NUREG/IA-0477

Thermal Hydraulic and Fuel Rod Mechanical 

Combination Analysis of Kuosheng Nuclear Power 

Plant with RELAP5 MOD3.3/FRAPTRAN/Python 

in SNAP Interface

Prepared by:

Jong-Rong Wang*, Chunkuan Shih*, Hao-Chun Chang*, Shao-Wen Chen*, Show-Chyuan Chiang**, 

Tzu-Yao Yu**

*Institute of Nuclear Engineering and Science, National Tsing Hua University; Nuclear and New 

Energy Education and Research Foundation

101 Section 2, Kuang Fu Rd., HsinChu, Taiwan

**Department of Nuclear Safety, Taiwan Power Company

242, Section 3, Roosevelt Rd., Zhongzheng District, Taipei, Taiwan

K. Tien, NRC Project Manager

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: February 2016

Date Published: November 2016

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

NUREG/IA-0477

Thermal Hydraulic and Fuel Rod Mechanical 

Combination Analysis of Kuosheng Nuclear Power 

Plant with RELAP5 MOD3.3/FRAPTRAN/Python 

in SNAP Interface

Prepared by:

Jong-Rong Wang*, Chunkuan Shih*, Hao-Chun Chang*, Shao-Wen Chen*, Show-Chyuan 

Chiang**, Tzu-Yao Yu**

*Institute of Nuclear Engineering and Science, National Tsing Hua University; Nuclear and New 

Energy Education and Research Foundation

101 Section 2, Kuang Fu Rd., HsinChu, Taiwan

**Department of Nuclear Safety, Taiwan Power Company

242, Section 3, Roosevelt Rd., Zhongzheng District, Taipei, Taiwan

K. Tien, NRC Project Manager

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: February 2016

Date Published: November 2016

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

After the measurement uncertainty recapture power uprates, Kuosheng nuclear power plant (NPP) 

was uprated the power from 2894 MWt to 2943 MWt. For this power upgrade, several analysis codes 

were applied to assess the safety of Kuosheng Nuclear Power Plant. In our group, there were a lot of 

effort on thermal hydraulic code, TRACE, had been done before. However, to enhance the reliability 

and confidence of these transient analyses, thermal hydraulic code, RELAP5/MOD3.3 will be applied 

in the future. The main work of this research is to establish a RELAP5/MOD3.3 model of Kuosheng 

NPP with SNAP interface. Model establishment of RELAP5 code is referred to the Final Safety 

Analysis Report (FSAR), training documents, and TRACE model which has been developed and 

verified before. After completing the model establishment, three startup test scenarios would be 

applied to the RELAP5 model. With comparing the startup test data and TRACE model analysis 

results, the applicability of RELAP5 model would be assessed. 

Recently, Taiwan Power Company is concerned in stretch power uprated plan and uprates the 

power to 3030 MWt. Before the stretch power uprates, several transient analyses should be done 

for ensuring that the power plant could maintain stability in higher power operating conditions. In 

this research, three overpressurization transients scenario including main steam isolation valves 

closure, turbine trip with bypass failure and load rejection with bypass failure would be performed

by RELAP5 MOD3.3 code. Further, the thermal hydraulic properties of the reactor core will be 

transferred as the boundary conditions of FRAPTRAN code. With the boundary conditions from 

RELAP5 code, the fuel rod mechanical properties during the transient could be determined. In 

this research, the SNAP interface is applied so that the transferring process between RELAP5 

and FRAPTRAN code can be completed automatically with Python Job Stream. That is, the 

researchers need not calculate and transfer the thermal hydraulic boundary conditions for 

FRAPTRAN analysis manually.

v

FOREWORD

U.S. NRC (United States Nuclear Regulatory Commission) is developing an advanced thermal 

hydraulic code named TRACE for nuclear power plant safety analysis. The development of 

TRACE is based on TRAC, integrating RELAP5 and other programs. U. S. NRC has determined 

that in the future, TRACE will be the main code used in thermal hydraulic safety analysis, and no 

further development of other thermal hydraulic codes such as RELAP5 and TRAC will be 

continued. A graphic user interface program, SNAP (Symbolic Nuclear Analysis Program) which 

processes inputs and outputs for TRACE is also under development. One of the features of 

TRACE is its capacity to model the reactor vessel with 3-D geometry. It can support a more 

accurate and detailed safety analysis of nuclear power plants. TRACE has a greater simulation 

capability than the other old codes, especially for events like LOCA. 

Taiwan and the United States have signed an agreement on CAMP (Code Applications and 

Maintenance Program). INER (Institute of Nuclear Energy Research, Atomic Energy Council, 

R.O.C.) is the organization in Taiwan responsible for the application of TRACE and RELAP in 

thermal hydraulic safety analysis, for recording user’s experiences of it, and providing suggestions 

for its development. To meet this responsibility, a RELAP5/MOD 3.3 and FRAPTRAN 1.5 

combination model of Kuosheng NPP which can transfer the data results of RELAP5 code into 

FRAPTRAN input deck automatically was developed with SNAP interface.

vii

TABLE OF CONTENTS

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

FOREWORD .............................................................................................................................V

LIST OF FIGURES ...................................................................................................................IX

LIST OF TABLES.....................................................................................................................XI

EXECUTIVE SUMMARY ........................................................................................................XIII

ABBREVIATIONS ...................................................................................................................XV

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

2 MODEL ESTABLISHMENT................................................................................................. 3

2.1 Overview of the RELAP5/MOD 3.3 model ...................................................................... 3

2.2 Overview of the FRAPTRAN analysis model.................................................................. 5

2.3 Job stream assignment in SNAP interface...................................................................... 9

3 HYPOTHETICAL ACCIDENT.............................................................................................15

3.1 Main Steam Line Isolation Valves Closure with Bypass Failure.....................................15

3.2 Turbine Trip with Bypass Failure ...................................................................................18

3.3 Load Rejection with Bypass Failure...............................................................................20

4 RESULTS ...........................................................................................................................23

4.1 Main Steam Line Isolation Valves Closure with Bypass Failure.....................................23

4.1.1 Thermal Hydraulic Analysis data results ...............................................................23

4.1.2 Fuel Rod Properties..............................................................................................25

4.2 Turbine Trip with Bypass Failure ...................................................................................30

4.2.1 Thermal Hydraulic Analysis data results ...............................................................30

4.2.2 Fuel Rod Properties..............................................................................................32

4.3 Load Rejection with Bypass Failure...............................................................................36

4.3.1 Thermal Hydraulic Analysis data results ...............................................................36

4.3.2 Fuel Rod Properties..............................................................................................39

6 CONCLUSIONS..................................................................................................................43

7 REFERENCES....................................................................................................................45

ix

LIST OF FIGURES

Figure 1 Flow chart of thermal hydraulic and fuel rod mechanism analysis model ................. 4

Figure 2 Overview of the Kuosheng NPP RELAP5/MOD3.3 model ....................................... 4

Figure 3 Setting of radial nodes in FRAPTRAN model........................................................... 7

Figure 4 Setting of axial nodes in FRAPTRAN model............................................................ 8

Figure 5 Reference model of the combination analysis.........................................................10

Figure 6 AptPlot job stream setting for power history and dome pressure ............................11

Figure 7 AptPlot job stream setting for heat transfer coefficient and coolant temperature.....12

Figure 8 Job stream connection............................................................................................13

Figure 9 Python scripts dialog ..............................................................................................14

Figure 10 Controlling system of reactor scram in MSIVC hypothetical accident......................16

Figure 11 Controlling system of safety valves in MSIVC hypothetical accident.......................17

Figure 12 Controlling system of reactor scram in TTBF hypothetical accident ........................19

Figure 13 Controlling system of relief valves in TTBF hypothetical accident...........................19

Figure 14 Controlling system of reactor scram in LRBF hypothetical accident........................21

Figure 15 Controlling system of safety/relief valves in LRBF hypothetical accident ................21

Figure 16 Steam flow variation during the MSIVC hypothetical accident.................................24

Figure 17 Dome pressure variation during the MSIVC hypothetical accident..........................24

Figure 18 Core power variation during the MSIVC hypothetical accident................................25

Figure 19 Cladding temperature of MSIVC transient...............................................................27

Figure 20 Cladding hoop strain of MSIVC transient ................................................................27

Figure 21 Fuel enthalpy of MSIVC transient ...........................................................................28

Figure 22 Cladding temperature comparison of MSIVC transient ...........................................28

Figure 23 Cladding hoop strain comparison of MSIVC transient.............................................29

Figure 24 Fuel enthalpy comparison of MSIVC transient........................................................29

Figure 25 Steam flow variation during the TTBF hypothetical accident...................................31

Figure 26 Dome pressure variation during the TTBF hypothetical accident ............................31

Figure 27 Core power variation during the TTBF hypothetical accident ..................................32

Figure 28 Cladding temperature of TTBF transient.................................................................33

Figure 29 Cladding hoop strain of TTBF transient ..................................................................34

Figure 30 Fuel enthalpy of TTBF transient..............................................................................34

Figure 31 Cladding temperature comparison of TTBF transient..............................................35

Figure 32 Cladding hoop strain comparison of TTBF transient ...............................................35

