NUREG/IA-0461 

International 

 Agreement Report

TRAC-BF1 to TRACE Model 

Semi-Automatic Conversion.

PBTT Example

 

 

Prepared by: 

R. Miró, A. Jambrina, C. Mesado, T. Barrachina, G. Verdú

Institute for Industrial, Radiophysical and Environmental Safety (ISIRYM) 

Universitat Politècnica de València (UPV) 

Camí de Vera, s/n 

46022 Valencia, SPAIN 

K. Tien, NRC Project Manager 

 

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: November 2015

Date Published: December 2015

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-0461 

International 

 Agreement Report

TRAC-BF1 to TRACE Model 

Semi-Automatic Conversion.

PBTT Example

 

 

Prepared by: 

R. Miró, A. Jambrina, C. Mesado, T. Barrachina, G. Verdú

Institute for Industrial, Radiophysical and Environmental Safety (ISIRYM) 

Universitat Politècnica de València (UPV) 

Camí de Vera, s/n 

46022 Valencia, SPAIN 

K. Tien, NRC Project Manager 

 

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: November 2015

Date Published: December 2015

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

This publication intends to summarize a larger work and aims to establish a semi-automatic 

procedure to translate input decks from TRAC-BF1 to TRACE code. This work was developed at 

Universitat Politècnica de València (UPV). The main reason for this work is the importance to 

keep the models updated and adapted to be used on new codes. As a result, it is possible to take 

advantage of these codes, which are more powerful because of the new techniques used in their 

development and the increase of computer resources. As a working example, the Peach Bottom 

II (PB) model is presented. This model was chosen because it is one of the most used plant 

models to validate new codes or calculation models. Moreover, data is easily available due to the 

NEA/OECD BWR Peach Bottom Turbine Trip (PBTT) benchmark. This methodology presented 

hereafter, could be used to translate other TRAC-BF1 models to keep them updated and therefore 

useful for other applications.

v

FOREWORD

This report represents one of the assessment or application calculations submitted to fulfill the 

bilateral agreement for cooperation in thermal-hydraulic activities between the Consejo de 

Seguridad Nuclear (CSN) and the U.S. Nuclear Regulatory Commission (NRC) in the form of a 

Spanish contribution to the NRC’s Code Assessment and Management Program (CAMP), the 

main purpose of which is to validate the TRAC/RELAP Advanced Computational Engine (TRACE) 

code.

CSN and the Asociación Española de la Industria Eléctrica (UNESA, Electric Industry Association 

of Spain), together with some relevant universities, have established a coordinated framework 

(CAMP-Spain) with two main objectives: to fulfill the formal CAMP requirements and to improve 

the quality of the technical support groups that provide services to the Spanish utilities, CSN, 

research centers, and engineering companies.

The AP-28 Project Coordination Committee has reviewed this report: the contribution of a Spanish 

University to the above-mentioned CAMP-Spain program, for submission to CSN.

ISIRYM - UPV

June 2015

vii

CONTENTS

Page

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

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

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

TABLES ..................................................................................................................................xi

EXECUTIVE SUMMARY.........................................................................................................xiii

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

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

2. PEACH BOTTOM MODEL............................................................................................. 2-1

2.1 Thermal-Hydraulic Model .......................................................................................... 2-1

2.2 Transient Model: PBTT Benchmark........................................................................... 2-3

2.3 Neutronic Model ........................................................................................................ 2-5

3. METHODOLOGY........................................................................................................... 3-1

3.1 Translation Process ................................................................................................... 3-1

3.2 Error Correction......................................................................................................... 3-3

3.3 Thermal-Hydraulic Adjustment for Steady State Simulation..................................... 3-10

3.4 Thermal-Hydraulic Adjustment for Transient State Simulation................................. 3-13

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

4.1 Steady State.............................................................................................................. 4-1

4.2 Transient State (Turbine Trip).................................................................................... 4-2

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

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

ix

FIGURES

Page

Figure 1 Thermal-Hydraulic Channels................................................................................. 2-1

Figure 2 Vessel Nodalization and Main Containment Building Components ....................... 2-2

Figure 3 Steam Line Scheme.............................................................................................. 2-2

Figure 4 Turbine Stop Valve (Left) and Bypass Valve (Right) Area Fraction as Function 

of Time.................................................................................................................. 2-3

Figure 5 Relative Total Reactor Power for all Benchmark Participants................................ 2-4

Figure 6 Fuel Radial Map (Left) and Initial Control-Rod Banks Position (Right) .................. 2-5

Figure 7 Reactor Area Components in SNAP Tool ............................................................. 3-3

Figure 8 Proposed Algorithm for Automatic Thermal-Hydraulic Adjustment in PBTT 

Model.................................................................................................................. 3-12

Figure 9 Pump-Impeller Rotational Speed, TRACE and TRAC-BF1 Comparison ............. 3-14

Figure 10 Core Flow Distribution in TRACE Model (Left) and Absolute Difference Between 

TRACE and TRAC-BF1 (Right)............................................................................. 4-1

Figure 11 Mean Axial Moderator Temperature (Left) and Void Fraction (Right) Distribution.. 4-2

Figure 12 Core Average Axial Power Profile TRACE/Plant Data (Left) and Relative Total 

Reactor Power (Right) TRACE/Benchmark Data .................................................. 4-3

Figure 13 Dome (Left) and Core Exit (Right) Pressure, TRACE/Benchmark Data................. 4-3

Figure 14 Doppler (left) and Moderator (Right) Reactivity, TRACE/Benchmark Data............ 4-4

Figure 15 Total Reactivity, TRACE and Benchmark Results................................................. 4-4

xi

TABLES

Page

Table 1 Main Boundary Conditions.................................................................................. 2-2

Table 2 Time for Main Events in Peach Bottom Turbine Trip Accident (s) ....................... 2-3

Table 3 Variables to Correct in Control Blocks ................................................................ 3-4

Table 4 Variables to Resize when ncrz is Changed......................................................... 3-5

Table 5 TRACE Variables in a CHAN Component not Defined in TRAC-BF1.................. 3-5

Table 6 TRACE Variables in a CHAN Component that Change Definition in TRAC-BF1 . 3-6

