NUREG/IA-0196

International 

Agreement Report 

Analysis of 

PANDA Experiments P3 and P6 

Using RELAP5/MOD3.2 

Prepared by 

L. Batet, F. Revent6s 

Department of Physics and Nuclear Engineering 

University Politecnica de Cataluna 

08028 Barcelona 

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-0196

International 

Agreement Report 

Analysis of 

PANDA Experiments P3 and P6 

Using RELAP5/MOD3.2 

Prepared by 

L. Batet, E Revent6s 

Department of Physics and Nuclear Engineering 

University Politecnica de Cataluna 

08028 Barcelona 

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

Between 1996 and 1998 Technical University of Catalonia (UPC) has been participating 

in the "Technology Enhancement for Passive Safety Systems" (TEPSS) project, in the 

Fourth Framework Program of the Commission of the European Communities. 

One of the purposes of TEPSS has been the study of the residual heat removal by 

passive means after a loss of coolant accident into a simplified boiling water reactor 

(ESBWR) type containment. To that end, eight experiments have been performed in 

PANDA experimental facility of Paul Scherrer Institute in Villigen (Switzerland). 

PANDA represent a ESBWR scaled 1:1 in elevation and 1:40 in volume. 

UPC has supplied analytical support of PANDA tests P3 and P6, using 

RELAP5/Mod3.2. P3 and P6 simulate Main Steam Line Break scenarios with different 

initial conditions and different passive safety systems available. 

This report shows the results obtained in the simulation of those experiments. 

Discrepancies between calculated and measured parameters are identified and 

discussed.

iii
Table of Contents 

A bstract .................................................................. 111 

Table of Contents ..................................................... v 

List of Figures . ........................................................... vii 

Executive Sum mr ......................................................... ix 

Acknowledgm ents .......................................................... xi 

1. Introduction ............................................................. 1 

2. The PANDA experiments in TEPSS project ................................... 1 

3. P3 experim ent .......................................................... 2 

3.1 Experimental results description .......................................... 3 

3.2 Analytical results description ............................................ 3 

4. P6 experim ent ...... ....... .......................................... 6 

4.1 Experimental results description .......................................... 7 

4.2 Analytical results description ............................................ 7 

5. Analysis of main discrepancies and anomalies ................................... 9 

6. Summary and additional considerations ....................................... 13 

7. Conclusions ............................................................. 14 

8. Run Statistics ........................................................... 15 

R eferences ................................................................ 16

V
List of Figures 

Figure 1. Drawing of PANDA facility ........................................................................ 2 

Figure 2. Nodalization of ECN RELAP5/Mod3.2 model for PANDA facility ........... 3 

Figure 3. Containment pressures evolution in test P3 ................................................. 4 

Figure 4. Simulated evolution of DW2 partial air pressures in test P3 ........................ 4 

Figure 5. Simulated evolution of DW1 partial air pressures in test P3 ........................ 5 

Figure 6. Measured partial air pressures in test P3 ..................................................... 5 

Figure 7. Containment pressures evolution in test P6 .................................................. 6 

Figure 8. Steam flow to IC and PCC1 in test P6 ........................................................... 7 

Figure 9. DW1 partial air pressure in test P6 simulation ............................................. 8 

Figure 10. Containment pressures evolution in test P3 (calculation using Colburn

H ougen m ethod) ................................................................................................... 9 

Figure 11. Temperatures in WW 1 gas space in test P3 (calculation using Colbum

H ougen m ethod) ................................................................................................. 10 

Figure 12. Containment pressures evolution in test P6 (calculation using Colburn

H ougen m ethod) ................................................................................................. 11 

Figure 13. WW2 liquid temperatures calculated using original nodalization. Test P3... 11 

Figure 14. WW2 liquid temperatures calculated using the final nodalization. Test P3.. 12 

Figure 15. Partial air pressure in WW2 (at different elevations). Test P3 ................. 12 

Figure 16. Evolution of pressures in drywell and wetwell. Test P3 .......................... 13

vii
Executive Summary

Between 1996 and 1998 Technical University of Catalonia (UPC) has been 

participating, among other European research institutions, in the "Technology 

Enhancement for Passive Safety Systems" (TEPSS) project, in the Fourth Framework 

Program of the Commission of the European Communities. 

The objective of TEPSS project was to make significant contributions to the base 

technology of Passive Advanced Boiling Water Reactors. To fulfil this objective, 

experimental work and analytical research has been undertaken in different areas, one of 

them being the study of the residual heat removal by passive means after a loss of 

coolant accident into a simplified boiling water reactor (ESBWR) type containment. 