Figure 33 Fuel enthalpy comparison of TTBF transient ..........................................................36

Figure 34 Steam flow variation during the LRBF hypothetical accident...................................37

Figure 35 Dome pressure variation during the LRBF hypothetical accident ............................37

Figure 36 Core power variation during the LRBF hypothetical accident..................................38

Figure 37 Core flow variation during the LRBF hypothetical accident .....................................38

Figure 38 Cladding temperature of LRBF transient.................................................................40

Figure 39 Cladding hoop strain of LRBF transient ..................................................................40

Figure 40 Fuel enthalpy of LRBF transient .............................................................................41

Figure 41 Cladding temperature comparison of TTBF transient..............................................41

Figure 42 Cladding hoop strain comparison of LRBF transient ...............................................42

Figure 43 Fuel enthalpy comparison of LRBF transient ..........................................................42

xi

LIST OF TABLES

Table 1 Fuel rod geometry ........................................................................................................ 6

Table 2 Initial conditions of TRACE and RELAP5 models ........................................................15

Table 3 Events sequence and comparison of MSIVC hypothetical accident.............................16

Table 4 Events sequence and comparison of TTBF hypothetical accident...............................18

Table 5 Events sequence and comparison of LRBF hypothetical accident...............................20

xiii

EXECUTIVE SUMMARY

RELAP5/MOD3.3Patch04 code, which was developed for light water reactor (LWR) transient 

analysis at Idaho National Engineering Laboratory (INEL) for U.S. NRC, is applied in this research. 

This code is often performed to support rulemaking, licensing audit calculations, evaluation of 

accident, mitigation strategies, evaluation of operator guidelines, and experiment planning 

analysis. Same as other thermal hydraulic analysis codes, RELAP5/MOD3.3 is based on 

nonhomogeneous and nonequilibrium model for the two-phase system. However, calculations in 

this code will be solved by a fast, partially implicit numerical scheme to permit economical 

calculation of system transients. It can produce accurate transient analysis results in relatively 

short time. 

Symbolic Nuclear Analysis Package (SNAP) is an interface of NPP analysis codes which 

developed by U.S. NRC and Applied Programming Technology, Inc. Different from the traditional 

input deck in ASCII files, the graphical control blocks and thermal hydraulic connections make 

researches comprehend the whole power plant and control system more easily. Due to these 

advantages, the RELAP5/MOD3.3 model of Kuosheng NPP was developed with SNAP interface.

Kuosheng NPP is located on the northern coast of Taiwan. Its nuclear steam supply system is a 

type of BWR/6 designed and built by General Electric on a twin unit concept. Each unit includes 

two loops of recirculation piping and four main steam lines, with the thermal rated power of 

2894MWt. Recently, Taiwan Power Company is concerned in stretch power uprated plan and 

uprates the power to 3030 MWt. Before the stretch power uprates, several transient analyses

should be done for ensuring that the power plant could maintain stability in higher power operating 

conditions. In this research, three overpressurization transients scenario including main steam 

isolation valves closure, turbine trip with bypass failure and load rejection with bypass failure 

would be performed by RELAP5 MOD3.3 code. Further, the thermal hydraulic properties of the 

reactor core will be transferred as the boundary conditions of FRAPTRAN code. With the 

boundary conditions from RELAP5 code, the fuel rod mechanical properties during the transient 

could be determined. In this research, the SNAP interface is applied so that the transferring 

process between RELAP5 and FRAPTRAN code can be completed automatically with Python 

Job Stream. That is, the researchers need not calculate and transfer the thermal hydraulic 

boundary conditions for FRAPTRAN analysis manually.

xv

ABBREVIATIONS

BWR Boiling-Water Reactor

BPV Bypass valve

INER Institute of Nuclear Energy Research Atomic Energy Council, R.O.C.

kg kilogram(s)

kW kilowatt(s)

LRBF Load Rejection with Bypass Failure

MPa Megapascal(s)

MSIVC Main Steam Isolation Valves Closure

MUR Measurement Uncertainty Recapture

NPP Nuclear Power Plant

NRC Nuclear Regulatory Commission

NRWL Narrow Range Water Level

Pa Pascal(s)

psi pounds per square inch

SNAP Symbolic Nuclear Analysis Program

SPU Stretch Power Uprated

SRV Safety/Relief Valves

TBV Turbine Bypass Valve

TCVC Turbine Control Valves Closure

TRACE TRAC/RELAP Advanced Computational Engine

TTBF Turbine Trip with Bypass Failure

W Watt(s)

US United States

1

1 INTRODUCTION

Kuosheng NPP is located on the northern coast of Taiwan. Its nuclear steam supply system is a 

type of BWR/6 designed and built by General Electric on a twin unit concept. Each unit includes 

two loops of recirculation piping and four main steam lines, with the thermal rated power of 

2894MWt. After the project of MUR for Kuosheng NPP, the operating power is 2943 MWt [1, 2].

Few years later, Unit 1 started SPU from Cycle 24 and Unit 2 started SPU from Cycle 23. The 

operating power will be 3030 MWt. To uprate the power ratio, the assessments of power plant 

transient should be analyzed. In the past, a TRACE model of Kuosheng NPP has been developed.

The analysis and simulation of the TRACE model for the startup tests and hypothetical transients

has been done. In this research, a RELAP5/MOD3.3 model of Kuosheng NPP is developed 

referred to FSAR [1], training documents [2], RELAP5 3D and TRACE model which had been 

developed before [3, 4, 5]. Further, the model in this research is built up with the SNAP interface 

rather than the text files. The visible thermal hydraulic components and control blocks in the SNAP 

interface help users build the model more efficiently and concretely. 

RELAP5/MOD3.3Patch04 code, which was developed for light water reactor (LWR) transient 

analysis at Idaho National Engineering Laboratory (INEL) for U.S. NRC, is applied in this research. 

This code is often performed to support rulemaking, licensing audit calculations, evaluation of 

accident, mitigation strategies, evaluation of operator guidelines, and experiment planning 

analysis [6]. Same as other thermal hydraulic analysis codes, RELAP5/MOD3.3 is based on 

nonhomogeneous and nonequilibrium model for the two-phase system. However, calculations in 

this code will be solved by a fast, partially implicit numerical scheme to permit economical 

calculation of system transients. It can produce accurate transient analysis results in relatively 

short time, which means large amounts of sensitivity or uncertainty analysis might be possible. 

Fuel Rod Analysis Program Transient (FRAPTRAN) is a light water reactor fuel rods simulation 

code which is conducted by U.S. NRC and developed by Pacific Northwest National Laboratory 

(PNNL) since 1997. FRAPTRAN is developed to calculate the single fuel rod behavior during 

transients and hypothetical accidents at burnup level up to 62 gigawatt-days per metric ton of 

Uranium (GWd/MTU) [7]. In this research, FRAPTRAN version 1.5 was applied. With FRAPTRAN 

1.5, the important criteria such as cladding temperature, cladding hoop strain, fuel enthalpy and 

radial gap can be determined. However, some long term properties such as fuel densification, 

swelling, radiation growth and cladding creep are not concluded in FRAPTRAN code. Hence, the 

FRAPCON-3 code is also applied in this research.

FRAPCON-3 is also conducted by U.S. NRC and developed by PNNL. Different from FRAPTRAN 

code, FRAPCON-3 analyzes the fuel rod behavior when the power and coolant conditions change 

slowly. The calculation results supply several fuel rods properties and burnup information after a 

long-term operation [8]. The restart file from FRAPCON-3 analysis can be the initial conditions of 

FRAPTRAN analysis. With this restart file, some long-term operating properties can be concerned 

in FRAPTRAN transient analysis. In this research, FRAPTRAN code referenced the restart file of 

18-month fuel operation which was calculated by FRAPCON-3 code.

Symbolic Nuclear Analysis Package (SNAP) is an interface of NPP analysis codes which 

developed by US NRC and Applied Programming Technology, Inc. Different from the traditional 

input deck in ASCII files, the graphical control blocks and thermal hydraulic connections make 

researches comprehend the whole power plant and control system more easily [9]. Due to these 

advantages, the RELAP5/MOD3.3 model of Kuosheng NPP was developed with SNAP interface.
2

In the past, the TRACE thermal hydraulic and FRAPTRAN fuel rod mechanical property combined 

analysis had been performed [10, 11]. In this kind combination analysis, the data results cannot 

only show the thermal hydraulic properties but also the fuel rod deformation during the transient. 