Table 7 Vessel Nodalization to Adjust Adjoin Components.............................................. 3-7

Table 8 Incorrect Friction Factor Translation in Vessel Component as Function of 

Nodal Level ........................................................................................................ 3-7

Table 9 Incorrect Hydraulic Diameter Translation in Vessel Component as Function of 

Nodal Level ........................................................................................................ 3-7

Table 10 Chan Component (ID 1 to 33) Variables to Change............................................ 3-8

Table 11 Tee Component (ID 71, 75, 92, 93 and 94) Variables to Change........................ 3-8

Table 12 Valve Component (ID 72, 76, 77, 86, 87 and 88) Variables to Change............... 3-9

Table 13 Separator Component (ID 80) Variables to Change............................................ 3-9

Table 14 Jet Pump Component (ID 82) Variables to Change ............................................ 3-9

Table 15 Pump Component (ID 83) Variables to Change.................................................. 3-9

Table 16 Fill Component (ID 85) Variables to Change....................................................... 3-9

Table 17 Actions to Supress TRACE Warnings, Alerts and Errors .................................. 3-10

Table 18 Changes Made to Adjust the Thermal-Hydraulic Steady State Model............... 3-11

Table 19 Changes Made to Adjust the Thermal-Hydraulic Transient State Model ........... 3-13

Table 20 Main Components Flow for TRACE and TRAC-BF1 Comparison....................... 4-1

xiii

EXECUTIVE SUMMARY

In order to keep old models updated, a methodology to translate models from TRAC-BF1 to 

TRACE is developed at Universitat Politècnica de València. An additional advantage for this 

translation is that current and future TRACE features can be applied to the translated model. The 

methodology makes use of the TRACE executable and its feature to read TRAC-BF1 input decks 

and create the appropriate restart. Then, the restart file can be imported to SNAP tool and later 

exported as ASCII file. However, the methodology is said to be semi-automatic because some 

errors are done in the process. These errors must be corrected by hand as explained in this 

document.

As an example of the semi-automatic methodology, the Peach Bottom (PB) model in TRAC-BF1 

is translated to TRACE. Nonetheless, the methodology is not exclusive for Peach Bottom. Rather, 

this study is presented to help any other TRAC-BF1 to TRACE translation, regardless of the plant 

model.

Two different cases are presented in this document, both modeling PB. First, the translated model 

is code-to-code compared in steady state. Moreover, to test the transient model, the second case 

is a modification of the translated original model to simulate the Peach Bottom Turbine Trip 

(PBTT). This case is compared with the Benchmark participant results. In general good 

agreements are obtained for both cases. However, thermal-hydraulic adjustments must be made 

prior to the comparisons. A schematic framework for these modifications is presented.

xv

ABBREVIATIONS

BPV Bypass Valve

BWR Boiling Water Reactor

NRC Nuclear Regulatory Commission

PBTT Peach Bottom Turbine Trip

PWR Pressurized Water Reactor

SNAP Symbolic Nuclear Analysis Package

TSV Turbine Stop Valve

1-1 

1. INTRODUCTION

Nuclear codes are being developed and improved permanently. Examples include new physic 

models, updating old codes or parallelization of algorithms. In this work, mainly two thermalhydraulic codes are used: TRAC-BF1 [1] and TRACE V5.0P3 [2]. PARCS v2.7 [3] is used as a 

coupled neutronic code for TRAC-BF1 and PARCS v3.0 [4] is used for TRACE. TRAC-BF1 is a 

code developed in the Idaho National Engineering Laboratory (USA) for boiling water reactors 

(BWR) simulation purpose. TRACE code is an updated version that could be used for both types 

of reactors: BWR and PWR simulations. Besides, PARCS is a 3D diffusion neutronic code, widely 

used, and developed at Purdue University (USA).

A Peach Bottom (PB) input deck for TRAC-BF1 is used to run plant transient simulations. It 

models not only the vessel and reactor core, but also other components, such as the jet pump, 

recirculation pump, separator and the steam line. However, to keep the PB model updated, a 

semi-automatic translation from TRAC-BF1 to TRACE is carried out with a methodology 

developed at Universitat Politècnica de València (UPV). This methodology makes use of the 

TRACE executable and SNAP tool [5]. 

Peach Bottom model is chosen for the translation purpose because a lot of data is available in 

the framework of the Peach Bottom Turbine Trip NEA Benchmark [6] and [6]. In this study, the 

methodology to translate the model from TRAC-BF1 to TRACE is presented. The main 

advantages of this translation process are explained next:

- Peach Bottom input deck model updated to new TRACE code, thus easier to maintain and 

improve. 

- Get advantage of new models and features in TRACE, the ones that already are

implemented, and the ones that will have TRACE in future versions.

- Nuclear Regulatory Commission (NRC) intends to license TRACE code in a middle-term 

future. Therefore, anticipation is made in this work in order to clear the path for new 

TRACE input decks.

It is worth to mention that this methodology is not specific to the Peach Bottom case. It is meant 

that the methodology, hereafter presented, could aid the translation for other TRAC-BF1 models.

Two cases are presented here, both modelling the Peach Bottom reactor. The first case is the 

direct translation of the TRAC-BF1 model in steady state, and thus, the TRAC-BF1 results are 

used for code-to-code comparison. The second case is the first model with modifications to 

simulate the Peach Bottom Turbine Trip NEA Benchmark, thus the benchmark results are used 

to validate the model.

The translation is done semi-automatically using a TRACE executable and the SNAP tool. TRACE 

executable has the capability to read input files from TRAC-BF1, but it does not execute the 

simulation. However, it could generate the restart file in TRACE format. Afterwards, SNAP tool 

could be used to read this restart file and then export the model in TRACE ASCII format. The 

methodology is said to be semi-automatic because the translation done by TRACE executable is 

not error-free. Thus, the mistakes, for now, must be corrected by hand.

