NUREG/IA-0195

International 

Agreement Report 

LBLOCA Analysis in a 

Westinghouse PWR 3-Loop Design 

Using RELAP5/MOD3 

Prepared by 

J.I. Sinchez, C.A. Lage, T. Nfifiez 

Empresa Nacional del Uranio S.A. 

Santiago Rusinol, 12 

28040 Madrid 

SPAIN 

Office of Nuclear Regulatory Research 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001 

January 2001 

Prepared as part of 

The Agreement on Research Participation and Technical Exchange 

under the International Code Application and Maintenance Program (CAMP) 

Published by 

U.S. Nuclear Regulatory Commission
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NUREG/IA-0195

International 

Agreement Report

LBLOCA Analysis in a 

Westinghouse PWR 3-Loop Design 

Using RELAP5/MOD3 

Prepared by 

J.I. Sinchez, C.A. Lage, T. Ndifiez

Empresa Nacional del Uranio S.A. 

Santiago Rusinol, 12 

28040 Madrid 

SPAIN 

Office of Nuclear Regulatory Research 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001 

January 2001 

Prepared as part of 

The Agreement on Research Participation and Technical Exchange 

under the International Code Application and Maintenance Program (CAMP) 

Published by 

U.S. Nuclear Regulatory Commission
ABSTRACT.

This report documents the analysis of a postulated Large Break Loss-of-Coolant 

Accident (LBLOCA) in a Westinghouse 3-Loop PWR design analysed using the "Best 

Estimate" code RELAP5/MOD3. This LBLOCA calculation represents ENUSA's 

contribution to the "Code Assessment and Maintenance Program" (CAMP). 

The code used for this analysis is RELAP5/MOD3.2 - the latest CAMP version that 

was available to ENUSA when this study was performed. Nevertheless, since this 

version lacked of a reflood axial mesh renoding model, a developmental version was 

also used to analyse the reflood portion of the accident. This developmental version is 

RELAP5/MOD3.2 fg. 

Five calculations were analysed, and the results of these were compared. The first 

case described in this document compares two runs made, using the same input deck, 

on two different platforms: the CRAY-YMP, and the ALPHA SERVER 4100. Both 

calculations were done with a basic nodalization: downcomer modelled with one 1-D 

component (collapsed downcomer), without the gap conductance model, and without 

the reflood model. Both cases were run with the original RELAP5/MOD3.2 version. 

The second case was run to check the impact of modelling a quasi-three-dimensional 

downcomer, modelled with three 1 -D components joined with cross-flow junctions. For 

this case, the base nodalization used in the first case was modified by adding the new 

downcomer model, and then compared with the previous results. This case was run on 

the ALPHA SERVER 4100 with version RELAP5/MOD3.2. 

Finally, for the third case analysed, two additional input decks were prepared. Both of 

these included the three-dimensional downcomer nodalization and the gap 

conductance model. The first calculation was done with RELAP5/MOD3.2 with its 

standard heat transfer package. The second calculation was done with the reflood 

model activated, and using the developmental version RELAP5/MOD3.2 fg. 

The first case analysed showed minimal differences between the results obtained on 

the two platforms used. The second case analysed showed the impact of the 

downcomer nodalization and the three-dimensional effects are shown to be non

negligible. The third case analysed clearly shows the need for a specific reflood model 

for this kind of transient, instead of the standard heat transfer package. The standard 

heat transfer package produces a very oscillatory behavior under reflooding conditions.

iii
INDEX OF CONTENTS 

SECTION TITLE PAGE 

A BSTRACT ....................................................................................... iii 

