NUREG/IA-0415 

International 

 Agreement Report

TRACE (V 5.0 Patch 2) Validation Based on the 

RELAP5-Calculation of FIX-III LOCA Experiments 

NO. 5052, 4011, 3051

 

Prepared by:

S. ChunHong Sheng

Studsvik Nuclear AB 

SE-61182 Nyköping Sweden

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: February 2013

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-0415 

International 

 Agreement Report

TRACE (V 5.0 Patch 2) Validation Based on the 

RELAP5-Calculation of FIX-III LOCA Experiments 

NO. 5052, 4011, 3051

 

Prepared by:

S. ChunHong Sheng

Studsvik Nuclear AB 

SE-61182 Nyköping Sweden

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: February 2013

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

iii

ABSTRACT

The purpose of this project is to (i) evaluate the functionality of the SNAP capability to 

convert RELAP5 input files to corresponding TRACE input, and (ii) to validate the 

TRACE code against selected FIX-II experiments where the blowdown phase of large 

and small break LOCA is the main focus. 

More specifically, the TRACE Version 5.0 Patch 2 code is validated against the FIX-II 

experiments, where the input is obtained by converting a legacy RELAP5 FIX-II model 

by means of SNAP, possibly accompanied by additional manual adjustments. All FIX-II 

components in the RELAP5 model are retained after being converted to TRACE. Three 

transients (FIX 5052, 4011 and 3051) are analyzed. The TRACE analysis results are 

compared with experimental data and with previously obtained results from analyses 

made with the RELAP5/MOD3.3 Patch 03 model.

This work has been performed with financial support from the Swedish Radiation Safety 

Authority. 

v 

TABLE OF CONTENTS

Page

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

TABLE OF CONTENTS................................................................................................. v

LIST OF FIGURES.........................................................................................................vii

LIST OF TABLES............................................................................................................ix

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

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

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

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

2. FACILITY AND TEST DESCRIPTION.................................................................... 2-1

2.1 Test Facility...................................................................................................... 2-1

2.2 Selected Test Cases........................................................................................ 2-8

3. CODE AND MODEL DESCRIPTION...................................................................... 3-1

3.1 Used Code Version.......................................................................................... 3-1

3.2 The TRACE Model........................................................................................... 3-1

3.3 The RELAP5 Model ....................................................................................... 3-12

4. ANALYSIS RESULTS............................................................................................. 4-1

4.1 Experiment 5052.............................................................................................. 4-1

4.2 Experiment 4011.............................................................................................. 4-4

4.3 Experiment 3051.............................................................................................. 4-7

5. CONCLUSIONS AND EVALUATIONS................................................................... 5-1

6. REFERENCES ....................................................................................................... 6-1

APPENDIX A RESULT PLOTS OF FIX-II EXPERIMENT 5052..................................A-1

APPENDIX B RESULT PLOTS OF FIX-II EXPERIMENT 4011..................................B-1

APPENDIX C RESULT PLOTS OF FIX-II EXPERIMENT 3051................................. C-1

vii

LIST OF FIGURES

Page

Figure 2-1 General view of the FIX-II facility. ......................................................... 2-2

Figure 2-2 Options of break simulations in FIX-II facility. ....................................... 2-5

Figure 2-3 Cross section at heated part of bundle. ................................................ 2-6

Figure 2-4 Measurement points in the circuit. ........................................................ 2-7

Figure 3-1 Display of SNAP Model Editor with imported RELAP5 model of 

 FIX-II. ................................................................................................... 3-2

Figure 3-2 TRACE input model of FIX-II in SNAP Model Editor after 

rearrangement ..................................................................................... 3-3

Figure 3-3 The method to get the correct control variable for time 

 after a trip becomes true. ................................................................... 3-11

Figure 3-4 The RELAP5 component diagram for FIX-II experiments................... 3-13

Figure 3-5 Layout of the lower plenum and core regions. .................................... 3-17

Figure 3-6 Layout of the upper plenum, steam separator 

 and steam condenser regions. ........................................................... 3-18

Figure 3-7 Layout of the downcomer, bypass and the main 

 recirculation pipe lines........................................................................ 3-19

Figure 3-8 Layout of the steam relief line and the break lines.............................. 3-20

Figure 3-9 Core axial heat flux distribution........................................................... 3-22

Figure 3-10 Bypass axial heat flux distribution....................................................... 3-23

ix

LIST OF TABLES

Page

Table 2-1 The test matrix for the first FIX-II LOCA experimental period. .................. 2-8

Table 3-1 Excerpt of conversion report from RELAP5 to TRACE. ............................ 3-4

Table 3-2 Problems encountered during RELAP5 -> TRACE conversion and the 

corresponding corrective method.............................................................. 3-9

Table 3-3 RELAP5 and TRACE nodalization information. ...................................... 3-12

Table 3-4 Modeled RELAP5 pipe regions............................................................... 3-14

Table 3-5 RELAP5 components other than pipe..................................................... 3-16

Table 4-1 Initial condition for experiment 5052. ........................................................ 4-2

Table 4-2 Events list for experiment 5052................................................................. 4-3

Table 4-3 Initial condition for experiment 4011. ........................................................ 4-6

Table 4-4 Events list for experiment 4011................................................................. 4-6

Table 4-5 Initial condition for experiment 3051. ........................................................ 4-8

Table 4-6 Events list for experiment 3051................................................................. 4-9

xi

EXECUTIVE SUMMARY

The SSM and the USNRC have an agreement (SSM 2008/158) regarding participation 

and work effort within the Code Applications and Maintenance Program (CAMP). The 

agreement grants Sweden the access to thermal-hydraulic and neutronics codes 

developed within CAMP, such as the RELAP5- and TRACE-codes, but also requires 

Sweden to contribute to CAMP with code calculations and assessments. 

Within the current project, the TRACE thermal hydraulic system code, Version 5.0 

Patch 2, is validated against some FIX-II experiments, where the associated FIX-II input 

is obtained by converting previously used legacy RELAP5 FIX-II models by means of 

the SNAP utility. The TRACE analysis results are compared with the corresponding 

experimental data but also with the results from the used RELAP5/MOD3.3 Patch 03

model.

The BWR integral test program FIX-II was performed at Studsvik during years 

1979−1986, with the Swedish BWR Oskarshamn 2 as the reference reactor unit. FIX-II 

capability included the simulation of double ended breaks in the main recirculation pipe 

for a non-jet pump external recirculation loop BWR, but also simulations of split breaks 

and pump trip transients.

The SNAP model editor should ideally be capable to convert any RELAP5 input model 

into corresponding TRACE input. This functionality of SNAP however does not work to 

a full extent and consequently some user effort had to be spent to complete the TRACE 

model into an acceptable level. After the manual corrections the TRACE analyses could 

nevertheless be accomplished with the same boundary condition as in RELAP5.

Three FIX-II transients (5052, 4011 and 3051) were analyzed. The performed TRACE 

analyses revealed in general quite good comparisons with RELAP5 results and 

corresponding measured parameters, however, not quite as good as the RELAP5 

results when it comes to the system depressurization rate and the rod PCT and dryout 

predictions. It was specifically noted that:

• The various differential pressures around the test facility loops were well 

simulated indicating an overall favorable simulation of the liquid inventory 

distribution. Also the loop pressure decrease as a result of the flow expelled 

through the break was acceptably calculated although the rate of the 

decrease was somewhat exaggerated especially in the TRACE analyses. 

This exaggeration could have been made less pronounced by more careful 

tuning of the break conditions, i.e. basically tuning of the discharge 

coefficients associated with the used break flow model but also adjustments

of the loss coefficients at and close to the break location.
xii

• For the large break LOCA tests 5052 and 4011, the TRACE analyses could 

predict almost as well as the RELAP5 analyses on the dryout and the flow 

reversal in the test section bundle, with about 50 o

C lower PCT in 

comparison to measurements from the tests 5052 (716 o

C) and 4011 

(520 oC). 

• For the small break LOCA test 3051, where forced steam blow down 

occurred, the RELAP5 model well predicted the transient sequences in 

which no dryout occurred in the test section bundle due to the small loss of 

mass through the break; while the TRACE predicted a dryout in the later 

period of the transient due to larger loss of mass through the break and 

higher void distribution in the core.
xiii

ACKNOWLEDGMENTS

The author acknowledges the financial support from the SSM.

The author is also very grateful to the OKG AB utility for providing access to the 

RELAP5 input for the FIX-II experiments.

Finally, the author also thanks the colleague Anders Sjöberg for his kind support and 

valuable guidance during the work. 

xv

ABBREVIATIONS

BWR boiling water reactor

LOCA loss of coolant accident

LP lower plenum

O2 Oskarshamn 2 nuclear power plant

PLEX plant life extension

RCL recirculation line

SNAP Symbolic Nuclear Analysis Package

SRV steam relief valve

SSM the Swedish Radiation Safety Authority

TH thermal-hydraulic

UP upper plenum

USNRC the United States Nuclear Regulatory Commission

1-1 

1. INTRODUCTION

The SSM and the USNRC have an agreement (SSM 2008/158) regarding participation

and work effort within the Code Applications and Maintenance Program (CAMP). The 

CAMP comprises different activities including international partners’ exchange of 

information on thermal-hydraulic safety issues related to nuclear reactor and plant 

systems, and analysis work activities within associated code development and 

assessments. The agreement grants Sweden the access to thermal-hydraulic and 

neutronics codes developed within CAMP, such as the RELAP5- and TRACE-codes, 

but also requires Sweden to contribute to CAMP with code calculations and 

assessments on a regularly basis. 

Within the current project, the TRACE thermal hydraulic system code, Version 5.0 

Patch 2, is validated against some FIX-II experiments, where the associated FIX-II input 

is obtained by converting previously used legacy RELAP5 FIX-II models by means of 

the SNAP utility. The TRACE analysis results are compared with the corresponding 

experimental data but also with the results from the used RELAP5/MOD3.3 Patch 03

model. Thus the purpose of the project is twofold: (i) to evaluate the functionality of the 

SNAP capability to convert RELAP5 input files to corresponding TRACE input, and (ii) 

to validate the TRACE code. 

The extensive BWR integral test program FIX-II was performed at Studsvik during years 

1979−1986, with the Swedish BWR Oskarshamn 2 as the reference reactor unit. FIX-II 

had been widely used as validation data base for e.g. the Westinghouse vendor codes

BISON and GOBLIN. FIX-II is the only test program available with the capability to 

simulate the double ended break in the main recirculation pipe for a non-jet pump 

external recirculation loop BWR.

Studsvik Nuclear AB has contributed to the LOCA analyses made within the 

Oskarshamn 2 modernization and power uprate project PLEX. Those analyses were 

made with the RELAP5/MOD3.3, Patch 03, and as part of the analysis effort some 

validation of the RELAP5 and used input models was performed against selected FIX-II 

experiments. 

This TRACE validation project comprises three main recirculation pipe break 

experiments from the FIX-II test program, which are the very same as included in the 

mentioned RELAP5 validation: 

• Experiment 5052 is a double ended break test in the main recirculation 

pipe. The test facility core heating power corresponds to 2 500 MW for the 

real plant, which is about the power after PLEX project finalization. 

• Experiment 4011 is also a double ended break test in the main recirculation 

pipe. The test facility core heating power corresponds to 1 800 MW for the 

real plant, which represents the current Oskarshamn 2 full power.
1-2 

• Experiment 3051 is a 10 % (in area) split break in the main recirculation 

pipe, where the forced blowdown occurs at 40 s, and this will greatly 

influence the LOCA transient sequences. The test facility core heating 

power corresponds to current 1 800 MW for the real plant.
2-1 

2. FACILITY AND TEST DESCRIPTION

The FIX-II project was an extensive BWR LOCA blowdown and pump trip test program 

carried out during the years 1979−1986. The reference nuclear power plant was the 

Oskarshamn 2 BWR equipped with external recirculation loops. FIX-II was carried out 

by Studsvik Energiteknik AB in cooperation with Asea-Atom AB, and with support from 

the Swedish Nuclear Power Inspectorate. 

The FIX-II experiments were very well documented, and some of the experimental data 

are available in modern electronic form. The experiments have been widely used as 

validation data base for the Westinghouse vendor codes BISON and GOBLIN. The FIXII is the only test series available related to the double ended break in the main 

recirculation pipe of an external circulation pump type BWR like the Oskarshamn 2 

nuclear power plant. 