The error correction could be a long task. Nevertheless, this publication is intended to make this 
1-2 

task easier and less painful. The publication is divided in six sections. First, in section 2, the PB 

model, both thermal-hydraulic and neutronic parts are explained. In section 3 the translation 

methodology is explained and also the mistakes found in the PB translation are presented with a 

possible solution. In order to validate this work, section 4 shows the results for steady and 

transient state. Some concluding remarks and conclusions are drawn in section 5. Finally, in 

section 6, the references are presented.
2-1 

2. PEACH BOTTOM MODEL

2.1 Thermal-Hydraulic Model

Peach Bottom reactor is composed of 764 fuel assemblies, in this model fuel assemblies are 

collapsed to 33 channel (CHAN) components [6], as shown in Figure 1. 

Figure 1 Thermal-Hydraulic Channels 

The channels are inside the vessel, together with the jet pump and the separator. The vessel has 

9 axial nodes and 2 radial nodes, the outer of which conforms the downcomer. The channels are 

between axial levels 4 and 5, in the inner radial node. The fill feeds water directly to the 

downcomer in the 7th axial node. The steam line starts in the 8th axial node, see Figure 2. The 

steam line contains several relief valves and breaks, the main ones are the bypass valve (id 77) 

and turbine stop valve (id 76), see Figure 3. 
2-2 

Figure 2 Vessel Nodalization and Main Containment Building Components

Figure 3 Steam Line Scheme

The boundary conditions for the main model components can be seen in Table 1. 

Power (W) 2.03·109

Fill

 Flow (kg/s)

 Velocity (m/s)

 Temperature (K)

680.26

11.80

442.31

Break pressure

 Before turbine (Pa)

 Before condenser (Pa)

6.65·106

1.25·105

Reactor 

 Pressure drop (Pa)

 Bypass flow (kg/s)

 Core flow (kg/s)

 Total core flow (kg/s)

1.14·106

841.38

9603.62

10445.00

Table 1 Main Boundary Conditions
2-3 

2.2 Transient Model: PBTT Benchmark

In order to validate the full methodology, both steady state and transient state are modelled. For 

the transient simulation, a turbine trip accident is chosen to compare both models. The model 

corresponds to the NEA Peach Bottom Turbine Trip PB2 Benchmark (Exercise 3, Extreme 

Scenario 2: turbine trip without reactor SCRAM), the reader is referred to [6] for more information. 

In this scenario, the turbine trip signal is send and the turbine stop valve (TSV) is closed. After 

0.06s, the bypass valve (BPV) starts to open. Moreover, regarding the transition simulation, a null 

transient simulation is set for 50 seconds prior to the real transient. This is convenient to check

the steady state convergence before the simulation of any kind of perturbation. The main events 

and the occurrence time in this scenario are presented in Table 2. 

TSV begins to close 0.000

BPV begins to open 0.060

TSV closed 0.096

BPV fully open 0.846

Turbine pressure initial response

 Steam line A

 Steam line D

0.102

0.126

Steam line pressure initial response

 Steam line A

 Steam line D

0.348

0.378

Vessel pressure initial response 0.432

Core exit pressure initial response 0.486

Table 2 Time for Main Events in Peach Bottom Turbine Trip Accident (s) 

In Figure 4 the area fraction for both valves (TSV and BPV) as a function of time is shown. Even 

though there is a null transient of 50 s, the transient time beginning resets to 0 s in all figures. It 

can be seen that the TSV is closed in less than a tenth of a second, besides the BPV is opened 

in almost one second.

Figure 4 Turbine Stop Valve (Left) and Bypass Valve (Right) Area Fraction as Function 

of Time

0 0.02 0.04 0.06 0.08 0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Turbine valve flow area fraction

Time (s)

Flow area fraction (-)

0 0.2 0.4 0.6 0.8 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Bypass valve flow area fraction

Time (s)

Flow area fraction (-)
2-4 

The relative total reactor power for all the Benchmark participants is depicted in Figure 5. It can 

be seen that the power peak for all participants is produced almost one second from the turbine 

trip signal. Afterwards, the power oscillates and the second peak is produced between 2 and 3 

seconds after the turbine trip signal. Finally, the power reaches a steady state after 10 seconds.

In Figure 5, the power evolution is shown only from 0 to 5 seconds to be able to see the differences 

in the first power peak.

Figure 5 Relative Total Reactor Power for all Benchmark Participants

0 1 2 3 4 5

-1

0

1

2

3

4

5

6

7

8

9

10

Relative total reactor power

Time (s)

Power (W)

FZD

PSU/PUR/NRC

TEPSYS

UPISA

UPV

WES
2-5 

2.3 Neutronic Model

The 3D neutronic model is simulated using PARCS v2.7. The core is discretized in 26 levels and 

each of them has 888 cells, of which 124 represents the reflector, (total cells are 23088). All cells 

have the same dimensions 15.24 cm x 15.24 cm x 15.24 cm, see Figure 6 (left). The total 

simulated neutronic compositions are 1203, three of them used for reflectors. Besides, there are 

eight different control rod banks, their distribution and initial position for the transient simulation is 

given in Figure 6 (right). Position 48 means control rod bank totally withdrawn (out) and 0 means 

bank totally inserted (in).

Figure 6 Fuel Radial Map (Left) and Initial Control-Rod Banks Position (Right)

3-1 

3. METHODOLOGY

3.1 Translation Process

First, it is important to know that TRAC-BF1 allows some freedom degree regarding the physical 

model conditions. However, TRACE is restrictive dealing with parameters between different 

components, i.e. flow area, height, angle, friction factors, etc. 

To start the methodology some adjustments need to be done to the TRAC-BF1 input file; these 

adjustments are explained hereafter.

• The input deck must be in lower case. This could be changed with any text editor with

automatic features.

• Even though water rods should not be represented as the first rod group, TRAC-BF1

allows it for CHAN component [8]. Nonetheless, TRACE does not allow it. Thus, in case

the first rod group is composed by water rods in TRAC-BF1, some changes are needed

in order to proceed with the translation. This change involve changing some variables, i.e.

CPOWR parameter (power distribution within the fuel bundle), RDPWR (radial rod power

distribution), RDX (number of actual rods in each group), MATRD (rod group material),

among other variables. The values corresponding to the water rod (first group) should be

moved. For example in case the water rod group is moved to the end, these values should

be moved as well to the end of the value list. In case of Peach Bottom model, there are

not water rods, thus no worry about this issue is needed.

