NUREG/IA-0419 

International 

Agreement Report

Analysis with TRACE Code 

of ROSA Test 1.1: ECCS 

Water Injection Under 

Natural Circulation

Condition 

Prepared by:

A. Julbe, J.L. Muñoz-Cobo, A. Escrivá, A. Romero

Instituto de Ingenierίa Energética

 Polytechnic University of Valencia

 Camino de Vera s/n. 46022 Valencia

A. Calvo, NRC Project Manager

 

 Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: July 2012 

Date Published: October 2012

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-0419 

International 

Agreement Report

Analysis with TRACE Code 

of ROSA Test 1.1: ECCS 

Water Injection Under 

Natural Circulation

Condition 

Prepared by:

A. Julbe, J.L. Muñoz-Cobo, A. Escrivá, A. Romero

Instituto de Ingenierίa Energética

 Polytechnic University of Valencia

 Camino de Vera s/n. 46022 Valencia

A. Calvo, NRC Project Manager

 

 Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: July 2012 

Date Published: October 2012

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

iii

ABSTRACT

The goal of this report is to explain the main results obtained in the simulation performed with the 

consolidated thermal-hydraulic code TRACE of the OECD/NEA natural circulation test ROSA 

1.1, conducted at the Large Scale Test Facility (LSTF) in Japan. To attain the initial conditions 

the power was reduced from 7.11 MW to 1.44 MW, followed by a shutdown of the primary 

coolant pump with the 100% of water inventory in the primary. After some time the test initial 

conditions are attained and steady-state natural circulation conditions are established in the 

primary loop. The test can be divided in three different stages, each one characterized by 

attaining natural circulation conditions under different two-phase states in the primary coolant 

circuit, achieved reducing the primary inventory to 80, 70 & 50% of its original value, discharging 

the primary coolant water via auto bleed line located near the bottom of the RPV. The main goal 

of this report is to analyze the ability of TRACE code to precisely simulate the stratification and 

natural circulation conditions of both single and two-phase flows inside the primary circuit. During 

the experiment, the secondary side conditions were obtained by manually controlling the steam 

and feed-water flow rates in order to maintain the secondary pressure at 6.7 MPa, while in the 

simulation the secondary conditions were automatically controlled. At the beginning of the 

conditioning phase the severe power reduction and the shutdown of the coolant pump produces

a mass flow rate reduction which is almost perfectly matched by the simulation with the TRACE 

code, with only a slightly higher mass flow rate than in the experiment. Meanwhile, the pressure 

remains practically constant in the experiment while it drops during the simulation with TRACE to 

recover after some time. After the first extraction (20% of main inventory) two-phase flow natural 

circulation conditions are attained in the primary loop. It is worth to note that TRACE properly 

models the pressure and temperature evolution with time in the primary system when the 

secondary pressure is kept constant at 6 MPa. The predictions of the natural circulation mass 

flow rates are good after the first extraction but after the second extraction are below the 

experimental values.

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 stateof-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

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/FAP

, 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.

2

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

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 a number of 

experimental facilities and programmes of potential interest for present or future international 

collaboration within the safety community during the coming decade. 

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

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 full-height 

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

EXECUTIVE SUMMARY ................................................................................................................. ix

ACKNOWLEDGMENTS..............................................................................................................xiii

ABBREVIATIONS............................................................................................................................ xv

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

2. INITIAL AND BOUNDARY CONDITIONS............................................................................. 2-1

2.1 Description of the LSTF facility........................................................................................2-1

2.2 Stationary initial conditions for ROSA 1.1 test and results of TRACE code .................2-2

2.3 Test conditions and set points .........................................................................................2-2

2.4 Divergence in bleed-line discharges between simulation and experimental results.....2-6

2.5 Divergence in ECCS injections between simulation and experimental results.............2-7

3. TEST RESULTS AND COMPARISON WITH TRACE ......................................................... 3-1

3.1 Overview of test results and comparison with TRACE...................................................3-1

3.1.1 Phase 1 analysis (-1539 s – 3822 s).................................................................3-1

3.1.2 Phase 2 analysis (3822 s – 8466 s)..................................................................3-2

3.1.3 Phase 3 analysis (8466 s – 13926 s)................................................................3-3

3.1.4 Phase 4 analysis (13926 – 20408 s).................................................................3-4

4. CONCLUSIONS ..................................................................................................................... 4-1

5. REFERENCES....................................................................................................................... 5-1
viii

FIGURES

2.1 Large Scale Test Facility (LSTF) used to perform the ROSA test 1.1. Figure (a) 

displays the points of injection of the emergency core cooling system and the location 

of the break/auto bleed line in the RPV. Figure (b) shows the different components of 

the facility...............................................................................................................................2-1

Page

2.2 Comparison of discharged mass flow rate between experimental and simulation data....2-7

2.3 Comparison between experimental and simulation data of injected mass flow rates 

into cold leg A........................................................................................................................2-8

2.4 Comparison between experimental and simulation data of injected mass flow rates 

into cold leg B........................................................................................................................2-9

3.1 Pressure evolution in the upper plenum of the RPV versus time .......................................3-6

3.2 Evolution of pressure versus time at the secondary of the SG in loop A...........................3-6

3.3 Evolution of pressure versus time at the secondary of the SG in loop B...........................3-7

3.4 Evolution of the temperature at the seal of cold leg A.........................................................3-7

3.5 Evolution of the temperature at the seal of cold leg B.........................................................3-8

3.6 Evolution of the liquid level at the secondary of the SG A ..................................................3-8

3.7 Evolution of the liquid level at the secondary of the SG B....................................................3-9

3.8 Evolution with time of the mass flow rate in cold leg A .......................................................3-9

3.9 Evolution with time of the mass flow rate in cold leg B .................................................... 3-10

TABLES

2.1 Comparison of stationary initial conditions measured in the ROSA experiment against 

data calculated using the TRACE code model ....................................................................2-2

Page

2.2 Sequence of events ..............................................................................................................2-4

2.3 Report events versus experimental measures from the LSTF............................................2-5

2.4 Auto bleed line discharges....................................................................................................2-6

2.5 Cold leg A injections..............................................................................................................2-8

2.6 Cold leg B injections..............................................................................................................2-9

3.1 Experiment phases................................................................................................................3-1
ix

EXECUTIVE SUMMARY

This report presents the main results obtained with the NRC consolidated code TRACE for the 

simulation of the OCDE natural circulation test ROSA 1.1 that was carried out at LSTF.

The LSTF facility represents a PWR reactor reduced to 1/48 scale in volume and power. The 

facility has two primary loops, A and B, with one steam generator per loop, one recirculation pump 

per loop and a pressurizer located in loop A. The emergency core cooling system (ECCS) is 

formed by the accumulator tanks and the high pressure injection system (HPIS). The amount of 

water injected by the ECCS system depends on pressure. Both systems, i.e. the HPIS and the 

accumulators, actuate when the primary pressure drops below specific set point values. In test 

ROSA 1.1 there are three main discharges, amounting for 20%, 10% and 20% of the water 

inventory, which are not followed by HPIS water injection. In any other instance, water injected by 

the HPIS is subsequently discharged from the system, keeping the water inventory constant.

Conditions in the secondary are manually controlled in the experiment in order to maintain the 

pressure and the water level constant, while in the code an automatic controller has been used.

The objective of this assessment is to compare code predictions with experimental data obtained 

during the ROSA 1.1 test in order to establish TRACE’s capability to predict complex transients 

with multidimensional natural two-phase flow convection, and, if possible, to improve the available 

LSTF TRACE model.

