NUREG/IA-0408 

International 

Agreement Report

IJS Procedure for Converting Input 

Deck from RELAP5 to TRACE 

Prepared by:

A. Prošek, O.A. Berar, B. Mavko

Jožef Stefan Institute

 Jamova cesta 39

 SI-1000 Ljubljana, Slovenia

A. Calvo, NRC Project Manager

 

 

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: October 2012 

Date Published: October 2012

Prepared as part of 

The Agreement on Research Participation and Technical Exchange

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

Published by 

U.S. Nuclear Regulatory Commission
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infringe privately owned rights. 
NUREG/IA-0408 

International 

Agreement Report

IJS Procedure for Converting Input 

Deck from RELAP5 to TRACE 

Prepared by:

A. Prošek, O.A. Berar, B. Mavko

Jožef Stefan Institute

 Jamova cesta 39

 SI-1000 Ljubljana, Slovenia

A. Calvo, NRC Project Manager

 

 

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: October 2012 

Date Published: October 2012

Prepared as part of 

The Agreement on Research Participation and Technical Exchange

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

Published by 

U.S. Nuclear Regulatory Commission

iii

ABSTRACT

Much of efforts have been done in the past to develop the RELAP5 input decks. The purpose of 

this study is to present the RELAP5 to TRACE conversion procedure, developed at Institut 

“Jožef Stefan” (IJS). For demonstrating the conversion procedure the ACHILLES and BETHSY

input decks (IJS legacy) were used using SNAP. The RELAP5 input decks of ACHILLES rig and 

BETHSY facility were developed in the frame of participation to international standard problem 

no. 25 (ISP-25) and international standard problem no. 27 (ISP-27). These RELAP5 legacy 

input decks were used for demonstration of the developed IJS conversion methodology, 

consisting of eleven steps. Besides demonstration of the methodology also the comparison 

between RELAP5 and TRACE has been done. It can be concluded that calculated results 

obtained by TRACE are as good as the results by RELAP5, thus suggesting that IJS conversion 

procedure may be used for conversion of separate and integral effect test legacy RELAP5 input 

decks to TRACE input decks.

v

CONTENTS

Page

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

FIGURES....................................................................................................................................vii

TABLES.....................................................................................................................................viii

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

ACKNOWLEDGEMENTS ...........................................................................................................xi

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

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

2. IJS CONVERSION PROCEDURE DESCRIPTION ................................................................3

3. ACHILLES RIG, TEST AND RELAP5 INPUT MODEL DESCRIPTION ................................7

3.1 Achilles rig......................................................................................................................7

3.2 ACHILLES Run A1B105 ................................................................................................8

3.3 RELAP5 Input Model of ACHILLES ...............................................................................8

4. BETHSY FACILTY, TESTS AND RELAP5 INPUT MODEL DESCRIPTION.......................11

4.1 BETHSY Facility...........................................................................................................11

4.2 BETHSY Test 9.1b.......................................................................................................12

4.3 BETHSY Test 6.2TC....................................................................................................13

4.4 RELAP5 Input Model of BETHSY ................................................................................13

5. ACHILLES EXAMPLE OF CONVERSION...........................................................................15

5.1 Adaptation of legacy RELAP5 input model to RELAP5/MOD3.3 (Step 1) ...................15

5.2 Importation of ASCII input model into SNAP (Step 2)..................................................15

5.3 Manually arrange Hydraulic Components View (Step 3) .............................................16

5.4 Export RELAP5 Model Notebook (Step 4)...................................................................17

5.5 Perform RELAP5 calculation (Step 5)..........................................................................18

5.6 Create animation model for RELAP5 (Step 6) .............................................................18

5.7 Convert RELAP5 model to TRACE model (Step 7) .....................................................18

5.8 Check for and resolve TRACE model errors (Step 8)..................................................19

5.9 Export TRACE Model Notebook (Step 9).....................................................................20

5.10Perform TRACE calculation (Step 10) .........................................................................20

5.11Create TRACE animation model (Step 11)..................................................................21

5.12Results for ACHILLES natural reflood test...................................................................22

6. BETHSY EXAMPLE OF CONVERSION ..............................................................................25
vi

6.1 Adaptation of legacy RELAP5 input model to RELAP5/MOD3.3 (Step 1) ...................25

6.2 Importation of ASCII input model into SNAP (Step 2)..................................................25

6.3 Manually arrange Hydraulic Components View (Step 3) .............................................26

6.4 Export RELAP5 Model Notebook (Step 4)...................................................................26

6.5 Perform RELAP5 calculation (Step 5)..........................................................................27

6.6 Create animation model for RELAP5 (Step 6) .............................................................28

6.7 Convert RELAP5 model to TRACE model (Step 7) .....................................................29

6.8 Check for and resolve TRACE model errors (Step 8)..................................................30

6.9 Export TRACE Model Notebook (Step 9).....................................................................30

6.10Perform TRACE calculation (Step 10) .........................................................................31

6.11Create TRACE animation model (Step 11)..................................................................31

6.12Results for BETHSY tests ............................................................................................32

6.12.1 RELAP5 and TRACE calculations of BETHSY 9.1b test.....................................32

6.12.2 RELAP5 and TRACE calculations of BETHSY 6.2TC test..................................40

7. CONCLUSIONS....................................................................................................................49

8. REFERENCES......................................................................................................................51
vii

FIGURES

Figure 1 IJS conversion procedure from RELAP5 model to TRACE model.............................6

Page

Figure 2 Schematic of Achilles test facility................................................................................7

Figure 3 Achilles facility RELAP5 nodalization (Ref. 9) ............................................................9

Figure 4 BETHSY facility ........................................................................................................12

Figure 5 RELAP5/MOD3.1 input model of BETHSY facility. ..................................................14

Figure 6 Model Editor screen after importing ASCII input file.................................................15

Figure 7 SNAP hydrodynamic component view for RELAP5 input model of ACHILLES rig -

original......................................................................................................................16

Figure 8 SNAP hydrodynamic components view for RELAP5 input model of ACHILLES rig –

arranged ...................................................................................................................17

Figure 9 Basic animation model for RELAP5 model of ACHILLES rig ...................................18

Figure 10 SNAP hydrodynamic components view for TRACE input model of ACHILLES rig ..19

Figure 11 Error report for TRACE model of ACHILLES rig.......................................................19

Figure 12 Basic animation model for RELAP5 model of ACHILLES rig ...................................21

Figure 13 SNAP additional animation mask for the Achilles TRACE model.............................22

Figure 14 Peak cladding temperature at 2.13 m elevation .......................................................23

Figure 15 Quench front progression.........................................................................................23

Figure 16 Pressure at the top of the upper plenum ..................................................................24

Figure 17 Core collapsed liquid level........................................................................................24

Figure 18 Downcommer collapsed liquid level .........................................................................24

Figure 19 SNAP hydrodynamic components view for RELAP5 input model of BETHSY facility -

original......................................................................................................................25

Figure 20 SNAP hydrodynamic component view for RELAP5 input model of BETHSY facility –

arranged ...................................................................................................................26

Figure 21 SNAP animation model for RELAP5 model of BETHSY facility ...............................28

Figure 22 SNAP hydrodynamic component view for converted (and adapted) TRACE input 

model of BETHSY facility .........................................................................................29

Figure 23 Error report for TRACE model of BETHSY facility....................................................30

Figure 24 SNAP basic animation model for TRACE model of BETHSY facility .......................32

Figure 25 Break mass flow – BETHSY 9.1b.............................................................................35

Figure 26 Integrated break mass flow – BETHSY 9.1b............................................................35

Figure 27 Pressurizer pressure – BETHSY 9.1b......................................................................36
viii

Figure 28 Steam generator 1 pressure – BETHSY 9.1b ..........................................................36

Figure 29 Maximum heater rod surface temperature – BETHSY 9.1b.....................................37

Figure 30 Core level – BETHSY 9.1b.......................................................................................37

Figure 31 Loop 1 Seal Downflow Side Differential Pressure....................................................38

Figure 32 Accumulator pressure – BETHSY 9.1b ...................................................................38

Figure 33 Integrated low pressure injection system mass – BETHSY 9.1b .............................39

Figure 34 Primary coolant mass – BETHSY 9.1b.....................................................................39

Figure 35 Break mass flow – BETHSY 6.2TC..........................................................................42

Figure 36 Integrated break mass flow – BETHSY 6.2TC .........................................................42

Figure 37 Pressurizer pressure – BETHSY 6.2TC ...................................................................43

Figure 38 Steam generator 1 pressure – BETHSY 6.2TC........................................................43

Figure 39 Heater rod surface temperature at the middle of the core – BETHSY 6.2TC...........44

Figure 40 Heater rod surface temperature at the top of the core – BETHSY 6.2TC ................44

Figure 41 Core collapsed liquid level – BETHSY 6.2TC...........................................................45

Figure 42 Total primary mass – BETHSY 6.2TC......................................................................45

Figure 43 Intermediate leg 1 DP (SG side) – BETHSY 6.2TC .................................................46

Figure 44 Intermediate leg 1 DP (pump side) – BETHSY 6.2TC..............................................46

