NUREG/IA-0417 

International 

 Agreement Report

Post-Test Thermal-Hydraulic Analysis of

PKL Tests F1.1 and F1.2

 

Prepared by:

J. Freixa, C. Pretel, L. Batet, F. Reventós

Institute of Energy Technologies 

Department of Physics and Nuclear Engineering 

Technical University of Catalonia 

ETSEIB, Av. Diagonal 647, Pav. C 

08028 Barcelona Spain 

K. Tien, NRC Project Manager

 

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: April 2011

Date Published: October 2014

Prepared as part of 

The Agreement on Research Participation and Technical Exchange

Under the Thermal-Hydraulic Code Applications and Maintenance Program (CAMP)

Published by

U.S. Nuclear Regulatory Commission
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 NUREG/IA-0417

Post-test Thermal-Hydraulic analysis of PKL 

Tests F1.1 and F1.2 

Manuscript Completed: April 2011 

Date Published: October 2014 

Prepared by

J. Freixa, C. Pretel, L. Batet, F. Reventós 

Institute of Energy Technologies 

Department of Physics and Nuclear Engineering

Technical University of Catalonia

ETSEIB, Av. Diagonal 647, Pav. C 

08028 Barcelona 

Spain

Prepared for:

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

iii

ABSTRACT

Two different post-test analyses have been performed by making use of the UPC’s model of the 

PKL facility. The nodalization has been developed using the RELAP5mod3.3 code and two tests 

of the OECD/PKL project, namely Test F1.1 and F1.2, have been simulated. For both 

transients, the simulations correctly reproduce the finalization of natural circulation and the 

beginning of the reflux condensation conditions with the consequent formation of low borated 

water slugs in the loop seals due to the phenomenon of boron dilution. 

Test F1.1 is a Small Break LOCA in the cold leg assuming the non-availability of the LPI system 

and the accumulators while Test F1.2 simulates a series of steady state situations for different 

primary inventories in order to study the phenomenon of boron dilution as a function of the mass 

inventory.

Different sensitivity analyses are also reported and the nodalization of the U-tubes bundle is 

discussed for both scenarios. The results cover the formation of slugs in the loop seals under 

reflux and condensation conditions and their movement through the loop when the U-tubes are 

refilled and natural circulation is re-established.

v

FOREWORD

Extensive knowledge and techniques have been produced and made available in the field of 

thermal-hydraulic responses during reactor transients and accidents, and major system 

computer codes have achieved a high degree of maturity through extensive qualification, 

assessment and validation processes. Best-estimate analysis methods are increasingly used in 

licensing, replacing the traditional conservative approaches. Such methods include an 

assessment of the uncertainty of their results that must be taken into account when the safety 

acceptance criteria for the licensing analysis are verified. 

Traditional agreements between the Nuclear Regulatory Commission of the United States of 

America (USNRC) and the Consejo de Seguridad Nuclear of Spain (CSN) in the area of nuclear 

safety research have given access to CSN to the NRC-developed best estimate 

thermalhydraulic codes RELAP5, TRAC-P, TRAC-B, and currently TRACE. These complex 

tools, suitable state-of-the-art application of current two-phase flow fluid mechanics techniques 

to light water nuclear power plants, allow a realistic representation and simulation of 

thermalhydraulic phenomena at normal and incidental operation of NPP. Owe to the huge 

required resources, qualification of these codes have been performed through international 

cooperation programs. USNRC CAMP program (Code Applications and Maintenance Program) 

represents the international framework for verification and validation of NRC TH codes, allowing 

to:

• Share experience on code errors and inadequacies, cooperating in resolution of 

deficiencies and maintaining a single, internationally recognized code version.

• Share user experience on code scaling, applicability, and uncertainty studies.

• Share a well documented code assessment data base.

• Share experience on full scale power plant safety-related analyses performed with codes 

(analyses of operating reactors, advanced light water reactors, transients, risk-dominant 

sequences, and accident management and operator procedures-related studies).

• Maintain and improve user expertise and guidelines for code applications.

Since 1984, when the first LOFT agreement was settled down, CSN has been promoting 

coordinated joint efforts with Spanish organizations, such as UNESA (the association of 

Spanish electric energy industry) as well as universities and engineering companies, in the aim 

of assimilating, applying, improving and helping the international community in the validation of 

these TH simulation codes1

, within different periods of the associated national programs (e.g., 

CAMP-España). As a result of these actions, there is currently in Spain a good collection of 

productive plant models as well as a good selection of national experts in the application of TH 

simulation tools, with adequate TH knowledge and suitable experience on their use.

Many experimental facilities have contributed to the today’s availability of a large thermalhydraulic database (both separated and integral effect tests). However there is continued need 

for additional experimental work and code development and verification, in areas where no 

emphasis have been made along the past. On the basis of the SESAR/FAP2 reports “Nuclear 

Safety Research in OECD Countries:Major Facilities and Programmes at Risk” (SESAR/FAP, 

2001) and its 2007 updated version “Support Facilities for Existing and Advanced Reactors 

(SFEAR) NEA/CSNI/R(2007)6”, CSNI is promoting since 2001 several collaborative 

international actions in the area of experimental TH research. These reports presented some 

findings and recommendations to the CSNI, to sustain an adequate level of research, identifying 

 

1 It’s worth to note the emphasis made in the application to actual NPP incidents. 2

 SESAR/FAP is the Senior Group of Experts on Nuclear Safety Research Facilities and Programmes of NEA Committee on the 

Safety of Nuclear Installations (CSNI).
vi

a number of experimental facilities and programmes of potential interest for present or future 

international collaboration within the safety community during the coming decade. 

CSN, as Spanish representative in CSNI, is involved in some of these research activities, 

helping in this international support of facilities and in the establishment of a large network of 

international collaborations. In the TH framework, most of these actions are either covering not 

enough investigated safety issues and phenomena (e.g., boron dilution, low power and 

shutdown conditions), or enlarging code validation and qualification data bases incorporating 

new information (e.g., multi-dimensional aspects, non-condensable gas effects). In particular, 

CSN is currently participating in the PKL and ROSA programmes.

The PKL is an important integral test facility operated by of AREVA-NP in Erlangen (Germany), 

and designed to investigate thermal-hydraulic response of a four-loop Siemens designed PWR. 

Experiments performed during the PKL/OECD program have been focused on the issues:

• Boron dilution events after small-break loss of coolant accidents.

• Loss of residual heat removal during mid-loop operation (both with closed and open 

reactor coolant system.

ROSA/LSTF of Japan Atomic Energy Research Institute (JAERI) is an integral test facility 

designed to simulate a 1100 MWe four-loop Westinghouse-type PWR, by two loops at fullheight and 1/48 volumetric scaling to better simulate thermal-hydraulic responses in large-scale 

components. The ROSA/OECD project has investigated issues in thermal-hydraulics analyses 

relevant to water reactor safety, focusing on the verification of models and simulation methods 

for complex phenomena that can occur during reactor transients and accidents such as:

• Temperature stratification and coolant mixing during ECCS coolant injection

• Water hammer-like phenomena

• ATWS

• Natural circulation with super-heated steam

• Primary cooling through SG depressurization

• Pressure vessel upper-head and bottom break LOCA

This overall CSN involvement in different international TH programmes has outlined the scope 

of the new period of CAMP-España activities focused on:

• Analysis, simulation and investigation of specific safety aspects of PKL/OECD and 

ROSA/OECD experiments.

• Analysis of applicability and/or extension of the results and knowledge acquired in these 

projects to the safety, operation or availability of the Spanish nuclear power plants.

Both objectives are carried out by simulating experiments and plant application with the last 

available versions of NRC TH codes (RELAP5 and TRACE). A CAMP in-kind contribution is 

aimed as end result of both types of studies.

Development of these activities, technically and financially supported by CSN, is being carried 

out by 5 different national research groups (Technical Universities of Madrid, Valencia and 

Cataluña). On the whole, CSN is seeking to assure and to maintain the capability of the national 

groups with experience in the thermal hydraulics analysis of accidents of the Spanish nuclear 

power plants.

