NUREG/IA-0441 

International 

 Agreement Report

Assessment Against ACHILLES Reflood Experiment 

with TRACE V5.0 Patch3 

Prepared by:

O. A. Berar, A. Prošek

Jožef Stefan Institute

Jamova cesta 39

Sl-1000 Ljubljana, Slovenia

Kirk 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 2013 

Date Published: February 2014

Prepared as part of 

The Agreement on Research Participation and Technical Exchange

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

Published by

U.S. Nuclear Regulatory Commission
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NUREG/IA-0441 

International 

 Agreement Report

Assessment Against ACHILLES Reflood Experiment 

with TRACE V5.0 Patch3 

Prepared by:

O. A. Berar, A. Prošek

Jožef Stefan Institute

Jamova cesta 39

Sl-1000 Ljubljana, Slovenia

Kirk 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 2013 

Date Published: February 2014

Prepared as part of 

The Agreement on Research Participation and Technical Exchange

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

Published by

U.S. Nuclear Regulatory Commission

 iii 

ABSTRACT 

At present time, the development of the U.S. Nuclear Regulatory Commission RELAP5 has 

been ceased in favor of the newer TRACE best estimate system code. The present work 

presents the assessment of the Achilles natural reflood experiment with the TRACE V5.0 Patch 

3. In addition, comparison calculations with TRACE V5.0 Patch 1 and latest RELAP5/MOD3.3 

Patch 4 were performed. The TRACE input deck has been obtained by conversion of the 

already existing RELAP5 input deck. The calculated results were compared against the 

experimental data. The results show that both TRACE and RELAP5 are capable of reproducing 

the reflood phenomena at a satisfactory level. However, some discrepancies between the 

predicted variables and the experimental data suggest further investigation of the TRACE 

reflood model. 

 v 

CONTENTS

Page

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

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

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

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

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

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

2. THE ACHILLES NATURAL REFLOOD EXPERIMENT DESCRIPTION ............................... 3

3. JSI RELAP5 TO TRACE INPUT DECK CONVERSION METHOD ....................................... 5

4. RELAP5 AND TRACE INPUT MODELS ................................................................................ 7

4.1 Achilles facility RELAP5 input model ............................................................................. 7

4.2 Achilles facility TRACE input deck conversion and model description ........................... 8

5. RESULTS ............................................................................................................................. 11

5.1 Achilles calculations ..................................................................................................... 11

5.2 Achilles natural reflood experiment animation model ................................................... 15

5.3 Results discussion ....................................................................................................... 19

6. RUN STATISTICS ................................................................................................................ 21

7. CONCLUSIONS .................................................................................................................... 23

8. REFERENCES ...................................................................................................................... 25

 vii 

FIGURES 

Page 

Figure 1 Schematic of Achilles separate effect test facility (adaptation from Ref. 5) ................ 3

Figure 2 Achilles facility fuel simulator section view (adapted from Ref. 5) .............................. 4

Figure 3 JSI RELAP5 to TRACE input deck conversion method.............................................. 6

Figure 4 Achilles facility RELAP5/MOD2 input model nodalization .......................................... 7

Figure 5 SNAP Hydraulic Components view of RELAP5/MOD3.3 Achilles facility model ........ 8

Figure 6 SNAP Hydraulic Components view of Achilles facility TRACE model ........................ 9

Figure 7 TRACE heat structure radial mesh points location for the Achilles facility fuel 

simulator (adapted from Ref. 5) ................................................................................ 10

Figure 8 Rod cladding temperature at 1.084 m elevation ....................................................... 11

Figure 9 Rod cladding temperature at 2.01 m elevation ......................................................... 12

Figure 10 Rod cladding temperature at 3.176 m elevation ....................................................... 12

Figure 11 Quench front progression ......................................................................................... 13

Figure 12 Core collapsed liquid level ........................................................................................ 13

Figure 13 Pressure evolution in the upper plenum ................................................................... 14

Figure 14 Downcomer collapsed liquid level ............................................................................ 14

Figure 15 Pressure in the nitrogen vessel ................................................................................ 15

Figure 16 Simple SNAP nodalization animation mask of the Achilles natural reflood TRACE 

calculation ................................................................................................................. 16

Figure 17 Basic SNAP nodalization animation mask of the Achilles natural reflood TRACE 

calculation ................................................................................................................. 16

Figure 18 Control system and mass balance SNAP animation mask for the Achilles natural 

reflood TRACE calculation ....................................................................................... 17