A reference steady state situation was first obtained with the TRACE code using the same initial 

boundary condition measured in the LSTF facility for this experiment. During this phase the power 

is reduced from an initial value of 7.11 MW to 1.44 MW (80% reduction) which is equivalent to a 

scaled value of 2% of the reactor power at nominal conditions, followed by a shutdown of the 

primary coolant pumps. Then there is a waiting period of 1.200 s until the system attains the 

single flow natural circulation conditions. Then the ECCS system injects water into the loop A, 

followed by a release of the injected mass through the bleed line, located at the bottom of the 

RPV, then there is a waiting period of 10 minutes followed by a new injection period now into loop 

B, and a new extraction of the injected mass through the bleed line. This sequence of operations 

(waiting time plus injection into loops A or B and release of the injected water) is repeated two 

more times. Finally at the end of this sequence the pressurizer is isolated from the rest of the 

system.

Then the primary coolant inventory is reduced in three time steps by 20%, 10% and 20% 

respectively of the initial inventory. Each inventory reduction period is followed by a ten minutes 

waiting period followed by injection into cold legs A/B at a rate of 0.3 kg/s during 80 seconds. 

Then a new 10 minutes period without coolant extraction is run in order to attain quasi-stationary 

conditions in the system. After, each quasi-stationary period there is a small injection that lasts 80 

s into loops A/B followed by an extraction period through the bleed line that releases the 

previously injected mass except at the beginning of the next extraction period.

The first reduction of the mass inventory to 80% of the initial one, starts at time 4032 s and 

finishes at time 6037 s. During the first 400 seconds of the extraction period an increase of the 

natural circulation mass flow rate is observed in the experiment, being predicted with some delay 

by the TRACE simulation. Then the TRACE code in cold leg A predicts at the beginning of the 

extraction an increase in the mass flow rate but then the simulation results show big oscillations, 

as well as an average mass flow rate that is smaller than the experimental one.

After the second release of water from 80% to 70% at time 9229 s there is a reduction in the 

natural circulation mass flow rate from 10 kg/s to 5 kg/s. The TRACE code predicts also a 

reduction from 7 kg/s to 3 kg/s but displays bigger oscillation in the mass flow rate circulating 
x

through the cold legs. The third inventory discharge that finishes at time 15.166 s provokes a 

reduction of the coolant inventory to 50% of the initial one and also the natural circulation mass 

flow rate diminishes from 5 kg/s to practically zero, in the simulation with TRACE this last 

reduction is also observed but the natural circulation mass flow rate stops earlier in time.

One of the main goals of the present experiment is to analyze the capability of TRACE code to 

predict natural circulation under two phase flow conditions. In this test the thermal stratification 

which appears produced by the injection of cold water by the ECCS system in the cold legs is 

also important. However to analyze the thermal stratification with 1D TRACE components is not 

possible. Only with a 3D modelling of the full primary circuit is possible to analyze this issue, and 

this is only possible with CFD codes.

In order to evaluate the capability of the TRACE code to model this scenario experimental data 

and simulation results have been compared for the following magnitudes: pressure in the RPV 

upper plenum and steam generators, temperatures in both cold leg’s seals, liquid level in the 

secondary of both steam generators and mass flow rates in cold legs A and B.

At the start of the test, pressure in the primary system (measured in the RPV upper plenum) 

decreases slightly before the pressurizer, and heat released by the core acts restoring pressure to 

its initial value. This is not correctly simulated by the TRACE code, where the pressure drop is 

larger than the experimental one and takes more time to recover. During the first main discharge, 

pressure in the system suffers an intense decrease, which is mostly matched by the TRACE 

model. After the first main discharge, pressure in the primary system suffers only slight variations, 

which are not exactly matched by the model, although it gives values not far away from the 

experimental ones. Pressure in the primary side of the steam generators follows a similar 

evolution to the evolution of the pressure in the RPV, while in the secondary side pressure is kept 

between 59 and 66 bars, and does not present a drastic decrease after the first main discharge. 

The TRACE code predicts this behaviour pretty well oscillating between 64 and 65 bars.

Temperature in the cold leg seals also experiment a decrease after reactor power output 

decreases at the start of the test, being this variation larger and longer in the simulations

predictions than in the experimental data. After stabilizing again, temperatures grow and later 

decrease between each of the main discharges, with small variations about 5 K, with 

temperatures being between 546 K and 551 K. However after the third main discharge, which

finishes at time 15166 s the experimental temperature in the cold leg displays temperature 

oscillations of about 75 K. In contrast, TRACE results becomes constant at 551 K between the 

first and second main discharges, then decreases steadily from 551 K to 545 K between the 

second and third main discharges and, after the aforementioned third discharge, increases to 

remain stable at a value higher than the experimental one, not matching the oscillations of the 

experimental data. However both experimental data and TRACE results are close in a band 

ranging from 545 K to 551 K, except at the end of the transient.

Experimental data on the liquid level in the secondary side of the steam generators starts at a 

level of 9.5 m that is maintained oscillating between 9 and 10.2 m. The TRACE simulation

predicts a value that is maintained practically constant at 10 m during the full transient. 

Mass flow rates measured in both cold leg A and B show similar patterns: After the original 

reactor power output decreases, the mass flow rate also decreases to become almost constant 

around a value of approximately 6 kg/s per loop. After the first discharge, the natural circulation 

mass flow rate increases to a value of approximately 10 kg/s per loop, remaining again constant 

until the next main discharge, in which it drops to a constant value of 5 kg/s. After the third and 

last main discharge, experimental mass flow rates shows a decrease in both loops to almost zero 

kg/s. TRACE results matches experimental data until the first main discharge. After this, mass 

flow rate in cold leg A presents a markedly oscillatory behaviour, with an average value of about 
xi

7.5 kg/s, which is lower that the experimental one. Model data are worse in cold leg B, with flow 

rates becoming almost zero after the first main discharge.

xiii

ACKNOWLEGMENTS

The thermal-hydraulic and nuclear engineering group of the UPV is indebted to the management 

board of the ROSA project and to the people of the Thermal-Hydraulics Safety Research Group of 

JAEA. The CSN financed this work, and helped to develop this report. Also we are indebted to 

Cesar Queral that provided a model of the LSTF facility for the TRACE code and many interesting 

suggestions for the improvement of the model.

xv

ABBREVIATIONS

CSN Spanish Nuclear Regulatory Commission

CRGT Control Rod Guide Tubes

ECCS Emergency Core Cooling System

HPIS High Pressure Injection System

LOCA Loss of coolant accident

LSTF Large Scale Test Facility

OCDE Organization for the Cooperation and Economic Development

PH High-Pressure Injection Pump

PJ Charging Pump

PTS Pressurized Thermal Shock

PV Pressure Vessel

PWR Pressurized Power Reactor

ROSA Rig of Safety Assessment 

RPV Reactor Pressure Vessel

SG Steam Generator

TH Thermal-Hydraulics

UNESA Spanish Electricity Producers Association

TRACE TRAC/RELAP Advanced Computational Engine

1-1

1. INTRODUCTION

The present work was performed as a contribution to the OCDE international collaborative 

research project ROSA. A consortium formed by the CSN and several Spanish Technical 

Universities developed the Spanish participation in the project that was coordinated by the CSN 

and a steering committee.

The analysis of the experiment ROSA 1.1 with the TRACE code was assigned to the “thermalhydraulics and nuclear engineering group” of the Polytechnic University of Valencia. This test was 

performed in the nuclear safety research centre of the Japan Atomic Energy Agency in the large 

scale test facility (LSTF) on October 26 and 27, 2006 [1,2].

The goal of the test series 1 was to obtain the multidimensional temperature distribution in the 

cold legs and the vessel downcomer during the ECCS injection for verification of computer codes 

and models, and to check the ability of the present generation of TH codes to model complex 

phenomena where stratification 3D effects are of relevance for the safety of the plants.

