NUREG/IA-0246

 International 

Agreement Report

RELAP5/MOD3.3 Assessment against 

PMK Test T3.1 – LBLOCA with 

Nitrogen in PRZ

Prepared by:

P. Kral

Nuclear Research Institute Rez

Husinec-Rez 130

250 68 Rez, Czech Republic 

A. Calvo, NRC Project Manager 

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

November 2010 

Prepared as part of

The Agreement on Research Participation and Technical Exchange 

Under the International Code Assessment and Maintenance Program (CAMP)

 

Published by

U.S. Nuclear Regulatory Commission
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NUREG/IA-0246

RELAP5/MOD3.3 Assessment against PMK 

Test T3.1 - LBLOCA with Nitrogen 

in PRZ

Manuscript Completed: November 2008

Date Published: October 2010

Prepared by: Pavel Kral

Nuclear Research Institute Rez

Husinec-Rez 130

250 68 Rez, Czech Republic

Prepared for:

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

iii

Abstract

The results of RELAP5 post-test analyses of test T3.1 performed on the PMK experimental 

facility are presented. The Hungarian facility PMK is a scaled-down model of NPP with VVER440/213 reactor. The code versions RELAP5/MOD3.3hg (post Patch03) and RELAP5/MOD3.3ef

(Patch02) have been assessed against the experimental data from the test T3.1. The test T3.1 

was a large-break LOCA with 30% break starting from shutdown conditions with nitrogen in 

PRZ. Generally, both prediction of system behavior and prediction of nitrogen transport are in 

very good agreement with measured data.
iv
v

FOREWORD

The RELAP5 is a very important computational tool for increasing nuclear safety also of the 

VVER reactors, especially in the Czech Republic. The Nuclear Research Institute (NRI) Rez has 

assessed the code against numerous experiments and consequently applied it to safety 

analyses of Czech NPP. The presented report documents one of the assessment works.
vi
vii

CONTENTS

Page

ABSTRACT.................................................................................................................................... III

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

CONTENTS ..................................................................................................................................VII

LIST OF FIGURES ......................................................................................................................VIII

LIST OF TABLES.........................................................................................................................VIII

EXECUTIVE SUMMARY ...............................................................................................................IX

ACKNOWLEDGMENTS .................................................................................................................X

ABBREVIATIONS, GREEK LETTERS..........................................................................................XI

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

2. DESCRIPTION OF THE PMK FACILITY ................................................................................. 3

3. UJV INPUT MODEL OF PMK FACILITY.................................................................................. 7

4. POST-TEST ANALYSIS OF T3.1 EXPERIMENT.................................................................... 9

4.1 Experiment description........................................................................................................ 9

4.2 Results of calculation ........................................................................................................ 12

4.3 Comparison of results ....................................................................................................... 14

5. ADDITIONAL CALCULATIONS AND ANALYSES................................................................. 15

5.1 Analysis of NC Gas Behavior during the Test T3.1 .......................................................... 15

5.1.1 Analysis of NC Gas Behavior during the Test T3.1.............................................. 15

5.1.2 Checking NCG Balance along the Primary Circuit ............................................... 20

5.1.3 Influence of NCG on SG heat transfer.................................................................. 24

5.2 Auxiliary calculation with older RELAP5 version MOD3.3ef............................................. 27

6. CONCLUSIONS...................................................................................................................... 29

7. REFERENCES........................................................................................................................ 31

APPENDIX A COMPLETE SET OF COMPARISON PLOTS FOR CASE T3.1 .................. A-1
viii

LIST OF FIGURES

Page

Figure 1 Elevation diagram of the PMK facility ........................................................................ 5

Figure 2 PMK measurement locations #1 – pressure and temperature ............................... 5

Figure 3 PMK measurement locations #2 – levels and flow .................................................. 6

Figure 4 PMK measurement locations #3 - void probes ........................................................ 6

Figure 5 Nodalization scheme of PMK for RELAP5 ................................................................. 8

Figure 6 Primary pressure (T3.1).............................................................................................. 11

Figure 7 Integrated break mass flow rate (T3.1)..................................................................... 11

Figure 8 Collapsed level in reactor (T3.1) ............................................................................... 13

Figure 9 Cladding temperature (T3.1)...................................................................................... 13

Figure 10 PMK measurement locations #3 - void probes .................................................... 15

Figure 11 Void fraction in hot leg between PRZ connection and SG inlet (LV41).............. 16

Figure 12 Void fraction in SG outlet to cold leg (LV42) ......................................................... 16

Figure 13 Void fraction in cold leg loop seal upward part (LV52)........................................ 17

Figure 14 Void fraction at core outlet (LV25).......................................................................... 17

Figure 15 Void fraction in upper plenum by outlet nozzle (LV21) ........................................ 18

Figure 16 NC-gas tracking in hot leg in EXPERIMENT (LV41).............................................. 19

Figure 17 NC-gas tracking in hot leg in CALCULATION (LV41)............................................ 19

Figure 18 Mass of NC-gas in pressurizer................................................................................ 20

Figure 19 Mass of NC-gas in main parts of primary circuit................................................... 21

Figure 20 Mass of NC-gas in main parts of primary circuit – DETAIL.................................. 21

Figure 21 Mass of NC-gas in SG primary ................................................................................ 22