Figure 19 Facility view SNAP animation mask for the Achilles natural reflood TRACE 

calculation ................................................................................................................. 18

Figure 20 Core condition animation mask for the Achilles natural reflood TRACE calculation 19

Figure 21 TRACE V5.0 Patch 3 predictions for core collapsed liquid level and downcomer 

collapsed liquid level using semi-implicit and SETS methods .................................. 20

 ix 

EXECUTIVE SUMMARY 

At present time, the development of RELAP5 has been ceased in favor of the newer US NRC 

TRACE best estimate code and it is expected that, in the future, TRACE will be one of the main 

codes used for performing nuclear power plant thermal-hydraulic safety analysis. Therefore, the 

importance of assessing the TRACE code capability to predict various thermal-hydraulic 

transients in reactor systems becomes evident. One such transient that can occur in 

pressurized water reactor is the core reflooding, following a large break loss of coolant accident. 

The core reflood is of particular interest for code assessment as it requires the system code to 

accurately predict specific core heat transfer and two-phase flow phenomena. 

The present work presents the assessment of the Achilles natural reflood experiment with the 

TRACE V5.0 Patch 3. In addition, comparison calculations with TRACE V5.0 Patch 1 and latest 

RELAP5/MOD3.3 Patch 4 were performed. Given the fact that the TRACE Achilles natural 

reflood input model was obtained from the previously developed RELAP5 input model using the 

JSI RELAP5 to TRACE input deck conversion method, a RELAP5 and TRACE code 

comparison was also performed, using the calculation results of the above mentioned code 

versions. Mainly, this conversion method is based on SNAP and enables more direct 

comparison between the RELAP5 and TRACE calculations, given the same origin of data 

(RELAP5 input deck) and the same model geometry. 

The Achilles natural reflood experiment was simulated using TRACE and RELAP5. Such 

TRACE and RELAP5 code comparison to experimental data can provide valuable insight into 

the advancement of code capability as the TRACE code is intended to consolidate and further 

extend the capabilities of the RELAP5 code. Thus the presented work contributes to the TRACE 

code assessment, by particularly addressing the TRACE V5.0 Patch 3 code capability to predict 

the reflood phenomena, and also by performing a code comparison against the latest 

RELAP5/MOD3.3 Patch 4 code. 

The results show that both TRACE and RELAP5 are capable of reproducing the reflood 

phenomena at a satisfactory level. However, some discrepancies between the predicted 

variables and the experimental data suggest further investigation of the TRACE reflood model. 

 xi 

ACKNOWLEDGEMENTS 

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

Nuclear Safety Administration within CAMP program (project no. POG-U3-KE-R4/104/12 – also 

NEK no.: 3120118) and from the state budget by the Slovenian Research Agency (program no. 

P2-0026). 

 xiii 

ABBREVIATIONS 

AEA Atomic Energy Authority 

ASCII American Standard Code for Information Interchange 

ISP International Standard Problem 

JSI Jožef Stefan Institute 

LB LOCA Large Break Loss of Coolant Accident 

NPP Nuclear Power Plant 

NRC Nuclear Regulatory Commission 

PWR Pressurized Water Reactor 

RBHT Rod Bundle Heat Transfer 

RELAP Reactor Excursion and Leak Analysis Program 

SNAP Symbolic Nuclear Analysis Package 

TRACE TRAC/RELAP Advanced Computational Engine 

US NRC United States Nuclear Regulatory Commission 

1 

1. INTRODUCTION

The RELAP5 thermal-hydraulic safety analysis code is one of the best-estimate system 

codes that were developed by the US NRC for light water reactors thermal-hydraulic 

analysis. The first RELAP5 version was released in 1979 and has evolved continuously, 

through the development of new models and improvement of existing ones. This 30 years 

continuous evolution culminated with the release of the latest version RELAP5/MOD 3.3 

Patch 4 in 2010 (Ref. 1). In the past, the RELAP5 computer code was one of the most used 

system codes in the international community. At present time, the development of RELAP5 

has been ceased in favor of the newer US NRC TRACE best estimate code (Ref. 2) and it is 

expected that, in the future, TRACE will be one of the main codes used for performing 

nuclear power plant (NPP) thermal-hydraulic safety analysis. Therefore, the importance of 

assessing the TRACE code capability to predict various thermal-hydraulic transients in 

reactor systems becomes evident. One such transient that can occur in pressurized water 

reactor (PWR) is the core reflooding, following a large break loss of coolant accident (LB 

LOCA). The core reflood is of particular interest for code assessment as it requires the 

system code to accurately predict specific core heat transfer and two-phase flow phenomena 

(Ref. 3, Ref. 4). 