Francisco Fernández Moreno, Commissioner

Consejo de Seguridad Nuclear (CSN)
vii

CONTENTS

Page

ABSTRACT................................................................................................................................iii

FOREWORD ..............................................................................................................................v

CONTENTS..............................................................................................................................vii 

FIGURES...................................................................................................................................ix

TABLES......................................................................................................................................x

EXECUTIVE SUMMARY ...........................................................................................................xi

ABBREVIATIONS.................................................................................................................... xiii

ACKNOWLEDGMENTS............................................................................................................ xv 

1 INTRODUCTION............................................................................................................... 1-1

 1.1 Boron Dilution Events ............................................................................................... 1-1

 1.2 Definitions of Boron Dilution...................................................................................... 1-2

 1.3 PKL Test Facility....................................................................................................... 1-2

 1.4 PKL II OECD (Organization for Economic Co-operation and Development) Project.. 1-4

 1.5 Code and Nodalization.............................................................................................. 1-7

 1.6 Notes about Boron Transport Models........................................................................ 1-9

2 TEST F1.1......................................................................................................................... 2-1

 2.1 Boundary Conditions................................................................................................. 2-1

 2.2 Conditioning Phase................................................................................................... 2-1

 2.3 Initial Conditions at Test Start ................................................................................... 2-3

 2.4 Test Phase ............................................................................................................... 2-4

 2.5 Analysis of Results Concerning Boron Dilution ....................................................... 2-11

 2.5.1 Formation of the Boron Diluted Slugs ........................................................... 2-11

 2.5.2 Evolution of the Boron Diluted Slugs Prior NC Establishment....................... 2-12

 2.5.3 Establishment of Natural Circulation............................................................. 2-14

3 TEST F1.2......................................................................................................................... 3-1

 3.1 Boundary Conditions................................................................................................. 3-1

 3.2 Initial Conditions at t=0s............................................................................................ 3-1

 3.3 Nodalization with 5 U-tubes ...................................................................................... 3-2

 3.4 Test Phase ............................................................................................................... 3-2

 3.5 Analysis of Results Concerning Boron Dilution ......................................................... 3-9

4 SENSITIVITY STUDIES ON TESTS F1.1 AND F1.2......................................................... 4-1
viii

 4.1 Upper Head Bypass Nodalization ............................................................................. 4-1

 4.2 Nodalizing 5 U-tubes................................................................................................. 4-3

5 CONCLUSIONS................................................................................................................ 5-1

6 RUN STATISTICS............................................................................................................. 6-1

7 REFERENCES ................................................................................................................. 7-1
ix

FIGURES

Page

1.1 Diagram of the PKL test facility ..................................................................................... 1-3

1.2 RELAP5 nodalization for the PKL test facility. Only loops 1 and 2 are shown................ 1-8

2.1 Primary and secondary pressures, conditioning phase of test F1.1............................... 2-2

2.2 Mass flow in loop 4, conditioning phase ........................................................................ 2-2

2.3 Boron concentration in the loop seal of loop 4, conditioning phase................................ 2-3

2.4 Primary and secondary pressures for test F1.1 ............................................................. 2-6

2.5 RPV level for test F1.1 during the conditioning and the test phases .............................. 2-7

2.6 Pressurizer level for test F1.1........................................................................................ 2-7

2.7 Break mass flow for test F1.1........................................................................................ 2-8

2.8 Mass flow in all loops for test F1.1................................................................................. 2-9

2.9 Boron concentration at the COMBOs located in: cold leg 2, LS 4 and both sides of 

cold leg 4. Test F1.1 ................................................................................................... 2-10

2.10 Post test calculation of test F1.1, evolution of boron concentration in loop 4. Left: 

start of transient. Right: reflux-condenser conditions established ................................ 2-11

2.11 Post test calculation of test F1.1, evolution of boron concentration in loop 4. Left 

and right: Boron dilution in hot leg and LS................................................................... 2-12

2.12 Post test calculation of test F1.1, evolution of boron concentration in loop 4. Left: 

ECC flows to LS. Right: upward displacement of the slug ........................................... 2-12

2.13 Post test calculation of test F1.1, evolution of boron concentration in loop 4. Left: 

upward displacement of the slug continues. Right: the slug is moved back to the 

LS. .............................................................................................................................. 2-13

2.14 Post test calculation of test F1.1, evolution of boron concentration in loop 4. Left 

and right: intermittent forward and backward movements of the slug. ......................... 2-14

2.15 Post test calculation of test F1.1, evolution of boron concentration in loop 4. Left 

and right: intermittent forward and backward movements of the slug. ......................... 2-14

2.16 Post test calculation of test F1.1, evolution of boron concentration in loop 4. Left 

and right: establishment of natural circulation. ............................................................ 2-15

3.1 Primary and secondary pressures for Test F1.2............................................................ 3-3

3.2 Pressurizer collapsed water level for Test F1.1 ............................................................. 3-4

3.3 Total main steam flow for Test F1.2 .............................................................................. 3-4

3.4 Primary mass inventory for Test F1.2............................................................................ 3-5

3.5 Break mass flow for Test F1.2....................................................................................... 3-6

3.6 RPV collapsed water level for Test F1.2........................................................................ 3-6

3.7 Mass flow of loops 1 to 4 for Test F1.2.......................................................................... 3-7

3.8 U-tube collapsed water levels of loops 1 to 4 for Test F1.2............................................ 3-8

3.9 Boron concentration at each COMBO measurement for Test F1.2................................ 3-9

4.1 Break mass flow during conditioning phase of Test F1.1, upper head bypass 

sensitivity ...................................................................................................................... 4-1

4.2 Downcomer Collapsed water level during the conditioning phase of Test F1.1, 

upper head bypass sensitivity ....................................................................................... 4-2

4.3 Primary pressure of Test F1.1, upper head bypass sensitivity....................................... 4-2

4.4 Primary pressure for Test F1.1 with 1 and 5 U-tube elevations...................................... 4-4

4.5 Mass flow at loop 4 for test F1.1 with 1 and 5 U-tube elevations ................................... 4-5

4.6 Loop mass flows for Test F1.2 with 1 and 5 U-tubes ..................................................... 4-6

4.7 Boron concentration in all LSs for Test F1.2 with 1 and 5 U-tubes................................. 4-7

6.1 CPU time usage for Test F1.1 ....................................................................................... 6-1

6.2 CPU time usage for Test F1.2 using 1 and 5 pipes to simulate the U-tube bundle ........ 6-2
x

TABLES

Page

1.1 Tests on boron dilution events performed at the PKL test facility in the recent 

years.......................................................................................................................... 1-6

2.1 Initial conditions of Test F1.1, experimental and calculated data ................................ 2-4

2.2 Chronology of the main events in Test F1.1 ............................................................... 2-5

3.1 Initial conditions of Test F1.2, experimental and calculated data ................................ 3-2

4.1 CPU times using 1 and 5 U-tubes. Calculations performed on a PC, Pentium 4 at 

3.4GHz and 512 RAM................................................................................................ 4-8
xi

EXECUTIVE SUMMARY

This report shows the development and the improvement of the UPC’s model of the PKL test 

facility that has been developed using the RELAP5mod3.3 system code and used for the 

simulation of the two OECD/PKL project tests F1.1 and F1.2.. For both transients, the 

simulations correctly reproduce the natural circulation finalization and the beginning of the reflux 

condensation conditions with the consequent formation of low borated water slugs in the loop 

seals due to the phenomenon of boron dilution.

In Test F1.1 (Small Break LOCA in cold leg assuming non-availability of the LPI system and the 

accumulators), the results of a sensitivity calculation shows the influence of the nodalization of 

the head of the vessel bypass on parameters like downcomer levels and mass flow at the break. 

Based on the good results obtained, the report also displays graphically the formation of the 

slugs in the loop seals under reflux and condensation and their movement through the loop 

during the process of system refilling and start of natural circulation.

In Test F1.2 (simulating various steady state situations for different primary inventories in order 

to study the phenomenon of boron dilution), a sensitivity analysis on the U-tube nodalization is 

reported. The conclusion is that the simulation of U-tubes of different lengths is only relevant for 

transients in which, for a considerable period of time, the water level stabilizes around the 

middle of the U-tube bundle or experiences very slow variations such that natural circulation 

may take place exclusively within the shorter U-tubes. In contrast, this development can result in 

an important increase of computing time for the rest of transients with no improvements to the 

simulation.

xiii

ABBREVIATIONS

Acc Accumulator

AFW Auxiliary FeedWater

COMBO COntinious Measurement of BOron concentration

CVCS Chemical and Volume Control System

DC DownComer

ECCS Emergency Core Cooling Systems

EOP Emergency Operational Procedure

HPSI High Pressure Safety Injection

INTE Institut de Tècniques Energμetiques (Institute of Energy Technologies)

LOCA Loss Of Coolant Accident

LP Lower Plenum

LPSI Low Pressure Safety Injection

LS Loop Seal

MS Main Steam

NC Natural Circulation

NPP Nuclear Power Plant

OECD Organization for Economic Cooperation and Development

PKL Primärkreislauf (Primary System)

PRZ Pressurizer

PS Primary System

PWR Pressurized Water Reactors

RC Reflux-Condensation

RCP Reactor Coolant Pump

RCS Reactor Coolant System

RELAP Reactor Excursion Leak Analysis Program

RHR Residual Heat Removal

RPV Reactor Pressure Vessel

SBLOCA Small Break LOCA

SESAR Senior Group of Experts on Nuclear Safety Research

SETH SESAR Thermal Hydraulics

SG Steam Generator

SIP Safety Injection Pump

SOT Start Of Transient

UPC Universitat Politècnica de Catalunya (Technical University of Catalonia)

xv

ACKNOWLEDGMENTS

This work contains findings that were produced within the OECD-NEA/PKL Project. The authors 

are grateful to the Management Board of the PKL Project for their consent to this publication, 

and thank the Spanish Nuclear Regulatory Body (CSN) for the technical and financial support 

under the agreement STN/1388/05/748.

