NUREG/IA-0224

 International 

Agreement Report

An Assessment of TRACE V5 RC1 

Code Separator Model with the 

Westinghouse Model Boiler 2 

Experiments 

Prepared by:

S. 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

April 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
iii 

ABSTRACT 

A TRAC/RELAP Advanced Computational Engine (TRACE) model of the Model Boiler 2 test 

facility has been built as part of the International Code Assessment and Maintenance 

Program (CAMP) validation case. Simulation cases with 100-percent and 50-percent power 

levels were run, and the results were compared to tests that were performed in the facility in 

the early 1980s. Because the main interest was in the functionality of the primary separator, 

the model includes no steam dryer assembly. The only heat structures built into the model 

were the primary-secondary heat transfer elements, since the model was run in a steady 

state and therefore the heat capacities of the structures had little effect on the results. 

The Model Boiler 2 test facility failed to achieve the intended primary separator efficiencies in 

an oscillation-free system. Primary separator drain conditions had a major effect on the 

stability of the system and on the efficiency of the separation. A bigger drain area resulted in 

separation levels that were closer to the set value, but the system drifted easily into heavy 

oscillation. When the drain area was smaller, the separation levels were not achieved, but 

the system became more stable. 
v

CONTENTS 

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

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

2. DESCRIPTION OF THE MODEL BOILER 2 ..........................................................3

3. TRACE ....................................................................................................................7

4. SIMULATIONS......................................................................................................11

5. RESULTS..............................................................................................................13

5.1 100-PERCENT POWER LEVEL..............................................................................13

5.2 50-PERCENT POWER LEVEL................................................................................14

6. CONCLUSIONS....................................................................................................17

7. REFERENCES......................................................................................................19

APPENDIX 1. TRACE MB-2 MODEL INPUT DATA................................................ A-1

Figures

Figure 1 Model Boiler 2 riser cross-section (Ref. 1) ...................................................3 

Figure 2 Model Boiler 2 (Ref. 1)..................................................................................5 

Figure 3 SNAP view of the Model Boiler 2 TRACE model..........................................8 

Figure 4 SNAP animation view of the Model Boiler 2 model ....................................10 

Figure 5 Primary separator deficiency as a function of power level (Ref. 1) ............11 

Figure 6 Liquid mass flow in the separator steam exit with two different timesteps 

(100-percent power)...................................................................................................14 

Figure 7 Water mass flow in separator steam outlet, 50-percent power level..........15 

Tables 

Table 1 Model Boiler 2 Geometric Data (Ref. 2)...............................................................4 

Table 2 Model Boiler 2 TRACE Model Geometry .............................................................9 

Table 3 100-Percent Power Level Simulation Results, 0.005-Second Timestep ......13 

Table 4 50-Percent Power Level Simulation Results, 0.005-Second Maximum 

Timestep .................................................................................................................................15 
1

1 INTRODUCTION 

As a part of the International Code Assessment and Maintenance Program (CAMP) 

Agreement, the Finnish organizations have agreed to perform validation cases for the 

TRAC/RELAP Advanced Computational Engine (TRACE) code (RELAP/TRACE codes). For 

one validation case, these organizations built a TRACE model of the Model Boiler 2 test 

facility. They ran simulation cases with 100-percent and 50-percent power levels and 

compared the results to test cases that were performed in the facility in the early 1980s. The 

main interest was in the functionality of the primary separator. 

Chapter 2 describes the Model Boiler 2, while Chapter 3 contains an introduction to TRACE 

and the TRACE model. Chapter 4 describes the simulation input data, and Chapter 5 

contains the results of the tests. 
2
3

2 DESCRIPTION OF THE MODEL BOILER 2 

The Model Boiler 2 test facility is a 1-percent scaled-down version of the Westinghouse 

Model F steam generator. The scaling was done by steam generation rate and volume, while 

the height of the facility did not change. The riser area of this facility is built in a rectangular 

shape (wrapper box) and two tubes act as downcomers. The riser area is not separated, and 

there is only one primary separator. A dryer assembly is also present. This facility has 

52 Inconel-600 U-tubes arranged in a rectangular array, with the same flow area and 

cladding thickness as the ones in the F model. Figure 1 represents the Model Boiler 2 riser 

cross section, Table 1 contains the geometric data, and Figure 2 shows the whole steam 

generator. 