However, the data transferring process from TRACE to FRAPTRAN was complicated and timeconsuming. In this research, a developed RELAP5/MOD 3.3 model [12, 13] was applied for the 

thermal hydraulic data. However, the automatic transferring process for FRAPTRAN input deck 

was developed. Thermal hydraulic data from RELAP5/MOD 3.3 analysis can be easily converted 

into FRAPTRAN input deck and the whole both of the thermal hydraulic and fuel rod mechanism 

analysis can be finished with one model in SNAP interface. 
3

2 MODEL ESTABLISHMENT

The performance of the RELAP5 model had been verified and assessed last year [13]. However, 

the thermal hydraulic analysis of Kuosheng NPP performed last year could only obtain the reactor 

vessel and pipeline information during the transient. Information of fuel rod mechanical variations 

is unobtainable for the developed RELAP5 model. As a result, combined analysis of RELAP5 and 

FRAPTRAN is required for more detailed information of fuel rods during transient. In this research, 

analysis models of different codes including RELAP5/MOD 3.3, FRAPTRAN 1.4 and FRAPCON 

3.4 are developed in SNAP interface which can directly and conveniently control the job streams 

of these codes. At first, the RELAP5 model of Kuosheng NPP was developed in SNAP interface 

singly. Then, this verified model would be referenced by “Eng-template” model which can combine 

different code analysis in SNAP interface. With this useful model type of SNAP interface, the 

combination of these two codes becomes possible. From the experience of our group, the 

combination of thermal hydraulic and fuel rod performance code was successfully performed with 

TRACE and FRAPTRAN codes. The same analysis flow is followed in this research. However, 

different form the TRACE-FRAPTRAN analysis, the RELAP5-FRAPTRAN analysis can be 

applied with single job submitting. That is, the thermal hydraulic information of RELAP5 code can 

be transferred into FRAPTRAN input deck automatically. To complete such job stream, the python 

script which can extract and transfer thermal hydraulic information is developed in the SNAP 

interface. The flow chart of this combination model is shown in Figure1. 

2.1 Overview of the RELAP5/MOD 3.3 model

Different from typical thermal hydraulic model establishment, the RELAP model in this research 

was developed in the SNAP interface. With SNAP interface, the components and control blocks 

are visible as shown in Figure 2. As a result, the users could set up component parameters and 

nodding diagram at same time. In this RELAP model, situation of NSSS in transient was mainly 

concerned. Hence, the turbines and feedwater pumps of Kuosheng NPP were assumed to be 

boundaries which were simulated by components Time dependent volume (TMDVOL). Four main 

steam pipe lines, which were consistent with the configuration of Kuosheng NPP, were developed. 

On these pipe lines, three important valves including main steam line isolation valves, turbine stop 

valves and turbine control valves are developed. Further, there are totally 16 safety/relief valves 

connected on the main steam pipe lines. All the opening and closing setpoints are also developed 

according to the arrangement of Kuosheng NPP. 

The reactor vessel is developed by several kinds of components including Branch, Pipe, Single 

junction and single volume. Four pipes which are developed to simulate fuel assemblies are 

connected to heat structures inside the reactor vessel. Two recirculation loops and recirculation 

pumps are set up according to the configuration of the power plant. Further, there are two control 

valves developed on two recirculation loops respectively to adjust the recirculation flow rate. 20 

jet pumps in the NPP are merged into two jet pump components to save the computational time. 

More details of the components properties and the setting of control systems are described below.

Source data of these heat structures are referred to the total reactor power. To simplified the 

model and save the computational time, Point Kinetics and Separable feedback types are chosen 

for the reactor kinetics. Further, GAMMA-AC fission product decay type is chosen to calculate the 

decay heat. The total reactor power was set according to different transient scenario respectively. 

Initial reactivity and value of Beta over Lambda were 0 and 125 respectively, which were referred

to the manual of RELAP5. In addition, with some unit converting, density and Doppler reactivity 

feedback table were referred to the TRACE and RETRAN model which had been verified before. 
4

To simplify the control model and reduce the computer time, the scram control systems were 

developed according to each case respectively. However, the scram reactivity feedback was also 

referred to the TRACE model verified before, to control the power after the reactor scram. As the 

table trip set in Table 900 was initiated by the power control system in each case, the negative

reactivity feedback, which was increasing with time, would be concerned in the power calculation. 

Collect 

Thermal Hydraulic Parameters

Collecting Fuel Rod Geometric 

Parameters

Develop 

Thermal Hydraulic Model

Verify and Assess 

Thermal Hydraulic Model

Apply Hypothetical

Transient Analysis

Develop 

Fuel Rod Model

Verify and Assess 

Fuel Rod Model

Apply Hypothetical

Transient Analysis

Thermal Hydraulic

Boundary Conditions

Basic Model

AptPlot Job Stream

Python Scripts

FRAPTRAN input deck

Figure 1 Flow chart of thermal hydraulic and fuel rod mechanism analysis model

Figure 2 Overview of the Kuosheng NPP RELAP5/MOD3.3 model
5

2.2 Overview of the FRAPTRAN analysis model

The fuel rod geometric information was shown in Table 1. Some steady state fuel rod information 

such as burn-up value and fission gas is calculated by FRAPCON-3. The long-term information 

of FRAPCON-3 data results will be combined into the restart file, which can be read and calculated 

by FRAPTRAN code. As mentioned before, the boundary conditions required would be performed 

by TRACE and transferred to FRAPTRAN code manually. 

In the FRAPTRAN analysis, the time interval was 0.01 second. The analysis time was set up 

according to the hypothetical accidents respectively. The gap pressure and temperature 

convergence are both 0.001. The maximum temperature variation between two iterations is 6K. 

After setting up the convergence index, the limitation of iteration numbers is also required to 

prevent the computer crashing if the calculations could not meet the convergence index. In this 

research, both the limitations of iterations in transient and steady state are set as 100. 

In this research, the fuel rod nodes were divided into radial and axial directions. For the radial 

directions, it can be further divided into two settings including nodes inside the fuel pellet and 

nodes inside the cladding. As shown in Figure 3 and Figure 4, there are 13 radial nodes with 

equal interval started from the center of the fuel pellet and ended at the surface of the fuel pellet. 

Besides, same in the radial directions, there are two nodes inside and outside the cladding. 

However, it should be noticed that the radials nodes would be only calculated in the code 

iterations. The output data would be normalized the radial nodes information. For the axial 

direction, in this research, the fuel rod is divided into 12 nodes with equal interval. The output 

information of temperature, pressure, strain, stress and so on will be recorded and plotted mainly 

according to these 12 axial nodes.
6

Table 1 Fuel rod geometry

Parameters Input codes value Unit Notes

Length RodLength 12.45 Feet

Diameter RodDiameter 0.03608 Feet

Gap thickness gapthk 0.00031 Feet Replaced with FRAPCON data

Upper spring coils ncs 59 N/A

Upper spring height spl 0.7513 Feet

Upper spring diameter scd 0.02583 Feet

Upper spring wire diameter swd 0.00425 Feet

Cold works coldw 0.5 N/A

Cladding roughness roughc 1 um

Helium fraction gfrac(1) 1 N/A

Fill pressure gappr0 74.7 psia

Pellet diameter FuelPelDiam 0.0305 Feet Replaced with FRAPCON data

Pellet height pelh 0.0366 Feet Replaced with FRAPCON data

Fuel surface roughness roughf 1 um Replaced with FRAPCON data

Pellet density frden 0.945 N/A

Fuel sintering temperature tsntrk 1773 K

Fuel grain size fgrns 10 um
7 

Figure 3 Setting of radial nodes in FRAPTRAN model
8 

Figure 4 Setting of axial nodes in FRAPTRAN model
9

2.3 Job stream assignment in SNAP interface

In the SNAP, there is a model named “Eng-template” which allows users create analysis process 

with different kind of computer code. As developing this model, the reference model with single 

computer code should be developed previously. As shown in Figure 5, in the Eng-template model, 

there are two different computer analysis model created including the RELAP5 and FRAPCON 

reference model. To be noticed, the developed FRAPTRAN model is not referenced in this way 

because there are a lot of boundary conditions which are from the RELAP5 results need to be 

modified with python scripts. After modified by the python scripts, ASCII input deck will be 

transferred into FRAPTRAN job stream to do the next analysis about fuel rods. 

To retrieve thermal hydraulic data results from the RELAP5 analysis efficiently, the AptPlot job 

stream is also developed in this model. In this job stream, there are four thermal hydraulic data 

retrieved including the power history, dome pressure, heat transfer coefficient and core 

temperature. In the RELAP5 model, there are control blocks which extract power history in unit 

MW and dome pressure in unit psi respectively; hence, the data channels “cntrlvar-5” (for power 

history) and “cntrlvar-650” (for dome pressure) are described in AptPlot job stream settings as 

shown in Figure 6. Moreover, the fuel rods are represented by the heat structure in RELAP5 code. 

As a result, the data channels “hthtc-” and “tempf-” are described in AptPlot job stream node by 

node to retrieve the heat transfer coefficient and coolant temperature for FRAPTRAN analyses, 

as shown in Figure 7. To be noticed, there are three group of fuel assemblies developed in the 

RELAP5 model. For safety, the hottest fuel assembly heat structure (1621) information is applied. 

To transfer these data results retrieved by the AptPlot job stream, the output file type is defined 

as “ASCI” file, which will make the python job stream conversion more efficiently.

In the python job stream, the input file type can be defined manually. In this case, there are 33 

input files including 1 power history, 1 dome pressure, 1 FRAPTRAN input sample file, 15 heat 

transfer coefficient and 15 temperature data results. All these input files are respectively 

connected to the corresponding extraction data files from the AptPlot job stream as shown in 

Figure 8. Moreover, users should also determine what kind of output file type is desired. In this 

case, the FRAPTRAN input file with filename extension “.inp” is chosen. After setting up the input 

and output characters of the python job stream, the python code can be developed in the “python 

scripts” dialog (shown in Figure 9). At the beginning of the python scripts, the data files from the 

AptPlot job stream should be read into the memory so that the data results can be processed in 

the following steps. To read the data file from the previous job stream, there is a python function 

“get_input_file()” which allows users transfer a data file into a python variable. Then, the variable

can be opened through the python “open()” function. In addition to the thermal hydraulic data 

results files, the FRAPTRAN sample input deck should also be included in the job stream. This 

file will be introduced into SNAP with external file which can also be input by the python job stream. 