Figure 45 Accumulator no. 2 pressure – BETHSY 6.2TC ........................................................47

Figure 46 Integrated accumulators injected mass – BETHSY 6.2TC.......................................47

TABLES

Table 1 Errors reported by RELAP5/MOD3.3 for ACHILLES input model ............................15

Page

Table 2 RELAP5 model report table for ACHILLES ..............................................................17

Table 3 TRACE model report table for ACHILLES................................................................20

Table 4 Errors reported by RELAP5/MOD3.3 for BETHSY input model ...............................25

Table 5 RELAP5 model report table for BETHSY .................................................................27

Table 6 TRACE model report table for BETHSY...................................................................31

Table 7 Comparison of initial conditions for BETHSY 9.1b test ............................................33

Table 8 Main sequence of events – BETHSY 9.1b ...............................................................34

Table 9 Comparison of initial conditions for BETHSY 6.2TC test..........................................40

Table 10 Main sequence of events – BETHSY 6.2TC ............................................................41
ix

EXECUTIVE SUMMARY

Much of efforts have been done in the past to develop the RELAP5 input decks. In this study the 

RELAP5 to TRACE conversion procedure, developed at Institut “Jožef Stefan” (IJS), is 

presented. The conversion procedure consists of eleven steps and is based on the use of 

SNAP. For demonstrating the conversion procedure the ACHILLES and BETHSY input decks 

(IJS legacy) were used. One separate effect test and two integral effect tests were simulated by 

both RELAP5 and TRACE computer codes. The best estimate ACHILLES natural reflood 

experiment was the basis for the International Standard Problem no. 25 (ISP-25). The BETHSY 

9.1.b test is a scaled 5.08 cm cold leg break without high pressure safety injection (HPSI) and

with delayed operator action for secondary system depressurization and was selected for ISP27. BETHSY 6.2TC test is a 15.24 cm (6 inch) cold leg break in the loop one without available 

high pressure and low pressure safety injection system and is counterpart test. Each of the 

eleven steps was demonstrated for ACHILLES and BETHSY RELAP5 input model conversion 

to TRACE input model. Before legacy RELAP5 input deck in ASCII format is converted, it had to 

be adapted to the RELAP5/MOD3.3 input deck, which can be imported by SNAP. After creating 

RELAP5 model in SNAP by importing ASCII legacy input deck, the SNAP is used for creating 

and arranging the views, creating model notebooks, performing RELAP5 calculation, conversion 

from RELAP5 to TRACE, checking for and resolving errors, and creating animation model.

Besides demonstration of the conversion methodology also the comparison between RELAP 

and TRACE has been done. It can be concluded that calculated results obtained by TRACE are 

as good as the results by RELAP5, thus suggesting that IJS conversion methodology may be 

used for separate and integral effect test legacy RELAP5 input deck conversions to TRACE 

input decks.

xi

ACKNOWLEDGEMENTS

The authors acknowledge the financial support from Krško Nuclear Power Plant and Slovenian 

Nuclear Safety Administration within CAMP program (project no. POG-3473) and from the state 

budget by the Slovenian Research Agency (program no. P2-0026).

xiii

ABBREVIATIONS

ASCII American Standard Code for Information Interchange

CAMP Code Applications and Maintenance Program

EM evaluation model

HPSI high pressure safety injection

IJS Institut “Jožef Stefan” (in Slovene; in English called Jožef Stefan Institute)

ISP international standard problem

LSTF large scale test facility

RELAP5 Reactor Excursion and Leak Analysis Program

SB LOCA small-break loss-of-coolant-accident

SI safety injection

SNAP Symbolic Nuclear Analysis Package

TRACE TRAC/RELAP Advanced Computational Engine

U.S. United States

U.S. NRC U. S. Nuclear Regulatory Commission

1

1. INTRODUCTION

The TRAC/RELAP Advanced Computational Engine (TRACE) is the latest in a series of 

advanced, best-estimate reactor systems codes developed by the U.S. Nuclear Regulatory 

Commission (Ref. 1). The advanced TRACE comes with a graphical user interface called 

SNAP (Symbolic Nuclear Analysis Package) (Ref. 2), which is intended for pre- and postprocessing, running the codes, RELAP5 to TRACE input deck conversion, input deck 

database generation etc. The TRACE code is still under development and it will have all 

capabilities of RELAP5. The TRACE has superior capabilities and accuracy for most 

applications compared to RELAP5. Although the TRACE is the future of U.S Nuclear 

Regulatory Commission (U.S. NRC), its use in countries members of Code Applications and 

Maintenance Program (CAMP) it is still not dominant against the RELAP5 computer code. 

However, TRACE it is now more and more used by the RELAP5 code users, in a great deal 

also because of better RELAP5 to TRACE conversion capability. The typical RELAP5 users 

start with legacy RELAP5 input decks, which are first automatically converted to TRACE 

input decks using SNAP and then manual corrections are done. Namely, much of efforts 

were done in the past to develop the RELAP5 input decks. The purpose of this report is to 

present the RELAP5 to TRACE conversion procedure (methodology), developed at Institut 

“Jožef Stefan” (IJS). For demonstrating the conversion methodology the ACHILLES and 

BETHSY input decks (IJS legacy input decks), were used. The ACHILLES and BETHSY 

RELAP5 input decks were developed in the frame of participation to international standard 

problem no. 25 (ISP-25) and international standard problem no. 27 (ISP-27). The ISP-25 

reflood experiment was performed on ACHILLES separate effects test facility and the input 

deck was developed for RELAP5/MOD2 ver. 36.05 computer code (Ref. 3). The 

RELAP5/MOD2 input model used for pre- and post-test calculation of ISP-27, which was 

small-break loss-of-coolant-accident (SB LOCA), was later upgraded to RELAP5/MOD3, 

RELAP5/MOD3.1 and RELAP5/MOD3.2 code versions. The legacy RELAP5/MOD3.2 input 

model (Ref. 4) was the starting point for conversion to TRACE input model. 

Although much work has been done to date on TRACE assessment, important part is also 

independent assessment performed by wide community. Therefore in the present study also 

the accuracy of the TRACE calculation of ACHILLES natural reflood experiment Run 

A1B105, BETHSY 9.1b and BETHSY 6.2TC tests using the converted and adapted RELAP5 

nodalizations, which were developed in the past for international standard problem no. 25

(ISP-25) and ISP-27 at Jožef Stefan Institute (Ref. 4). The RELAP5 legacy input deck of 

BETHSY facility has different origin than the one, which has been used for conversion to 

TRACE in the original TRACE code assessment study (Ref. 5). When comparing the TRACE 

calculation to the RELAP5 calculation and to TRACE calculation described in the code 

assessment manual (Ref. 1), one can more easily see the peculiarities of the TRACE code. 

Finally, both RELAP5 and TRACE calculations were compared to the experimental data.

In Section 2 the IJS conversion procedure is described. In Sections 3 and 4 the facilities, 

scenarios and legacy RELAP5 input decks are described. In Sections 5 and 6 examples of 

conversion for ACHILLES and BETHSY are given. Besides, the calculations were performed 

by both RELAP5 and TRACE computers codes and compared to experimental data. Finally, 

in Section 7 conclusions are given.

3

2. IJS CONVERSION PROCEDURE DESCRIPTION

The IJS conversion procedure from RELAP5 to TRACE is based on the SNAP (Symbolic 

Nuclear Analysis Package) (Ref. 2) and the experience gained in the past. Not much open 

literature was available in this area. An exception is the work done for Almaraz nuclear power 

plant (Refs. 6 and 7). The IJS RELAP5 to TRACE conversion procedure is shown in Figure 1.

The procedure consists of 11 steps. The starting point is legacy RELAP5 input model in ASCII 

form and the final result is TRACE model in SNAP. The steps of IJS conversion procedure are 

the following:

Step 1

The first requirement is the existing legacy RELAP5 input deck in ASCII form to adapt for use by 

RELAP5/MOD3.3 computer code. Namely, when input decks of older RELAP5 code versions 

are imported, errors are reported for RELAP5 input deck. It is helpful, if this is done with the 

analyst, familiar with previous code versions of RELAP5 and ASCII editing of input model. The 

modifications in the input model are needed mainly because of new options included in higher 

versions of RELAP5 code. Once having running input deck for RELAP5/MOD3.3, the second 

step may be performed.

: Adapt legacy RELAP5 input deck to RELAP5/MOD3.3 input deck

Step 2

In the SNAP Model Editor the command Import, RELAP5 ASCII is used. The RELAP5 model is 

generated by SNAP in Model Editor, including optional Hydraulic Components View and Control 

Systems View. You may select also Job Streams View, however old legacy input decks does 

not have such information. When checking RELAP5 model, there should be no errors in 

principle.

: Import adapted RELAP5/MOD3.3 ASCII input model into SNAP

Step 3

When automatically generating Hydraulic Components View, the layout may not be good

enough for the user. Therefore it is highly recommended to help with the drawing of original 

nodalization representing legacy RELAP5 input model in ASCII and to manually rearrange the 

layout of components, add labels etc. This step is very important to be done before conversion, 

because during RELAP5 to TRACE conversion also Hydraulic Components View is converted 

and there is no need to repeat the work done for RELAP5 view. Good hydraulic components 

view helps to locate TRACE components, being at the same location as corresponding RELAP5

components. In addition, from Hydraulic Components View the basic animation model is also 

created.

