NUREG/IA-0403

International 

Agreement Report

Full Scale Loop Seal Experiments with

TRACE V5 Patch 1

Prepared by:

Seppo Hillberg

 VTT – Technical Research Centre of Finland

Nuclear Power Plant Modeling

P.O.B. 1000

02044 VTT

Finland

A. Calvo, NRC Project Manager

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: January 2009

Date Published: December 2011

Prepared as part of 

The Agreement on Research Participation and Technical Exchange

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

Published by

U.S. Nuclear Regulatory Commission
NUREG/IA-0403

 International 

Agreement Report

Full Scale Loop Seal experiments with 

TRACE V5 Patch 1

Prepared by:

Seppo Hillberg

VTT – Technical Research Centre of Finland

Nuclear Power Plant Modelling

P.O.B. 1000

02044 VTT

Finland

Antony Calvo, NRC Project Manager 

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Date Printed: December 2011

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
iii

ABSTRACT

Full scale loop seal experiments have been simulated with thermal hydraulic simulation code 

TRACE V5 Patch 1. Multiple nodalizations were created different geometric accuracy. The 

main interest in the simulations was the residual water level in the horizontal pipe. Also 

pressure behaviour during the air blow to the loop seal and effects of different maximum time 

steps and initial liquid levels were studied.

Simulations revealed differencies in results obtained with different nodalizations. Most of the 

nodalizations produced reasonable results except a simple 90o bend that used grav terms 

elevation option (namelist variable ielev=0). This model cleared too much water out of the 

loop seal. A very similar model using cell angle elevation option didn't suffer from this 

problem.
iv
v

CONTENTS

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

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

2 THE LOOP SEAL FACILITY.......................................................................................................... 2

3 MODELED PIPE GEOMETRIES FOR THE SIMULATIONS......................................................... 3

4 RESULTS OF THE SIMULATIONS............................................................................................... 9

5 SUMMARY ................................................................................................................................... 12

6 REFERENCES ............................................................................................................................. 13
vi
vii

FIGURES

TABLES

Figure 1. Cross section of a partially water filled pipe................................................. 1

Figure 2. The loop seal facility /1/. .............................................................................. 2

Figure 3. Pressure oscillations in the loop seal experiment (Ref. 2). .......................... 3

Figure 4. The model with single corner node (grav terms elevation option). .............. 3

Figure 5. The model with three corner nodes (grav terms elevation option)............... 4

Figure 6. The model with no dedicated corner nodes (cell angle elevation option). ... 5

Figure 7. The model with three corner nodes (cell angle elevation option)................. 6

Figure 8. The model with short nodes, no dedicated corner nodes (cell angle 

elevation option). ........................................................................................................ 7

Figure 9. Residual water level with different simulated geometries. ........................... 9

Figure 10. Residual water level. The effect different maximum time steps 

(IELEV=0, 3 corner nodes). ...................................................................................... 10

Figure 11. Residual water level. The effect of different initial water levels 

(IELEV=0, 1 corner node, 1 ms maximum timestep). ............................................... 10

Figure 12. Pressure difference over the loop seal ( IELEV=0, 1 corner node, 1 ms).11

Figure 13. Pressure difference over the loop seal (IELEV=0, 3 nodes, 1 ms) .......... 11

Table 1. Node geometries: Grav terms elevation option, one corner node................. 4

Table 2. Node geometries: Grav terms elevation option, three corner nodes............. 5

Table 3. Cell geometries: Cell angle elevation option, no corner node, long nodes. .. 6

Table 4. Cell geometries: Cell angle elevation option, three corner nodes................. 7

Table 5. Cell geometries: Cell angle elevation option, no corner node, short nodes. . 8

1

1 INTRODUCTION

In a Loss of Coolant (LOCA) situation loop seal behaviour has potentially a strong impact on 

core cooling as it causes pressure difference which disturbs flows in the core area. This 

pressure difference is at its highest when the ascending pipe of the loop seal is full of water 

and much lower when the flow in the pipe is stratified.

In this report full scale Loop Seal Facility tests are simulated with TRACE (V5p1) thermal 

hydraulic simulation code and the results are compared to the facility data. Multiple 

nodalizations were created different geometric accuracy. The main interest in the simulations 

was the residual water level in the horizontal pipe. Also the effects of different maximum 

timesteps and different initial liquid levels were studied.

For better comparability the water levels in these simulations are defined as a division of 

water level and pipe diameter (Figure 1).

D

h

h

~ l

l

(1)

Figure 1. Cross section of a partially water filled pipe.