With the sample input deck file, data results from the RELAP5 code can be written into the 

corresponding input structure of the FRAPTRAN input deck. Moreover, it should be noted that the 

units of data results from REALP5 code may different from the FRAPTRAN input deck. For 

instance, the unit of power from the RELAP5 code is MW for the whole reactor core while the unit 

of power in the FRAPTRAN deck is kw/ft for single fuel rod. A proper unit conversion should be 

concerned in corresponding cases. 

After combined with the thermal hydraulic data results and fuel rod design input deck, the output 

file of python job stream will be delivered to FRAPTRAN code to do the further analysis. Hence, 

the output file type setting of python job stream should be defined as FRAPTRAN input which can 

be connected to FRAPTRAN job stream. In addition, the burn-up value is also an important 

concerned in the FRAPTRAN analysis. In this Eng template, the “restart” file was generated with 
10

FRAPCON 3.4 code and connected to the FRAPTRAN job stream. In this research, 18-month 

burn-up calculation is assumed in FRAPCON, which is the same period of re-fueling.

Figure 5 Reference model of the combination analysis
11

Figure 6 AptPlot job stream setting for power history and dome pressure
12

Figure 7 AptPlot job stream setting for heat transfer coefficient and coolant temperature
13

Figure 8 Job stream connection
14

Figure 9 Python scripts dialog
15

3 HYPOTHETICAL ACCIDENT

To verify the applicability of RELAP5-FRAPTRAN analysis model, hypothetical accidents of 

Kuosheng NPP were performed. With these accidents performed in the new model, there are two 

main advantages. First, these accidents had been performed with TRACE and FRAPTRAN codes 

(with manually data transferring) so there were a lot of data results which could be compared. 

Second, the transient duration of these hypothetical accidents are short, which are convenient for 

model modification. 

As mentioned above, a series of overpressurization transient test of TRACE code had been 

performed before by our group. In this research, same hypothetical accidents will be performed 

again by RELAP5 code and then these data results will be transferred as FRAPTRAN input deck 

with python job stream in SNAP interface. Hence, the initial conditions such as power ratio, 

feedwater flow rate, steam flow rate and dome pressure of RELAP5 code should be same as that

of TRACE code so that the comparison would be useful and meaningful. The hypothetical 

accidents assumption and event sequence are described below.

Table 2 Initial conditions of TRACE and RELAP5 models

Parameters Unit FSAR TRACE RELAP5

Power (MWt) 3030 3030 3030

Dome Pressure (MPa) 7.17 7.17 7.17

Feedwater Flow (kg/sec) 1645.3 1641.2 1640.6

Steam Flow (kg/sec) 1645.3 1641 1640.6

Core inlet flow (kg/sec) 10645 10704 10674

Recirculation flow (Single loop) (kg/sec) 1549.5 1538.6 1548.4

Core exit pressure (MPa) 7.23 7.3 7.23

NRWL (m) 0.934 0.934 0.933

3.1 Main Steam Line Isolation Valves Closure with Bypass Failure

In this hypothetical accident, a 210-second steady state was performed for ensuring that all the 

parameters matched the operating conditions. At time point 210 second, the main steam line 

isolation valves (MSIVs) closed in three seconds. In order to promote the difficulties of this 

transient for the plant, the closure time was assumed to be 3 seconds in this case, which was 

shorter than 5 seconds that described in FSAR. According to the FSAR description, reactor scram 

signal in this case might come from two ways including neutron flux exceeding 122% or dome 

pressure exceeding 7.66 MPa. The reactor scram control system is shown in Figure 10. Further, 

as the dome pressure reached to 7.82 MPa, recirculation pump trip signal would be sent out with 

delayed time 0.14 second. For the conservation reason, only 11 safety valves works even though 

that there are 16 safety/relief valves on the main steam pipelines. All these 11 safety valves were 

still divided into three groups, as shown in Figure 11, with different set-points including 8.38 MPa 

for two valves, 8.48 MPa for five valves and 8.55 MPa for four valves. Besides, after the closure 

signal sent out, the safety valves would fully close in 0.15 second with an electronic delayed time 

0.4 second. Table 3 lists the MSIVC events sequence and important set-points. From the data 
16

results, the reactor scrammed due to neutron flux exceeding 122% in this transient. Further, the 

safety valves will fully open in 0.15 second with delayed time 0.4 second. Once the safety valves 

open, the dome pressure was under controlled and the NPP was back to the safe situation.

Table 3 Events sequence and comparison of MSIVC hypothetical accident

Events RELAP5 (sec) TRACE (sec) Notes

Steady state 0~210 0~210

Power 3030 MW

Feedwater flow rate 1641 kg/sec

Feedwater temperature 488.7K

Core inlet flow rate 11177.3 kg/sec 

Dome pressure 7.17MPa

MSIVs closure 210 210 Fully closure time 3 seconds

MSIVs fully closed 213.210 213.06

Reactor scram signal initiated 213.211 213.14 Power ratio reached to 122%

Reactor scram 213.310 213.23 Delayed time 0.09 second

Recirculation pumps tripped 213.315 213.46 Dome pressure 7.82MPa

Safety valves group 1 opened 

(8.38MPa,Group 1) 213.915 214.53 Delayed 0.4 second

Safety valves group 2 opened 

(8.48MPa,Group 2) 214.015 214.65 Delayed 0.4 second

Safety valves group 3 opened 

(8.55MPa,Group 3) 214.115 214.73 Delayed 0.4 second

End of the analysis 220 220

Figure 10 Controlling system of reactor scram in MSIVC hypothetical accident
17

Figure 11 Controlling system of safety valves in MSIVC hypothetical accident
18

3.2 Turbine Trip with Bypass Failure

In the hypothetical accident of TTBF, a 500 second steady state was performed to ensure all the 

parameters matched the operation conditions. Same as the MSIVC hypothetical accident, the 

reactor scram signal might come from neutron flux exceeding 122% or dome pressure exceeding

7.66 MPa. However, in the TTBF case, the scram signal might also come from the TSVs reaching 

90% open. In the RELAP5 model, all the possibilities were concerned as shown in Figure 12. 

At 500 second, the turbine tripped and the turbine stop valves started to close with closure time 

0.1 second. As the TSVs reached to 90% open, the reactor scram signal was initiated. According 

to the FSAR, there would be 0.08 second delayed time. After the TSVs closure, the dome 

pressure increased and the void fraction inside the reactor core decreased. Hence, a positive 

reactivity feedback functioned and the power ratio increased. 

For conservative reason, on 6 safety relief valves opened with delayed time 0.4 second at the 

dome pressure 7.94 MPa and closed at dome pressure 7.62 MPa. The control system of SRVs 

was shown in Figure 13. Further, the safety relief valves would fully open in 0.15 second. Once 

the safety relief valves opened, the dome pressure and the power plants would be under 

controlled. Table 4 listed the event sequences and important setpoints of the TRACE model 

during TTBF transient.

Table 4 Events sequence and comparison of TTBF hypothetical accident

Events RELAP5 (sec) TRACE (sec) Notes

Steady state 0~499 0~499

Power 3030 MW

Feedwater flow rate 1640 kg/sec

Feedwater temperature 488.7K

Core inlet flow rate 11177.3 kg/sec

Dome pressure 7.17MPa

TSVs start to close 500 500 Fully closure time 0.1 second

Reactor scram signal 

initiated 500.02 500.01 Signal initiated at TSVs 90% open 

Reactor scram 500.11 500.09 Delayed time 0.08 second

TSVs fully closed 500.1 500.1

Relief valves opened 501.34 501.77 Open at dome pressure 7.94MPa and 

closed at dome pressure 7.63MPa

Peak dome pressure (MPa) 503.81 (8.53MPa) 502.27

(8.2MPa)

End of the analysis 505 505
19

Figure 12 Controlling system of reactor scram in TTBF hypothetical accident

Figure 13 Controlling system of relief valves in TTBF hypothetical accident
20

3.3 Load Rejection with Bypass Failure

In the TCVC hypothetical accident, a 210-second steady state simulation was performed to 

ensure all the parameters reached to the operating conditions. At 210 second, the load rejection 

happened and the TCVs closed immediately. Once the turbine control valves closed, the main 

steam line flow decreased immediately which increased the dome pressure. The positive void 

fraction reactivity feedback functioned and the power increased. Different from the previous two 

cases, the recirculation pumps trip signal and the reactor scram signal were sent out immediately 

as the TCVs closed. However, due to the signal delayed time, the reactor would scram 0.07 

second later as shown in Figure 14. Further, the recirculation pumps would be out of service 0.14 

second later. For conservative reason, there were only 7 safety valves and 6 relief valves 

functioned in this case with signal delayed time 0.4 second. The control system of the SRVs is 

shown in Figure 15. Same as previous two cases, once the safety/relief valves open, the dome 

pressure decreased and the NPP was under controlled. Table 5 lists the important setpoints and 

transient events of the TCVC hypothetical accident.