In the present work the simulation of the Achilles natural reflood experiment (Ref. 5) with 

RELAP5/MOD3.3 Patch 4, TRACE V5.0 Patch 1 (version V5.141) and the latest released 

TRACE V5.0 Patch 3 (version V5.333) is presented and the calculation results are compared 

against experimental data and discussed. Given the fact that the TRACE Achilles natural 

reflood input model was obtained from the previously developed RELAP5 input model using 

the JSI RELAP5 to TRACE input deck conversion method (Refs. 6 and 7), a RELAP5 and 

TRACE code comparison was also performed, using the calculation results of the above 

mentioned code versions. Mainly, this conversion method is based on SNAP and enables 

more direct comparison between the RELAP5 and TRACE calculations, given the same 

origin of data (RELAP5 input deck) and the same model geometry. 

The assessment of the TRACE reflood model was performed in the past against the GOTA 

Reflood test 43, FLECHT-SEASET Forced Reflood Tests and RBHT tests (Ref. 8) and the 

Achilles natural reflood experiment (Ref. 9). However, the latter work presents the Achilles 

natural reflood TRACE V5.211 calculation results using a model of the test bundle and 

shroud vessel only, with inlet and outlet boundary conditions imposed by a FILL and BREAK 

component respectively. The published works did not address the Achilles natural reflood 

experiment simulation using the latest TRACE V5.0 Patch 3 code employing a detailed 

model of the Achilles test rig. Also, TRACE and RELAP5 code comparison can provide 

valuable insight into the advancement of code capability as the TRACE code is intended to 

consolidate and further extend the capabilities of the RELAP5 code. Thus the presented work 

contributes to the TRACE code assessment, by particularly addressing the TRACE V5.0 

Patch 3 code capability to predict the reflood phenomena, and also by performing a code 

comparison against the latest RELAP5/MOD3.3 Patch 4 code. 

3 

2. THE ACHILLES NATURAL REFLOOD EXPERIMENT 

DESCRIPTION 

The Achilles natural reflood experimental study was performed at AEA Winfrith Technology 

Center, UK in 1991 aiming at investigating the end phase of the accumulator injection in the 

primary system of a PWR and the heat transfer in the core during the reflood phase of a 

postulated LB LOCA. The Achilles natural reflood experiment was the basis for the International 

Standard Problem no. 25 (ISP-25) (Ref. 5). A simplified schematic of Achilles separate effect 

test facility is presented in Figure 1. 

Figure 1 Schematic of Achilles separate effect test facility (adaptation from Ref. 5) 
4 

The simulated core test section comprised of 69 rods corresponding to Westinghouse 17x17 

geometry was electrically heated over a length of 3.66 m. The electrical rod simulators had a 

diameter of 9.5 mm and were comprised of a kanthal wire heating coil, an inner insulating layer 

of boron nitride and an inconel 600 sheath (see Figure 2). The rod simulators, together with the 

temperature and pressure measurement instrumentation were housed inside the shroud vessel. 

A centrifugal steam separator located at the top of the shroud vessel discharges the test section 

output liquid flow. Additionally, a steam separator located downstream of shroud vessel, 

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 flow losses. The liquid is separated from the flow exiting the upper part of the 

downcomer by means of another steam separator and the output gas phase flows through a 

second orifice that simulates the cold leg associated flow losses. The experimental procedure 

started with a full water inventory in downcomer and no water in the test section vessel. The rig 

is heated and circulated with steam until saturation temperature and pressure are attained 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 simulated 

core. After the decay of the flow oscillations that are occurring at this point in the transient, the 

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

been quenched. 