The ROSA TEST 1.1 consists of several stages. At the start of the transient, core power was 

decreased to 20% of nominal power (from 7.11 to 1.44 MW), followed within 20 seconds by the 

stop of the primary coolant pump. Conditions in the system are then maintained until variables 

stabilize, resulting in a single-phase natural circulation stage. Thereafter, the pressurizer is 

isolated and the primary water inventory is reduced three times to 80, 70 and 50% of its original 

value, keeping secondary side conditions constant by manually maintaining pressure and liquid 

level in the SGs secondary sides. This is done in order to attain three different two-phase natural 

circulation states in the primary loop. Discharges are produced at a valve located at the end of an 

auto bleed line connected to the RPV near its bottom. The high pressure injection system (HPIS) 

is activated automatically with a delay of 12 seconds when the pressure falls below 12.27 MPa. 

The accumulator injection is activated when the pressure falls below 4.51 MPa. Cold water 

injected into the cold legs by the ECCS system mixes partially with the hot primary coolant flow 

and flows into the downcomer of the pressure vessel. As injected water is colder than the one 

already present in the coolant system, it does not mix completely with the fluid of the cold leg and 

accumulates in the bottom of the pipe, forming a cold layer which moves towards the downcomer, 

while the upper part contains coolant fluid at higher temperature and some steam above the 

interface. This steam partially condenses on the lower boundary layer interface and on the sides 

of the injected jet, and the local pressure variations triggered can cause unstable flow oscillations 

that in turn promote coolant mixing. Such multidimensional and non-equilibrium flow phenomena 

are of concern for pressurized thermal shock (PTS) [3,4].

Aside from the three main aforementioned coolant extractions, there are several ECCS water 

injections which are subsequently mirrored by a mass release via the auto bleed line in order to 

maintain the actual secondary system inventory constant.

The goal of this work is to compare the experimental results with the predictions of the TRACE 

code simulation and to analyze the set of phenomena that take place at the primary and 

secondary loops. Special emphasis is devoted to the analysis between the code results and the 

experimental data.

This work has been divided into four sections: section 1 is an introduction to the ROSA test series 

1; section 2 described the LSFT facility, the initial and boundary conditions of test 1.1 and the 

study of the transient sequence; section 3 deals with the comparison of TRACE simulation results 

for the main physical magnitudes versus the experimental data; finally the conclusions are 

presented in section 4.

2-1

2. INITIAL AND BOUNDARY CONDITIONS

2.1

The LSTF facility as displayed in figure 2.1 simulates a Westinghouse-type four-loop PWR on fullheight and scaled by a factor of 1/48 in volume and power. The flow areas are scaled with a 

factor of 1/48 in the vessel, and 1/24 in the steam generator. As displayed in figure 2.1 the facility 

has two primary loops denoted by A, and B, respectively, which are of similar length and diameter 

but have different shape and location of the ECCS injection nozzles. The hot and cold legs, with 

an inner diameter of 207 mm, have been dimensioned to conserve the volumetric scaling 2/48,

and the ratio of the length to the square root of the diameter, i.e.

Description of the LSTF facility

L / D , in order to properly

simulate the flow regime transitions in the horizontal legs which is one of the goals of this test.

(a) (b)

Figure 2.1 Large Scale Test Facility (LSTF) used to perform the ROSA test 1.1. Figure 

(a) displays the points of injection of the emergency core cooling system and 

the location of the break/auto bleed line in the RPV. Figure (b) shows the 

different components of the facility.

The LSTF represents the reference PWR bypasses by including eight upper-head spray nozzles 

(inner-diameter of 3.4 mm each) and the hot leg nozzle leakage. The spray nozzles allow bypass 

flow that amounts to 0.3% of the total core flow rate during the initial steady-state, while bypass 

area of the hot-leg nozzle is set to allow 0.2% bypass flow for each loop.

Control rod guide tubes (CRGT) represent the flow paths between the upper-head and the upper 

plenum. Eight CRGTs are attached to the upper core plate and pass through the upper core 
2-2

support plate to simulate the CRGT in the reference PWR.

The accumulator tanks have an initial gas volume of 0.46 m3 for both loops. The initial water level 

and volume above the standpipe are 1.58 m3 and 1.12 m3 respectively for both loops. The volume

of the accumulator injection lines with and without pressurizer are 0.2 and 0.14 m3 respectively.

2.2 Stationary initial conditions for ROSA 1.1 test and results of TRACE code

Third column of table 2.1 displays the stationary initial conditions for Test 1.1 specified by the final 

data report, while second and fourth columns show the experimental measured conditions in 

loops A and B and the results of the TRACE code simulation respectively. These initial conditions 

were the operating ones prior to the beginning of data recording.

Table 2.1 Comparison of stationary initial conditions measured in the ROSA experiment 

against data calculated using the TRACE code model

Items Experimental A/B Specified Simulation (TRACE)

Pressure Vessel

Core Power (MW) 7.11 10.0 7.11

Upper Plenum Pressure 15.26 15.4

Primary loop

Hot leg fluid temperature (K) 581.3/580.8 598.0 583

Cold leg fluid temperature (K) 555.3/555.0 562.0 553

Mass flow rate (kg/s/loop) 24.83/24.45 24.3 23.8

Pump rotation speed (rps) 13.8/13.6 13.3 13.8

Pressurizer (PZR)

Pressurizer pressure (MPa) 15.36 15.5 15.3

Liquid level (m) 7.68 7.2 5

Steam Generator (SG)

Secondary side pressure (bar) 6.41/6.39 6.5 6.2

Secondary side liquid level (m) 9.11/9.11 9.1 10.5

Steam mass flow rate (kg/s) 1.77/1.73 2.74 1.8

Main Feed-water flow rate (kg/s) 1.87/1.65 2.74 1.8

Main Feed-Water Temperature (K) 483.9/483.0 495.2 495

2.3 Test conditions and set points

After initial conditions have been fully established, core power is reduced from a measured value 

of 7.11 MW to 1.44 MW, which corresponds to 20% of the measured power output and 2% of the 

scaled nominal power, respectively. Primary coolant pumps are shutdown and thereafter steadystate single-phase natural circulation conditions are established in the primary loops, at a 

pressure of 15.5 MPa. The pressure and water level in the secondary side were kept manually 

constant at 6.4 MPa and 9 meters during the experiment, a point which could not be reproduced 

adequately in the simulation due to lack of information. Instead of a manual control, the model 

incorporates an automatic control which turns on when secondary pressure goes outside the 

6.0/6.4 MPa interval. During this phase of the experiment, ECCS water is injected alternatively in 

the cold leg A and B of the system using the charging pumps (PJ), as injections cannot be 

performed simultaneously in both cold-legs due to a limitation of the pump head. Each of these 

injections is followed between 80 and 100 seconds later by a discharge through the bleed line of 

the same amount of water introduced in the system by the last injection, thus maintaining the 

primary coolant water inventory constant.

After letting the system attain quasi-stationary conditions, a second stage is started by isolating 
2-3

the pressurizer from the rest of the primary loop, and then consecutively reducing the primary 

inventory to 80, 70 and 50% of its initial value, in order to force a two-phase flow regime in the 

primary loop with three different condition sets. Each one of these discharges is not coupled with 

a previous injection, and is always followed by a time period in which conditions in the system are 

left intact in order to let them to reach a quasi-stationary state. During this stage, ECCS water is 

injected using the high pressure injection pumps (PH), which means that they can operate in both 

cold leg A and B at the same time as displayed in tables 2.2 and 2.3. Throughout the experiment, 

injection mass flow rates are of only two types: 0.3 kg/s (which corresponds to the actual injection 

mass flow rate of power plants) and 1 kg/s (which is almost the maximum flow rate attainable at 

15.5 MPa for the LSTF), aside from an additional 1.8 kg/s injection performed in cold-leg B at 

70% and 50% water inventory levels, with these additional injections being the only ones not 

followed by a discharge. The duration of each injection is approximately 80 seconds, and the 

temperature of the ECCS water around 300 K.

