, and 

Debris Screens

1.1.1.1 A minimum of two sumps should 

be provided, each with sufficient capacity 

to service one of the redundant trains of the 

ECCS and CSS. The distribution of water 

sources and containment spray between the 

sumps should be considered in the 

calculation of boron concentration in the 

sumps for evaluating post-LOCA 

subcriticality and shutdown margins. 

Typically, these calculations are performed 

assuming minimum boron concentration 

and minimum dilution sources. Similar 

considerations should also be given in the 

calculation of time for Hot Leg 

Switchover, which is calculated assuming 

maximum boron concentration and a 

minimum of dilution sources.

PWR provided with 

specific guidance on 

details of sump system 

design in Regulatory 

Positions 1.1.1.1 

through 1.1.1.5, 1.1.1.7, 

1.1.1.10, and 1.1.1.15.

BWR provided no 

guidance beyond the 

general statement of 

Regulatory Position 2.1.

1.1.1.2 To the extent practical, the 

redundant sumps should be physically 

separated by structural barriers from each 

other and from high-energy piping systems 

to preclude damage from LOCA, and, if 

within the design basis, main steam or 

main feedwater break consequences to the 

components of both sumps (e.g., trash 

racks, sump screens, and sump outlets) by 

whipping pipes or high-velocity jets of 

water or steam.
A.2

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.1.1.3 The sumps should be located on the 

lowest floor elevation in the containment 

exclusive of the reactor vessel cavity to 

maximize the pool depth relative to the 

sump screens. The sump outlets should be 

protected by appropriately oriented (e.g., at 

least two vertical or nearly vertical) debris 

interceptors: (1) a fine inner debris screen 

and (2) a coarse outer trash rack to prevent 

large debris from reaching the debris 

screen. A curb should be provided 

upstream of the trash racks to prevent 

high-density debris from being swept 

along the floor into the sump. To be 

effective, the height of the curb should be 

appropriate for the pool flow velocities, as 

the debris can jump over a curb if the 

velocities are sufficiently high. 

Experiments documented in NUREG/CR6772 and NUREG/CR-6773 have 

demonstrated that substantial quantities of 

settled debris could transport across the 

sump pool floor to the sump screen by 

sliding or tumbling.

1.1.1.4 The floor in the vicinity of the ECC 

sump should slope gradually downward 

away from the sump to further retard floor 

debris transport and reduce the fraction of 

debris that might reach the sump screen.

1.1.1.5 All drains from the upper regions 

of the containment should terminate in 

such a manner that direct streams of water, 

which may contain entrained debris, will 

not directly impinge on the debris 

interceptors or discharge in close 

proximity to the sump. The drains and 

other narrow pathways that connect 

compartments with potential break 

locations to the ECC sump should be 

designed to ensure that they would not 

become blocked by the debris; this is to 

ensure that water needed for an adequate 

NPSH margin could not be held up or 

diverted from the sump.
A.3

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.1.1.7 Where consistent with overall sump 

design and functionality, the top of the 

debris interceptor structures should be a 

solid cover plate that is designed to be 

fully submerged after a LOCA and 

completion of the ECC injection. The 

cover plate is intended to provide 

additional protection to debris interceptor 

structures from LOCA-generated loads. 

However, the design should also provide 

means for venting of any air trapped 

underneath the cover.

1.1.1.10 The debris interceptor structures 

should include access openings to facilitate 

inspection of these structures, any vortex 

suppressors, and the sump outlets.

1.1.1.15 Advanced strainer designs (e.g., 

stacked disc strainers) have demonstrated 

capabilities that are not provided by simple 

flat plate or cone-shaped strainers or 

screens. For example, these capabilities 

include built-in debris traps where debris 

can collect on surfaces while keeping a 

portion of the screen relatively free of 

debris. The convoluted structure of such 

strainer designs increases the total screen 

area, and these structures tend to prevent 

the condition referred to as the thin bed 

effect. It may be desirable to include these 

capabilities in any new sump 

strainer/screen designs. The performance 

characteristics and effectiveness of such 

designs should be supported by appropriate 

test data for any particular intended 

application.
A.4

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.1 Net Positive Suction Head of ECCS 

and Containment Heat Removal Pumps

2.1.1 Net Positive Suction Head of ECCS 

and Containment Heat Removal Pumps

1.3.1.1 ECC and containment heat removal 

systems should be designed so that 

sufficient available NPSH is provided to 

the system pumps, assuming the maximum 

expected temperature of pumped fluid and 

no increase in containment pressure from 

that present prior to the postulated LOCA. 

(See Regulatory Position 1.3.1.2.) For 

sump pools with temperatures less than 

212 F, it is conservative to assume that the 

containment pressure equals the vapor 

pressure of the sump water. This ensures 

that credit is not taken for the containment 

pressurization during the transient. For 

subatmospheric containments, this 

guidance should apply after the injection 

phase has terminated. For subatmospheric 

containments, prior to termination of the 

injection phase, NPSH analyses should 

include conservative predictions of the 

containment atmospheric pressure and 

sump water temperature as a function of 

time.

2.1.1.1 ECC and containment heat 

removal systems should be designed so 

that adequate available NPSH is provided 

to the system pumps, assuming the 

maximum expected temperature of the 

pumped fluid and no increase in 

containment pressure from that present 

prior to the postulated LOCAs. (See 

Regulatory Position 2.1.1.2.)

Guidance essentially the 

same for PWR and 

BWR, but PWR 

guidance has more 

details on specific 

conservatisms for the 

analysis.

1.3.1.2 For certain operating PWRs for 

which the design cannot be practicably 

altered, conformance with Regulatory 

Position 1.3.1.1 may not be possible. In 

these cases, no additional containment 

pressure should be included in the 

determination of available NPSH than is 

necessary to preclude pump cavitation. 

Calculation of available containment 

pressure and sump water temperature as a 

function of time should underestimate the 

expected containment pressure and 

overestimate the sump water temperature 

when determining available NPSH for this 

situation.

2.1.1.2 For certain operating BWRs for 

which the design cannot be practicably 

altered, conformance with Regulatory 

Position 2.1.1.1 may not be possible. In 

these cases, no additional containment 

pressure should be included in the 

determination of available NPSH than is 

necessary to preclude pump cavitation. 

Calculation of available containment 

pressure should underestimate the 

expected containment pressure when 

determining available NPSH for this 

situation. Calculation of suppression pool 

water temperature should overestimate the 

expected temperature when determining 

available NPSH.

Guidance the same for 

PWR and BWR.
A.5

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.1.3 For certain operating reactors for 

which the design cannot be practicably 

altered, if credit is taken for operation of 

an ECCS or containment heat removal 

pump in cavitation, prototypical pump tests 

should be performed along with post-test 

examination of the pump to demonstrate 

that pump performance will not be 

degraded and that the pump continues to 

meet all the performance criteria assumed 

in the safety analyses. The time period in 

the safety analyses during which the pump 

may be assumed to operate while 

cavitating should not be longer than the 

time for which the performance tests 

demonstrate that the pump meets 

performance criteria.

2.1.1.3 For certain operating BWRs for 

which the design cannot be practicably 

altered, if credit is taken for operation of 

an ECCS or containment heat removal 

pump in cavitation, prototypical pump 

tests should be performed along with posttest examination of the pump to 

demonstrate that pump performance will 

not be degraded and that the pump 

continues to meet all the performance 

criteria assumed in the safety analyses. 

The time period in the safety analyses 

during which the pump may be assumed to 

operate while cavitating should not be 

longer than the time for which the 

performance tests demonstrate the pump 

meets performance criteria.

Guidance the same for 

PWR and BWR.

1.3.1.4 The decay and residual heat 

produced following accident initiation 

should be included in the determination of 

the water temperature. The uncertainty in 

the determination of the decay heat should 

be included in this calculation. The 

residual heat should be calculated with 

margin.

2.1.1.4 The decay and residual heat 

produced following accident initiation 

should be included in the determination of 

the water temperature. The uncertainty in 

the determination of the decay heat should 

be included in this calculation. The 

residual heat should be calculated with 

margin.

Guidance the same for 

PWR and BWR.