• TRACE does not allow the clad to be composed of only one node, whereas TRAC-BF1

does. This restriction is set to get a better radiation model results. If this parameter is

changed, then the vector TW (initial wall temperature) must be resized. This is not the

case for Peach Bottom model.

• Comment the first TRAC-BF1 line (=3D). This line appears when TRAC-BF1 is coupled

with a neutronic code. Comments in TRAC-BF1 are done with the asterisk sign. This is

the title line and it is not interesting in order to get the TRACE input file (steady state).

• When TRAC-BF1 is coupled with a neutronic code, the kinetics options should be 1D.

Therefore, in this case, the parameter IRPOP (kinetic model option) must be changed

from 8 (1D) to 1 (constant power). For this kinetic model option, NDG (delayed neutron

groups) parameter must be 0. Both parameters are found in POWER0001X card.

• All cards related to the 1D kinetics model are deleted. With the exceptions of

POWER0001X card (already modified in previous step) and POWER00ZPOWRX card

(axial power distribution). Cards to delete are: POWDECAY, POWER00020,

POWER00030, POWONKIN.

• For fill components, if IFTY parameter is equal to 4 (constant velocity until trip is on, then

table), then the control block associated (card cntrl20268) must have ncbout (control block

output identifier) equal to 0. Otherwise, the following error is thrown:
3-2 

*WireCBOut*

CB output not compatible with input IFTY

 Fill Component= 85

Control Block ID= 268

icbout= -42

 ***************** ** fatal error ** *****************

*ConBlkDataF* call WireCbOut

• For pump components, if IPMPTY parameter is equal to 1 (constant speed until trip is on,

then table), then the control block associated (card cntrl20104) is not compatible. The

IPMPTY must be changed to 3 (pump motor torque controlled by control system).

Otherwise, the following error is thrown:

*WireCBOut*

CB output not compatible with input IPMPTY

 Pump Component= 83

Control Block ID= 104

icbout= -1 

 ***************** ** fatal error **

 *****************

*ConBlkDataF* call WireCbOut

Afterwards, the modified input is run with a TRACE executable. Even though the simulation is 

stopped with a warning output, some output files are created. Specifically it is of interest the restart 

file (.tpr). This file can be imported as a TRACE file by SNAP tool (using tpr option), and therefore, 

after successfully reading the tpr file, a TRACE input deck file can be exported as an ASCII file. 

In Figure 7, the main model components (from tpr file) are shown as seen in SNAP tool.
3-3 

Figure 7 Reactor Area Components in SNAP Tool

3.2 Error Correction

From now on, next modifications could be made either on SNAP or in the ASCII TRACE input file. 

The main mistakes done by TRACE/SNAP translation are explained next, they must be corrected 

by hand. This section is exclusively for Peach Bottom Turbine Trip model case. However, it is 

expected that this section helps in order to achieve a good translation in other inputs.

• In trip section, since a null transient is desired for an appropriate steady state

convergence, the set point for trip with ID 10 is set to 50 seconds.

• In control system blocks, parameter cbcon2 (control constant 2) has bad values for most

of the controls. Table 3 shows the control block IDs for which cbcon2 value is

mistranslated, the correct cbcon2 value, and other mistranslated variables if any.

Jet pump

Fuel assemblies

Vessel

Separator
3-4 

CB ID cbcon2 Other variables

2 9.834

54 0.672

50 0.208

52 -0.381

54 0.208

58 0.88 icbn=3

100 cbgain=cbxmin=cbmax

=cbcon1=cbcon2= 9603.32

101, 102, 

103 0.0

105 to 137 icbn=59 cbcon1=cbon2=1.0

138 7.4884E6

139 7.5553E6

140 7.6222E6

141 8.3245E6

206 0.0

208, 210, 

214,

220, 222

0.9568

224 0.0

228 1.0

230 0.0

234, 236 62.37

238 26.90

240 cbcon1=1.17647

248, 250, 

252,

256, 262

64.3

254 1.0

264 1.036

266 1046.0

267 1.3

268 0.0

512 0.226

516 0.0

Table 3 Variables to Correct in Control Blocks
3-5 

• In channel components, in order to facilitate the coupling with a neutronic code, ncrz

(number of heated nodes) is reduced to 24, and icrnk (number of CHAN cells below the

powered region) and icrlh (number of CHAN cells below the lower tie plate) are set to 1.

Because of these changes, some vector variables for channels components must be

resized. These vector variables are summarized in Table 4.

Variable Comp. location Meaning

nfax Channel Number of fine-mesh cells added in each coarse 

mesh cell

rftn Channel Rod element temperatures, initial distribution

radpw Channel Core wide radial chan-to-chan power peaking factor

burn Channel Fuel burnup, initial condition

zpwtb1 Power Axial power-shape, initial condition

Table 4 Variables to Resize when ncrz is Changed

• Some channel variables in TRACE are not defined in TRAC-BF1. However, TRACE

assigns some default values to them, see Table 5. Other channel variables change their

definition between codes or need some comments, see Table 6.

Variable Default value Meaning

iaxcnd 0 Specification of axial conduction (0 means off)

liqlev 0 Liquid-level tracking (currently not used).

reflood 0 Reflood flag, redundant parameter (use fmon)

nff 1 Friction-factor correlation option

ilev -1 Level tracking (-1 means off)

Table 5 TRACE Variables in a CHAN Component not Defined in TRAC-BF1
3-6 

TRAC-BF1 TRACE

Variable Comment Variable Comment

hgapo

Fuel rod gap conductance coefficient

Value in card 30 is not used. Instead, 

variable HGAP vector is used. It specifies a 

value for each cell in CHAN component.

hgapo Rod gas gap HTC. Constant for each CHAN 

component.

nrod Number of fuel rods in a single bundle row. nrow Number of rods in a row. Same definition, the 

name is changed.

bundw Inside channel width. width Inside perimeter of the canister wall.

epsc Fuel rod surface emissivity. Constant. emcof Rod Surface, second order polynomial.

epsr Channel wall inside emissivity. Constant. emcif Canister Wall, second order polynomial.

irad

Radiation heat transfer option.

IRAD = 0 includes steam and droplets in 

radiation heat transfer calculation.