1-1

1 INTRODUCTION

Safety studies for low power and shut-down conditions of nuclear plants performed in several 

countries during the last decade directed the interest to accident sequences for which a local 

boron dilution in the reactor core by a plug of pure water or water of reduced boron 

concentration might occur. These investigations comprise events during shut-down conditions 

including event sequences with boron dilution. Also, for loss-of-coolant accidents with small 

leaks, sequences that involve boron dilution have been identified.

This report presents an analysis of two different PKL experiments. Test F1.1 which represents a 

small break loss of coolant accident with boron dilution and Test F1.2 which analyses the boron 

dilution rate as a function of the primary mass inventory.

1.1 Boron Dilution Events 

Boric acid is used as a soluble neutron absorber in the primary coolant of pressurized water 

reactors. The main functions of boric acid are to compensate for fuel burn up and xenon 

poisoning with the required reactivity margin during normal operation, and to provide the 

necessary subcriticality of the core during refueling and maintenance. During normal reactor 

operation, the specified boron concentration is maintained by the volume control system. In 

case of a loss-of-coolant accident, borated water is injected by the emergency core cooling 

system (high-pressure safety injection pumps, accumulators, and low-pressure pumps).

Inadvertent boron dilution might occur in two different scenarios:

• Unintentional injection of unborated water from outside the primary cooling system by 

malfunctions of injecting systems.

• Separation of primary coolant into highly borated water and almost boron-free water by 

evaporation and condensation.

The first kind of events would progress slowly in normal reactor conditions. In other words, it can 

be detected and corrected easily. Transients of the second kind could result in event sequences 

that are more difficult to analyze and constitute a challenge to the analytical methods.

For the event sequences with boron dilution multiple failures of systems are assumed. In 

scenarios where the primary coolant inventory has been reduced, for example by a small break, 

and the reactor is shut down, the water evaporates in the core and condensates in the steam 

generators. Boron is mainly retained in the liquid phase so that the vapor phase and the 

condensate are almost boron-free. This condensated water totally unborated precipitates to the 

loop seals. If these conditions are maintained during a long period of time, the loop seals and 

the cold legs will be full of unborated water; meanwhile the core cooling inventory will be highly 

borated. These amounts of unborated water form plugs in the primary system. If a later time 

circulation in the cooling system is re-established either by natural circulation following refilling 

or by restarting of a main coolant pump, water of low boron concentration may be driven 

through downcomer and lower plenum into the core giving a reactor supercriticality.

The most important analysis referred to boron dilution events considers a small leak with 

multiple failures in the emergency core cooling system. Differences between tests are referred 
1-2

to leak size, leak location and the different characteristics and distribution of the emergency 

injection systems.

1.2 Definitions of Boron Dilution

Rapid boron dilution transients supposes a nonhomogeneous distribution of the boron in the 

primary system. There could be boron dilution in one part of the RCS although the mean boron 

concentration is maintained at its normal value. It is clear that for inherent boron dilution two 

different aspects have to be analyzed as there is normally coolant exiting the RCS (in the case 

of a SBLOCA) and, therefore the average boron concentration will also vary. It is necessary 

here to introduce two definitions that will be helpful throughout this paper in order to differentiate 

two different types of boron reduction involved in rapid boron dilution events:

Boron dilution The process where a local decrease in boron concentration in PS 

(primary system) occurs 

De-boration The process where a net loss of boron from PS occurs that implies a 

decrease in average boron concentration 

In the same way, the following definitions are accepted as well:

Boration The opposite of de-boration

Boron concentration recovery The opposite of boron dilution

De-boration rate The rate of de-boration

Dilution rate The rate of boron dilution

These definitions were first introduced by D'Auria [3] and are nowadays accepted in the 

international community.

1.3 PKL Test Facility

PKL is an experimental power facility plant designed to simulate pressurized water reactors 

(PWR) under accidental conditions. The plant, situated in Erlangen (Germany), has been the 

scenario of several experiments in the last 25 years. The PKL facility replicates the entire 

primary system and most of the secondary system (except for the turbine and condenser) of a 

1300-MW PWR plant, with elevations scaled 1:1 and diameters reduced by a factor of 12. 

Therefore volumes and power have been reduced by a factor of 145. The number of rods in the 

core and the U-tubes in the steam generator has been divided by 145 too. The core has been 

modeled by 314 electrical heater rods. Unlike many experimental facilities with only two 

available loops (one for the break loop and one to simulate the other three loops) PKL simulates 

all four loops separately. This is very important in order to analyze asymmetrical transients, e.g. 

with injection in two out of four loops. A diagram of the PKL test facility is shown in Figure 1.
1-3

Figure 1.1 Diagram of the PKL test facility

The operating pressure of the PKL facility is limited to 45 bars on the primary side and to 56 

bars on the secondary side. This allows simulation over a wide temperature range (522K to 

322K) that is particularly applicable to the cooldown procedures investigated. 

All emergency systems are represented and have a wide versatility referred to their functions 

and positions. There are 8 accumulators (one for each cold and hot leg). The pump injection 

system is available in all the hot and cold legs. Many break locations are available too.

PKL test facility has about 1500 measurement points that permit an exhaustive analysis of the 

tests. There are measurement devices for cladding, wall and fluid temperature, absolute and 

differential pressure, one and two phase mass flow, density and boron concentration.

Sixty of the measurement devices are identical to those that are used in a commercial plant to 

simulate what an operator would control in case of accident.
1-4

There are 4 measurement devices for the boron concentration; these are called the COMBO 

devices. They can be placed in any point of the facility, around pipes or even in big vessels 

(more than 150 mm). The COMBO devices measure the boron density from the outside so it is 

quite easy to change their location. The COMBO uses an irradiation method to calculate the 

proportion of boron in the coolant. A neutron source is placed in the outside of the pipe or the 

vessel. Some limitations arise when using the COMBO methodology. In order to get reliable 

data, the volume in which the boron is measured has to be totally filled with water. Moreover, 

results show high oscillations due to the error induced by the methodology.

It is important to mention that the break discharge experimental data have to be computed from 

other values. One can calculate the break mass flow by differentiating the level variation of the 

two separator vessels (where the discharged water goes) and subtracting the mass released 

through the separator relief line. Hence, the break mass flow is expected to be just an 

estimation of the experimental value as other effects like fluid density and vaporization might 

interact.

1.4 PKL II OECD (Organization for Economic Co-operation and Development) 

Project

OECD/SETH program included four tests performed in the PKL test facility; two of them were 

carried out in 2001 and the reminders during 2002. Three of these tests were designed to study 

boron dilution after a small break in the primary system. Before the OECD program started, a 

previous test of boron dilution was performed: Test E1.1, which served to define the 

configuration of the following tests handling the same phenomena. Table 1 shows the main 

characteristics of each of the SBLOCA with boron dilution tests.

OECD/PKL project comprises the F series of tests, which are a continuation of the SETH 

program. There are four groups of tests that cover the following issues:

• Boron dilution events after small break loss of coolant accident (first series F1, tests F1.1 

and F1.2).

• Loss of residual heat removal during mid-loop operation with closed (series F2) and 

open (series F3) reactor coolant system.

In this paper a post test analysis of tests F1.1 and F1.2 is brought out covering the study of 

boron dilution events of the OECD/PKL project. 

Test F1.1 is a clear continuation of the E series (SETH project) related with SBLOCA with boron 

dilution. The test is complementary to tests E1.1 and E2.2 with similar break location/size and 

same sequence of events. In this case the test is configured as to resemble a PWR 

Westinghouse design, e.g. steam generator cool down rate of about 50 K/h and ECC injection 

into all 4 cold legs. The main objective of the test is to gather the maximum size of low borated 

water in the loop seals and study the restart of natural circulation providing the minimum boron 

concentration at the reactor pressure vessel inlet. Three COMBO measurements were located 

along loop 4 improving the study of the mixing and the real shape of the boron diluted slug in 

this loop. This had not been done previously.