The sequence of events of this test, as described in the final data report, is shown in table 2.3. In 

this work, only the data presented in the OECD/NEA final data report regarding ROSA project 1-1 

was taken into account to simulate the experiment using the TRACE code.
2-4

Table 2.2 Sequence of events

Time (s) Event

-1901 Data record start

-1535 Core power decreased from 7.11 to 1.44 MW

-1519 Primary coolant pump stopped

18 - 100 0.225 kg/s injection into cold leg A

275 – 314 Discharge

1128 – 1211 0.203 kg/s injection into cold leg B

1315 - 1349 Discharge

2097 - 2184 0.98 kg/s injection into cold leg A

2267 – 2421 Discharge

3192 – 3275 0.851 kg/s injection into cold leg B

3442 – 3600 Discharge

3822 Pressurizer isolation

4032 – 6107 Discharge: primary inventory 100% -> 80%

6929 – 7004 0.253 kg/s injection into cold leg A

6926 - 7004 0.265 kg/s injection into cold leg B

7155 – 7281 Discharge

8237– 8318 0.997 kg/s injection into cold leg A

8232 - 8319 0.87 kg/s injection into cold leg B

8466 – 9229 Discharge: primary inventory 80% -> 70%

10835 – 10917 0.253 kg/s injection into cold leg A

10828 – 10918 0.279 kg/s injection into cold leg B

11007 – 11135 Discharge

12114 – 12193 1.0 kg/s injection into cold leg A

1212 – 12196 0.889 kg/s injection into cold leg B

12328 – 12547 Discharge

13703 – 13776 1.576 kg/s injection into cold leg B

13926 – 15166 Discharge: primary inventory 70% -> 50%

15834 – 15909 0.254 kg/s injection into cold leg A

15825 - 15913 0.269 kg/s injection into cold leg B

16499 – 16571 Discharge

17339 – 17414 1.0 kg/s injection into cold leg A

17335 – 17417 0.901 kg/s injection into cold leg B

17855 - 18085 Discharge

18876 – 19252 1.565 kg/s injection into cold leg B

19764 Core power off

20408 Data record end
2-5

Table 2.3 Report events versus experimental measures from the LSTF

Event 

Number Report Event Experimental Event Experimental Duration (s)

1 Core power reduced from 10 to 

1.426 MW

Core power reduced from 7.11 to 

1.44 MW NA

2 Primary coolant pump stopped Primary coolant pump stopped NA

3 0.3 kg/s injection into cold-leg A 0.225 kg/s injection into cold-leg A 82

4 Mass release through PV auto 

bleed line

Mass release through PV auto 

bleed line 39

5 0.3 kg/s injection into cold-leg B 0.203 kg/s injection into cold-leg B 83

6 Mass release through PV auto 

bleed line

Mass release through PV auto 

bleed line 34

7 1 kg/s injection into cold-leg A 0.98 kg/s injection into cold-leg A 87

8 Mass release through PV auto 

bleed line

Mass release through PV auto 

bleed line 154

9 1 kg/s injection into cold-leg B 0.851 kg/s injection into cold-leg B 83

10 20% of inventory release through 

PV auto bleed line

20% of inventory release through 

PV auto bleed line 2075

11 0.3/0.3 kg/s injection through 

PJ/PH into cold-leg A/B

0.253/0.265 kg/s injection through 

PJ/PH into cold-leg A/B 75/72

12 Mass release through PV auto 

bleed line

Mass release through PV auto 

bleed line 126

13 1.0 kg/s injection through PJ/PH 

into cold-leg A/B

0.997/0.87 kg/s injection through 

PJ/PH into cold-leg A/B 81/87

14 10% of inventory release through 

PV auto bleed line

10% of inventory release through 

PV auto bleed line 763

15 0.3/0.3 kg/s injection through 

PJ/PH into cold-leg A/B

0.253/0.279 kg/s injection through 

PJ/PH into cold-leg A/B 82/90

16 Mass release through PV auto 

bleed line

Mass release through PV auto 

bleed line 128

17 1.0 kg/s injection through PJ/PH 

into cold-leg A/B

1.0/0.889 kg/s injection through 

PJ/PH into cold-leg A/B 79/76

18 Mass release through PV auto 

bleed line

Mass release through PV auto 

bleed line 219

19 1.8 kg/s injection through PJ into 

cold-leg B

1.576 kg/s injection through PJ

into cold-leg B 73

20 20% of inventory release through 

PV auto bleed line

20% of inventory release through 

PV auto bleed line 1240

21 0.3/0.3 kg/s injection through 

PJ/PH into cold-leg A/B

0.254/0.269 kg/s injection through 

PJ/PH into cold-leg A/B 75/88

22 Mass release through PV auto 

bleed line

Mass release through PV auto 

bleed line 72

23 1.0 kg/s injection through PJ/PH 

into cold-leg A/B

1.0/0.901 kg/s injection through 

PJ/PH into cold-leg A/B 75/82

24 Mass release through PV auto 

bleed line

Mass release through PV auto 

bleed line 130

25 1.8 kg/s injection through PJ into 

cold-leg B

1.565 kg/s injection through PJ

into cold-leg B 76
2-6

2.4 Divergence in bleed-line discharges between simulation and experimental 

results

In figure 2.2 it can be clearly seen that there are divergences between model results (provided in 

the final data report) and the experimental data (contained in the experimental files): although any 

single discharge in the experiment is mirrored by the simulation, with start and end set at the 

same time, there are clear differences in the actual mass flow rate of each discharge. Also, it can 

be seen that the experimental data gives values for the discharge mass flow rate that are different 

from zero between discharges, which could be accounted as noise or even systematic error, as 

the non-zero value tends to be almost constant (near 0.024 kg/s). The systematic error 

assumption seems also to be in line with the actual discharge data, as almost every discharge 

(with the sole exception of the fifth one) is bigger in the experiment than in the model, so a 

systematic error would take both results closer. Disregarding this error, the experimental water 

discharge mass surpasses that of the model by 197 kg, which accounts for around 6% of the 

total, experimental discharged water mass. Taking the 0.023 permanent discharge flow rate into 

account from the start of the experiment at -1901 seconds, adding it to the modelled discharge 

will result in the modelled discharge being 498 kg higher than the experimental one, which will 

account for roughly 14% of the experimentally discharged water mass. In the end, it was decided

not to take into account the noise/systematic error, in order to get modelled the total water mass 

discharge closer to the one in the experiment. Even so, it can be seen in table 2.4 that this 6% 

difference between the experimental and model data is not evenly distributed among every 

discharge, with the three main discharges (those which start at 4032, 8466 and 13926 seconds, 

accounting for 20, 10 and 20% of the original primary coolant water inventory, respectively) being 

farthest away from the intended values in absolute numbers.

Table 2.4 Auto bleed line discharges

Time

[s]

Duration

[s]

Flow rate

[kg/s]

Total mass

(Experiment)

[kg]

Total mass

(Simulation)

[kg]

Difference 

(SimulationExperiment)

[kg]

Accumulated

difference

[kg]

275-314 39 0.473 22.347 18.447 -3.9 -3.9

1315-1349 34 0.496 18.87 16.864 -2.006 -5.906

2267-2421 154 0.55 92.423 85.162 -7.261 -13.167

3442-3600 158 0.447 94.2 70.179 -24.021 -37.188

4032-6107 2075 0.519 960.365 1076.925 116.56 79.372

7155-7281 126 0.315 54.3 39.69 -14.61 64.762

8466-9229 763 0.70684 662.51 536.2 -126.31 -61.548

11007-11135 128 0.3568 52.0945 45.568 -6.5265 -68.0745

12328-12547 219 0.6694 170.721 146.73 -23.991 -92.0655

13926-15166 1240 0.869877 1118.255 1068.36 -49.895 -141.9605

16499-16571 72 0.548 54.82 42.744 -12.076 -154.0365

17855-18085 230 0.647 192.601 148.81 -43.791 -197.8275
2-7

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Mass Flow Rate (kg/s)

Time (s)

Model Data Experimental Data

Figure 2.2 Comparison of discharged mass flow rate between experimental and simulation

data.