The aim of the project was to increase the knowledge of and improve the data base for 

calculations of certain accident transients for Swedish BWRs. The transients studied 

were those which provided a reduced circulation of coolant in the reactor core, caused 

by a break in a main recirculation loop (LOCA) for external pump type reactors, or by a 

loss of power supply to the pumps (pump trip).

 

2.1 Test Facility

The test facility layout is shown in Figure 2-1. FIX-II was designed with a volume scaling 

of 1:777, relative to the reference reactor. A detail description of the FIX-II test facility is 

provided in [1], therefore, only some fundamental aspects of the facility are presented 

here.

The test facility mainly includes the following components in representing the reactor 

pressure vessel internals and the main recirculation pipe lines:

1 Steam condenser with internal steam separator

2 Downcomer

3 Two main recirculation pipe lines: one representing the three intact loops 

and the other one representing the broken loop. 

4 Lower plenum

5 Rod channel (6×6 rod simulators electrically heated, 12.25 mm OD, full 

length 3.68 m)

6 Bypass channel with heating simulating the heat transfer from fuel 

assemblies

7 Upper plenum
2-2 

8 Spray lines

9 Feed water line

Figure 2-1. General view of the FIX-II facility. 
2-3 

The test section comprises a lower plenum with connections to the two recirculation 

lines, a middle part with the heated rod bundle, and an upper plenum (Figure 2-1). A 

spray condenser is placed on top of the test section. A simplified steam separator is 

located in the lower part of the condenser, and the upper part is used as a steam dome. 

The steam relief line is connected to the steam dome. During steady state operation the 

main steam outlet line is closed. 

The external downcomer line represents the actual power plant downcomer annulus 

surrounding the core. An external pipe connects the downcomer and the recirculation 

lines. The turbine power associated condenser effects are modeled by the partial circulation of water from the downcomer through an external 6 MW cooler with feedback

to three spray lines and feed water line. The flow rate in the two branches with cooled 

water is adjusted to control the condenser pressure, the liquid level and the core inlet 

subcooling. The flow through the cooler is shut off immediately after the initiation of the 

break.

The remaining downcomer flow representing the recirculation coolant flow splits at the 

lower downcomer end into two loops (both loops have its own recirculation pump):

• One loop representing three intact recirculation lines of the reference 

reactor.

• The other loop representing the fourth recirculation line, with break devices.

In order to simulate both the double ended break and the split break, the break 

recirculation line was varied by a combination of three break locations, i.e. the valves 

V117, V120 and V123. The size of the break is determined by the flow area in the break

restriction nozzle. The break flow is discharged into the water filled tanks, designed to 

provide an efficient condensation of steam. Three different configurations A, B and C 

were used as shown in Figure 2-2. 

The core channel is equipped with a full length rod bundle (3.68 m), which is closely 

adhered to the fuel assembly of Oskarshamn 2 in geometry. However, there are only 

6×6 rod simulators (12.25 mm O.D.) instead of 8×8 rods in the typical fuel element of 

that time period. Figure 3-5 shows an assembly drawing of the test section. The cross 

section of the heated part of the bundle is shown in Figure 2-3. The 36 fuel rods were 

directly heated by electric current through the cladding material (Inconel 600). The 

cladding is filled with MgO2 which has about the same heat capacity as the referenced 

reactor fuel rod. A mercury rheostat was used for power regulation; this allows 

simulating both the decay heat and the latent heat stored in the fuel rods.

As mentioned above FIX-II has as part of the core model an external bypass simulator. 

Through this bypass about 12 % of the recirculation mass flow is diverted through a 

control valve. This bypass is heated separately to represent the channel wall heat 

transfer. At the lower end of the bypass, Figure 2-1, there is a stagnant water volume to 

simulate the reference reactor space for the control rod guide tubes.
2-4 

Since the FIX-II facility has been designed only for blowdown experiments, no 

emergency core cooling equipment is installed.

Data from the comprehensive data measurement acquisition system (Figure 2-4) 

includes: 

• Pressure (PT)

• Several differential pressures (dPT) measurement points along the loop 

which provide information of the flow distribution during the course of the 

transient sequences

• Fluid temperature (TE)

• Cladding temperature (about 100 thermocouples distributed at 16 axial 

levels of the 36 rods)
2-5 

Figure 2-2. Options of break simulations in FIX-II facility. 
2-6 

 

Figure 2-3. Cross section of heated part of bundle. 
2-7 

Figure 2-4. Measurement points in the circuit. 
2-8 

2.2 Selected Test Cases

This validation project comprises three main recirculation pipe break experiments from 

the FIX-II test program with break simulations according to Figure 2-2: 

• Experiment 5052 is a double ended break test in the main recirculation 

pipe. The test heating power is corresponding to 2 500 MW core power of

the real plant, which is about the power after PLEX.

• Experiment 4011 is also a double ended break test in the main recirculation 

pipe. The test heating power is corresponding to 1 800 MW core power of 

the real plant, which represents the present O2 full power.

• Experiment 3051 is a 10 % (in area) split break in the main recirculation 

pipe, where the forced blowdown occurs at 40 s, and this will greatly 

influence the LOCA transient sequences. The test heating power is 

corresponding to 1 800 MW core power of the real plant.

Table 2-1 provides short information about the different FIX-II experiments.

Table 2-1. The test matrix for the first FIX-II LOCA experimental period. 

Break classification Split breaks Guillotine

Type of simulation 

(see Figure 2-2)

A B C 

Relative break area 

(%)

10 31 48 100 150 200 155 200

Break I.D. (mm) 6.8 12.0 15.0 21.6 26.4 30.5 16.0+21.6 21.6+21.6

Initial bundle power 

(MW)

-hot channel 3.35 3.35 3.35

-average 2.35 2.35 2.35 2.35 2.35 2.35 2.35

LOCA test ident. 

No.

3051 3013 3024

3025

3026

3027

3031 3061 3071 3041 4011 5061 5051

5052
3-1 

3. CODE AND MODEL DESCRIPTION

3.1 Used Code Version

The analyses of the FIX-II experiment 5052, 4011 and 3051 are performed with the

TRACE code, which is available for safety analyses through the agreement between 

the SSM and the USNRC. The version being used is TRACE Version 5.0 Patch 2 with

the associated code manual [2]. 

3.2 The TRACE Model

Previous RELAP5 models of FIX-II 5052, 4011 and 3051 experiments were converted 

to TRACE Version 5.0, Patch 2 by means of SNAP (note that the RELAP5 input model 

of FIX-II was developed in ASCII, i.e. without SNAP, and is described in chapter 3.3). 

All FIX-II components in RELAP5 are retained after converted to TRACE.

For the conversion the latest SNAP-version was used as available at the time when the 

project activities were pursued, i.e. the SNAP version 1.2.6 was used. Importing of the 

RELAP5 ASCII input file into SNAP resulted in a little bit unstructured mask as shown in 

Figure 3-1. The user had to spend some time in rearranging the objects, changing the 

“Width Scale Factor” and “Pixel per Meter” and making other changes to obtain a more 

clearly arranged SNAP mask (See Figure 3-2).

Ideally the SNAP model editor should be capable to convert any RELAP5 input model 

into corresponding TRACE input. This functionality of SNAP however does not work to 

a full extent and consequently some user effort was necessary to complete the model 

into a “runable” status. A conversion report is generated by running the “convert to 

TRACE” tool in SNAP and an excerpt of the report is shown in Table 3-1, in this report 

table corresponding components in RELAP5 and TRACE are listed. Table 3-3 shows 

some nodalization information.

The problems encountered during the RELAP5 to TRACE conversion are listed in 

Table 3-2, in which also is shown the corrective action by the code user to handle the 

problem and revise the model. 

After the manual corrections the TRACE calculations could be run with the same 

boundary conditions as previous calculations with RELAP5:

• The same core and bypass power.

• The same boundary flow including spray, feed water and cooling flow.

• The same pump characteristics.
3-2 

• The same trip time to activate the break flow, the steam flow, the decay 

power, the pump coast down, and the time to close the spray, feed water 

and cooling water flow.

Figure 3-1. Display of SNAP Model Editor with imported RELAP5 model of FIX-II.
3-3 

Figure 3-2. TRACE input model of FIX-II in SNAP Model Editor after 

rearrangement. 
3-4 

Table 3-1. Excerpt of conversion report from RELAP5 to TRACE.

Original RELAP5 component Resulting TRACE component

CB 10 tripunit (ssunit) Trip -10 (ssunit)

CB 20 sum (fd1) Sum -20 (fd1)

CB 30 mult (ssmul1) Multiply -30 (ssmul1)

CB 40 integral (integ1) Integrate -40 (integ1)

CB 50 function (p1sptab) Function -50 (p1sptab)

CB 60 mult (p1vel) Multiply -60 (p1vel)

CB 70 sum (fd2) Sum -70 (fd2)

CB 80 mult (ssmul2) Multiply -80 (ssmul2)

CB 90 integral (integ2) Integrate -90 (integ2)

CB 100 function (p2sptab) Function -100 (p2sptab)

CB 190 mult (p2vel) Multiply -190 (p2vel)

CB 200 function (srvtab) Function -200 (srvtab)

CB 320 sum (bpdf) Sum -320 (bpdf)

CB 330 mult (ssmulbp) Multiply -330 (ssmulbp)

CB 340 integral (integbp) Integrate -340 (integbp)

CB 500 sum (tot-pow) Sum -500 (tot-pow)

CB 501 function (cor-pow) Function -501 (cor-pow)

CB 502 function (bp-pow) Function -502 (bp-pow)

CB 1181 constant (romeslin) Constant -1181 (romeslin)

CB 1183 sum (dpcnd) Sum -1183 (dpcnd)

CB 1184 sum (xp) Sum -1184 (xp)

CB 1185 stdfnctn-sqrt (sqxp) Square Root -1185 (sqxp)

CB 1186 sum (xdro) Sum -1186 (xdro)

CB 1187 sum (nomin) Sum -1187 (nomin)

CB 1188 div (zofcnd) Divide -1188 (zofcnd)

CB 1189 lag (zxcnd) First Order Lag -1189 (zxcnd)

CB 1199 sum (zcnd) Sum -1199 (zcnd)

CB 3001 sum (p_dp0) Sum -3001 (p_dp0)

CB 3002 sum (p_dp1) Sum -3002 (p_dp1)

CB 3003 sum (p_dp2) Sum -3003 (p_dp2)

CB 3004 sum (p_dp3) Sum -3004 (p_dp3)

CB 3005 sum (p_dp4) Sum -3005 (p_dp4)

CB 3006 sum (p_dp5) Sum -3006 (p_dp5)

CB 3007 sum (p_dp6) Sum -3007 (p_dp6)

CB 3008 sum (p_dp7) Sum -3008 (p_dp7)
3-5 

Table 3-1. Excerpt of conversion report.......(continued)

CB 3009 sum (p_dp8) Sum -3009 (p_dp8)

errmax 0 Generic Signal Variable (R5:errmax) 147

systms 1 Generic Signal Variable (R5:systms) 148

time 0 Problem Time 149

timeof 501 Trip Time -3 

timeof 504 Trip Time -4 

General Table 1 GeneralTable 1

General Table 2 GeneralTable 2

General Table 3 GeneralTable 3

General Table 5 GeneralTable 5

General Table 201 GeneralTable 201

General Table 202 GeneralTable 202

httemp 132202505 Slab/Rod Temperature 152

httemp 132202405 Slab/Rod Temperature 153

httemp 132202305 Slab/Rod Temperature 154

httemp 132202205 Slab/Rod Temperature 155

httemp 132202105 Slab/Rod Temperature 156

httemp 132202005 Slab/Rod Temperature 157

httemp 132201905 Slab/Rod Temperature 158

Heatstructure 1021 Heat Structure 1021

Heatstructure 1022 Heat Structure 1022

Heatstructure 1121 Heat Structure 1121

Heatstructure 1122 Heat Structure 1122

Heatstructure 1221 Heat Structure 1221

Heatstructure 1222 Heat Structure 1222

Heatstructure 1223 Heat Structure 1223

Heatstructure 1224 Heat Structure 1224

Heatstructure 1225 Heat Structure 1225

Heatstructure 1226 Heat Structure 1226

Heatstructure 1227 Heat Structure 1227

Heatstructure 1228 Heat Structure 1228

Heatstructure 1321 Heat Structure 1321

Heatstructure 1322 Heat Structure 1322

 Power 11

Heatstructure 1323 Heat Structure 1323

 Power 21

Heatstructure 1324 Heat Structure 1324

 Power 31

Heatstructure 1325 Heat Structure 1325

Heatstructure 1421 Heat Structure 1421
3-6 

Table 3-1. Excerpt of conversion report.....(continued)