IRAD = 1 does not include steam and 

droplets in radiation heat transfer calculation.

norad

Radiation heat transfer option.

norad = 0, radiation heat transfer model is 

turned on.

norad = 1, radiation heat transfer model is 

turned off.

iani

Anisotropic radiation reflection flag.

iani = 0, No anisotropic correction.

iani = 1, Correct view factors for anisotropic 

reflection.

noani

Anisotropic option.

noani = 0, anisotropic view factor corrections 

are applied to the view factor calculation.

noani = 1, anisotropic view factor corrections 

are not applied.

epsd

Ratio, surface roughness/hydraulic diameter. 

It specifies a value for each cell in CHAN 

component.

epsw Wall surface roughness. Constant for each 

CHAN component.

ihts

Indicator for heat transfer to the fluid of this 

component from the outer wall of one or 

more other components.

nhcom Component number receiving outside wall 

energy.

ichoke

Choking calculation flag.

ichoke = 0, no choking.

ichoke = 1, choking calculation.

icflg

Cell edge choked flow model option.

icflg = 0, no choked flow model calculation.

icflg = 1, choked flow model using default 

multipliers.

icflg = 2 to 5, using NAMELIST variable 

defined multipliers.

iccfl

Countercurrent Flow (CCFL) control flag.

iccfl = 0, Turn off CCFL model.

iccfl = 1, CCFL upper tie plate constants.

iccfl = 2, CCFL side entry orifice constants.

lccfl

Countercurrent flow limitation option.

lccfl = 0, no countercurrent flow limitation 

calculation.

lccfl = N, the countercurrent flow limitation 

parameter set number used to evaluate 

countercurrent flow limitation.

rdx Number of rods in each rod group. rdx Number of actual rods in each rod group. 

Water rods must not be the first group.

ichf Choke Flow calculation flag.

Negative value means that CHF model is off. ichf Choke Flow calculation flag.Negative value not permitted.

alptst Threshold void fraction for radiation 

calculation. - Do not exist in TRACE.

nrad Number of time steps between radiation 

calculations. - Do not exist in TRACE.

Table 6 TRACE Variables in a CHAN Component that Change Definition in TRAC-BF1
3-7 

• Parameter nff (friction factor option), present in most of the thermal-hydraulic components

in TRACE is not defined in TRAC-BF1. It is chosen to have value 0 (constant friction factor)

for all components since it reduces the oscillations in the solution.

• Vessel nodal height is recalculated in order to fit node faces height with components

length, see Table 7. Some vessel friction factors and hydraulic diameters need to be

cheeked since some of them are not the same as in TRAC-BF1, see Table 8 and Table

9. In these tables, values shown are the correct TRAC-B values, cells in blank correspond

to correct translated values

Vessel Component, ID = 99

Level Upper height (m) Adjoin component

R=1 inner radial cell R=2 outer radial cell

9 22.250

8 19.9585 SEPARATOR

7 13.9990 Δz = 7.78256 m

6 12.17594

5 9.67320 CHANNELS

4 9.09682 Δz = 3.9624 m JET PUMP

3 5.71080 Δz = 5.01072 m

2 4.08610

1 1.26760

Table 7 Vessel Nodalization to Adjust Adjoin Components

level cfzlz cfzlxr cfzvz cfzvxr cfrlz cfrlxr cfrvz cfrvxr

4 0.5 0.5 0.5 0.5 1.5 0.5 1.5 0.5

5 1.5 0.5 1.5 0.5 1.5 0.5 1.5 0.5

6 5.0 5.0 5.0 5.0

8 1.0 0.0 1.0 0.0

9 5.0 5.0 5.0 5.0

Table 8 Incorrect Friction Factor Translation in Vessel Component as Function of Nodal 

Level

level hdz hdxr

1 4.079495 1.0 2.8776495 0.0

2 4.0795116 1.0 5.7633977 0.0

3 0.0 0.0

4

5 0.0 0.0

6 1.0 1.2577906 0.0 0.0

7 5.7409076 0.0

8 0.0 0.0

9 1.0 1.0 10.175143 0.0

Table 9 Incorrect Hydraulic Diameter Translation in Vessel Component as Function of 

Nodal Level
3-8 

• Changes in other components are detailed in Table 10 to Table 16.

Variable Correct Value Comment

ibeam 0 View factors not translated.

noani 1 The anisotropic view factor corrections are not applied 

to the view factor calculation.

dznht, dznhtw

dtxht1, dtxht2 0 TRAC-BF1 value.

toutl, toutv 559.71 TRAC-BF1 value.

nzmax, nzmaxw 27 TRAC-BF1 value.

hgapo 4542.56 TRAC-BF1: an array, TRACE: constant value, see 

Table 6.

epsw See comment

Depending on the fuel type.

1.467E-4 in CHANS with 49 rods.

1.314E-4 in CHANS with 64 rods.

Different definition, see Table 6. 

TRAC-BF1: an array, TRACE: constant value.

rdx 64

For TRAC-BF1 CHANS with rdx=63 (water rod not 

included), corresponding TRACE CHANS must have 

rdx=64 (8*8). See Table 6.

cpowr See comment

Depending on the fuel type.

2.04081633E-02 in CHANS with 49 rods.

1.58730159E-02 in CHANS with 64 rods.

radpw See comment

Depending on the fuel type.

8.67373056E+03 in CHANS with 49 rods.

1.13962021E+04 in CHANS with 64 rods.

ncrz 24 For couple easiness.

icrnk, icrlh 1 For couple easiness.

nfax, rftn

rdpwr, burn See comment Reshape arrays to ncrz.

Table 10 Chan Component (ID 1 to 33) Variables to Change

Variable Correct Value Comment

ichf 0 Negative values not allowed. See Table 6.

Table 11 Tee Component (ID 71, 75, 92, 93 and 94) Variables to Change
3-9 

Variable Correct Value Comment

radin 0.0011 Only valves with ID 72, 86, 87 and 88. th 0.0011

hd(2) 1E-5

vl(2) 1E-10 Only valves with ID 72, 77, 86, 87 and 88.

vv(2) 1E-10

ichf 0 Only valves with ID 76 and 77.

kfac(1) 325.0

kfacr(1) 325.0 Only valve with ID 77.

grav(1) 1.0

ivsv 36 Only valve with ID 72.

ivsv 37 Only valve with ID 86.

ivsv 38 Only valve with ID 87.

ivsv 39 Only valve with ID 88.