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

1. INTRO DUCTION ................................................................................ 1 

2. PLANT DESCRIPTION AND SCENARIO ......................................... 2 

3. CODE VERSION AND NODALIZATION ........................................... 5 

4. CASE 1: COMPARISON BETWEEN CRAY-YMP AND 

ALPHA SERVER 4100 RESULTS ................................................. 10 

5. CASE 2: RESULTS OBTAINED BY MODELING A 

COLLAPSED AND A THREE- DIMENSIONAL 

DOW NCOM ER ................................................................................. 29 

6. CASE 3: REFLOOD MODEL VS. STANDARD HEAT 

TRANSFER PACKAGE ................................................................... 44 

7. RUN STATISTICS ............................................................................ 59 

8. SUMMARY AND CONCLUSIONS .................................................. 60 

9. ACKNOWLEDGMENTS .................................................................. 61 

10. LIST OF REFERENCES .................................................................. 61

V
INDEX OF TABLES 

TABLE TITLE PAGE 

1. INITIAL CONDITIONS AND RELEVANT INPUT 

PA R A M ET E R S .................................................................................. 4 

2. SEQUENCE OF EVENTS (CASE 1) ................................................. 11 

3. ANALYSIS OF RESULTS (CASE 1) .................................................. 11 

4. SEQUENCE OF EVENTS (CASE 2) ................................................ 30 

5. ANALYSIS OF RESULTS (CASE 2) .................................................. 30 

6. SEQUENCE OF EVENTS (CASE 3) ................................................. 45 

7. ANALYSIS RESULTS (CASE 3) ...................................................... 46 

8. RUN STATISTICS FOR THE FIRST CASE ANALYSED ................ 59 

9. RUN STATISTICS FOR THE SECOND CASE ANALYSED .............. 60 

10. RUN STATISTICS OF THE THIRD CASE ANALYSED .................... 60

vi
INDEX OF FIGURES

FIGURE TITLE PAGE 

1. THREE LOOPS NODALIZATION ............................................................ 5 

2. VESSEL NODALIZATION (COLLAP. DOWNCOMER) ....................... 6 

3. BREAK MASS FLOW (PUMP SIDE) ..................................................... 12 

4 INTEGRATED MASS FLOW AT BREAK (PUMP SIDE) .................. 13 

5. BREAK MASS FLOW (VESSEL SIDE) ............................................. 14 

6. INTEGRATED MASS FLOW AT BREAK (VESSEL SIDE) ............... 15 

7. PRESSURIZER PRESSURE ............................................................. 16 

8. ACCUMULATOR MASS FLOW (INTACT LOOP 1) ......................... 17 

9. CORE COLLAPSED LIQUID LEVEL (HOT CHANNEL) ................... 18 

10. DOW NCOMER LIQUID LEVEL ........................................................ 19 

11. CORE INLET FLOW (HOT CHANNEL) ............................................. 20 

12. CORE OUTLET FLOW (HOT CHANNEL) ......................................... 21 

13. CLAD TEMPERATURE AT POS. 7 ................................................. 22 

14. CLAD TEMPERATURE AT POS. 8 ................................................. 23 

15. CLAD TEMPERATURE AT POS. 9 ................................................. 24 

16. CLAD TEMPERATURE AT POS. 10 ............................................... 25 

17. CLAD TEMPERATURE AT POS. 11 ................................................. 26 

18. CLAD TEMPERATURE AT POS. 12 ............................................... 27 

19 . C P U T IM E .............................................................................................. 2 8 

20. PRESSURIZER PRESSURE .............................................................. 31 

21. INTEGRATED MASS FLOW AT BREAK (PUMP SIDE) .................. 32 

22. INTEGRATED MASS FLOW AT BREAK (VESSEL SIDE) ............... 33 

23. LIQUID FRACTION IN THE VESSEL LOWER PLENUM ................. 34 

24. CORE INLET FLOW (HOT CHANNEL) ................................................. 35 

25. COLLAPSED CORE LIQUID LEVEL (HOT CHANNEL) ................... 36 

26. CLAD TEMPERATURE AT POS. 7 (HOT ROD) ............................... 37

vii
INDEX OF FIGURES 

FIGURE TITLE PAGE 

27. CLAD TEMPERATURE AT POS. 8 (HOT ROD) .................................... 38 

28. CLAD TEMPERATURE AT POS. 9 (HOT ROD) .................................... 39 

29. CLAD TEMPERATURE AT POS. 10 (HOT ROD) ............................. 40 

30. CLAD TEMPERATURE AT POS. 11 (HOT ROD) ............................. 41 

31. CLAD TEMPERATURE AT POS. 12 (HOT ROD) ............................. 42 

32. DOWNCOMER COLLAPSED LEVEL. 3D ......................................... 43 

33. CORE INLET MASS FLOW (HOT CHANNEL) ................................. 47 

34. CORE OUTLET MASS FLOW (HOT CHANNEL) .............................. 48 

35. CORE COLLAPSED LIQUID LEVEL (HOT CHANNEL) ................... 49 

36. HEAT TRANSFER COEFFICIENT AT POS. 4 (HOT ROD ............... 50 

37. HEAT TRANSFER COEFFICIENT AT POS. 8 (HOT ROD) ............... 51 

38. HEAT TRANSFER COEFFICIENT AT POS. 9 (HOT ROD) .............. 52 

39. CLAD TEMPERATURE AT POS. 7 (HOT ROD) ................................ 53 

40. CLAD TEMPERATURE AT POS. 8 (HOT ROD) ................................ 54 

41. CLAD TEMPERATURE AT POS. 9 (HOT ROD) ................................ 55 

42. CLAD TEMPERATURE AT POS. 10 (HOT ROD) ............................. 56 

43. CLAD TEMPERATURE AT POS. 11 (HOT ROD) ............................. 57 

44. CLAD TEMPERATURE AT POS. 12 (HOT ROD) ............................. 58 

45. CLAD RADIUS (HOT ROD) .............................................................. 59

viii
EXECUTIVE SUMMARY

This report documents the analysis of a postulated Large Break Loss-of-Coolant 

Accident (LBLOCA) in a Westinghouse 3-Loop PWR design analysed using the "Best 

Estimate" code RELAP5/MOD3. This LBLOCA calculation represents ENUSA's 

contribution to the "Code Assessment and Maintenance Program" (CAMP). 

The code used for this analysis is RELAP5/MOD3.2 - the latest CAMP version that 

was available to ENUSA when this study was performed. Nevertheless, since this 

version lacked a reflood axial mesh renoding model, a developmental version was 

also used to analyse the reflood portion of the accident. This developmental version is 

RELAP5/MOD3.2 fg. 

Five calculations were done, and the results of these compared. The first case 

described in this document compares two runs made, using the same input deck, on 

two different platforms: the CRAY-YMP, and the ALPHA SERVER 4100. Both 

calculations were done with a basic nodalization: downcomer modelled with one 1-D 

component (collapsed downcomer), without the gap conductance model, and without 

the reflood model. Both cases were run with the RELAP5/MOD3.2 version. Minimal 

differences between the results obtained in both machines have been showed. 

The second case run was to check the impact of modelling a quasi-three-dimensional 

downcomer (modelled with three 1 -D components joined with cross-flow junctions). For 

this case, the basic nodalization used in the first case was modified by adding the new 

downcomer model, and then compared with the previous results. This case was run on 

the ALPHA SERVER 4100 with version RELAP5/MOD3.2. This case show a non

negligible impact of the downcomer nodalization on the ECCS bypass phenomenon. 

The quasi-three-dimensional approximation reduces the ECCS bypass and, 

consequently, the core recovery in the reflood period begins earlier. 

Finally, for the third case analysed, two additional input decks were prepared. Both of 

these included the three-dimensional downcomer nodalization and the gap 

conductance model. The first calculation was done with RELAP5/MOD3.2 with its 

standard heat transfer package. The second calculation was done with the reflood 

model activated, and using the developmental version RELAP5/MOD3.2 fg. The 

results with the heat transfer package show a oscillatory behaviour when compared 

with the reflood model calculation. 

Due to the highly dependence of the downcomer model in this accident, where 3-D 

phenomena are very important, further work is considered necessary to assess the 

capability of RELAP5/MOD3.2 to model this accident. 

All cases analyzed show ample margin to the 10CFR50.46 limits. That is, the peak 

cladding temperature (PCT) is well bellow 2200 2F, the maximum local oxidation is well 

bellow 17 % of the initial cladding thickness, and the overall hydrogen generation is 

well bellow 1%. A coolable core geometry is maintained, and long term cooling of the 

core is assured.

ix
1. INTRODUCTION.

This report documents the "best estimate" code RELAP5/MOD3 studies done by ENUSA 

as the "in kind" contribution to the "Code-Assessment and Maintenance Program" 

(CAMP). Taking as basis the simulation of a Large Break Loss of Coolant Accident 

(LBLOCA) for a Spanish 3-loop PWR, the study considered the different aspects and 

scope that are indicated next: 

"* To verify the effect of the downcomer nodalization in a LBLOCA study, in which three

dimensional phenomena are expected to be important. 

"* To verify the gap conductance model during the steady state calculation, by 

comparison against results from the steady-state fuel rod design codes used at 

ENUSA. 

"* To compare the results obtained with the standard heat transfer package and the new 

reflood model under reflooding conditions (the latter available in the 3.2fg version). 

"* To check the code performance for a given analysis, using two different computer 

platforms on which the code is currently installed at ENUSA. 

The plant selected for this LBLOCA analysis is a typical three-loop Westinghouse 17x17 

fueled PWR design. 

RELAP5/MOD3 was developed at the Idaho National Engineering Laboratory (INEL) 

for the U. S. Nuclear Regulatory Commission (USNRC). The code version in this study 

is RELAP5/MOD3.2, the latest version released under the CAMP program at the time 

of this study. 

The postulated accident consists in a 200% double-ended guillotine break in the cold leg, 

which is considered a design-basis accident in licensing applications. It must be 

demonstrated from the analysis that the Emergency Core Cooling System (ECCS) 

provides sufficient protection to keep the core below the limits that are defined in the 

10CFR50.46 regulation. 