1.3.1.5 The hot channel correction factor 

specified in ANSI/HI 1.1-1.5-1994 should 

not be used in determining the margin 

between the available and required NPSH 

for ECCS and containment heat removal 

system pumps.

2.1.1.5 The hot channel correction factor 

specified in ANSI/HI 1.1-1.5-1994 should 

not be used in determining the margin 

between the available and required NPSH 

for ECCS and containment heat removal 

system pumps.

Guidance the same for 

PWR and BWR.

1.3.1.6 The calculation of available NPSH 

should minimize the height of water above 

the pump suction (i.e., the level of water 

on the containment floor). The calculated 

height of water on the containment floor 

should not consider quantities of water that 

do not contribute to the sump pool (e.g., 

atmospheric steam, pooled water on floors 

and in refueling canals, spray droplets and 

other falling water, etc.). The amount of 

water in enclosed areas that cannot be 

readily returned to the sump should not be 

included in the calculated height of water 

on the containment floor.

2.1.1.6 The level of water in suppression 

pools should be the minimum value given 

in the technical specifications reduced by 

the drawdown due to suppression pool 

water in the drywell and the sprays.

Differences due to 

differences between 

BWR and PWR 

systems; intent of 

guidance the same 

(i.e., obtain a 

conservative [low]

estimate of gravity head 

seen by strainers).

1.3.1.7 The calculation of pipe and fitting 

resistance and the calculation of the 

nominal screen resistance without 

blockage by debris should be done in a 

recognized, defensible method or 

determined from applicable experimental 

data.

2.1.1.7 Pipe and fitting resistance and the 

nominal screen resistance without 

blockage by debris should be calculated in 

a recognized, defensible method or 

determined from applicable experimental 

data.

Guidance the same for 

PWR and BWR.
A.6

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.1.8 Sump screen flow resistance that is 

due to blockage by LOCA-generated 

debris or foreign material in the 

containment, which is transported to the 

suction intake screens, should be 

determined using Regulatory 

Position 1.3.4.

2.1.1.8 Suction strainer screen flow 

resistance caused by blockage by LOCAgenerated debris or foreign material in the 

containment that is transported to the 

suction intake screens should be 

determined using the methods in 

Regulatory Position 2.3.3.

Guidance essentially the 

same for PWR and 

BWR.

1.3.1.9 Calculation of available NPSH 

should be performed as a function of time 

until it is clear that the available NPSH 

will not decrease further.

2.1.1.9 Calculation of available NPSH 

should be performed as a function of time 

until it is clear that the available NPSH 

will not decrease further.

Guidance the same for 

PWR and BWR.

1.1.1 ECC Sumps, Debris Interceptors, and 

Debris Screens

(Guidance for PWRs in Section 1.1.1 

addresses issues covered in Section 2.1.2 

for BWRs. )

2.1.2 Passive Strainer

The inlet of pumps performing the above 

functions should be protected by a suction 

strainer placed upstream of the pumps; this 

is to prevent the ingestion of debris that 

may damage components or block 

restrictions in the systems served by the 

ECC pumps. The following items should 

be considered in the design and 

implementation of a passive strainer.

Difference in 

organization of BWR 

and PWR sections of the 

Regulatory Guide;

overall intent of 

guidance essentially the 

same

1.1.1.6 The strength of the trash racks 

should be adequate to protect the debris 

screens from missiles and other large 

debris. Trash racks and sump screens 

should be capable of withstanding the 

loads imposed by expanding jets, missiles, 

the accumulation of debris, and pressure 

differentials caused by post-LOCA 

blockage under design-basis flow 

conditions. When evaluating impact from 

potential expanding jets and missiles, 

credit for any protection to trash racks and 

sump screens offered by surrounding 

structures or credit for remoteness of trash 

racks and sump screens from potential high 

energy sources should be justified.

2.1.2.5 The strength of the suction 

strainers should be adequate to protect the 

debris screen from missiles and other large 

debris. The strainers and the associated 

structural supports should be adequate to 

withstand loads imposed by missiles, 

debris accumulation, and hydrodynamic 

loads induced by suppression pool 

dynamics. To the extent practical, the 

strainers should be located outside the 

zone of influence of the vents, 

downcomers, or spargers to minimize 

hydrodynamic loads. The strainer design, 

vis-a-vis the hydrodynamic loads, should 

be validated analytically or 

experimentally.

Differences due mainly 

to differences between 

BWR and PWR 

systems; BWR systems 

are expected to 

experience more severe 

dynamic loads in 

postulated LOCA 

events; intent of 

guidance the same. 

1.1.1.8 The debris interceptors should be 

designed to withstand the inertial and 

hydrodynamic effects that are due to 

vibratory motion of a safe shutdown 

earthquake (SSE) following a LOCA 

without loss of structural integrity.

2.1.2.6 The suction strainers should be 

designed to withstand the inertial and 

hydrodynamic effects that are due to 

vibratory motion of a safe shutdown 

earthquake (SSE) without loss of 

structural integrity.

PWR postulates a 

LOCA before the 

earthquake, or there 

would not be water in 

the sump; BWR has 

water in suppression 

pool at all times. Intent 

of the guidance is the 

same.
A.7

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.1.1.9 Materials for debris interceptors 

and sump screens should be selected to 

avoid degradation during periods of both 

inactivity and operation and should have a 

low sensitivity to such adverse effects as 

stress-assisted corrosion that may be 

induced by chemically reactive spray 

during LOCA conditions.

2.1.2.7 Material for suction strainers 

should be selected to avoid degradation 

during periods of inactivity and operation 

and should have a low sensitivity to such 

adverse effects as stress-assisted corrosion 

that may be induced by coolant during 

LOCA conditions.

Differences due to 

differences between 

BWR and PWR 

systems; intent of 

guidance the same. 

1.1.1.11 A sump screen design (i.e., size 

and shape) should be chosen that will 

avoid the loss of NPSH from debris 

blockage during the period that the ECCS 

is required to operate in order to maintain 

long-term cooling or maximize the time 

before loss of NPSH caused by debris 

blockage when used with an active 

mitigation system (see Regulatory Position 

1.1.4).

2.1.2.1 The suction strainer design (i.e., 

size and shape) should be chosen to avoid 

the loss of NPSH from debris blockage 

during the period that the ECCS is 

required to operate in order to maintain 

long-term cooling or maximize the time 

before loss of NPSH caused by debris 

blockage when used with an active 

mitigation system (see Regulatory Position 

2.1.5).

Guidance essentially the 

same for PWR and 

BWR. 

1.1.1.12 The possibility of debris-clogging 

flow restrictions downstream of the sump 

screen should be assessed to ensure 

adequate long term recirculation cooling, 

containment cooling, and containment 

pressure control capabilities. The size of 

the openings in the sump debris screen 

should be determined considering the flow 

restrictions of systems served by the ECCS 

sump. The potential for long thin slivers 

passing axially through the sump screen 

and then reorienting and clogging at any 

flow restriction downstream should be 

considered. Consideration should be given 

to the buildup of debris at downstream 

locations such as the following: 

containment spray nozzle openings, HPSI 

throttle valves, coolant channel openings in 

the core fuel assemblies, fuel assembly 

inlet debris screens, ECCS pump seals, 

bearings, and impeller running clearances. 

If it is determined that a sump screen with 

openings small enough to filter out 

particles of debris that are fine enough to 

cause damage to ECCS pump seals or 

bearings would be impractical, it is 

expected that modifications would be 

made to ECCS pumps or ECCS pumps 

would be procured that can operate long 

term under the probable conditions.

2.1.2.2 The possibility of debris clogging 

flow restrictions downstream of the 

strainers should be assessed to ensure 

adequate long-term ECCS performance. 

The size of openings in the suppression 

pool suction strainers should be based on 

the minimum restrictions found in systems 

served by the suppression pool. The 

potential for long thin slivers passing 

axially through the strainer and then 

reorienting and clogging at any flow 

restriction downstream should be 

considered. Consideration should be given 

to the buildup of debris at the following 

downstream locations: spray nozzle 

openings, throttle valves, coolant channel 

openings in the core fuel assemblies, fuel 

assembly inlet debris screens, ECCS pump 

seals, bearings, and impeller running 

clearances. If it is determined that a 

strainer with openings small enough to 

filter out particles of debris that are fine 

enough to cause damage to ECCS pump 

seals or bearings would be impractical, it 

is expected that modifications would be 

made to ECCS pumps or ECCS pumps 

would be procured that can operate long 

term under the probable conditions.