2.5 Divergence in ECCS injections between simulation and experimental results

As ECCS injections were different in cold leg A and B, it was deemed necessary to analyze the 

ECCS injections in each cold leg separately. Figure 2.3 shows water injections into cold leg A, 

while figure 2.4 shows water injections into cold leg B.

In figure 2.3 it can be seen that every programmed injection into cold leg A appears also in the 

model. The experimental data again presents an injection flow rate different from zero between 

programmed injections, which could be due to data noise or a systematic error, as it fluctuates 

around a value of 0,.061 kg/s. The figure also shows that there are five flow rate peaks 

(corresponding to the second, fourth, ninth and twelfth injections) which do not correspond to 

programmed injections, being attributed to data acquisition errors. Also, most injections have a 

top value greater in the experiment than in the simulation, which could be accounted for an 

inertial effect. At the end, even if the noise/systematic error is not taken into account, the 

experimental mass injected in cold leg A exceeds the model injected mass by 70 kg, which 

accounts for roughly 17% of the simulated total mass.
2-8

Table 2.5 Cold leg A injections

Time

[s]

Duration

[s]

Flow rate

[kg/s]

Total mass

(Experiment)

[kg]

Total mass

(Simulation)

[kg]

Difference 

(SimulationExperiment)

[kg]

Accumulated

difference

[kg]

18-100 82 0.225 20.25 19.575 -0.675 -0.675

2097-2184 87 0.98 91.63 85.26 -6.37 -7.045

6929-7004 75 0.253 27.61 21.75 -5.86 -12.905

8237-8318 81 0.997 92.606 79.76 -12.846 -25.751

10835-10917 82 0.253 26.035 22.77 -3.265 -29.016

12114-12193 79 1 93.5 79 -14.5 -43.516

15834-15909 75 0.254 23.749 19.05 -4.699 -48.215

17339-17414 75 1 97 75 -22 -70.215

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Mass Flow Rate (kg/s)

Time (s)

Model Data Experimental Data

Figure 2.3 Comparison between experimental and simulation data of injected mass flow 

rates into cold leg A.

In figure 2.4 it can be seen that every programmed injection into cold leg B appears also in the 

model. The experimental data again present an injection flow rate different from zero between 

programmed injections, which could be due to data noise or a systematic error, with an almost 

constant value of 0.08 kg/s. Unlike the data regarding cold leg A, in cold leg B there are no 

unplanned flow rate peaks, and generally injections are almost identical in experimental data and 

simulation results (with the 7th and 10th injections presenting the greater deviations). Even so, the 

experimental total mass injected into cold leg B exceeds the model injected mass by 120 kg, 

which accounts for 11% of the simulation total mass.
2-9

Table 2.6 Cold leg B injections

Time

[s]

Duration

[s]

Flow rate

[kg/s]

Total mass

(Experiment)

[kg]

Total mass

(Simulation)

[kg]

Difference 

(SimulationExperiment)

[kg]

Accumulated

difference

[kg]

1128-1211 83 0.203 16.6 16.6 0 0

3192-3275 83 0.851 72.675 70.55 -2.125 -2.125

6926-7004 78 0.265 21.38 20.67 -0.71 -2.835

8232-8318 86 0.87 79.17 75.69 -3.48 -6.315

10828-10918 90 0.279 24.823 23.157 -1.666 -7.981

12120-12193 73 0.889 76.985 67.64 -9.345 -17.326

13703-13776 73 1.576 154.9895 115.048 -39.9415 -57.2675

15825-15913 88 0.269 25.824 24.21 -1.614 -58.8815

17335-17417 82 0.901 88.2 76.5 -11.7 -70.5815

18876-19252 376 1.565 650.52 600.6 -49.92 -120.5015

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Mass Flow Rate (kg/s)

Time (s)

Model Data Experimental Data

Figure 2.4 Comparison between experimental and simulation data of injected mass flow 

rates into cold leg B.

In the end, although there is a divergence between experimental and simulation data, the 

differences between both injections and discharges seem to compensate partially each other: 

The model injects less water into the system, but it does so in almost the same proportion for 

both injections and discharges.

3-1

3. TEST RESULTS AND COMPARISON WITH TRACE

3.1 Overview of test results and comparison with TRACE

This section is devoted to review the main experimental results and to discuss the results 

obtained with the TRACE code. Steady-state conditions were attained prior to the start of the 

experiment. Table 3.1 shows these initial conditions as they were originally intended to be, as 

registered during the experiment and the ones calculated by the TRACE model. Concerning to 

the experimental data obtained, the most important events are the core power decrease, the 

primary coolant pump stop, the pressurizer isolation and the three main discharges. According to 

this, the experiment can be divided into the following phases:

Table 3.1 Experiment phases

PHASE Starting event / time Ending event / time

1 Core power decrease / -1539 Pressurizer isolation / 3822

2 Pressurizer isolation / 3822 Discharge 80 -> 70% inventory / 8466

3 Discharge 80 -> 70% inventory / 8466 Discharge 70 -> 50% inventory / 13926

4 Discharge 70 -> 50% inventory / 13926 End of test / 20408

3.1.1 Phase 1 analysis (-1539 s – 3822 s)

Phase 1 of the experiment starts at -1539 seconds with a core power output reduction from 7.11 

to 1.44 MW. The conditions at the start of this phase are those previously listed in table 2.1. The 

power reduction is followed in 20 seconds by the shutdown of the primary coolant pump, thus 

prompting the establishment of natural circulation in the primary side. It’s followed by 4 injections 

(two in leg A, two in leg B) and their respective discharges. The phase ends at 3822 seconds with 

the isolation of the pressurizer in cold leg A.

At the beginning of this phase, pressure in the upper plenum of the RPV (displayed at figure 3.1) 

decreases in the experiment from 15.45 to 15 MPa, recovering original conditions shortly

afterwards (around 156 seconds). In the other hand, the simulated pressure gets a more 

remarkable decrease, going down to 12.7 MPa, and then needing 3436 seconds to match

experimentally measured conditions. This difference could be explained by the use of an 

automatic controller for the secondary side flow rate. Pressure in the secondary side of the steam 

generators A and B (shown respectively in figure 3.2 and figure 3.3) behave in a similar way: in 

both cases, the experimental pressure decreases from 6.3 to 5.9 MPa, returning in around 1200 

seconds to the original value, while the pressure in the model drops to a lower value (3.75 MPa) 

and takes more time to recover the intended experimentally determined conditions (around 2000 

seconds in both cases).

Temperature in the seal sections of both cold legs A and B (shown in figures 3.4 and 3.5) evolves 

in a way similar to the cold leg pressures: experimental temperature in cold leg A goes down from 

555 to 549 K, after the power decrease to recover the initial condition value in 1100 seconds. 

After 440 seconds, temperature decreases again, and begins to oscillate between 547 and 551 K. 

The simulation results show a more pronounced decrease, arriving to 524 K, then increases to 

541 K at t = 550 seconds. From that point onwards the model temperature oscillates around 551 

K close to the experimental value.

Experimental temperature in cold leg B decreases from 555 to 550 K, to recover the initial 

condition value in around 1000 seconds. After 520 seconds, temperature decreases again, and 

begins to oscillate between 549 and 551 K, in a way similar to that of cold leg A. The simulation 

results again display a more pronounced decrease at the beginning of the transient after the 

shutdown arriving to 523 K, then the temperature increases to 550 K at t = 550 seconds close to 

the experimental values. The temperature obtained with TRACE oscillates around 551 K, as in 
3-2

cold leg A, and close to the experimental value.