UPC has been working in this area, in the analytical support of the experiments 

performed in PANDA experimental facility of Paul Scherrer Institute in Villigen 

(Switzerland). PANDA represent a ESBWR scaled 1:1 in elevation and 1:40 in volume. 

Eight experiments were performed in PANDA facility for TEPSS project, with the 

following general objectives: 

• to investigate the effect of new designs in the containment's short term 

response following a loss of coolant accident 

. to demonstrate that residual heat removal passive systems perform as 

anticipated in different accidental scenarios 

• to generate a data base usable to code validation and to the analysis of 

particular phenomena. 

UPC provided analytical support of the experiments P3 and P6 using 

RELAP5/MOD3.2. Test P3 simulates a main steam line break (MSLB) scenario in 

which the drywell is initially full of air and the facility configuration is highly 

asymmetrical. Two out of three condensers of the Passive Containment Cooling System 

(PCCS) are operative. Test P6 simulates a MSLB scenario with the three PCCS units 

available during the whole test and the IC available during the first seven hours. A 

leakage path from the suppression chamber to the drywell is postulated. 

The results obtained in the analyses are reasonably consistent from an overall point of 

view, but some important discrepancies with experimental data have been identified, the 

main causes being the stratification of both temperatures and air-steam mixture and the 

accumulation of air in anomalous locations. In the considered scenarios it has been 

experimentally observed that, though the air-steam mixture is not uniform in the large 

volumes, its behaviour is homogeneous, in the sense that it exhibits a smooth spatial 

variation. On the contrary, as a monodimensional nodalization has been used in 

RELAP5 calculation, anomalous accumulations of air and sharp stratifications of the 

air-steam mixture have been produced in the simulation. These phenomena are 

dominant over other causes of discrepancy. 

Nevertheless, RELAP5/Mod3.2 has shown to be a robust code, reproducing quite well 

the global behaviour of the system with reasonable computation times. So, it has been 

possible to perform some sensitivity studies which have lead to improvements in the 

model used. The fact of studying two tests in parallel has been of great usefulness in

ix
these analyses in order to have a more general scope on the effect of the changes 

introduced into the model. 

Computer codes used by conventional nuclear plant owners are requested to be able to 

simulate scenarios that are far from those classically studied. So, it is necessary to test 

the codes in situations they are not designed for. Participation in TEPSS, has been a 

chance for using RELAP5 to analyze a experiment series involving the containment and 

the heat removal passive safety systems and implying phenomena such as natural 

circulation and steam and non-condensable gas mixing, among others. In authors 

opinion, the results contributed by this study and similar ones are a reference point to be 

taken in consideration in the development process of future consolidated codes.

X
Acknowledgements

TEPSS project has been undertaken with the financial support of the Commission of the 

European Communities into the Forth Framework Program. 

PANDA experiments, performed by Paul Scherrer Institut, have been supported by 

Swiss Federal Bureau for Education and Science. 

Authors wish to acknowledge the organizations participating in TEPSS for their co

operation in order to get and publish the experimental results presented here.

xi
1. Introduction

Between 1996 and 1998 Technical University of Catalonia (UPC) has been participating 

in the "Technology Enhancement for Passive Safety Systems" (TEPSS) project, in the 

Fourth Framework Program of the Commission of the European Communities. The 

following institutions participated in TEPSS Project: the research centre ECN (The 

Netherlands), Paul Scherrer Institute (PSI) of Switzerland, the Dutch companies KEMA 

and Stork NUCON B. V. and, from the Spanish side, the research centre CIEMAT, 

Technical University of Valencia (UPV) and UPC. 

The objective of TEPSS project was to make significant contributions to the base 

technology of Passive Advanced Boiling Water Reactors (General Electric's ESBWR is 

the reference design of the project). To fulfil this objective, experimental work and 

analytical research have been undertaken in three areas. The first one is the study of 

countermeasures to the thermal stratification in the suppression pool in order to mitigate 

the system's pressure increase following the steam and gas transfer from the drywell 

and from the passive containment cooling system (PCCS). The second area is the study 

of the residual heat removal by passive means after a loss of coolant accident into an 

ESBWR type containment. Last, the third area consists in the study of the behaviour of 

the aerosol particles that would be generated in a severe accident, their elimination from 

the containment atmosphere by retention in the PCCS and the performance degradation 

of that system due to the deposition of aerosols in the internal face of the passive 

condensers' tubes [1]. 