Test F1.2 was designed to analyze the rate of boron dilution depending on the mass inventory 

in the primary system. The test follows a series of steady states with different mass inventory 
1-5

and with a high SG level maintained in the U-Tubes thus showing at what U-tube levels and 

mass inventories the accumulation of condensate in the loop seals occurs. One of the main 

objectives of the test is to analyze the transport of borated water from the SG inlet to the SG 

outlet when the SG levels are maintained in the U-tubes. The test can be used as to see the 

performance of the code and model to simulate the dilution of the loop seals during the different 

steady states.
1-6

Table 1.1 Tests on boron dilution events performed at the PKL test facility in the recen

National Program 

2000 OECD/SETH Program 2001-2002 OECD/NEA 

E1.1 E2.1 E2.2 E2.3 F1.1 

Break size 40 cm2 40 cm2 32 cm2 50 cm2 21 cm2

Break location Hot leg in loop 

with PRZ

Hot leg in loop 

with PRZ Cold leg 1 Hot leg 1 Cold leg 1 

HPSI All 4 into cold 

legs

All 4 into hot 

legs

2 out of 4 SIPs, 

cold legs (1 in 

broken loop)

2 out of 4 SIPs, 

cold legs (1 in 

broken loop)

All 4 into cold 

legs 

CVCS --- --- --- --- ---

ACCs --- --- --- All 4 into hot 

legs --- 

LPSI All 4 into cold 

legs

SG secondary 

side 100K/h 100K/h 100K/h 100K/h 56K/h 

Central topics of 

investigation

Symmetry, boron 

concentration

Symmetry, 

boron 

concentration

Maximum

condensate 

accumulation

Hybrid state 

with condensate 

accumulation in 

two loops

Simulating a 

Westinghouse 

design
1-7

1.5 Code and Nodalization

The experience gained in the UPC-INTE group during the last years with the use of PWR 

models of real plants as well as of other experimental facilities has been very useful to develop 

a new nodalization for the PKL Test Facility. This nodalization was improved during the 

simulation of the different tests as various problems were faced at each transient. All 

improvements were included in the previous post test calculations so the final model was able to 

represent all tests at a similar accuracy. During this process, the information sent by Framatome 

ANP was always useful to improve this model. Some of the control systems and some 

coefficients were taken from the Framatome nodalization, which was available for all 

OECD/SETH participants.

Calculations were initially performed with RELAP5/MOD3.2gamma but all simulations were 

finally done with the newest version (RELAP5/MOD3.3) with an improved boron transport model 

implemented by the UPC-INTE group. New code improvements along with the possibility to 

modify the source code have encouraged the group to use this version. The step forward 

implied some modifications and some new expected discrepancies. 

In order to test the effect of the U-tube bundle3 nodalization, two different models were 

generated: one with a single pipe for the U-tubes and another one with five different pipes at 5 

elevations. The differences and results between these two models will be explained in section

4.2. Figure 2 shows both nodalizations of the PKL test facility. The nodalization is especially 

detailed along the legs. As a guidance reference, the length of the volumes was chosen to be 

around 0.5 meters for a good performance of the models although the lengths vary from 0.1 to 

1.04

 . 

 

3 In the PKL test facility there are 7 U-tube bundles with different elevations 4 meterSome volumes in the secondary system are larger
1-8

Figure 1.2 RELAP5 nodalization for the PKL test facility. Only loops 1 and 2 are 

shown.

The number of components used is:

• 438 volumes and 775 for the 5 U-tube nodalization

• 468 junctions and 817 for the 5 U-tube nodalization
1-9

• 331 heat structures 651 for the 5 U-tube nodalization

• 1356 mesh points in the heat structures 2636 for the 5 U-tube nodalization

All walls and superficies are modeled by heat structures to take into account any heat loss or 

exchange. The core fuel rods and other heaters (upper head and pressurizer) are modeled by 

means of heat structures.

Most auxiliary, safety and control systems are modeled by means of time dependent volumes 

and junctions. The turbine, main steam line and the pressurizer relief system (PORV) are all 

simulated using time dependent volumes and valves. The AFW system, the high and low 

pressure injection systems and the pressurizer spray system are modeled with time dependent 

volumes and junctions. Accumulators are nodalized with acc components and the accumulator 

discharge line is nodalized with the same length and elevation as in the facility. 

The core region is nodalized by a single pipe with 7 axial volumes. A core bypass is modeled by 

a pipe of three axial volumes. The downcomer is modeled with two separate pipes since some 

recirculation phenomena were detected in some of the experiments. On the other hand, the 

downcomer of the SGs are simulated with a single pipe as the required detail in the secondary 

system is not as high. There are four bypasses from the upper plenum to the downcomer which 

are nodalized with two different lines to allow asymmetry phenomena. In these bypass the 

throat is located at the same elevation as in the real facility.

1.6 Notes about Boron Transport Models

Second order Godunov scheme was not working properly in RELAP5 MOD3.2gamma and 

MOD3.3 versions. Macian-Juan (ref. [11]) pointed out the incapability of first order upwind 

models (the RELAP5 default model) to simulate boron dilution transients. UPC-INTE group 

detected and fixed a mistake in one of the RELAP subroutines that was disabling the accurate 

model. This modification was reported and accepted by the RELAP group, the model will be 

available in new released versions. Moreover, a new model for the boron transport to reduce 

numerical diffusion and introduce physical diffusion was implemented and was thereby used for 

all calculations in this paper. The new model is extensively explained in reference [9]. 

2-1

2 TEST F1.1

Test F1.1 is a boron dilution transient with a small break in cold leg 1. The test is similar from its 

precedent series (E), although this one was meant to simulate a Westinghouse design. 

Therefore, the cooldown of the secondary system is slower and each HPSI pump injects 

symmetrically in all loops. For this test, unavailability of LPSI and accumulators is supposed. 

Besides, one of the HPSI pumps is also not operable.

Since the maximum operating pressure of the PKL test facility is 45 bars, it is not possible to 

simulate the entire transient of a normal PWR (normal operating pressure of 160 bars). Hence, 

in the test, the transient starts at a primary pressure of less than 45 bar and with initial 

conditions corresponding to those that would prevail in a real plant at this time. The initial 

conditions for Test F1.1 consist of a partially emptied primary system at a pressure of 39 bars. 

2.1 Boundary Conditions

The hardware configuration of test F1.1 is described in references [10, 2]. Some important 

points are the following:

1. Break assembly. Small break (21 cm2

/145) in cold leg 1 between RCP and RPV.

2. Accumulator and LPSI. Unavailability of LPSI and accumulators.

3. High pressure ECCs. Primary feed by 1 out of 2 HPSI pumps, injection via header 

symmetrically into all 4 cold legs ([B] = 2200 ppm).

4. Steam generators secondary side. Cooldown of all 4 SGs at 56 K/h using all MS-RCVs 

(SGs connected via main steam header).

2.2 Conditioning Phase

The conditioning phase can be subdivided into 3 parts:

Phase 1 was initiated at t=-8730s by stopping the feedwater supply and closing the main steam 

lines of all 4 SG secondaries in order to increase the secondary pressure (fig. 3). Secondaries 

remained connected via the main steam header. Due to the increase of secondary temperature, 

the primary temperature rose and approached saturation at core outlet. Primary pressure 

increased.

Phase 2 was started by opening the break in cold leg 1 (at t=-7750 s). Primary pressure 

decreased to a value approx. 3 bar above the secondary pressure. The RPV level decreased to 

the coolant line and the PZR to the surge line, whereby the primary started to void. When 

secondary pressure had reached 38 bar, the main steam lines were opened again (t=-6610 s) 

and the secondary pressure was controlled as to keep the primary pressure at approx. 40 bar. 

The control implemented in the calculation is finer than the experimental one although, it did not 

affect the results and the final initial conditions were reached. RPV level stabilized around RCL 

as it was in the test facility. Phase 2 was finished at t=-5500 s by closing the break valve. By this 

time, natural circulation in all loops had stopped and reflux condenser conditions had been 

established. The SG inlet chambers were still filled by water/steam mixture and the residual 

primary coolant inventory was 1230 kg.
2-2

-9000 -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0

2.5

3.0

3.5

4.0

4.5

Pressure (MPa)

Time (s)

 Experimental primary

 Experimental secondary

 RELAP primary

 RELAP secondary

Figure 2.1 Primary and secondary pressures, conditioning phase of test F1.1

-9000 -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0

0.0

0.5

1.0

1.5

2.0

2.5

Mass flow (kg/s)

Time (s)

 Experimental

 RELAP

Figure 2.2 Mass flow in loop 4, conditioning phase
2-3

-9000 -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0

0

400

800

1200

1600

2000

Boron concentration (ppm)

Time (s)

 Experimental

 RELAP

Figure 2.3 Boron concentration in the loop seal of loop 4, conditioning phase

These conditions were kept constant during phase 3. The break was closed during the whole 

phase and water evaporated in the core and condensated in the U-tubes. From t = -5500 s until 

test started, approx. 300 kg of condensate were accumulated in the loop seals.

The transition from conditioning phase to test was performed stepwise in order to avoid sudden 

changes in the differential pressure distribution at test start due to condensation caused by the 

ECC water injected. All test actions, but the break opening, were performed prior the start of the 

transient.

Figure 4 shows the loop 4 mass flow during the conditioning phase that increased when the 

break was opened. Figure 5 shows the boron concentration in loop seal 4 and how the boron 

concentration was reduced in both, the simulation and the test.

Finally, the test initial conditions were reached with a quite good agreement with the 

experimental ones. Table 2 presents a comparison between the initial conditions achieved and 

the actual ones.

2.3 Initial Conditions at Test Start

Initial conditions used for Test F1.1 consist of a partially emptied primary system at a pressure 

of 38.9 bars. Low borated water has been accumulating in all loop seals.