Table 5 Events sequence and comparison of LRBF hypothetical accident

Events RELAP5 (sec) TRACE (sec) Notes

Steady state 0~209 0~209 Power 3030 MW

Feedwater flow rate 1641 kg/sec

Feedwater temperature 488.7K

Core inlet flow rate 11177.3 kg/sec

Dome pressure 7.17MPa

Turbine control 

valves closed

210 210 Start of the transient; valve closed in 0.15 

second

Reactor scram 210.07 210.07 Signal initiated at TCVs close with delayed 

0.07 second

Recirculation 

pumps trip

211.4 211.4 Signal initiated at TCVs close with delayed for 

0.14 second

Safety/relief valves 

open

211.36 211.76 7 safety valves and 6 relief valves functioned in 

this model with delayed time 0.4 second

Peak dome 

pressure (MPa)

213.75

(8.5 MPa)

211.83

(8.2 MPa)

End of analysis 215 215
21

Figure 14 Controlling system of reactor scram in LRBF hypothetical accident

Figure 15 Controlling system of safety/relief valves in LRBF hypothetical accident

23

4 RESULTS

In each hypothetical accident, there are two parts of data results will be illustrated. For the 

previous part, the analysis results of TRACE and RELAP5 will be compared to realize the 

computational difference of two codes. In fact, the comparison had been described in the NUREG 

report published by our group last year. However, to explain the FRAPTRAN analysis results 

comparisons more clearly, analysis results comparisons of TRACE and RELAP5 codes are 

described again in current report. 

The main FRAPTRAN analysis results of current report will be described in the second part. All 

the 12 nodes data results of regulatory criteria including cladding temperature, cladding hoop 

strain and fuel enthalpy will be plotted. With these figures, it can be easily determined that if the 

fuel rod keeps good integrity during the transient. In addition, the FRAPTRAN analysis results 

which boundary conditions were from TRACE analysis results will also be compared to the 

FRAPTRAN analysis results of current research.

4.1 Main Steam Line Isolation Valves Closure with Bypass Failure

4.1.1 Thermal Hydraulic Analysis data results

Figure 16 shows the comparison of steam flow rate between TRACE and RELAP5 model. From 

this figure, it is obviously that the steam flow rate dropped rapidly once the MSIVs closed at 210 

second. At 213 second, the MSIVs fully closed and the dome pressure increased greatly as shown 

in Figure 17. Hence, the void fraction inside the reactor core decreased which increased the power 

greatly as shown in Figure 18. In this figure, the core power of RELAP5 model was much lower 

than that of TRACE model. This difference might came from the neutron kinetic feedback table 

setting. In the RELAP5 model, the neutron slowing effect would reference the table which includes 

density and reactivity. However, in the TRACE model, the reactor power iteration would reference 

the table containing void fraction and reactivity. As a result, the power calculation might have 

some differences during high-power transient.

Despite the fact that the reactor had scrammed at 213.23 second, the decay heat still heated the 

reactor vessel and generated steam in both the RELAP5 and TRACE model. As a result, the 

dome pressure kept increasing. At 214.53 second, the dome pressure was higher enough to meet 

the setpoint of group 1 safety valves. However, 3 opened safety valves discharged the steam 

insufficiently. The dome pressure kept increasing until the group 2 and group 3 safety valves 

opened at 214.65 and 214.73 second respectively in TRACE model. Further, because the dome 

pressure varied near the setpoints, the safety valves opened and closed for three times. On the 

other hand, in the RELAP5 model, safety valves of group 1 opened at 214.31 second, which was 

not far from that in TRACE model. Safety valves of group 2 and group 3 also opened at 214.40 

second and 214.51 second. However, the dome pressure prediction of RELAP5 model was higher 

than that of TRACE model. As a result, the dome pressure variation did not trigger the setpoints 

of safety valves closure in the RELAP5 model. Hence, as shown in Figure 17, the reactor vessel 

in the RELAP5 model maintained a higher pressure than that in the TRACE model. 

From both the RELAP5 TRACE data results, it is known that the core power could be controlled 

by the scram system. In addition, as long as the safety valves opened, the dome pressure would 

not exceed the ASME criteria 9.58 MPa. Kuosheng NPP was safe in the MSIVC hypothetical 

transient.
24

Figure 16 Steam flow variation during the MSIVC hypothetical accident

Figure 17 Dome pressure variation during the MSIVC hypothetical accident

-500

0

500

1000

1500

2000

210 212 214 216 218 220 222

Flow rate (kg/sec)

Time (Sec)

Steam Flow

Main steam flow rate_RELAP5

Main steam flow rate_TRACE

7

7.5

8

8.5

9

9.5

210 212 214 216 218 220 222

Pressure (MPa)

Time (Sec)

Dome Pressure (MPa)

Dome Pressure_RELAP5 Dome Pressure_TRACE
25

Figure 18 Core power variation during the MSIVC hypothetical accident

4.1.2 Fuel Rod Properties

Figure 19 shows the cladding temperature variation of single fuel rod which was divided into 12 

nodes during the MSIVC transient. During the MSIVC transient, the cladding temperature 

increased because of the increasing power ratio. In this figure, it is obvious that the temperature 

of node 11 and 12 increased differently compared to temperature of other nodes. It is because 

that as the SRVs started to open at 214 seconds, the sudden pressure decreasing would cause 

the water flashing inside the reactor pressure vessel. The water flashing might influence the heat 

transfer efficiency of the top of fuel rods. To ensure this phenomenon, the RELAP5 analysis 

results were checked and it is found that heat transfer coefficient of the top of fuel rods is indeed 

decreased from 214 seconds to 218 seconds. In addition, the power ratio of node 12 is lower than 

that of node 11 so that the cladding temperature of node 12 did not increase so much as that of 

node 11. 

Figure 20 shows the average cladding hoop strain variation during the MSIVC transient. Once the 

power and temperature increased, the gap gas would inflate and the fuel cladding would expand 

with heat, which was known as the thermal hoop strain. However, the increasing dome pressure 

would squeeze the fuel cladding which would cause negative elastic hoop strain, especially at the 

lower position such as Node 1. Based on the interaction of these two types of hoop strain, it can 

be determined that the cladding expanded or shrank during the transient. From Figure 4, it shows 

that most part of the fuel rod shrank but for the node 11 and 12, the cladding expanded because 

of the increasing cladding temperature. Nonetheless, even node 11 of fuel rod expanded 

obviously, the hoop strain is still not higher than the acceptance criteria 0.01. In addition to the 

cladding hoop strain, the fuel pellet enthalpy should also be noticed in overpressurization 

transient. Figure 21 shows the enthalpy variation during the transient. The peak value is about 

250 kJ/kg (52.08 cal/g), which is much lower than the criteria 170 cal/g. From these three criteria 

mentioned above (temperature, hoop strain and enthalpy), it is known that the fuel rods would 

keep good integrity during the MSIVC transient.

0

1000

2000

3000

4000

5000

6000

7000

8000

210 212 214 216 218 220 222

Power (MW)

Time (Sec)

Core Power (MW)

Power_RELAP5 Power_TRACE
26

In addition to obtaining the mechanical variation of fuel rod during the transient, comparing these 

analysis results with past research is another main goal of current research. Figure 22 shows the 

comparison of cladding temperature of RELAP5-FRAPTRAN 1.5 and TRACE-FRATRAN 1.4 

analyses. To simplify the comparison, 12-node data is averaged. From this figure, it is obvious 

that at the steady state, the cladding temperature of RELAP5-FRAPTRAN 1.5 analysis is higher 

than that of TRACE-FRATRAN 1.4 analysis because the RELAP5 calculation predicted a higher 

coolant temperature which is an input condition for FRAPTRAN code. Further, the cladding 

temperature difference of steady state might also come from different FRATRAN code version. 

However, the influence of version should be checked with more research. 

As the transient started and power increased, both cladding temperature increased. At 213.31 

seconds, reactor scrammed and hence the fuel pellet stopped generating heat. Further, the power 

scrammed at 214 seconds, the SRVs opened and released vapor which would efficiently 

decreased the energy inside the reactor pressure vessel. For TRACE-FRAPTRAN 1.4 analysis, 

at about 215 seconds, the cladding temperature increased again because the SRVs returned to 

close and the main steam flow decreased so that the heat could not be carried with vapor until 

the SRVs opened and mains steam flow increased again at 216 seconds. On the contrary, SRVs 

of the RELAP5 model were kept open from 214 to 218 seconds. The steam flow kept releasing 

energy of the reactor core. Hence, the cladding temperature of RELAP5-FRAPTRAN analysis 

kept decreasing during the transient. 