Heatstructure 1422 Heat Structure 1422

Heatstructure 1521 Heat Structure 1521

Heatstructure 1522 Heat Structure 1522

Heatstructure 1523 Heat Structure 1523

Heatstructure 1524 Heat Structure 1524

Heatstructure 1525 Heat Structure 1525

Heatstructure 1526 Heat Structure 1526

Heatstructure 1527 Heat Structure 1527

Heatstructure 1528 Heat Structure 1528

Heatstructure 1621 Heat Structure 1621

Heatstructure 1721 Heat Structure 1721

Heatstructure 1722 Heat Structure 1722

Heatstructure 1723 Heat Structure 1723

Heatstructure 1724 Heat Structure 1724

Heatstructure 1725 Heat Structure 1725

Heatstructure 1726 Heat Structure 1726

Heatstructure 1727 Heat Structure 1727

mflowj 272020000 Mix Mass Flow Across the X Axis 51

mflowj 312020000 Mix Mass Flow Across the X Axis 52

mflowj 193000000 Mix Mass Flow Across the X Axis 53

Logical Trip 601 Trip -601

 Trip Set Status Value 202

 Trip Set Status Value 203

 Logical AND -7 

Logical Trip 604 Trip -604

 Trip Set Status Value 204

 Trip Set Status Value 205

 Logical AND -11

Logical Trip 606 Trip -606

 Trip Set Status Value 206

 Trip Set Status Value 207

 Logical AND -12

Logical Trip 607 Trip -607

 Trip Set Status Value 208

 Trip Set Status Value 209

 Logical AND -13

Logical Trip 608 Trip -608

 Trip Set Status Value 210

 Trip Set Status Value 211

 Logical AND -14
3-7 

Table 3-1. Excerpt of conversion report...(continued)

Logical Trip 609 Trip -609

 Trip Set Status Value 212

 Trip Set Status Value 213

 Logical AND -16

Logical Trip 610 Trip -610

 Trip Set Status Value 214

 Trip Set Status Value 215

 Logical AND -18

Material 1 Material 50

Material 2 Material 51

Material 3 Material 52

Material 4 Material 53

Material 5 Material 54

PIPE 102 (dskri) Pipe 102 (dskri)

PIPE 112 (dskrb) Pipe 112 (dskrb)

PIPE 122 (lp) Pipe 122 (lp)

PIPE 132 (core) Pipe 132 (core)

PIPE 142 (core_ut) Pipe 142 (core_ut)

PIPE 152 (up) Pipe 152 (up)

PIPE 162 (ss) Pipe 162 (ss)

PIPE 172 (cnd) Pipe 172 (cnd)

PIPE 182 (bp) Pipe 182 (bp)

PIPE 192 (bpin1) Pipe 192 (bpin1)

PIPE 212 (bpin2) Pipe 212 (bpin2)

PIPE 222 (bput) Pipe 222 (bput)

PIPE 232 (dc) Pipe 232 (dc)

PIPE 242 (dco) Pipe 242 (dco)

PIPE 252 (dc1) Pipe 252 (dc1)

PIPE 262 (hci_in) Pipe 262 (hci_in)

PIPE 282 (hci_u1) Pipe 282 (hci_u1)

PIPE 292 (hci_u2) Pipe 292 (hci_u2)

PIPE 302 (hcb_in) Pipe 302 (hcb_in)

PIPE 322 (hcb_u1) Pipe 322 (hcb_u1)

PIPE 332 (hcb_u2) Pipe 332 (hcb_u2)

PIPE 342 (hcb_u3) Pipe 342 (hcb_u3)

PIPE 352 (hcb_u4) Pipe 352 (hcb_u4)

PIPE 362 (sl1) Pipe 362 (sl1)

PIPE 372 (sl2) Pipe 372 (sl2)

PIPE 382 (bl1_1) Pipe 382 (bl1_1)

PIPE 392 (bl1_2) Pipe 392 (bl1_2)
3-8 

Table 3-1. Excerpt of conversion report....(continued)

PIPE 402 (bl2_1) Pipe 402 (bl2_1)

PIPE 412 (bl2_2) Pipe 412 (bl2_2)

PIPE 422 (bl3_1) Pipe 422 (bl3_1)

PIPE 432 (bl3_2) Pipe 432 (bl3_2)

PUMP 272 (pmp1) Pump 272 (pmp1)

PUMP 312 (pmp2) Pump 312 (pmp2)

SNGLJUN 103 (dskri_o) Hydraulic [5] from Pipe 102 to Pipe 122

SNGLJUN 113 (dskrb_o) Hydraulic [6] from Pipe 112 to Pipe 122

SNGLJUN 131 (core_i) Hydraulic [8] from Pipe 132 to Pipe 122

SNGLJUN 133 (core_o) Hydraulic [9] from Pipe 132 to Pipe 142

SNGLJUN 143 (core_u_o) Hydraulic [10] from Pipe 142 to Pipe 152

SNGLJUN 153 (up_o) Hydraulic [11] from Pipe 152 to Pipe 162
3-9 

Table 3-2. Problems encountered during RELAP5 -> TRACE conversion and the 

corresponding corrective method.

Error Revise 

Generic Signal Variable (R5:errmax) 147 

Error

Error: Contains an invalid variable 

reference: R5:errmax

Delete Generic Signal Variable 147

Generic Signal Variable (R5:systms) 148 

Error

Error: Contains an invalid variable 

reference: R5:systms

Delete Generic Signal Variable 148

Junction Flow Area (Pipe 192: Edge 5) 

exceeds 1.1 * the flow area of the adjacent 

cells. 

Set the NOFAT namelist variable to 

TRUE.

Large area change between Junction Flow 

Area and Adjacent cells detected at pipe 

142, 152, 182, 192 and 342. 

Specify Friction factor correlation 

option NFF = -1, the abrupt area flag. 

material error RELAP5 uses Cv, the volumetric heat 

capacity (J/m3

K), while TRACE uses Cp, 

the specific heat [J/(kg K)]. So the

density needs to be used in order to 

convert RELAP5 material property to 

TRACE material property.

Conversion of RELAP5 valve with special 

choking discharge coefficient resulted in 

TRACE to valve with default choking 

option.

In namelist variables, set CHM12=0.8, 

CHM22=0.8

The constant heat transfer coefficient 

(HTC) boundary condition could not be 

converted correctly; it resulted in a normal 

convective boundary condition.

Set outer surface boundary condition 

option for heat structure 1325, 

IDBCON = 4; add table 203 for 

constant HTC, where (in table 203) 

also correct signal source SCBSVID is 

provided according to Figure 3-3; and 

add table 204 for sink temperature.

This kind of trip “0000508 time 0 ge 

timeof 504 0. n” cannot be converted 

correctly. If the time is not zero, as 

“0000508 time 0 ge timeof 504 10. 

n” then it would be ok.

Connect it with right trip signal.

Wrong conversion of control variable for 

“time after trip become true” 

See Figure 3-3. 
3-10

Table 3-2. Problems encountered during RELAP5....(continued)

Wrong conversion of core power (the 

scaling factor was lost, and the decay 

power table did not come into function due 

to wrong control variable conversion)

• Set reactivity scale factor 

RPWSCL to correct value.

• Do as Figure 3-3. 

Wrong conversion of pump speed control, 

control function 50 for pump1 and control 

function 100 for pump2 are connecting 

with wrong control variable, should be time 

after trip 604 become true.

Set correct control variable.

Control function 1188 “divided” was wrongly 

converted; the numerator and denominator 

were in wrong order.

Set it in the right order.

Wrong conversion of feed water boundary 

flow control (TDJ453), ICBVL is connecting 

with wrong control variable.

See Figure 3-3 

Wrong conversion of steam relief flow 

control (valve 503), IVSV is connecting with 

wrong control variable.

See Figure 3-3 

Wrong conversion of cooling water 

boundary flow control (TDJ483), ICBVL is 

connecting with wrong control variable.

See Figure 3-3 

Wrong conversion of spray water boundary 

flow control (TDJ473), ICBVL is connecting 

with wrong control variable.

See Figure 3-3 
3-11

Figure 3-3. The method to get the correct control variable for time after a trip 

becomes true. 
3-12

3.3 The RELAP5 Model

The FIX-II test facility model is mainly based on the detail geometric information from 

Table 2 in [1], and the modeling principle is to include all regions and to model the 

geometry in adequate high resolution. 

The RELAP5 input data is generated by the help of a separately developed input data 

generation program. This provides a major advantage in generating for instance inputs 

models with different variation in nodalization. The system is divided into regions, each 

region is connected to the others by flow junction. By the help of the input generation 

program, each region will be divided automatically into different amount of cells 

depending on the node length selected by the user. The nodalization information for the 

different experiments is shown in Table 3-3. The RELAP5 heat structure input could 

also be generated by means of the input generation program. 

Table 3-3. RELAP5 and TRACE nodalization information.

Experiment Nodalization Node 

length

Number of 

volumes

Number of 

junctions

Code

5052 

regular 0.3 m 219 221 RELAP5

fine 0.15 m 371 373 RELAP5 

and 

TRACE

4011 fine 0.15 m 371 373 RELAP5 

and 

TRACE

3051 fine 0.08 m 655 657 RELAP5 

and 

TRACE

The region component diagram for the geometric modeling is shown in Figure 3-4. A list 

of all pipe regions is shown in Table 3-4 and the other components as pumps, tanks, 

etc. are listed in Figure 3-5 through Figure 3-8 illustrate different regions and their 

geometry of the modeled test facility.
3-13

Figure 3-4. The RELAP5 component diagram for FIX-II experiments. 

Time dependent junction

Time dependent volume

cnd

ss

up

core_ut

core

lp

hci_in

hcb_u1

bl1_1

bl1_2

bl2_1

hci_u2 hci_u1 pmp1

hcb_u4 hcb_u3 hcb_u2 pmp2 hcb_in

bl2_2

bl3_1

bl3_2

t3 t2 t1

dco

dc1

dskri dskrb

bpin2 bpin1

bp

spbcvol

bput fwbcvol

t0 sl2 sl1

dc

clbcflw

extrfw

fillvol

V117 V120 V123

V103

V114

V112
3-14

Table 3-4. Modeled RELAP5 pipe regions.

Region pipe 

name

Desciption

RELAP5 pipe no. Reference figure

bl1_1 Break line to tank 1, part 1 382 Figure 3-8

bl1_2 Break line to tank 1, part 1 392 Figure 3-8

bl2_1 Break line to tank 2, part 2 402 Figure 3-8

bl2_2 Break line to tank 2, part 2 412 Figure 3-8

bl3_1 Break line to tank 3, part 3 422 Figure 3-8

bl3_2 Break line to tank 3, part 3 432 Figure 3-8

bp Bypass 182 Figure 3-7

bpin1 Bypass inlet, part 1 192 Figure 3-7

bpin2 Bypass inlet, part 2 212 Figure 3-7

bput Bypass outlet 222 Figure 3-7

cnd Steam condenser 172 Figure 3-6

core Core 132 Figure 3-5

core_ut Core outlet 142 Figure 3-5

dc Downcomer 232 Figure 3-7

dc1 Downcomer before break line 252 Figure 3-7

dco The end of downcomer 242 Figure 3-7

dskrb Inlet in lower plenum inside the 

baffle (from break line)

112 Figure 3-5

dskri Inlet in lower plenum inside the 

baffle (from intact loop)

102 Figure 3-5

hcb_in Main recirculation break line inlet 302 Figure 3-7

hcb_u1 Main recirculation break line 

outlet part 1

322 Figure 3-7

hcb_u2 Main recirculation break line 

outlet part 2

332 Figure 3-7

hcb_u3 Main recirculation break line 

outlet part 3

342 Figure 3-7

hcb_u4 Main recirculation break line 

outlet part 4

352 Figure 3-7

hci_in Main recirculation intact loop 

inlet

262 Figure 3-7

hci_u1 Main recirculation intact loop 

outlet part 1

282 Figure 3-7

hci_u2 Main recirculation intact loop 

outlet part 2

292 Figure 3-7

lp Lower plenum 122 Figure 3-5

pmp1 Recirculation pump in intact loop 272 Figure 3-7

pmp2 Recirculation pump in break line 312 Figure 3-7

sl1 Steam line part 1 362 Figure 3-8
3-15

Table 3-4. Modeled RELAP5 pipe regions....(continued)

sl2 Steam line part 2 372 Figure 3-8

ss Steam separator 162 Figure 3-6

up Upper plenum 152 Figure 3-6
3-16

Table 3-5. RELAP5 components other than pipe.