The initial conditions obtained in the post-test calculation (Table 2) of test F1.1 after the 

conditioning phase are in quite close agreement with the experimental values. The most 

significant discrepancy in the initial conditions is a difference of 70 kg in the initial inventory 

mass.
2-4

Table 2.1 Initial conditions of Test F1.1, experimental and calculated data

Initial conditions at t=0s

Experimental Data RELAP

Core power 485 kW 485 kW

Primary pressure 39.2 bar 38.9 bar

Temperature at core outlet 522 K 522 K

Subcooling at core outlet 0 K 0 K

Fluid temperature pressurizer 522 K 522 K

Pressurizer level 0.8 m 0.65 m

Boron concentration

<50 ppm in loop seals and 

downcomer

Approx. 4000 ppm in the core

<50 ppm

Approx. 4000 ppm

Main steam pressure in all 4 SGs 38.1 bar 37.4 bar 

Main steam temperature 519 K 520 K

Collapsed level in all 4 SGs 12.2 12.2

Feedwater temperature 383-393 K 388 K

2.4 Test Phase

The test is started by:

• Starting the HPSI to inject into all 4 cold legs (t=-140 s, [B]= 2500 ppm)

• Start of the rod bundle power decrease (t=-60s)

• Start of cooldown of all SG-secondaries by 56 K/h (t=-50s)

• Opening of the break in cold leg 1 (t=0 s)

Until SOT, the primary coolant inventory has increased up to a value of 1280 kg as the break 

has remained closed while the HPSI have injected water during 140 seconds. The HPSI rates 

are controlled according to the correct pump characteristics.

During the first part of the test, the primary and secondary pressures decrease due to the 

secondary cooldown and the power decay. As the quantity of injected mass is larger than the 

discharged mass, the RPV level increases. At t = 5000 s the primary system is almost full again 

and natural circulation is established in all loops asymmetrically. Times for establishment of NC 

in all loops are shown in Table 3. Primary pressure rises and levels off. The PRZ level sharply 

increases.

Table 3 shows the chronology of the main events occurred in test F1.1, comparing the 

experimental values with the calculated ones.
2-5

Table 2.2 Chronology of the main events in Test F1.1

Event Experimental 

data (s) RELAP (s)

Starting the HPSI injection into all 4 cold legs -140 -140

Start of the rod bundle power decrease -60 -60

Start of 56K/h cooldown of all 4 SGs -50 -50

Opening of the break in cold leg 1 0 0 

Establishment of NC in loop 2 4651 5080

Establishment of NC in loop 4 4991 5080

Establishment of NC in loop 3 5556 4890

Establishment of NC in loop 1 6851 5020

End of transient 11911 11911

All actions taken in this test happen just before SOT. The HPSI starts to inject symmetrically into 

all 4 cold legs 140 seconds before SOT. Afterwards, the rod bundle power decrease starts 

according to a heat decay table. Fifty seconds before the test started the SGs initiate the cool 

down. At SOT, the break cross section is opened again. There are no further actions during the 

transient. Most of the difficulties found during the simulation occurred at the conditioning phase, 

once the initial coolant mass is correctly matched the rest of the transient presents acceptable 

results.

Figure 6 shows the primary and secondary pressures. The primary pressure has small 

discrepancies after the primary system is filled again (5000 seconds) although it stabilizes at 

similar values. 
2-6

0 2000 4000 6000 8000 10000 12000

0.0

0.8

1.6

2.4

3.2

4.0

4.8

Pressure (MPa)

Time (s)

 Experimental primary

 Experimental secondary

 RELAP primary

 RELAP secondary

Figure 2.4 Primary and secondary pressures for test F1.1

Differences shown in the RPV level (Figure 7) are similar to the ones seen on previous post-test 

calculations [4]. For the pressurizer level (Figure 8) the results have a good agreement with the 

experimental data. There is a 500 seconds difference on the level increase, although the rise 

ratio is similar. Break mass flow is shown in Figure 9. As said in Section 1.3, the experimental 

break flow was indirectly calculated from other variables. The simulation flow appears to be all 

the time slightly greater. That is consistent with the RPV level differences. In the simulation 

there is stagnant vapor in the upper head, which causes that the final mass at NC establishment 

is lesser than the experimental. 
2-7

-8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000 12000

4.8

6.0

7.2

8.4

9.6

10.8

12.0

level (m)

Time (s)

 Experimental

 RELAP

Figure 2.5 RPV level for test F1.1 during the conditioning and the test phases

0 2000 4000 6000 8000 10000 12000

0.0

2.4

4.8

7.2

9.6

12.0

level (m)

time (s)

 Experimental

 RELAP

Figure 2.6 Pressurizer level for test F1.1
2-8

0 2000 4000 6000 8000 10000 12000

0.0

0.2

0.4

0.6

0.8

Mass flow (kg/s)

Time (s)

 Experimental

 RELAP

Figure 2.7 Break mass flow for test F1.1

Figure 10 show the mass flow in all loops. Discrepancies simulating loop 1 were found. These 

differences were also observed in loop 2 of test E2.2, even by different groups performing 

analytical studies of the experiment [13] and are still an open issue. Natural circulation is 

correctly matched for the rest of loops although even small discrepancies hinder the analyses of 

the boron concentration at the COMBOs for the slugs are moved at different times and speed.
2-9

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

2.0

Loop 1

Time (s)

Mass flow (kg/s)

Loop 2

Loop 3

 Experimental RELAP

Loop 4

Figure 2.8 Mass flow in all loops for test F1.1
2-10

Boron concentration at the COMBOs is displayed in Figure11. The tracking of the boron 

concentration will be explained in the next section along with a complete analysis of the 

evolution of the boron diluted slugs.

-8000 -6000 -4000 -2000 0 2000 4000 6000 8000

0

700

1400

2100

2800

Time (s)

0

700

1400

2100

2800

Cold leg 4, RCP exit

 Experimental RELAP5

Boron concentration (ppm)

0

700

1400

2100

2800

0

700

1400

2100

2800

3500

Cold leg 4, RPV entrance

Loop seal 4

Cold leg 2, RPV entrance

Figure 2.9 Boron concentration at the COMBOs located in: cold leg 2, LS 4 and both 

sides of cold leg 4. Test F1.1
2-11

2.5 Analysis of Results Concerning Boron Dilution

Figures 12 to 18 show the boron concentration evolution along loop 4 of post-test F1.1 to 

illustrate the formation process and the behavior of the slugs prior the re-start of NC. In all these 

plots, the color criterion is red for non borated water, blue for highly borated water and grey for 

steam (null liquid void fraction). The values range from 0 to 2500 ppm. There is a plot in each 

figure with the boron concentration evolution of the volume marked with a circle.

2.5.1 Formation of the Boron Diluted Slugs 

Figure 2.10 Post test calculation of test F1.1, evolution of boron concentration in loop 4. 

Left: start of transient. Right: reflux-condenser conditions established

After the break is opened the primary mass inventory decreases until the primary level stabilizes 

around mid loop. At this point, if the SG levels are high enough, reflux-condensation conditions 

are established (Figure 12). Water evaporates in the core and condenses in the U-tubes. This 

condensed water precipitates either to the loop seals or to the hot legs, forming boron diluted 

slugs (left picture of Figure 13). If reflux-condensation conditions are maintained for a long 

period of time the LSs can be totally filled up with pure water (right picture of Figure 13). The 

behavior of the hot leg slugs is different; condensed water flows back to the core where it mixes 

with highly borated water so only a small amount of condensate is gathered in this region. 

On the other hand, water accumulated in the loop seals will be pushed to the downcomer for 

very long periods of reflux-condensation conditions. In that case, coolant in the downcomer will 

be highly diluted. Generally the size of the slug is limited by the volume of the LSs although 

small slugs can be formed in hot legs and downcomer.
2-12

Figure 2.11 Post test calculation of test F1.1, evolution of boron concentration in loop 4. 

Left and right: Boron dilution in hot leg and LS

Right picture of Figure 13 shows full boron dilution in the LS and partial dilution in the hot legs. 

For this test the downcomer was also full of un-borated water5

. It is worth mentioning that Figure 

10 of the final report of test F1.1 (ref. [12]) illustrates very similar results.

Figure 2.12 Post test calculation of test F1.1, evolution of boron concentration in loop 4.

Left: ECC flows to LS. Right: upward displacement of the slug 

2.5.2 Evolution of the Boron Diluted Slugs Prior NC Establishment 

 

5 During the conditioning phase of the test both the break and the ECC valves remain closed 

and the facility is kept under stationary reflux-condensation conditions for a long period of time 

to enhance the formation of condensate
2-13

The simulation of test F1.1 gave quite good results and permitted to follow up the movements of 

the slug during the filling up process. After the conditioning phase, with the start-up of the cold 

leg injection, reverse flow occurred only in the cold legs. High borated water from the ECC 

flowed down the LS and mixed with the slug which was not pushed upward to the U-tubes 

(Figure 14, left). 

Figure 2.13 Post test calculation of test F1.1, evolution of boron concentration in loop 4. 

Left: upward displacement of the slug continues. Right: the slug is moved 

back to the LS. 