The steam separation of Model Boiler 2 takes place in three physical areas. The first 

separation stage is the primary separator. After this stage, the steam-water mixture flows 

into an open cavity, where gravitation causes some of the water drops to return to circulation 

through a channel located on the upper deck plate. The last stage of separation is the dryer, 

from which the saturated steam exits to the steam dome. 

Figure 1 Model Boiler 2 riser cross-section (Ref. 1)
4

Table 1 Model Boiler 2 Geometric Data (Ref. 2) 

Steam generator 

Total height 

Height from bottom to primary separator 

Height from bottom to upper deck plate 

13.93 m 

11.60 m 

12.12 m 

Riser 

Height of wrapper box 

Length and width of wrapper box (inner dim.) 

Height of heat transfer area 

8.72 m 

68.4 x 9.96 cm 

6.92 m 

U-tubes 

Number of tubes 

Cladding thickness 

Outer diameter 

Material of tubes 

52 

1 mm 

1.75 cm 

Inconel 600 

Downcomers 

Number of downcomers 

Height of downcomer pipes 

Inside diameter of downcomer pipes 

2 

8.58 m 

7.79 cm 

Primary separator 

Height of separator 

Inside diameter of separator 

0.51 m 

17.8 cm 

Steam shroud 

Height 

Volume 

0.94 m 

0.111 m3

Steam dome 

Height 

 Volume 

0.94 m 

0.379 m3
5

Figure 2 Model Boiler 2 (Ref. 1)
6
7

3 TRACE 

The TRACE model retains all the functionality of RAMONA, RELAP5, TRAC-PWR, and 

TRAC-BWR codes. The most recent release at the time when this report was written was 

Version 5, Release Candidate 3 (launched in December 2006), and the final version was 

expected to be released at the end of July 2007 (realeased August 2007). The simulations in 

this report used Version 5, Release Candidate 1. At the time of these simulations The 

U.S. Nuclear Regulatory Commission was stating that making the documentation is the 

priority and focus of its attention until the final release (Ref. 3). 

For the simulations in this report, the separator component of TRACE disables itself if one or 

more of three conditions are recognized. These conditions are backward flow, void fraction 

of under 5 percent, or flow which is essentially all vapor ( 6

vapor total 1 q /q 10− − < ). If these 

conditions are no longer present later in the simulation, the separator is automatically turned 

on (Ref. 4). 

The thermal-hydraulic model of the Model Boiler 2 facility was created with the Symbolic 

Nuclear Analysis Package (SNAP), and the code used in the simulation was TRACE. The 

parts of the facility included in the model were the downcomer, riser, separator, steam 

shroud, and steam dome. The steam dryer assembly was not modeled, since the main 

interest in these tests was in the functionality of the primary separator. Also, there were few 

data showing just how much water passes through the primary separator but returns into 

circulation through openings in the upper deck plate. This was problematic, since the 

intended test data contained the liquid flow entering the steam dryer (i.e., liquid flow passing 

through both of the previous separation stages). Therefore, the separator component of the 

built model actually represents not only the primary separator but also the gravitational 

separation that occurs in the steam shroud. 

The built model consists of the primary and secondary circuits. The primary circuit has a 

simple design that includes only the inlet and outlet components and four tube components. 

Two of the tubes represent the U-tubes and are equipped with heat structures, while two 

shorter tubes are only added for measuring purposes. In these studies, where the power 

level was constant, the circuit had constant mass flow, pressure, and inlet temperature. 

The secondary circuit consists of an inlet for feedwater, an outlet for steam, a riser, a 

downcomer, a separator, and a dome. There are also two small pipes that connect the dome 

and separator components to the downcomer. 

In the selected tests, the feedwater flow was directed evenly to both downcomers. This, and 

the fact that the riser was not separated, allowed the modeling of the downcomer as a single 

pipe. The Model Boiler 2 downcomer consists of five physical areas: barrel, funnel, pipes, 

ducts, and annulus. The downcomer barrel surrounds the riser barrel and separator, while 

the funnel is the volume that spreads the water to both downcomers. In the built model, the 

barrel and funnel heights and volumes were treated as one, and the volume was evenly split 

into three. The downcomer pipes were split into four nodes. The ducts exist as one node. 