Chapter 2 provides a description of the reactor and describes the postulated accident 

scenario analysed in this study. Chapter 3 describes briefly both the standard and 

developmental code versions used for this study. Chapter 4 describes the base case 

(first case) calculation on the two different computer platforms (CRAY-YMP and ALPHA

SERVER 4100). A sensitivity analysis considering three-dimensional effects in the 

downcomer during the blowdown phase is documented in Chapter 5 (second case). The 

performance of the dynamic gap conductance and clad deformation models, and a 

comparison of the standard heat transfer package and the reflood model is included in 

Chapter 6 (third case). Since the as-released RELAP5/MOD3 version 3.2 does not 

include a fine mesh reflood capability, a new version of the code having this capability 

was requested from the USNRC. This version, still under development, is the 3.2fg

I
version which was used for the third case analysed. Finally, run statistics are given in 

Chapter 7. Finally, summary and main conclusion are given in chapter 8. 

2. PLANT DESCRIPTION AND SCENARIO. 

The plant analysed is a 3-Loop PWR of Westinghouse design, with a rated power of 

2686 MWth and a thermal design flow rate of 270,000 gpm. The core is composed of 

157 fuel assemblies of the 17x17 lattice design and has an active fuel height of 12 feet. 

The accident scenario consists of a LBLOCA 200% double-ended guillotine break 

located in the cold leg in the loop to which the pressurizer surge line is connected. This 

break location and size produces the most limiting results in the safety analysis for this 

reactor. At the time of the break, loss of offsite power is assumed with the reactor 

operating at 100% rated power. 

For this transient, the depressurization of the reactor coolant system (RCS) produces a 

pressure decrease in the pressurizer until the reactor trip signal is reached. The ECCS 

is subsequently activated when this low pressure trip setpoint is reached. 

In the initial stages of the transient, the core is covered with subcooled liquid, and the 

heat transfer between the fuel rod wall and the coolant is provided by the mechanism 

of forced convection, with some nucleate boiling. Very early in the transient, departure 

from nucleate boiling is reached, at which point the heat transfer becomes very 

unstable and a sudden increase in cladding temperature is experienced, due mainly to 

the redistribution of the internal stored energy in the fuel. 

When the primary pressure drops below the accumulators pressure, subcooled water 

from the accumulator enters the pressure vessel. Part of this water, however, is lost 

through the break to the containment building. 

The blowdown phase ends when the pressure in the primary system reaches the 

pressure of the containment building. Water discharged from the accumulators begins 

to replenish the pressure vessel lower head and lower plenum, and reaches the 

bottom of the core. This represents the beginning of the reflood phase. 

The downcomer water column provides the driving force needed to reflood the core 

and return the cladding temperature to saturation. The ECCS continues refilling the 

downcomer to maintain the pressure vessel water inventory. 

Table 1 summarises important input parameters and the initial conditions at the 

beginning of the transient that were assumed in the calculations. Although 

RELAP5/MOD3 is a best-estimate code, some conservative assumptions in the initial 

and boundary conditions were made in the calculations. These assumptions are listed 

below:

2
"* Peaking factors at the maximum values specified in the plant's Technical 

Specifications. 

"* Chopped Cosine Axial Power Shape. 

"* Maximum stored energy in the fuel rods (only in the third case analysed; see 

Chapter 6). 

"* Decay power taken to be 120% of the 1971 ANS standard. 

"* Constant Containment Pressure at 1 bar. 

"* Single failure criterion for the ECCS assumed (the failure of one diesel engine 

precludes the actuation of two of the three safety injection pumps). 

"* Conservative delays in the ECCS actuation assumed (25 seconds). 

"* Reactor coolant pumps tripped at the beginning of the transient. 

"* ECCS in Broken loop not modelled.

3
Table 1. Initial Conditions and Relevant Input Parameters

Core Power 

Peak linear Power 

Total Core Peaking Factor FJ 

Core Axial Power Shape 

Hot Assembly Average Rod Radial 

Peaking Factor F,, 

Fuel Type Analyzed 

Rod Internal Pressure 

Secondary Steam Pressure 

Steam Generator Tube Plugging Level 

Initial Loop Mass Flow Rate 

Number of LHSI/HHSI Pumps Running 

Assumed Delay for full Safety Injection Flow 

Accumulator Tank Volume 

Accumulator Water Volume 

Accumulator Water Temperature 

Barrel-Baffle Configuration

100% of 2686 Mwt 

12.9 kw/ft 

2.40 

Chopped Cosine 

1.60 

17xl 7 

7.29 Mpa 

6.72 Mpa 

0% 

4416 kg/sec. 

1 

25 sec. 

41.05 m3 

27.6 m3 

305 K 

Up-flow

4
3. CODE VERSION AND NODALIZATION.

The code version used in this study is RELAP5/MOD3.2, the latest version officially 

released under the CAMP agreement at the time of this study. 

Since the as-released RELAP5/MOD3.2 does not include a fine mesh reflood capability, 

a new version of the code was requested from the USNRC. This version is the 3.2fg 

version (Internal INEL name), a developmental version of RELAP5 that includes the 

modifications made in the Mod3.2.1.2 version, and the updates made since April 1996, 

plus a fix for reflood memory and the Paul Scherrer Institute (PSI) reflood model (Ref. 1). 

This version was used for the reflood sensitivity studies (third case analysed). 

The nodalization diagram is shown in Figures 1 and 2. This nodalization follows the 

general recommendations given in the RELAP user's guide for this kind of analysis (Ref. 

2).

Figure 1. Three Loops Nodalization

5
upper head 200 

upper head 2o0 5 2Z guide tubes 

L. J2= upper plen. 

I 2S5 no-active fuel 

downcom. 220 

! - . / Zcore bypass 

* /- m average chan. 

210 hot chan. 

2z no-active fuel 

lower plen.225 & lower sup. plate 

Figure 2. Vessel Nodalization (collap. downcomer). 

The final nodalization scheme used the following groupings of components, in which the 

three coolant loops were modelled explicitly: 

0 The primary side was modelled with 219 hydrodynamic volumes (241 in the 

three-dimensional downcomer case), and 233 junctions (284 in the three

dimensional downcomer case). 

* The secondary side contains 51 hydrodynamic volumes and 57 junctions. 

* The number of heat structures are 81 and the total number of mesh points used 

are 558. 

The detail of this nodalization and the maximum time step selected (0.05 seconds during 

the blowdown period and 0.0125 sec. during the reflood period) are considered to be fine 

enough to give an accurate and stable solution for a LBLOCA transient. 

The following nodalization numbering scheme was used: 

"* Components 200 to 295 to model the reactor vessel. 

"* Components 10 to 13 to model the broken loop, including the pressurizer surge line. 

"* Components 40 to 43 to model the pressurizer 

"* Components 14 to 38 to model the intact loops 

"* Components 110 to 130 to model the steam generators (primary side) 

"* Components 500 to 912 to model the steam generators (secondary side) 

A brief description of the nodalization is given below.

6
Reactor Vessel

The reactor pressure vessel consists of the vessel wall, downcomer, core barrel, lower 

head, inlet plenum and internals, the core, upper plenum and internals, the guide tubes 

and support columns, and the upper head. The vessel is modelled using several 

components to describe the multiple flowpaths inside the vessel (see Fig. 2). 