As the filling up process advanced the slug was displaced upward to the SG outlet plenum and 

ECC water reached the horizontal region of the LS (figure 14, right). The slug remained there 

(Figure 15, left) only for 200 seconds. Some mass flow was detected even before the filling up 

process that pushed the slug back to the LS (Figure 15, right). The reason was that water could 

be transported from SG inlet to outlet side by entrainment. The same phenomenon occurred to 

a small extent in test E2.2 although in that case it only took place in the loops with no injection

[12]. 
2-14

Figure 2.14 Post test calculation of test F1.1, evolution of boron concentration in loop 4.

Left and right: intermittent forward and backward movements of the slug. 

Afterwards, intermittent movements of the slug were seen in both the calculation and the 

experiment. The slug advanced to the injection point and came back to the LS at least three 

times increasing the mixing (Figures 16 and 17). 

Figure 2.15 Post test calculation of test F1.1, evolution of boron concentration in loop 4. 

Left and right: intermittent forward and backward movements of the slug. 

Before the establishment of NC, the slug was placed below the pump and the minimum boron 

concentration was around 600 ppm in the calculation (Figure 17, right), similar to the results 

obtained in the experiment (approx. 600 ppm; as shown in Figure 10 of reference [2]).

2.5.3 Establishment of Natural Circulation 
2-15

It is clear that, it is difficult to represent large asymmetries between loops and usually some of 

the NC establishments are not matched with RELAP5. Still, in loop 4 of test F1.1 the agreement 

allowed a good analysis of the main phenomena. Intermittent mass flows were observed in the 

test facility and in the calculation. The slug moved forward and backward before crossing the 

cold leg.

Figure 2.16 Post test calculation of test F1.1, evolution of boron concentration in loop 4.

Left and right: establishment of natural circulation. 

Figure 18 shows the continuation of the simulation of Test F1.1. As pointed out in the previous 

section, a slug of 600 ppm was placed below the RCP before natural circulation establishment 

(Figure 17, right). A last reverse movement of the slug was detected 10 seconds before the full 

natural circulation was established (Figure 18, left). The slug was cut in two due to the ECC 

water injection as shown in the right picture of Figure 18. 

3-1

3 TEST F1.2

Test F1.2 is a parametric study for the investigation of inherent boron dilution due to refluxcondensation conditions as a function of the primary coolant inventory [1]. The test was 

composed by a sequence of steady states with intermediate transition phases consisting of the 

actual change of parameters and subsequent relaxation phases. The test was started with the 

primary system completely full of borated water and natural circulation in all loops. The mass 

inventory was first reduced stepwise and thereafter the CVCS was used also stepwise to 

replenish the primary system. The test was interesting as to investigate the dependence of the 

boron dilution with the mass inventory and the RPV level.

3.1 Boundary Conditions

• Test start with the primary system completely filled with water, 1-diameter natural 

circulation and homogeneous boron concentration

• Stepwise reduction and replenishment of the primary coolant inventory at constant 

primary pressure, drain and injection via lower plenum

• Additional steady states with varied pressure and core power

• Isolation of the pressurizer at the beginning of the transient as to eliminate its influence 

in the test.

• Main steam and feedwater mass flows were adjusted manually to constant values and 

corrected only from time to time.

3.2 Initial Conditions at t=0s

The initial conditions of the test and the calculation are shown in Table 4. The test was initiated 

at steady-state conditions. The test facility was completely filled with water at high 

homogeneous boron concentration and a constant core power. The secondary sides were kept 

in operation. There was subcooled natural circulation in all 4 loops. The primary and secondary 

pressures were 12 and 6 bar, respectively. There was no special conditioning phase.

The main difference is the distribution of the mass inventory. During the steady state calculation 

the primary coolant was redistributed, i.e. the pressurizer level increased while the RPV 

dropped. The same occurred in the first part of the experiment, even before that any action had 

been taken except for the secondary pressure increase. This disagreement did not affect the 

simulation since, when the pressurizer was isolated at 4650 seconds, the level of the PRZ and 

the mass inventory in the primary system (without the PZR) were the same in both the test and 

the RELAP5 simulation.
3-2

Table 3.1 Initial conditions of Test F1.2, experimental and calculated data

Initial conditions at t=0s

Experimental Data RELAP

Primary coolant inventory 2540 kg (40 kg in PZR) 2550 kg (130 kg in PZR)

Boron concentration 2000 ppm 2000 ppm

Core power 600 kW 600 kW

Primary pressure 12 bar 12 bar

Temperature at core outlet 457.3 K 458.1 K

Subcooling at core outlet 4 K 3 K

Fluid temperature pressurizer 462.3 K 462.1 K

Pressurizer level 1.3 m 3.1 m

Mass flow in all 4 loops 1.23 kg/s 1.12 kg/s

Main steam pressure in all 4 SGs 5.9 bar 5.9 bar

Main steam temperature 431 K 434 K

Collapsed level in all 4 SGs 12 m 12.15

3.3 Nodalization with 5 U-tubes

This test goes through different steady states with different natural circulation mass flows. 

During its simulation it was found out that during the last steady state period, just before the NC 

resumed, there was natural circulation only in some of the U-tubes and whereby boron dilution 

started only slightly. This behavior could not be simulated using a single U-tube and therefore, a 

nodalization with 5 U-tubes was used for this test. This nodalization was able to simulate this 

phenomenon and the end of natural circulation occurred at the same time as in the experiment. 

The comparison between the two nodalizations will be reported and discussed in Section 4.2. 

3.4 Test Phase

Test F1.2 was started at steady state flow conditions with the initial conditions just described. 

Throughout the test, the secondary pressure was controlled by partly opening and closing the 

main steam relief valves in order to adjust the primary pressure to the desired values; these 

actions were performed manually and without any specific control system. The primary and 

secondary pressures are plotted in Figure 19 displaying a quite good agreement between the 

experiment and the calculation.
3-3

0 10000 20000 30000 40000 50000 60000 70000 80000

0.48

0.72

0.96

1.20

1.44

1.68

1.92

2.16

2.40

pressure (MPa)

Time (s)

 Experimental primary 

 Experimental secondary

 RELAP primary 

 RELAP secondary

Figure 3.1 Primary and secondary pressures for Test F1.2

After a first accumulation of steam in the RPV upper head, the pressurizer was isolated by 

closing an isolation valve in the surge line (t=4650 s). Afterwards the PZR had no influence to 

the test. Figure 20 displays the pressurizer level evolution. Even though the initial level displays 

a large discrepancy the values are very similar when the PZR is isolated so the primary total 

mass is not affected. Variations after the isolation have no influence to the rest of the system 

and are therefore meaningless.
3-4

0 10000 20000 30000 40000 50000 60000 70000 80000

0

1

2

3

4

5

6

Level (m)

Time (s)

 Experimental

 RELAP

Figure 3.2 Pressurizer collapsed water level for Test F1.1

0 10000 20000 30000 40000 50000 60000 70000 80000

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Mass flow (kg/s)

Time (s)

 Experimental

 RELAP

Figure 3.3 Total main steam flow for Test F1.2

There were no control systems for the feedwater and main steam valves. One of the main 

objectives of the test was to establish real steady states whereby control loops for feedwater 

injection, main steam pressure and secondary levels were removed in order to restrain their 

influence. The SG relief valve areas and the feedwater mass flows were manually modified from 

time to time. Figure 21 shows the total main steam flow. As the information about the 
3-5

modifications performed at each time on the main steam valves was not given, the section of 

these valves was modified from time to time so the secondary pressure had a similar evolution 

as in the experiment. 

The mass inventory was reduced stepwise as shown in Figure 22 by temporarily openings of a 

drain line in the lower plenum. The calculation value does not take into account the pressurizer 

mass hereby the discrepancy at the beginning of the test. Once the pressurizer had been 

isolated, the mass inventory shows similar results. There was no specific section for the drain 

line, the actions were performed as to open the valve (with no determined section) until the new 

desired mass inventory value had been reached. Hence, in the calculation the areas of each 

opening were matched by a process of try and check until the mass was reduced to the correct 

value and with the same amount of time.