Figure 2 Achilles facility fuel simulator section view (adapted from Ref. 5) 
5 

3. JSI RELAP5 TO TRACE INPUT DECK CONVERSION METHOD

The JSI RELAP5 to TRACE input deck conversion method (Refs. 6 and 7) is based on SNAP 

and covers a wide range of aspects related to input deck conversion, input data verification and 

post-processing of the calculation results. This method also brings some additional benefits to 

the analyst. Firstly, more direct comparison between RELAP5 and TRACE analysis is possible, 

given the same origin of data (RELAP5 input deck) and consequently, the same model 

geometry. Secondly, using SNAP post-processing capabilities enables displaying the calculation 

results in an advanced way, in a straight foreword procedure, using the Hydraulic Components 

view model in SNAP (Refs. 10 and 12). The JSI RELAP5 to TRACE input model conversion 

method consists of 15 steps and is illustrated in Figure 3. 
6 

Figure 3 JSI RELAP5 to TRACE input deck conversion method 
7 

4. RELAP5 AND TRACE INPUT MODELS 

4.1 Achilles facility RELAP5 input model 

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

JSI, in the framework of ISP-25 participation (Ref. 13). The original RELAP5/MOD2 nodalization 

consists of 50 volumes, 50 junctions and 13 heat slabs and is presented in Figure 4. The 

nitrogen vessel is modeled by an accumulator component and the assumption is made that this 

component is not initially in injection mode. For that reason a valve was placed in between the 

accumulator and top of the downcomer. The accumulator was modeled as containing nitrogen 

only. The separators were modeled by RELAP5 branch components. 

So prepared, the RELAP5/MOD2 model served for adaptation to RELAP5/MOD3.3 model 

(Figure 3, step 1). Some additional errors were identified while running the modified Achilles 

facility input deck under RELAP5/MOD3.3 code, mostly related to heat structure boundary data. 

Consequently, some additional left and right boundary data were added for the heat structure. 

The RELAP5/MOD3.3 ASCII input deck was imported into SNAP Model Editor (Figure 3, step 2) 

and the Hydraulic Components view was generated and manually arranged (Figure 3, Step 3) in 

order to provide a clear graphical representation of the RELAP5/MOD3.3 model nodalization. 

The RELAP5/MOD3.3 SNAP nodalization is presented in Figure 5. 

Figure 4 Achilles facility RELAP5/MOD2 input model nodalization 
8 

Figure 5 SNAP Hydraulic Components view of RELAP5/MOD3.3 Achilles facility model 

4.2 Achilles facility TRACE input deck conversion and model description 

The conversion of the RELAP5 input model to TRACE input model was performed in SNAP 

using the RELAP5 to TRACE conversion tool (Figure 3, Step 7). After checking for and 

resolving all errors reported in SNAP (Figure 3, Step 8), a model notebook for the TRACE 

model (Figure 3, Step 9) was generated in order to perform a comparison of the RELAP5 model 

against the converted TRACE model and several inconsistencies were identified in the 

converted model. The nitrogen vessel modeled by an Accumulator component in the RELAP5 

input model was converted to Liquid separator pipe type in TRACE. The pipe type was manually 

changed to Accumulator type. The Accumulator surge line present in the RELAP5 input model 

as a part of the Accumulator component was not preserved in the converted TRACE model and 

was modeled using a Pipe component. The nitrogen non-condensable gas option and reflood 

model option were not successfully converted to the TRACE input model, and were specified 

accordingly. The hydraulic diameters for the Hydraulic Components in the TRACE model were 

either recalculated or obtained from the RELAP5/MOD3.3 calculation output file. 

The Achilles facility SNAP Hydraulic Components view for the TRACE input is presented in 

Figure 6 and it comprises of 36 hydraulic components with a core test section nodalization 

divided in 13 cells, and a downcomer nodalization of 11 cells. 
9 

A TRACE heat structure component is used to model the 69 electrical rod simulators. As the 

outside surface boundary condition for each axial level is given by the thermal-hydraulic cells of 

the Pipe component that models the test section, the heat structure is subdivided into 13 axial 

nodes. In the radial direction, 6 radial mesh points are used, corresponding to 5 material regions 

(Figure 7). The rod simulator inconel 600 sheath, boron nitride insulator and kanthal heating coil 

are modeled in TRACE by specific material properties tables comprising of density, specific heat 

and thermal conductivity as a function of temperature. The bundle power in the core simulator is 

specified using a TRACE POWER component. The reflood phenomena are simulated using the 

TRACE heat structure reflood model. The fine mesh reflood option was specified employing 500 

maximum axial nodes with a minimum node distance of 7 mm. The semi-implicit numerical 

method is used for the TRACE calculation. 