Components Name Description

pump 272 pmp1 Recirculation pump in break line

pump 312 pmp2 Recirculation pump in intact loop

tmdpvol 452 fwbcvol Feed water boundary condition volume

tmdpvol 462 extrfw Extra feed water using for regulating 

condenser liquid level

tmdpvol 472 spbcvol Spray water boundary condition 

volume

tmdpvol 482 clbcvol fluid drawn from downcomer for 

cooling

tmdpvol 492 fillvol Volume between filler body and 

pressure vessel

tmdpvol 502 t0 Time dependent volume where main 

steam flows to

tmdpvol 512 t1 Tank 1

tmdpvol 522 t2 Tank 2

tmdpvol 532 t3 Tank 3

tmdpjun 453 fwbcflw Time dependent junction connecting 

to tmdpvol 452

tmdpjun 463 fwbcregf Time dependent junction connecting 

to tmdpvol 462

tmdpjun 473 spbcflw Time dependent junction connecting 

to tmdpvol 472

tmdpjun 483 clbcflw Time dependent junction connecting 

to tmdpvol 482

tmdpjun 493 zeroflw Time dependent junction connecting 

to tmdpvol 492

valve 193 bpin1_o Valve 114 locating at bypass inlet pipe

valve 333 hcb_u2_o Valve 103 used to separate the break 

line in order to simulate double ended 

break

valve 503 slv112 Steam relief valve 112

valve 513 brkv123 Break valve 123 to tank 1

valve 523 brkv120 Break valve 120 to tank 2

valve 533 brkv117 Break valve 117 to tank 3
3-17

Figure 3-5. Layout of the lower plenum and core regions. 
3-18

Figure 3-6. Layout of the upper plenum, steam separator and steam condenser 

regions. 
3-19

Figure 3-7. Layout of the downcomer, bypass and the main recirculation pipe 

lines.
3-20

Figure 3-8. Layout of the steam relief line and the break lines. 
3-21

Cross section geometrical data for the 36-rods bundle are provided in Figure 2-3. An 

axial power distribution comprising eight steps has been introduced by variations of the 

rod cladding wall thickness. In each rod there are five thermocouples for measurement 

of the cladding temperature. The power steps and the locations of the thermocouples 

are shown in Figure 3-9. By a combination of four different patterns of thermocouple 

locations, cladding temperatures can be measured at sixteen axial levels. The axial 

power distribution for the bypass is shown in Figure 3-10. The radial power distribution 

should be uniform but minor deviations exist due to variations in the electrical 

resistance of the fuel rods. However, the radial power distribution is not considered in 

the model, the same power is introduced in all the rods, and the core is modeled as one 

pipe in this RELAP5 model.

Only limited pump data was provided in [1] and [6] so the pump input data from a 

previous analysis [7] is adapted. The pump is modeled by homologous curves, with the 

data from GOBLIN calculations. 

The condenser liquid level measurement is modeled according to the real 

measurement principle, i.e. the liquid level is calculated from the pressure difference 

measured between two axial locations in the condenser. The basic assumption of the 

condition is that it is saturated liquid in the lower part and saturated steam above it. The 

measured liquid level is a function of the measured pressure difference over the 

distance between the two measurement locations, the liquid and steam density in the 

condenser at the condenser pressure and the reference density of the fluid in the 

measurement pipe line. 
3-22

Figure 3-9. Core axial heat flux distribution. 
3-23

Figure 3-10. Bypass axial heat flux distribution. 
3-24

To reproduce the fundamental measured steady state quantities, the input for the 

steady state calculations is modeled by some additional components and control 

systems: 

• The steady state steam dome pressure is achieved by regulating the core 

heat loss transfer coefficient through the filler body.

• The speed of the main recirculation pumps P1 and P2 are controlled by the 

RELAP5 control system to reproduce the measured mass flows.

• To divert the correct mass flow into the bypass, the junction between the 

two bypass inlet pipes is modeled as a motor valve. By trip logic, the 

amount of valve opening is controlled to provide the measured bypass 

mass flow rate. When entering into the transient calculation, the valve 

setting is fixed.

• The steam condenser liquid level was controlled to obtain the measured 

value by connecting an auxiliary time dependent volume and time 

dependent junction to the condenser. The flow through the time dependent 

junction is controlled according to the offset liquid level.

The steady state input model is made as complete as possible in order to also cover 

the transient actions. The steady state controls were disconnected by a first trip and the 

additional trips followed to simulate the system actions when the transient starts. In the 

transient restart, the first trip is set at time zero with a new initiation of the time scale. 

Until the second trip, which is the one initiating the actual transient, the transient 

calculation is run at the initial steady state as to confirm the initial steady state stability. 
4-1 

4. ANALYSIS RESULTS 

4.1 Experiment 5052

Experiment 5052 simulates a guillotine break in one of the four main recirculation lines. 

The total break area, equally divided between the two broken discharge pipe line 

sections, was 200 % of the scaled down pipe area of the reactor recirculation line. The 

initial heating power of the 36-rod bundle in the FIX-II loop was 3.356 MW, 

corresponding to 2500 MW of the real plant, which is about the power after PLEX. The 

detail experiment description can be found in [3]. 

Experiment 5052 is a C type break shown in Figure 2-2, the initial condition is shown in 

Table 4-1. The list of sequence of events is shown in Table 4-2. 

The following is a summary of the main experiment and analysis results: The large size 

of the break resulted in reversed core flow and in dryout conditions along the whole 

heated length of the rod bundle. The void fraction increased towards the lower part of 

the bundle, which resulted in the highest rod temperatures in that region (the 

phenomena are well predicted in the analyses). A peak clad temperature of 716 oC 

(about 650 o

C in TRACE and 630 oC in RELAP5 analyses) was measured 12 s after the 

break at rod No 29, level 4, Figure A-12. However, the temperatures decreased again 

after around 10 s, in several cases due to rewetting (no rewetting could be predicted in 

the analyses). The maximum temperature at the end of the blowdown, at 27.4 s, was 

620 o

C (well predicted in the analyses shown in Figure A-12 to Figure A-14). The steam 

dome pressure had then decreased to 1.9 MPa (1.35 MPa in the RELAP5 analyses and 

1.25 MPa in the TRACE analyses, the latter which predicted somehow faster 

depressurization).

Some main experimental and analysis results of pressure, differential pressure, mass 

flow rates and temperatures at different locations are shown in figures in Appendix A 

and discussed briefly in relation to the figures. The detail experiment transient 

description can be found in [8]. Measurement locations are shown in Figure 2-4. 

In general, the results are quite similar between RELAP5 and TRACE analyses and 

they are in good agreement with the experimental results, although some differences 

were revealed: 

• Almost the same pressure decrease rate in the system (Figure A-1, 

Figures A-7 and A-8).

• TRACE provided a faster depressurization rate and higher break flow rate 

at the very beginning of the transient, but later on, almost the same break 

flow rate (Figures A-2 and A-3), the same steam relief flow (Figure A-4) 

and pump flow rate (Figures A-5 and A-6) as for the RELAP5 analysis. 

Although the discharge flow rates are quite similar between RELAP5 and 
4-2 

TRACE analyses, the small difference is nevertheless one of the reasons 

resulting in the different rod temperatures as discussing below. This will 

also be discussed in detail related to experiment 3051 where the discharge 

flow rate revealed bigger differences between the TRACE and RELAP5 

analyses. 

• There are quite big differences in rod temperatures between the two 

analyses (Figures A-9 through Figure A-20) due to different void distribution 

(Figures A-21 and A-22) in the core both during the steady-state and the 

course of the transient, although the same power and total flow rate through 

the core were applied. Thus RELAP5 and TRACE analyses resulted in 

different distribution of the liquid and vapour mass flow rate, and this is 

most likely due to somewhat different correlations applied in the two codes 

for interfacial friction and interfacial area concentration. A more thorough 

examination of the causes to the found differences is however beyond the 

scope of the current effort, and has to be left to the future.

Table 4-1. Initial condition for experiment 5052.

Break configuration (Figure 2-2) C

Break restriction nozzles I.D. 21.6 + 21.6 

mm

Steam relief valve area, V112 0.56 cm2

Pressure in the steam dome 6.9 MPa

Power to the 36-rod bundle 3.356 MW

Power to the bypass heaters 56.6 kW

Mass flow rate through pump 1 4.4 kg/s

Mass flow rate through pump 2 1.68 kg/s

Mass flow rate in the bypass 0.57 kg/s

Mass flow rate in the 36-rod bundle 5.51 kg/s

Mass flow rate in the spray line 5.1 kg/s

Mass flow rate in the feed water line 2.3 kg/s

Temperature of water at the bundle inlet (TE2) 268.7 oC

Temperature of feed and spray water 181.3 oC

Water level in the spray condenser 6.223 m

Rotation speed of pump 1 24.78 revs/s

Rotation speed of pump 2 32.51 revs/s
4-3 

Table 4-2. Events list for experiment 5052.

Event Time (s)

Break valve V120 starts to open 0

Start of coast down of pump 1 0

Break valve V117 starts to open 0.2

Valve V103 in the broken RCL starts to close 0.3

Start of power decay in the bundle 0.4

The SRV starts to open 0.7

Start of power decay in the bypass heaters 1.2

The SRV is fully open 1.2-27.5

The spray flow is closed 2.1

The feed water flow is closed 2.2

Valve V104 to the evaporation cooler is closed 2.3

Tripping of the bundle power 27.4

The break valves are closed 27.8

Termination of data acquisition 37.7
4-4 

4.2 Experiment 4011

Experiment 4011 simulates a guillotine break in one of the four main recirculation lines. 

A simplified loop configuration was applied, with only one recirculation line in operation, 

the other recirculation lines being shut off. The simulated break was located at 

extensions from the lowermost part of the downcomer and from the lower plenum, with 

break areas of 55 and 100 percent of the scaled down area of one recirculation line, 

respectively. The initial heating power of the 36-rod bundle was 2.324 MW, 

corresponding to 1800 MW for the real plant, which represents the current full power. 

The detail experiment description can be found in [4]. 

Experiment 4011 is a B type break shown in Figure 2-2, the initial condition is shown in 

Table 4-3. The list of sequence of events is shown in Table 4-4. 

The following is a summary of the main experiment and analysis results: The large size 

of the break resulted in a reversed flow in the bundle throughout the blowdown phase. 

The dryout conditions therefore became more serious in the lower part (the phenomena 

are well predicted in the analyses). A maximum rod temperature of 520 o

C (460 oC in 

TRACE analyses, 478 o

C in RELAP5 analyses) was measured at 12 s after the break at 

rod No 21, level 4, Figure B11. However, the temperatures decreased again after about 

12 to 15 s, also at not rewetted rod positions, for which the final maximum temperature, 

451 oC was obtained at the end of the blowdown. The steam dome pressure was then 

at 1.55 MPa (1.2 MPa in the RELAP5 analyses and 0.85 MPa in the TRACE analyses,

the latter which predicted somehow faster depressurization).

Some main experimental and analysis results of pressure, differential pressure, mass 

flow rates and temperatures at different locations are presented in figures in Appendix 

B and discussed briefly in relation to the figures. The detail experiment transient 

description can be found in [8]. Measurement locations are shown in Figure 2-4. 

 

In general, the results are quite similar between RELAP5 and TRACE analyses and 

they are in good agreement with the experimental results, although some differences 

were revealed. The analysis here is very close to the analysis of the previous 

experiment 5052:

• Almost the same depressurization rate in the system (Figure B-1, 

Figures B-6 and B-7).

• TRACE provided a faster depressurization rate and higher break flow rate 

at the very beginning of the transient, but later on, almost the same break 

flow rate (Figures B-2 and B-3), a somewhat less steam relief flow 

(Figure B-4) and pump flow rate (Figure B-5) as for the RELAP5 analysis. 