Figure 22 Mass of NC-gas in SG primary - DETAIL................................................................ 22

Figure 23 Mass of NC-gas in reactor ....................................................................................... 23

Figure 24 Balance of NC-gas mass in RCS............................................................................. 23

Figure 25 Temperature on SG secondary – bottom layer of TB ........................................... 24

Figure 26 Temperature on SG secondary – middle layer of TB............................................ 25

Figure 27 Temperature on SG secondary – upper layer of TB ............................................. 25

Figure 28 Calculation heat transfer at SG layers.................................................................... 26

Figure 29 Primary pressure in final (Mod3.3hg) and in auxiliary calc. (Mod3.3ef) ............. 27

Figure 30 Mass of NC-gas and voids in pressurizer in auxiliary calc. with MOD3.3ef....... 28

Figure 31 Mass of NC-gas and voids in pressurizer in final calc. with MOD3.3hg............ 28

LIST OF TABLES

Page

Table 1 Initial conditions of test T3.1...................................................................................... 10

Table 2 Boundary conditions of test T3.1 .............................................................................. 10

Table 3 Timing of main events of test T3.1 ............................................................................ 12
ix

EXECUTIVE SUMMARY

The PMK-2 facility [3] is a scaled down model of the VVER-440/213 and it had been primarily 

designed for investigation of small-break loss of coolant accidents (SBLOCA) and transient 

processes of this type of NPP. Nowadays the facility is also widely used for assessment of 

advanced computer code, that are used for safety analysis in VVER-operating countries.

One of the most important and world-widespread computer codes is the RELAP5 code. In the 

Czech Republic, the RELAP5 is installed under agreement between US NRC and Czech 

regulatory body (SONS). The main user of the code is the Nuclear Research Institute (NRI, UJV) 

Rez, where the code is widely assessed and applied to NPP safety analyses.

The test T3.1 used in this report for assessment of RELAP5/MOD3.3 computer code is largebreak LOCA with 30% break in cold leg starting from shutdown conditions with nitrogen in PRZ.

Comparison of the measured test data and the RELAP5/MOD3.3 results showed very good 

overall agreement of all major system parameters as primary pressure, reactor level, reactor 

coolant and clad temperature etc. Also the prediction of nitrogen transport in primary system was 

in very good agreement with the measured data.

:
x

ACKNOWLEDGMENTS

The authors acknowledge the support of the Czech regulatory body - the State Office of Nuclear 

Safety (SONS) - in acquiring the advanced thermal hydraulic codes. We also acknowledge the 

support of the Ministry of Industry and Trade of the Czech Republic within the national programs 

and grants focused on increase of the nuclear safety and the level of knowledge in branch of 

thermal hydraulics. 
xi

ABBREVIATIONS, GREEK LETTERS

BE best-estimate 

CL cold leg

D diameter

DC downcomer

ECCS Emergency Core Cooling System

EOP Emergency Operating Procedures

HA hydroaccumulator

HL hot leg 

HPIS High Pressure Injection System

HPSI high pressure safety injection 

ID inner diameter

LOCA loss-of-coolant accident 

LOOP loss-of-offsite power 

LPIS Low Pressure Injection System

LPSI low pressure safety injection

MBLOCA medium-break LOCA 

N/A not applicable 

EOP emergency operating procedures

PCT peak clad temperature 

PRZ pressurizer

RCP reactor coolant pump 

SBLOCA small-break LOCA 

SCRAM reactor trip (―safety control rod ax man‖)

SG steam generator

SIT safety injection tank

UP upper plenum

VVER Russian type of PWR (with horizontal SGs)
xii
1

1. INTRODUCTION

The test used in this report for assessment of RELAP5/MOD3.3 computer code was carried out 

in frame of the IMPAM-VVER project. The project was focused on different problems 

encountered during the development of EOPs for VVER reactors. The participants of the project 

performed both pre- and post-test analyses of the test with computer codes CATHARE, ATHLET 

and RELAP5. 

Objective the work presented in this report is assessment of RELAP5/MOD3.3 computer code 

against the PMK test T3.1 performed in frame of the IMPAM project. The test is a large-break 

LOCA with 30% break starting from shutdown conditions with nitrogen in PRZ.

The objective of our assessment work was at one side verify RELAP5 capability to predict 

overall system behavior in LOCA conditions, which is a usual objective. And at the other side to 

test the RELAP5 capability to simulate system behaviour starting from shutdown conditions and 

to predict nitrogen transport in primary system, which are less usual tasks for a system TH 

computer codes.
2
3

2. DESCRIPTION OF THE PMK FACILITY

The PMK-2 facility [3] is a scaled down model of the VVER-440/213 and it was primarily 

designed for investigating small-break loss of coolant accidents (SBLOCA) and transient 

processes of this type of NPP. The specific features of VVER-440/213 are as follows: 6-loop 

primary circuit, horizontal steam generators, loop seal in hot and cold legs, safety injection tank 

(SIT) set-point pressure higher than secondary pressure (nowadays modified at majority of 

VVER-440/213), the coolant from SITs directly injected to the upper plenum and downcomer. As 

a consequence of the differences the transient behavior of such a reactor system should be 

different from the usual PWR system behavior.