Table 12 Valve Component (ID 72, 76, 77, 86, 87 and 88) Variables to Change

Variable Correct Value Comment

ichf 0 Negative values not allowed. See Table 6.

toutl, toutv 561.4 TRAC-BF1 value toutl2, toutv2 561.4

Table 13 Separator Component (ID 80) Variables to Change

Variable Correct Value Comment

toutl, toutv 549.3

kfac1, kfacr1 0.02 TRAC-BF1 value

kfac2, kfacr2 0.03

Table 14 Jet Pump Component (ID 82) Variables to Change

Variable Correct Value Comment

ichf 2 TRAC-BF1 value

kfacr(10) 0.024 TRAC-BF1 value

Table 15 Pump Component (ID 83) Variables to Change

Variable Correct Value Comment

tvin 442.31 TRAC-BF1 value

Table 16 Fill Component (ID 85) Variables to Change

Finally, some adjustments are needed to supress some TRACE warnings, alerts, and errors, 

see Table 17. 
3-10 

Warning/Note Comment

Warning: Model Options. The water packing model is currently 

disabled Model Flag ipak = 1.

Break 73 (relief valve break)

Alert: The maximum rate of change of the break pressure is set to 

0.0

For all breaks, rbmx variable is 

set to 1E+20.

Warning: Hydraulic [22] from Pump 83 to Vessel 99

Inconsistent connection from Pump 83 (intact loop recirc pump) to 

3D Cell ( 2, 1, 3) of Vessel 99 (bwr-4 254 inch ID vessel).

1D Connections to a radial face of a 3D Cell must have a GRAV 

term of 0.0. (-0.57 != 0.0)

For pump, grav array variable, 

first element is set to 0.0.

Error: Valve 77 (bypass valve): Inconsistent GRAV terms on 

junction 59 from Tee 75 edge 7 (-0.074) to Valve 77 edge 1 (1.0). 

Tee 75 will be used.

For valve with ID 77, grav array 

variable, first element is set to

-0.074.

Separator 80 (separator/dryer (simple) 226) [2] Errors

Error: Wall Power Table

The independent variable signal has not been set.

Error: Wall Power Table

The independent variable signal has not been set.

For separator, variable iqsv is 

set to 59.

JetPump 82 (intact loop jetpump) [2] Errors

Error: Wall Power Table

The independent variable signal has not been set.

Error: Wall Power Table

The independent variable signal has not been set.

For jet pump, variable iqsv is set 

to 59.

namelist option useSJC must be 2 when jun1=0

*****************

** warning **

*****************

*rfill* disconnected fill input error

Namelist variable usesjc is set 

to 2.

*****************

** warning **

*****************

*chbd* Junction boundary error detected

 Comp= valve ;num= 77; junction= 59; variable= rev. friction 

factor 

 Junction left side= 3.568685E+00; right side= 1.976502E-02

For tee with ID 75, kfacr2 array 

variable, second element is set 

to 163.4

For valve with ID 77, kfacr array 

variable, first element is set to 

163.4

*****************

** warning **

*****************

*chbd* Junction boundary error detected

 Comp= valve ;num= 77; junction= 59; variable= friction factor 

 Junction left side= 3.568685E+00; right side= 1.976502E-02

For tee with ID 75, kfac2 array 

variable, second element is set 

to 163.4

For valve with ID 77, kfac array 

variable, first element is set to 

163.4

Table 17 Actions to Supress TRACE Warnings, Alerts and Errors

3.3 Thermal-Hydraulic Adjustment for Steady State Simulation

This case (steady state simulation) begins with the input deck obtained directly following all steps 

in section 3.1 and 3.2. When all model translation is properly checked, the input deck is further 

modified. To this end, some flow adjustments must be made. The code-to-code comparison for 

this model is done using the TRAC-BF1 steady state results. To get a good agreement in flow 

distribution some friction factors are changed. See Table 18 for detailed information.
3-11

Component Variable Element Original value Changed value

Channels with ID: 2, 4,6, 

10, 12, 15, 26, 28, 30, 31, 

32

kfac 1 31.2 34.0

Channels with ID: 17, 18, 

33 kfac 1 191.07 113.07

All Channels kfac 15, 18, 21,

24 and 27

1.08497

1.2141

1.2341

0.0

Jet Pump 82

fa1 1 2.5084E-2 3.6484E-2

2 2.9358E-2 4.8358E-2

fa2 1 4.2740E-3 3.9050E-3

2 3.0434E-2 1.5434E-2

Pump 83

ipmpty

ipmptr

npmpmt

pmpmt

- 

3

0 

2 

-

1

1 

delete variable

delete table

Vessel 99

level 4 cfzlz

cfzvz

1 

1 

9.0

9.0

6.0

6.0

level5 cfzlz

cfzvz

1 

1 

30.0

30.0

18.5

18.5

Table 18 Changes Made to Adjust the Thermal-Hydraulic Steady State Model

As seen in Table 18, for all channel components, the last five friction factors, representing last 

grids, were removed. The reason for this change was that otherwise some oscillations were 

observed in the axial void fraction distribution, obviously unreal in a BWR.

In consideration of the effort done to adjust the model, and for sake of simplicity, hereafter an 

algorithm for automatic model adjustment is proposed for future works, see Figure 8. Please, note 

that ∝ is used to indicate that two values are proportional.

1. The pump flow is adjusted changing the side tube flow area in the jet pump (first node

only).

2. The main tube flow area in the jet pump (first and second nodes only) is modified to

achieve the total core flow rated value.

3. The bypass flow is adjusted by means of the friction factor in vessel levels 4 and 5. Steps

1 to 3 are repeated iteratively until TRAC-BF1 values were obtained.

4. The flow in each channel is modified using the friction factor in first level, until a good flow

distribution through the whole core is obtained.