Guidance essentially the 

same for PWR and 

BWR. 

PWR specifies adequate 

long term recirculation 

cooling, containment 

cooling, and 

containment pressure 

control capabilities.

BWR mentions only 

adequate long-term 

ECCS performance.
A.8

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.1.1.13 ECC and containment spray pump 

suction inlets should be designed to 

prevent degradation of pump performance 

through air ingestion and other adverse 

hydraulic effects (e.g., circulatory flow 

patterns, high intake head losses).

2.1.2.3 ECC pump suction inlets should be 

designed to prevent degradation of pump 

performance through air ingestion and 

other adverse hydraulic effects (e.g., 

circulatory flow patterns, high intake head 

losses).

Guidance essentially the 

same for PWR and 

BWR.

1.1.1.14 All drains from the upper regions 

of the containment building, as well as 

floor drains, should terminate in such a 

manner that direct streams of water, which 

may contain entrained debris, will not 

discharge downstream of the sump screen, 

thereby bypassing the sump screen.

2.1.2.4 All drains from the upper regions 

of the containment should terminate in 

such a manner that direct streams of water, 

which may contain entrained debris, will 

not impinge on the suppression pool 

suction strainers. 

Differences due to 

differences between 

BWR and PWR 

systems.

1.1.2 Minimizing Debris

The debris (see Regulatory Position 1.3.2) 

that could accumulate on the sump screen 

should be minimized.

2.1.3 Minimizing Debris

The amount of potential debris (see 

Regulatory Position 2.3.1) that could clog 

the ECC suction strainers should be 

minimized.

Guidance essentially the 

same for PWR and 

BWR.

1.1.2.1 Cleanliness programs should be 

established to clean the containment on a 

regular basis, and plant procedures should 

be established for control and removal of 

foreign materials from the containment.

2.1.3.1 Containment cleanliness programs 

should be instituted to clean the 

suppression pool on a regular basis, and 

plant procedures should be established for 

control and removal of foreign materials 

from the containment.

Differences due to 

differences between 

BWR and PWR 

systems; intent of 

guidance is essentially 

the same

1.1.2.2 Insulation types (e.g., fibrous and 

calcium silicate) that can be sources of 

debris that is known to more readily 

transport to the sump screen and cause 

higher head losses may be replaced with 

insulations (e.g., reflective metallic 

insulation) that transport less readily and 

cause less severe head losses once 

deposited onto the sump screen. If 

insulation is replaced or otherwise 

removed during maintenance, abatement 

procedures should be established to avoid 

generating latent debris in the containment.

2.1.3.3 Insulation types (e.g., fibrous and 

calcium silicate) that can be sources of 

debris that is known to more readily 

transport to the strainer and cause higher 

head losses should be avoided. Insulations 

(e.g., reflective metallic insulation) that 

transport less readily and cause less severe 

head losses once deposited onto the 

strainers should be used. If insulation is 

replaced or otherwise removed during 

maintenance, abatement procedures should 

be established to avoid generating latent 

debris in the containment.

Guidance essentially the 

same for PWR and 

BWR.

1.1.2.3 To minimize potential debris 

caused by chemical reaction of the pool 

water with metals in the containment, 

exposure of bare metal surfaces (e.g., 

scaffolding) to containment cooling water 

through spray impingement or immersion 

should be minimized either by removal or 

by chemical-resistant protection (e.g., 

coatings or jackets).

2.1.3.4 To minimize potential debris 

caused by chemical reaction of coolant 

with metals in the containment, exposure 

of bare metal surfaces (e.g., scaffolding) to 

spray impingement or immersion should 

be minimized either by removal or by 

using chemical-resistant protection (e.g., 

coatings or jackets).

Guidance essentially the 

same for PWR and 

BWR.
A.9

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.1.3 Instrumentation

If relying on operator actions to mitigate 

the consequences of the accumulation of 

debris on the ECC sump screens, safetyrelated instrumentation that provides 

operators with an indication and audible 

warning of impending loss of NPSH for 

ECCS pumps should be available in the 

control room.

2.1.4 Instrumentation

If relying on operator actions to mitigate 

the consequences of the accumulation of 

debris on the suction strainers, safetyrelated instrumentation that provides 

operators with an indication and audible 

warning of impending loss of NPSH for 

ECCS pumps should be available in the 

control room.

Guidance essentially the 

same for PWR and 

BWR.

1.1.4 Active Sump Screen System

An active device or system (see examples 

in Appendix B) may be provided to 

prevent the accumulation of debris on a 

sump screen or to mitigate the 

consequences of accumulation of debris on 

a sump screen. An active system should be 

able to prevent debris that may block 

restrictions found in the systems served by 

the ECC pumps from entering the system. 

The operation of the active component or 

system should not adversely affect the 

operation of other ECC components or 

systems. Performance characteristics of an 

active sump screen system should be 

supported by appropriate test data that 

address head loss performance.

2.1.5 Active Strainers

An active component or system (see 

Appendix B) may be provided to prevent 

the accumulation of debris on a suction 

strainer or to mitigate the consequences of 

accumulation of debris on a suction 

strainer. An active system should be able 

to prevent debris that may block 

restrictions found in the systems served by 

the ECC pumps from entering the system. 

The operation of the active component or 

system should not adversely affect the 

operation of other ECC components or 

systems. The use of active strainers should 

be validated by adequate testing.

2.3.3.6 The performance characteristics of 

a passive or an active strainer should be 

supported by appropriate test data that 

addresses, at a minimum, (1) suppression 

pool hydrodynamic loads and (2) head loss 

performance.

Guidance essentially the 

same for PWR and 

BWR.

1.1.5 Inservice Inspection

To ensure the operability and structural 

integrity of the trash racks and screens, 

access openings are necessary to permit

inspection of the ECC sump structures and 

outlets. Inservice inspection of racks, 

screens, vortex suppressors, and sump 

outlets, including visual examination for 

evidence of structural degradation or 

corrosion, should be performed on a 

regular basis at every refueling period 

downtime. Inspection of the ECC sump 

components late in the refueling period 

will ensure the absence of construction 

trash in the ECC sump area.

2.1.6 Inservice Inspection

Inservice inspection requirements should 

be established that include (1) inspection 

of the cleanliness of the suppression pool, 

(2) a visual examination for evidence of 

structural degradation or corrosion of the 

suction strainers and strainer system, and 

(3) an inspection of the wetwell and the 

drywell, including the vents, downcomers, 

and deflectors, for the identification and 

removal of debris or trash that could 

contribute to the blockage of suppression 

pool suction strainers. These inservice 

inspections should be performed on a 

regular basis at every refueling period 

downtime.

Differences due to 

differences between 

BWR and PWR 

systems; intent of 

guidance is essentially 

the same
A.10

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.2 Evaluation of Alternative Water 

Sources

To demonstrate that a combination of the 

features and actions listed above are 

adequate to ensure long-term cooling and 

that the five criteria of 10 CFR 50.46(b) 

will be met following a LOCA, an 

evaluation using the guidance and 

assumptions in Regulatory Position 1.3 

should be conducted. If a licensee is 

relying on operator actions to prevent the 

accumulation of debris on ECC sump 

screens or to mitigate the consequences of 

the accumulation of debris on the ECC 

sump screens, an evaluation should be 

performed to ensure that the operator has 

adequate indications, training, time, and 

system capabilities to perform the 

necessary actions. If not covered by plantspecific emergency operating procedures, 

procedures should be established to use 

alternative water sources that will be 

activated when unacceptable head loss 

renders the sump inoperable. The valves 

needed to align the ECCS and containment 

spray systems (taking suction from the 

recirculation sumps) with an alternative 

water source should be periodically 

inspected and maintained.