The annulus was spread to 80-percent and 20-percent parts, and the smaller bottom part 

was bent towards the riser. The length of the object was slightly increased to compensate for 

the height loss that resulted from bending it. Figure 3 presents the built model, and Table 2 

contains the model geometry. 
8

Figure 3 SNAP view of the Model Boiler 2 TRACE model
9

Table 2 Model Boiler 2 TRACE Model Geometry 

 Volume Length Hydraulic Area Additive Number

 [m3

] [m] diam. [m] [m2] loss of 

nodes 

RISER 

pipe 0.067 2.734 0.177 0.025 1.1 1 

cone 0.012 0.152 0.321 0.081 0.1 1 

risertop 0.116 1.703 0.295 0.068 0.1 1 

tube area 0.297 7.017 0.232 0.042 0.2 4 

total 0.493 11.606 1.5 7 

 

SEPARATOR 

separator 0.013 0.510 0.177 0.025 2.5 1 

DOWNCOMER 

barrel+funnel 0.255 2.750 0.344 0.093 0.1 3 

pipes 0.082 8.582 0.110 0.010 0.1 4 

ducts 0.008 0.191 0.224 0.040 0.1 1 

annulus 80% 0.004 0.475 0.102 0.008 0.1 1 

annulus 20% 0.001 0.119 0.102 0.008 10.1 1 

total 0.350 12.116 10.5 10 

 

DOME 

dome 0.379 0.940 0.716 0.403 0.2 1 

shroud 0.111 0.946 0.387 0.117 1 1 

total 0.490 1.886 1.2 2 

 

CONNECTION 

PIPES 

sep. -> downc. 0.066 1.657 0.010 0.040 1 1 

shroud -> 

downc. 0.008 0.005 0.150 0.150 1 1 

The separator was used in “carryover/carryunder” mode. In this mode, a controller is defined 

for both carryover and carryunder. However, in this case, the controller was a constant 

arithmetic block. Different values were manually set for different power levels. The same 

functionality would have been achieved by using the separator in ideal separator mode 

(Ref. 5) and by changing the carryover and carryunder parameters when the power level 

changes. In this mode, the separation efficiency stays constant, regardless of the state of the 

incoming flow. 

The heat transfer from the primary to the secondary circuit was determined by two means. 

The first was to define the tube wall and the second was to use heat structures. In this 

model, the heat transfer was arranged with heat structures. They were connected between 

the eight cells of primary tubes and the first four cells of the riser. The model included no 

other heat structures, because the conditions remained constant during the tests. 
10

In addition to the primary and secondary circuit components, Figure 3 shows the number of 

signal variable modules and control blocks. These are used to extract relevant information 

from the model and also to calculate primary separator deficiency, carryover, carryunder, 

and heating power variables. Equations 1, 2, and 3 define these variables. There is also a 

proportional-integral (PI) controller that controls the feed water inlet component. The 

controller observes the collapsed water level signal variable, which measures the water level 

in all 10 downcomer nodes. A constant block sums 0.877 meters to the collapsed water level 

signal to correct the misreading produced by the downcomer being connected to the 

crossflow face of the riser’s bottom node. A constant signal variable is attached to the 

controller; its purpose is to define the desired water level. 

Totalmass flow invapor outlet

Water mass flow invapor outlet Carryover = (1) 

Totalmass flow inliquidoutlet

Vapor mass flow inliquidoutlet Carryunder = (2) 

Liquid mass flow entering separator node

Liquid mass flow carried to dryer inlet Primary separator deficiency = (3) 

Once the simulation had been run, the results were opened in the animation model, which 

offers a graphic view of the process. The animation view was built by copying the 

components from the simulation model and adding some playback control and indicator 

components. In this case, a fluid conditions range was also added to the view. Figure 4 

presents the simulation model. 

Figure 4 SNAP animation view of the Model Boiler 2 model
11

4 SIMULATIONS 

Extensive transient and steady-state testing was performed in a Model Boiler 2 facility in the 

early 1980s, and some of the data can be found in the ATHOS2 code verification report 

(“Thermal and Hydraulic Code Verification: ATHOS2 and Model Boiler 2 Data. Final Report,” 

issued 1983 (Ref. 6)). The material contained data from the steady-state runs at 25-percent, 

50-percent, and 100-percent power levels. The most important pieces of information were 

the recirculation ratio, pressures, primary side temperatures, and heat transfer rates. The 

feedwater flow was also very useful in verifying the correct heat transfer rate. These data 

allowed adjustments to the secondary-side form losses and heat transfer rates. However, 

the data gave little information regarding the state of the primary separator. More useful 

information was found from the report “MB-2 Steam Generator Transient Response 

Program: Loss of Feed Flow, Steam Generator Tube Rupture and Steam Line Break 

Thermohydraulic Experiments” (Ref. 2), which provided far more geometric data and 

construction drawings. 