The volume and power scaling of PMK facility are 1:2070. Transients can be started from 

nominal operating conditions. The ratio of elevations is 1:1 except for the lower plenum and 

pressurizer. The six loops of the plant are modeled by a single active loop. In the secondary side 

of the steam generator the steam/water volume ratio is maintained. The coolant is water under 

the same operating conditions as in the nuclear power plant.

The core model consists of 19 electrically heated rods, with uniform power distribution. Core 

length, elevation and flow area are the same as in the Paks NPP.

In the modeling of the steam generator primary side, the tube diameter, length and number were 

determined by the requirement of keeping the 1:2070 ratio of the product of the overall heat 

transfer coefficient and the equivalent heat transfer area. The elevations of tube rows and the 

axial surface distribution of tubes are the same as in the reference system. On the secondary 

side the water level and the steam to water volume ratios are kept. The temperature and 

pressure are the same as in the NPP. The horizontal design of the VVER steam generator 

affects the primary circuit behavior during a small break LOCA in quite a different way to the 

usual vertical steam generators.

Cold and hot legs are volume scaled and care was taken to reproduce the correct elevations of 

the loop seals in both the cold and the hot legs. Cold and hot leg cross section areas if modeled 

according to volume scaling principles would have produced much too high pressure drops. 

Since, for practical reasons, length could not be maintained 1:1, relatively large cross sections 

were chosen for the PMK loop. On the one hand this results in smaller cold and hot leg frictional 

pressure drops than in the NPP, on the other hand, however, it improves the relatively high 

surface to volume ratio of the PMK pipework. As to the former effect, the small frictional pressure 

drop of the PMK cold and hot legs will have a negligible effect on small-break processes. 

However, the pressure drop is increased using orifices around the loop. 

For the pressurizer the volume scaling, the water to steam volume ratio and the elevation of the 

water level is kept. For practical reasons the diameter and length ratios cannot be realized. The 

pressurizer is connected to the same point of the hot leg as in the reference system. Electrical 

heaters are installed in the model and the provision of the spray cooling is similar to that of Paks 

NPP.

For the hydroaccumulators, the volume scaling and elevation is kept. They are connected to the 

downcomer and upper plenum similar to those of the reference system. The four 

hydroaccumulators of the VVER-440/213 are modeled by 2 SIT vessels.
4

The HPIS and LPIS systems are modeled by controlling the coolant flow rate in the lines by 

control valves. The flow rates measured during the start-up period of the Paks NPP are used to 

control the valves.

The main circulating pump of the PMK serves to produce the nominal operating conditions 

corresponding to that of the NPP prior to break initiation as well as to simulate the flow coastdown following pump trip early in the transient. For this reason the pump is accommodated in a 

by-pass line. Flow coast-down is modeled by closing a control valve in an appropriate manner 

and if flow rate is reduced to that of natural circulation, the valve in the by-passed cold leg part is 

opened while the pump line is simultaneously closed.

PMK Test Facility Characteristics:

Reference NPP:

Paks Nuclear Power Plant with VVER-440/213 (6 loops)

1375 MWt - hexagonal fuel arrangement

General Scaling factor:

Power, volumes: 1/2070, loops 1/345

Elevations: 1/1

Primary coolant system (1 loop representation):

- Pressure: 12.3 MPa (nominal), 16 MPa (max.)

- Nominal core inlet temperature: 540K

- Nominal core power: 664 kW

- Nominal flow rate: 4.5 kg/s

Special features:

- 19 heater rods, uniform axial and radial power distribution

- 2.5 m heated length

- External downcomer

- Pump is accommodated in by-pass line

-- flow rate 0 to nominal value

-- NPP pump coast down simulation

- Loop piping: 46 mm ID

Secondary system:

- Pressure: 4.6 MPa, feed water temperature: 496 K

- Nominal steam and feed water mass flow: 0.36 kg/s

Special features:

- Horizontal steam generator

- Controlled heat removal system

Safety injection systems:

- High Pressure Injection System (HPIS) and Low Pressure Injection System (LPIS)

- Safety Injection Tanks (SITs)

- Emergency feed water
5

3.325

3.494

0.994

0.0

3.225

10

9

8

7

6

5

4

3

2

1

0

-1

10.26

9.533

7.800

6.33

8.485 8.485

6.225

4.825

Figure 1 Elevation diagram of the PMK facility

Figure 2 PMK measurement locations #1 – pressure and temperature
6

Figure 3 PMK measurement locations #2 – levels and flow

Figure 4 PMK measurement locations #3 - void probes
7

3. UJV INPUT MODEL OF PMK FACILITY

The RELAP5 input deck of PMK used for the post-test analyses is a modified version of our 

older deck [1, 2] used for modeling of PMK-NVH in early 90-ties, when we analyzed the IAEA 

organized SPE tests. 

The modeling approach used in development of PMK model is similar to the approach applied in 

development of input models of Czech NPPs with VVER reactors. Generally, geometry and 

nodalization of primary circuit except of SG is very similar to those of standard PWR. There are 

only 3 major specific features of VVER-440/213, that should be reflected in nodalization –

horizontal SG (reflected in multi-layer nodalization of SG tubing), loop seal in hot leg (reflected in 

detailed nodalization of HL), and direct HA/LPIS injection to reactor (we don’t expect any multidimensional effects in small-scale facility like PMK, so simple 1-D modeling of reactor vessel 

was used).