The downcomer was modelled with an "annulus" component with 8 axial nodes. In the 

three-dimensional downcomer model, three "annulus" components were modelled with 

three multiple cross flow junction components. Three "sngljun" components connect the 

downcomer with each separate cold leg. The lower head is modelled with a single 

"branch" component. Another "branch" component is used to model the lower plenum up 

to the elevation where the active fuel portion of the core begins. 

The core is divided in two regions: the first, representative of the average power zone 

with 148 fuel assemblies, and the second operating at a higher power and comprised of 

9 fuel assemblies. Both regions are modelled with "pipe" components of 15 axial cells, 

interconnected to model crossflow paths with a "mtpljun" component. With this 

nodalization the hot channel is reasonably modelled and the hot channel/average 

channel core volumes maintain a ratio of 1/16 (in accordance with rules of Ref. 2). 

Three 15 axial nodes heat structures are modelled to represent the average fuel rod in 

the average power region (39,072 rods). Another rod is modelled to represent the 

average fuel rod in the high power region (2,375 rods) with a rod power 1.125 times 

below the hot rod power. Finally, a single 104 kW hot rod (the hot assembly average rod 

radial peaking factor FAH is 1.60). All rods are modelled with a chopped cosine axial 

power shape with an axial peaking factor (FNz) equal to 1.50. The fuel rod was modelled 

with ten radial mesh points to represent the different constitutive materials in the rod. A 

parabolic radial flux depression profile, representative of beginning of life conditions, was 

assumed. For the first and second cases, which were not run with the gap conductance 

model, an average pellet temperature of 1243 K was obtained. For the third case 

analysed (comparison of the reflood model and the standard heat transfer package), run 

with the conductance gap model, the average pellet temperature obtained was 1454 K, 

in good agreement with results from ENUSA's steady-state fuel performance codes. 

A core bypass flow path was modelled between the lower and upper plena with a single 

cell "pipe" component. The bypass flow is about 4.5% of the total core flow in steady 

state conditions. 

The upper plenum was modelled with 2 "branch" components and 1 "snglvol" 

component. The CCFL model was used, with the appropriate parameters for this 

geometry in the upper plenum. 

The upper guide tubes were modelled with a four cell "pipe" component that connects the 

upper plenum region with the upper head. 

The upper head was modelled with two branch components. 

Primary Loops. 

Each of the three loops was modelled separately, including the hot leg, the pressurizer 

and surge line, the steam generator, the reactor coolant pump, the cross-over leg, cold

7
leg and the accumulator tanks. The ECCS was modelled as a boundary condition (time 

dependent junctions). 

Hot Leg 

The hot leg for the intact loops was modelled with "pipe" components with a total of six 

cells. The hot leg in the broken loop was also modelled with a total of six cells, but using 

three components: two "branch" components and a central "pipe" component where the 

pressurizer surge line is connected. The hot leg elbow to the steam generator plenum 

using the CCFL option has been explicitly modelled. 

Pressurizer and Surge Line 

The pressurizer, connected to the broken loop through the surge line, was modelled with 

one 4 cells "pipe" component for the top section of the pressurizer, and one "branch" 

component for the bottom section of the pressurizer. 

The surge line was modelled with one "pipe" component with a total of 15 cells. 

A suitable control logic was included in the model to reach the desired initial pressurizer 

water level and initial pressure. 

Steam Generator 

- Primary Side. 

Five components were used to represent the primary side of each steam generator. Four 

"branch" components were used to model the inlet and outlet steam generator 

chambers. The CCFL option, with the constant parameters representative of this 

geometry, was used in the connection of the inlet plenum with the steam generator 

tubes. A single tube, representing the whole tube bundle, was modelled using a "pipe" 

component with 12 cells. 

- Secondary Side. 

Up to 23 components were used to model the secondary side of each steam generator 

and the steam lines, including the suitable elements to adjust the downcomer level at the 

desired value. 

The boiler was simulated with a "pipe" component with 6 cells. The bundle option was 

selected. Hydraulic diameters were calculated and included in the input deck to allow the 

code to properly determine the interfacial drag. 

The transition region from the boiler to the separator was modelled with a branch 

component. 

The primary separators were modelled using the RELAP separator component. 

The steam dome was modelled with a branch component. 

The steam generator downcomer was modelled with a branch component for the upper 

downcomer, and with an annulus component with 6 cells for the lower downcomer. In

8
order to get the specified circulation ratio, appropriate form loss energy coefficients were 

included in the downcomer and in its connection with the boiler. 

Time-dependent volume, time-dependent junction and valve components were used to 

simulate the steam generator secondary boundary conditions (pressure and level 

proportional-integral controls, steam and feedwater boundary conditions) for the steady 

state condition and during the transient. The steam generator relief and safety valves. 

were also modelled. The Auxiliary Feed Water System was not included in this model. 

- Heat Structures 

Heat structures were used to model the tube wall. 

Cross-Over Leg 

Each cross-over leg portion of the primary piping was modelled using a "pipe" component 

with 10 cells. 

Reactor Coolant Pump 

The Reactor Coolant Pumps (RCP's) were simulated using the RELAP "pump" 

components, with specific head and torque homologous curves for a Westinghouse 7000 

HP one-stage centrifugal pump design. 

Cold Leg 

The cold legs in the intact loops were modelled with one "branch" component to which 

the ECCS is connected and one "pipe component" with 2 cells. The cold leg in the 

broken loop was modelled with two "pipe" components with 2 (pump side) and 3 cells 

(vessel side). 

Accumulator and discharge lines 

The Accumulators were simulated with the RELAP "accum" components. The 

Accumulator discharge surge lines were modelled with "pipe" components. A check valve 

connects the discharge line with the cold legs. The Accumulator in the broken cold leg 

was not modelled. 

Break 

The break was modelled with two instantaneously opening "trip valves" connected to a 

constant pressure "tmdpvol". Choked flow multipliers equal to 1.0 were assumed for 

these valves, with the homogeneous option activated, as recommended in the RELAP 

manuals.

9
4. CASE 1: COMPARISON BETWEEN CRAY-YMP and ALPHA SERVER 4100 

RESULTS. 

Two calculations were done, using the same input deck, on two different computer 

platforms and the results were compared. In this way, the effect of the machine 

accuracy and the code compilation method on the results could be determined. 

The idea to perform these calculations arose from the CAMP May 1996 meeting in 

Madrid. In this meeting, some participants reported very different results, in some 

cases, running the same input deck on different platforms. 

For this comparison case, version RELAP5/MOD3.2 was installed with the default 

values on both the CRAY-YMP and ALPHA SERVER 4100 platforms, and the same 

input deck run on both machines. 

The input deck used for these calculations is the most simple one: the reflood model 

was not selected (it is not implemented in RELAP5/MOD3.2), a one-dimensional 

downcomer was modelled, and the gap conductance model was not turned on 

(average pellet temperature of 1243 K obtained at the hottest position). 