The data for adjusting the steam separator were taken from “Prototypical Steam Generator 

Testing Program: Test Plan/Scaling Analysis,” issued 1984 (Ref. 1). The report included a 

chart where the primary separator deficiency was plotted as a function of power. As the 25-

percent, 50-percent, and 100-percent power levels were obtained from the other report, 

these data allowed the correct adjustment of the primary separator for these three power 

levels. Figure 5 contains the chart. The primary separator deficiencies for 100-percent, 50-

percent, and 25-percent power levels are approximately 2.5 percent, 0.6 percent, and 

0.4 percent, respectively. It should be noted that the 25-percent power level is slightly 

outside the chart, and therefore, the value was considered unreliable. 

Figure 5 Primary separator deficiency as a function of power level (Ref. 1) 
12
13

5 RESULTS 

With each power level, there were difficulties in obtaining as high a recirculation ratio as was 

measured in the actual tests. It seemed that form losses along the flowpath had a major 

impact on model behavior. If the form losses were too small, the downcomer water level 

dropped and led to a situation where the target water level was never achieved and too 

much feedwater flowed into the system. The system also had a tendency to oscillate if not 

enough friction was present. With higher additional losses, recirculation was lower than 

desired. 

5.1 100-Percent Power Level

In this simulation, a pattern was observed where the carryunder vapor instantly condensed 

when it came in contact with the feedwater. This produced a small but very noticeable 

oscillation in the whole system. Because of this, the feedwater inlet was moved downwards 

from its original position. With this arrangement, the inlet was positioned under the water 

level, and the problem was avoided. 

Getting the steam generator model to work properly at all power levels proved to be difficult, 

and ultimately, good results were not achieved. The targeted primary separator deficiency 

levels were not achieved in a stable system. The problem seemed to be related to the 

separator drain. There were no specific data regarding the drain flow area, and therefore, 

many geometries were tried. If the flow area of this pipe was set to be large, the system and 

especially the gas mass flow (in the particular pipe) oscillated. If the flow area was set to be 

small, the separator could no longer achieve the targeted separation levels. With a geometry 

and form losses that produced a stable system, a primary separator deficiency level of 

7.6 percent was achieved. Table 3 contains the results from this simulation. 

Table 3 100-Percent Power Level Simulation Results, 0.005-Second 

Timestep 

Simulated Measured 

Pressure in steam dome [bar] 

Transferred power [MW] 

Feedwater flow [kg/s] 

Water level [m] 

Recirculation ratio 

Primary separator deficiency [%] 

Carryover [%] 

Carryunder [%] 

Primary circuit pressure [bar] 

Primary circuit flow [kg/s] 

Primary circuit inlet temperature [°C] 

Primary circuit outlet temperature [°C] 

69.64 

6.86 

3.83 

11.23 

2.0 

7.6 

6.7 

0.7 

156 

36.064 

325.2 

291.7 

69.57 

6.625 

3.76 

11.23 

3.0 

2.5 

- 

- 

155 

36.064 

325.1 

293.3 
14

The fact that decreasing the carryunder value led to flow oscillations in the system was 

studied. In this model, the behavior became clearer after the set value decreased to below 

0.005. Since there were no primary separator carryunder data available from the tests, and 

since, with this carryunder, the flows were still stable, it was decided to use this value. 

This simulation run required use of a relatively small timestep. As shown in Figure 6, the 

water mass flow in the primary separator steam exit vibrated even with a 10-millisecond 

maximum timestep. The system was completely free of oscillations when the maximum 

timestep was reduced to 5 milliseconds. The minimum timestep time was 10-6 seconds in all 

the simulation runs, and plotting was set to 1-second intervals. 