Our RELAP5 input model of PMK experimental facility consists of:

134 volumes

144 junctions

126 heat structures (with 553 mesh points)

62 control variables

68 trips

Nodalization scheme can be seen in Figure 5. Comparing to our „old― model of PMK-NVH [1, 2], 

the major modifications of PMK nodalization implemented during work on this report, are as 

follows: more exact modeling of lower plenum, remodeled core outlet and upper plenum, and 

modified nodalization of PRZ and PRZ surge line (incl. location of PRZ surge line connection to 

the hot leg). 

Listing of the PMK input deck for RELAP5 developed in the NRI Rez can be found in the 

NUREG/IA-0229 report [12].
8

Figure 5 Nodalization scheme of PMK for RELAP5
9

4. POST-TEST ANALYSIS OF T3.1 EXPERIMENT

4.1 Experiment description

The Test 3.1 (T3.1-CD) [5, 6] experiment simulates a large break LOCA during the cool-down 

state of the plant. According to the original VVER-440 cool-down procedures neither passive, 

nor active ECCS could be automatically activated below 2.5 MPa. This may lead to core heat-up 

in case of a larger LOCA. The test should help to answer the question, whether a single LPSI 

train started by the operator – as now foreseen in the Paks NPP – can effectively prevent core 

heat-up. The break size is about 30%.

The test is defined by the following steps:

Initial conditions correspond to the plant state during cool-down with nitrogen in 

PRZ,

The PMK core power relevant for the shutdown state is 6 kW, however the core 

power in steady state was increased to 21 kW in order to compensate for heat 

losses of the facility - power ―correction‖ 21 6 kW was performed at the 

beginning of modelled accident,

The experiment is started by opening the 30% break in the cold leg with

simultaneous initiation of

secondary side isolation,

switch off of pressurizer heaters,

pump coast down,

LPIS starts at p < 0,7 MPa and time > 1800 s, or Twall > 450 °C,

Test is terminated at Twall > 500 °C.

The main objective of the test is to get experimental evidence about the effectiveness of the 

plant procedure to prevent core heat-up. As a consequence of the large break size the 

pressurizer is emptied in a few seconds and N2 gas enters the primary circuit. The special void 

probes installed in PMK make it possible to track the N2 propagation along the circuit. 

The initial conditions of the test are nearly the same as the nominal operating parameters of the 

plant considering the scaling ratio. In Table 1 below these conditions are given. Specified data 

are compared with measured data and the steady-state calculation results. 
10

Table 1 Initial conditions of test T3.1

Unit Specified Measured Calculation NRI

Primary system pressure (PR21) MPa 2.6 2.82 2.87

Primary loop flow (FL53) kg/s 4.5 4.54 4.54

Core inlet temperature (TE63) K 150.0 152.5 152.8

Core power (PW01) kW 21.0 21.0 21.00

Coolant level in PRZ (LE71) m 9.27 8.98 8.98

Pressure (PR81) MPa 1.0 1.32 1.319

Feedwater flow (FL81) kg/s 0.31 1.0 1.1

Feedwater temperature (TE81) °C 146.8 148.0 146.8

Coolant level in SG (LE81) m completely filled

The boundary conditions of the test and at the calculations are nearly the same as specified, 

except for the pump coast-down time, which had to be shortened in order to save the pump from 

running too long in two-phase conditions. The LPIS flow rate was specified for 0.7 MPa. The 

increased flow rate is the consequence of the fact that the injection took place at nearly 

atmospheric conditions. The boundary conditions are listed in Table 2 below.

Table 2 Boundary conditions of test T3.1

Unit Specified Measured Calculation NRI

Break orifice diameter mm 6.0 6.0 6.0

Break opens at s 0.0 1.0 0.0

Core power linearly reduced to 6 kW s 0.0 1.0 *10.0

Isolation of feedwater and steam lines *

2

s 3.0 0.0 0.0

Pump coast-down initiated at s 0.0 2.0 *1 2.0

Pump coast-down end s 150 86 150

LPIS flow rate (1 system assumed) kg/s 0.042 0.070 0.070

LPIS injection starts if clad temperature

 or time

°C

s

450

1800

N/A

1777

N/A

1777

Notes: *1There is a slight inconsistency in the test results – start of core power reduction is 

 reported either at 1.0 s or at 2.0 s, start of pump coastdown is reported either at 2.0 or 

 at 4.0 s.

 *

2

To get acceptable prediction of secondary pressure (in condition of SG full of water), 

 we modelled for T3.1 not simple steam line isolation, but pressure boundary condition 

 at steam line end.
11

000,0E+0

500,0E+3

1,0E+6

1,5E+6

2,0E+6

2,5E+6

3,0E+6

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] pressure [Pa]

measured data R5/M3.3 (NRI) 

Figure 6 Primary pressure (T3.1)

0

50

100

150

200

250

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] mass [kg]

measured data R5/M3.3 (NRI) 

Figure 7 Integrated break mass flow rate (T3.1)
12

4.2 Results of calculation

The main events of the Test 3.1 and the RELAP5 calculations are listed in Table 3 below:

Table 3 Timing of main events of test T3.1

Timing [s]

Event Measured Calculation 

NRI

Comment

Break opens 0.0 0.0 Break D6 mm (30%)

