POLICY ISSUE

(Information)

March 31, 2006 SECY-06-0078

FOR: The Commissioners

FROM: Luis A. Reyes

Executive Director for Operations /RA/

SUBJECT: STATUS OF RESOLUTION OF GSI-191, “ASSESSMENT OF [EFFECT OF]

DEBRIS ACCUMULATION ON PWR SUMP PERFORMANCE”

PURPOSE:

The purpose of this paper is to inform the Commission of the status of the resolution of Generic

Safety Issue (GSI) 191, “Assessment of [Effect of] Debris Accumulation on Pressurized-Water

Reactor (PWR) Sump Performance.” This paper does not address any new commitments.

SUMMARY:

The industry is making progress in developing plant-specific specifications for the type and size

of PWR containment emergency core cooling system (ECCS) sump screens. Most PWR

licensees intend to substantially enlarge the sump screens with passive designs, and the

remaining licensees are planning to replace sump screens with active designs. As of

March 24, 2006, the staff has received five requests to delay installation of sump screen

modifications beyond December 31, 2007, and intends to evaluate those requests with the

criteria provided in this paper.

This paper discusses (1) the status of the resolution of GSI-191, (2) staff plans for resolving

GSI-191, and (3) staff activities for communicating Nuclear Regulatory Commission (NRC)

expectations to licensees.

CONTACT: Hanry Wagage, NRR/DSS/SSIB

 301-415-1840
The Commissioners 2

BACKGROUND:

GSI-191 concerns the possibility that debris generated by a loss-of-coolant accident (LOCA)

could accumulate on the ECCS sump screen, resulting in a loss of net positive suction head

margin. Debris passing through the screen may degrade downstream components such as

pumps, valves, and heat exchangers or plug or restrict heat exchanger or fuel flow channels. 

These phenomena may prevent the ECCS from meeting the criteria of Section 50.46 of

Title 10 of the Code of Federal Regulations, “Acceptance Criteria for Emergency Core Cooling

Systems for Light-Water Nuclear Power Reactors.” Section 50.46(b)(5) requires that licensees

design their ECCS systems with capability for long-term cooling. After a successful system

initiation, the ECCS must be able to provide cooling to maintain the core temperature at an

acceptably low value for a sufficient duration.

During a high-energy primary coolant line break inside the containment of a PWR, energetic

pressure waves and fluid jets would impinge on materials near the break such as thermal

insulation, coatings, and concrete, damaging and dislodging them. In addition to debris

generated by jet forces from the pipe rupture or latent debris in containment entrained by break

or spray water flows, 

chemical

products could be created by reactions between the materials in containment and the

containment environment following a LOCA. A fraction of the generated debris, latent debris,

disbonded coatings, and chemical products might be transported to the pool of water on the

containment floor. For a number of postulated LOCA scenarios, the ECCS and containment

spray system (CSS) pumps take suction from the recirculation sump, and the debris in the

containment pool could subsequently accumulate on the sump screen or be transported into the

ECCS and CSS. 

The accumulation of the debris on the sump screen would create a debris bed, which would

increase the head loss across the screen through a filtering action. If enough debris

accumulated, the debris bed could reach a critical thickness such that the head loss through the

screens would exceed the net positive suction head margin required to ensure the successful

operation of the ECCS and CSS pumps in the recirculation mode. This sump screen clogging

could result in severely degraded pump performance, eventual pump failure, and loss of the

ECCS and CSS function.

Debris that passes through the sump screen could plug or cause excessive wear of

close-tolerance components, deposit on surfaces within the ECCS or CSS systems, or impede

coolant flow or cooling capability within the reactor vessel. These phenomena are referred to

as “downstream effects.” The plugging or wear might cause a component to degrade to the

point where it could not perform its designated function (e.g., pump fluid, maintain system

pressure, remove heat, or pass and control system flow). Debris that passes through the

recirculation sump screen could lodge at a downstream flow restriction such as a high-pressure

safety injection throttle valve or the fuel assembly inlet area. Debris blockage at such flow
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restrictions would impede or prevent the recirculation of coolant through the reactor core,

leading to inadequate core cooling. Debris could also deposit on the reactor fuel, reducing the 

efficiency of decay heat transfer to the recirculating fluid. Debris blockage at flow restrictions in

the CSS flowpath, such as containment spray nozzles, would impede or prevent CSS

recirculation, leading to inadequate containment heat removal. Debris could also accumulate in

close-tolerance subcomponents of pumps and valves. The effect could be either to plug the

subcomponent, thereby rendering the component unable to perform its function, or to wear

critical close-tolerance subcomponents to the point of degrading component or system function.

Containment coating debris is generated from destruction of coatings within the zone of

influence and from postulated failure of degraded qualified coatings and unqualified coatings

outside the zone of influence. The zone of influence is the volume of space affected by the

impact of energetic pressure waves and fluid jets from a high-energy line break.