: Manually arrange Hydraulic Components View

Step 4

RELAP5 Model Notebook is useful, because information about RELAP5 input deck is presented 

in well-organized way. Model Options, Hydraulic Components, Control Systems, Heat Structure 

and Materials are described. For example, for General Tables, which are parts of Control 

Systems, the table data are presented also graphically and this helps among other things also 

the users not too familiar with RELAP5 legacy input deck. The RELAP5 Model Notebook is also 

used for verification of converted TRACE model.

: Export RELAP5 Model Notebook

Step 5

Running the RELAP5 is important, because in this way the calculated data may be checked 

against old calculations (performed by ASCII legacy RELAP5 input decks) and may be used as 

source data for animation model. Moreover, from the output file the data on hydraulic diameter 

: Perform RELAP5 calculation
4

may be extracted. These data are needed for checking hydraulic diameters in converted 

TRACE models.

Step 6

This step is optional. It turned out to be very useful for old legacy input decks not having much 

documentation available. The animation model helps to understand the RELAP5 calculated 

results, what is very useful for later TRACE calculation. The basic animation model consists of 

RELAP5 model Views copied to animation model. The components are already connected to 

source data (no need to specify channels). It should also be noted that this basic RELAP5 

animation model cannot be converted to TRACE basic animation model, but there is no need for 

this, because it can be created after conversion of RELAP5 model to TRACE model (again just 

Copy Paste is needed). On the other hand, several additional animation masks may be created. 

These are the candidates to be copied to TRACE animation model. Namely, not much effort is 

needed for converting the RELAP5 animation masks to TRACE animation masks. In principle, 

the channels with source data need to be adjusted to TRACE and the possible geometry 

changes.

: Create animation model for RELAP5 (optional)

Step 7

In this step the Convert to TRACE command is used in RELAP5 Analysis Code plugin. Besides 

RELAP5 input model also SNAP Model editor Views are converted, including Hydraulic 

Components View. The Views can be further adapted in TRACE model.

: Convert RELAP5 model to TRACE model

Step 8:

After converting RELAP5 model to TRACE model, checking a TRACE model for errors is done

using model editor Check Model menu command. Usually, the converted TRACE model is not 

error free. First all errors identified by SNAP are manually corrected. Once corrected, the first 

TRACE run can be done.

Check for and resolve TRACE model errors

Step 9

Before running the error free TRACE input model, the converted TRACE input model need to be 

compared to RELAP5 input model. Consistency checks may be done for geometry data, control 

systems etc. 

: Export TRACE Model Notebook

Step 10

In this step, the first calculation is performed to check for errors by TRACE computer code. The 

errors, not identified by SNAP Check Model button need to be corrected and this may be quite 

time consuming. The information about errors is obtained directly from the TRACE output file for 

specific calculation. As it is difficult to eliminate all errors at once, the work is iterative (repeating 

Steps 9 and 10). Once all errors are eliminating and TRACE is running, the calculated results 

present Source Data for animation model.

: Perform TRACE calculation

Step 11

It is recommended to develop TRACE animation model. The simplest model is created by 

copying TRACE Hydraulic View to Animation model. For the purpose of this report it is called 

basic animation model. Such Copy Paste is very efficient, because hydraulic components are 

already connected to Source Data. More advanced animation masks may be created later by 

importing drawings of facilities and use them as template to put SNAP Display Beans over. In 

this respect Polygon is very useful.

: Create TRACE animation model (optional)

The first calculation will probably not satisfy the analyst, therefore manual work may be needed 

to further modify the model or add additional components. The above procedure was described 
5

for the whole model only. However, the RELAP5 transient input model is typically in the 

separate ASCII restart file and steady-state is calculated first. The difficulty is that restart input 

models cannot be imported as full plant model. Therefore TRACE transient input model has to 

be built manually. For example, the RELAP5 ASCII restart input model may be appended to 

steady-state input model and after conversion to TRACE the components needed for transient 

calculation may be copied into TRACE transient model. The TRACE calculation is then 

performed and the results can be compared to experimental, plant or other data and RELAP5 

calculations. The RELAP5 calculated results provide several variables, for which no measured 

data exists.

Important is also to mention that when steady state is achieved by RELAP5 and such model is 

converted, the TRACE model does not require steady-state controllers. The converted TRACE 

model can be used directly for transient calculation. 

The IJS procedure for converting input deck from RELAP5 to TRACE will be demonstrated on 

IJS legacy RELAP5 input models for ACHILLES rig and BETHSY facility.
6

Figure 1 IJS conversion procedure from RELAP5 model to TRACE model
7

3. ACHILLES RIG, TEST AND RELAP5 INPUT MODEL 

DESCRIPTION

Before conversion of legacy RELAP5 input deck for ACHILLES, the ACHILLES rig is described 

first. Then scenario of the test is described and finally the legacy RELAP5 input model. This 

information is very useful to analyst before conversion is done.

3.1 Achilles rig

The Achilles test experiment was an experimental study performed at AEA Winfrith Technology 

Center, Dorchester, UK in 1991. This experiment investigated the end phase of the accumulator 

injection in the primary system of a Pressurized Water Reactor and the heat transfer in the core 

during the reflood phase of a postulated large break loss of coolant accident (LOCA). The best 

estimate ACHILLES natural reflood experiment was also the basis for the ISP-25 (Ref. 8). A 

simplified schematic of the test facility configuration for best estimate natural reflood test series 

is presented in Figure 2.

Figure 2 Schematic of Achilles test facility
8

The simulated core or the test section comprised of 69 rods corresponding to Westinghouse 

17x17 geometry, electrically heated over a length of 3.66 m. Aside from these nuclear fuel rods 

simulators, the test section is also housing the heat shroud and temperature and pressure 

measurement instrumentation. A centrifugal steam separator located at the top of the test 

section discharges the test section output liquid flow. Furthermore, a separator located 

downstream, ensures the separation and collection of the liquid phase flowing out of the upper 

plenum, and outputs the single phase steam flow through an orifice, designed to simulate the 

hot leg associated losses. Thus, the liquid is separated from the flow out the upper part of the 

downcomer by means of another separator and outputs the gas through a second orifice that 

simulates the cold leg associated losses.

3.2 ACHILLES Run A1B105

The natural reflood experiment Run A1B105 was selected for ISP-25 (Ref. 8). The experimental 

procedure is started with the downcomer full with water and with no water in the simulated core. 

The rig is heated and circulated with steam until saturation temperature and pressure is reached

and the rods and shroud have reached the required temperatures. When all initial conditions are 

reached, the valve between the downcomer and the shroud vessel is opened. Under the effect 

of both gravity head and nitrogen pressure, the water in the downcomer enters the core. After 

the flow oscillations that are occurring at this point in the transient decay, the reflooding of the 

core continues by means of pumped water injection, until all the test section rods have been 

quenched.

The nature of the ISP-25 transient is such that it was divided into two periods (Ref. 8). The first 

period lasted around 20 seconds (when the nitrogen vessel discharged into the top of the 

downcomer) was characterized by a highly oscillatory flow between the downcomer and core. 

The high flooding rates during this early period quenched the bottom of the rod bundle. In the 

second period a steady rate of reflood was established by the pumped injection and conditions 

in the rod bundle are then similar to a conservative evaluation model (EM) transient of the type 

prescribed by the appendix K rules of 10CFR.50. The main difference between the latter part of 

the ISP-25 experiment and an appendix K type of EM transient is that the lower elevations of 

the rod bundle quench during the initial surge of water from the downcomer. This combined with 

rewetting of the grids during the initial surge lead to an overall improvement in heat transfer from 

the surface of the fuel rods.

3.3 RELAP5 Input Model of ACHILLES

The RELAP5/MOD3.3 input model originated from a RELAP5/MOD2 input model, created at 

IJS, during the participation to the ISP-25. The original RELAP5/MOD2 nodalization consists of 

50 volumes, 50 junctions and 13 heat slabs and is shown in Figure 3. The nitrogen vessel is 

modeled by an accumulator component. For this component it is assumed that it is not initially in 

injection mode. For that reason the valve was placed in between the accumulator and top of the 

downcomer. The accumulator was filled with nitrogen only. The separators were modeled by 

RELAP5 branch components to avoid calculation problems. In the RELAP5/MOD3.3 input 

model only some slight changes were needed. For heat structure some additional left and right 

boundary data were added and separators were modeled by RELAP5 “separatr” component. 

The rest of the model was unchanged. So prepared, the RELAP5/MOD3.3 input model served 

for calculation and RELAP5 to TRACE conversion.
9

Figure 3 Achilles facility RELAP5 nodalization (Ref. 9)

11

4. BETHSY FACILTY, TESTS AND RELAP5 INPUT MODEL 

DESCRIPTION

Before conversion of legacy RELAP5 input deck for BETHSY, the BETHSY facility is described 

first. Then scenario is described and finally the legacy RELAP5 input model. This information is 

very useful to the analyst before conversion is done.