Core power linearly reduced (0.0) 0.0

Isolation of feedwater and steam lines 0.0 0.0

Pump coast-down initiated 2.0 2.0

Pressurizer empty 8 8

Pump coast-down ended 86 150

Hot collector empty 88 90

Hot leg loop seal cleared 150 192

Cold leg loop seal cleared (reactor side) 172 180

1st core overheat - 220-250 Maximal PCT in calc. 

was 186 °C

2nd core overheat starts 1281 1360

LPIS starts 1777 1777 Flow rate 0.07 kg/s

Vessel level minimum during major core 

overheating

1763

(1.40 m)

1780 

(1.08 m)

Fuel rod temperature maximum 1806 

(405 °C)

1800 

(417 °C)

2nd core overheat end = end of reflood 1950 1920

Transient end 3543 3500

The defined LOCA scenario starts with opening of the break valve D6 mm (30% of cold leg flow 

area) at reactor downcomer top. At the same time the reactor power reduction, trip of RCP, and 

isolation of SG secondary side occurs. 

The pump coast-down time had to be in the actual test reduced to 86 s in contrast to the 

specified 150 s, in order to save the pump from consequences of cavitations. Due to the large 

break size and consequently strongly two-phase character of the process, this has limitted effect 

on the overall system behavior.

Outflow of primary coolant through large break leads to fast decrease of primary pressure. 

Because of shutdown initial conditions, there is neither hydroaccumulators injection start after 

pressure drop under 6.0 MPa nor automatic actuation of active safety injection systems.

As there is no compensation of coolant leak through the break, the primary inventory is depleted 

pretty fast – the reactor collapsed level drops under 3.5 m (approximate elevation of core top) 

both in experiment and calculation before 200 s. In the calculation, there is even an early 

temporary core overheat in this phase – in time 220-250 s with clad temperature maximum 

186 °C.
13

0

2

4

6

8

10

12

14

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] level [m]

measured data R5/M3.3 (NRI) 

Figure 8 Collapsed level in reactor (T3.1)

100

150

200

250

300

350

400

450

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] temperature [

oC]

measured clad temp. computed clad temp (R5/M3.3) 

Figure 9 Cladding temperature (T3.1)
14

The ultimate core uncovery and fuel heat-up starts around 1300 s. Increase of clad temperature 

is fast and so the operator starts at 1777 s the LPIS injection, which limits the maximal clad 

temperature in calculation to 417 °C at 1800 s (measured values were similar).

The core reflood was finished at about 1920 s and later on a stable core cooling was ensured up 

to the end of test at 3500 s.

4.3 Comparison of results

The most important comparison plots of the measured data and the post-test UJV calculations 

are shown in Figure 6 - Figure 9. Complete set of T3.1 comparison plots can be found in 

Appendix III.

Most calculated parameters are in very good agreement with measured data, especially the 

most important system parameters like primary and sec. pressure, coolant and clad temperature 

etc.

The initial primary pressure drop is well predicted. In later phases of the accident the calculated 

course is slightly overpredicted against the measured pressure course.

The integrated break flow is slightly overpredicted in the first 200 s of the transient and on the 

contrary, partially underpredicted in interval 200-2100 s. After start of LPIS and refilling of the 

system, predicted mass outflow is again higher than the measured one. One can conclude, that 

calculated break flow is overpredicted in single phase liquid and two phase outflow phase, while 

underpredicted in single phase steam outflow phase of the accident.

As for the cladding temperature, the major heat-up period was very well predicted, both in timing 

and in maximal PCT value (417 °C compared to measured 405 °C). In the calculation, there was 

even an early small core heat-up period in time 220-250 s with clad temperature maximum 186 

°C, which was not measured in the test.

Both the experiment and calculations show that in this LBLOCA scenario the Accident 

Management represented by operators start of LPIS can effectively stop the core heat-up and 

cool down the system. 

Further comments to results:

Correct prediction of core overheat was sensitive on the used break model and discharge 

coefficients – for the final calculation we used coefs 1.1 and H-Fauske choked flow model.

Results were also very sensitive on initial coolant temperature (connected here strongly with FW

temperature), initial PRZ temperature (not properly specified in test data) etc.

A very surprising and positive finding of the analysis was a minimal mass error, although the 

calculated process (LBLOCA) was very dynamic and further complicated by presence of noncondensable gas in primary system.
15

5. ADDITIONAL CALCULATIONS AND ANALYSES 

5.1 Analysis of NC Gas Behavior during the Test T3.1

The LOCA test T3.1 gives us a chance to assess capability of current version of RELAP5 

computer code to compute presence of noncondensable gas(es) in the primary system. The 

follosing chaptes focus on this topic.

5.1.1 Analysis of NC Gas Behavior during the Test T3.1

Tracking of non-condensable gases in PMK test T3.1 was done with help of special void probes 

containing micro-thermocouple. There were installed 8 probes of traditional type (measuring void 

only) and 8 advanced probes with integrated thermocouple.

Advanced void probes with thermocouple enables to distinguish portion of subcooled gas from 

sub-cooled liquid, which can be quantified as non-condensable gas.

For faster orientation we place also here the figure with PMK void probes – see the Figure 10

below with location of the 16 void probes. In the following figures, one can see comparison of 

measured and calculated voids in selected positions of primary circuit. 