In 2004, the staff developed a generic letter (GL) to assist the resolution of GSI-191. In a

June 1, 2004, letter commenting on a draft of the proposed GL, the Nuclear Energy Institute

(NEI) requested a 5-month extension of the due date (from April 1, 2005, to

September 1, 2005) for the requested information, in part to allow licensees time to address

chemical effects (ML041550866). The NRC granted this request and changed the due date for

licensees submitting the information requested in the GL to September 1, 2005.

On September 13, 2004, the NRC issued the final version of the GL as GL 2004-02, “Potential

Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at

Pressurized-Water Reactors (PWRs).” This GL requested all PWR licensees (1) to use an

NRC-approved methodology to perform a mechanistic evaluation of the potential for postaccident debris blockage and operation with debris-laden fluids to impede or prevent the

recirculation functions of ECCS and CSS following all postulated accidents for which these

recirculation functions are required and (2) to implement plant modifications or other corrective

actions which the evaluation identifies as necessary to ensure system functionality by

December 31, 2007. If they could not complete all corrective actions by December 31, 2007,

licensees were asked to describe how they would meet the applicable regulatory requirements

referenced in the GL until the corrective actions were completed.

On December 6, 2004, the staff issued a safety evaluation of a May 28, 2004, NEI report

(ML041550279) on “Pressurized Water Reactor Containment Sump Evaluation Methodology.” 

The NEI report, in conjunction with the staff’s safety evaluation (ML043280007 and

ML043280008), provided a method acceptable to the staff for evaluating PWR sump

performance as requested in the GL.

DISCUSSION:

Status of Licensee GL responses

The GL requested licensees to provide a significant amount of information by

September 1, 2005, so that the NRC would have assurance that effective corrective actions

were being taken. The September 2005 licensee GL responses revealed that licensees of 66

of the 69 PWRs committed to replace sump screens; the licensees of the remaining 3 PWRs

had previously replaced their sump screens.
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However, despite the fact that the staff gave the industry an additional 5 months to respond to

the GL at the NEI’s request, much of the information submitted by licensees was incomplete. 

For example, the September 2005 licensee GL responses revealed that licensees did not

appear to have made significant progress in addressing chemical, downstream, and coatings

issues. Two PWR licensees indicated that they would not meet the December 2007 deadline

for completing sump modifications.

The staff recently sent requests for additional information to PWR licensees to supply the

information missing from the September 2005 GL responses. In a letter dated

February 28, 2006, NEI stated that the effort necessary to prepare responses to the requests

for additional information on the originally requested schedule would divert resources and

attention from the plant strainer modifications and jeopardize the current modification

schedules. The staff agreed with the NEI proposal and licensees will now provide supplements

to their GL responses by December 31, 2006, for licensees that complete sump screen

modifications by that date; and within 90 days of outage completion (but not later than

December 31, 2007) for licensees that complete sump screen modifications after 2006.

Criteria for Evaluating Delay of Hardware Changes

The industry has been developing an understanding of certain aspects of chemical and

coatings behavior in a post-LOCA environment and establishing an acceptable methodology for

evaluating downstream effects. We believe that some licensees may find it difficult to meet the

December 2007 deadline for installing a sump screen system that fully demonstrates

conformance with the functional requirements. Several licensees have either requested or are

planning to request extensions to delay implementation of sump modifications to beyond

December 31, 2007. Although the GL stated that the new mechanistic licensing basis should

be effective on December 31, 2007, when modifications were to be completed, the staff will

consider reasonable extension requests to delay the implementation of final hardware

modifications. Provided such extension requests are found acceptable, the new mechanistic

licensing basis will be invoked after the modifications are completed. 

Proposed extensions to permit changes at the next outage of opportunity after December 2007

may be acceptable if, based on the licensee’s request, the staff determines that:

• the licensee has a plant-specific technical/experimental plan with milestones and

schedule to address outstanding technical issues with enough margin to account for

uncertainties and

• the licensee identifies mitigative measures to be put in place prior to

December 31, 2007, and adequately describes how these mitigative measures will

minimize the risk of degraded ECCS and CSS functions during the extension period.

For proposed extensions beyond several months, a licensee’s request will more likely be

accepted if the proposed mitigative measures include temporary physical improvements to the

ECCS sump or materials inside containment to better ensure a high level of ECCS sump

performance.
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Status of Research Activities

The NRC, in collaboration with the Electric Power Research Institute, conducted testing to

address concerns about the formation of chemical reaction products in the ECCS containment

pool. The testing showed that (1) chemical products/precipitates and gelatinous-like material

can form under certain chemical environments in a PWR containment pool during the

post-LOCA recirculation phase and (2) changes to important containment parameters such as

pool temperature and pH, insulation debris type, and debris concentrations can affect the type

and nature of chemical byproducts that form.

A series of NRC-sponsored head loss tests are underway to determine the potential for

chemical products to increase the head loss associated with sump screen debris beds. The

first test series is investigating the head loss caused by the formation of calcium phosphate due

to the reaction of dissolved calcium in environments buffered with trisodium phosphate. 

Calcium silicate insulation is a prominent source of dissolved calcium in some plants, but other

insulation materials and uncoated concrete are also sources. The objective of this test series is

to realistically encompass expected post-LOCA containment pool conditions. 