The Loop Seal facility tests were made in atmospheric pressure with cold water (~20oC) and 

air. These conditions are on the low end of the operational range of the simulation code. 

Therefore it should be kept in mind that the results may not be fully applicable at higher 

pressures and temperatures.

The Loop Seal Facility is presented in Chapter 2. The used geometries in TRACE simulations 

and their results are presented in Chapter 3. Similarly the APROS geometries and results can 

be found from Chapter 4. Summary is in Chapter 5.
2

2 THE LOOP SEAL FACILITY 

The Loop Seal facility models a VVER-1000 primary circuit with a rupture in the cold leg. It 

has a speed-controlled fan which provides up to 9 m/s superficial velocities and a 10 m3

buffer tank to dampen the pressure oscillations. The loop seal part of the facility has a inner 

diameter of 0.85 m and length of 6,98 m between the vertical pipe centers. The vertical pipe 

which rises to the (non-existant) reactor coolant pump has elevation difference of 2,9 meters. 

The loop seal bottom bends have a radius of 1,34 m (Ref. 1). The facility is presented in 

Figure 2.

Figure 2. The loop seal facility /1/.

The main parameter in the tests was the residual water level. Also pressure difference over the 

loop seal was measured. In a publication "Two-Phase Flow in a Full-Scale Loop Seal 

Facility" (Ref. 2) Tuomisto and Kajanto present the Figure 3 which represents the pressure 

oscillations at superficial velocity of 5-6 m/s. The marked flow regimes in the figure are A)

initial wavy stratified flow, B) transition to slug flow, C) slug flow period, D) transition back 

to stratified flow and E) stratified flow.
3

Figure 3. Pressure oscillations in the loop seal experiment (Ref. 2).

3 MODELED PIPE GEOMETRIES FOR THE SIMULATIONS

Five different nodalizations were used. All of the nodalizations had the same flow area of 

0,567 m2and the same hydraulic diameter of 0,85 m. All of the models also had the same 

elevations changes of 2,9 m and length between vertical pipe center lines was 6,98 m. 

Because the bottom bends of the pipe were modeled differently the straight horizontal and 

vertical lengths varied between the models.

The first geometry was modeled using grav terms elevation option (IELEV=0), using 

approximately 1 m long nodes and had its bottom bends modeled with a single 90o bent node. 

This geometry is presented in Figure 4 and Table 1.

Figure 4. The model with single corner node (grav terms elevation option).
4

Table 1. Node geometries: Grav terms elevation option, one corner node.

Cell

number

Cell

volume [m3]

Cell 

length [m]

Cell flow

area [m2]

Cell 

elevation

change [m]

1 0,567 1,000 0,567 0,000

2 0,567 1,000 0,567 0,000

3 0,454 0,800 0,567 -0,400

4 0,567 1,000 0,567 -1,000

5 0,567 1,000 0,567 -1,000

6 0,567 1,000 0,567 -0,500

7 0,562 0,990 0,567 0,000

8 0,567 1,000 0,567 0,000

9 0,567 1,000 0,567 0,000

10 0,567 1,000 0,567 0,000

11 0,567 1,000 0,567 0,000

12 0,562 0,990 0,567 0,000

13 0,567 1,000 0,567 0,500

14 0,567 1,000 0,567 1,000

15 0,567 1,000 0,567 1,000

16 0,454 0,800 0,567 0,400

17 0,567 1,000 0,567 0,000

18 0,567 1,000 0,567 0,000

In the second geometry grav terms elevation option was also used, but the bottom bends were 

modeled with three nodes. This was one of the two nodings that were close to the original 

since they had the same bend radius of 1,34 m. The geometry is presented in Figure 5 and 

Table 2.

Figure 5. The model with three corner nodes (grav terms elevation option).

1,34m 

1,34m 
5

Table 2. Node geometries: Grav terms elevation option, three corner nodes.

Cell

number

Cell

volume [m3]

Cell 

length [m]

Cell flow

area [m2]

Cell 

elevation

change [m]

1 0,567 1,000 0,567 0,000

2 0,567 1,000 0,567 0,000

3 0,567 1,000 0,567 -0,500

4 0,301 0,530 0,567 -0,530

5 0,301 0,530 0,567 -0,530

6 0,407 0,718 0,567 -0,670

7 0,407 0,718 0,567 -0,491

8 0,407 0,718 0,567 -0,180

9 0,407 0,717 0,567 0,000

10 0,407 0,717 0,567 0,000

11 0,407 0,717 0,567 0,000

12 0,407 0,717 0,567 0,000

13 0,407 0,717 0,567 0,000

14 0,407 0,717 0,567 0,000

15 0,407 0,718 0,567 0,180

16 0,407 0,718 0,567 0,491

17 0,407 0,718 0,567 0,670

18 0,301 0,530 0,567 0,530

19 0,301 0,530 0,567 0,530

20 0,567 1,000 0,567 0,500

21 0,567 1,000 0,567 0,000

22 0,567 1,000 0,567 0,000

The third geometry is using cell angle elevation option (IELEV=2) and is, along with the first 

geometry (grav terms, one corner node), one of the simpliest of the tested geometries. This 

geometry is presented in Figure 6 and Table 3.