Figure 10 PMK measurement locations #3 - void probes
16

Figure 11 Void fraction in hot leg between PRZ connection and SG inlet (LV41)

Figure 12 Void fraction in SG outlet to cold leg (LV42)

000,0E+0

20,0E+0

40,0E+0

60,0E+0

80,0E+0

100,0E+0

0 100 200 300 400 500

time [s] void fraction 

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

measure data R5/M3.3 (NRI) 

000,0E+0

20,0E+0

40,0E+0

60,0E+0

80,0E+0

100,0E+0

0 100 200 300 400 500

time [s] void fraction 

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

measure data R5/M3.3 (NRI) 
17

Figure 13 Void fraction in cold leg loop seal upward part (LV52)

Figure 14 Void fraction at core outlet (LV25)

000,0E+0

20,0E+0

40,0E+0

60,0E+0

80,0E+0

100,0E+0

0 100 200 300 400 500

time [s] void fraction 

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

measure data R5/M3.3 (NRI) 

000,0E+0

20,0E+0

40,0E+0

60,0E+0

80,0E+0

100,0E+0

0 100 200 300 400 500

time [s] void fraction 

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

measured data R5/M3.3 (NRI) 
18

Figure 15 Void fraction in upper plenum by outlet nozzle (LV21)

Tracking of non-condensable gases in PMK by help of detecting of location of void and 

subcooling is well proved by calculation results, where we can work not only with both VOIDG, 

TEMPG and SATTEMP variables, but also directly quantify mass fraction of noncondensable 

gas in vapor phase by help of QUALA variable. See below some examples with NC gas tracking 

in hot leg.

000,0E+0

20,0E+0

40,0E+0

60,0E+0

80,0E+0

100,0E+0

0 100 200 300 400 500

time [s] void fraction 

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

measured data R5/M3.3 (NRI) 
19

Figure 16 NC-gas tracking in hot leg in EXPERIMENT (LV41)

Figure 17 NC-gas tracking in hot leg in CALCULATION (LV41)

0,00

0,20

0,40

0,60

0,80

1,00

0 100 200 300 400 500

time [s] void or mass fraction 

0

50

100

150

200

250 temperature [C]

local void (R5) NC-gas mass fraction (R5) local temp. (R5) sat. temp. (R5) 

000,0E+0

20,0E+0

40,0E+0

60,0E+0

80,0E+0

100,0E+0

0 100 200 300 400 500

time [s] void fraction 

0,00

50,00

100,00

150,00

200,00

250,00 temperature [C]

local void local temperature measured sat. temp. (UP) 
20

5.1.2 Checking NCG Balance along the Primary Circuit

As a next step we added into the PMK input deck a number of control variables for checking of 

NC gas balance along primary circuit, like the following: 

MASSNCG,VOL VVOL VOIDG RHOG QUALA

t

NCG JUN dt

VOIDFJ VELFJ RHOFJ VOIDGJ VELFGJ RHOGJ

VOIDGJ VELFGJ RHOGJ MFLOWJ QUALAJ MASS ,

(Note: In the latest version of RELAP5/MOD3.3/Patch4, the checking of NC balance and 

transport would be easier as there are new Minor Edit variables available.)

These variables enable us to watch the nitrogen transport from pressurizer to various parts of 

primary system and/or to the break:

Figure 18 Mass of NC-gas in pressurizer

0,000

0,025

0,050

0,075

0,100

0,125

0,150

0,175

0,200

0,225

0,250

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] mass [kg]

PRZ vessel PRZ surge line 
21

Figure 19 Mass of NC-gas in main parts of primary circuit

Figure 20 Mass of NC-gas in main parts of primary circuit – DETAIL

0,000

0,010

0,020

0,030

0,040

0,050

0,060

0,070

0,080

-20 0 20 40 60 80 100 120 140 160 180 200

time [s] mass [kg]

PRZ surge line hot leg SG CL reactor 

0,000

0,010

0,020

0,030

0,040

0,050

0,060

0,070

0,080

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] mass [kg]

PRZ surge line hot leg SG CL reactor 
22

Figure 21 Mass of NC-gas in SG primary

Figure 22 Mass of NC-gas in SG primary - DETAIL

0,000

0,005

0,010

0,015

0,020

0,025

0,030

0,035

0,040

0,045

-50 0 50 100 150 200 250 300 350 400 450 500

time [s] mass [kg]

SG hot collector SG tubing - bottom layer SG tubing - middle layer SG tubing - bottom layer SG cold collector 

0,000

0,005

0,010

0,015

0,020

0,025

0,030

0,035

0,040

0,045

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] mass [kg]

SG hot collector SG tubing - bottom layer SG tubing - middle layer SG tubing - bottom layer SG cold collector 
23

Figure 23 Mass of NC-gas in reactor

Figure 24 Balance of NC-gas mass in RCS

0,000

0,050

0,100

0,150

0,200

0,250

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] mass [kg]

mass of NC gas in RCS (w/out HA) integral of NC outflow through break sum check 

0,000

0,005

0,010

0,015

0,020

0,025

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] mass [kg]

DC LP core UP UH 
24

Conclusions from additional analysis of NC-gas transport in Test T3.1:

Very good qualitative and good quantitative (depending on location) prediction of void 

and nitrogen mass transport from PRZ to primary circuit and partially out through 

break.

Verification of non-condensable gases tracking method based on void probes with 

integrated micro- thermocouples.