Although the discharge flow rates are quite similar between RELAP5 and 

TRACE analyses, the small difference is nevertheless one of the reasons 

resulting in the different rod temperatures as discussing below. This will 

also be discussed in detail related to experiment 3051 where the discharge 
4-5 

flow rate revealed bigger differences between the TRACE and RELAP5 

analyses.

• There are quite big differences in rod temperatures between the two 

analyses (Figure B-8 through Figure B-15) due to different void distribution 

(Figures B-16 and B-17) in the core both during the steady-state and the 

course of the transient, although the same power and total flow rate through 

the core were applied. Thus RELAP5 and TRACE analyses resulted in 

different distribution of the liquid and vapour mass flow rate, and this is 

most likely due to somewhat different correlations applied in the two codes 

for interfacial friction and interfacial area concentration. A more thorough 

examination of the causes to the found differences is however beyond the 

scope of the current effort, and has to be left to the future.
4-6 

Table 4-3. Initial condition for experiment 4011.

Break configuration (Figure 2-2) B

Break restriction nozzles I.D. 16.0 + 21.6 

mm

Steam relief valve area, V112 0.56 cm2

Pressure in the steam dome 6.79 MPa

Power to the 36-rod bundle 2.324 MW

Power to the bypass heaters 54.8 kW

Mass flow rate through pump 1 6.05 kg/s

Mass flow rate in the bypass 0.43 kg/s

Mass flow rate in the 36-rod bundle 5.62 kg/s

Mass flow rate in the spray line 3.18 kg/s

Mass flow rate in the feed water line 1.9 kg/s

Temperature of water at the bundle inlet (TE2) 266.6 oC

Temperature of feed and spray water 180.5 oC

Water level in the spray condenser 6.31 m

Rotation speed of pump 1 26.9 revs/s

Table 4-4. Events list for experiment 4011.

Event Time (s)

Start of coast down of pump 1 -0.1

Break valve V123 starts to open 0.0

Start of power decay in the bundle 0.0

Break valve V117 starts to open 0.1

The SRV starts to open 0.3

The SRV is fully open 0.7-32.6

Start of power decay in the bypass heaters 0.8

The spray flow is closed 1.8

The feed water flow is closed 1.9

Valve V104 to the evaporation cooler is closed 1.9

The break valves are closed 32.6

Termination of data acquisition 43.0
4-7 

4.3 Experiment 3051

Experiment 3051 simulates a split break case, and had the smallest break area of all 

FIX-II tests, the break area was 10 % of the scaled down pipe area of the reactor

recirculation line. The initial heating power of the 36-rod bundle in the FIX-II loop was 

2.38 MW, corresponding to 1800 MW of the real plant, which represents the current full 

power. The detail experiment description can be found in [5]. 

Experiment 3051 is an A type break shown in Figure 2-2, the initial condition is shown 

in Table 4-5. The list of sequence of events is shown in Table 4-6. 

The following is a description of the main experiment transient: The small break area 

plus the low average channel power did not lead to any initial dryout in the 36-rod 

bundle and no associated bundle heat up indicating no core uncovery. In test 3051 a 

comparatively large fraction of energy, was lost through the steam relief and contributed 

to the depressurization, although the second valve opening was delayed to 39.6 s after 

the break. The steam dome pressure was then decreased to 2.2 MPa in the 

experiment. 

Some main experimental results along with RELAP5 and TRACE analysis results of 

pressure, differential pressure, mass flow rates and temperatures at different locations 

are shown and discussed in figures in Appendix C. The detail experiment transient 

description can be found in [8]. The measurement locations are shown in Figure 2-4. 

In general, the results are quite similar between RELAP5 and TRACE analyses and 

they are in good agreement with the experimental results, although some differences 

were revealed: 

• Almost the same pressure decrease rate in the system (Figure C-1, 

Figures C-6 and C-7).

• As for the other two experiments, TRACE provided faster depressurization

rate and higher break flow rate at the very beginning of the transient, but 

later on, quite similar break flow rate (Figure C-2) as for the RELAP5 

analysis, and both analyses revealed almost the same steam relief flow 

(Figure C-3) and pump flow rate (Figures C-4 and C-5). 

• Although the flow rate through the break appears to be quite similar 

between RELAP5 and TRACE analysis results (Figure C-2), the discharged 

mass differs quite much as shown in Figure C-19, and Figure C-18. This is 

one of the reasons for providing the different rod temperatures as 

discussing below.

• Figure C-18 also shows that it is somewhat different mass inventory in the 

system even during the steady state. Since the same liquid level in the 

condenser, the same system pressure, and the same core inlet liquid 
4-8 

temperature prevailed, it is reasonable to assume that different two-phase 

distribution in the core and the bypass is a major cause for the differences, 

and this is also proved by the void distribution curve from Figures C-16 and

C-17.

• For the rods temperatures, the RELAP5 shows very good agreement with 

the experimental data (Figure C-8 to Figure C-15), whereas TRACE 

provided slightly higher temperature already at steady state and predicted

even a dryout along the whole rod length in the core during the last period 

of the transient. The reason to the different rods temperature results of the 

TRACE and RELAP5 analyses is most probably due to the following: First, 

due to quite different void distribution (Figure C-16, 17) in the core both 

during the steady-state and during the course of the transient, although the 

same power and total flow rate through the core were applied. This meant 

that RELAP5 and TRACE analyses resulted in different distribution of the 

liquid and vapour mass flow rate due to most likely somewhat different 

correlations applied in these two codes for interfacial friction and interfacial 

area concentration. Second, different discharge flow rate correlations which 

resulted in different mass inventory remaining in the system.

Table 4-5. Initial condition for experiment 3051.

Break configuration (Figure 2-2) A

Break restriction nozzles I.D. 6.82 mm

Steam relief valve area, V112 0.94 cm2

Pressure in the steam dome 6.99 MPa

Power to the 36-rod bundle 2.38 MW

Power to the bypass heaters 61.1 kW

Mass flow rate through pump 1 4.85 kg/s

Mass flow rate through pump 2 1.59 kg/s

Mass flow rate in the bypass 0.69 kg/s

Mass flow rate in the 36-rod bundle 5.75 kg/s

Mass flow rate in the spray line 3.08 kg/s

Mass flow rate in the feed water line 1.95 kg/s

Temperature of water at the bundle inlet (TE2) 266 oC

Temperature of feed and spray water 180 oC

Water level in the spray condenser 6.349 m

Rotation speed of pump 1 24.63 revs/s

Rotation speed of pump 2 31.64 revs/s
4-9 

Table 4-6. Events list for experiment 3051.

Event Time (s)

Break valve V120 starts to open 0

Start of coast down of pump 1 0

Start of power decay in the bundle and the bypass 0.0

The SRV starts to open 0.5

The SRV is fully open 1.1

The SRV starts to close 1.5

Break valve V117 starts to open 0.2

Valve V103 in the broken RCL starts to close 0.3

The spray flow is closed 2.0

The feed water flow is closed 2.1

Valve V104 to the evaporation cooler is closed 2.2

The SRV is closed 2.8

The SRV starts to open 39.6

The SRV is fully open 40.3-137.2

The break valves are closed 137.0

Termination of data acquisition 146

5-1 

5. CONCLUSIONS AND EVALUATIONS

The TRACE Version 5.0 Patch 2 is validated against some FIX-II experiments, where 

the input is obtained by converting the legacy RELAP5 FIX-II model by means of SNAP. 

The SNAP model editor should ideally be capable to convert any RELAP5 input model 

into TRACE input. This functionality of SNAP however does not work to a full extent 

and consequently some user effort was needed to complete the model into a “runable”

status. After the manual corrections the TRACE analyses could nevertheless be run 

with the same boundary condition as in RELAP5: the same core and bypass power, the 

same boundary flow rate, the same pump characteristics, the same trip time to activate 

the break flow, the decay power, the pump coast down, etc.

Three FIX-II transients (5052, 4011 and 3051) were analyzed. The performed TRACE 

analyses revealed in general quite good comparisons with RELAP5 results and 

corresponding measured parameters, however, not quite as good as the RELAP5 

results when it comes to the system depressurization rate and the rod PCT and dryout 

predictions. It was specifically noted that: 

• The various differential pressures around the test facility loops were well 

simulated indicating an overall favorable simulation of the liquid inventory 

distribution. Also the loop pressure decrease as a result of the flow expelled 

through the break was acceptably calculated although the rate of the 

decrease was somewhat exaggerated especially in the TRACE analyses. 

This exaggeration could have been made less pronounced by more careful 

tuning of the break conditions, i.e. basically tuning of the discharge 

coefficients associated with the used break flow model but also adjustments

of the loss coefficients at and close to the break location.

• For the large break LOCA tests 5052 and 4011, the TRACE analyses could 

predict almost as well as the RELAP5 analyses on the dryout and the flow 

reversal in the test section bundle, with about 50 o

C lower PCT in 

comparison to measurements from the tests 5052 (716 oC) and 4011 

(520 oC). 

• For the small break LOCA test 3051, where forced steam blow down 

occurred, the RELAP5 model well predicted the transient sequences in 

which no dryout occurred in the test section bundle due to the small loss of 

mass through the break; while the TRACE predicted a dryout in the later 

period of the transient due to larger loss of mass through the break and 

higher void distribution in the core. 

It is likely that the critical flow and the heat transfer are modeled somewhat differently in 

TRACE and RELAP5, resulting in different PCT and dryout behavior of the core rods.

6-1 

6. REFERENCES

1. Nilsson, L. et. al. (1983). “FIX-II – LOCA blowdown and pump trip heat transfer 

experiments, summary report of phase 2: description of experimental equipment. 

Parts 1, 2 and 3. Studsvik AB, Sweden (Technical Note NR-83/238).

2. TRACE V5.0 USER’S MANUAL. 

3. Nilsson, L. et. al. (1983). FIX-II – LOCA blowdown and pump trip heat transfer 

experiments, experimental results from LOCA test No. 5052. Studsvik AB, 

Sweden (Technical Note NR-83/323).

4. Nilsson, L. et. al. (1983). FIX-II – LOCA blowdown and pump trip heat transfer 

experiments, experimental results from LOCA test No. 4011. Studsvik AB, 

Sweden (Technical Note NR-83/321).

5. Nilsson, L. et. al. (1984). FIX-II – LOCA blowdown and pump trip heat transfer 

experiments, experimental results from LOCA test No. 3051. Studsvik AB, 

Sweden (Technical Note NR-84/486).

6. Nilsson, L. et. al. (1984). FIX-II – LOCA blowdown and pump trip heat transfer 

experiments, final report for phase 3: Results of running-in and pilot experiments. 

Studsvik AB, Sweden (Technical Note NR-84/363).

7. Eriksson, J. (1986). Assessment of RELAP5/MOD2, CYCLE 36.04 against FIX-II 

guillotine break experiment No.5061. Studsvik AB, Sweden (Technical Note NP86/109).

8. Sheng, C. (2010). Validation of RELAP5/MOD3.3 against FIX-II LOCA 

experiments No. 5052, 4011, 3051. Studsvik Nuclear AB, Sweden (N-09/215).

A-1 

APPENDIX A RESULT PLOTS OF FIX-II EXPERIMENT 5052
A-2 

 1

 2

 3

 4

 5

 6

 7

-10 0 10 20 30 40 50

Pressure (MPa)

Time (s)

pt1 (TRACE) 

pt1 (RELAP5) 

FIX EXP. 5052: PT1

Figure A-1 Pressure in the steam dome. 

The steam dome pressure dropped rapidly. The pressure decrease became less 

pronounced after the closure of the spray and feed water flows at 2.2 s. The pressure 

drops a little faster in the analysis compared to the experimental result.
A-3 

-10

 0

 10

 20

 30

 40

 50

 60

-10 0 10 20 30 40 50

Flow rate (kg/s)

Time (s)

W Break to T2 (TRACE)

W Break to T2 (RELAP5)

FIX EXP. 5052: Break flow to tank 2

Figure A-2 Break mass flow rate into tank T2.

The break valve V120 (restriction nozzle I.D. 21.6 mm) starts to open at 0 s. A 

maximum mass flow rate of about 12.5 kg/s was obtained in the experiment. However, 

the initial break mass flow rate based on DPT 42 was very uncertain, since tank T2 had 

a too low initial water level, below the expansion pipe. The discharge flows through the 

breaks reached their maxima within 1 s after the break incident. 