The work carried out by UPC, linked to the second area, has consisted in supplying 

analytical support, both pre and post-test, to the experiments performed in PANDA 

experimental facility of Paul Scherrer Institute in Villigen (Switzerland). 

2. The PANDA experiments in TEPSS project 

PANDA facility represents, in its TEPSS configuration, a simplified boiling water 

reactor (ESBWR) scaled 1:1 in elevation and 1:40 in volume (figure 1). The 

configuration includes the reactor vessel (the core is simulated by electrical resistances 

which power is adjustable to the temporal evolution of residual heat), the drywell, the 

suppression chamber (wetwell), the isolation condenser (IC), the passive containment 

cooling system (PCCS) and the gravity driven core cooling system (GDCS). It also 

includes the main vent lines and the vacuum breaker valves (VB), that prevent the 

pressure in the suppression chamber (WW) to be excessively higher than the pressure in 

the drywell (DW). The PANDA asymmetrical layout makes possible the study of tri

dimensional effects [2]. 

Eight experiments were performed in PANDA facility for TEPSS project. The general 

objectives of such tests were: (1) to investigate the effect of new designs in the 

containment's short term response following a loss of coolant accident (LOCA), (2) to 

demonstrate that residual heat removal passive systems perform as anticipated in

1
different accidental scenarios and (3) to generate a data base usable to code validation 

and to the analysis of particular phenomena.

PCC Pool IC Pool

VBI

25 m

Figure 1. Drawing of PANDA facility.

Among all PANDA tests, UPC provided analytical support of the experiments P3 and 

P6, which will be outlined later on. Calculations were performed with 

RELAP5/MOD3.2 using a base input deck originally prepared by ECN [1] (a drawing 

of the nodalization can be seen in figure 2). UPC, taking advantage of the information 

received during the project, modified the model in order to better predict the behaviour 

of the experimental facility in its final configuration [1,3]. 

3. P3 experiment 

Test P3 simulates a main steam line break (MSLB) scenario in which the drywell is 

initially full of air. All the steam flow generated in the vessel is directed to DW2, where 

two condensers of the passive containment cooling system (PCC2 and PCC3) are 

connected. PCCl and IC are not used in this experiment and are isolated by valving 

them out from the system. Heating power is maintained at a constant level (850 kW) 

during the eight hours the test lasts. Both available PCCs are initially full of air [2,4]. 

The particular objective of this test is to demonstrate PCCS start-up capability under 

such challenging conditions.

2
IC no? 95> - ;31" Mr<ý> 424 77 957~I: *,P004 744'**,~ 

p~_P00-I 72..._ M6 PCC-2 K _ 

l 2 2 757 4 74 

,W I 1, M724 753 17,0 741 

602 1 8 0,712 1 717.7761716 1 722 1 2 8 3 1 T737 2 2 2 I2~79lI2 

2 2 22 2 LEI 2 2 2~ 2 2 2 

II 7 3 

....... 

2.. 

2] 47 3064 M26 4 42 

76 64 M75 7,46 4 

697 61 624 21376 

'26713,

Figure 2. Nodalization of ECN RELAP5/Mod3.2 model for PANDA facility. 

3.1. Experimental results description 

The test is initiated by adjusting the heat power and opening the valves simulating the 

main steam line break. The steam flow causes drywell pressure to rise, forcing air to 

circulate to the suppression chamber through the PCCS condensers, so that the pressure 

in that space also increases. During a first pressurization stage, air contained in the 

drywell prevents the effective start up of PCC system. 

Once air in DW2 has been purged, PCCS condensers are able to start condensing steam, 

so that pressures become stable and system reaches a quasi-steady state. The capability 

of the two PCCS operative units is less than heat power supplied electrically to the 

simulated reactor core, the excess vapour circulates to the suppression pool through the 

main steam lines, causing an increase in water temperature [5]. 

3.2. Analytical results description 

The results obtained in pre and post-test analyses are reasonably consistent from an 

overall point of view, but some important discrepancies with experimental data have 

been identified. 

Figure 3 shows the pressures evolution in the containment. During the initial stage of 

the test, calculated pressure values increase more rapidly than experimental ones. This

3

RF

• A A
behaviour is related to the fact that RELAP5 predicts the existence in DW2 of a quite 

strong stratification in the air-vapour mixture (see figure 4) that delays the steam 

entrance into the PCCS condensers.

DW measured 

--- WW measured 

DW calculated 

WW calculated

0 5000 10000 15000 

Time (s)

I ' I 3 I 

20000 25000 30000

Figure 3. Containment pressures evolution in test P3.