Experimental values of the water level in the secondary of the steam generator of cold leg A 

(figure 3.6) change constantly during this phase, dropping from 9.36 meters at -1129 seconds to 

roughly 7.2 meters at 402 seconds, only to rise again to 9.5 meters at 1482 seconds, oscillating

from that point onwards in response to the operator actions, varying from 9 to 10 meters. On the 

other hand the liquid level in steam generator A computed by the TRACE code don’t change until 

second -1091, in which it drops from 10.4 to 10 meters, a value around which the water level will 

slightly oscillate for the remainder of the test. 

In the case of water level in cold leg B in the secondary of the steam generator (figure 3.7),

experimental values are almost identical to those measured in cold leg A. On the other hand, 

TRACE results for liquid level in steam generator B don’t change until second -1232, in which it 

increases from 10.4 to 11.7 meters at -888 seconds, to immediately start decreasing again to 10 

meters at 514 seconds, a value at which the water level slightly oscillates for the remainder of the 

test. 

In this phase, the mass flow rates on both legs seem to be adequately predicted by TRACE. The 

mass flow rate in cold leg A (figure 3.8) decreases from its initial value immediately after the 

reactor power reduction, reaching an experimental value of roughly 6.2 kg/s while TRACE model 

yields a value of around 7.37 kg/s, to decrease to a value around 6.7 kg/s short after the power 

decrement. Mass flow rate in cold leg B (figure 3.9) follows a similar pattern, with experimental 

and computed mass flow rates in this phase stabilizing around 6.2 and 6.7 kg/s respectively, 

therefore TRACE results for the natural circulation mass flow rates are close to experimental 

ones in loop B and slightly higher in loop A. So TRACE model predicts adequately the single 

phase natural circulations mass flow rates during this initial phase.

3.1.2 Phase 2 analysis (3822 s – 8466 s)

Phase 2 of the experiment starts at t=3822 seconds with the pressurizer’s isolation, and finished 

at time 8466 s with the beginning of the second main discharge. At time 4032 s starts the first 

main discharge of 20% of the main water inventory, which finishes at time 6107 s. This phase 

also comprises 4 minor injections in loops A/B and one single discharge through the bleed line as 

shown in tables 2.4, 2.5 and 2.6.

Once the pressurizer becomes isolated, it is no longer able to regulate the pressure in the primary 

coolant circuit and when the first main discharge starts the pressure registered in the upper 

plenum begin to decrease. When the extraction finishes at time 6107 s the pressure stabilizes at 

a lower level of 7.0 MPa. TRACE code predicts pretty well the decrement of the pressure while 

the extraction is taking place with the same slope at the beginning of the extraction and with a 

slightly different slope between 4500 s and 5500 s, to finish at the same pressure than the 

experiment short after the end of the bleeding period (figure 3.1).

The pressure at the secondary of the SG presents, as displayed in figures 3.2 and 3.3, the same 

behavior in both loops, displaying a pressure decrement during the first extraction period. The 

experimental pressure diminishes during this phase from 6.7 MPa at the beginning to 6.2 MPa at 

the end. The pressure decrement during this extraction phase is governed by the manual actions 

taken by the operator to compensate for the pressure diminishment caused by the coolant 

extraction in the primary that produces less heat transference from the primary to the secondary 

coolant in the SG. However, TRACE results oscillates between 6 MPa and 6.4 MPa as result of 

the action of the automatic controller implemented in the TRACE model that maintains the 

pressure at an average value of 6.2 MPa.

Temperature in the seal section of loop A starts at 556 K at the beginning of this phase, then 

drops to 550 K, at 8415 seconds with small variations, maintaining this value until the end of the 

phase II (figure 3.4). On the other hand, TRACE results during this phase display small 

oscillations around 551 K, maintaining practically this value until the end of this phase. 
3-3

Temperature in the seal section of loop B follows a similar pattern as that on loop A, although with 

slightly smaller temperatures (555 and 550 K at start and end of the phase). TRACE results for 

loop B behave in a similar way as that for seal section A, although it more clearly reproduces 

experimental temperature’s rises and drops, matching experimental data for most of the phase

(figure 3.5).

The experimental values of the water level in the secondary of the steam generator A remain 

almost constant for the entire phase, with an average value of about 9.7 meters while TRACE

simulation gives during this phase a constant value of 10 meters (figure 3.6). Water level in the 

secondary of the steam generator B starts this phase decreasing from 10.2 meters to 9 meters, 

then rises again to a value of 10 meters, to again fall. TRACE results displayed at figure 3.7 show 

small oscillations around an average value of 10 meters.

Finally the experimental mass flow rate in cold leg A during this phase increases from a 

practically constant value of 5.9 kg/s to 11 kg/s at t= 5880 seconds, maintaining this value until 

the end of the phase. The mass flow rate predicted by the TRACE code, increases at the 

beginning of this phase from 7.37 kg/s to 14 kg/s and then begins to oscillate around an average 

value of 7.5 kg/s, maintaining this average value until the end of the phase, see figure 3.8. The 

experimental results in cold leg B are pretty similar to those observed in cold leg A, see figure 3.9. 

However, TRACE predictions show an increase of the mass flow rate from 6.7 kg/s at the 

beginning of the phase to 14.7 kg/s at time 4600 s, then the mass flow rate begin to oscillate with 

an average value of 9 kg/s and finally falls to 0.2 kg/s at the end of the extraction (t=6107 s). Two 

phase flow natural circulation conditions are established during this phase.

3.1.3 Phase 3 analysis (8466 s – 13926 s)

Phase 3 of the experiment starts at 8466 seconds with the start of the second main discharge 

that lasts for 763 s (reducing primary coolant water inventory from 80 to 70%). This phase

finishes at 13926 seconds with the start of the last main discharge (reduction of the coolant water 

inventory from 70 to 50%). During this phase, two cold leg A+B injections and one cold leg singleB injection take place. 

Experimental data on pressure in the upper plenum of the RPV shows that pressure starts at a 

value of around 6.8 MPa and decreases slightly to 6.7 MPa during the main discharge of this 

phase, then it remains practically constant with small variations between 6.7 MPa and 6.9 MPa 

during the rest of this phase (figure 3.1). TRACE results for pressure show average values close 

to the experimental ones, decreasing slightly from 6.9 MPa to 6.8 MPa.

Pressure in the secondary of both steam generators depends on the manual actions taken by the 

operator during this period and behaves experimentally in an almost identical way in both SG. 

The pressure starts with a value of 6 MPa at the beginning of this phase, and then increases for 

1800 seconds up to 6.5 MPa. This value is maintained until t=10588 seconds, when pressure 

begins to decrease again to 6 MPa at the end of the phase. The predicted values by the TRACE 

code as displayed in figures 3.2 and 3.3, show that the pressure is maintained between 6 MPa 

and 6.4 MPa during this phase with an average value of 6.2 MPa, close to the average 

experimental result.

Experimental liquid temperature in the seal section of cold leg A starts decreasing at the 

beginning of this phase, to stabilize at a value of 549 K at t = 8550 s, then increases up to 555 K

at 10500 seconds, with the increase becoming markedly stronger after the end of the main 

discharge, see figure 3.4. Temperature maintains constant for a while until t = 11005, when it 

decreases again to a temperature of 549 K. Experimental temperature in the seal section of cold 

leg B, see figure 3.5, evolves in the same way as temperature in cold leg A. TRACE results 

predicts a decrease in the liquid temperature from 551 K at the beginning of the second extraction 

to 547 K at the end of this phase, this reduction in temperature is mainly a consequence of the 

extraction through the bleeding line. Also it is worth nothing that the injection through the cold leg 
3-4

A/B is cold water and the extractions through the bleeding line is coolant with more enthalpy, 

because the water injected by the ECCS partially mixes with hotter coolant in the cold leg and in 

the downcomer.