Both the TRACE and the RELAP5 analysis results are based on the critical flow model 

with both the subcooled and the two-phase discharge coefficient equal to 0.8. The used 

discharge coefficients reveal acceptable calculated flow rates compared to measured 

flow rates. 
A-4 

 0

 10

 20

 30

 40

 50

 60

 70

-10 0 10 20 30 40 50

Flow rate (kg/s)

Time (s)

W Break to T3 (TRACE)

W Break to T3 (RELAP5)

FIX EXP. 5052: Break flow to tank 3

Figure A-3 Break mass flow rate into tank T3.

The break valve V117 (restriction nozzle I.D. 21.6 mm) starts to open at 0.2 s. A 

maximum mass flow rate of about 16.5 kg/s was obtained in the experiment. The 

discharge flows through the breaks reached their maxima within 1 s after the break 

incident. 

Both the TRACE and the RELAP5 analysis results are based on the critical flow model 

with both the subcooled and the two-phase discharge coefficient equal to 0.8. The used 

discharge coefficients reveal acceptable calculated flow rates compared to measured 

flow rates.
A-5 

-0.1

 0

 0.1

 0.2

 0.3

 0.4

 0.5

 0.6

 0.7

 0.8

-10 0 10 20 30 40 50

Flow rate (kg/s)

Time (s)

W Steam relief (TRACE)

W Steam relief (RELAP5)

FIX EXP. 5052: steam relief flow

Figure A-4 Mass flow rate of steam through orifice meter K6 in the steam relief 

valve.

The steam relief valve (flow area 0.56 cm2

) starts to open at 0.7 s, was fully open at 

1.2 s and remained so for the rest of the test. A maximum mass flow rate of 0.48 kg/s 

was obtained in the experiment. The analyses mass flow rate is a little bit less than 

measurement after about 4 s in the later period of experiment, and is caused by the 

steam dome being depressurized a little bit faster in the analyses compared to the

experiment in that period (see Figure A-1).
A-6 

-2

 0

 2

 4

 6

 8

 10

 12

 14

 16

-10 0 10 20 30 40 50

Flow rate (kg/s)

Time (s)

W pmp 1 (TRACE)

W pmp 1 (RELAP5)

Figure A-5 Mass flow rate through pump 1 in the intact loop. 

4.4 kg/s in steady state. The speed of the main recirculation pump P1 is controlled by 

the control system to reproduce the measured mass flows.
A-7 

 0

 2

 4

 6

 8

 10

 12

 14

-10 0 10 20 30 40 50

Flow rate (kg/s)

Time (s)

W pmp 2 (TRACE)

W pmp 2 (RELAP5)

Figure A-6 Mass flow rate through pump 2 in the break pipe line. 

1.68 kg/s in steady state. The speed of the main recirculation pump P2 is controlled by 

the control system to reproduce the measured mass flows.
A-8 

-5

 0

 5

 10

 15

 20

 25

 30

 35

 40

 45

 50

-10 0 10 20 30 40 50

Differential pressure (kPa)

Time (s)

dp2 (TRACE) 

dp2 (RELAP5) 

FIX EXP. 5052: DPT02

Figure A-7 Differential pressure in the lower plenum. 

In the lower plenum, the flow from the intact recirculation line entered from one side, 

mixed with the downwards directed two-phase flow from the bundle, and passed out on 

the opposite side into the broken recirculation line. The fluid in the LP was quickly 

heated by the bundle flow to saturation temperature, which was reached after only 

1.8 s, flashing took consequently place, which depleted the LP of most of its water after 

5 s. 
A-9 

-400

-350

-300

-250

-200

-150

-100

-50

 0

 50

 100

-10 0 10 20 30 40 50

Differential pressure (kPa)

Time (s)

dp4 (TRACE) 

dp4 (RELAP5) 

FIX EXP. 5052: DPT04

Figure A-8 Differential pressure over the bundle inlet restriction. 

The core inlet flow reversed immediately after opening of the break valve 120 and the 

negative flow direction was then maintained throughout the blowdown period. 
A-10

 200

 250

 300

 350

 400

 450

 500

 550

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 02 core (TRACE)

Ti 02 core (RELAP5)

FIX EXP. 5052: ROD 36 LEVEL 1

FIX EXP. 5052: ROD 30 LEVEL 1

FIX EXP. 5052: ROD 19 LEVEL 1

FIX EXP. 5052: ROD 17 LEVEL 1

FIX EXP. 5052: ROD 22 LEVEL 1

Figure A-9 Rod temperature at cell 2 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 

The TRACE analysis reveals good prediction of dryout onset and the maximum rod 

temperature at this level, and no rewetting was predicted.
A-11

 250

 300

 350

 400

 450

 500

 550

 600

 650

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 05 core (TRACE)

Ti 05 core (RELAP5)

FIX EXP. 5052: ROD 2 LEVEL 2

FIX EXP. 5052: ROD 23 LEVEL 2

FIX EXP. 5052: ROD 22 LEVEL 2

Figure A-10 Rod temperature at cell 5 level 

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-12

 250

 300

 350

 400

 450

 500

 550

 600

 650

 700

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 06 core (TRACE)

Ti 06 core (RELAP5)

FIX EXP. 5052: ROD 17 LEVEL 3

FIX EXP. 5052: ROD 2 LEVEL 3

FIX EXP. 5052: ROD 24 LEVEL 3

FIX EXP. 5052: ROD 22 LEVEL 3

Figure A-11 Rod temperature at cell 6 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-13

 200

 250

 300

 350

 400

 450

 500

 550

 600

 650

 700

 750

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 08 core (TRACE)

Ti 08 core (RELAP5)

FIX EXP. 5052: ROD 3 LEVEL 4

FIX EXP. 5052: ROD 29 LEVEL 4

FIX EXP. 5052: ROD 17 LEVEL 4

FIX EXP. 5052: ROD 27 LEVEL 4

FIX EXP. 5052: ROD 2 LEVEL 4

FIX EXP. 5052: ROD 19 LEVEL 4

FIX EXP. 5052: ROD 24 LEVEL 4

FIX EXP. 5052: ROD 12 LEVEL 4

FIX EXP. 5052: ROD 21 LEVEL 4

Figure A-12 Rod temperature at cell 8 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-14

 200

 250

 300

 350

 400

 450

 500

 550

 600

 650

 700

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 09 core (TRACE)

Ti 10 core (TRACE)

Ti 09 core (RELAP5)

Ti 10 core (RELAP5)

FIX EXP. 5052: ROD 12 LEVEL 6

FIX EXP. 5052: ROD 21 LEVEL 6

FIX EXP. 5052: ROD 27 LEVEL 6

FIX EXP. 5052: ROD 31 LEVEL 6

FIX EXP. 5052: ROD 32 LEVEL 6

FIX EXP. 5052: ROD 28 LEVEL 6

FIX EXP. 5052: ROD 28 LEVEL 5

FIX EXP. 5052: ROD 21 LEVEL 5

FIX EXP. 5052: ROD 27 LEVEL 5

Figure A-13 Rod temperature at cell 9, 10 levels. 

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-15

 200

 250

 300

 350

 400

 450

 500

 550

 600

 650

 700

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 11 core (TRACE)

Ti 13 core (TRACE)

Ti 11 core (RELAP5)

Ti 13 core (RELAP5)

EXP.: ROD 6 LEVEL 8

EXP.: ROD 9 LEVEL 8

EXP.: ROD 31 LEVEL 8

EXP.: ROD 21 LEVEL 8

EXP.: ROD 32 LEVEL 8

EXP.: ROD 5 LEVEL 8

EXP.: ROD 11 LEVEL 8

EXP.: ROD 16 LEVEL 8

EXP.: ROD 34 LEVEL 8

EXP.: ROD 3 LEVEL 8

EXP.: ROD 15 LEVEL 7

EXP.: ROD 8 LEVEL 7

EXP.: ROD 22 LEVEL 7

EXP.: ROD 18 LEVEL 7

EXP.: ROD 29 LEVEL 7

EXP.: ROD 21 LEVEL 7

EXP.: ROD 32 LEVEL 7

EXP.: ROD 2 LEVEL 7

EXP.: ROD 28 LEVEL 7

EXP.: ROD 24 LEVEL 7

EXP.: ROD 3 LEVEL 7

EXP.: ROD 35 LEVEL 7

EXP.: ROD 23 LEVEL 7

EXP.: ROD 1 LEVEL 7

EXP.: ROD 14 LEVEL 7

Figure A-14 Rod temperature at cell 11, 13 levels. 

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-16

 250

 300

 350

 400

 450

 500

 550

 600

 650

 700

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 15 core (TRACE)

Ti 15 core (RELAP5)

FIX EXP. 5052: ROD 15 LEVEL 9

FIX EXP. 5052: ROD 8 LEVEL 9

FIX EXP. 5052: ROD 35 LEVEL 9

FIX EXP. 5052: ROD 18 LEVEL 9

FIX EXP. 5052: ROD 1 LEVEL 9

FIX EXP. 5052: ROD 14 LEVEL 9

Figure A-15 Rod temperature at cell 15 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-17

 200

 250

 300

 350

 400

 450

 500

 550

 600

 650

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 16 core (TRACE)

Ti 16 core (RELAP5)

FIX EXP. 5052: ROD 25 LEVEL 10

FIX EXP. 5052: ROD 6 LEVEL 10

FIX EXP. 5052: ROD 10 LEVEL 10

FIX EXP. 5052: ROD 9 LEVEL 10

FIX EXP. 5052: ROD 5 LEVEL 10

FIX EXP. 5052: ROD 11 LEVEL 10

FIX EXP. 5052: ROD 16 LEVEL 10

FIX EXP. 5052: ROD 34 LEVEL 10

Figure A-16 Rod temperature at cell 16 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-18

 200

 250

 300

 350

 400

 450

 500

 550

 600

 650

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 18 core (TRACE)

Ti 18 core (RELAP5)

FIX EXP. 5052: ROD 7 LEVEL 11

FIX EXP. 5052: ROD 14 LEVEL 11

FIX EXP. 5052: ROD 35 LEVEL 11

Figure A-17 Rod temperature at cell 18 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-19

 200

 250

 300

 350

 400

 450

 500

 550

 600

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 19 core (TRACE)

Ti 20 core (TRACE)

Ti 19 core (RELAP5)

Ti 20 core (RELAP5)

FIX EXP. 5052: ROD 6 LEVEL 12

FIX EXP. 5052: ROD 5 LEVEL 12

FIX EXP. 5052: ROD 4 LEVEL 12

FIX EXP. 5052: ROD 9 LEVEL 12

Figure A-18 Rod temperature at cell 19, 20 levels. 

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-20

 200

 250

 300

 350

 400

 450

 500

 550

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 21 core (TRACE)

Ti 22 core (TRACE)

Ti 23 core (TRACE)

Ti 21 core (RELAP5)

Ti 22 core (RELAP5)

Ti 23 core (RELAP5)

FIX EXP. 5052: ROD 14 LEVEL 13

FIX EXP. 5052: ROD 1 LEVEL 13

FIX EXP. 5052: ROD 35 LEVEL 13

FIX EXP. 5052: ROD 18 LEVEL 13

FIX EXP. 5052: ROD 15 LEVEL 13

FIX EXP. 5052: ROD 11 LEVEL 14

FIX EXP. 5052: ROD 6 LEVEL 14

FIX EXP. 5052: ROD 9 LEVEL 14

FIX EXP. 5052: ROD 10 LEVEL 14

FIX EXP. 5052: ROD 16 LEVEL 14

Figure A-19 Rod temperature at cell 21, 22, 23 levels. 