Figure 6 SNAP Hydraulic Components view of Achilles facility TRACE model 
10 

Figure 7 TRACE heat structure radial mesh points location for the Achilles facility fuel 

simulator (adapted from Ref. 5) 
11 

5. RESULTS 

5.1 Achilles calculations 

The TRACE calculations data were compared against the experimental data and the 

RELAP5/MOD3.3 Patch4 calculation data as shown in Figures 8 through 14. The experimental 

data are labeled as “exp”, RELAP5/MOD3.3 Patch4 calculation data as “R5/MOD3.3”, TRACE 

V5.0 Patch 1 calculation data as “TRACEp1” and the TRACE V5.0 Patch 3 calculation data as 

“TRACEp3”. 

During the reflood phase, the thermal behavior of the electrically heated rods is characterized by 

an initial gradual rise in temperature, followed by a temperature turn-around time when the peak 

cladding temperature is reached and a quench time, when the temperature of the rod drops in a 

very steep manner. This sudden drop in temperature is a consequence of an increase in heat 

removal rate due to the transition from dispersed droplet flow film boiling to nucleate boiling 

(Ref. 4). The TRACE V5.0 Patch 1 and Patch 3 calculations provide good agreement with the 

experimental data for the peak cladding temperatures at low and medium elevations in the test 

section, as presented in Figure 8 and Figure 9, showing rod cladding temperature evolution at 

1.084 meters and 2.01 meters elevation in the test section, respectively. Although the 

RELAP5/MOD3.3 Patch4 is slightly over-predicting the peak cladding temperature for the 1.084 

meters elevation in the test section (Figure 8), the predictions of the peak cladding temperature 

for middle (Figure 9) and higher test section locations (Figure 10) are in very close agreement 

with the experimental values. The peak cladding temperature at 2.01 meters (Figure 9) is of 

particular interest as the highest experimental peak cladding temperature was achieved at this 

elevation. For higher elevations in the test section the TRACE V5.0 Patch 1 calculation is 

slightly over-predicting the peak cladding temperature but the overall agreement with the 

experimental data in the first part of the heating up phase is maintained, as shown in Figure 10 

that describes the cladding temperature evolution at 3.176 meters elevation in the test section. 

Figure 11 presents the quench front progression. For this parameter, the RELAP5/MOD 3.3 

provides a closer agreement to the experimental values for the middle transient interval. 

Figure 8 Rod cladding temperature at 1.084 m elevation 
12 

Figure 9 Rod cladding temperature at 2.01 m elevation 

Figure 10 Rod cladding temperature at 3.176 m elevation 
13 

Figure 11 Quench front progression 

The core collapsed liquid level predicted by RELAP5/Mod3.3 Patch4 and the TRACE versions is 

in reasonable agreement with the experimental results, although the magnitude of the level 

oscillations is greater than in the experimental results, as shown in Figure 12. The core 

collapsed liquid level is in close connection with the pressure conditions predicted in the core as 

the phase shift occurs in the coolant that is entering the test section. The pressure condition in 

the upper plenum predicted with RELAP5 and TRACE is presented in Figure 13. The higher 

pressure predicted by RELAP5/MOD3.3 Patch4 in the first 30 seconds of the transient 

represents the reason for the initial core collapsed liquid level mismatch in comparison to the 

experimental results (Figure 12). TRACE Patch 1 and Patch 3 calculations of pressure at the 

top of the upper plenum (Figure 13) and core collapsed liquid level (Figure 12) provide closer 

agreement to the experimental data for the early stages of the transient. 

Figure 12 Core collapsed liquid level 
14 

Figure 13 Pressure evolution in the upper plenum 

The evolution of the downcomer collapsed liquid level is presented in Figure 14. The oscillatory 

behavior observed in the RELAP5 and TRACE calculated core collapsed liquid level is also 

noted in the calculated downcomer collapsed liquid level. However, it is observable that the 

TRACE calculations, and in particular the TRACE V5.0 Patch 1 results, provide the best match 

to the initial level swelling. 

Figure 14 Downcomer collapsed liquid level 

Finally, the pressure in the nitrogen vessel is shown in Figure 15. Calculated pressure of both 

TRACE versions is in good agreement with the experimental data. 
15 

Figure 15 Pressure in the nitrogen vessel 

5.2 Achilles natural reflood experiment animation model 

Several SNAP animation masks were developed for the Achilles natural reflood experiment 

TRACE model, graphically displaying the evolution of different phenomena during the 400 

seconds transient. 

The simple animation mask shown in Figure 16 contains the SNAP Hydraulic Components View 

of the TRACE model transferred to a SNAP animation model by simple “Copy-Paste” method. 