0 10000 20000 30000 40000 50000 60000 70000 80000

500

1000

1500

2000

2500

Mass inventory (kg)

Time (s)

 Experimental

 RELAP

Figure 3.4 Primary mass inventory for Test F1.2

When the core was uncovered the primary mass inventory was increased again by injection of 

borated water in the lower plenum. Figure 24 shows the RPV level. The initial level is different 

since part of the RPV inventory had flown to the PZR before SOT in the calculation. After the 

isolation of the pressurizer the RPV levels equalize. Due to the reduction of the mass inventory, 

the mass flow of the loops increased (Figure 25) and became twice and thrice of the initial 

natural circulation. With high natural circulation the RPV level in the calculation was lower.
3-6

0 10000 20000 30000 40000 50000 60000 70000 80000

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Mass flow (kg/s)

Time (s)

 Experimental

 RELAP

Figure 3.5 Break mass flow for Test F1.2

0 10000 20000 30000 40000 50000 60000 70000 80000

4.8

7.2

9.6

12.0

level (m)

Time (s)

 Experimental

 RELAP

Figure 3.6 RPV collapsed water level for Test F1.2
3-7

0 10000 20000 30000 40000 50000 60000 70000 80000

0

1

2

3

Time (s)

Loop 4

0

1

2

3

Loop 3

0

1

2

3

End of natural  circulation

End of natural  circulation

0

1

2

3

4

 Experimental RELAP

Loop 2

Mass flow (kg/s)

Loop1

Figure 3.7 Mass flow of loops 1 to 4 for Test F1.2
3-8

0 10000 20000 30000 40000 50000 60000 70000 80000

0

2

4

6

8

10

Time (s)

0

2

4

6

8

10

0

2

4

6

8

10

Level (m)

0

2

4

6

8

10

 Experimental RELAP

Figure 3.8 U-tube collapsed water levels of loops 1 to 4 for Test F1.2

The different mass flows reached in the steady states were basically achieved by the 

calculation. Results shown in Figure 25 were smoothed from the real values since the mass 

flows in some periods had large oscillations. Although there was no special reason, the test 

showed asymmetries between loops which were not represented by the nodalization. The last 

steady state before the end of natural circulation was only achieved when using 5 different 

elevations for the U-tubes. Natural circulation ended at similar times.
3-9

3.5 Analysis of Results Concerning Boron Dilution

Figure 27 shows the boron concentration in each loop seal. Boron dilution starts at similar time 

and the dilution rate is similar in both the test and the calculation. Reflux-condensation 

conditions were detected in both the test and the calculation even with low mass flow in the 

loops. There was a two phase natural circulation that slightly diluted the loop seals, at a very low 

rate. The circulation occurred only in some of the U-tubes and vapor flowed boron free in the 

rest of the U-tubes, thus boron dilution took place only partially. This could, certainly, not be 

simulated with only one U-tube. After NC was broken the dilution rate was similar in both the 

calculation and the test.

0 10000 20000 30000 40000 50000 60000 70000 80000

0

700

1400

2100

2800

Time (s)

0

700

1400

2100

2800

 Experimental RELAP

LS 4, below SG outlet

LS 3, below RCP

0

700

1400

2100

2800

LS 2, below SG outlet

0

700

1400

2100

2800

Boron concentration (ppm)

LS 1, below SG outlet

Figure 3.9 Boron concentration at each COMBO measurement for Test F1.2

4-1

4 SENSITIVITY STUDIES ON TESTS F1.1 AND F1.2

4.1 Upper Head Bypass Nodalization

One of the problems when simulating test F1.1 was the first uncovery of the break during the 

conditioning phase. In the first simulations the downcomer level decreased under the cold legs 

and the break was uncovered too early. Therefore the break mass flow had a sudden plunge 

(Figure28). The DC level (Figure 29) in the facility does not decrease under the cold legs and 

the break mass flow is still high during the second part of the conditioning phase. Hence, the 

coolant mass at the beginning of the transient was about 330 kg greater than expected. That 

mistake made the primary system fill up before the experimental data and so the primary 

pressure had an early increase (Figure 30). That discrepancy caused also an early 

establishment of natural circulation. 

-9000 -8500 -8000 -7500 -7000 -6500 -6000 -5500 -5000 -4500 -4000

0.00

0.18

0.36

0.54

0.72

0.90

1.08

Mass flow (kg/s)

Time (s)

 Experimental

 RELAP nodalization 1

 RELAP nodalization 2

Figure 4.1 Break mass flow during conditioning phase of Test F1.1, upper head 

bypass sensitivity

In order to better simulate that behavior, a sensitivity study on the bypass nodalization was 

performed. Improving the detail in the bypass nodalization changed the downcomer level and, 

therefore, the total mass at the end of the conditioning phase. The four bypasses were 

previously modeled as two parallel volumes and a small junction at the upper part. Those 

volumes were subdivided into two so the small orifice was in its real position. All changes 

performed were done in order to better represent the real geometry so the final input deck is 

more realistic. The changes on this part of the nodalization improved the simulation as the 

downcomer level stabilized around the cold leg line (Figure 29) and the coolant mass at the end 
4-2

of the conditioning phase was almost matched. Initial coolant mass for the first nodalization was 

1560 kg whereas the experimental mass was 1230 kg. After the bypass changes the final mass 

is 1163 kg. This ended up with a better simulation of the primary pressure during the transient 

(Figure 30).

-10000 -9000 -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0

6.3

7.2

8.1

9.0

Level (m)

Time (s)

 Experimental

 RELAP nodalization 1

 RELAP nodalization 2

Figure 4.2 Downcomer Collapsed water level during the conditioning phase of Test 

F1.1, upper head bypass sensitivity

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

0.0

0.9

1.8

2.7

3.6

4.5

Pressure (MPa)

Time (s)

 Experimental

 RELAP nodalization 1

 RELAP nodalization 2

Figure 4.3 Primary pressure of Test F1.1, upper head bypass sensitivity
4-3

4.2 Nodalizing 5 U-tubes

During the OECD/SETH project, one of the most relevant discussions was whether the U-tube 

bundle should be nodalized using more than one pipe at different elevations to capture all 

phenomena involved in SBLOCA with boron dilution. The actual U-tube bundle has 7 different 

elevations, allowing a smooth transition from natural circulation to a static state. Thus, NC might 

be occurring in the lowest tubes of the facility while water in the highest tubes is not able to 

cross to the descending part. It is clearly not possible to simulate such behavior using a single 

U-tube pipe, however what is not so obvious is to what extension the use of several tubes 

improves results.

Most of the groups used 3 different pipes at different elevations. Other participants used a single 

pipe and a few nodalized the region with 5 pipes. Although most participants using more than 

one pipe stated that it was necessary to nodalize the U-tube in this way, other participants, e. g. 

the UPC-INTE, brought in results that had an agreement with the experimental data with the 

same degree of accuracy. Dr. J. Macek from the Nuclear Research Institute (Czech Republic) 

stated that ``the increase of number of SG U-tubes (from 1 to 5) had no significant effect on the 

results'' when simulating tests E2.1, E2.2 and E2.3 [14]. Similarly, a nodalization with 5 different 

U-tubes for each loop was used by the UPC-INTE team in order to assure that using a single 

tube was a suitable description of the U-tube bundle. 

The 5-Utube nodalization was first generated for test E2.3 due to discrepancies found in the 

natural circulation of the loops [7]. It was thought that a nodalization with 5 U-tubes would help 

to hold NC in loops 1 and 3 at SOT. The results showed very little or no differences between the 

two nodalizations as shown in ref. [7]. This nodalization was hence rejected. 

Figures 31 and 32 show results on test F1.1 with very small or inappreciable differences. The 

results were similar for the other SBLOCA with boron dilution tests (test E1.1, E2.2 and E2.3 

[4]).
4-4

0 2000 4000 6000 8000 10000 12000

0.0

0.8

1.6

2.4

3.2

4.0

4.8

Pressure (MPa)

Time (s)

 Experimental primary

 RELAP primary (1 U-tube)

 RELAP primary (5 U-tubes)

Figure 4.4 Primary pressure for Test F1.1 with 1 and 5 U-tube elevations

The reason is that when natural circulation starts, the level in the U-tubes rises sharply. Hence, 

there is not much difference between using 1 or 5 elevations. That behavior was already studied 

in ref. [13], where U-tube levels of all SETH participants where compared for test E2.2. Results 

showed that it takes about a hundred seconds for the U-tube level to rise or fall from the coolant 

line to the top of the U-tubes. The error induced by using a single pipe is of the order of 10 

seconds6

. As the transition is so fast, there is no intermediate state and the use of 5 elevations 

seems to be not necessary. 

The simulation of test F1.2 was first performed with a single U-tube. The results obtained with 

the model had a good agreement with the experimental data although boron dilution started 

earlier according to the RELAP5 results. Analyzing the experimental data, it was seen that 

during the last steady state the test had very low mass flows while, on the other hand the UPCINTE nodalization did not. As there was no liquid crossing to the descending part of the U-tubes 

(only vapor flowed) boron dilution started in the simulation. Moreover, even though some mass 

flow was detected, the loop seals were slightly diluted in the test during this last steady state, 

thus it was thought that NC occurred only in the lowest U-tubes while in the highest ones, boron 

free vapor was circulating. Bearing this in mind, the 5 U-tubes nodalization would be needed. 

The change of nodalization improved the results impressively. Natural circulation stopped just at 

the same time as in the experiment (Figure 33) and the last steady state with mass flow (by 

entrainment) only in the lowest U-tubes was simulated. Therefore full boron dilution started also 

as it did in the experiment (Figure 34).