2.2 Evaluation of Alternative Water 

Sources

To demonstrate that a combination of the 

features and actions listed above are 

adequate to ensure long-term cooling and 

that the five criteria of 10 CFR 50.46(b) 

will be met following a LOCA, an 

evaluation using the guidance and 

assumptions in Regulatory Position 2.3 

should be conducted. If a licensee is 

relying on operator actions to prevent the 

accumulation of debris on suction strainers 

or to mitigate the consequences of the 

accumulation of debris on the suction 

strainers, an evaluation should be 

performed to ensure that the operator has 

adequate indications, training, time, and 

system capabilities to perform the 

necessary actions. If not covered by plantspecific emergency operating procedure, 

procedures should be established to use 

alternative water sources. The valves 

needed to align the ECCS with an 

alternative water source should be 

periodically inspected and maintained.

Differences due to 

differences between 

BWR and PWR 

systems; intent of 

guidance is essentially 

the same
A.11

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3 Evaluation of Long-Term 

Recirculation Capability

The following techniques, assumptions, 

and guidance should be used in a 

deterministic, plant-specific evaluation to 

ensure that any implementation of a 

combination of the features and 

capabilities listed in Regulatory Position 

1.1 are adequate to ensure the availability 

of a reliable water source for long-term 

recirculation following a LOCA. The 

assumptions and guidance listed below can 

also be used to develop test conditions for 

sump screens. Evaluation and confirmation 

of (1) sump hydraulic performance (e.g.,

geometric effects, air ingestion), (2) debris 

effects (e.g., debris transport, interceptor 

blockage, head loss), and (3) the combined 

impact on NPSH available at the pump 

inlet should be performed to ensure that 

long-term recirculation cooling can be 

accomplished following a LOCA. Such an 

evaluation should arrive at a determination 

of NPSH margin calculated at the pump 

inlet. An assessment should also be made 

of the susceptibility to debris blockage of 

the containment drainage flow paths to the 

recirculation sump; this is to protect 

against reduction in available NPSH if 

substantial amounts of water are held up or 

diverted away from the sump. An 

assessment should be made of the 

susceptibility of the flow restrictions in the 

ECCS and CSS recirculation flow paths 

downstream of the sump screens and of the 

recirculation pump seal and bearing 

assembly design to failure from particulate 

ingestion and abrasive effects to protect 

against degradation of long-term 

recirculation pumping capacity.

2.3 Evaluation of Long-Term 

Recirculation Capability

During any evaluation of the susceptibility 

of a BWR to debris blockage, the 

considerations and events shown in 

Figures 4 and 5 should be addressed. The 

following techniques, assumptions, and 

guidance should be used in a deterministic 

evaluation to ensure that any 

implementation of a combination of the 

features and capabilities listed in 

Regulatory Position 2.1 are adequate to 

ensure the availability of a reliable water 

source for long-term recirculation after a 

LOCA. An assessment should be made of 

the susceptibility to debris blockage of the 

containment drainage flowpaths to the 

suppression pool, flow restrictions in the 

ECCS, and containment spray 

recirculation flowpaths downstream of the 

suction strainer to protect against 

degradation of long-term recirculation 

pumping capacity. Unless otherwise noted, 

the techniques, assumptions, and guidance 

listed below are applicable to an 

evaluation of passive and active strainers. 

The assumptions and guidance listed 

below can also be used to develop test 

conditions for suction strainers or strainer 

systems.

Differences due to 

differences between 

BWR and PWR 

systems; intent of 

guidance is essentially 

the same
A.12

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.2 Debris Sources and Generation 2.3.1 Debris Sources and Generation

1.3.2.1 Consistent with the requirements of 

10 CFR 50.46, debris generation should be 

calculated for a number of postulated 

LOCAs of different sizes, locations, and 

other properties sufficient to provide 

assurance that the most severe postulated 

LOCAs are calculated. The level of 

severity corresponding to each postulated 

break should be based on the potential 

head loss incurred across the sump screen. 

Some PWRs may need recirculation from 

the sump for licensing basis events other 

than LOCAs. Therefore, licensees should 

evaluate the licensing basis and include 

potential break locations in the main steam 

and main feedwater lines as well in 

determining the most limiting conditions 

for sump operation.

2.3.1.1 Consistent with the requirements 

of 10 CFR 50.46, debris generation should 

be calculated for a number of postulated 

LOCAs of different sizes, locations, and 

other properties sufficient to provide 

assurance that the most severe postulated 

LOCAs are calculated.

Differences due to 

differences between 

BWR and PWR 

systems; intent of 

guidance is essentially 

the same
A.13

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.2.2 An acceptable method for estimating the 

amount of debris generated by a postulated 

LOCA is to use the zone of influence (ZOI). 

Examples of this approach are provided in 

NUREG/CR-6224 and Boiling Water Reactor 

Owners‘ Group (BWROG) Utility Resolution 

Guidance (NEDO-32686 and the staff‘s Safety 

Evaluation on the BWROG‘s response to NRC 

Bulletin 96-03). A representation of the ZOI for 

commonly used insulation materials is shown in 

Figure 3. 

• The size and shape of the ZOI should be 

supported by analysis or experiments for the 

break and potential debris. The size and shape 

of the ZOI should be consistent with the debris 

source (e.g., insulation, fire barrier materials, 

etc.) damage pressures, i.e., the ZOI should 

extend until the jet pressures decrease below the 

experimentally determined damage pressures 

appropriate for the debris source. 

• The volume of debris contained within the 

ZOI should be used to estimate the amount of 

debris generated by a postulated break. 

• The size distribution of debris created in the 

ZOI should be determined by analysis or 

experiments. 

• The shock wave generated during the 

postulated pipe break and the subsequent jet 

should be the basis for estimating the amount of 

debris generated and the size or size distribution 

of the debris generated within the ZOI. Certain 

types of material used in a small quantity inside 

the containment can, with adequate 

justification, be demonstrated to make a 

marginal contribution to the debris loading for 

the ECC sump. If debris generation and debris 

transport data have not been determined 

experimentally for such material, it may be 

grouped with another like material existing in 

large quantities. For example, a small quantity 

of fibrous filtering material may be grouped 

with a substantially large quantity of fibrous 

insulation debris, and the debris generation and 

transport data for the filter material need not be 

determined experimentally. However, such 

analyses are valid only if the small quantity of 

material treated in this manner does not have a 

significant effect when combined with other 

materials (e.g., a small quantity of calcium 

silicate combined with fibrous debris).

2.3.1.2 An acceptable method for determining 

the shape of the zone of influence (ZOI) of a 

break is described in NUREG/CR-6224 and 

NEDO-32686. The volume contained within 

the ZOI should be used to estimate the amount 

of debris generated by a postulated break. The 

distance of the ZOI from the break should be 

supported by analysis or experiments for the 

break and potential debris. The shock wave 

generated during postulated pipe break and the 

subsequent jet should be the basis for 

estimating the amount of debris generated and 

the size or size distribution of the debris 

generated within the ZOI. Certain types of 

material used in a small quantity inside the 

containment can, with adequate justification, be 

demonstrated to make a marginal contribution 

to the debris loading for the ECC sump. If 

debris generation and debris transport data 

have not been determined experimentally for 

such material, it may be grouped with another 

like material existing in large quantities. For 

example, a small quantity of fibrous filtering 

material may be grouped with a substantially 

larger quantity of fibrous insulation debris, and 

the debris generation and transport data for the 

filter material need not be determined 

experimentally. However, such analyses are 

valid only if the small quantity of material 

treated in this manner does not have a 

significant effect when combined with other 

materials (e.g., a small quantity of calcium 

silicate combined with fibrous debris).

Guidance essentially the 

same for PWR and 

BWR.

Note that the text of 

1.3.2.2 is a severe 

condensation of a 

complex topic, and can 

be confusing if read out 

of context. Expanded 

discussion of the ZOI 

(describing how it is 

defined and how it is 

used) can be found in 

the two cited sources, 

NUREG/CR-6224 and 

NEDO-32686, which 

are common to the 

guidelines for both 

BWRs and PWRs. 

2.3.1.3 All sources of fibrous materials in 

the containment such as fire protection 

materials, thermal insulation, or filters that 

are present during operation should be 

identified.

No specific guidance on 

this topic for PWR; 

however, it appears to 

be covered in 

Regulatory Position 

1.3.2.2 above.
A.14

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.2.3 A sufficient number of breaks in 

each high-pressure system that relies on 

recirculation should be considered, to 

reasonably bound variations in debris 

generation by the size, quantity, and type 

of debris. As a minimum, the following 

postulated break locations should be 

considered.