I, 

*1°

0 2000 4000

Time (s)

- node 8 (top) 

node 7 

node 6 

node 5 

........ node 4 

- *node 3 

node 2 

node 1 (bottom)

6000 8000

Figure 4. Simulated evolution of DW2 partial air pressures in test P3. 

In the long term, calculated system pressures remain always below measured values due 

to the large amount of air that remains trapped in the upper part of DW1, which acts as a 

dead end volume. The simulation predicts that steam penetrating DWl, in spite of being 

lighter than air, turn towards the lower part of that vessel, driving the air of the bottom

4

350000 -

300000 -

1a

C) 

=3 

C).

250000 

200000

150000 

100000 1*

C) CU 

CL a2

C-.

250000 

200000 

150000 

100000 

50000 

0

10000

I
nodes to the suppression chamber through the main vent line, while allowing an 

important mass of air to remain trapped in the upper part (figure 5). The fact that the 

pipe connecting both drywell vessels in PANDA has been modelled as a mono

dimensional volume prevents, in RELAP5 calculation, air convection from DWI to 

DW2 and from there to the wetwell through PCCS. In the experiment, air only 

accumulated in the lower part of DWl (figure 6). The remainder of the air was purged 

to the wetwell, causing the system pressure to increase.

- node 8 (top) 

node 7 

node 6 

node 5 

-........ node 4 

- -------- node 3 

node 2 

node 1 (bottom)

0 5000 10000 15000 

Time (s)

20000 25000

Figure 5. Simulated evolution of DWI partial air pressures in test P3. 

300000 

250000 

/ DW1 top / 

200000 / DW1 bottom 

1 DW2 top 

DW2 middle 

100000 -DW2 bottom

50000 

0

0 5000 10000 15000 

Time (s)

20000 25000 30000

Figure 6. Measured partial air pressures in test P3.

5

I 250000 

200000

C) 

-,l 

Co 

C, 

10_ 

C

°--

150000 

100000 

50000 

0 3 

30000

C) 

C) 

en 

Co 

C) 

a

C) 

C) 

0

I I
In a less extent, the lower calculated pressures are also related to the predicted 

stratification of water temperatures in the suppression pool, that leads to lower surface 

temperature and, consequently, to lower vapour partial pressure. That stratification 

results from the injection of hot steam through the main and PCCS vent lines into some 

specific nodes of the suppression pool. Instrumentation in PANDA facility indicates 

that no actual stratification occurs. 

As it is deduced from the stated above, observed discrepancies (stratifications and air 

accumulation in anomalous sites) have to do with the fact of using a mono-dimensional 

model of the vessels that, in PANDA, simulate the containment spaces. 

4. P6 experiment 

Test P6 simulates a MSLB scenario with the three PCCS units available during the 

whole test and the IC available during the first seven hours. Steam flow is 

symmetrically directed to both drywell vessels. The test is assumed to begin one hour 

after the postulated break, when GDCS injection phase has finished and boiling in the 

reactor pressure vessel (RPV) resumes. A leakage through one vacuum breaker (VB) 

valve, connecting wetwell and drywell, is postulated to start at four hours after the 

beginning of the experiment [2,6]. 

The particular objectives of the test are, on the one hand, to investigate the effect of the 

interaction between the PCCS and the IC on the containment and the passive safety 

systems performance and, on the other hand, to study the effect of a possible leakage 

path between drywell and wetwell on the containment performance. 

300000 

DW measured 

WW measured --.- 

280000 

DW calculated 7

----WW calculated 7 ¶ 260000 ....... 

t. 00 0 2 .. .............. 

S240000 

220000 

200000 I 

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 

Time (s) 

Figure 7. Containment pressures evolution in test P6.

6
4.1. Experimental results description

Figure 7 shows the evolution of main pressures, for both the experiment and the 

simulation. After the initial pressurization, pressure values stabilize due to the combined 

effect of PCCS and IC, that starts operating very soon, as it can be seen in figure 8. The 

observed valley in the drywell pressure plot was caused by a momentary loss of core 

heaters' electrical power. In this first stage of the test, drywell pressure plot has a 

negative slope because of the efficiency of cooling systems. 

Four hours after the beginning of the test the leakage from drywell to wetwell started. 

Due to the fact that, at that moment, the pressure difference between both vessels was 

quite small, pressures become rapidly equal. The leakage path had little relevance in the 

behaviour of PANDA facility. 

Three hours later IC was valved out of service. Containment pressure rised until the 

PCCS condensers assumed the power fraction corresponding to IC [7].

200

c, 

-

0 

LL 

a,

150 

100 

50

0

PCCl calculated

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 

Time (s) 

Figure 8. Steam flow to IC and PCCM in test P6. 