Figure 6 Liquid mass flow in the separator steam exit with two different 

timesteps (100-percent power) 

5.2 50-Percent Power Level

The test demonstrated that the returning water pipe geometry and form losses that were set 

for a 100-percent power level were not suitable for the 50-percent power level. In this 

simulation, many parts of the system were oscillated heavily. This behavior was best shown 

in the gas and liquid flows of the pipe that returns water from the separator to the 

downcomer. Also, the water and steam flows in the separator steam exit were not stable 

(water flow presented in Figure 7). Table 4 presents the results of the simulation. 
15

Table 4 50-Percent Power Level Simulation Results, 0.005-Second 

Maximum Timestep 

Simulated Measured 

Pressure in steam dome [bar] 

Transferred power [MW] 

Feedwater flow [kg/s] 

Water level [m] 

Recirculation ratio 

Primary separator deficiency [%] 

Carryover [%] 

Carryunder [%] 

Primary circuit pressure [bar] 

Primary circuit flow [kg/s] 

Primary circuit inlet temperature [°C] 

Primary circuit outlet temperature [°C] 

72.02 

3.16 

1.60 

11.23 

4.1 

<1 

- 

- 

154.8 

37.78 

308.6 

293.2 

73.01 

3.25 

1.699 

11.23 

6.8 

0.9 

- 

- 

154.9 

37.78 

308.6 

292.9 

Figure 7 Water mass flow in separator steam outlet, 50-percent power level 

It was possible to stabilize the system at this power level by widening the flow area of the 

small pipe that connects the separator back to the downcomer. This kind of geometry, 

however, led to heavy oscillation when used in 100-percent power level runs. Having faced 

difficulties in adjusting the separator for both 50-percent and 100-percent power levels, it 

was decided not to simulate the lowest power level. 
16
17

6 CONCLUSIONS 

Many attempts to find a working geometry for the returning water pipe and suitable form 

losses were made, but, ultimately, good results were not achieved. The 100-percent power 

level run fell behind its target separation level. Although the separation level in the 50-

percent power level run may seem satisfactory at first glance, it must be noted that the 

system oscillated heavily and the results are unreliable. 

The TRACE theory manual (Ref. 4) urges caution when using the TRAC-P separator 

component that was brought to TRAC-M (V3.0) and states that the functionality of the 

component will be improved in TRAC-M/F90. TRAC-M was renamed TRACE in 2003, so it is 

likely that the separator model has been improved. The results of these simulations, 

however, indicate that the component might not work as expected. 

If the built Model Boiler 2 model is to be used again, the following aspects should be noted. 

First, in this study, it was decided that the simulation model would not include the dryer 

assembly. The transient test report (Ref. 2) included some carryover data for the whole 

separator, but this data was not used, since achieving the proper functionality of the primary 

separator component proved to be complicated. Secondly, the only built-in heat structures 

are the ones that transfer heat from the primary to the secondary circuit. This was sufficient, 

since the tests were all run in constant steady-state conditions and the heat capacities of the 

structures did not affect the results. If, however, a transient were to be simulated, more heat 

structures should be used. 
18
19

7 REFERENCES 

1. “Prototypical Steam Generator Testing Program: Test Plan/Scaling Analysis,” 

Westinghouse Electric Corporation, 1984. NUREG/CR-3661 

2. Mendler O. J., “MB-2 Steam Generator Transient Response Program: Loss of Feed 

Flow, Steam Generator Tube Rupture and Steam Line Break Thermohydraulic 

Experiments.” U.S. Nuclear Regulatory Commission, 1986. 

3. Christopher Murray, “Overview of TRACE V5.0,” Regulatory Information Conference 

slides, U.S. Nuclear Regulatory Commission, February 21, 2007. Available at: 

http://www.nrc.gov/public-involve/conference-symposia/ric/slides/

CMurraytraceoverview.pdf.

4. J.W. Spore, J.S. Elson, et al., “TRAC-M/Fortran 90 (Version 3.0) Theory Manual,” 

Los Alamos National Laboratory and Pennsylvania State University, 2000. 

5. “Trace v 5.0 User’s Manual, Field Equations, Solution Methods, 

and Physical Models” U.S. Nuclear Regulatory Commission, August 2007. 

6. “Thermal and Hydraulic Code Verification: ATHOS2 and Model Boiler No. 2 Data, 

Final Report,” Westinghouse Electric Corporation, 1983. 
A-1

APPENDIX 1. TRACE MB-2 MODEL INPUT DATA 

  

 

 

 

    

 

 

      

      

 

 

 

    

 

 

  

  

  

  

  

  

  

 

 

    

 

 

   

   

      

      

   

   

      

      

      

      

 

 

    

 

 

     

           

          

          

          

           

         

         

         

         

          

          

            

            

            

        

        

         

            

 

 

 

        

 

 

      

      

 

      

      

 

      

      

 

      

      

 

      

      

 

      

      

 

      

      
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