The basic animation mask is similar, however the mask was further elaborated in the Model 

Editor. A snapshot of such animation mask for the Achilles natural reflood TRACE V5.0 Patch 3 

calculation at 152.3 seconds into the transient is presented in Figure 17. 

In Figure 18 is presented a snapshot of the control system and fluid mass balance animation 

mask at 39.1 seconds into the transient. Figure 19 presents a snapshot of the TRACE V5.0 

Patch 3 calculation at 244.5 seconds into the transient. In a similar approach, a core condition 

animation mask was generated, displaying the void fraction in the test section, core collapsed 

liquid level, rod temperatures and quench front progression. A snapshot of this animation mask 

is presented in Figure 20, displaying the TRACE V5.0 Patch 3 calculation data for 98.1 seconds 

into the transient. 
16 

Figure 16 Simple SNAP nodalization animation mask of the Achilles natural reflood 

TRACE calculation 

Figure 17 Basic SNAP nodalization animation mask of the Achilles natural reflood 

TRACE calculation 
17 

Figure 18 Control system and mass balance SNAP animation mask for the Achilles 

natural reflood TRACE calculation 
18 

Figure 19 Facility view SNAP animation mask for the Achilles natural reflood TRACE 

calculation 
19 

Figure 20 Core condition animation mask for the Achilles natural reflood TRACE 

calculation 

5.3 Results discussion 

Our previous studies employing RELAP5 to TRACE conversions using SNAP usually show that 

the TRACE calculation results are in general comparable to the RELAP5 calculation results 

(Ref. 6). This finding also applies in the case of Achilles natural reflood experiment TRACE and 

RELAP5 calculation results. 

Nevertheless in the case of TRACE computer code Achilles natural reflood calculation, few 

observations can be made. Both TRACE V5.0 Patch 1and Patch 3 calculations present a 

tendency to under predict the quench time for the test sections located at low and middle 

elevations in the core (Figure 8 and Figure 9). This under-prediction of the quench times 

calculated by TRACE V5.0 Patch 1 and Patch 3 is in agreement with the conclusions of the 

TRACE assessment report (Ref. 8) which states that TRACE has a tendency to under predict 

quench times for low flow, low power cases. The tendency of TRACE to under-predict the 

quench times can be also observed in Figure 11 presenting the quench front progression. The 

RELAP5/MOD3.3 Patch4 calculation is providing closer agreement to the experimental values 

for the 40 seconds to 300 seconds interval. However, the RELAP5 calculation data deviates 

from the experimental trend towards the end of the transient, while the TRACE V5.0 Patch 3 

calculation is converging to the experimental quench front location. Compared to the TRACE 

V5.0 Patch 3, the TRACE V5.0 Patch 1 calculation shows slightly better overall estimation of the 

quench front progression but exceeds the experimental quenching time for the last part of the 

transient. 
20 

The inherent oscillatory behavior of the reflood phenomena that was also observed in the 

experiment is present in both RELAP5 and TRACE calculations. The calculated magnitude of 

oscillations for the downcomer and core collapsed liquid levels is higher than in the 

experimental results. In the case of the TRACE V5.0 Patch 3 computer code several attempts 

were made to reduce the magnitude of the oscillations. Firstly, two different numerical methods 

for solution of the two-phase-flow equations were used. These methods are the semi-implicit 

method and the stability-enhancing two-step numerics (SETS). It was found that apart from 

some shift that is occurring in the phase of the predicted oscillations, the oscillation magnitude 

for the core and downcomer collapsed liquid level is maintained during the transient for both 

numerical methods (Figure 21). A sensitivity study was also performed in order to investigate 

the influence of the time-step size on the oscillations magnitude predicted by the TRACE V5.0 

Patch 3. In order for the code to pass the nitrogen injection phase, the variation in the time-step 

size is performed only after the initial 50 seconds of the transient, however the variation in timestep size had minimum impact on the TRACE calculation results. Taking this into consideration, 

the oscillatory behavior observed in the codes calculations could relate to the ability of the 

codes to predict heat transfer mechanisms and two-phase flow regimes that characterize the 

reflood phenomena. 

Figure 21 TRACE V5.0 Patch 3 predictions for core collapsed liquid level and 

downcomer collapsed liquid level using semi-implicit and SETS methods 
21 

6. RUN STATISTICS 

The calculations were performed using Intel® CoreTM i5 750 @ 2.676 GHz processor. The 

operating system is Windows 7 Professional. 