4.2. Analytical results description 

Except for the loss of heaters power (without effect on the remainder of the test), the 

behaviour of containment pressures is quite well predicted in the RELAP5 simulation 

for the first part of the scenario (up to four hours). The loss of heaters' electrical power 

was not simulated to avoid the opening of vacuum breaker (not produced during the 

experiment). Sensitivity calculations performed with the theoretical scaled decay power 

curve showed no difference with those performed using the actual power experimental 

data, except when VB opened. For that reason it was decided to perform the final 

TEPSS analyses using the theoretical power curve.

7
For the first hour and a half, the simulation predicts a heat extraction greater than in the 

experiment, as suggests the greater negative slope in the drywell pressure plot. This 

effect is related to the retention of air in the bottom part of the drywell vessels, below 

the main steam lines connection (see figure 9). Gas so accumulated should have been 

driven to PCCS condensers, slightly degrading their efficiency. Although data on partial 

air pressure evolution in the drywell are not totally reliable for test P6, temperature 

measurements suggest that such accumulation of gas does not occur in PANDA (in 

addition to that, PSI calculations using the GOTHIC code also show an homogeneous 

air-vapour mixture in the drywell). 

The predicted excess cooling rate of PCCS and IC is compensated by the release (due to 

the depressurization) of the trapped air. When air reaches the PCCs' tubes, the 

performance degradation of PCCS leads the pressure, after a quick increase, to stabilize 

at a value near the experimental one. 

At four hours the leakage through vacuum breaker (VB) starts and the wetwell pressure 

increase (much greater than in the experiment) makes air venting from PCCs difficult. 

Air accumulation in PCCS condensers hinders heat transfer to the pools, causing 

drywell pressure to rise. Main discrepancies between simulated an experimental results 

are found in this phase of the scenario. The abnormal behaviour has two possible 

explanations. On one hand, the pressure difference between DW and WW is greater 

than in the test, causing a greater mass flow through the leakage path. On the other hand 

a non negligible amount of air is in PCCs when the leakage starts, due to the slow 

release of air trapped in the bottom part of the drywell during the depressurization (as 

seen in figure 9). 

Simulation reflects quite well the effect of stopping IC, though system pressures 

stabilize at values greater than experimental ones. This difference is due to the fact that 

pressures are too high when IC stops. 

12000 

10000 

ý" node 8 (top) 

8000 6000 _ node 6 

------- node 4 

0 0 ' 

o_. 4000 < , 

2000 1 

0 - - -' -- ' T - • 

0 5000 10000 15000 20000 25000 

Time (s) 

Figure 9. DWJ partial air pressure in test P6 simulation.

8
Despite of the above mentioned discrepancies, predicted inlet steam flows to PCCS 

condensers and IC are not far from the experimental ones (as shown in figure 8) and 

temperatures in PCCS and IC tubes are in close agreement with the test. 

5. Analysis of main discrepancies and anomalies 

As it has been shown in the two previous sections, the main causes of discrepancy 

between experiment and simulation results are, on one hand, the stratification of both 

temperatures and air-steam mixture and, on the other hand, the accumulation of non

condensable gas (air in this case) in anomalous locations. Both effects are closely 

connected to the fact that a monodimensional nodalization has been used for simulating 

the big containment volumes [8]. 

Other causes of distortion between analytical and experimental results have been 

encountered. Some of them have been corrected without excessive work, for example 

the effect of the time step and the errors induced by the RELAP5/MOD3.2 default 

condensation model. Other discrepancies, like heat transfer in both sides of condensers' 

tubes, seem to have little influence in the final results when compared to stratifications 

and air accumulations. 

Regarding condensation in presence of non-condensable gas, RELAP5/Mod3.2 build-in 

subroutine contains the Colburn-Hougen iteration method to find the inter-phase 

temperature and, from it, the heat transfer coefficient. Such method caused huge 

distortions in the results when, too often, the iteration didn't converge, producing 

unphysical results. 

350000 

300000 

"" 2500000 

0 00 DW measured 

.WW -------- measured 

150000 DW calculated 

WW calculated 

100000 I 1 - I I I 

0 5000 10000 15000 20000 25000 30000 

Time (s) 

Figure 10. Containment pressures evolution in test P3 (calculation using 

Colburn-Hougen method).

9
To overcome that problem, the source program was modified in order to use the so 

called UCB model (Vierow and Shrock, University of California, Berkeley), which is a 

simpler model that reduces the pure steam heat transfer coefficient using a degradation 

factor depending on mass fraction of non-condensable gas. This model, though it 

overestimates heat transfer, is quite robust and doesn't produce non-physical results. 