Figs. 3 and 4 show the break flow and the integrated break flow, respectively, both on 

the pump side. In Figs. 5 and 6, the break flow and the integrated flow on the vessel 

side are shown. This mass flow rate is very high at the beginning of the transient due 

to the subcooled choked flow at the break location. Nevertheless, the liquid in the RCS 

reaches saturation conditions early in the transient, and the choked flow decreases 

drastically. It can be observed that the results obtained on the two computer platforms 

are very close to each other. The same can be said from Fig. 7 where the pressurizer 

pressure is plotted. 

Figs. 11 and 12 show the core inlet and outlet mass flow rate, respectively. During the 

blowndown period of the transient, the core flows shown in Figs. 11 and 12 produce 

two quenchings. The first is a bottom-up quench, as can be observed in Fig. 11, which 

finishes approximately 10 seconds after the beginning of the transient. The second one 

is a top-down quench that can be observed in Fig. 12. This quench begins 10 sec. 

after the beginning of the transient, and finishes at t = 20 seconds. Both quench 

phenomena produces the characteristic two peaks in the clad temperature during the 

blowdown period that can be observed in Figs. 13 through 18. 

Fig. 8 shows the accumulator mass flow rate. The accumulator injection begins at time 

(t) equals 11 seconds. The accumulator empties at t = 42 sec. Figures 9 and 10 show 

the core collapsed liquid level and the downcomer collapsed liquid level, respectively, 

(data obtained by multiplying the cell liquid fraction by the cell length). After the 

blowdown, the liquid level in the core begins to increase at t = 31 sec., which marks the 

beginning of the reflood period. It can be observed that, during the reflood period, the 

differences obtained on the two computer platforms increase. These differences are 

displayed in Figures 11 and 12 (core inlet flow and core outlet flow rate, respectively). 

Figures 13 through 18 show the clad temperature at different axial positions along the 

hot rod (note that the core axial positions are numbered from 1 to 15, and that position 

1 corresponds to the lower elevation of the active fuel region). The differences 

between the two platforms are found to be small and the impact in the conclusion is

10
null. As shown in Fig. 9, globally the amount of liquid calculated in the core is very 

close between the two computer platforms. 

Nevertheless, the CPU time spent on both platforms differs drastically, as can be 

observed in Fig. 19. With the installation of the default RELAP5 version on the CRAY

YMP, the CPU time needed is near 24,000 seconds, while on the ALPHA-SERVER 

4100, the CPU time is lower than 4,000 seconds. The number of time advancements 

were 47190 and 49388 on the CRAY-YMP and ALPHA-SERVER 4100, respectively. A 

summary of the run statistics for these two cases can be found in Chapter 7 . Due to 

the excessive CPU consumption on the CRAY-YMP, it was decided to continue this 

CAMP study running all cases on the ALPHA-SERVER 4100 workstation. 

Table 2 shows the sequence of events during the transient, and in Table 3, the result 

analysis for this first case is included. The local oxidation and the burst location are not 

calculated, because the gap conductance and oxidation models were not used. 

Table 2. Sequence of Events (Case 1)

Reactor trip signal 2.0 sec. 2.0 sec. 

SI signal generated 3.0 sec. 3.0 sec. 

Accumulator injection 11 sec. 11 sec. 

End of bypass 19 sec. 19 sec. 

End of blowdown 26 sec. 26 sec. 

Safety injection 26 sec. 26 sec. 

Beginning of reflood 31 sec. 31 sec. 

Accumulator empties 42 sec. 42 sec. 

Table 3. Analysis of Results (Case 1) 

CRAY-YMP run ALPHA-SERVER run 

Peak clad temperature 1140 K 1125 K 

Time of peak clad temp. 31 sec. 31 sec. 

Peak clad temp. elevation 1.9512 m 1.9512 m 

Local oxidation _ _ 

Hot rod burst time 

Hot rod burst location

II

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5. CASE 2: RESULTS OBTAINED BY MODELING A COLLAPSED AND A 

PSEUDO THREE-DIMENSIONAL DOWNCOMER. 

Traditionally, one of the most important limitations for using RELAP5/MOD2 in the 

LBLOCA analysis has been the lack of a three-dimensional component to model the 

three- dimensional effects that are anticipated to occur in the Reactor Pressure Vassel 

and, in particular, in the downcomer. 

The use of cross flow components allows the user to model three-dimensional 

geometries, but the momentum flux terms are neglected in these cross flow junctions. 

In practice, the use of cross flow junctions has been limited to leak flow paths with low 

flows compared with the main stream. 

Several improvements to the cross flow model have been made since the release of 

RELAP5/MOD2. The latest version, RELAP5/MOD3.2, includes an improved cross 

flow model. Momentum in the cross flow junctions is conserved, but it's not a full three

dimensional model, because transport of the momentum between one direction and 

another is not modelled. 

Taking into consideration these limitations, a new nodalization was made that only 

differs in the modelling of the downcomer. In the first run, the nodalization models the 

downcomer as a single pipe of 8 cells (previous nodalization used in Case 1). In the 

second run, the downcomer is modelled as made up of three pipes (of 8 cells) joined 

by cross flow junctions. The results of this second run with the three-dimensional 

downcomer was then compared with the first run to check the effect of the downcomer 

nodalization on the ECCS bypass flow and the lower plenum swept out. 

Fig. 20 shows the pressurizer pressure for both runs. 

Figs. 21 and 22 show the integrated mass flow at the break location on the pump and 

vessel side, respectively. It can be observed that the integrated mass flow is, on the 

pump side, very close in both cases. Nevertheless, on the vessel side of the break, the 

mass flow delivered differs for both cases, with the break mass flow rate for the 

collapsed nodalization model being higher than when the three-dimensional 

nodalization is used. 

Looking at Fig. 22, it would be expected that the three-dimensional nodalization model 

would predict more liquid in the vessel. This can be seen in Fig. 23 which plots the 

liquid fraction in the lower plenum. In the three dimensional nodalization, the lower 

plenum is refilled earlier than in the collapsed nodalization model. The ECCS bypass 

flow is higher in the collapsed nodalization model, than in the three dimensional 

nodalization. Fig. 24 shows the core inlet mass flow in the hot channel. The liquid 

enters the core earlier in the three-dimensional nodalization. The core liquid level can 

be observed in Fig. 25 (obtained by multiplying the cell lengths by their liquid void 

fractions). 

Figures 26 through 31 show the cladding temperatures obtained at different elevations 

(the core was modelled with 15 axial elevations). Due to the higher amount of liquid 

calculated with the three-dimensional nodalization, the cladding temperatures are 

clearly lower than for the collapsed nodalization downcomer model. Finally, Fig. 32 

shows the collapsed downcomer level (obtained by multiplying the cell lengths by their

29
liquid void fractions) in the three downcomer pipes used to model the three

dimensional downcomer. The labels "broken", "intact1" and "intact2" are used to 

indicate to which of the downcomer pipe components, the broken cold leg is joined to. 