Figure 23 shows the difference of cladding hoop strain between RELAP5-FRAPTRAN 1.5 and 

TRACE-FRAPTRAN 1.4 analysis models. At about 213 seconds, the reactor scrammed and the 

gap gas pressure decreased as a result. However, the dome pressure started increased. Hence, 

the cladding hoop strain decreased from 213 seconds. After that, the cladding temperature started 

to increase at 214 seconds which means the thermal hoop strain would against the elastic hoop 

strain. The cladding hoop strain increased at 214 seconds for RELAP5-FRAPTRAN 1.5 model 

and at about 215 seconds for TRACE-FRAPTRAN 1.4 model. Then, the cladding temperature 

decreased and the elastic hoop strain (coolant pressure) dominated the total hoop strain. The 

cladding hoop strain kept decreasing to the end of the analysis. However, it is worth noticing that 

the cladding hoop strain of TRACE-FRAPTRAN 1.4 model vibrated from 215 to 220 seconds 

because the dome pressure from TRACE analysis varied in this duration. On the contrast, the 

dome pressure from the RELAP5 analysis decreased with a smooth trend. As a result, the 

cladding hoop strain after SRVs open of these two models is slightly different. 

The last compared data is fuel enthalpy. With a higher coolant temperature, temperature of the 

fuel pellet of RELAP5-FRATRAN model is also higher than that of TRACE-FRAPTRAN model. 

Hence, the fuel enthalpy of RELAP5-FRATRAN model is higher than that of TRACE-FRAPTRAN 

model. However, from Figure 24, it can be noticed that the enthalpy curves of these two models 

come closer at the end of transient because the cladding temperature of RELAP5-FRATRAN 

model decreased more than that of TRACE-FRAPTRAN model, which implies that the fuel 

temperature of RELAP5-FRAPTRNA model reached to a lower value. The fuel enthalpy 

decreased as a result. 
27

Figure 19 Cladding temperature of MSIVC transient

Figure 20 Cladding hoop strain of MSIVC transient

550

570

590

610

630

650

670

210 212 214 216 218 220

Temperature (K)

Time(sec)

Cladding Temperature (MSIVC)

Node 1

Node 2

Node 3

Node 4

Node 5

Node 6

Node 7

Node 8

Node 9

Node 10

Node 11

Node 12

0.001

0.0012

0.0014

0.0016

0.0018

0.002

210 212 214 216 218 220

Strain

Time (sec)

Cladding Hoop Strain (MSIVC)

Node 1

Node 2

Node 3

Node 4

Node 5

Node 6

Node 7

Node 8

Node 9

Node 10

Node 11

Node 12
28

Figure 21 Fuel enthalpy of MSIVC transient

Figure 22 Cladding temperature comparison of MSIVC transient

0

50000

100000

150000

200000

250000

300000

210 212 214 216 218 220

Enthalpy (J/kg)

Time (sec)

Average Fuel Enthalpy (MSIVC)

Node 1

Node 2

Node 3

Node 4

Node 5

Node 6

Node 7

Node 8

Node 9

Node 10

Node 11

Node 12

575

580

585

590

595

600

605

610

210 212 214 216 218 220 222

Temperature (K)

Time (sec)

Average Cladding Temperature (MSIV)

R5 + FRAPTRAN 1.5 TRACE + FRAPTRAN 1.4
29

Figure 23 Cladding hoop strain comparison of MSIVC transient

Figure 24 Fuel enthalpy comparison of MSIVC transient

0.0014

0.00145

0.0015

0.00155

0.0016

0.00165

210 212 214 216 218 220 222

Strain

Time (sec)

Average Cladding Hoop Strain (MSIVC)

R5 + FRAPTRAN 1.5 TRACE + FRAPTRAN 1.4

100000

110000

120000

130000

140000

150000

160000

170000

180000

190000

200000

210 212 214 216 218 220 222

Enthalpy (kJ/kg)

Time (sec)

Average Fuel Enthalpy (MSIVC)

R5 + FRAPTRAN 1.5 TRACE + FRAPTRAN 1.4
30

4.2 Turbine Trip with Bypass Failure

4.2.1 Thermal Hydraulic Analysis data results

The TSVs started to close at 500 second and fully closed at 500.1 second. Figure 25 shows the 

comparison of steam flow rate in TRACE and RELAP5 model. At the beginning of the transient, 

the steam flow dropped rapidly in both models. As mentioned in previous case, because the 

friction data is hard to be determined in the RELAP5 model, the steam flow rate might decreased 

(or increased) faster than that in TRACE model. The steam inside the reactor vessel was trapped; 

hence, the dome pressure increased as shown in Figure 26. Due to the increasing dome pressure, 

the void fraction of the reactor core would decrease, which caused a positive reactivity feedback. 

As a result, the power increased as shown in Figure 27. Once the TSVs reached to 90% open, 

reactor scram signal was sent out with a delayed time 0.08 second. That is, the negative scram 

feedback would function at 500.09 second. However, the scram reactivity feedback needed some 

time to dominate the core power. Hence, as shown in Figure 27, the core power kept increasing 

until the scram feedback dominated the power at 501.4 second. Despite that the reactor scram 

control system in RELAP5 model was same as that in TRACE model, the negative reactivity 

feedback dominated the core power with a timing difference. Although the reactor scrammed at 

501.4 second, the dome pressure still increased as shown in Figure 26 because the decay heat 

still produced steam inside the reactor core both in TRACE and RELAP5 model. 

As the dome pressure reached to 7.94 MPa, the relief valves open signal was sent out with 

delayed time 0.4 second. Hence, as shown in Figure 26, the relief valves really functioned until 

the dome pressure reached to 8.2 MPa. Once the relief valves opened, the dome pressure 

decreased in TRACE model. However, in the RELAP5 model, the dome pressure would maintain 

at 8.5 MPa, which was still under the ASME regulation 9.58MPa. 

Due to the opening of the relief valves, the steam flow rate increased again as shown in Figure 

25. However, only six relief valves functioned in this hypothetical accident, the steam flow was 

only 0.3 times of the normal operating flow rates. From the analysis results of RELAP5 and 

TRACE models, it is noticed that the reactor scram system can successfully inhibit positive 

reactivity which comes from the decreasing void fraction. In addition, even though the bypass 

valve is failed and only six relief valves are functioned, the dome pressure can still be controlled 

sufficiently.
31

Figure 25 Steam flow variation during the TTBF hypothetical accident 

Figure 26 Dome pressure variation during the TTBF hypothetical accident

-1000

-500

0

500

1000

1500

2000

500 501 502 503 504 505 506

Flow Rate (kg/sec)

Time (Sec)

Steam flow

Main steam flow_RELAP5 Main steam flow_TRACE

7

7.2

7.4

7.6

7.8

8

8.2

8.4

8.6

500 501 502 503 504 505 506

Pressure (MPa)

Time (Sec)

Dome pressure

Dome pressure_RELAP5 Dome P_TRACE
32

Figure 27 Core power variation during the TTBF hypothetical accident

4.2.2 Fuel Rod Properties

As shown in Figure 28, the cladding temperature increased with power increasing. From this 

figure, it is obvious that the cladding temperature is far away from the criterion 1088K. Due to the 

increasing temperature of the fuel rod, the gas inside would be inflated. As a result, the gap gas 

pressure inside cladding would expand the cladding hoop strain. Further, the increasing cladding 

temperature would also cause the thermal cladding hoop strain expansion. The cladding hoop 

strain would increase with increasing temperature. However, at the beginning of the transient 

(500.3 to 500.5 seconds), the influence of dome pressure increasing is much more than that of 

fuel rod temperature. The dome pressure dominated the cladding deformation. After that, the 

influence of temperature became large and as a result the cladding hoop strain increased until 

501.2 seconds, the time step of reactor scram. After the reactor scram, core power dropped 

rapidly and the temperature decreased. The dome pressure outside cladding then dominated the 

hoop strain variation again. During the transient, the cladding hoop strain was not larger than 

criterion 0.01 (as shown in Figure 29). In addition to the cladding temperature and hoop strain, 

fuel pellet enthalpy is also an important criterion during the transient. Figure 30 shows that the 

maximum enthalpy in the TTBF transient of 12 nodes of fuel rod is about 231800 J/kg, which was 

much lower than the criteria 710600 J/kg. 

Same as the MSIVC case, the fuel rod properties are compared to past research. As mentioned 

in 4.1 section, the variation of 12 positions on the fuel rod was averaged to make the figure clear. 

Figure 31 is the cladding temperature comparison of RELAP5-FRAPTRNA 1.5 and TRACEFRAPTRAN 1.4 analysis. From this figure, it shows that the cladding temperature variation of 

these two analysis was quiet similar. The temperature of RELAP5-FRAPTRAN 1.5 case 

increased slightly earlier than that of TRACE-FRAPTRAN 1.4 case. It is because the power of 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

500 501 502 503 504 505 506

Power (MW)

Time (Sec)

Core Power (MW)

Power_RELAP5 Power_TRACE
33

RELAP5 analysis increased earlier as shown in Figure 41. Figure 32 shows that the cladding 

hoop strain of these two cases is similar but the strain of the RELAP5-FRAPTRNA 1.5 case is 

lower at the end of the transient. It is reasonable because the dome pressure of RELAP5 analysis 

kept in a higher value than that of TRACE analysis. Figure 33 shows the fuel enthalpy comparison. 

For both analysis, the enthalpy variation and value are quiet similar. Both curves in Figure 33 

decreased slowly due to the power scram. 