4.1 BETHSY Facility

BETHSY is an integral test facility, which was designed to simulate most pressurized water 

reactor accidents of interest, to study accident management procedures and to validate the 

computer codes. BETHSY facility was located at Centre D'Etudes Nucleaires de Grenoble,

France (Ref. 10). It was a scaled down model of three loop Framatome nuclear power plant with 

the thermal power 2775 MW. Six important choices have been made which characterize indeed 

the general design of the BETHSY facility. They concern: the number of loops, the rated 

pressure of both the primary and the secondary side, the maximum core power level, the 

maximum flow rate of primary pumps, the general scaling factors and the connected circuits and 

systems. Volume, mass flow and power were scaled to 1:96.9, while the elevations and the 

pressure of the primary and secondary system were preserved. The design pressure on the 

primary side was 17.2 MPa and on the secondary side 8 MPa. The power was limited to the 

decay heat level; therefore the transient without reactor trip could not be simulated. The facility 

was equipped with all important systems and measurement system, needed for performance 

and observing the analyzed transients. The facility consisted of pressure vessel, reactor coolant 

pumps and piping, heat tracing system, the system for break simulation, instrumentation and the 

control systems. The core power was 3 MW, what is 10% of the reference power considering 

scaling. The break system enabled simulation of the break in different locations, i.e. in the cold 

leg, the lower plenum, the pressurizer, the steam generator U tubes and the feedwater pipe. 

The instrumentation data system measured all data needed for the transient analysis. The 

control system could simulate the plant control systems and operator actions.
12

Figure 4 BETHSY facility

4.2 BETHSY Test 9.1b

The BETHSY 9.1.b test is a scaled 5.08 cm cold leg break without high pressure safety injection 

(HPSI) and with delayed operator action for secondary system depressurization (Ref. 11). This 

transient leads to a large core uncovery and fuel heat-up, requiring the implementation of an 

ultimate procedure. The scenario of the test started at 10% nominal power. At time 0 s the break 

was opened (initiation of the transient). The scram signal was obtained when pressurizer 

pressure dropped below 13.1 MPa and was delayed 17 s. The safety injection (SI) signal is 

triggered at 11.9 MPa. However, high pressure safety injection, turbine bypass and main 

feedwater were assumed to be off. Thirty seconds after SI signal the auxiliary feedwater started. 

Three hundred seconds after SI the reactor coolant pump started to coast down. When the 

maximum core cladding temperature reaches 723 K, the ultimate procedure was started, i.e. full 

opening of three steam dumps to atmosphere. Accumulators were available in the intact loops 

only. They started to inject when pressurizer pressure dropped below 4.2 MPa and were 

isolated at pressurizer pressure 1.5 MPa. The low pressure safety injection system started at 

pressurizer pressure 0.91 MPa and injected in the two intact loops. When stable residual heat 

removal system operating condition prevail (core outlet fluid temperature < 450 K, primary 

pressure < 2.5 MPa, saturation margin > 20 K), the transient was terminated.
13

4.3 BETHSY Test 6.2TC

BETHSY 6.2TC test was a 15.24 cm (6 inch) cold leg break in the loop one without available 

high pressure and low pressure safety injection system (Ref. 12). Accumulators were 

available in the intact loops. The main aims of this test were to compare the counterpart test 

data from BETHSY and LSTF facilities and qualification of CATHARE 2 computer code. The 

experiment scenario was the following: opening of the valve simulating the break in the cold 

leg no. 1, accumulator injection in the intact loops when a primary circuit pressure was lower 

than 4.2 MPa and end of transient, when the primary circuit pressure was below 0.7 MPa.

4.4 RELAP5 Input Model of BETHSY

The RELAP5/MOD2 input model was developed, when participating to ISP-27. It was initialized 

according to the specified data for each test. Each of the three coolant loops were represented 

explicitly without taking into account the small asymmetry between the loops. The base 

RELAP5/MOD2 model of Bethsy facility for pre-test calculations contained 196 volumes, 207 

junctions and 191 heat structures (see Ref. 4). This base RELAP5/MOD2 input model was 

further upgraded to RELAP5/MOD3.1, which is shown in Figure 5 (Ref. 4). The base 

RELAP5/MOD2 input model was renodalized, increasing the number of nodes in reactor coolant 

system piping (from 9 to 45 nodes), reactor coolant pumps (2 nodes more not shown in Figure 

5), core bypass section (from 1 to 12 nodes), reactor vessel (two more in the head) and 

downcomer (from 5 to 14 nodes). The elevations of parallel volumes of the reactor downcomer, 

in bypass, reactor core, hot leg and cold leg were preserved. Nodalization of the reactor core, 

pressurizer, reactor head, upper plenum and lower plenum remained the same. This RELAP5 

input model of BETHSY facility, called middle input model and shown in Figure 5, contains 332 

volumes, 343 junctions and 330 heat structures. This RELAP5 model was further refined, 

increasing the number of nodes in the steam generator. The U-tubes were modeled with 20 

nodes instead of 10, and the downcomer and riser region of steam generator were modeled 

with 11 nodes instead of five, what gives 22 more nodes per steam generator and 66 more 

nodes in total. The common input model for RELAP5/MOD3.2 has increased number of nodes 

in the upper head and consisted of 398 volumes, 408 junctions and 402 heat structures (Ref. 4). 

With the exception of the number of nodes, the layout is the same as for middle 

RELAP5/MOD3.1 input model shown in Figure 5. The RELAP5/MOD3.2 input model was 

adapted for the use with RELAP5/MOD3.3, with no changes to the geometry and the number of 

hydrodynamic components and heat structures. 
14

Figure 5 RELAP5/MOD3.1 input model of BETHSY facility.
15

5. ACHILLES EXAMPLE OF CONVERSION

The IJS conversion procedure, described in Section 2, is first demonstrated step by step on the 

RELAP5 input model of ACHILLES separate effects test rig.

5.1 Adaptation of legacy RELAP5 input model to RELAP5/MOD3.3 (Step 1)

The RELAP5/MOD2 input model was first adapted to RELAP5/MOD3.3. First the model was 

changed to lower case. When running lower case RELAP5/MOD2 input model by 

RELAP5/MOD3.3 Patch 3, several errors were reported because of heat structure component, 

as shown in Table 1. All these error were first manually corrected.

Table 1 Errors reported by RELAP5/MOD3.3 for ACHILLES input model

0******** word 5 on card 10020801 should be in floating point format

******** Default values being entered.

0******** word 5 on card 10020901 should be in floating point format

******** Default values being entered.

0******** Right boundary of heat structure 20001 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20002 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20003 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20004 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20005 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20006 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20007 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20008 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20009 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20010 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20011 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20012 does not have chf-heat transfer correlation data entered.

0******** Right boundary of heat structure 20013 does not have chf-heat transfer correlation data entered.

0******** Alphanumeric part, ztrwt , of variable request , 20800001, is not in table of legal variables.

0******** Errors detected during input processing.

5.2 Importation of ASCII input model into SNAP (Step 2)

Once Import RELAP5 ASCII is done, the RELAP5 model is created in SNAP. The Model Editor 

screen (see Figure 6) shows Message Window, which displays a running list of error, warning, 

alert, and notice messages. The Navigator on the left shows the RELAP5 model content. 

Figure 6 Model Editor screen after importing ASCII input file
16

5.3 Manually arrange Hydraulic Components View (Step 3)

Figure 6 shows also a part of Hydraulic Components View (whole model is visible Figure 7). It 

may be seen that the view can be significantly improved when comparing to nodalization shown 

in Figure 3. After manually arranging components, mostly by moving and scaling components 

and adding labels, the Hydraulic Components View shown in Figure 8 was obtained.

Figure 7 SNAP hydrodynamic component view for RELAP5 input model of ACHILLES rig 

- original
17

Figure 8 SNAP hydrodynamic components view for RELAP5 input model of ACHILLES

rig – arranged

5.4 Export RELAP5 Model Notebook (Step 4)

The RELAP5 plug-in allows generating model-wide reports as a single annotated document, 

called a model notebook. Information such as calculations, export data, model status, attribute 

descriptions, etc. are all included. Table 2 shows, which components are included in the 

RELAP5 model for ACHILLES.

Table 2 RELAP5 model report table for ACHILLES

Component Count

Hydraulic Accumulators 1

Branches 10

Pipes 6

Single Junctions 17

Single Volumes 7

Time Dependent Volumes 2

Time Dependent Junctions 1

Valves 1

Total: 45

Control System Variable Trips 3

Control Blocks 14

General Tables 1

Total: 18

Heat Structures Materials 3

Total: 3
18

5.5 Perform RELAP5 calculation (Step 5)

The calculation may be performed from Model Editor or directly by the computer used. In this 

way the data needed are obtained, especially RELAP5 junction areas, from which hydraulic 

diameters can be calculated.

5.6 Create animation model for RELAP5 (Step 6)

Having Hydraulic Component View of RELAP5 model for ACHILLES, the basic animation model 

was created using Copy Paste (in animation model) technique as shown in Figure 9. The void 

fraction color map was selected. 