It can be observed that the amount of liquid in the downcomer pipe connected to the 

broken cold leg is lower during the blowdown, and slightly higher than in the rest of the 

downcomer pipes due to the sweep phenomena to the break. During refill 

(approximately 20 sec. after the beginning of the transient), once terminated the ECCS 

bypass, the liquid in the two downcomer pipes connected to the two intact cold legs 

are higher due to the ECCS addition, showing the non-symmetric behaviour expected. 

The sequence of events and the analysis results are given in Tables 4 and 5. Local 

oxidation and the burst variables have not been calculated because the gap 

conductance model and the oxidation model were not used for this second case. 

Table 4. Sequence of Events (Case 2).

*1

Reactor trip signal 2.0 sec. 2.0 sec. 

Sl signal generated 3.0 sec. 3.0 sec. 

Accumulator injection 11 sec. 11 sec. 

End of bypass 19 sec. 18 sec 

End of blowdown 26 sec. 26 sec. 

Safety injection 26 sec. 26 sec. 

Beginning of reflood 31 sec. 28 sec. 

Accumulator empties 42 sec. 42 sec. 

Table 5. Analysis of Results (Case 2) 

Collap. Downcomer run 3D downcomer run 

Peak clad temperature 1125 K 1080 K 

Time of peak clad temp. 31 sec. 29 sec. 

Peak clad temp. elevation 1.9512 m 1.9512 m 

Local oxidation 

Hot rod burst time 

Hot rod burst location'

30

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6. CASE 3: REFLOOD MODEL vs. STANDARD HEAT TRANSFER PACKAGE. 

In Case 3 two different runs were made. The first run was made with the three

dimensional downcomer model and using the gap conductance model of 

RELAP5/MOD3.2. Due to the lack of a reflood model in this CAMP version, the second 

run was made using a development version: RELAP5/MOD3.2fg. This second run was 

also made using the three-dimensional downcomer modelling, the gap conductance 

model, as well as the new reflood model provided in this version. Therefore, both runs 

differ only in the use of the reflood model in the second run (and also, in the code 

version). Nevertheless, in order to only check the impact of the reflood model, the new 

models implemented in the beta version (that can be requested by the user via the 

developmental input card) were shut off. 

RELAP5/MOD3.2fg includes the reflood improvements made at the Paul Scherrer 

Institute (PSI) in Switzerland in RELAP5/MOD3.2 to improve the quench front 

behaviour during the reactor core reflood portion of the transient. Changes were made 

to the interfacial drag, interfacial heat transfer and wall heat transfer. If the code user 

activates "reflood" on word 6 of the heat structure data card, then the code uses these 

updates. 

With the cold fuel dimensions, the gap conductance model calculated for both runs an 

average fuel temperature in the hot rod at the hottest position (12.9 kw/ft) of 1454.1 K, 

in good agreement with the steady state fuel design codes used at ENUSA. The hot 

fuel dimensions obtained in the steady state were 2.2837*10" m in the gas gap and 

4.579*103 m in the clad outer radius. 

The objective of Case 3 is to strictly compare the effect of the reflood model. A careful 

review of the comparison between both runs (e.g., using reflood model vs. using the 

default heat transfer package) shows that the standard heat transfer package is more 

oscillatory than the reflood model. 

The figures labeled with -ref show the results obtained with the reflood model. Figures 

labeled with -wref show the results obtained with the standard heat transfer package 

(without reflood model). Figure 33 through 35 show the core inlet mass flow, core 

outlet flow, and the collapsed core liquid level. It can be observed that the core mass 

flow and the liquid amount is very oscillatory without the reflood model. A more detailed 

review of Fig. 34, for instance, shows that between t = 20 and 40 seconds (just at the 

beginning of the reflood period), large amounts of water are swept out to the vessel 

upper plenum in the standard heat transfer package calculation. 

The oscillatory hydrodynamic behaviour caused by the standard heat transfer package 

is produced by the non-smooth behavior of the heat transfer coefficient. Figs. 36 

through 38 show the heat transfer coefficient at three different core elevations. It can 

be observed that large peaks are calculated by the standard heat transfer package in 

the heat transfer coefficient. This is due to the lack of a heat transfer quench model 

and a fine renodalization to properly characterize the quench level. 

In the standard heat transfer package, heat transfer coefficients in the rewetting zone 

(film boiling - transition boiling - nucleate boiling) are computed based on the local fluid 

conditions. In the refIlood model, the heat transfer coefficient is also space- dependent. 

This means that the heat transfer coefficient is gradually forced to decrease in the

44
transition film boiling regime, upstream of the quench level. This reduction is done by 

multiplying the heat transfer coefficient obtained with a standard correlation by an 

empirical factor exponentially dependent on the distance to the quench position. 

The exponential reduction of the heat transfer coefficient in the transition film boiling 

regime is, in concept, based on the stability of the vapor film formed just above the 

quench front. The continuous steam generation in the quench front causes the 

formation of a steam film that reduces the heat transfer upstream of the quench front. 

A large amount of water can exist just upstream of the quench front, but the vapor film 

precludes a high heat transfer coefficient. The standard package does not take this 

effect into account, with the heat transfer coefficient dependent only on the bulk fluid 

conditions. 

Figs. 39 through 44 show the cladding temperature at selected core elevations (elev. 7 

to 12). The run made with the standard heat transfer package (label -wref) and the run 

made with the reflood model (label -ref) calculate similar cladding temperatures at the 

lower elevations (see Figs. 39 & 40), with the standard heat transfer package 

calculation being more oscillatory. Nevertheless, at the higher elevations, the results 

using the reflood model differ from those obtained using the standard heat transfer 

package. In the run made with the standard heat transfer package, the rewetting is 

delayed. As can be seen in Fig. 35, for the run made with the reflood model, more 

water remains in the core at t = 100 seconds from the beginning of transient (because 

lower amounts of water are swept to the upper plenum). 

The clad outer radius is shown in Fig. 45, at t = 200 seconds, in the hot rod for both 

runs. Due to the different cladding temperatures obtained with the reflood model and 

the standard heat transfer package, the cladding rupture positions and the clad plastic 

deformation varies. In the reflood model run, the clad fails at position 10 at t = 21.84 

seconds. With the standard heat transfer package, rupture is calculated at t = 21.6 

seconds, at elevation 9. The sequence of events and the analysis results are given in 

Tables 6 and 7, respectively. 

Table 6. Sequence of Events (Case 3) 

Reflood model run Standard HT run

Reactor trip signal 2.0 sec. 2.0 sec. 

SI signal generated 3.0 sec. 3.0 sec. 

Accumulator injection 11 sec. 11 sec. 

End of bypass 18 sec. 18 sec 

End of blowdown 26 sec. 26 sec. 

Safety injection 26 sec. 26 sec. 

Beginning of reflood 28 sec. 28 sec. 

Accumulator empties 42 sec. 42 sec.

45
Table 7. Analysis Results (Case 3)

R~flnod model run Standard HT run

Peak clad temperature 1190 K 1193 K 

Time of peak clad temp. 30 sec. 3 sec. 