In order to illustrate the reasons underlying the change in the condensation model, 

figure 10 reproduces the results of a test P3 calculation using the original subroutine. 

From the analysis of pressure evolution it could be erroneously deduced that Colburn

Hougen method gives better results than UCB model. Such feeling disappears when 

looking at wetwell space temperatures (figure 11): with the original subroutine, 

RELAP5 predicts gas phase temperature values that are well above of pure steam 

saturation temperature at the maximum pressure in the experiment. Figure 12 shows the 

pressures evolution calculated with Colbum-Hougen method for test P6; it's clear that 

with UCB model better results are obtained (figure 7). 

150 

125 

a,100 

E 75 node 8 (top) 

node 7 

50 node 6 

node 5 

node 4 (surface) 25 I 1 1 -i 

0 5000 10000 15000 20000 25000 30000 

Time (s) 

Figure 11. Temperatures in WW] gas space in test P3 (calculation using 

Colburn-Hougen method). 

After some sensitivity studies it was observed that small modifications in the 

nodalization helped, in some cases, to mitigate stratification effects. Two solutions of 

this kind that were incorporated into the RELAP model of PANDA [3]: 

improvements in the connection between suppression pools and 

change in the orientation of the junctions connecting the main vent lines to the 

suppression pools. 

In the original model, the pipe connecting both suppression pools had only junctions to 

the upper face of node 3 of both wetwell vessels. In such way, part of the air vented 

through the main vent lines accumulated in the aforementioned pipe, moved the liquid 

into the pools and caused a sharp increase in the pools level. Such phenomenon has 

been avoided by complementing the downward oriented junctions with upward oriented 

ones (from the pipe to the lower face of node 4 of both wetwell vessels).

10
400000 

350000

DW measured 

---------. WW measured 

DW calculated 

WW calculated

. I I I I I . I I I I I I I I I . I 

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 

Time (s)

Figure 12. Containment pressures evolution in test P6 (calculation using 

Colburn-Hougen method).

node 4 (surface) 

node 3 

node 2 

node 1 (bottom)

I I I I 

0 5000 10000

! 

15000 

Time (s)

/

I I 1 1 

20000 25000 30000

Figure 13. WW2 liquid temperatures calculated using original 

nodalization. Test P3. 

The effects of the calculated temperatures stratification in the suppression pool have 

been mitigated reorienting the junctions at the exit of the main vent lines (originally 

connected downwards the upper face of node 2 of both wetwells), connecting them to 

the lower face of node 3. From figure 13 it can be seen that, with the original noding, 

the energy transferred through the vent lines warmed only the liquid in node 2 and the 

surface temperature remained constant during the whole simulation. Figure 14 shows 

the evolution of the temperatures calculated using the final nodalization, as compared 

with the experimental ones; it can be observed that, although a strong stratification

11

-

- - -

/ 7 I.

Cl) 

CO) 

0 

a..

300000 -

250000 

200000

105 

100

a 

0.= 

E 

CD

95

90 

85 

ort I -

___'Zý -- -- -- -- -- -- - -- -- -- -- -- -- -- -- --

----------

,,Iy 

- --- -- ---- ---- ----

J

.

Q•
persists, the calculated surface temperature (upper node) exhibits a behaviour closer to 

the real one. 

95 

Top measured 

.. - -. Mid measured 

S90 Bottom measured / , S-------- Top calculated . 

Mid calculated - .. 

CL_---- Bottom calculated E 

- 85 

------------- -- -- -- -- -- -- -- -- -- -- -- -- - 80 Ii 

0 5000 10000 15000 20000 25000 30000 

Time (s) 

Figure 14. WW2 liquid temperatures calculated using the final 

nodalization. Test P3. 

Although a suitable monodimensional nodalization can sensitively improve the global 

behaviour of the model, some causes of anomaly (i.e. stratification of air-steam mixture 

in gas spaces and accumulation of non-condensable gas in some nodes of those spaces) 

require stronger solutions like the use of three-dimensional models, but RELAP5 shows 

some limitations in this field. 

70000 

/ 

60000 

o_ 50000 

CO 40000 Measured 

69 

Calculated 1 D 

- - Calculated pseudo-3D 

30000 1 1 1 I 

0 5000 10000 15000 20000 25000 30000 

Time (s) 

Figure 15. Partial air pressure in WW2 (at different elevations). Test P3.