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-21

 100

 150

 200

 250

 300

 350

 400

-10 0 10 20 30 40 50

Rod temperature (C)

Time (s)

Ti 24 core (TRACE)

Ti 25 core (TRACE)

Ti 24 core (RELAP5)

Ti 25 core (RELAP5)

FIX EXP. 5052: ROD 5 LEVEL 16

FIX EXP. 5052: ROD 18 LEVEL 15

FIX EXP. 5052: ROD 35 LEVEL 15

FIX EXP. 5052: ROD 10 LEVEL 16

FIX EXP. 5052: ROD 15 LEVEL 15

FIX EXP. 5052: ROD 16 LEVEL 16

FIX EXP. 5052: ROD 11 LEVEL 16

FIX EXP. 5052: ROD 9 LEVEL 16

FIX EXP. 5052: ROD 4 LEVEL 16

FIX EXP. 5052: ROD 6 LEVEL 16

Figure A-20 Rod temperature at cell 24 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
A-22

 0

 0.2

 0.4

 0.6

 0.8

 1

-10 0 10 20 30 40 50

Core vapor void (-)

Time (s)

Void 01 core RELAP5

Void 05 core RELAP5

Void 10 core RELAP5

Void core 01 TRACE

Void core 05 TRACE

Void core 10 TRACE

Figure A-21 Void distribution along the core at nodes 1, 5, 10 

The core is divided into 26 nodes, node 1 is at the bottom level and node 26 is at the 

top of the core. 
A-23

 0.5

 0.55

 0.6

 0.65

 0.7

 0.75

 0.8

 0.85

 0.9

 0.95

 1

-10 0 10 20 30 40 50

Core vapor void (-)

Time (s)

Void 15 core RELAP5

Void 20 core RELAP5

Void 25 core RELAP5

Void core 15 TRACE

Void core 20 TRACE

Void core 25 TRACE

Figure A-22 Void distribution along the core at nodes 15, 20, 25.

The core is divided into 26 nodes, node 1 is at the bottom level and node 26 is at the 

top of the core. 

B-1 

APPENDIX B RESULT PLOTS OF FIX-II EXPERIMENT 4011
B-2 

 0

 1

 2

 3

 4

 5

 6

 7

-10 0 10 20 30 40 50 60

Pressure (MPa)

Time (s)

pt1 (RELAP5) 

pt1 (TRACE) 

FIX EXP 4011: PT1

Figure B-1 Pressure in the steam dome. 

The steam dome pressure dropped rapidly. The pressure decrease became less 

pronounced after the closure of the spray and feed water flows at about 2 s. The 

pressure drops a little faster in the analysis compared to the experimental result. 
B-3 

 0

 10

 20

 30

 40

 50

 60

-10 0 10 20 30 40 50 60

Flow rate (kg/s)

Time (s)

W Break to T1 (RELAP5) 

W Break to T1 (TRACE)

FIX EXP 4011: Break flow to tank 1

Figure B-2 Break mass flow rate into tank T1.

The break valve V123 (restriction nozzle I.D. 16 mm) starts to open at 0 s. A maximum 

mass flow rate was obtained immediately after the valve opened. However, the break 

mass flow rate measurement was uncertain as is mentioned in [4].

 

The analysis results are based on the critical flow model with both the subcooled and 

the two-phase discharge coefficient equal to 0.8. The used discharge coefficients reveal 

acceptable calculated flow rates compared to measured flow rates.
B-4 

-10

 0

 10

 20

 30

 40

 50

 60

 70

 80

-10 0 10 20 30 40 50 60

Flow rate (kg/s)

Time (s)

W Break to T3 (RELAP5) 

W Break to T3 (TRACE)

FIX EXP 4011: Break flow to tank 3

Figure B-3 Break mass flow rate into tank T3.

The break valve V117 (restriction nozzle I.D. 21.6 mm) starts to open at 0.1 s. A 

maximum mass flow rate was obtained immediately after the valve opened. However, 

the break mass flow rate measurement was uncertain as is mentioned in Ref. 4.

 

The analysis results are based on the critical flow model with both the subcooled and 

the two-phase discharge coefficient equal to 0.8. The used discharge coefficients reveal 

acceptable calculated flow rates compared to measured flow rates.
B-5 

-0.1

 0

 0.1

 0.2

 0.3

 0.4

 0.5

 0.6

 0.7

 0.8

-10 0 10 20 30 40 50 60

Flow rate (kg/s)

Time (s)

W Steam relief (RELAP5) 

W Steam relief (TRACE)

FIX EXP 4011: steam relief flow

Figure B-4 Mass flow rate of steam through orifice meter K6 in the steam relief 

valve.

The steam relief valve (flow area 0.56 cm2

) starts to open at 0.3 s, was fully open at 

0.7 s and remained so for the rest of the test. A maximum mass flow rate of 0.47 kg/s 

was obtained in the experiment. 
B-6 

-2

 0

 2

 4

 6

 8

 10

 12

 14

 16

 18

-10 0 10 20 30 40 50 60

Flow rate (kg/s)

Time (s)

W pmp 1 (RELAP5) 

W pmp 1 (TRACE) 

Figure B-5 Mass flow rate through pump 1 in the intact loop. 

6.05 kg/s in steady state. The speed of the main recirculation pump P1 is controlled by 

the control system to reproduce the measured mass flows.
B-7 

-700

-600

-500

-400

-300

-200

-100

 0

 100

-10 0 10 20 30 40 50 60

Differential pressure (kPa)

Time (s)

dp4+dp5 (RELAP5) 

dp4+dp5 (TRACE) 

FIX EXP 4011: DPT05+DPT04

Figure B-6 Differential pressure over the bundle inlet restriction. 

The depressurization through the breaks made the inlet flow to the bundle be reversed 

immediately. The negative flow direction was then maintained throughout the blowdown

period. 
B-8 

-40

-35

-30

-25

-20

-15

-10

-5

 0

 5

-10 0 10 20 30 40 50 60

Differential pressure (kPa)

Time (s)

dp6 (RELAP5) 

dp6 (TRACE) 

FIX EXP 4011: DPT06

Figure B-7 Differential pressure over subdivisions in the lower part of the 

bundle. 

The core flow reversed immediately after opening of the break valve 123, and 

decreased quickly to almost to 0.
B-9 

 100

 150

 200

 250

 300

 350

 400

 450

 500

-10 0 10 20 30 40 50 60

Rod temperature (C)

Time (s)

Ti 05 core (RELAP5) 

Ti 05 core (TRACE) 

FIX EXP 4011: ROD 23 LEVEL 2

FIX EXP 4011: ROD 22 LEVEL 2

Figure B-8 Rod temperature at cell 5 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 

The rod temperatures increased to a maximum, then decreased again either slowly with 

decaying power (for example Rod 23, level 2 in Figure B-8), or rapidly due to rewetting 

(for example Rod 22, level 2 in Figure B-8). The highest temperature, 520 oC was 

obtained on rod No 21, at level 4 after 12 s (Figure B-9). Large variations in the course 

of the rod temperatures were observed for thermocouple positions at the same level but 

at different rods.
B-10

 200

 250

 300

 350

 400

 450

 500

 550

-10 0 10 20 30 40 50 60

Rod temperature (C)

Time (s)

Ti 08 core (RELAP5) 

Ti 08 core (TRACE) 

FIX EXP 4011: ROD 3 LEVEL 4

FIX EXP 4011: ROD 17 LEVEL 4

FIX EXP 4011: ROD 27 LEVEL 4

FIX EXP 4011: ROD 19 LEVEL 4

FIX EXP 4011: ROD 24 LEVEL 4

FIX EXP 4011: ROD 12 LEVEL 4

FIX EXP 4011: ROD 21 LEVEL 4

Figure B-9 Rod temperature at cell 8 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
B-11

 200

 250

 300

 350

 400

 450

 500

 550

-10 0 10 20 30 40 50 60

Rod temperature (C)

Time (s)

Ti 10 core (RELAP5) 

Ti 10 core (TRACE) 

FIX EXP 4011: ROD 12 LEVEL 6

FIX EXP 4011: ROD 21 LEVEL 6

FIX EXP 4011: ROD 27 LEVEL 6

FIX EXP 4011: ROD 31 LEVEL 6

FIX EXP 4011: ROD 32 LEVEL 6

FIX EXP 4011: ROD 28 LEVEL 6

Figure B-10 Rod temperature at cell 10 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
B-12

 200

 250

 300

 350

 400

 450

 500

-10 0 10 20 30 40 50 60

Rod temperature (C)

Time (s)

Ti 13 core (RELAP5) 

Ti 13 core (TRACE) 

FIX EXP 4011: ROD 6 LEVEL 8

FIX EXP 4011: ROD 9 LEVEL 8

FIX EXP 4011: ROD 31 LEVEL 8

FIX EXP 4011: ROD 21 LEVEL 8

FIX EXP 4011: ROD 32 LEVEL 8

FIX EXP 4011: ROD 5 LEVEL 8

FIX EXP 4011: ROD 11 LEVEL 8

FIX EXP 4011: ROD 16 LEVEL 8

FIX EXP 4011: ROD 34 LEVEL 8

FIX EXP 4011: ROD 3 LEVEL 8

Figure B-11 Rod temperature at cell 13 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
B-13

 200

 250

 300

 350

 400

 450

 500

-10 0 10 20 30 40 50 60

Rod temperature (C)

Time (s)

Ti 16 core (RELAP5) 

Ti 16 core (TRACE) 

FIX EXP 4011: ROD 6 LEVEL 10

FIX EXP 4011: ROD 10 LEVEL 10

FIX EXP 4011: ROD 9 LEVEL 10

FIX EXP 4011: ROD 5 LEVEL 10

FIX EXP 4011: ROD 11 LEVEL 10

FIX EXP 4011: ROD 16 LEVEL 10

FIX EXP 4011: ROD 34 LEVEL 10

Figure B-12 Rod temperature at cell 16 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
B-14

 200

 250

 300

 350

 400

 450

 500

-10 0 10 20 30 40 50 60

Rod temperature (C)

Time (s)

Ti 19 core (RELAP5) 

Ti 20 core (RELAP5) 

Ti 19 core (TRACE) 

Ti 20 core (TRACE) 

FIX EXP 4011: ROD 6 LEVEL 12

FIX EXP 4011: ROD 5 LEVEL 12

FIX EXP 4011: ROD 4 LEVEL 12

FIX EXP 4011: ROD 9 LEVEL 12

Figure B-13 Rod temperature at cell 19, 20 levels. 

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
B-15

 200

 220

 240

 260

 280

 300

 320

 340

 360

 380

 400

-10 0 10 20 30 40 50 60

Rod temperature (C)

Time (s)

Ti 23 core (RELAP5) 

Ti 23 core (TRACE) 

FIX EXP 4011: ROD 11 LEVEL 14

FIX EXP 4011: ROD 6 LEVEL 14

FIX EXP 4011: ROD 9 LEVEL 14

FIX EXP 4011: ROD 10 LEVEL 14

FIX EXP 4011: ROD 16 LEVEL 14

Figure B-14 Rod temperature at cell 23 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
B-16

 210

 220

 230

 240

 250

 260

 270

 280

 290

 300

 310

-10 0 10 20 30 40 50 60

Rod temperature (C)

Time (s)

Ti 25 core (RELAP5) 

Ti 25 core (TRACE) 

FIX EXP 4011: ROD 5 LEVEL 16

FIX EXP 4011: ROD 10 LEVEL 16

FIX EXP 4011: ROD 16 LEVEL 16

FIX EXP 4011: ROD 11 LEVEL 16

FIX EXP 4011: ROD 9 LEVEL 16

FIX EXP 4011: ROD 4 LEVEL 16

FIX EXP 4011: ROD 6 LEVEL 16

Figure B-15 Rod temperature at cell 25 level.

The heated rod is divided into 25 cells, cell 1 is at the bottom level and cell 25 is at the 

top of the core. 
B-17

 0

 0.2

 0.4

 0.6

 0.8

 1

-10 0 10 20 30 40 50 60

Core vapor void (-)

Time (s)

Void 10 core (RELAP5)

Void 05 core (RELAP5)

Void 01 core (RELAP5)

Void 10 core (TRACE)

Void 05 core (TRACE)

Void 01 core (TRACE)

Figure B-16 Void distribution along the core at nodes 01, 10, 15.

The core is divided into 26 nodes, node 1 is at the bottom level and node 26 is at the 

top of the core. 
B-18

 0.4

 0.5

 0.6

 0.7

 0.8

 0.9

 1

-10 0 10 20 30 40 50 60

Core vapor void (-)

Time (s)

Void 25 core (RELAP5)

Void 20 core (RELAP5)

Void 15 core (RELAP5)

Void 25 core (TRACE)

Void 20 core (TRACE)

Void 15 core (TRACE)

Figure B-17 Void distribution along the core at nodes 15, 20, 25.

The core is divided into 26 nodes, node 1 is at the bottom level and node 26 is at the 

top of the core.

 
C-1 

APPENDIX C RESULT PLOTS OF FIX-II EXPERIMENT 3051
C-2 

 2

 3

 4

 5

 6

 7

 8

-20 0 20 40 60 80 100 120 140 160

Pressure (MPa)

Time (s)

pt1 (RELAP5) 

pt1 (TRACE) 

FIX EXP 3051: PT1

Figure C-1 Pressure in the steam dome. 