Figure 6. The model with no dedicated corner nodes (cell angle elevation option).
6

Table 3. Cell geometries: Cell angle elevation option, no corner node, long nodes.

Cell

number

Cell

volume [m3]

Cell 

length [m]

Cell flow

area [m2]

Cell 

elevation

change [m]

1 0,567 1,000 0,567 0,000

2 0,567 1,000 0,567 0,000

3 0,511 0,900 0,567 -0,900

4 0,567 1,000 0,567 -1,000

5 0,567 1,000 0,567 -1,000

6 0,562 0,990 0,567 0,000

7 0,567 1,000 0,567 0,000

8 0,567 1,000 0,567 0,000

9 0,567 1,000 0,567 0,000

10 0,567 1,000 0,567 0,000

11 0,567 1,000 0,567 0,000

12 0,562 0,990 0,567 0,000

13 0,567 1,000 0,567 1,000

14 0,567 1,000 0,567 1,000

15 0,511 0,900 0,567 0,900

16 0,567 1,000 0,567 0,000

17 0,567 1,000 0,567 0,000

Also a three corner node version was modeled using the cell angle elevation option. Along 

with the grav terms using three corner node version, this was closest to the test facility 

geometry. This geometry is presented in Figure 7 and Table 4.

Figure 7. The model with three corner nodes (cell angle elevation option).

1,34m 

1,34m 
7

Table 4. Cell geometries: Cell angle elevation option, three corner nodes.

Cell

number

Cell

volume [m3]

Cell 

length [m]

Cell flow

area [m2]

Cell 

elevation

change [m]

1 0,567 1,000 0,567 0,000

2 0,567 1,000 0,567 0,000

3 0,443 0,780 0,567 -0,780

4 0,443 0,780 0,567 -0,780

5 0,378 0,665 0,567 -0,615

6 0,378 0,665 0,567 -0,470

7 0,378 0,665 0,567 -0,255

8 0,407 0,717 0,567 0,000

9 0,407 0,717 0,567 0,000

10 0,407 0,717 0,567 0,000

11 0,407 0,717 0,567 0,000

12 0,407 0,717 0,567 0,000

13 0,407 0,717 0,567 0,000

14 0,378 0,665 0,567 0,255

15 0,378 0,665 0,567 0,470

16 0,378 0,665 0,567 0,615

17 0,443 0,780 0,567 0,780

18 0,443 0,780 0,567 0,780

19 0,567 1,000 0,567 0,000

20 0,567 1,000 0,567 0,000

The last modeled geometry is a variation of the cell angle version without dedicated corner 

nodes. That version was renodalized and had most of its nodes split in three. This geometry is 

presented in Figure 8 and Table 5.

Figure 8. The model with short nodes, no dedicated corner nodes (cell angle elevation

option).
8

Table 5. Cell geometries: Cell angle elevation option, no corner node, short nodes.

Cell

number

Cell

volume [m3]

Cell 

length [m]

Cell flow

area [m2]

Cell 

elevation

change [m]