12
To illustrate how the predictions could be improved by departing from 

monodimensionality, test P3 has been simulated using a pseudo-3D noding (two parallel 

channels with cross-flow junctions) of wetwell gas space. Figure 15 shows the time 

evolution of partial air pressure at different elevations in WW2, as compared with the 

experimental value (each wetwell in PANDA is equipped with a single air probe). It can 

be noticed that the stratification is clearly reduced by using a non-monodimensional 

model. The behaviour of system pressures (figure 16) also shows a tendency closer to 

the real one in the simulation with a pseudo-3D model, though the large differences 

between calculation and experiment have not been corrected (due to the retention of air 

in DW1 in the simulation)

= -

Measured 

Calculated 1D 

Calculated pseudo-3D

I I I I 

5000 10000 15000 

Time (s)

-J I * I I

20000 25000 30000

Figure 16. Evolution of pressures in drywell and wetwell. Test P3. 

6. Summary and additional considerations 

In the considered scenarios and, actually, in all the experiments performed in PANDA 

for the TEPSS project, it has been observed that, though the air-steam mixture is not 

uniform in the large vessels, its behaviour is homogeneous, in the sense that it exhibits a 

smooth spatial variation. This fact has been corroborated with experimental 

measurements (partial air pressures and temperatures) as well as with 3D fluid

dynamics codes (CFD) [1,9]. 

On the contrary, the simulations with RELAP5/Mod3.2 performed by UPC, as well as 

those performed by ECN and PSI, give anomalous accumulations of air and sharp 

stratifications of the air-steam mixture. These phenomena are dominant over other 

causes of discrepancy, such as, i.e., the inaccuracy of the condensation models when 

non-condensable gases are present.

13

350000 

300000 

250000 

200000 

150000

CL, 

C,.

100000

0

I

-----------------------------

-----------------------------

- -- - - -- - - - - - - --

--------------
Nevertheless, RELAP5 has shown to be a robust code, reproducing quite well the global 

behaviour of the system (in the sense that it correctly predicts the function of the facility 

components and systems) with reasonable computation times [10]. So, it has been 

possible to perform with relative economy different sensitivity studies, as the above 

mentioned regarding time step and noding. About the latter ones, it is worth to mention 

that the fact of studying two tests in parallel has been of great usefulness. On one hand 

it has been possible to detect which renodalizations, though improving the results of 

some of the parameters calculated in one of the experiments, hindered the predictions 

from the overall point of view. On the other hand, it has also been possible to make 

changes in the model that, although penalizing some partial results, were beneficial to 

the simulation of the general scenario. In other words, studying both tests has allowed 

us to better validate the final model. 

In addition to the aforementioned, the sensitivity of the model (and of the facility) to the 

initial conditions (temperatures, partial air pressures, levels...), to the heat losses, etc. 

have been analyzed. 

7. Conclusions 

UPC has been co-operating for three years in TEPSS project, giving analytical support 

of PANDA experiments. Actually, UPC task has been to perform the pre- and post-test 

analyses of the P3 and P6 experiments using RELAP5/Mod3.2. Despite of the 

difficulties encountered in the simulations, because of code limitations and of the fact of 

using a monodimensional model of the facility, results so obtained are considered 

acceptable from a global point of view, as the goal of reproducing the global behaviour 

of the facility (functionality of components and systems) has been reached. 

RELAP5 as well as the model used have shown to be useful tools in order to analyze 

transients of the kind of PANDA experiments. It is important to take advantage of the 

code capabilities to predict the general behaviour of accidental scenarios as well as to 

perform sensitivity studies. 

Computer codes used by conventional nuclear plant owners are requested to be able to 

simulate scenarios that are far from those classically studied. So it is necessary to test 

the codes in challenging situations they are, in principle, not designed for. Thus, authors 

consider very positive their participation in TEPSS, as well as the chance of using 

RELAP5 to analyze an experimental facility simulating a passive boiling water reactor, 

in a experiment series involving the containment and the heat removal passive safety 

systems and implying phenomena such as natural circulation and steam and non

condensable gas mixing, among others. 

To conclude, in authors opinion, the results contributed by this study and similar ones 

are a reference point to be taken in consideration in the development process of future 

consolidated codes.

14
8. Run Statistics

Calculations were carried out on a SUN UltraSPARC 450, with SunOS 5.6 operating 

system. 

RELAP5/Mod3.2 was used in all the calculations, with the changes in the condensation 

model mentioned in section 5.

15
References

[1] Hart, J. et al, Technology Enhancement for Passive Safety Systems-TEPSS. Final 

Report ThNNO-TEPSS(99)-D20, 1999. 