Figure 9 Basic animation model for RELAP5 model of ACHILLES rig

5.7 Convert RELAP5 model to TRACE model (Step 7)

After conversion the TRACE model was obtained. The Hydrodynamic Components View of 

TRACE model is shown in Figure 10. The only modification was down scaling the junction

components (converted from RELAP5 Branch component). The Achilles facility TRACE

Hydrodynamic Components View is presented in Figure 10 and it comprises of 37 hydraulic 

components with a core test section nodalization divided in 13 cells, and a downcomer 

nodalization geometry of 11 cells. When comparing TRACE Hydrodynamic Components View 

(Figure 10) to RELAP5 Hydrodynamic Components View (Figure 8), it may be seen that 

component numbering was mostly preserved.
19

Figure 10 SNAP hydrodynamic components view for TRACE input model of ACHILLES

rig

5.8 Check for and resolve TRACE model errors (Step 8)

Once the model is imported, check for errors is done. The errors are shown in Error Report, 

which is shown in Figure 11. All these error need to be resolved, before TRACE can be run. 

SNAP helps a lot to locate the error.

Figure 11 Error report for TRACE model of ACHILLES rig
20

5.9 Export TRACE Model Notebook (Step 9)

The TRACE plug-in also allows generating a model notebook. Table 3 shows, which 

components are included in the TRACE model for ACHILLES.

Table 3 TRACE model report table for ACHILLES

Component Count

Hydraulic Components Breaks 1

Fills 1

Pipes 23

Pumps 1

Separators 2

Single Junctions 7

Tees 1

Valves 1

Total: 37

Control Systems Trips 3

Control Blocks 16

Signal Variables 64

General Tables 1

Total: 84

Thermal User Defined Materials 3

All Heatstructures 1

Total: 4

Power Components Powers 1

Total: 1

After having both RELAP5 and TRACE Model Notebooks the comparison is performed. For 

example, in the case of ACHILLES wall-roughness for some hydraulic components was not 

converted and the nitrogen non-condensable gas option was not included. The nitrogen vessel 

represented by an Accumulator component in the RELAP5 input model was automatically 

converted to Pipe component - Liquid separator type instead of Accumulator type. It was 

manually changed to Accumulator type of pipe (Ref. 14). Hydraulic diameters can be obtained 

from RELAP5 output file and then comparison with TRACE values is performed.

5.10 Perform TRACE calculation (Step 10)

This step helps to identify any other possible errors remaining in the TRACE model. In the 

example of ACHILLES (using SNAP 2.0.8) the TRACE code start to run (this was not the case 

with SNAP 1.2.6). This does not mean that the model perform the same as RELAP5. To check 

the correctness, the first calculation is performed and the results needed for plotting the graphs 

and animation model are generated.
21

5.11 Create TRACE animation model (Step 11)

Similarly as for RELAP5 the TRACE basic animation model can be created from Hydrodynamic 

Components View using Copy Paste. It is shown in Figure 12. In the basic animation mask the 

fluid condition color map is shown. Additional Animation Mask was also created as shown in 

Figure 13.

Figure 12 Basic animation model for RELAP5 model of ACHILLES rig
22

Figure 13 SNAP additional animation mask for the Achilles TRACE model

5.12 Results for ACHILLES natural reflood test

Once having running the TRACE model, the basic conversion process from RELAP5 to TRACE 

is completed. Now the analysis using TRACE can be started. The analysis reveals the need for 

several other modification and improvements of the TRACE model. This is not described here, 

as this is not part of IJS conversion procedure. Nevertheless, based on the converted TRACE 

model the 1D and 3D models of ACHILLES rig were created and calculations were performed. 

These two TRACE calculations were compared against the experimental data and the 

RELAP5/MOD3.3 calculation data obtained in Step 5 of IJS conversion procedure. The TRACE 

calculation has a tendency to under predict the quench front time for the test sections, Figure 

14, although, compared to RELAP5/MOD3.3, the TRACE overall quench front progression, is

closer to the experimental results, as shown in Figure 15. The under prediction of quench front 

time is in an agreement with the TRACE assessment report conclusions where it is stated that 

TRACE has a tendency to under predict quench times for low flow, low power cases (Ref. 5). 

The test section peak cladding temperatures predicted by the codes are in agreement with the 

experimental data although TRACE has a tendency to over predict the peak cladding 

temperatures in higher elevations, but this is also part of the TRACE assessment report 

conclusions (Ref. 5). The peak cladding temperatures code predictions, for the highest 

temperature test section are presented in Figure 14. The closest result to the experimental 
23

value is provided by the TRACE 1D model, followed closely by the TRACE 3D model result.

The results for static pressure at various elevations in the core show an oscillatory behaviour for 

TRACE, and RELAP5/MOD3.3 calculations, manifesting particularly during the early stages of 

the transient. Nevertheless, the TRACE prediction for the pressure at the top of the upper 

plenum is in good agreement with the experimental data, and RELAP5/Mod3.3 calculation data, 

although the initial pressure peak is under predicted and an oscillatory behaviour is observable, 

Figure 16.

The downcomer collapsed liquid level transient proved difficult to be predicted by both codes, 

Figure 8, but the core collapsed liquid level predicted by RELAP5/MOD3.3 and TRACE is in 

good agreement to the experimental results, although some oscillations are present, as shown 

in Figure 17.

It is important to mention that the results of both the TRACE 1D model calculations and TRACE 

calculation with 3D test section model are in close agreement. 

Figure 14 Peak cladding temperature at 2.13 m elevation

Figure 15 Quench front progression

300

400

500

600

700

800

900

1000

1100

1200

0 50 100 150 200 250 300 350 400

Rod Clading Temperature (K)

Time (s)

Exp.

R5M3.3

TRACE 1D

TRACE 3D

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

0 50 100 150 200 250 300 350 400

Quench Front Location (m) 

Time (s)

Exp.

R5M3.3

TRACE 1D

TRACE 3D
24

Figure 16 Pressure at the top of the upper plenum

Figure 17 Core collapsed liquid level

Figure 18 Downcommer collapsed liquid level

A SNAP animation model for the Achilles TRACE model with 3D VESSEL component was also 

developed, for the purpose of graphically displaying the evolution of different phenomena during 

the 400 seconds transient. In addition it is important to mention that this model is using both the 

TRACE nodalization (basic animation model) as well as the Achilles facility simplified schematic 

as a layout for the various hydraulic components (additional animation mask). A screen capture 

of the animation model at 214.7 seconds in the transient is presented in Figure 13.

Finally, it should be noted, that for demonstration of APTPlot capabilities Figures 14 through 18

were created by APTPlot.

2.90E+05

3.40E+05

3.90E+05

4.40E+05

4.90E+05

0 50 100 150 200 250 300 350 400

Pressure (Pa)

Time (s)

R5M3.3

TRACE 1D

TRACE 3D

Exp.

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 50 100 150 200 250 300 350 400

Level(m)

Time (s)

Exp.

R5M3.3

TRACE 1D

TRACE 3D

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 50 100 150 200 250 300 350 400

Level (m)

Time (s)

Exp.

R5M3.3

TRACE 1D

TRACE 3D
25

6. BETHSY EXAMPLE OF CONVERSION

6.1 Adaptation of legacy RELAP5 input model to RELAP5/MOD3.3 (Step 1)

The RELAP5/MOD3.2 input model was first adapted to RELAP5/MOD3.3. When running 

RELAP5/MOD3.2 computer code, several errors were reported because of junction options (vflag), as shown in Table 4.

Table 4 Errors reported by RELAP5/MOD3.3 for BETHSY input model

0******** Junction 125010000: Neither the FROM volume 125010002 or the TO volume 131010001 are horizontal

0******** Junction 221000000: Neither the FROM volume 212030002 or the TO volume 203010001 are horizontal

0******** Junction 308010000: Neither the FROM volume 319010001 or the TO volume 308010001 are horizontal

0******** Junction 308020000: Neither the FROM volume 308010002 or the TO volume 309010001 are horizontal

0******** Junction 313010000: Neither the FROM volume 301210002 or the TO volume 313010001 are horizontal

0******** Junction 319010000: Neither the FROM volume 310010001 or the TO volume 319010002 are horizontal

0******** Junction 408010000: Neither the FROM volume 419010001 or the TO volume 408010001 are horizontal

0******** Junction 408020000: Neither the FROM volume 408010002 or the TO volume 409010001 are horizontal

0******** Junction 413010000: Neither the FROM volume 401210002 or the TO volume 413010001 are horizontal

0******** Junction 419010000: Neither the FROM volume 410010001 or the TO volume 419010002 are horizontal

0******** Junction 508010000: Neither the FROM volume 519010001 or the TO volume 508010001 are horizontal

0******** Junction 508020000: Neither the FROM volume 508010002 or the TO volume 509010001 are horizontal

0******** Junction 513010000: Neither the FROM volume 501210002 or the TO volume 513010001 are horizontal

0******** Junction 519010000: Neither the FROM volume 510010001 or the TO volume 519010002 are horizontal

0******** Junction 535000000: Neither the FROM volume 509110002 or the TO volume 503010001 are horizontal

0******** Junction 663000000: is not connected to face 5 of horizontal stratification volume 653010000

It is connected from face 1 (Requirement for junction v-flag = 2)

0******** Junction 863000000: is not connected to face 5 of horizontal stratification volume 853010000

It is connected from face 1 (Requirement for junction v-flag = 2)

0******** Junction 963000000: is not connected to face 5 of horizontal stratification volume 953010000

It is connected from face 1 (Requirement for junction v-flag = 2)

6.2 Importation of ASCII input model into SNAP (Step 2)

The hydrodynamic components view was generated by SNAP from RELAP5 ASCII input model 

as shown in Figure 19 and then arranged manually using Model Editor of SNAP (see Figure 20).