Experimental values of the water level in the secondary of SG A start this phase decreasing to a 

level of 9.2 meters at t = 9395 seconds (figure 3.6). It then increases to a level of 9, 8 meters at 

10450 seconds, to again decrease in two steps to a level of 9 meters at t = 12800 seconds, from 

which it will steadily increase for the remainder of the experiment. Experimental values of water 

level in steam generator B follow a similar evolution during this phase, see figure 3.7. TRACE 

results show the same behavior in both cold legs, showing an almost constant value of 10 meters

as result of the control system implemented to control the secondary pressure and the secondary 

collapsed level. 

Experimental mass flow rate in cold leg A presents an oscillating behavior around a constant 

value of 10.2 kg/s until t = 8700 seconds as displayed in figure 3.8, at this time and as 

consequence of the second extraction it decreases to stabilize around 5 kg/s, oscillating around 

that value until the end of the phase. This reduction in the natural circulation mass flow rate is a 

consequence of the reduction of the natural circulation driving force between the cold and hot 

legs. In cold leg B experimental data follows a similar evolution pattern as that of cold leg A, see 

figure 3.9. TRACE results also display a reduction of the mass flow rate during the second 

extraction. But the reduction of the mass flow rate is stronger than that observed experimentally;

the natural circulation mass flow rate drops to 2.5 kg/s. It can be observed in the simulation 

results that steam accumulates in the upper part of the U-tubes of the SG and this reduces the 

natural circulation mass flow rate. TRACE results in loop B also predict a reduction in the mass 

flow rate to an average value 0.2 kg/s, as it did in the previous phase, and continues this way for 

the rest of the phase.

3.1.4 Phase 4 analysis (13926 s – 20408 s)

Phase 4 of the experiment starts at 13926 seconds with the beginning of the last main discharge 

(reducing primary coolant water inventory from 70 to 50% of its original value), and finishes at 

20408 seconds with the end of the experiment. During this phase, in addition the are two 

simultaneous cold leg A+B injections and one single-injection in cold leg B, with only the A+B 

injections being mirrored by proportional discharges through the bleed line.

Experimental data on pressure in the upper plenum of the RPV starts at a value of around 6.5

MPa, which is maintained, with a small increase, practically constant until 18600 s, as displayed 

in figure 3.1. From that point onwards, pressure decreases until the end of the experiment, with 

this decrease becoming stronger after the power gets shutdown. TRACE results oscillate around 

an average value of 6.5 MPa for almost all the duration of the phase, only showing an evident 

decreasing tendency after power is off. It can be concluded that experimental and predicted

values match during this phase.

Experimental pressure in the secondary of both steam generators follows a similar pattern as 

pressure in the upper plenum, starting at a minimum initial value of 5.9 MPa, and then this value 

increases slowly until 18615 seconds with a maximum pressure of 6.2 MPa, followed by a twostep decrease until the end of the experiment, with the first step being seemingly related to the 

last injection into cold leg B, and the second step being related to reactor shutdown. TRACE 

results predictions are around a center value (6.20 MPa) not very far from the experimental 

values, only decreasing at t = 19600 seconds, seemingly due to the reactor shutdown (see 

figures 3.2 and 3.3). 

Experimental temperature in the seal section of cold leg A (figure 3.4) starts at a value of 549.4 K, 

then, after a slight increase, decreases to a value of 545.5 K at t = 15653 seconds, which is 

roughly 200 seconds after the last main injection finishes. Subsequently temperature suffers an

increase, reaching a value of 552 K at 16113 seconds, and from then on it decreases to 543 K at 
3-5

16700 seconds. This temperature will be maintained for 500 seconds. After this, experimental 

temperature shows oscillations as displayed in figure 3.4. The maximum and minimum values of 

these oscillations are 550, 525 and 553 K in just 55 seconds, which seem related with the last 

simultaneous cold injections into cold legs A/B, then the temperature returns to 543 K.

Temperature then remains almost constant until t = 18650, when it increases to 550 K, to 

decrease until the end of the experiment as consequence of the last cold injection and the power 

shut off. Experimental temperatures in cold leg B follow a similar pattern until the beginning of the 

last cold water injection which produces a reduction in the temperature, see figure 3.5. TRACE 

results show the same behavior in both cold legs, starting at 545 K at the beginning of the phase

and decreasing to 544.8 K at 14397 seconds close to the experimental value. Then the predicted 

temperature rises to 552.7 K, a value around which it will oscillate until t = 18836 s, and that is 

about between 5 and 7 K above the experimental value. The only major change of temperature 

occurs when reactor is shutdown at t = 19700 s, and the temperature drops to a value of 550.93 

K. In general the predicted temperatures in the loop seals during last step of phase 4 are between 

5 and 7 K above the experimental values. This effect could be partially explained by the increase 

of the SG level during the last part of the transient, due to the manual operator actions. This rise 

of the water level in both SG, mainly in SG B, increase the primary to secondary heat transfer, 

reducing the experimental temperature in the primary. So TRACE predicted temperatures 

practically match the experimental ones except during the last part of phase 4 when a difference 

of 5 K appears.

Experimental values of water level in steam generator A (figure 3.6) start this phase at an almost 

constant value of 9.14 meters, then increase for the remainder of the test, with this increase 

becoming more pronounced after t = 18700 seconds, when reactor is shutdown, finishing the 

experiment at a value of 10.41 meters. Experimental liquid level in steam generator B (figure 3.7)

follows the same pattern as the one in SG A, although with slightly higher values (it starts at 9.5 

meters and ends at 11.2 meters). TRACE results for both steam generators show a value which

oscillates around 10 meters close to the average experimental value during this phase except at 

the end.

Experimental mass flow rate in cold leg A starts this phase decreasing to a value of practically 

zero mass flow rate that sometimes becomes slightly negative (-0.332 kg/s), after which it 

presents three small increases (which seemingly correspond to the minor injections which take 

place during this phase) followed by a short constant period, to again drop to zero mass flow rate, 

a value which will be maintained until the end of the test, see figure 3.8. The experimental 

average mass flow rate reported during this period is located between 0.5 kg/s and 1.3 kg/s per 

loop. TRACE results for the mass flow rate started this phase oscillating around an average value 

of 1.2 kg/s, and slightly oscillate during this phase until reaching an almost stable value of around 

0.2 kg/s.

Experimental mass flow rate in cold leg B also decreases during the last extraction to a value of 

1.15 kg/s at t = 14391 seconds, see figure 3.9. Then, remains almost constant, displaying one 

main increase in the mass flow rate that starts at time 18.800 seconds which is related to the last 

injection. TRACE results display an oscillating behavior during this phase with an average mass 

flow rate of 0.2 kg/s. TRACE code also predicts a small increase in the mass flow rate during the 

last extraction period. Mass flow rates predicted by TRACE in cold leg B are below the 

experimental ones. 
3-6

0

2

4

6

8

10

12

14

16

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Pressure (MPa)

Time (s)

Model Data Experimental Data

Figure 3.1 Pressure evolution in the upper plenum of the RPV versus time.

0

1

2

3

4

5

6

7

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Pressure (MPa)

Time (s)

Model Data Experimental Data

Figure 3.2 Evolution of pressure versus time at the secondary of the SG in loop A.
3-7

0

1

2

3

4

5

6

7

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Pressure (MPa)

Time (s)

Model Data Experimental Data

Figure 3.3 Evolution of pressure versus time at the secondary of the SG in loop B.

300

350

400

450

500

550

600

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Temperature (K)

Time (s)

Model Data Experimental Data

Figure 3.4 Evolution of the temperature at the seal of cold leg A.
3-8

300

350

400

450

500

550

600

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Temperature (K)

Time (s)

Model Data Experimental Data

Figure 3.5 Evolution of the temperature at the seal of cold leg B.