[2] Huggenberger, M., TEPSS Project: PANDA P-Series Test Specification. PSI, 

ALPHA-703-0, TM-42-97-05, 1997. 

[3] Batet, L., Revent6s, F., Relevant results of pre and post-test analysis of P3 and P6 

PANDA experiments. IAEA Technical Committee Meeting on Experimental Tests and 

Qualification of Analytical Methods to Address Thermalhydraulic Phenomena in 

Advanced Water Cooled Reactors, PSI (Switzerland), 1998. 

[4] Polo, J., Post Test Analysis of PANDA Test P3. TEPSS Project (WP2). 

CIEMAT, INNO-TEPSS(98)-D- 14, DFN/SN- 17/IF-98, 1999. 

[5] Huggenberger, M. et al., TEPSS Project: Integral System Test P3. Test Report. PSI, 

INNO-TEPSS (98) D8, ALPHA-819, 1998. 

[6] Polo, J., Post Test Analysis of PANDA Test P6. TEPSS Project (WP2). 

CIEMAT, INNO-TEPS S(98)-D- 16, DFN/SN-01/IF-99, 1999. 

[7] Huggenberger, M. et al., TEPSS Project: Integral System Test P6. Test Report. PSI, 

INNO-TEPSS (98) Dll, ALPHA-819, 1998. 

[8] Batet, L., Chiva, S., Mufioz-Cobo, J.L., Revent6s, F., Modelling activities of 

experimental facilities related to advanced reactors. Considerations on ID/3D issues. 

ICONE7, Tokyo (Jap6n), 1999. 

[9] Bandurski, T., Putz, F., Andreani, M., Analytis, M. Modelling of particular 

phenomena observed in PANDA with GOTHIC. IAEA TCM on Experimental Tests and 

Qualification of Analytical Methods to Address Thermalhydraulic Phenomena in 

Advanced Water Cooled Reactors, PSI (Switzerland), 1998. 

[10] Ltibbesmeyer, D., Post test analyses of TEPSS tests P2, P3, P5 and P7 using the 

system code RELAP5/Mod3.2. IAEA TCM on Experimental Tests and Qualification of 

Analytical Methods to Address Thermalhydraulic Phenomena in Advanced Water Cooled 

Reactors, PSI (Switzerland), 1998.

16
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 instructions on the reverse) NUREG/IA-0196 

2. TITLE AND SUBTITLE 

Analysis of PANDA Experiments P3 and P6 Using RELAP5/MOD3.2 3. DATE REPORT PUBLISHED 

MONTH YEAR 

January 2001 

4. FIN OR GRANT NUMBER 

5. AUTHOR(S) 6. TYPE OF REPORT 

L. Batet, F. Reventos Technical 

7. PERIOD COVERED (Inclusive Dates) 

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

provide name end mailing address.) 

Department of Physics and Nuclear Engineering 

University Politecnica de Cataluna 

08028 Barcelona 

SPAIN

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (If NRC, typ '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) 

Between 1996 and 1998 Technical University of Catalonia (UPC) has been participating in the 'Technology enhancement for 

Passive Safety Systems" (TEPSS) project, in the Fourth Framework Program of the Commission of the European Communities. 

One of the purposes of TEPSS has been the study of the residual heat removal by passive means after a loss of coolant 

accident into a simplified boiling water reactor (ESBWR) type containment. To that end, eight experiments have been 

performed in PANDA experimental facility of Paul Scherrer Institute in Villigen (Switzerland). PANDA represent a ESBWR 

scaled 1:1 in elevation and 1:40 in volume. UPC has supplied analytical support of PANDA tests P3 and P6, using 

RELAP5/MOD3.2 P3 and P6 simulate Main Steam Line Break scenarios with different initial conditions and different passive 

safety systems available, this report shows the results obtained in the simulation of those experiments. Discrepancies between 

calculated and measured parameters are identified and discussed.

12. KEY WORDS/DESCRIPTORS (List words or phrases that will assist researchers in locating the report.) 13. AVAILABILITY STATEMENT 

unlimited 

RELAP5N MOD3.2 14. SECURITY CLASSIFICATION 

SBWR (This Page) 

unclassified 

(This Repo"t) 

unclassified 

15. NUMBER OF PAGES 

16. PRICE

NRC FORM 335 (2-89)
Federal Recycling Program
NUREG/IA- 0196 ANALYSIS OF PANDA EXPERIMENTS P3 AND P6 USING RELAP5/MOD3.2 JANUARY 2001

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

WASHINGTON, DC 20555-0001 

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