Peak clad temp. elevation 1.9512 m 1.9512 m 

Local oxidation 0.95 % 0.8 % 

Hot rod burst time 22 sec. 21.6 sec. 

Hot rod burst location 2.43 m 2.187 m

46
i.ue+ui

mflowi-ref 

-4 - I-i mflowj-wref 

7.5e+02 

iB 

1 6 7 .5 e + 0 2 . . . . . .. .. . .. .. . ......... .... . . . . ... : .... .. 

O.O +0 it 1: 

5.5e+02 

:11 

.._. II N, 

° 'ii 

"• 2 .5e+ 02 . . . ... .. .... : ... .. .. ... . . .. .!........ ..... 

O.Oe+O0O 

-2,5e+02 . . . . ...... .

-5.0e+02 .... 

-20.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 

Time (s)

- -- -I
1 .Ue+03 

7.5e+02 

5.0e+02 

2.5e+02 

O.Oe+00 

-2.5e+02 

-5.0e+02 

-20.0

. . ..

0.0 20.0 40.0 60.0 80.0 

Time

100.0 120.0 140.0 160.0 

(s)

180.0

-- mflowj-ref - 4- mflowi-wref 

4 

200.0

00

co 

0

It, 

Oo,, 

T

I
4.0e+O0 ... 

- I level-ret 

1 -4 - flevel-wret 

3 .0 e + 0 0 .. .. . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . .. 

2.0e+O 0 ........................ . . .... . ., 

1.0e+O0 U)G 0 

O.Oe+O0 

-20.0 0.0 20. 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0

Time (s)
8.0e+04 

7.0e+04 

6.0e+04 

5.0e+04 

4.0e+04 

3.0e+04 

2.0e+04 

1.0e+04 

O.Oe+00 

-20.0 40.0 60.0 80.0 100.0 

Time (s)

.-'--, hthtc-ret 

[-4 - F ýhthtc-wret

120.0 140.0 160.0 180.0 200.0

U'=

0 

.5o 

0o 0 

00 

C: 

Cu L,... 

IU) 

"I"

N= 

ft 

ft 

N=
/.ue+U4 

6.0e+04 

5.0e+04 

4.0e+04 

3.0e+04 

2.0e+04 

1.0e+04 

O.Oe+O0 

-20.0 80.0 100.0 

Time (s)

120.0 140.0 160.0 L.0 180.0

tCR

E 

0• 

0 

4'

U, 

I--

- hthtc -ret 

-4-f-ihthlc-wref 

200.0

*2

N 

ft 

N 

ft 

9 

N 

lb N 

ft 

lb 

Ce 

N 

N0.0
f.ue+uq r

4 - Ihthtc-wret 

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0 N 

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I-- . . 

•-" 2.0e+04. .. 

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O.Oe+O0 

-20.0 0.0 20.040'o 60.0 80.0 100.0 120.0 140.0 160.0 180.0 2000

Time (s)
2 

4) 

I.

L.. 

{3. 

E 

t

-C

ftJ wA

(I4--- tdemp -ref 

4 -FItemp-wrel

1.1e+U3 

1.0e+03 

1,.Oe+03 ... 

9.0e+02 

8.0e+02 

7.0e+02 

6.0e+02 

5.0e+02 

4.0e+02 

-20.0

-I a~ 

0. .... 0 6 

* t ! * I 

* . 

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0

Time (s)

'1 

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ft 

0 

-I ft I 

N 

ft 

-4 

a

N

..........
cv 

-

a) 

a) 

I (A 

1

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1.3e+03 

1.2e+03 

1.1e+03 

1.0e+03 

9.0e+02 

8.0e+02 

7.0e+02 

6.0e+02 

5.0e+02 

4.0e+02 

-20.0

±-e-)temp-ref 

-4 - i1temp-wret

N 

N 

N 

Co 

N

I 'I 

II 

II

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 

Time (s)

I - ---
CL 

IE 

U) 

H 

0 

(n

i.,ie+uJi 

1 .2e+03 

1.1e+03 

1.Oe+03 

9.0e+02 

8.0e+02 ..... 

7.0e+02 

6.0e+02 

5.0e+02 

4.0e+02 

-20.0

-- )temp-ref 

-4 - f]]temp-wref

'I 

I'

" ................

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 

Time (s)

*4 

0ý

................ 

........................
,- lemp-rcf 

-~- -lemp-wref

1.je+0U3 

1.2e403 

1.1 e+03 

1.0e+03 

9.0e+02 

8.Oe+02 

7.0e+02 

6.0e+02 

5.0e+02 

4.0e+02 

-20.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 

Time (s)

as 

0) 

E 

0) 

I

0~ 

-C 

ciN 

N 

-I 

N 

N 

I., 

N
1.2e+03 

1.le+03 

1.0e+03 

9.0e+02 

8.0e+02 

7.0e+02 

6.Oef02 

5.0e+02 

4.0e+02 

-20.0 20.0

£ 

L_ 

1_r (n

60.0 80.0 

Time

'I /' N

t 

V1 

*1 

'I

100.0 

(s)

(- )emp- rel 

F- - 4-temp-wref

... .......

120.0 140.0 160.0 180.0 200.0

-- 4

0.0 40.0

070 

fz 

ft. 

P

.........
4--+)temp-ref 

-t - i Itemp-wret

1.2e+03 

1.1e+03 

1.0e+03 

9.0e+02 

8.0e+02 

7.0e+02 

6.0e+02 

5.0e+02 

4.0e+02 -... -20.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 

Time (s)

t. O0

7 

I

0

U) 

C

E 

0~ 

-C

N 

-i 

N 

N 

N

'I '

0.0 2 0.0 40.0 60.0 I .L
CLAD RADIUS

9 00E-03 -

8 00E-03 

7 00E-03 

6 00E-03 

5 OOE-03 

_ 400E-03

.8. 

UU 

5

/ S I

- -" without reflood

J OCE-03

2 00E-03 

1 00E-03 

0 O0E -0C

2 3 4 5 6 7 8 9 10 11 12 13 14 15 

axial poftlions

Figure 45. Clad radius (hot rod) 

7. RUN STATISTICS 

The first case analysed was run both on a CRAY-YMP computer, and on a work

station ALPHA SERVER 4100. The run statistics are given in Table 8. 

Table 8. Run statiscs for the first case analysed. 

Number of cells 270 

Number of time steps (CRAY/ALPHA) 47190/49388 

Maximum time step size 0.05 sec. 

Problem time 400 sec. 

CPU time. (CRAY/ALPHA) 23552/3816 sec 

CPU x 1000 / (n vol x n time steps) (CRAY/ALPHA) 1.85/0.29 

The second case analysed was run on an ALPHA SERVER 4100. For this second 

case, two runs were made, using a downcomer collapsed model in the first and using a 

three-dimensional downcomer model in the second. The run statistics for these two 

runs are given in Table 9.

59
Table 9. Run statistics for the second case analysed. 

Number of cells (collapsed/3D) 270/292 

Number of time steps (collapsed/3D) 49388/39849 

Maximum time step size 0.05 sec. 