No mass error in noncondensable balance in RELAP5/MOD3.3 calculation.

5.1.3 Influence of NCG on SG heat transfer

As for the influence of NC gas on heat transfer in SG (it was naturally not measured), we can 

compare only the SG temperatures (see the figures below and also the Figure 21 and Figure 22

above):

Figure 25 Temperature on SG secondary – bottom layer of TB

100

110

120

130

140

150

160

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] temperature [

oC]

measured temp by hot collector at 6.66 m measured data in middle at 6.66 m measured data by cold collector at 6.66 m 

computed temp in TB bottom volume (522) computed temp in DC bottom volume (532) 
25

Figure 26 Temperature on SG secondary – middle layer of TB

Figure 27 Temperature on SG secondary – upper layer of TB

100

110

120

130

140

150

160

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] temperature [

oC]

measured temp by hot collector at 8.16 m measured temp in SG centre at 8.16 m measured temp by cold collector at 8.16 m 

computed temp in TB upper volume 526 computed temp in DC upper volume 536 

100

110

120

130

140

150

160

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] temperature [

oC]

measured temp by hot collector at 7.13 m measured temp in SG middle at 7.13 m measured temp by cold collector at 7.13 m 

computed temp in TB middle volume 524 computed temp in DC middle volume 534 
26

Figure 28 Calculation heat transfer at SG layers

-25000

-20000

-15000

-10000

-5000

0

5000

10000

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] power [W]

SG bottom layer (calc.) SG middle layer (calc.) SG upper layer (calc.) 
27

5.2 Auxiliary calculation with older RELAP5 version MOD3.3ef

The base analysis presented in this report (above) was done with help of the RELAP5 version 

MOD3.3hg (post Patch03 – our latest version at time of T3.1 analysis). 

Running the identical input model with older version RELAP5/MOD3.3ef (Patch02) led to 

substantial differences in initial phase of the process - sudden drop of primary pressure caused 

probably by NC-gas mass error in PRZ – see comparison figures below.

Figure 29 Primary pressure in final (Mod3.3hg) and in auxiliary calc. (Mod3.3ef)

000,0E+0

500,0E+3

1,0E+6

1,5E+6

2,0E+6

2,5E+6

3,0E+6

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] pressure [Pa]

measured data R5/M3.3hg (NRI) - final case R5/M3.3ef (NRI) - auxi calc 
28

Figure 30 Mass of NC-gas and voids in pressurizer in auxiliary calc. with MOD3.3ef

Figure 31 Mass of NC-gas and voids in pressurizer in final calc. with MOD3.3hg

0,000

0,025

0,050

0,075

0,100

0,125

0,150

0,175

0,200

0,225

0,250

-2 0 2 4 6 8 10 12 14 16 18 20

time [s] mass [kg]

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0 liquid void fraction

PRZ vessel PRZ surge line liquid void fraction in PRZ volume 420-03 

liquid void fraction in PRZ volume 420-02 liquid void fraction in PRZ bottom volume 420-01 

0,000

0,025

0,050

0,075

0,100

0,125

0,150

0,175

0,200

0,225

-2 0 2 4 6 8 10 12 14 16 18 20

time [s] mass [kg]

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0 liquid void fraction

PRZ vessel PRZ surge line liquid void fraction in PRZ volume 420-03 

liquid void fraction in PRZ volume 420-02 liquid void fraction in PRZ bottom volume 420-01 
29

6. CONCLUSIONS

As a part of the assessment of new version of RELAP5 (the MOD3.3) in UJV Rez, we have 

performed a set of post-test analyses of new PMK experiments. The tests T2.1, T2.2, T2.3, and 

T3.1 were performed in 2003-2004 in frame of the IMPAM-VVER project and presented in 

NUREG/IA-0229 report. The presented report is focused on the test T3.1 - a large-break LOCA 

with 30% break starting from shutdown conditions with nitrogen in PRZ. 

The PMK facility is a scaled down model of the VVER-440/213 and it was primarily designed for 

investigating small-break loss of coolant accidents (SBLOCA) and transient processes of this 

type of NPP. The volume and power scaling of PMK facility are 1:2070. Transients can be 

started from nominal operating conditions. The ratio of elevations is 1:1 except for the lower 

plenum and pressurizer. The six loops of the plant are modeled by a single active loop. In the 

secondary side of the steam generator the steam/water volume ratio is maintained. The coolant 

is water under the same operating conditions as in the nuclear power plant.

The RELAP5 input deck used for the post-test analyses is a modified version of the older UJV 

input deck used for modeling of PMK-NVH in early 90-ties, when we analyzed the IAEA 

organized SPE tests. Listing of the current version of the deck used for the presented analyses 

is in the Appendix I.

Comparison of the measured test data and the calculation results showed very good overall 

agreement of all major system parameters as primary pressure, reactor level, reactor coolant 

and clad temperature etc. Also, prediction of nitrogen mass balance and transport was in very 

good agreement with measured data.
30
31

7. REFERENCES

1. Král, P.: Introductory Calculation with RELAP5/MOD2 Computer Code - Analysis of 

Primary-to-Secondary Leak Test Performed in PMK-NVH Facility, UJV-9393T, UJV Rez, 

June 1993.