Figure 28 Cladding temperature of TTBF transient

560

565

570

575

580

585

590

595

600

605

500 501 502 503 504 505 506

Temperature (K)

Time (sec)

Cladding Temperature (TTBF)

Node 1

Node 2

Node 3

Node 4

Node 5

Node 6

Node 7

Node 8

Node 9

Node 10

Node 11

Node 12
34

Figure 29 Cladding hoop strain of TTBF transient

Figure 30 Fuel enthalpy of TTBF transient

0.0013

0.00135

0.0014

0.00145

0.0015

0.00155

0.0016

0.00165

0.0017

500 501 502 503 504 505 506

Strain

Time (sec)

Cladding Hoop Strain (TTBF)

Node 1

Node 2

Node 3

Node 4

Node 5

Node 6

Node 7

Node 8

Node 9

Node 10

Node 11

Node 12

0

50000

100000

150000

200000

250000

500 501 502 503 504 505 506

Enthalpy (kj/kg)

Time (sec)

Average Fuel Enthalpy (TTBF)

Node 1

Node 2

Node 3

Node 4

Node 5

Node 6

Node 7

Node 8

Node 9

Node 10

Node 11

Node 12
35

Figure 31 Cladding temperature comparison of TTBF transient

Figure 32 Cladding hoop strain comparison of TTBF transient

580

582

584

586

588

590

592

594

596

500 501 502 503 504 505 506

Temperature (K)

Time (sec)

Average Cladding Temperature (TTBF)

R5 + FRAPTRAN 1.5 TRACE + FRAPTRAN

1.40E-03

1.45E-03

1.50E-03

1.55E-03

1.60E-03

1.65E-03

500 501 502 503 504 505 506

Strain

Time (sec)

Average Cladding Hoop Strain (TTBF)

R5 + FRAPTRAN 1.5 TRACE + FRAPTRAN
36

Figure 33 Fuel enthalpy comparison of TTBF transient

4.3 Load Rejection with Bypass Failure

4.3.1 Thermal Hydraulic Analysis data results

Figure 34 shows the comparison of steam flow rate in TRACE and RELAP5 models. Once the 

turbine control valves closed, the steam flow rate dropped in both models. However, the steam 

flow in RELAP5 model dropped more rapidly than that in TRACE model even with same TCVs 

closure time. Due to the declination of the steam flow, the dome pressure increased immediately 

as shown in Figure 35. As a result, the void fraction inside the reactor core decreased which 

caused a positive reactivity feedback. The power increased as shown in Figure 36. Different from 

previous two cases, the reactor scram signal was generated at once as the turbine control valves 

closed. The power ratio would decrease at the beginning of the transient both in the TRACE and 

RELAP5 models. After 210.5 second, the power increased because the positive void fraction 

reactivity feedback dominated the power ratio. However, at 211.1 second for RELAP5 model and 

211.2 second for TRACE model, the negative scram reactivity feedback dominated the power 

ration again. The power dropped rapidly. After 211.76 second, the safety/relief valves opened and 

released the steam inside the reactor vessel. Same as previous two cases, the dome pressure of 

RELAP5 model would maintain at a higher pressure and the dome pressure of TRACE model 

would decrease a little after the SRVs opening. Nonetheless, both the dome pressures of TRACE 

and RELAP5 model were under controlled and the steam flow rate was back to a stable value. 

On the other hand, the recirculation pump and feedwater pump trip signal would be sent out to 

100000

110000

120000

130000

140000

150000

160000

170000

180000

190000

500 501 502 503 504 505 506

Enthalpy (kJ/kg)

Time (sec)

Average Fuel Enthalpy (TTBF)

R5 + FRAPTRAN 1.5 TRACE + FRAPTRAN
37

avoid the void fraction increased after reactor scram. As a result, Figure 37 shows that the core

flow rate decreased. 

From those data results mentioned above, the dome pressure did not exceed the criteria 9.58Mpa 

which was regulated by ASME. Further, all the inside and outside flow rates became stable. The 

Kuosheng NPP was under control in the LRBF transient.

Figure 34 Steam flow variation during the LRBF hypothetical accident 

Figure 35 Dome pressure variation during the LRBF hypothetical accident

-1000

-500

0

500

1000

1500

2000

2500

Flow Rate (kg/sec)

210 211 212 213 214 215 216

Time (Sec)

Steam flow

Main steam flow_RELAP5 Main steam flow_TRACE

7

7.2

7.4

7.6

7.8

8

8.2

8.4

8.6

210 211 212 213 214 215 216

Pressure (MPa)

Time (Sec)

Dome Pressure

Dome pressure_RELAP5 Dome pressure_TRACE
38

Figure 36 Core power variation during the LRBF hypothetical accident

Figure 37 Core flow variation during the LRBF hypothetical accident

0

1000

2000

3000

4000

5000

210 211 212 213 214 215 216

Power (MW)

Time (Sec)

Core Power

Power_RELAP5 Power_TRACE

6000

7000

8000

9000

10000

11000

12000

13000

210 211 212 213 214 215 216

Flow rate (kg/sec)

Time (Sec)

Core Flow

Core flow_RELAP5 Core flow_TRACE
39

4.3.2 Fuel Rod Properties

Once the TCVs started to close, the power ratio increased and as a result, the cladding 

temperature of all positions increased as shown in Figure 38. Theoretically, the increasing power 

and temperature would inflate the gap gas and cause the thermal hoop strain which expanded 

the cladding hoop strain. However, the TCVs closed in a short time and as a result the dome 

pressure increasing and elastic hoop strain would dominate the cladding deformation at the 

beginning of the transient. Hence, as shown in Figure 39, the cladding hoop strain decreased at 

the beginning of transient until 211 seconds. At about 211.1 seconds, the temperature was high 

enough to dominate the cladding deformation. However, the temperature expanded the cladding 

just for 0.2 to 0.3 second because the reactor scrammed and hence the fuel and cladding 

temperature decreased. The dome pressure squeezed the cladding again so that the cladding 

hoop strain kept decreasing to the end of analysis. From the figure, it is obvious that cladding 

hoop strain of LRBF transient was not larger than criteria 0.01. In addition to the cladding 

temperature and hoop strain, fuel enthalpy is another important criterion for fuel rod analysis. 

From Figure 40, maximum of the fuel enthalpy for all positions is just 230750 J/kg, which is far 

from the acceptance limit 710600 J/kg.

In addition to obtaining the mechanical variation of fuel rod during the transient, comparing these 

analysis results with past research is another main goal of current research. Figure 41 shows the 

comparison of cladding temperature of RELAP5-FRAPTRAN 1.5 and TRACE-FRATRAN 1.4 

analyses. To simplify the comparison, 12-node data is averaged. Same as comparisons of the other 

two cases, as shown in Figure 42, it is obvious that at the steady state, the cladding temperature of 

RELAP5-FRAPTRAN 1.5 analysis is higher than that of TRACE-FRATRAN 1.4 analysis because the 

RELAP5 calculation predicted a higher coolant temperature which is an input condition for 

FRAPTRAN code. In addition, for the cladding hoop strain comparison shown in Figure 42, the 

variation predicted by RELAP5-FRAPTRAN 1.5 model is higher than that of TRACE-FRATRAN 1.4 

model due to the cladding temperature difference. Further, in this figure, a peak value of RELAP5-

FRAPTRAN 1.5 variation was found at about 211.3 seconds because the dome pressure of RELAP5 

analysis was in an obvious decreasing trend during 210.98 to 211.2 seconds so that the cladding 

expanded in this period. After that, variations of both models are quiet similar. However, the cladding 

hoop strain obtained from RELAP5-FRAPTRAN 1.5 model decreased more than that obtained from 

TRACE-FRATRAN 1.4 model because the dome pressure of RELAP5 analysis kept in a higher value 

while the dome pressure of TRACE analysis decreased.

As for the fuel enthalpy, the RELAP5-FRAPTRAN 1.5 model predicted a higher value than TRACEFRATRAN 1.4 model predicted as shown in Figure 43. The main reason of this difference would also 

be the temperature. Though the RELAP5-FRAPTRAN 1.5 model predicted a higher enthalpy, in 

general, the variation is quiet similar than that TRACE-FRAPTRAN 1.4 model predicted. 
40

Figure 38 Cladding temperature of LRBF transient

Figure 39 Cladding hoop strain of LRBF transient

560

565

570

575

580

585

590

595

600

605

210 211 212 213 214 215 216

Temperature (K)

Time(sec)

Cladding Temperature (LRBF)

Node 1

Node 2

Node 3

Node 4

Node 5

Node 6

Node 7

Node 8

Node 9

Node 10

Node 11

Node 12

0.0013

0.00135

0.0014

0.00145

0.0015

0.00155

0.0016

0.00165

0.0017

210 211 212 213 214 215 216

Strain

Time (sec)

Cladding Hoop Strain (LRBF)

Node 1

Node 2

Node 3

Node 4

Node 5

Node 6

Node 7

Node 8

Node 9

Node 10

Node 11

Node 12
41

Figure 40 Fuel enthalpy of LRBF transient

Figure 41 Cladding temperature comparison of TTBF transient

0

50000

100000

150000

200000

250000

210 211 212 213 214 215 216

Enthalpy (kj/kg)

Time (sec)

Average Fuel Enthalpy (LRBF)