1 0,567 1,000 0,567 0,000

2 0,567 1,000 0,567 0,000

3 0,170 0,300 0,567 -0,300

4 0,170 0,300 0,567 -0,300

5 0,170 0,300 0,567 -0,300

6 0,189 0,333 0,567 -0,333

7 0,189 0,333 0,567 -0,333

8 0,189 0,333 0,567 -0,333

9 0,189 0,333 0,567 -0,333

10 0,189 0,333 0,567 -0,333

11 0,189 0,333 0,567 -0,333

12 0,187 0,330 0,567 0,000

13 0,187 0,330 0,567 0,000

14 0,187 0,330 0,567 0,000

15 0,189 0,333 0,567 0,000

16 0,189 0,333 0,567 0,000

17 0,189 0,333 0,567 0,000

18 0,189 0,333 0,567 0,000

19 0,189 0,333 0,567 0,000

20 0,189 0,333 0,567 0,000

21 0,189 0,333 0,567 0,000

22 0,189 0,333 0,567 0,000

23 0,189 0,333 0,567 0,000

24 0,189 0,333 0,567 0,000

25 0,189 0,333 0,567 0,000

26 0,189 0,333 0,567 0,000

27 0,189 0,333 0,567 0,000

28 0,189 0,333 0,567 0,000

29 0,189 0,333 0,567 0,000

30 0,187 0,330 0,567 0,000

31 0,187 0,330 0,567 0,000

32 0,187 0,330 0,567 0,000

33 0,189 0,333 0,567 0,333

34 0,189 0,333 0,567 0,333

35 0,189 0,333 0,567 0,333

36 0,189 0,333 0,567 0,333

37 0,189 0,333 0,567 0,333

38 0,189 0,333 0,567 0,333

39 0,170 0,300 0,567 0,300

40 0,170 0,300 0,567 0,300

41 0,170 0,300 0,567 0,300

42 0,567 1,000 0,567 0,000

43 0,567 1,000 0,567 0,000
9

4 RESULTS OF THE SIMULATIONS

The inlet component had a state of 20 oC temperature and 1 bar pressure. Its void fraction was 

set to be 1 and the gas was fully noncondensible. The simulations initiated with a 50 second 

ramp from zero velocity to the currently simulated superficial velocity. Then followed a 

steady 650 second period when the flow was kept steady. After this the velocity was dropped 

back to zero and once the possible oscillations had soothed down the residual void fractions 

were read. After this the residual water levels were solved numerically. 

Figure 9 shows the residual water levels of simulations with different geometries. In all of the 

cases the maximum time step was 1 ms and only the horizontal tube was filled with water. 

The only model that produced results that clearly differed from the rest was the simple 90o 

bend using grav terms elevation option (the first nodalization).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10

m/s

hl/D

IELEV=0, 1 node

IELEV=0, 3 nodes

IELEV=2, 0, short nodes

IELEV=2, 0, long nodes

IELEV=2, 3 nodes

measured

Figure 9. Residual water level with different simulated geometries.

The effect of maximum time steps (1 ms, 10 ms and 100 ms) was tested with the 90o bend 

model that used grav terms. All of the the results, however, turned to be identical. This same 

simulation was then run with the 3 corner nodes grav terms using model. The results of the 

latter simulation are presented in Figure 10.
10

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9

m/s

hl/D

0, 3, 1 ms

0, 3, 10 ms

0, 3, 100 ms

measured

Figure 10. Residual water level. The effect different maximum time steps 

(IELEV=0, 3 corner nodes).

The effect of initial water level was studied with the 90o bend model that used grav terms. In 

the first case the loop seal was fully filled with water (zero void fraction in the vertical pipes). 

In the second case only the horizontal pipe was filled with water. In the third case the 

horizontal pipe had initial void fraction of 20%. All of the simulated cases produced identical 

results (Figure 11).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10

m/s

hl/D

vertical pipes full

horizontal pipe full

20% initial void fraction

measured

Figure 11. Residual water level. The effect of different initial water levels 

(IELEV=0, 1 corner node, 1 ms maximum timestep).
11

The pressure oscillation behaviour that was observed in the facility (Figure 3) was studied 

with the 90o bend and three corner node models that used grav terms. The results are 

presented in Figures 12 and 13. Transitions to slug flow and back to stratified flow are best 

seen in 5 m/s simulation of the three corner node model.

Figure 12. Pressure difference over the loop seal ( IELEV=0, 1 corner node, 1 ms).

Figure 13. Pressure difference over the loop seal (IELEV=0, 3 nodes, 1 ms)
12

5 SUMMARY

Full scale loop seal experiments have been simulated with thermal hydraulic simulation code 

TRACE V5 Patch 1. Multiple nodalizations were created with different geometric accuracy. 

The main interest in the simulations was the residual water level in the horizontal pipe. Also 

pressure behaviour during the air blow to the loop seal and effects of different maximum time 

steps and initial liquid levels were studied.

Simulations revealed differencies in results obtained with different nodalizations. Most of the 

nodalizations produced reasonable results except a simple 90o bend that used grav terms 

elevation option (namelist variable ielev=0). This model cleared too much water out of the 

loop seal. A very similar model using cell angle elevation option didn't suffer from this 

problem.
13

6 REFERENCES

/1/ Kymäläinen O. 1992. The assesment of RELAP5/MOD2 Against IVO Loop 

Seal Tests. Imatran Voima Oy, Finland. NUREG/IA-0082.

/2/ Tuomisto H., Kajanto P. 1988. TWO-PHASE FLOW IN A FULL SCALE 

LOOP SEAL FACILITY. Nuclear Engineering and Design 107 295-305.