• Breaks in the reactor coolant system 

(e.g., hot leg, cold leg, pressurizer surge 

line) and, depending on the plant licensing 

basis, main steam and main feedwater lines 

with the largest amount of potential debris 

within the postulated ZOI,

• Large breaks with two or more different 

types of debris, including the breaks with 

the most variety of debris, within the 

expected ZOI,

• Breaks in areas with the most direct path 

to the sump,

• Medium and large breaks with the 

largest potential particulate debris to 

insulation ratio by weight, and 

• Breaks that generate an amount of fibrous 

debris that, after its transport to the sump 

screen, could form a uniform thin bed that 

could subsequently filter sufficient 

particulate debris to create a relatively high 

head loss referred to as the ―thin-bed 

effect.‖ The minimum thickness of fibrous 

debris needed to form a thin bed has 

typically been estimated at 1/8 inch thick 

based on the nominal insulation density 

(NUREG/CR-6224).

2.3.1.5 A sufficient number of breaks in 

each high-pressure system that relies on 

recirculation should be considered to 

reasonably bound variations in debris 

generation by the size, quantity, and type 

of debris. As a minimum, the following

postulated break locations should be 

considered.

• Breaks in the main steam, feedwater, 

and recirculation lines with the largest 

amount of potential debris within the 

postulated ZOI,

• Large breaks with two or more different 

types of debris, including the breaks with 

the most variety of debris, within the 

expected ZOI,

• Breaks in areas with the most direct path 

between the drywell and wetwell,

• Medium and large breaks with the 

largest potential particulate debris to 

insulation ratio by weight, and 

• Breaks that generate an amount of 

fibrous debris that, after its transport to the 

suction strainer, could form a uniform thin 

bed that could subsequently filter 

sufficient particulate debris to create a 

relatively high head loss referred to as the 

―thin-bed effect.‖ The minimum thickness 

of fibrous debris needed to form a thin bed 

has typically been estimated at 1/8 inch 

thick based on the nominal insulation 

density (NUREG/CR-6224).

Differences due to 

differences between 

BWR and PWR 

systems; specific 

guidance on types of 

breaks to be considered 

is essentially identical, 

and the intent of 

guidance is clearly the 

same.

1.3.2.4 All insulation (e.g., fibrous, 

calcium silicate, reflective metallic), 

painted surfaces, fire barrier materials, and 

fibrous, cloth, plastic, or particulate 

materials within the ZOI should be 

considered a debris source. Analytical 

models or experiments should be used to 

predict the size of the postulated debris. 

For breaks postulated in the vicinity of the 

pressure vessel, the potential for debris 

generation from the packing materials 

commonly used in the penetrations and the 

insulation installed on the pressure vessel 

should be considered. Particulate debris 

generated by pipe rupture jets stripping off 

paint or coatings and eroding concrete at 

the point of impact should also be 

considered.

2.3.1.4 All insulation, painted surfaces, 

and fibrous, cloth, plastic, or particulate 

materials within the ZOI should be 

considered debris sources. Analytical 

models or experiments should be used to 

predict the size of the postulated debris.

Differences due to 

differences between 

BWR and PWR 

systems; specific 

guidance on postulated 

debris is identical and 

the intent of guidance is 

clearly the same.
A.15

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.2.5 The cleanliness of the containment 

during plant operation should be 

considered when estimating the amount 

and type of debris available to block the 

ECC sump screens. The potential for such 

material (e.g., thermal insulation other than 

piping insulation, ropes, fire hoses, wire 

ties, tape, ventilation system filters, 

permanent tags or stickers on plant 

equipment, rust flakes from unpainted steel 

surfaces, corrosion products, dust and dirt, 

latent individual fibers) to impact head loss 

across the ECC sump screens should also 

be considered.

2.3.1.6 The cleanliness of the suppression 

pool and containment during plant 

operation should be considered when 

estimating the amount and type of debris 

available to block the suction strainers. 

The potential for such material (e.g., 

thermal insulation other than piping 

insulation, ropes, fire hoses, wire ties, 

tape, ventilation system filters, permanent 

tags or stickers on plant equipment, rust 

flakes from unpainted steel surfaces, 

corrosion products, dust and dirt, latent 

individual fibers) to impact head loss 

across the suction strainer should also be 

considered.

Guidance essentially the 

same for PWR and 

BWR.

Differences due to 

differences between 

BWR and PWR 

systems.

2.3.1.7 The amount of particulates 

estimated to be in the pool prior to a 

LOCA should be considered to be the 

maximum amount of corrosion products 

(i.e., sludge) expected to be generated 

since the last time the pool was cleaned. 

The size distribution and amount of 

particulates should be based on plant 

samples.

Guidance for BWR 

only; PWR sump is 

normally dry. This issue 

covered in 1.3.2.5 for 

PWR, and intent of 

guidance is clearly the 

same for the two types 

of LWR.

1.3.2.6 In addition to debris generated by 

jet forces from the pipe rupture, debris 

created by the resulting containment 

environment (thermal and chemical) 

should be considered in the analyses. 

Examples of this type of debris would be 

disbondment of coatings in the form of 

chips and particulates or formation of 

chemical debris (precipitants) caused by 

chemical reactions in the pool.

2.3.1.8 In addition to debris generated by 

jet forces from the pipe rupture, debris 

created by the resulting containment 

environment (thermal and chemical) 

should be considered in the analyses. 

Examples of this type of debris would be 

disbondment of coatings in the form of 

chips and particulates or formation of 

chemical debris (precipitants) caused by 

chemical reactions in the pool.

Guidance the same for 

PWR and BWR. 

1.3.2.7 Debris generation that is due to 

continued degradation of insulation and 

other debris when subjected to turbulence 

caused by cascading water flows from 

upper regions of the containments or near 

the break overflow region should be 

considered in the analyses.

Guidance for PWR only; 

BWR containment is not 

as tall or 

compartmentalized. 

This issue adequately 

covered in 2.3.1.8 for 

BWR, and intent of 

guidance is clearly the 

same for the two types 

of LWR.
A.16

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.3 Debris Transport 2.3.2 Debris Transport

1.3.3.1 The calculation of debris quantities 

transported from debris sources to the 

sump screen should consider all modes of 

debris transport, including airborne debris 

transport, containment spray washdown 

debris transport, and containment sump 

pool debris transport. Consideration of the 

containment pool debris transport should 

include (1) debris transport during the fillup phase, as well as during the 

recirculation phase, (2) the turbulence in 

the pool caused by the flow of water, water 

entering the pool from break overflow, and 

containment spray drainage, and (3) the 

buoyancy of the debris. Transport analyses 

of debris should consider: (1) debris that 

would float along the pool surface, (2) 

debris that would remain suspended due to 

pool turbulence (e.g., individual fibers and 

fine particulates), and (3) debris that 

readily settles to the pool floor.

2.3.2.1 It should be assumed that all debris 

fragments smaller than the clearances in 

the gratings will be transported to the 

suppression pool during blowdown. Credit 

may be taken for filtration of larger pieces 

of debris by floor gratings and other

interdicting structures present in a drywell 

(NEDO-32686 and NUREG/CR-6369). 

However, it should be assumed that a 

fraction of large fragments captured by the 

gratings would be eroded by the combined 

effects of cascading break overflow and 

the drywell spray flow. The fraction of the 

smaller debris generated and thus 

transported to the suppression pool during 

the blowdown, as well as the fraction of 

the larger debris that may be eroded 

during the washdown phase, should be 

determined analytically or experimentally.

PWR guidance outlines 

specific requirements of 

analytical models 

developed to calculate 

the amount of debris 

transported to the sump 

screens. 

BWR guidance requires 

the potentially more 

conservative assumption 

that all debris below a 

certain size is 

transported to the 

suppression pool. 

Analytical or empirical 

models must then be 

developed to determine 

the amount and size 

distribution of debris 

generated, including 

accounting for the 

various processes that 

could erode larger debris 

fragments into smaller 

fragments.

Differences reflect 

slightly different 

approaches to the 

problem, but the intent 

of the guidance is 

clearly the same; 

developing a 

conservative estimate of 

the amount of debris that 

reaches the sump 

screens or suction 

strainers.