The initial results demonstrate that there could be a significant head loss contribution from

chemical products associated with calcium silicate/trisodium phosphate containment pool

environments. Testing of calcium silicate concentrations representative of reported plant

conditions has resulted in significant head loss in these chemical environments due to the

formation of chemical products. The NRC issued Information Notice 2005-26 (ML052570220)

and follow-on Supplement 1 (ML060170102) to provide the results of head loss tests in an

environment containing calcium silicate and trisodium phosphate. The staff is continuing head

loss testing for chemical byproducts that developed in simulated PWR sump pool environments

that use chemical species other than trisodium phosphate to buffer pH. This testing will be

completed in spring 2006.

In addition to research on chemical effects, research is being conducted to address concerns

about head loss from debris and about coatings transport. Testing is underway to evaluate

head loss associated with standard PWR containment debris materials and to provide data that

will be used to develop analytical head loss correlations. Testing is also underway to evaluate

the transportability of coating chips to the sump screen and to understand chip characteristics

that may affect transportability. Both testing activities will be completed in spring 2006.

Public Meetings With Licensees, Industry, and Advisory Committee on Reactor Safeguards

(ACRS) 

The staff has held regular public meetings with PWR licensees to share information on the

NRC-sponsored test program and to provide feedback on related industry activities. Such

meetings were held in December 2004 and January, April, June, September, and

November 2005. On February 2, 2006, the staff met with the Palisades nuclear power plant

licensee to discuss its plan to temporarily remove trisodium phosphate buffering agent from the

containment. This change is intended to reduce uncertainty with regard to sump screen 
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performance resulting from chemical effects associated with calcium silicate/trisodium

phosphate until the licensee makes a permanent change to the screen, plant insulation

material, and/or the buffering agent. During the meeting the staff asked questions regarding

various technical aspects of the proposed change. The licensee stated that they would 

address these aspects in a license amendment request. 

On February 9, 2006, the staff held a public meeting with the industry regarding the status of

resolution of GSI-191. The staff expressed concerns with the September 2005 licensee

responses to the GL and reemphasized the need for the licensees to meet the December 2007

date for completing sump performance modifications and corrective actions. The staff and

industry discussed results from recent chemical effects head loss testing for calcium

silicate/trisodium phosphate environments and the status of ongoing research. Also, the

licensees of 6 PWRs using calcium silicate insulation and trisodium phosphate buffering agent

inside containment discussed actions taken or planned relevant to the Information Notice

2005-26 Supplement. The staff considered the meeting effective in communicating the staff’s

concerns and expectations regarding licensee GL responses.

The staff continues to interact with the ACRS on GSI-191. In February 2006, the staff met with

the ACRS Thermal-Hydraulics Phenomena Subcommittee to discuss progress and future plans

on GSI-191 issues. The staff briefed the ACRS Full Committee on March 9, 2006. On

March 24, 2006, the ACRS provided a letter to the Commission on GSI-191. In general, the

letter supports the staff’s emphasis on near-term improvements to containment sumps to

reduce the risk of sump screen clogging. In addition, the ACRS calls for the staff to develop

improved predictive methods and guidance in several technical areas related to GSI-191. The

staff is reviewing the letter and developing a response.

Audits of Selected Licensees

The staff is conducting audits to verify the adequacy of sump modifications and corrective

actions in response to the GL. The staff audited two volunteer pilot plants (Crystal River and

Fort Calhoun) in 2005. The pilot audits were intended to contribute to the resolution of GSI-191

by providing timely feedback on the implementation of the NRC-approved methodology. For

example, the first pilot audit conducted at Crystal River revealed that the licensee used an

approach that seemed reasonable to design replacement sump screens. The staff is currently

conducting two audits (at Oconee and Watts Bar) and plans to conduct six more audits during

2006 and 2007.

RESOURCES:

The staff plans for resolving GSI-191 discussed in this paper are not expected to require

additional resources. The impact of the application of the revised GL extension criteria on NRC

resources is expected to be minimal in FY 2007 and beyond.
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COORDINATION:

The Office of the General Counsel has no legal objection to this paper.

/RA/

Luis A. Reyes

Executive Director 

 for Operations
The Commissioners 7

COORDINATION:

The Office of the General Counsel has no legal objection to this paper.

/RA/

Luis A. Reyes

Executive Director 

 for Operations

Accession No.: ML053620174 SECY-012

OFFICE NRR/DSS/SSIB Tech Editor NRR/DPR/PSPB NRR/DSS/SSIB: NRR/DSS: DD

NAME HAWagage PEKleene JBHopkins DLSolorio JNHannon

DATE 01/13/2006 01/23/2006 01/18/2006 01/17/2006 01/19/2006

OFFICE NRR/DCI: D NRR/DSS: D OGC RES: D NRR: D

NAME JAGrobe TOMartin APHodgdon CJPaperiello

(JTWiggins for)

JEDyer

DATE 02/24/2006 02/24/2006 01/17/2006 01/27/2006 03/27/2006

OFFICE EDO

NAME LAReyes

DATE 03/31/2006

OFFICIAL RECORD COPY