0

2

4

6

8

10

12

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

SG A Liquid Level (m)

Time (s)

Model Data Experimental Data

Figure 3.6 Evolution of the liquid level at the secondary of the SG A.
3-9

0

2

4

6

8

10

12

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

SG B Liquid Level (m)

Time (s)

Model Data Experimental Data

Figure 3.7 Evolution of the liquid level at the secondary of the SG B.

-5

0

5

10

15

20

25

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Mass Flow Rate (kg/s)

Time (s)

Model Data Experimental Data

Figure 3.8 Evolution with time of the mass flow rate in cold leg A.
3-10

-5

0

5

10

15

20

25

-2500 0 2500 5000 7500 10000 12500 15000 17500 20000

Mass Flow Rate (kg/s)

Time (s)

Model Data Experimental Data

Figure 3.9 Evolution with time of the mass flow rate in cold leg B.
4-1

4. CONCLUSIONS

In this report ROSA test 1.1 has been analyzed using a model of the LSFT created for the 

TRACE code. Initial steady-state conditions were achieved by maintaining every variable of the 

model at the intended initial value for as long as seven thousand seconds in order to assure that 

every relevant variable stabilized at a value close to the shown in the final data report. This is not 

reflected in the data shown in this report, as it is irrelevant for its purpose.

At the beginning of the test, core power is decreased and primary coolant pump stopped, forcing 

one-phase natural circulation in the primary system. After 5200 seconds, the pressurizer is 

isolated from the rest of the secondary system and the first main discharge (accounting for 20% 

of the original primary water inventory) takes place, forcing two-phase natural circulation into the 

primary side of the system. 2400 seconds after the end of this first main discharge, a second 

main discharge accounting for 10% of the original primary water inventory takes place, the effect 

of this discharge is to reduce the natural circulation mass flow rate flowing through the primary 

system. Finally 5000 seconds later, the third and last main discharge (again accounting for 20% 

of the original primary water inventory) takes place; after this last extraction is performed, natural 

circulation mass flow rate reduces to very small values in loop A, and to 1.2 kg/s in loop B. 

At the start of the test, and after the drop of power and the shutdown of the coolant pumps, 

pressure in the primary system (measured in the RPV upper plenum) decreases slightly before 

the pressurizer and the operator actions in the secondary, closing the steam valves and restoring 

pressure to its initial value. This is not correctly predicted by the TRACE simulation, where the 

pressure drop is larger and takes more time to recover to its initial value. This difference is 

caused by the different methods used to control the pressure in the secondary that is by the 

operator actions in the experiment, and implemented by means of an automatic-control system in 

the TRACE code. During the first main discharge, pressure in the system suffers an intense 

decrease, which is mostly matched by the TRACE simulations. After the first main discharge, 

pressure in the primary system suffers only slight variations, which are not exactly matched by 

the TRACE results, although it offers values not far away from the experimental ones. Pressure in 

the secondary side of the steam generators follows a similar evolution pattern during the transient 

maintaining an average pressure of 6.2 MPa, although it does not present a drastic decrease 

after the first main discharge, due to the operator actions, maintaining an almost constant value 

for the entire test. However, TRACE predictions for the pressure in the secondary of the steam 

generators displays a larger decrease immediately after the power reduction due to the type of 

control implemented that gives good results for the rest of the transient but is not able to restore 

immediately the pressure to the initial value when the heat transferred to the secondary side 

drops suddenly after the power reduction.

Temperature in the cold leg seals diminish after reactor power output decreases at the start of the 

test, being this decrement larger and longer in the TRACE simulation than in the experimental 

data. After stabilizing again, temperatures grow and later decrease between each one of the main 

discharges, to display some oscillations after the third main discharge. After the first discharge

and during the second phase of the transient the temperature is maintained at an average value 

of 552.5 K, while TRACE results give a practically constant value of 551 K, close to the 

experimental one. During the third phase of the transient the experimental data shows an 

increase of the liquid temperature and then the temperature start to decrease continuously until 

the end of this phase. TRACE results show that the temperature diminishes during this phase 

with a constant slope approaching the experimental value at the end of the phase. During the 

fourth phase and after the last extraction the TRACE simulation displays an increase of the 

temperature in the seal of the cold leg at a value around 554 K in both legs maintaining this value 

practically constant during the rest of the test. However, the experimental value displays

temperature oscillations caused by small injections and discharges that affect to the temperatures 

in the seal due to the small collapsed water level in the system.
4-2

Experimental data on liquid level in the secondary side of the steam generators starts at 9.5 

meters which is lower than the one simulated by the TRACE code. Then both experimental and 

TRACE results decrease but while TRACE results decrease to a 10 m level, experimental data

show a bigger decrement to then recover to a level of 9.6 m. Then, in the experiment the operator 

action maintains the level oscillating around an average value of 9.55 m in loop A, while the Level 

calculated by TRACE is maintained practically constant at 10 m. Water level in loop B follows a 

similar pattern to that of loop A, but experimental and calculated values are closer, experimental 

average water level is around 9.7 m while TRACE results are closer to 9.9 m.

Mass flow rates measured in both cold leg A and B show similar patterns: After the original 

reactor power output decrease, and the shutdown of the pumps, mass flow rate decreases to 

become almost constant around a value of approximately 6 kg/s, this stage is well predicted by 

the TRACE code. After the first discharge, the mass flow rate increases to a value of 

approximately 10 kg/s, remaining again constant until the next main discharge, in which it drops 

to a constant value of 5 kg/s. After the third and last main discharge, experimental flow rate 

shows a decrease to almost zero kg/s. TRACE results match the experimental data until the first 

main discharge. After this, the mass-flow-rate in cold leg A present a markedly oscillatory 

behaviour, with average values lower than the experimental ones. TRACE results are even worse 

in cold leg B, with small mass flow rates after the first main discharge.

In sight of the current results, it can be concluded that TRACE code can satisfactorily reproduce 

temperature and pressure changes under two-phase natural circulation conditions over long time 

periods. At the beginning of the test and before the first extraction, TRACE properly simulates the 

single-phase natural circulation mass flow rate. Also, TRACE predicts the increase in the natural 

circulation mass flow rate that takes place when the extraction begins but when the first extraction 

finishes TRACE predictions are below the experimental values for the mass flow rate. In general 

after the second and third extractions TRACE simulation are lower than the experimental value 

for the mass flow rate.

Finally, it seems that TRACE code simulations show consistent results for pressure, temperature, 

liquid level and also mass flow rate in single flow natural circulation conditions. But these results 

are not so good regarding mass flow rate calculations under two phase flow natural circulation 

conditions. Further analysis and reviewing of the ROSA facility model for TRACE are necessary 

to know the reason of this last discrepancy.
5-1

5. REFERENCES

1. Thermal-Hydraulic Safety Research Group. “Final Data Report of OECD/NEA ROSA 

Project Test 1-1 (ECCS water injection under natural circulation condition: ST-NC-34 in 

JAEA)”. Nuclear Safety Research Centre, Japan Atomic Energy Agency, January. 2008.

2. The ROSA-V Group. “ROSA-V Large Scale Test Facility (LSTF) System Description for the 

Third and Fourth Simulated Fuel Assemblies”, JAERI-Tech 2003-037, Japan Atomic Energy 

Agency. 2003.

3. Reyes, J.N., “Flow stagnation and thermal stratification in single and two phase natural 

circulation loops”, IAEA-TECDOC-1474, Appendix-12. 2005.

4. Lele, A.L., S.K. Gupta, H.S. Kushawaha and R. Venkat. “Modelling of thermal and flow 

stratification for reactor pressure vessel pressurized thermal shock”. Nuclear Engineering 

and Design. Vol. 212: pp 75-84. 2002.




UNITED STATES 

NUCLEAR REGULATORY COMMISSION

WASHINGTON, DC 20555-0001

--------------------

OFFICIAL BUSINESS
NUREG

/IA-0419 Analysis with TRACE CODE of ROSA Test 1.1: 

ECCS Water Injection Under Natural Circulation Conditions