Figure 19 SNAP hydrodynamic components view for RELAP5 input model of BETHSY 

facility - original
26

The RELAP5/MOD3.3 input model in terms of SNAP consists of 141 hydrodynamic components 

and 72 heat structures. The difference comparing to RELAP5 output file information on volumes 

is that pipes consist of several volumes; however they are considered as one component by 

SNAP. Similar is the case with the heat structures.

6.3 Manually arrange Hydraulic Components View (Step 3)

Figure 20 shows Hydraulic Components View for arranged model. It may be seen that the view 

can be significantly improved when comparing to nodalization shown in Figure 19. After 

manually arranging components, mostly by moving and scaling components and adding labels, 

the Hydraulic Components View shown in Figure 20 was obtained.

Figure 20 SNAP hydrodynamic component view for RELAP5 input model of BETHSY 

facility – arranged

6.4 Export RELAP5 Model Notebook (Step 4)

The RELAP5 plug-in allows generating model-wide reports as a single annotated document, 

called a model notebook. Information such as calculations, export data, model status, attribute 

descriptions, etc. are all included. Table 5 shows, which components are included in the 

RELAP5 model for BETHSY.
27

Table 5 RELAP5 model report table for BETHSY

Component Count

Hydraulic Accumulators 2

Branches 31

Pipes 40

Pumps 3

Single Junctions 27

Single Volumes 6

Separators 3

Time Dependent Volumes 13

Time Dependent Junctions 9

Valves 7

Total: 141

Control System Logical Trips 3

Variable Trips 5

Control Blocks 38

General Tables 16

Total: 62

Heat Structures Materials 3

Heat Structures 72

Total: 75

6.5 Perform RELAP5 calculation (Step 5)

The RELAP5 calculations were performed for BETHSY 9.1b and 6.2TC tests. The results are 

presented in Section 5.12 where comparison between RELAP5 and TRACE computer code 

calculations is shown.
28

6.6 Create animation model for RELAP5 (Step 6)

Animation model for RELAP5 was created, consisting from basic animation model and 

additional animation masks. In Figure 21 the additional animation mask is shown, representing 

the facility view, the loops arrangement and core part. The results used are RELAP5 

calculations performed in Step 5 for BETHSY 9.1b test.

Figure 21 SNAP animation model for RELAP5 model of BETHSY facility
29

6.7 Convert RELAP5 model to TRACE model (Step 7)

After conversion from RELAP5 model the TRACE model was obtained. This model was then 

further adapted (reactor vessel model was moved to left, break was added etc.). The final 

Hydrodynamic Components View of TRACE model for BETHSY 6.2TC test is shown in Figure 

22. When comparing TRACE Hydrodynamic Components View (Figure 22) to RELAP5 

Hydrodynamic Components View (Figure 20), it may be seen that component numbering was 

mostly preserved.

Figure 22 SNAP hydrodynamic component view for converted (and adapted) TRACE

input model of BETHSY facility
30

6.8 Check for and resolve TRACE model errors (Step 8)

Once the model is imported, check for errors is done. The errors are shown in Error Report, 

which is shown in Figure 23. All these error need to be resolved, before TRACE can be run. 

SNAP helps a lot to locate the error.

Figure 23 Error report for TRACE model of BETHSY facility

6.9 Export TRACE Model Notebook (Step 9)

In Table 6 a model notebook generated by TRACE plug-in shown. When comparing to RELAP5 

model report table shown in Table 5, it may be seen that the number of hydraulic components 

are similar and that some hydraulic components are specific for codes. From Table 6 can also 

be seen that after conversion there are many more Control Systems components in TRACE 

model than in RELAP5 model.
31

Table 6 TRACE model report table for BETHSY

Component Count

Hydraulic Components Breaks 19

Pipes 74

Pumps 18

Separators 3

Single Junctions 42

Tees 6

Valves 11

Total: 173

Control Systems Trips 13

Control Blocks 236

Signal Variables 318

General Tables 20

Total: 587

Thermal User Defined Materials 3

All Heatstructures 71

Total: 74

Power Components Powers 1

Total: 1

Global CCFL Models 1

6.10 Perform TRACE calculation (Step 10)

This step helps to identify any other possible errors remaining in TRACE model. To check the 

correctness, the first calculation is performed and the results needed for plotting the graphs and 

animation model are generated. The results of final calculations are shown in Section 6.12.

6.11 Create TRACE animation model (Step 11)

Similarly as for RELAP5 the basic animation model for TRACE can be creating from 

Hydrodynamic Components View (see Figure 22) using Copy command in Model Editor, then

opening Animation Model and pasting the copied content, which results in basic animation 

mask. Each view has to be copied separately and in this way the basic animation model is 

obtained. Once the basic animation model is connected to TRACE calculated source data, the 

animation model may be run. In Figure 24 the fluid condition color map is used to animate the 

calculated results.
32

Figure 24 SNAP basic animation model for TRACE model of BETHSY facility

6.12 Results for BETHSY tests

The results are shown for two BETHSY tests, 9.1b and 6.2TC. Subsection 6.12.1 shows the 

results for BETHSY 9.1b test and Subsection 6.12.2 shows the results for BETHSY 6.2TC test.

6.12.1 RELAP5 and TRACE calculations of BETHSY 9.1b test

The TRACE V5.0 Patch 1 and RELAP5/MOD3.3 Patch 3 were used for calculations of BETHSY 

9.1b test. Only transient calculation was performed.

Table 7 shows initial and boundary conditions for BETHSY 9.1b test. The initial conditions for 

TRACE come from converted model and are practically the same as for RELAP5. No steadystate calculation was therefore performed. The RELAP5 model was originally initialized to cold 

leg temperature; therefore the secondary pressure is not exactly matched. The steam generator 

levels and masses were matched to average measured values. The pressurizer pressure and 

level were also matched to average measured values. The core power was input value. In the 

experiment the electrical trace heating system was installed of the power of 107.5 kW and was 

operating till ultimate procedure start. In the calculations the heat losses were modeled only 

after the electrical heat system was off. Before ultimate procedure start there were no heat 

losses, what is equal to experiment which compensates the heat losses by electrical heat 

system.
33

Table 7 Comparison of initial conditions for BETHSY 9.1b test

Parameter Measured RELAP5 TRACE

core thermal power 2864 ± 30 kW 2864 kW 2864 kW

cold leg temperature (per loop) 559.9 ± 0.5 K 559.9 K (core inlet) 559.4 K (core inlet)

downcomer mass flow rate 150.0 ± 5.0 kg/s 155.2 kg/s 155.2 kg/s

reactor coolant pump speed (per 

loop)

2940 ± 30 rpm 2940 rpm 2970 rpm

pressurizer pressure 15.51 ± 0.09 MPa 15.51 MPa 15.51 MPa

pressurizer level 4.08 ± 0.1 m 4.08 m 4.08 m

reactor coolant system mass 1960 kg 1948 kg 1948 kg

secondary side pressure (per SG) 6.91 ± 0.04 MPa 6.77 MPa 6.77 MPa

steam generator level (per SG) 13.45 ± 0.05 m 13.41 m 13.18 m

feedwater temperature 491.1 ± 2.0 K 491.0 K 491.0 K

secondary coolant mass (per SG) 820 ± 30 kg 820 kg 804 kg

RCS trace heating 107.5 kW no heat losses 

considering the trace 

heating system on

no heat losses 

considering the trace 

heating system on

The TRACE transient calculation was performed with already verified restart input model after 

conversion. The main sequence of events is shown in Table 8. As can be seen the RELAP5 

calculation using standard BETHSY input model is in a good agreement with the experiment in 

the initial phase, while in later phase the TRACE calculation using converted model has better 

agreement. The reason is the break flow. For RELAP5 original Ransom-Trapp break flow model 

the values of 0.8, 1.0 and 1.1 were used for subcooled, two phase and superheated discharge 

coefficients, respectively. For TRACE break model the values of 1.0 and 1.1 were used for 

subcooled and two phase discharge coefficients, respectively. The values for TRACE were 

selected after some sensitivity studies and the aim was to use the values as close as possible to 

the default vales. Decreasing of discharge coefficients delays the ultimate procedure initiation. 