5. To get the desired vapor mass flow in the dome, the reverse friction factor for vapor in

levels 7 and 8 are changed. The whole process (steps 1 to 5) is repeated iteratively until

proper convergence is achieved.
3-12

Figure 8 Proposed Algorithm for Automatic Thermal-Hydraulic Adjustment in PBTT 

Model

YES

Component: jetp 82

Variable: fa2 (1st node)

Until: agreement in Qpump

fa2 ∝ Qpump

Component: vessel 99

Variable: cfzlz(1), cfzvz(1)

Level: 4 & 5

Until: agreement in Qbypass

cfzlz, cfzvz ∝ 1/Qbypass

Flow convergence criteria 

met? (Flow in pump, jet pump 

and bypass)

NO

Component: all channels

Variable: kfaci(1)

Until: agreement in Qchan_i

kfac ∝ 1/Qchan_i

Component: jetp 82

Variable: fa1 (1st & 2nd nodes)

Until: agreement in Qcore

fa1 ∝ Qcore

Component: vessel 99

Variable: cfrvz(1)

Level: 7 & 8

Until: agreement in Qdome,vapor

cfrvz ∝ Qdome,vapor
3-13

3.4 Thermal-Hydraulic Adjustment for Transient State Simulation

For the second case (transient state simulation), as in the first case, the input deck obtained 

following steps in section 3.1 and 3.2 is used. The modifications in the thermal-hydraulic model 

to simulate the Peach Bottom Turbine Trip NEA Benchmark are presented in this section, see 

Table 19. Therefore, the second case is validated using the Benchmark results. In order to obtain 

a good steady state convergence, a null transient spanning 50 seconds is simulated prior to the 

turbine trip accident. Thus, all times in TRACE model are shifted 50 seconds.

Component Variable Element Original value Changed value

All Channels kfac 15, 18, 21,

24 and 27

1.08497

1.2141

1.2341

0.0

Jet Pump 82

fa1 1 2.5084E-2 3.5794E-2

2 2.9358E-2 3.7258E-2

fa2 1 4.2740E-3 3.9540E-3

2 3.0434E-2 3.0434E-2

Pump 83

ipmpty

ipmptr

ipmpsv

npmptb

pmptb

npmpmt

pmpmt

- 

3

0 

1 

0 

- 

2 

-

1

1 

45

14

see Figure 9

delete variable

delete table

Vessel 99

level 7 cfrlz

cfrvz

1 

1 

5.0

5.0

0.0

1E4

level8 cfrvz 1 5.0 1E4

Table 19 Changes Made to Adjust the Thermal-Hydraulic Transient State Model

Due to the fact that there are no information about rotational pump speed in Benchmark 

specifications, the rotational pump velocity in TRACE table (second case) is obtained from TRACBF1 transient simulation results (first case), see Figure 9. Thus, in order to simulate a real 

transient state in the recirculation pump with ID 83, according to Table 19, ipmpty variable was 

changed from 3 (pump controlled by control block, TRAC-B value) to 1 (pump speed obtained 

from table). Then, a table containing pump-impeller rotational speed vs time is set in TRACE input 

deck. This table makes use of npmptb and pmptb variables, number of data pairs and its time vs. 

velocity points respectively.
3-14

Figure 9 Pump-Impeller Rotational Speed, TRACE and TRAC-BF1 Comparison

0 1 2 3 4 5

100

105

110

115

120

125

130

135

140

Pump-impeller rotational speed

Time (s)

Speed (rad/s)

TRACB

TRACE
4-1 

4. RESULTS

4.1 Steady State

In this section, the results for the first case (steady state) are code-to-code compared against 

TRAC-BF1. In Table 20, the mass flow through the main system points are compared.

Location TRACE (kg/s) TRAC-BF1 (kg/s) Abs Difference (kg/s)

Fill 982 982 0

Downcomer 9895 9845 50

Jet Pump inlet 7075 7038 37

Pump 2820 2820 0

Bypass 597 606 9

Core 9297 9245 52

Separator

Inlet (vap.) 991 759 232

Inlet (liq.) 8903 9076 173

Main Outlet (vap.) 960 979 19

Main Outlet (liq.) 0 95 95

Lateral Outlet (vap.) 27 34 7

Lateral Outlet (liq.) 8908 8713 195

Steam line 986 979 7

Table 20 Main Components Flow for TRACE and TRAC-BF1 Comparison

Once the total flow inside vessel agrees in both models, the flow inside each channel is compared. 

As explained in Figure 8, to modify the flow through one channel, the first friction factor is modified 

according to the desired change. Figure 10 shows the core flow distribution in TRACE model and 

its absolute difference compared with TRAC-BF1.

Figure 10 Core Flow Distribution in TRACE Model (Left) and Absolute Difference 

Between TRACE and TRAC-BF1 (Right)
4-2 

Other variables, such as the liquid temperature or the void fraction could be compared. The latter

is important for BWR because it has a strong effect on the separator and the neutronic 

calculations. It is also important to consider the axial distribution, because it is the dimension 

where the gradients are bigger. Figure 11 shows the mean core liquid temperature (left) and mean 

void distribution (right), both inside the core. Both charts compare TRAC-BF1 and TRACE codes.

Figure 11 Mean Axial Moderator Temperature (Left) and Void Fraction (Right) 

Distribution

4.2 Transient State (Turbine Trip)

In Figure 12, the axial power profile for the core average before the accident is depicted in the left 

side. It corresponds to the initial transient simulation time. Results are shown for TRACE/PARCS 

coupled codes and reference (plant) code. On the right side, the total reactor power (relative to 

initial power) for the whole transient simulation. A 50-seconds null transient is simulated.

However, the accident is shown at time 0 s in subsequent figures. Results are shown for 

TRACE/PARCS and Benchmark data.

5 10 15 20 25

544

546

548

550

552

554

556

558

Mean moderator temperature distribution

Node

Temperature (K)

TRACB

TRACE

5 10 15 20 25

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Mean void fraction distribution

Node

Void fraction (-)

TRACB

TRACE
4-3 

Figure 12 Core Average Axial Power Profile TRACE/Plant Data (Left) and Relative 

Total Reactor Power (Right) TRACE/Benchmark Data

Moreover, Figure 13 shows the pressure at dome (upper vessel cell) and core exit (vessel cell 

just after the channels). TRACE/PARCS and Benchmark results are shown. Besides, Doppler 

reactivity, moderator reactivity and total reactivity are shown in Figure 14 and Figure 15. 