Problem time 400 sec. 

CPU time. (collapsed/3D) 3816/3520 sec 

CPU x 1000 / (n vol x n time steps) (collapsed/3D) 0.29/0.30 

The third case analysed was run on an ALPHA SERVER 4100. For this third case, two 

runs were made, using the RELAP5/MOD3.2fg developmental version, with the reflood 

model activated in the first and the standard heat transfer package of 

RELAP5/MOD3.2 (CAMP release) used in the second. The run statistics for these two 

runs are given in Table 10. 

Table 10. Run statistics of the third case analysed. 

Number of cells 292 

Number of time steps (reflood/standard) 33558/43057 

Maximum time step size 0.05 sec. 

Problem time 400 sec. 

CPU time. (reflood/standard) 3411/3887 sec 

CPU x 1000 / (n vol x n time steps) (reflood/standard) 0.35/0.31 

8. SUMMARY AND CONCLUSIONS 

The objectives set forth in Chapter 1 have been accomplished, namely: 

The effect of the downcomer nodalization (collapsed vs. three-dimensional) has 

been checked. 

The results obtained using the gap conductance model have been compared with 

those from the steady-state fuel performance design codes used at ENUSA. 

The results obtained with the standard heat transfer package have been 

compared with the new reflood model implemented in RELAP5/MOD3.2fg 

(developmental version). 

Two different runs obtained on different computer platforms, using the same input 

deck, were compared. 

The comparison of two identical runs, on two different computer platforms where 

RELAP5/MOD3.2 was installed , shows the choice of platform to have very little effect on 

the results for this transient.

60
The downcomer model, 1 -D, 3-D, has an effect on the transient results. In this study, two 

different downcomer models were compared: a collapsed nodalization downcomer and 

a three- dimensional nodalization of the downcomer, using cross flow junctions. With the 

3-D downcomer model, the ECCS bypass is reduced. The initiation of the reflood period 

is delayed in the collapsed nodalization compared to the 3-D nodalization model. 

Finally, the results show good agreement in the steady-state average fuel temperature 

when the gap conductance model is used. The comparison made between the standard 

heat transfer package (RELAP5/MOD3.2) and the new reflood model implemented in the 

developmental version (RELAP5/MOD3.2 fg) shows the very important effect of this new 

model on the results. The standard heat transfer package, applied to reflooding 

phenomena, produces a very oscillatory behavior of the heat transfer coefficients, and 

consequently, in the hydraulic behaviour as well. The reflood model has a more smooth 

and reliable behaviour. 

All cases analyzed show ample margin to the 10CFR50.46 limits. That is, the peak 

cladding temperature (PCT) is well below 1477 K (2200 2F), the maximum local oxidation 

is bellow 17% of the initial cladding thickness, and the overall hydrogen generation is 

bellow 1%. A coolable core geometry is maintained, and the long term cooling of the core 

is assured. 

9. ACKNOWLEDGMENTS 

This document is a small example of the on-going work being done at ENUSA, related to 

the validation of the thermal-hydraulics codes. Many people of our organisation have 

been involved in this project. In particular, the authors want to acknowledge Rail Arias 

OrduFia, and Marina Trueba Alonso for their special contributions. 

10. LIST OF REFERENCES 

Ref. 1 INEL-96/005. PSI Reflood Changes for RELAP5/MOD3. Rev. 0. 

December 1996. 

Ref. 2. NUREG/CR-5535. RELAP5/MOD3 Code Manual. June 1995.

61
NRC FORM 335 U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER 

(2-89) (Assigned by NRC, Add Vol., Supp.. Rev., 

NRCM 1102. and Addendum Numbers, If any.) 

3201, 3202 BIBLIOGRAPHIC DATA SHEET 

(See nk os on U rever) NUREG/IA-0195 

2. TITLE AND SUBTITLE 

LBLOCA Analysis in a Westinghouse PWR 3-Loop Design Using RELAP5/MOD3 3. DATE REPORT PUBLISHED 

MONTH YEAR 

January 2001 

4. FIN OR GRANT NUMBER 

5. AUTHOR(S) 6. TYPE OF REPORT 

J.1. Sanchez, C.A. Lage, T. Nunez Technical 

7. PERIOD COVERED (Inclusive Dates) 

8. PERFORMING ORGANIZATION - NAME AND ADDRESS (if NRC. provide Division, Office or Region, U.S. Nuclear Regulatory Comnission, and mailing address; ifcontrectot. 

provide name and mailing address.) 

Empresa Nacional del Uranio S.A. 

Santiago Rusinol, 12 

28040 Madrid 

SPAIN 

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (If NRC, tpe 'Same as above'; if contractor, provide NRC Division. Office or Region, U.S. Nuclear Regulatory COmmission, 

and mailing address.) 

Division of Systems Analysis and Regulatory Effectiveness 

Office of Nuclear Regulatory Research 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001 

10. SUPPLEMENTARY NOTES 

11. ABSTRACT (200 words or less) 

This report documents the analysis of a postulated Large Break Loss-of-Coolant Accident (LBLOCA) in a Westinghouse 3-Loop 

PWR design analysed using the "Best Estimate" code RELAP5IMOD3. This LBLOCA calculation represents ENUSA~s 

contribution to the "Code Assessment and Maintenance Program" (CAMP). The code used for this analysis is 

RELAP5/MOD3.2 - the latest CAMP version that was available to ENUSA when this study was performed. Nevertheless, since 

this version lacked a reflood axial mesh renoding model, a developmental version was also used to analyse the reflood portion 

of the accident. This developmental version is RELAP5/MOD3.2 fg. Five calculations were analysed, and the results of these 

were compared. The first case described in this document compares two runs made, using the same input deck, on two 

different platforms: the CRAY-YMP, and the ALPHA SERVER 4100. Both calculations were done with a basic nodalization: 

downcomer modelled with one 1-D component (collapsed downcomer), without the gap conductance model, and without the 

reflood model. Both cases were run with the original RELAP5/MOD3.2 version. The second case was run to check the impact 

of modelling a quasi-three-dimensional downcomer, modelled with three 1 -D components joined with cross-flow junctions. For 

the case, the base nodalization used in the first case was modified by adding the new downcomer model, and then compared 

with the previous results. This case was run on the ALPHA SERVER 4100 with version RELAP5/MOD3.2. Finally for the third 

case analysed, two additional input decks were prepared. Both of these included the three-dimensional downcomer 

nodalization and the gap conductance model.

12. KEY WORDS/DESCRIPTORS (List words orphisses Moat wi assist reseamhes mncalg the report) 13. AVAILABILITY STATEMENT 

PANDA unlimited 

RELAP5/MOD3.2 14. SECURITY CLASSIFICATION 

SBWR (This Page) 

unclassified 

(This Report) 

unclassified 

15. NUMBER OF PAGES 

16. PRICE

NRC FORM 335 (2-89)
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3-LOOP DESIGN USING RELAP5/MOD3

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

WASHINGTON, DC 20555-0001 

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