To match the initial period the TRACE subcooled discharge coefficient should be around 0.85, 

but this would require larger value of two phase coefficient. This also explains why the timing of 

RELAP5 calculation was better in the initial phase. Our goal was to as closely as possible to 

match the start of ultimate heat procedure. In Figures 25 and 26 are shown the break flow and 

integrated break mass flow, respectively. There are periods with some disagreements. In 

general the agreement is satisfactory. TRACE and RELAP5 are practically the same until 

accumulator injection. During accumulator injection TRACE is better, while during low pressure 

injection the slightly higher secondary pressure calculated by TRACE causes lower injection 

flow and therefore also lower break flow. The pressurizer pressure is shown in Figure 27. Due to 

selected break discharge coefficients the timing of pressure drop is better in TRACE, while after 

ultimate procedure initiation pressure drop is slightly slower in TRACE calculation than in 

experiment, what causes lower low pressure injection system flow. As can be seen from 

Figure 28, the secondary pressure is better predicted by TRACE. In the period before ultimate 

procedure initiation the pressure in both calculations is constant, because in the experiment the 

pressure was controlled to be constant at 6.91 MPa.
34

Table 8 Main sequence of events – BETHSY 9.1b

Events Time (s)

Experiment RELAP5 TRACE

Break opening 0 0 0

Scram signal (13.1 MPa) 41 31 21

Safety injection signal (11.9 MPa) 50 54 35

Main feedwater off, turbine bypass 54 58 39

Core power decay start (17 s after scram) 58 48 38

Auxiliary feedwater on (30 s after SI signal) 82 84 65

Pump coastdown start (300 s after SI signal) 356 354 335

End of pump coastdown 971 969 950

Start of the first core level depletion 1830 2020 1820

Start of second core uncovery 2180 2130 2183

Ultimate procedure initiation 2562 2508 2573

Accumulator injection starts (4.2 MPa) 2962 2880 2930

Primary mass inventory is minimum 2970 2880 2932

Maximum core clad heatup 3053 3009 3002

Accumulator isolation (1.5 MPa) 3831 3865 3957

Low pressure injection system start (0.91 MPa) 5177 5235 5330

End of test/calculation 8200 to 8330 8330 8500

The maximum heater rod temperature is shown in Figure 29. The heater rods start to heatup 

when core uncovers, as it is shown in Figure 30. TRACE calculation has very good timing, while 

the peak cladding temperature is slightly underpredicted. The core level (see Figure 30) shows 

that the core level depression occurred twice and the core level recovered respectively at about 

1940 seconds and 3020 seconds in RELAP5 calculation, concurring with the first and second 

loop seal reformation. The TRACE calculation also predicts the core level depression twice with 

about the same minimum collapsed liquid levels as in the test. In the TRACE calculation, the 

core level recovers respectively at about 1820 seconds and 2990 seconds, corresponding with 

the times of the first and second loop sealing clearings. Both in the test and the calculation, 

there is an instantaneous core level recovery at the moment the loop seal clearing occurs (see 

Figure 31). TRACE calculation of core level is better than RELAP5 calculation. For quenching 

the rod the primary depressurization was needed to enable accumulator injection. Figure 32

shows the accumulator pressure drop due to discharging. TRACE calculation is in better 

agreement. During accumulator injection the core recovers. After accumulator injection the 

primary mass start to decrease again until the low pressure injections start as shown in 

Figure 33. Due to the slightly higher primary pressure prediction in TRACE the injection started 

a bit later and the injected flow is also lower. Finally, in Figure 34 the primary system mass is 

shown. The agreement is good both for RELAP5 and TRACE until accumulator injection start. 

Initially is slightly better RELAP5 because of the selected subcooled discharge coefficient, 

however later TRACE is superior.
35

Figure 25 Break mass flow – BETHSY 9.1b

Figure 26 Integrated break mass flow – BETHSY 9.1b

0

0.5

1

1.5

2

2.5

0 2000 4000 6000 8000

Mass flow (kg/s)

Time (s)

exp

RELAP5

TRACE

0

1000

2000

3000

4000

0 2000 4000 6000 8000

Mass (kg)

Time (s)

exp

RELAP5

TRACE
36

Figure 27 Pressurizer pressure – BETHSY 9.1b

Figure 28 Steam generator 1 pressure – BETHSY 9.1b

0

4

8

12

16

0 2000 4000 6000 8000

Pressure (MPa)

Time (s)

exp

RELAP5

TRACE

0

2

4

6

8

0 2000 4000 6000 8000

Pressure (MPa)

Time (s)

exp

RELAP5

TRACE
37

Figure 29 Maximum heater rod surface temperature – BETHSY 9.1b

Figure 30 Core level – BETHSY 9.1b

0

200

400

600

800

1000

1200

0 2000 4000 6000 8000

Temperature (K)

Time (s)

exp

RELAP5

TRACE

0

2

4

6

8

10

0 2000 4000 6000 8000

Level (m)

Time (s)

exp

RELAP5

TRACE
38

Figure 31 Loop 1 Seal Downflow Side Differential Pressure

Figure 32 Accumulator pressure – BETHSY 9.1b

-10

0

10

20

30

40

50

0 2000 4000 6000 8000

Pressure drop (KPa)

Time (s)

exp

RELAP5

TRACE

0

1

2

3

4

5

0 2000 4000 6000 8000

Pressure (MPa)

Time (s)

exp

RELAP5

TRACE
39

Figure 33 Integrated low pressure injection system mass – BETHSY 9.1b

Figure 34 Primary coolant mass – BETHSY 9.1b

0

500

1000

1500

2000

2500

3000

3500

0 2000 4000 6000 8000

Mass (kg)

Time (s)

exp

RELAP5

TRACE

0

500

1000

1500

2000

2500

0 2000 4000 6000 8000

Mass (kg)

Time (s)

exp

RELAP5

TRACE
40

6.12.2 RELAP5 and TRACE calculations of BETHSY 6.2TC test

Table 9 shows the initial and boundary conditions for BETHSY 6.2TC test. The RELAP5 and 

TRACE input model were initialized to the cold leg temperature. This is different from 9.1b test 

where no stead-state calculation was performed. In the case of TRACE the secondary pressure 

is not exactly matched. The difference comes from the geometry and the code models. In the 

TRACE assessment report (Ref. 5) the cold leg temperature was not matched for the sake of 

matching secondary pressure. The steam generator levels and masses were matched to 

average measured values both for RELAP5 and TRACE. The pressurizer pressure and level 

were also matched to average measured values. The core power was boundary condition. In 

the experiment the electrical trace heating system was installed of the power of 54.82 kW and 

was operating till the transient start. Therefore in the calculations the heat losses were modeled 

after the electrical heat system was off.

Table 9 Comparison of initial conditions for BETHSY 6.2TC test

Parameter Measured RELAP5 TRACE

core thermal power (kW) 2863 ± 30 2864 2863

pressurizer pressure (MPa) 15.38 ± 0.15 15.38 15.38

pressurizer level (m) 7.45 ± 0.2 7.45 7.45

total flow (kg/s)

16.81 (calculated from 

core power)

16.84 16.61

core inlet temperature (K) 557.2 ± 0.4 557.2 557.2

core outlet temperature (K) 588.2 ± 0.4 588.1 588.8

reactor coolant system mass (kg) 1984 ± 50 1948 1948

secondary side pressure - per SG 

(MPa)

6.84 ± 0.07 6.83 6.69

steam generator level - per SG (m) 11.1 ± 0.05 11.1 11.1

feedwater temperature (K) 523.2 ± 4 523.2 523.2

heat loss (kW) 54.82 N.A. N.A.

downcomer to upper head flow (kg/s) 0.047 0. 47 0.047

The main sequence of events for BETHSY 6.2TC test is shown in Table 10. The graphical 

comparison between the experiment, RELAP5 and TRACE for main variables is shown in 

Figures 35 through 46. The calculation results showed that occurrences and trends of key 

transient phenomena are reasonably predicted by both computer codes.

As shown in Table 10 most of times were reasonably captured. The times of reactor trip and 

safety injection signals are similar for both RELAP5 and TRACE calculation. As the pressure 

drop in TRACE calculation is slower, the primary to secondary pressure reversal is delayed in 

case of TRACE. The main reason is probably the secondary side behavior. Namely, the mass 

released through atmospheric relief valves in the initial period greatly influenced the primary 

pressure drop. Higher secondary pressure indicated that in first 100 s the atmospheric relief 

valves were open few tens of seconds. The overall accumulator time performance is better by 

TRACE than by RELAP5.
41

Table 10 Main sequence of events – BETHSY 6.2TC

Events Time (s)

Experiment RELAP5 TRACE

Break opening 0 0 0

Scram signal (13.1 MPa) 8 2 3

Safety injection signal (11.9 MPa) 11 8 9

First core uncovery 92 90 136

Loop seal clearing 134 155 173

Primary/secondary pressure reversal 172 175 203

Second core uncovery 334 280 253

Accumulator injection starts (4.2 MPa) 363 365 329

Accumulator isolation (1.5 MPa) 895 1125 801

Pressurizer pressure < 0.7 MPa 2065 2230 2167

The timing of the transient very much depends on the break mass flow. For RELAP5 original 

Ransom-Trapp break flow model the values of 0.85, 1.25 and 0.75 were used for subcooled, 

two phase and superheated discharge coefficients, respectively. For TRACE break model the 

values of 0.8 and 0.9 were used for subcooled and two phase discharge coefficients, 

respectively. The values for TRACE were selected after some sensitivity studies. In Figure 35

and Figure 36 are shown the break flow and the integrated break mass flow. It can be seen that 

the calculated break flows are quite well matched, in the range of 10% uncertainty. The 

integrated break flow better agree for the TRACE calculation. Primary pressure is shown in

Figure 37. In spite of larger RELAP5 break flow than TRACE break flow the pressure drop is 

faster in the case of TRACE calculation. Secondary pressure is shown in Figure 38. The 

experimental values indicated that atmospheric relief valves were open a few tens of seconds. 