Figure 13 Dome (Left) and Core Exit (Right) Pressure, TRACE/Benchmark Data

0 5 10 15 20 25 30

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Normalized axial power profile

Node

Power (-)

TRACE/PARCS

SIMULATE

0 1 2 3 4 5

0

2

4

6

8

10

12

Relative total reactor power

Time (s)

Power (W)

FZD

PSU/PUR/NRC

TEPSYS

UPISA

UPV

WES

TRACE/PARCS

0 1 2 3 4 5

0

0.2

0.4

0.6

0.8

1

1.2

Dome pressure

Time (s)

Pressure delta change (MPa)

CEA

FANP

FZD

GRS

NFI

NUPEC

PSI

PSU/PUR/NRC

TEPSYS

UPISA

UPV

WES

TRACE/PARCS

0 1 2 3 4 5

0

0.2

0.4

0.6

0.8

1

1.2

Core exit pressure

Time (s)

Pressure delta change (MPa)

CEA

FANP

FZD

GRS

NFI

NUPEC

PSI

PSU/PUR/NRC

TEPSYS

UPISA

UPV

WES

TRACE/PARCS
4-4 

Figure 14 Doppler (Left) and Moderator (Right) Reactivity, TRACE/Benchmark Data

Figure 15 Total Reactivity, TRACE and Benchmark Results

0 1 2 3 4 5

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Doppler reactivity

Time (s)

Reactivity (-)

Benchmark

TRACE/PARCS

0 1 2 3 4 5

-1.5

-1

-0.5

0

0.5

1

Moderator reactivity

Time (s)

Reactivity (-)

Benchmark

TRACE/PARCS

0 1 2 3 4 5

-2

-1.5

-1

-0.5

0

0.5

1

Total reactivity

Time (s)

Reactivity (-)

Benchmark

TRACE/PARCS
5-1 

5. CONCLUSIONS

A new semi-automatic translation procedure for translating models from TRAC-BF1 to TRACE is 

presented in this publication. It is explained how TRACE executable and SNAP tool could be used 

in order to achieve this translation. However, these tools are not completely error-free and 

afterwards several handmade corrections need to be made. The main corrections are also 

explained in this study. Finally, two different cases are simulated to withdraw some conclusions: 

a steady state case (TRACE and TRAC-BF1 comparison) and a transient state case (TRACE and 

Benchmark comparison).

As explained in this publication, after the conversion, some adjustments need to be made to the

TRACE input deck in order to achieve a good thermal-hydraulic agreement between codes, see 

Table 7 to Table 17. Nevertheless, a huge amount of time is saved in comparison to begin a 

complete handmade translation from scratch. In addition, an algorithm for an automatic thermalhydraulic adjustment is proposed, see Figure 8. 

According to the data shown in Table 20, the flow through the main components is in agreement 

between codes. Due to the fact that the separator component has a strong effect on the thermalhydraulic calculations, some discrepancies could be found on Table 20 regarding this component. 

The algorithm depicted in Figure 8 was followed to reach flow concordance. Thus, a good 

agreement can be seen in the core flow distribution (Figure 10), mean axial moderator 

temperature distribution and mean axial void fraction distribution (Figure 11). Special attention 

must be paid on the void fraction distribution since this variable determines the separator behavior 

after the core outlet. Moreover, in case of neutronic coupling, it can have a strong effect on the 

neutronic calculations. It is important the fact that the last five friction factors in channel 

components were suppressed, see Table 18, otherwise, some oscillations were observed in the 

axial void fraction distribution, obviously unreal in a BWR.

Regarding the transient simulation, a turbine trip accident is simulated successfully. Power 

profiles (1D and 0D) are presented in Figure 12. According to these figure, TRACE/PARCS result 

has a good agreement with the reference code (plant code). However, the total reactor power, 

shown reveals that the first power peak is slightly over predicted with respect to Benchmark 

participants. Besides, the dome pressure, exit core pressure and different reactivity components 

are in line with Benchmark participants (Figure 13 to Figure 15).

In general, the obtained results are in agreement with TRAC-BF1 for the steady state case, and 

Benchmark results for the transient state case.

Finally, just to mention that this work tried to present a fast general procedure for the conversion 

of legacy TRAC-BF1 input decks to TRACE, and in any case represents a new contribution to the 

results of the NEA Boiling Water Reactor Turbine Trip (TT) Benchmark. This Benchmark was 

selected for this work because availability of the data for both steady state and transient

scenarios.

6-1 

6. REFERENCES

[1] TRAC-BF1/MOD1: An Advanced Best Estimate Computer Program for Boiling Water 

Reactor Accident Analysis. Volume 1: Model Description. 1992.

[2] TRACE v5.0 User’s Manual. Volume 1: Input Specification. Nuclear Regulatory 

Commission.

[3] PARCS v2.7 U.S. NRC Core Neutronics Simulator USER MANUAL. August, 2006.

[4] PARCS v3.0 U.S. NRC Core Neutronics Simulator USER MANUAL. November, 2010.

[5] Symbolic Nuclear Analysis Package (SNAP). User's Manual. 2007.

[6] Boiling Water Reactor Turbine Trip (TT) Benchmark - Volume I, Benchmark Specification 

(25 June 2001) by J. Solis, K. Ivanov, B. Sarikaya, A. Olson and K.W. Hunt (final printed 

version), NEA/NSC/DOC(2001)1, ISBN 92-64-18470-8.

[7] Boiling Water Reactor Turbine Trip (TT) Benchmark - Volume IV: Summary Results of 

Exercise 3, by Bedirhan Akdeniz, Kostadin N. Ivanov and Andy M. Olson, OECD 2010, 

NEA/NSC/DOC(2010)1, ISBN 978-92-64-99137-8.

[8] TRAC-BF1/MOD1: An Advanced Best Estimate Computer Program for Boiling Water 

Reactor Accident Analysis. Volume 2: User’s Guide. 1992.





NUREG/IA-0461 TRAC-BF1 to TRACE Model Semi-Automatic Conversion. PBTT Example