1.3.3.2 The debris transport analyses 

should consider each type of insulation 

(e.g., fibrous, calcium silicate, reflective 

metallic) and debris size (e.g., particulates, 

fibrous fine, large pieces of fibrous 

insulation). The analyses should also 

consider the potential for further 

decomposition of the debris as it is 

transported to the sump screen.

2.3.1.4 All insulation, painted surfaces, 

and fibrous, cloth, plastic, or particulate 

materials within the ZOI should be 

considered debris sources. Analytical 

models or experiments should be used to 

predict the size of the postulated debris.

Organization of 

guidance is different, but 

requirements to consider 

all types of debris source 

materials and full range 

of possible debris size 

are essentially the same 

for both types of 

systems.
A.17

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.3.3 Bulk flow velocity from 

recirculation operations, LOCA-related 

hydrodynamic phenomena, and other 

hydrodynamic forces (e.g., local 

turbulence effects or pool mixing) should 

be considered for both debris transport and 

ECC sump screen velocity computations.

2.3.2.5 Bulk suppression pool velocity 

from recirculation operations, LOCArelated hydrodynamic phenomena, and 

other hydrodynamic forces (e.g., local 

turbulence effects or pool mixing) should 

be considered for both debris transport and 

suction strainer velocity computations.

Guidance essentially the 

same for PWR and 

BWR.

1.3.3.4 An acceptable analytical approach 

to predict debris transport within the sump 

pool is to use computational fluid 

dynamics (CFD) simulations in 

combination with the experimental debris 

transport data. Examples of this approach 

are provided in NUREG/CR-6772 and 

NUREG/CR-6773. Alternative methods 

for debris transport analyses are also 

acceptable, provided they are supported by 

adequate validation of analytical 

techniques using experimental data to 

ensure that the debris transport estimates 

are conservative with respect to the 

quantities and types of debris transported 

to the sump screen.

2.3.2.2 It should be assumed that LOCAinduced phenomena (i.e., pool swell, 

chugging, condensation oscillations) will 

suspend all the debris assumed to be in the 

suppression pool at the onset of the 

LOCA.

2.3.2.3 The concentration of debris in the 

suppression pool should be calculated 

based on the amount of debris estimated to 

reach the suppression pool from the 

drywell and the amount of debris and 

foreign materials estimated to be in the 

suppression pool prior to a postulated 

break.

2.3.2.5 Bulk suppression pool velocity 

from recirculation operations, LOCArelated hydrodynamic phenomena, and 

other hydrodynamic forces (e.g., local 

turbulence effects or pool mixing) should 

be considered for both debris transport and 

suction strainer velocity computations.

PWR guidance 

explicitly states that 

analysis based on CFD 

modeling or other 

―alternative methods‖

could be 

acceptable approaches to 

predicting debris 

transport within the 

sump pool. 

BWR guidance provides 

specific assumptions 

regarding the amount 

and location of debris, 

but does not give 

specific guidance on the 

types of analyses that 

would be acceptable to 

determine debris 

transport.

1.3.3.5 Curbs can be credited for removing 

heavier debris that has been shown 

analytically or experimentally to travel by 

sliding along the containment floor and 

that cannot be lifted off the floor within the 

calculated water velocity range.

2.3.2.1 It should be assumed that all debris 

fragments smaller than the clearances in 

the gratings will be transported to the 

suppression pool during blowdown. Credit 

may be taken for filtration of larger pieces 

of debris by floor gratings and other 

interdicting structures present in a drywell 

(NEDO-32686 and NUREG/CR-6369). 

However, it should be assumed that a 

fraction of large fragments captured by the 

gratings would be eroded by the combined 

effects of cascading break overflow and 

the drywell spray flow. The fraction of the 

smaller debris generated and thus 

transported to the suppression pool during 

the blowdown, as well as the fraction of 

the larger debris that may be eroded 

during the washdown phase, should be 

determined analytically or experimentally.

Differences due mainly 

to differences between 

BWR and PWR 

systems; intent of 

guidance appears to be 

essentially the same.
A.18

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.3.6 If transported to the sump pool, all 

debris (e.g., fine fibrous, particulates) that 

would remain suspended due to pool 

turbulence should be considered to reach 

the sump screen.

2.3.2.2 It should be assumed that LOCAinduced phenomena (i.e., pool swell, 

chugging, condensation oscillations) will 

suspend all the debris assumed to be in the

suppression pool at the onset of the 

LOCA.

2.3.2.4 Credit should not be taken for 

debris settling until LOCA-induced 

turbulence in the suppression pool has 

ceased. The debris settling rate for the 

postulated debris should be validated 

analytically or experimentally.

Guidance for PWR for 

BWR essentially the 

same.

1.3.3.7 The time to switch over to sump 

recirculation and the operation of 

containment spray should be considered in 

the evaluation of debris transport to the 

sump screen.

Guidance applies to 

PWR only.

1.3.3.8 In lieu of performing airborne and 

containment spray washdown debris 

transport analyses, it could be assumed that 

all debris will be transported to the sump 

pool. In lieu of performing sump pool 

debris transport analyses (Regulatory 

Position 1.3.3.4), it could be assumed that 

all debris entering the sump pool or 

originating in the sump will be considered 

transported to the sump screen when 

estimating screen debris bed head loss. If it 

is credible in a plant that all drains leading 

to the containment sump could become 

completely blocked, or an inventory 

holdup in containment could happen 

together with debris loading on the sump 

screen, these situations could pose a worse 

impact on the recirculation sump 

performance than the assumed situations 

mentioned above. In this case, these 

situations should also be assessed.

2.3.2.1 It should be assumed that all debris 

fragments smaller than the clearances in 

the gratings will be transported to the 

suppression pool during blowdown. Credit 

may be taken for filtration of larger pieces 

of debris by floor gratings and other 

interdicting structures present in a drywell 

(NEDO-32686 and NUREG/CR-6369). 

However, it should be assumed that a 

fraction of large fragments captured by the 

gratings would be eroded by the combined 

effects of cascading break overflow and 

the drywell spray flow. The fraction of the 

smaller debris generated and thus 

transported to the suppression pool during 

the blowdown, as well as the fraction of 

the larger debris that may be eroded 

during the washdown phase, should be 

determined analytically or experimentally.

Guidance for PWR and 

BWR is essentially the 

same. For PWR it is an 

alternative option, but 

for BWR, it is the main 

assumption for all debris 

transport analysis.

1.3.3.9 The effects of floating or buoyant 

debris on the integrity of the sump screen 

and on subsequent head loss should be 

considered. For screens that are not fully 

submerged or are only shallowly 

submerged, floating debris could 

contribute to the debris bed head loss. The 

head loss due to floating or buoyant debris 

could be minimized by a design feature to 

keep buoyant debris from reaching the 

sump screen.

BWR guidelines do not 

address the possible 

effects of floating 

debris, or partially 

uncovered suction 

strainers. The 

suppression pool is 

expected to remain deep 

enough to preclude such 

issues.
A.19

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.4 Debris Accumulation and Head Loss 2.3.3 Strainer Blockage and Head Loss

1.3.4.1 ECC sump screen blockage should 

be evaluated based on the amount of debris 

estimated using the assumptions and 

criteria described in Regulatory Position 

1.3.2 and on the debris transported to the 

ECC sump per Regulatory Position 1.3.3. 

This volume of debris should be used to 

estimate the rate of accumulation of debris 

on the ECC sump screen.

2.3.3.1 Strainer blockage should be based 

on the amount of debris estimated using 

the assumptions and guidance described in 

Regulatory Position 2.3.1 and on the 

debris transported to the wetwell per 

Regulatory Position 2.3.2. This volume of 

debris, as well as other materials that 

could be present in the suppression pool 

prior to a LOCA, should be used to 

estimate the rate of accumulation of debris 

on the strainer surface.

2.3.3.2 The flow rate through the strainer 

should be used to estimate the rate of 

accumulation of debris on the strainer 

surface.

Guidance for PWR and 

BWR appears to be 

essentially the same for 

estimating the rate of 

accumulation of debris. 

The explicit guidance 

for BWR, specifying use 

of the flow rate to 

estimate rate of 

accumulation of debris, 

may be redundant, and 

could potentially be 

inconsistent with 

Regulatory Position 

2.3.3.1.