Later, the agreement between experiment and calculation is better for TRACE than for RELAP5. 

This is due to better heat losses modeling in the case of TRACE. However, in general after 

initial period the secondary side has small influence on the primary side and by this on the 

overall calculation. Figure 39 and Figure 40 show the heater rod surface temperatures in the 

middle and at the top of the core, respectively. The core heatup corresponds by the minimum 

core collapsed liquid level shown in Figure 41. Both calculations predicted with delay the first 

peak of heater rod surface temperature at the middle of the core. The second rod heatup was 

better calculated by TRACE. In the case of heater rod surface temperature at the top of the core 

the timing of heatup prediction was better in the case of TRACE, while heatup rate was better in 

the case of RELAP5. The primary mass is shown in Figure 42. In spite of correct TRACE 

calculated mass discharged through the break the TRACE calculated primary mass is smaller 

than the experimental. Similar is the situation in the case of RELAP5 calculation. The 

information on the loop seal clearing can be obtained from Figure 43 and Figure 44, showing 

the differential pressures on the steam generator and pump side, respectively. It may be seen 

that some further adjustment is needed for TRACE pressure drop on the pump side. Finally, the 

accumulator behavior is shown in Figure 45 showing the accumulator pressure and Figure 46

shown the integrated accumulator injected mass. Again the accumulator injected mass was very 

close to measurement value in the case of TRACE. The trend for RELAP5 is very good with 

exception that approximately 10% more mass was discharged. The difference in the calculated 

masses originates partly from a bit smaller initial primary mass, while the rest of difference may 

be attributed to the measurement uncertainty.
42

Figure 35 Break mass flow – BETHSY 6.2TC

Figure 36 Integrated break mass flow – BETHSY 6.2TC

0

3

6

9

12

15

0 500 1000 1500 2000 2500

Mass flow (kg/s)

Time (s)

exp

RELAP5

TRACE

0

500

1000

1500

2000

2500

0 500 1000 1500 2000 2500

Mass (kg)

Time (s)

exp

RELAP5

TRACE
43

Figure 37 Pressurizer pressure – BETHSY 6.2TC

Figure 38 Steam generator 1 pressure – BETHSY 6.2TC

0

4

8

12

16

0 500 1000 1500 2000 2500

Pressure (MPa)

Time (s)

exp

RELAP5

TRACE

0

2

4

6

8

0 500 1000 1500 2000 2500

Pressure (MPa)

Time (s)

exp

RELAP5

TRACE
44

Figure 39 Heater rod surface temperature at the middle of the core – BETHSY 6.2TC

Figure 40 Heater rod surface temperature at the top of the core – BETHSY 6.2TC

400

450

500

550

600

650

700

750

800

850

0 500 1000 1500 2000 2500

Temperature (K)

Time (s)

exp

RELAP5

TRACE

400

450

500

550

600

650

700

750

800

850

0 500 1000 1500 2000 2500

Temperature (K)

Time (s)

exp

RELAP5

TRACE
45

Figure 41 Core collapsed liquid level – BETHSY 6.2TC

Figure 42 Total primary mass – BETHSY 6.2TC

0

1

2

3

4

0 500 1000 1500 2000 2500

Level (m)

Time (s)

exp

RELAP5

TRACE

0

500

1000

1500

2000

0 500 1000 1500 2000 2500

Mass (kg)

Time (s)

exp

RELAP5

TRACE
46

Figure 43 Intermediate leg 1 DP (SG side) – BETHSY 6.2TC

Figure 44 Intermediate leg 1 DP (pump side) – BETHSY 6.2TC

0

10

20

30

40

0 500 1000 1500 2000 2500

Pressure drop (KPa)

Time (s)

exp

RELAP5

TRACE

0

5

10

15

20

0 500 1000 1500 2000 2500

Pressure drop (kPa)

Time (s)

exp

RELAP5

TRACE
47

Figure 45 Accumulator no. 2 pressure – BETHSY 6.2TC

Figure 46 Integrated accumulators injected mass – BETHSY 6.2TC

0

1

2

3

4

5

0 500 1000 1500 2000 2500

Pressure (MPa)

Time (s)

exp

RELAP5

TRACE

0

100

200

300

400

500

600

0 500 1000 1500 2000 2500

Mass (kg)

Time (s)

exp

RELAP5

TRACE

49

7. CONCLUSIONS

In the report the IJS procedure for converting input deck from RELAP5 to TRACE is described. 

Besides, examples of conversion are given for separate effects test ACHILLES rig and integral 

effects test BETHSY facility. The conversion methodology consists of eleven steps and is based 

on SNAP. The ACHILLES and BETHSY legacy RELAP5 input decks were developed at IJS in 

the frame of participation to international standard problem no. 25 and international standard 

problem no. 27. These legacy input decks were successfully converted from RELAP5 to 

TRACE. Besides demonstration of the conversion methodology also the comparison between 

RELAP and TRACE has been done. It can be concluded that calculated results obtained by 

TRACE are as good as the results by RELAP5, thus suggesting that IJS conversion 

methodology may be used for separate and integral effect test legacy RELAP5 input deck 

conversions to TRACE input decks.

51

8. REFERENCES

1. U. S. Nuclear Regulatory Commission, “TRACE V5.0 User Manual”, Division of Risk 

Assessment and Special Projects, Office of Nuclear Regulatory Research, Washington, 

DC.

2. APT, “Symbolic Nuclear Analysis Package (SNAP), User's Manual”, Applied 

Programming Technology (APT), Inc., 2007.

3. A. Stritar, A. Prošek, “The analysis of the reflood experiment”, Z. Angew. Math. Mech., 

1991, vol. 71, pp. 525-527.

4. S. Petelin, B. Mavko, O. Gortnar, I. Ravnikar, G. Černe, “Result of BETHSY test 9.1.b 

using RELAP5/MOD3”, International agreement report NUREG/IA-0141, U.S. Nuclear 

Regulatory Commission, Washington, U.S.A, 1998.

5. U. S. Nuclear Regulatory Commission, “TRACE V5.0 Assessment Manual”, Division of 

Risk Assessment and Special Projects, Office of Nuclear Regulatory Research, 

Washington, DC.

6. C. Queral, J. Mulas, I. Collazo, A. Concejal, N. Burbano, “Conversion of the thermal 

hydraulics components of ALMARAZ NPP model from RELAP5 into TRAC-M”, 

International Conference Nuclear Energy for New Europe 2002, Kranjska Gora, 

September 9-12. 2002.

7. C. Queral, J. Mulas, I. Collazo, A. Concejal, N. Burbano, “Conversion of control systems, 

protection and engineering safeguard system-signals of ALMARAZ NPP model from 

RELAP5 into TRAC-M”, International Conference Nuclear Energy for New Europe 2002, 

Kranjska Gora, September 9-12. 2002.

8. Holmes, B. J., “ISP 25 Comparison Report”, (NEA/CSNI/R/(1991)/11), AEE Winfrith, 

Dorchester, UK, 1991.

9. A. Stritar, A. Prošek, “Analysis of ISP-25 Achilles Experiment by Computer Codes 

RELAP4/MOD6 and RELAP5/MOD2”, IAEA Technical Committee meeting on Computer 

Aided Safety Analysis, Moscow, 14-17 May 1990.

10. CEA, “BETHSY, General Description”, No. SETh/LES/90-97, CEA (Commissariat à 

l'énergie atomique et aux énergies alternatives), Grenoble, France, April 1990.

11. T. Chataing, P. Clement, “2" Cold Leg Break without HPSI and with Delayed Ultimate 

Procedure”, No. SETh/LES/90-104, CEA (Commissariat à l'énergie atomique et aux 

énergies alternatives), Grenoble, France, June 1990.

12. G. Briday, D. Juhel, “BETHSY – Test 6.2 TC, 6 inch cold leg counterpart test”, Test 

Analysis Report no. STR/LES/91-034, CEA (Commissariat à l'énergie atomique et aux 

énergies alternatives), Grenoble, France, October 1991.

13. S. Hrvatin, A. Prošek, I. Kljenak, “Quantitative assessment of the BETHSY 6.2 TC test 

simulation”, Proceedings of the Eighth International Conference on Nuclear Engineering, 

The American Society of Mechanical Engineers, New York, U.S.A., 2000.

14. A. Prošek, B. Mavko, “Status of CAMP activities in Slovenia”. Fall 2010 CAMP Meeting, 

October, 27-29, 2010, State College, Pennsylvania, USA. Proceedings, 2010.




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/IA-0408 IJS Procedure for Converting Input Deck from RELAP5 to TRACE