 

6

 It takes 10 seconds for the liquid level to raise from the top of the lowest U-tube to the highest
4-5

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

2.0

Loop 1

Time (s)

Mass flow (kg/s)

Loop 2

Loop 3

 Experimental RELAP 1 U-tube RELAP 5 U-tubes

Loop 4

Figure 4.5 Mass flow at loop 4 for test F1.1 with 1 and 5 U-tube elevations
4-6

0 10000 20000 30000 40000 50000

0

1

2

3

Time (s)

Loop 4

0

1

2

3

Loop 3

0

1

2

3

End of natural  circulation

End of natural  circulation

0

1

2

3

4

Loop 2

Mass flow (kg/s)

 Experimental RELAP 1 U-tube RELAP 5 U-tube

Loop1

Figure 4.6 Loop mass flows for Test F1.2 with 1 and 5 U-tubes
4-7

0 10000 20000 30000 40000 50000 60000 70000 80000

0

700

1400

2100

2800

Time (s)

0

700

1400

2100

2800

 Experimental RELAP 1 U-tube RELAP 5 U-tubes

LS 4, below SG outlet

LS 3, below RCP

0

700

1400

2100

2800

LS 2, below SG outlet

0

700

1400

2100

2800

Boron concentration (ppm)

LS 1, below SG outlet

Figure 4.7 Boron concentration in all LSs for Test F1.2 with 1 and 5 U-tubes

The 5 U-tubes nodalization was necessary for test F1.2 because, unlike in the other tests where 

NC was stopped or initiated abruptly, in this case the reduction of the mass inventory was 

performed steadily.
4-8

After these results, all tests were performed again with 5 U-tubes to see the influence on the 

results. The new nodalization barely modified the results, the differences were either negligible 

or could not be observed at all. Both nodalizations were able to represent tests E1.1 [6], E2.2

[5], E2.3 [7] and F1.1 [8]; however, test F1.2 had to be nodalized with 5 U-tubes to obtain better 

results [4]. 

Another issue arose at that point. It is important to consider that the number of volumes was 

increased from 438 to 775; therefore the time step was dramatically pumped up. Table 5 shows 

the CPU times of all tests when using one or the other nodalization. The computer time was 

doubled or tripled when using the 5 U-tubes nodalization. Therefore, it is suitable to keep both 

nodalizations and decide when it is necessary to use one or the other. For instance, it is very 

convenient to use the simplest nodalization in order to perform sensitivity studies.

Table 4.1 CPU times using 1 and 5 U-tubes. Calculations performed on a PC, Pentium

4 at 3.4GHz and 512 RAM

Test Time (s) CPU time (s), 1 Utube

CPU time (s), 5 Utubes

Test E1.1 15000 3530 8470

Test E2.2 16700 3892 32714

Test E2.3 19145 4707 14325

Test F1.1 20000 3825 10920

Test F1.2 88000 19070 51250
5-1

5 CONCLUSIONS

A model of the PKL Test Facility has been set up and it has proved to be a suitable tool to 

simulate the behavior of this facility.

Simulation of test F1.1 was in close agreement with the experimental data. A modification of the 

nodalization of the upper head bypass was done improving the results of the test.

The post-test calculation of test F1.2 produced satisfactory results. The different steady states 

were accomplished along with their correspondent natural circulation mass flows. The reduction 

of the boron concentration occurred at similar times and with matching rates. The model proved 

to be able to simulate the boron dilution phenomena when the reflux-condensation conditions 

were established and can be, therefore, used to study this phenomenon in commercial power 

plants.

The issue of the U-tube bundle nodalization turned out to be a key point. The interpretation of 

the experimental data pointed out that during the last steady state before the end of NC, liquid 

water would flow through the lowest U-tubes and vapor in the highest ones. This behavior can 

certainly not be simulated with a single U-tube. Therefore, the nodalization with 5 different Utubes was used for this test.

Two different sensibility studies were performed in order to improve the response of the 

nodalization in both tests:

• U-tube bundle. To properly simulate the U-tube test bundle was one of the main issues 

within the OECD/SETH and OECD/PKL projects. The onset of natural circulation is 

supposed to be greatly affected by how the U-tubes are nodalized since, geometrically, 

the U-tubes have different elevations and therefore natural circulation might be 

established in some of them while circulation in other U-tubes remain still. Simulations 

carried out for various tests where the U-tube bundle was simulated with 5 pipes 

revealed different conclusions. In most cases, i.e. e. in SBLOCA with boron dilution the 

results were barely affected. The computer time was increased as much as 200%, thus 

in these cases the use of more than one U-tube is optional. On the other hand, in test 

F1.2 the use of 5 different elevations improved the results and circulation in some of the 

U-tubes was simulated successfully. Hence, in some cases, when the U-tube level might 

stabilize amidst the top of the U-tubes, an accurate nodalization is needed.

• Upper head bypass nodalization It is well known that the upper head bypass 

nodalization highly affects the behavior of the model and specially the downcomer level. 

A sensitivity study was focused on the nodalization of this part of the facility. The 

changes performed were designed to represent more accurately the geometry yielding 

improved results. In the middle of the four bypass of PKL there is a throat. The 

calculations revealed the importance of both the elevation position of the throat and the 

distance from the downcomer and upper head. Detailed nodalization of this area is 

hereby required. The changes highly affected results of test F1.1.
5-2

The formation of boron diluted slugs was correctly simulated in both PKL tests. Refluxcondensation conditions were reached in all tests with a very similar behavior toward the ones 

observed in the tests. Dilution occurred at both sides of the U-tubes and small boron diluted 

slugs were placed in the hot legs as well. These slugs were formed in both the calculations and 

the experiments although their size was larger in the UPC-INTE simulation. It can be concluded 

that the UPC-INTE nodalization was able to reproduce the necessary conditions and with such 

conditions the boron dilution in the loop seals occurred with the same rates as in the 

experiments.
6-1

6 RUN STATISTICS

The calculations have been performed using the NRC RELAP5 3.3 code, running on a PC 

Pentium 4 3.6MHz. The operating system was Windows 2000. The base case calculation for 

tests F1.1, F1.2 (one U-tube) and F1.2 (5 U-tubes) took 1.8, 5.3 and 14.2 hours respectively of 

CPU time. Figures 35 and 36 display the CPU time for these three calculations. The time step 

remained quite constant and was only substantially increased when the core level was at its 

minimum.

Figure 6.1 CPU time usage for Test F1.1
6-2

Figure 6.2 CPU time usage for Test F1.2 using 1 and 5 pipes to simulate the U-tube 

bundle
7-1

7 REFERENCES 

1. Framatome ANP. PKL III F1.2 CD. Technical report, Framatome ANP report, December

2004.

2. Framatome ANP. PKL III F1.1 CD. Technical report, Framatome ANP report, January 

2005.

3. F. D'Auria, G.M. Galassi, W. Giannotti, D. Araneo, M. Cherubini, and A. Del Nevo. The 

Boron Issue in PWR and VVER-1000. In OECD/NEA/CSNI PKL Project, PKL Analytical 

Workshop | University of Pisa, Pisa, Italy, 2005.

4. J. Freixa. SBLOCA with Boron Dilution in Pressuirzed Water Reactors, Impact to the

Operation and Safety. PhD thesis, Universitat Politècnica de Catalunya, 2007.

5. J. Freixa, C. Pretel, L. Batet, and F. Reventós. Post-test Thermal-Hydraulic Analysis of 

PKL experiment E2.2. Technical report, Institut de Tμecniques Energμetiques, UPC,

2004.

6. J. Freixa, C. Pretel, L. Batet, and F. Reventós. Thermal-Hydraulic study of a final posttest E1.1. Technical report, Institut de Tμecniques Energμetiques, UPC, 2004.

7. J. Freixa, C. Pretel, L. Batet, and F. Reventós. Post-test Thermal-Hydraulic Analysis of 

PKL test E2.3. Technical report, Institut de Tμecniques Energμetiques, UPC, 2005.

8. J. Freixa, C. Pretel, L. Batet, and F. Reventós. Post-test Thermal-Hydraulic Analysis of 

PKL test F1.1 using RELAP5. Technical report, Institut de Tμecniques Energμetiques,

UPC, 2006.

9. J. Freixa, F. Reventós, C. Pretel, and L. Batet. Boron Transport Model with Physical

Diffusion for RELAP5. Nuclear Technology, to appear.

10. H. Kremin, H. limprecht, R. Güneysu, and K. Umminger. Description of the PKL III Test 

Facility. Technical report, Framatome ANP report, july 2001.

11. R. Macian-Juan and J. H. Mahaffy. Numerical diffusion and the tracking of solute fields 

in system codes. Part I: One-dimensional flows. Nuclear Engineering and Design, 

179:297, 1997.

12. T. Mull, S. Bernhard, and K. Umminger. Test PKL III F1.1: Inherent Boron Dilution during 

SB-LOCA. Technical Report FANP-NGTT1/05/en/05, Framatome ANP, 2005.

13. F. Reventós, J. Freixa, L. Batet, C. Pretel, D. Luebbesmeyer, D. Spaziani, J. Macek, F. 

Kasahara, and K. Umminger. An analytical comparative exercise on the OECD-SETH 

PKL 2.2 experiment. Nuclear Engineering and Design, to appear.

14. Technical University Of Catalonia. Proceedings of the First Workshop on Analytical

Activities related to the SETH-OECD project", Barcelona, Spain, September 2003.




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