1.3.4.2 Consideration of ECC sump screen 

submergence (full or partial) at the time of 

switchover to ECCS should be given in 

calculating the available (wetted) screen 

area. For plants in which containment heat 

removal pumps take suction from the ECC 

sump before switchover to the ECCS, the 

available NPSH for these pumps should 

consider the submergence of the sump 

screens at the time these pumps initiate 

suction from the ECC sump. Unless 

otherwise shown analytically or 

experimentally, debris should be assumed 

to be uniformly distributed over the 

available sump screen surface. Debris mass 

should be calculated based on the amount 

of debris estimated to reach the ECC sump 

screen. (See Revision 1 of NUREG-0897, 

NUREG/CR-3616, and NUREG/CR6224.)

2.3.3.3 The suppression pool suction 

strainer area used in determining the 

approach velocity should conservatively 

account for blockage that may result. 

Unless otherwise shown analytically or 

experimentally, debris should be assumed 

to be uniformly distributed over the 

available suction strainer surface. Debris 

mass should be calculated based on the 

amount of debris estimated to reach or to 

be in the suppression pool. (See Revision 

1 of NUREG-0897, NUREG/CR-3616, 

and NUREG/CR-6224.)

Differences are due to 

differences between 

PWR and BWR 

systems; intent of 

guidance is the same for 

both designs. 

1.3.4.3 For fully submerged sump screens, 

the NPSH available to the ECC pumps 

should be determined using the conditions 

specified in the plant‘s licensing basis.

2.3.3.4 The NPSH available to the ECC 

pumps should be determined using the 

conditions specified in the plant‘s 

licensing basis.

Guidance for PWR and 

BWR appears to be 

essentially the same.
A.20

Regulatory Guide 1.82 (contd)

PWR BWR Difference(s)

1.3.4.4 For partially submerged sump 

[screen]s, NPSH margin may not be the 

only failure criterion, as discussed in 

Appendix A. For partially submerged 

sumps, credit should only be given to the 

portion of the sump screen that is expected 

to be submerged, as a function of time. 

Pump failure should be assumed to occur 

when the head loss across the sump screen 

(including only the clean screen head loss 

and the debris bed head loss) is greater 

than one-half of the submerged screen 

height or NPSH margin.

Guidance for PWR only; 

assumes BWR suction 

strainers are always 

fully submerged.

1.3.4.5 Estimates of head loss caused by 

debris blockage should be developed from 

empirical data based on the sump screen 

design (e.g., surface area and geometry), 

postulated combinations of debris (i.e., 

amount, size distribution, type), and 

approach velocity. Because debris beds 

that form on sump screens can trap debris 

that would pass through an unobstructed 

sump screen opening, any head loss 

correlation should conservatively account 

for filtration of particulates by the debris 

bed, including particulates that would pass 

through an unobstructed sump screen.

2.3.3.5 Estimates of head loss caused by 

debris blockage should be developed from 

empirical data based on the strainer design 

(e.g., surface area and geometry), 

postulated debris (i.e., amount, size 

distribution, type), and velocity. Any head 

loss correlation should conservatively 

account for filtration of particulates by the 

debris bed.

Guidance for PWR and 

BWR is essentially the 

same.

1.3.4.6 Consistent with the requirements of 

10 CFR 50.46, head loss should be 

calculated for the debris beds formed of 

different combinations of fibers and 

particulate mixtures (e.g., minimum 

uniform thin bed of fibers supporting a 

layer of particulate debris) based on 

assumptions and criteria described in 

Regulatory Positions 1.3.2 and 1.3.3.

2.3.3.5 Estimates of head loss caused by 

debris blockage should be developed from 

empirical data based on the strainer design 

(e.g., surface area and geometry), 

postulated debris (i.e., amount, size 

distribution, type), and velocity. Any head 

loss correlation should conservatively 

account for filtration of particulates by the 

debris bed.

Organization of 

guidance is different, but 

intent is same for both

PWR and BWR.
APPENDIX B

TIME-LINE OF EVOLUTION OF POST-LOCA ECCS SAFETY ISSUES IN 

COMMERCIAL LWRS
B.1

Appendix B: Time-Line of Evolution of Post-LOCA ECCS 

Safety Issues in Commercial LWRs

Date Event

January 

1979

Unresolved Safety Issue A-43, Containment Emergency Sump Performance 

(included in NUREG-0510, Identification of Unresolved Safety Issues Relating to 

Nuclear Power Plants); initiated to address concerns about adequate recirculation 

water following a LOCA for long-term cooling; initially raised for PWRs, 

technically also applied to BWRs, but scope later included BWRs as well.

October 

1985

USI A-43 considered resolved with publication of NUREG-0896 USI A-43 

Regulatory Analysis and NUREG-0897 Containment Emergency Sump 

Performance, revision of the Standard Review Plan (Section 6.2.2).

November 

1985

NRC issued Regulatory Guide 1.82, Revision 1, addressing Water Sources for 

Long-Term Recirculation Cooling Following a Loss-of-Coolant Accident.

December 

1985

NRC issued Generic Letter 85-22, Potential for Loss of Post-LOCA Recirculation 

Capability Due to Insulation Debris Blockage to inform licensees of BWRs and 

PWRs of the closure of USI A-43, the updates made to regulatory guidance for 

strainer performance, and the regulatory analysis which determined that backfitting 

the revised regulatory guidance on operating plants was not justified

1988,1989 Strainer blocking events at Grand Gulf Nuclear station (a BWR/6, Mark III plant) 

in Mississippi during testing of RHR pump suction strainers.

May 1992 Strainer clogging event at Perry Nuclear Station (a BWR/6, Mark III plant) in Ohio.

July 1992 Strainer blockage incident at Barsebäck Unit 2 BWR plant in Sweden (similar to 

BWR/4, Mark II containment).

May 1993 NRC issued Bulletin 93-02, Debris Plugging of Emergency Core Cooling Suction 

Strainers to all nuclear power plant licensees.

September 

1993

Second strainer clogging event at Perry Nuclear Station in Ohio;

strainers deformed due to excessive head loss in one incident at Perry.

January 

1994

Divers discover cloth-like material partially blocking ECCS strainers at Browns 

Ferry Unit 2 (a BWR/4, Mark I plant).

February 

1994

NRC issued Supplement 1 to Bulletin 93-02, requesting further interim actions by 

licensees.

August 

1994

NRC published draft-for-comment version of NUREG/CR-6224, Parametric 

Study of Potential for BWR ECCS Strainer Blockage Due to LOCA Generated 

Debris, documenting results of detailed study of BWR/4 Mark I ―reference plant,‖ 

initiated in September 1993.

September 

1995

Strainer blockage incident at Limerick Unit 1 (a BWR/4, Mark II plant);

pump cavitation in ECCS loop indicated at Limerick.

October 

1995

NRC published final report NUREG/CR-6224, Parametric Study of the Potential 

for BWR ECCS Strainer Blockage Due to LOCA Generated Debris.
B.2

Date Event

NRC issued Bulletin 95-02, Unexpected Clogging of a Residual Heat Removal 

Pump Strainer While Operating in Suppression Pool Cooling Mode to all operating 

BWR licensees, requesting licensees to take action to ensure that 

unacceptable build-up of debris in suppression pool would not occur during normal 

operation.

Time-Line of Post-LOCA ECCS Safety Issues (contd)

Date Event

March 

1996

Strainer blockage event at LaSalle plant (BWR/4; Mark II )

May 1996

NRC issued Revision 2 of Regulatory Guide 1.82, altering the debris blocking 

evaluation guidance for BWRs; operational events, analyses, and research work 

indicated that Revision 1 guidance was not comprehensive enough.

May 1996

NRC issued Bulletin 96-03, Potential Plugging of Emergency Core Cooling 

Suction Strainers by Debris in Boiling Water Reactors to all holders of operating 

licenses, requiring that they implement ―appropriate measures‖ to ensure ECCS 

performance following a LOCA.

November 

1996

BWROG submitted Utility Resolution Guidance in NEDO-32686, Rev. 0, Utility 

Resolution Guidance for ECCS Suction Strainer Blockage, for NRC review and 

approval.