The steam dome pressure dropped from 6.99 MPa to 6.7 MPa at 1.9 s. As the initial 

opening of the steam relief valve was closed almost simultaneously with the closing of 

the spray and feed water flows, the pressure increased again to a maximum of 7.1 MPa 

at 8.9 s in the experiment. The pressure decrease, caused by the relatively small 

bottom break, was then rather slow, until the steam relief valve opened a second time 

at 39.6 s, at which time the depressurization rate suddenly increased. The steam relief 

continued until the test was terminated.
C-3 

-5

 0

 5

 10

 15

 20

-20 0 20 40 60 80 100 120 140 160

Flow rate (kg/s)

Time (s)

W Break to T2 (RELAP5)

W Break to T2(TRACE)

FIX EXP 3051: Break flow to tank 2

Figure C-2 Break mass flow rate into tank T2.

The break valve V120 (restriction nozzle I.D. 6.82 mm) started to open at 0 s. A 

maximum mass flow rate was obtained immediately after the valve opened. 

Both the TRACE and RELAP5 analysis results are based on the critical flow model with 

both the subcooled and the two-phase discharge coefficient equal to 0.8. The used 

discharge coefficients reveal acceptable calculated flow rates compared to measured 

flow rates.
C-4 

-0.2

 0

 0.2

 0.4

 0.6

 0.8

 1

 1.2

-20 0 20 40 60 80 100 120 140 160

Flow rate (kg/s)

Time (s)

W Steam relief (RELAP5)

W Steam relief (TRACE)

FIX EXP 3051: steam relief flow

Figure C-3 Mass flow rate of steam through orifice meter K6 in the steam relief 

valve.

The steam relief valve (flow area 0.94 cm2

) started to open at 0.5 s, and closed soon 

after that, at 39.6 s it was opened again, and remained so for the rest of the test. A 

maximum mass flow rate of 0.83 kg/s was obtained in the experiment. 
C-5 

-3

-2

-1

 0

 1

 2

 3

 4

 5

 6

 7

 8

-20 0 20 40 60 80 100 120 140 160

Flow rate (kg/s)

Time (s)

W pmp 1 (RELAP5) 

W pmp 1 (TRACE) 

Figure C-4 Mass flow rate through pump 1 in the intact loop. 

4.85 kg/s in steady state. The speed of the main recirculation pump P1 is controlled by 

the control system to reproduce the measured mass flows.
C-6 

 0

 1

 2

 3

 4

 5

 6

-20 0 20 40 60 80 100 120 140 160

Flow rate (kg/s)

Time (s)

W pmp 2 (RELAP5) 

W pmp 2 (TRACE) 

Figure C-5 Mass flow rate through pump 2 in the break pipe line. 

1.59 kg/s in steady state. The speed of the main recirculation pump P2 is controlled by 

the control system to reproduce the measured mass flows.
C-7 

-80

-60

-40

-20

 0

 20

 40

-20 0 20 40 60 80 100 120 140 160

Differential pressure (kPa)

Time (s)

dp4+dp5 (RELAP5) 

dp4+dp5 (TRACE) 

FIX EXP 3051: DPT05+DPT04

Figure C-6 Differential pressure over the bundle inlet restriction. 
C-8 

-20

-15

-10

-5

 0

 5

 10

 15

-20 0 20 40 60 80 100 120 140 160

Differential pressure (kPa)

Time (s)

dp6 (RELAP5) 

dp6 (TRACE) 

FIX EXP 3051: DPT06

Figure C-7 Differential pressure over subdivisions in the lower part of the 

bundle. 
C-9 

 200

 220

 240

 260

 280

 300

 320

-20 0 20 40 60 80 100 120 140 160

Rod temperature (C)

Time (s)

Ti 08 core (RELAP5) 

Ti 08 core (TRACE) 

FIX EXP 3051: ROD 23 LEVEL 2

FIX EXP 3051: ROD 22 LEVEL 2

Figure C-8 Rod temperature at cell 8 level.

The heated rod is divided into 46 cells, cell 1 is at the bottom level and cell 46 is at the 

top of the core. 

Since the total loss of water was comparatively small there was never any uncovery of 

the rod bundle during the blowdown period in the experiment and consequently no heat 

up of the rods. The RELAP5 could predict the rod temperature profile very well, while 

the TRACE got somehow slightly higher temperature already at steady state and 

predicted even a dry out along the whole rod length in the core during the last period of 

the transient. The reason is described in chapter 4.3.
C-10

 200

 220

 240

 260

 280

 300

 320

 340

-20 0 20 40 60 80 100 120 140 160

Rod temperature (C)

Time (s)

Ti 15 core (RELAP5) 

Ti 15 core (TRACE) 

FIX EXP 3051: ROD 3 LEVEL 4

FIX EXP 3051: ROD 29 LEVEL 4

FIX EXP 3051: ROD 17 LEVEL 4

FIX EXP 3051: ROD 27 LEVEL 4

FIX EXP 3051: ROD 19 LEVEL 4

FIX EXP 3051: ROD 12 LEVEL 4

FIX EXP 3051: ROD 21 LEVEL 4

Figure C-9 Rod temperature at cell 15 level.

The heated rod is divided into 46 cells, cell 1 is at the bottom level and cell 46 is at the 

top of the core. 

See comments in Figure C-8.
C-11

 200

 220

 240

 260

 280

 300

 320

 340

-20 0 20 40 60 80 100 120 140 160

Rod temperature (C)

Time (s)

Ti 18 core (RELAP5) 

Ti 18 core (TRACE) 

FIX EXP 3051: ROD 12 LEVEL 6

FIX EXP 3051: ROD 21 LEVEL 6

FIX EXP 3051: ROD 27 LEVEL 6

FIX EXP 3051: ROD 31 LEVEL 6

FIX EXP 3051: ROD 32 LEVEL 6

FIX EXP 3051: ROD 28 LEVEL 6

Figure C-10 Rod temperature at cell 18 level.

The heated rod is divided into 46 cells, cell 1 is at the bottom level and cell 46 is at the 

top of the core. 

See comments in Figure C-8.
C-12

 200

 220

 240

 260

 280

 300

 320

 340

 360

-20 0 20 40 60 80 100 120 140 160

Rod temperature (C)

Time (s)

Ti 23 core (RELAP5) 

Ti 23 core (TRACE) 

FIX EXP 3051: ROD 6 LEVEL 8

FIX EXP 3051: ROD 9 LEVEL 8

FIX EXP 3051: ROD 31 LEVEL 8

FIX EXP 3051: ROD 21 LEVEL 8

FIX EXP 3051: ROD 32 LEVEL 8

FIX EXP 3051: ROD 11 LEVEL 8

FIX EXP 3051: ROD 16 LEVEL 8

FIX EXP 3051: ROD 34 LEVEL 8

FIX EXP 3051: ROD 3 LEVEL 8

Figure C-11 Rod temperature at cell 23 level.

The heated rod is divided into 46 cells, cell 1 is at the bottom level and cell 46 is at the 

top of the core. 

See comments in Figure C-8. 
C-13

 200

 220

 240

 260

 280

 300

 320

 340

 360

-20 0 20 40 60 80 100 120 140 160

Rod temperature (C)

Time (s)

Ti 29 core (RELAP5) 

Ti 29 core (TRACE) 

FIX EXP 3051: ROD 6 LEVEL 10

FIX EXP 3051: ROD 9 LEVEL 10

FIX EXP 3051: ROD 5 LEVEL 10

FIX EXP 3051: ROD 11 LEVEL 10

FIX EXP 3051: ROD 16 LEVEL 10

FIX EXP 3051: ROD 34 LEVEL 10

Figure C-12 Rod temperature at cell 29 level.

The heated rod is divided into 46 cells, cell 1 is at the bottom level and cell 46 is at the 

top of the core. 

See comments in Figure C-8. 
C-14

 200

 220

 240

 260

 280

 300

 320

 340

 360

-20 0 20 40 60 80 100 120 140 160

Pressure (MPa)

Time (s)

Ti 35 core (RELAP5) 

Ti 36 core (RELAP5) 

Ti 35 core (TRACE)

Ti 36 core (TRACE)

FIX EXP 3051: ROD 6 LEVEL 12

FIX EXP 3051: ROD 5 LEVEL 12

FIX EXP 3051: ROD 4 LEVEL 12

Figure C-13 Rod temperature at cell 35, 36 levels. 

The heated rod is divided into 46 cells, cell 1 is at the bottom level and cell 46 is at the 

top of the core. 

See comments in Figure C-8. 
C-15

 200

 220

 240

 260

 280

 300

 320

 340

-20 0 20 40 60 80 100 120 140 160

Rod temperature (C)

Time (s)

Ti 42 core (RELAP5) 

Ti 42 core (TRACE) 

FIX EXP 3051: ROD 11 LEVEL 14

FIX EXP 3051: ROD 6 LEVEL 14

FIX EXP 3051: ROD 9 LEVEL 14

FIX EXP 3051: ROD 10 LEVEL 14

FIX EXP 3051: ROD 16 LEVEL 14

Figure C-14 Rod temperature at cell 42 level.

The heated rod is divided into 46 cells, cell 1 is at the bottom level and cell 46 is at the 

top of the core. 

See comments in Figure C-8. 
C-16

 200

 220

 240

 260

 280

 300

 320

 340

-20 0 20 40 60 80 100 120 140 160

Rod temperature (C)

Time (s)

Ti 46 core (RELAP5) 

Ti 46 core (TRACE) 

FIX EXP 3051: ROD 5 LEVEL 16

FIX EXP 3051: ROD 10 LEVEL 16

FIX EXP 3051: ROD 16 LEVEL 16

FIX EXP 3051: ROD 11 LEVEL 16

FIX EXP 3051: ROD 4 LEVEL 16

FIX EXP 3051: ROD 6 LEVEL 16

Figure C-15 Rod temperature at cell 46 level.

The heated rod is divided into 46 cells, cell 1 is at the bottom level and cell 46 is at the 

top of the core. 

See comments in Figure C-8. 
C-17

 0

 0.1

 0.2

 0.3

 0.4

 0.5

 0.6

 0.7

 0.8

 0.9

 1

-20 0 20 40 60 80 100 120 140 160

Core vapor void (-)

Time (s)

Void 47 core (RELAP5)

Void 40 core (RELAP5)

Void 30 core (RELAP5)

Void 47 core (TRACE)

Void 40 core (TRACE)

Void 30 core (TRACE)

Figure C-16 Void distribution along the core at nodes 30, 40, 47.

The core is divided into 47 nodes, node 1 is at the bottom level and node 47 is at the 

top of the core. 

TRACE analysis showed a more significant void increase along the whole length of the 

core in comparison to the RELAP5 results, and this is one of the reasons why TRACE 

predicted the rod dryout. 
C-18

 0

 0.2

 0.4

 0.6

 0.8

 1

-20 0 20 40 60 80 100 120 140 160

Core vapor void (-)

Time (s)

Void 20 core (RELAP5)

Void 10 core (RELAP5)

Void 01 core (RELAP5)

Void 20 core (TRACE)

Void 10 core (TRACE)

Void 01 core (TRACE)

Figure C-17 Void distribution along the core at nodes 01, 10, 20.

The core is divided into 47 nodes, node 1 is at the bottom level and node 47 is at the 

top of the core. 

TRACE analysis showed a more significant void increase along the whole length of the 

core in comparison to the RELAP5 results, and this is one of the reasons why TRACE 

predicted the rod dryout. 
C-19

Figure C-18 The total fluid mass in the whole system.

TRACE predicted more mass inventory during the steady state, and it also lost more 

mass through the break and the steam relief line compared to the RELAP5 results. This 

is one of the reasons why TRACE predicted a higher rod temperature and even a 

dryout.
C-20

Figure C-19 The total fluid mass loss through the break and the steam relief line.

TRACE results showed that more mass was lost through the break and the steam relief 

line compared to the RELAP5 results. This is one of the reasons why TRACE predicted

a higher rod temperature and even a dryout. 



UNITED STATES

NUCLEAR REGULATORY COMMISSION

WASHINGTON, DC 20555-0001

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

OFFICIAL BUSINESS
NUREG/IA-0415 TRACE (V 5.0 Patch 2) Validation Based on the RELAP5-Calculation of FIX-II 

LOCA Experiments NO. 5052, 4011, 3051