Node 1

Node 2

Node 3

Node 4

Node 5

Node 6

Node 7

Node 8

Node 9

Node 10

Node 11

Node 12

576

578

580

582

584

586

588

590

210 211 212 213 214 215 216

Temperature (K)

Time (sec)

Average Cladding Temperature (LRBF)

R5 + FRAPTRAN 1.5 TRACE + FRAPTRAN 1.4
42

Figure 42 Cladding hoop strain comparison of LRBF transient

Figure 43 Fuel enthalpy comparison of LRBF transient

5 

0.00142

0.00144

0.00146

0.00148

0.0015

0.00152

0.00154

0.00156

0.00158

0.0016

0.00162

210 211 212 213 214 215 216

Strain

Time (sec)

Average Cladding Hoop Strain (LRBF)

R5 + FRAPTRAN 1.5 TRACE + FRAPTRAN 1.4

100000

110000

120000

130000

140000

150000

160000

170000

180000

190000

210 211 212 213 214 215 216

Enthalpy (kJ/kg)

Time (sec)

Average Fuel Enthalpy (LRBF)

R5 + FRAPTRAN 1.5 TRACE + FRAPTRAN 1.4
43

6 CONCLUSIONS

Before this research, the RELAP5/MOD 3.3 thermal hydraulic model and the TRACE-FRAPTRAN 

combination model for Kuosheng NPP have been developed. However, for the RELAP5 model, 

the fuel rods mechanical response during the transient cannot be determined. As for the TRACEFRAPTRAN combination model, the manual data transferring process is quiet complicate. Hence, 

a RELAP5/MOD 3.3 and FRAPTRAN 1.5 combination model of Kuosheng NPP which can 

transfer the data results of RELAP5 code into FRAPTRAN input deck automatically was 

successfully developed with SNAP interface. To verify the capability of this model, three 

overpressurization hypothetical transient are performed. From the analysis data, it is known that 

the fuel rod properties would not exceed the criteria among these three cases. Further, the data 

results are also compared to the past research which was done by TRACE-FRAPTRAN model. 

From the comparisons, it is known that in general, fuel rod mechanical response of these two 

models are quiet similar but the RELAP5-FRAPTRAN 1.5 predicted higher values because of the 

higher coolant temperature from RELAP5 analysis results. This research also verified the 

applicability of AptPlot and python script job stream. With proper code descripts and settlement, 

the job streams of SNAP interface can save many human resources and time to perform analysis. 

With developing the thermal hydraulic and fuel rod mechanical combination model, the 

FRAPTRAN 1.5 is recommended because it allows more boundary condition data sets so that 

the pressure, power, heat transfer coefficient and coolant temperature from thermal hydraulic 

analysis need just few data reduction to meet the FRAPTRAN code requirements for boundary 

conditions. With the useful python scripts, thermal hydraulic and fuel rod mechanism combination 

analysis such as RELAP5-FRAPTRAN can be easily applied. In the future, researchers may apply 

this job stream on uncertainty analysis and determine how the thermal hydraulic uncertainties 

terms affect the mechanical response of fuel rods. 

45

7 REFERENCES

[1] Taiwan Power Company, Final Safety Analysis Report for Kuosheng Nuclear Power 

Station Units 1&2 (FSAR), Taiwan Power Company, Republic of China (Taiwan), 2001.

[2] Taiwan Power Company, BWR-6 MARK-III Containment of Kuosheng NPS, Taiwan 

Power Company, Republic of China (Taiwan), 1995.

[3] K.Y. Lin et al., “Verification of the Kuosheng BWR/6 TRACE Model with Load Rejection 

Startup Test”, ASME verification and validation, Las Vegas, Neveda, 2012.

[4] C. Shih et al., The Development and Application of Kuosheng (BWR/6) Nuclear Power 

Plant TRACE/SNAP Model, NUREG/IA-0450, 2014.

[5] J.M. Lyu, “Development of KUOSHENG Nuclear Power Plant RELAP5-3DK Loss of 

Coolant Accident Evaluation Model Input Deck”, Graduate Thesis, National Tsing Hua 

University, 2001.

[6] Information Systems Laboratories, Inc., RELAP5/MOD3.3 Code Manual Volume I: Code 

Structure, System Models, and Solution Methods, Information Systems Laboratories, Inc., 

Rockville Maryland, 2010.

[7] K.J. Geelhood, W.G.L., C.E. Beyer, FRAPTRAN 1.5: A Computer Code for the Transient 

Analysis of Oxide Fuel Rods, NUREG/CR-7023, Pacific Northwest National Laboratory, 

Richland, Washington, 2014.

[8] K.J. Geelhood, W.G.L., C.E. Beyer, FRAPCON-3.4: A Computer Code for the Calculation 

of Steady-State Thermal-Mechanical Behavior of Oxide Fuel Rods for High Burnup, 

NUREG/CR-7022, Pacific Northwest National Laboratory, Richland, Washington, 2011.

[9] Applied Programming Technology, Symbolic Nuclear Analysis Package (SNAP) User's 

Manual, Applied Programming Technology Inc., Bloomsburg, 2007.

[10] H.C. Chang et al., “The Turbine Trip without Bypass Analysis of Kuosheng BWR/6 Using 

TRACE/FRAPCON/FRPATRAN”, American Nuclear Society Annual Meeting, Reno 

Nevada, 2014.

[11] H.C. Chang et al., Fuel Rod Performance Uncertainty Analysis During Overpressurization 

Transient for Kuosheng Nuclear Power Plant with TRACE/ FRAPTRAN/ DAKOTA Codes 

in SNAP Interface, NUREG/IA-0465, 2015.

[12] H.C. Chang et al., “Model Establishment and application of Kuosheng Nuclear Power 

Plant with RELAP5 MOD 3.3 code through the SNAP Interface”, American Nuclear 

Society Winter Meeting, Washington DC, 2015.

[13] H.C. Chang et al., RELAP5/MOD3.3 Model Assessment and Hypothetical Accident 

Analysis of Kuosheng Nuclear Power Plant with SNAP Interface, NUREG/IA-0464, 2015.

Thermal Hydraulic and Fuel Rod Mechanical Combination Analysis 

of Kuosheng Nuclear Power Plant with RELAP5 

MOD3.3/FRAPTRAN/Python in SNAP Interface

NUREG/IA-0477

Jong-Rong Wang*, Chunkuan Shih*, Hao-Chun Chang*, Shao-Wen Chen*, 

Show-Chyuan Chiang**, Tzu-Yao Yu**

*Institute of Nuclear Engineering and Science, 

National Tsing Hua University; Nuclear and New 

Energy Education and Research Foundation

101 Section 2, Kuang Fu Rd., HsinChu, Taiwan

K. Tien

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

After the measurement uncertainty recapture power uprates, Kuosheng nuclear power plant (NPP) was uprated the power from 2894

MWt to 2943 MWt. For this power upgrade, several analysis codes were applied to assess the safety of Kuosheng Nuclear Power 

Plant. In our group, there were a lot of effort on thermal hydraulic code, TRACE, had been done before. However, to enhance the 

reliability and confidence of these transient analyses, thermal hydraulic code, RELAP5/MOD3.3 will be applied in the future. The main 

work of this research is to establish a RELAP5/MOD3.3 model of Kuosheng NPP with SNAP interface. Model establishment of RELAP5 

code is referred to the Final Safety Analysis Report (FSAR), training documents, and TRACE model which has been developed and

verified before. After completing the model establishment, three startup test scenarios would be applied to the RELAP5 model. With 

comparing the startup test data and TRACE model analysis results, the applicability of RELAP5 model would be assessed. 

Recently, Taiwan Power Company is concerned in stretch power uprated plan and uprates the power to 3030 MWt. Before the stretch 

power uprates, several transient analyses should be done for ensuring that the power plant could maintain stability in higher power 

operating conditions. In this research, three overpressurization transients scenario including main steam isolation valves closure, 

turbine trip with bypass failure and load rejection with bypass failure would be performed by RELAP5 MOD3.3 code. Further, the 

thermal hydraulic properties of the reactor core will be transferred as the boundary conditions of FRAPTRAN code. With the boundary 

conditions from RELAP5 code, the fuel rod mechanical properties during the transient could be determined. In this research, the SNAP 

interface is applied so that the transferring process between RELAP5 and FRAPTRAN code can be completed automatically with 

Python Job Stream. That is, the researchers need not calculate and transfer the thermal hydraulic boundary conditions for FRAPTRAN 

analysis manually.

November 2016

Technical

**Department of Nuclear Safety

Taiwan Power Company

242, Section 3, Roosevelt Road

Zhongzheng District, Taipei, Taiwan

Measurement Uncertainty Recapture (MUR)

Stretch Power Uprate (SPU)

RELAP5/MOD3.3

Main Steam Isolation Valve Closure (MSIV)

Turbine Trip with Bypass Failure

Load Rejection with Bypass Failure

Kuosheng Nuclear Power Plant (NPP)



NUREG/IA-0477 Thermal Hydraulic and Fuel Rod Mechanical Combination Analysis of Kuosheng Nuclear Power 

Plant with RELAP5 MOD3.3/FRAPTRAN/Python in SNAP Interface

November 1, 2016