2. Král, P.: RELAP5/MOD2 Post-Test Analysis of PMK-NVH Cold Leg 7.4% Loss of Coolant 

Accident – Depth of Nodalization Parametric Study, UJV-9429T, UJV Rez, August 1991.

3. Szabados, L. et al: PMK-2 HANDBOOK, Technical Specification of the Hungarian 

Integral Test Facility for VVER-440/213 Safety Analysis. KFKI Atomic Energy Research 

Institute. Budapest, 1996. 

4. Lahovský, F.: Pre-Test Calculation for PMK-2 Test 2.2 with ATHLET code: 7.4% Cold 

Leg Break with Secondary Bleed and Primary Bleed and Feed. Rež, April 2003.

5. Guba, A. et al: Analyses of PMK Experiments – Summary Report, IMPAM-VVER Project, 

KFKI-AEKI, February 2005.

6. Tóth, I. et al: PMK Experiments – Summary Report, IMPAM-VVER project, KFKI-AEKI, 

May 2005. 

7. Král, P.: Results of RELAP5 Calculations of LOCA for VVER-1000, IMPAM-VVER, UJV 

Rez, 2005. 

8. Král, P.: Results of RELAP5 Calculations of LOCA D136 and D60 mm for VVER-440/213, 

IMPAM-VVER, UJV Rez, 2005. 

9. Král P.: REL RELAP5/MOD3.3 Assessment Against New PMK Experiments, prezentation 

at Fall 2006 CAMP Meeting.

10. Král P.: RELAP5/MOD3.3 Assessment Against PMK Test T3.1 - LOCA with Nitrogen in 

PRZ, prezentation at Fall 2008 CAMP Meeting.

11. Král P.: RELAP5/MOD3.3 Assessment Against PMK Test T3.1 - LOCA with Nitrogen in 

PRZ, UJV Z 2545 T, November 2008.

12. Král P.: RELAP5/MOD3.3 Assessment Against New PMK Experiments, NUREG/IA-0229, 

June 2010. 

APPENDIX A COMPLETE SET OF COMPARISON PLOTS FOR 

CASE T3.1
A-2

000,0E+0

500,0E+3

1,0E+6

1,5E+6

2,0E+6

2,5E+6

3,0E+6

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] pressure [Pa]

measured data R5/M3.3 (NRI) 

Fig.A-1 Primary pressure (T3.1)

000,000E+0

200,000E+3

400,000E+3

600,000E+3

800,000E+3

1,000E+6

1,200E+6

1,400E+6

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] pressure [Pa]

measured data R5/M3.3 (NRI) 

Fig.A-2 Secondary pressure (T3.1)
A-3

0

20

40

60

80

100

120

140

160

180

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] temperature [

oC]

measured data R5/M3.3 (NRI) 

Fig.A-3 Core inlet temperature (T3.1)

0

20

40

60

80

100

120

140

160

180

200

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] temperature [

oC]

measured data R5/M3.3 (NRI) core exit temp. (058-02) 

Fig.A-4 Core outlet temperature (T3.1)
A-4

100

150

200

250

300

350

400

450

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] temperature [

oC]

measured clad temp. computed clad temp (R5/M3.3) 

Fig.A-5 Cladding temperature (T3.1)

0

2

4

6

8

10

12

14

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] level [m]

measured data R5/M3.3 (NRI) 

Fig.A-6 Collapsed level in reactor (T3.1)
A-5

0

1

2

3

4

5

6

7

8

9

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] level [m]

measured data R5/M3.3 (NRI) 

Fig.A-7 Collapsed level in reactor downcomer (T3.1)

7,80

8,00

8,20

8,40

8,60

8,80

9,00

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] level [m]

measured data R5/M3.3 (NRI) 

Fig.A-8 Collapsed level in PRZ (T3.1)
A-6

4,50

5,00

5,50

6,00

6,50

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] level [m]

measured data R5/M3.3 (NRI) 

Fig.A-9 Collapsed level in hot leg loop seal – reactor side (T3.1)

4,50

5,00

5,50

6,00

6,50

7,00

7,50

8,00

8,50

9,00

9,50

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] level [m]

measured data R5/M3.3 (NRI) 

Fig.A-10 Collapsed level in hot leg loop seal – SG side (T3.1)
A-7

0,00

1,00

2,00

3,00

4,00

5,00

6,00

7,00

8,00

9,00

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] level [m]

measured data R5/M3.3 (NRI) 

Fig.A-11 Collapsed level in cold leg loop seal – SG side (T3.1)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

-500 0 500 1000 1500 2000 2500 3000

time [s] flow rate [kg/s]

measured data T2.1 R5/M3.3 (NRI) 

Fig.A-12 Loop mass flow rate (T3.1)
A-8

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] flow rate [kg/s]

measured data T2.1 R5/M3.3 (NRI) 

Fig.A-13 Break mass flow rate (T3.1)

0

50

100

150

200

250

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] mass [kg]

measured data R5/M3.3 (NRI) 

Fig.A-14 Integrated break mass flow rate (T3.1)
A-9

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

0,18

-500 0 500 1000 1500 2000 2500 3000 3500

time [s] DT [s]

0,000

0,005

0,010

0,015

0,020

0,025

0,030

0,035

0,040

0,045 mass [kg]

DT DTCRNT emass 

Fig.A-15 Parameters of calculation (T3.1)