October 

1997

NRC issued Generic Letter 97-04, Assurance of Sufficient Net Positive Suction 

Head for Emergency Core Cooling and Containment Heat Removal Pumps to all 

nuclear power plant licensees, requesting current information on net positive 

suction head (NPSH) analyses.

July 1998

NRC issued Generic Letter 98-04, Potential for Degradation of the Emergency 

Core Cooling System and Containment Spray System after Loss-of-Coolant 

Accident Because of Construction and Protective Coatings Deficiencies and 

Foreign Material in Containment, to all licensees of operating nuclear power plants 

requesting information on licensees‘ programs for ensuring that protective coatings 

do not detach from substrate during DBLOCA.

August 

1998

NRC issued Safety Evaluation on BWROG Utility Resolution Guidance, granting 

approval of the document, but with specific limitations.

(w)

October 

1998

BWROG published NEDO-32686-A, Utility Resolution Guidance for ECCS 

Suction Strainer Blockage, consisting of the original NEDO-32686, Revision 0, 

plus the SE from NRC and a summary of the elements NRC did not accept or 

approve.

 

(w) BWROG did not directly address the NRC limitations. NEDO-32686-A (October 1998) summarizes the 

guidelines that NRC did not accept and suggests that the individual utilities can address the relevant issues that 

pertain to their plant-specific remediation proposals when they submit them for NRC approval.
B.3

Time-Line of Post-LOCA ECCS Safety Issues (contd)

Date Event

1998

NRC defined Generic Safety Issue 191 (GSI-191), Assessment of Debris 

Accumulation on PWR Sump Performance in footnotes 1691 and 1692 of NUREG0933, in response to NRR request for re-examination of USI A-43, made in 1996;

initiated studies to determine if debris in containment after LOCA would impede 

ECCS operation in PWRs.

September 

1999

NRC published NUREG/CR-6368, Vol. 1, Drywell Debris Transport Study, 

documenting efforts to determine transport rates for debris generated in BWR 

containment.

March 

2001

NRC issued LA-UR-01-1595, BWR ECCS Strainer Blockage Issue: Summary of 

Research and Resolution Actions, a report summarizing technical basis for 

resolution of the BWR strainer blockage issue.

September 

2001

NRC issued review draft of NUREG/CR-6762, Vol. 1, GSI-191 Technical 

Assessment: Parametric Evaluations for Pressurized Water Reactor 

Recirculation Sump Performance, documenting technical basis showing that 

sump blockage is a credible concern for PWR operation.

October 

2001

NRC Memorandum Completion of Staff Reviews of NRC Bulletin 96-03 – Potential

Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling 

Water Reactors, and NRC Bulletin 95-02 – Unexpected Clogging of a Residual 

Heat Removal (RHR) Pump Strainer while Operating in Suppression Pool 

Cooling Mode, documenting closure of multi-plant actions (MPAs) related to NRC 

Bulletins 95-02 and 96-03

May 2002 PWR owners group submitted NEI 02-01, Condition Assessment Guidelines: 

Debris Sources Inside PWR Containments, to NRC for review and approval

August 

2002

NRC published NUREG/CR-6762, GSI-191 Technical Assessment:

Vol. 1, GSI-191 Technical Assessment: Parametric Evaluations for 

Pressurized Water Reactor Recirculation Sump Performance,

Vol. 2 Summary and Analysis of US Pressurized Water Reactor Industry 

Survey Responses and Responses to GL 97-04,

Vol. 3 Development of Debris Generation Quantities in Support of the 

Parametric Evaluation,

Vol. 4 Development of Debris Transport Fractions in Support of the 

Parametric Evaluation.

February 

2003

NRC published NUREG/CR-6808, Knowledge Base for Effect of Debris on 

Pressurized Water Reactor Emergency Core Cooling Sump Performance.

November 

2003

NRC issued Revision 3 of Regulatory Guide 1.82, clarifying and reconciling the 

requirements for ensuring ECCS water supply in BWRs and PWRs.

May 2004 PWR owners group submitted NEI 04-07, Pressurized Water Reactor Sump 

Performance Evaluation Methodology, to NRC for review and approval.

September 

2004

NRC issued Generic Letter 2004-02, Potential Impact of Debris Blockage on 

Emergency Recirculation During Design Basis Accidents at Pressurized Water 

Reactors, to all licensees.
B.4

Time-Line of Post-LOCA ECCS Safety Issues (contd)

Date Event

November 

2004

NRC issued SE on NEI 04-07, approving the general Baseline Methodology for 

sump performance issues, with certain reservations and requirements for additional 

work.

December 

2004

NRC issued Volume 2 – Safety Evaluation by the Office of Nuclear Reactor 

Regulation Related to NRC Generic Letter 2004-02, Revision 0, December 6, 

2004

July 2005

NRC published NUREG/CR-6877, Characterization and Head Loss Testing of 

Latent Debris from Pressurized Water Reactor Containment Buildings.

Study to quantify parameters critical to application of NUREG/CR-6224 head loss 

correlation for sump screens.

August 22, 

2006

NRC agreed to review PWROG technical report WCAP-16406-P, Rev. 1, 

Evaluation of Downstream Sump Debris Effects in Support of GSI-191.

December 

2006

NRC published NUREG/CR-6917, Experimental Measurements of Pressure 

Drop Across Sump Screen Debris Beds in Support of Generic Safety Issue 191.

June 2007

WCAP-16793-NP, Evaluation of Long-Term Cooling Considering Particulate, 

Fibrous and Chemical Debris in the Recirculating Fluid, submitted by the PWR 

Owners Group in response to NRC staff determination in the review of WCAP16306-P that additional information is needed on downstream effects of debris on 

primary system components

September 

12, 2007

NRC agreed to review PWROG technical report WCAP-16530-NP, Evaluation of 

Post-Accident Chemical Effects in Containment Sump Fluids to Support GSI191, February 2006.

September 

2007

NRC issued draft guidance for review of licensee responses to GL 2004-02.

November 

2007

NRC issued revised content guide for GL 2004-02 supplemental responses.

December 

2007

NRC approved PWROG guidance reports WCAP-16406-P, Rev. 1, Evaluation of 

Post-Accident Chemical Effects in Containment Sump Fluids to Support GSI191, and WCAP-16530-NP, Evaluation of Post-Accident Chemical Effects in 

Containment Sump Fluids to Support GSI-191, both with supporting 

supplemental information (see Section 4 of SEs).
B.5

Time-Line of Post-LOCA ECCS Safety Issues (contd)

Date Event

March 

2008

NRC issued revised guidance for review of final licensee responses to GL 2004-02, 

via letter dated March 28, 2008 to A.R. Pietrangelo of Nuclear Energy Institute 

from W.H. Ruland, NRC; subject: Revised Guidance for Review of Final Licensee 

Response to Generic Letter 2004-02, Potential Impact of Debris Blockage on 

Emercency Recirculation During Desing Basis Accidents at Pressurized-Water 

Reactors:

- Enclosure I: NRC Staff Review Guidance Regarding Generic Letter 

2004-02 Closure in the Area of Strainer Head Loss and Vortexing

- Enclosure II: NRC Staff Review Guidance Regarding Generic Letter 

2004-02 Closure in the Area of Coatings Evaluation

- Enclosure III: NRC Staff Review Guidance Regarding Generic Letter 

2004-02 Closure in the Area of Plant-Specific Chemical Effect 

Evaluations

May 2008 NRC developed draft white paper on differences in treatment of containment sump 

screen and suppression pool suction strainer clogging issues for PWRs and BWRs.

June 2008 NRC contracted with PNNL for study of disparate treatment of debris issues in 

BWRs and BWRs.

July 24, 

2008

NRC issued a recision of the draft SE for WCAP-16793-NP, having determined 

that additional information was needed on certain subject areas in the report, which 

the PWROG agreed to address by submitting Revision 1of WCAP-16793-NP.

April 2009

WCAP-16793-NP, Revision 1, Evaluation of Long-Term Cooling Considering 

Particulate, Fibrous and Chemical Debris in the Recirculating Fluid, submitted by 

the PWR Owners Group in response to NRC staff determination that additional 

information was needed to supplement Revision 0 of the report.