Table 1 shows the run statistics for the codes TRACE Patch 1, TRACE Patch 3 and 

RELAP%/MOD3.3 Patch 4 calculations. As it can be seen RELAP5 is more than ten times 

faster in calculating the Achilles reflood experiment than TRACE. 

Table 1 Run statistics 

Code Transient Time 

(s) 

CPU Time 

(s) 

CPU/Transient 

Time 

Number of Time 

Steps 

TRACE Patch 1 399.9 727.68 1.82 120448 

TRACE Patch 2 399.84 506.18 1.27 130924 

RELAP5/MOD3.3 

Patch 4 400.01 55.15 0.14 34123 

23 

7. CONCLUSIONS 

The Achilles natural reflood experiment was calculated using RELAP5/MOD3.3 Patch4, TRACE 

V5.0 Patch 1 and the latest TRACE V5.0 Patch 3. The calculation results were compared to the 

experimental data. In general the TRACE calculations reasonably agree with experimental data. 

This is especially true for peak cladding temperatures in the simulated core, proving that the 

TRACE code is capable of predicting the reflood phenomena in sufficient detail. The TRACE 

predictions for the core and downcomer collapsed liquid levels reproduce the reflood oscillatory 

nature. However, the calculated magnitudes were higher in comparison with the experimental 

data, suggesting further investigation of the TRACE code reflood model. The TRACE V5.0 

Patch 1 and Patch 3 calculation results are comparable to the RELAP5/MOD3.3 Patch4 

calculation implying the similar capabilities of the codes to model the reflood phenomena. 

Finally, the calculated results were animated using SNAP masks, which provide clear insight 

into the phenomena during transient. 

25 

8. REFERENCES

1. USNRC, 2010. RELAP5/MOD3.3 code manual Patch 04. Information Systems 

Laboratories, Inc., Rockville, Maryland, Idaho Falls, Idaho. 

2. USNRC, 2012. TRACE V5.0 Theory Manual, in: Division of Risk Assessment and 

Special Projects, Washington, DC. 

3. Abe, Y., Sudo, Y., Osakabe, M., 1983. Experimental Study of Upper Core Quench in 

PWR Reflood Phase. Journal of Nuclear Science and Technology 20, pp. 571-583. 

4. Cho, S., Park, H.-S., Choi, K.-Y., Kang, K.-H., Baek, W.-P., Kim, Y.-S., 2009. Core 

Thermal Hydraulic Behavior During the Reflood Phase of Cold-Leg LBLOCA 

Experiments using the Atlas Test Facility. Nuclear Engineering and Technology vol. 41 

pp. 1263-1274. 

5. Holmes, B.J., 1991. ISP 25 Comparison Report. AEE, Winfrith, Dorchester, UK. 

6. Prošek, A., Berar, O.-A., 2012. IJS procedure for RELAP5 to TRACE input model 

conversion using SNAP, 2012 International Congress on Advances in Nuclear Power 

Plants, Chicago, Illinois, USA, pp. 2032-2041. 

7. Prošek, A., Berar, O.A., Mavko, B., 2012. IJS procedure for converting input deck from 

RELAP5 to TRACE, in: USNRC International Agreement Report, NUREG/IA-0408. 

8. USNRC, 2008. TRACE V5.0 Assessment Manual Main Report. Division of Risk 

Assessment and Special Projects, Office of Nuclear Regulatory Research, U. S. Nuclear 

Regulatory Commission, Washington, DC. 

9. Neykov, B., Palazov, V., Downar, T., Ivanov, K., 2011. TRACE Assessment of the 

Achilles ISP-25 Reflood Transient, The 14'th International Topical Meeting on Nuclear 

Reactor Thermal Hydraulics, Toronto, Ontario, Canada. 

10. Applied Programming Technology Inc., 2011. Symbolic Nuclear Analysis Package 

(SNAP) User's Manual. 

11. Applied Programming Technology Inc., 2011. R52TRACE Plug-in User’s Manual. 

12. Prošek, A., Mavko, B., 2011. Animation model of Krško nuclear power plant for RELAP5 

calculations. Nuclear Engineering and Design 241, 1034-1046. 

13. Stritar, A., Prošek, A., 1991. The analysis of the reflood experiment. Z. Angew. Math. 

Mech., 71, 525-527. 






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