NUREG/IA-0454 

International 

 Agreement Report

Modelling of ROCOM Mixing Test 2.2 with 

TRACE v5.0 Patch 3

 

 Prepared by: 

Joona Kurki

VTT – Technical Research Centre of Finland

Nuclear Power Plant Modelling

P.O.B. 1000

02044 VTT

Finland

K. Tien, NRC Project Manager 

 

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: April 2015

Date Published: August 2015

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
AVAILABILITY OF REFERENCE MATERIALS

IN NRC PUBLICATIONS

NRC Reference Material

As of November 1999, you may electronically access 

NUREG-series publications and other NRC records at 

NRC’s Library at www.nrc.gov/reading-rm.html. Publicly 

released records include, to name a few, NUREG-series

publications; Federal Register notices; applicant, 

licensee, and vendor documents and correspondence; 

NRC correspondence and internal memoranda; bulletins 

and information notices; inspection and investigative 

reports; licensee event reports; and Commission papers 

and their attachments.

NRC publications in the NUREG series, NRC regulations, 

and Title 10, “Energy,” in the Code of Federal Regulations

may also be purchased from one of these two sources.

1. The Superintendent of Documents

U.S. Government Publishing Office

Mail Stop IDCC

Washington, DC 20402-0001

Internet: bookstore.gpo.gov

Telephone: (202) 512-1800

Fax: (202) 512-2104

2. The National Technical Information Service

5301 Shawnee Rd., Alexandria, VA 22312-0002

www.ntis.gov

1-800-553-6847 or, locally, (703) 605-6000

A single copy of each NRC draft report for comment is

available free, to the extent of supply, upon written

request as follows:

Address: U.S. Nuclear Regulatory Commission

Office of Administration

Publications Branch

Washington, DC 20555-0001

E-mail: distribution.resource@nrc.gov

Facsimile: (301) 415-2289

Some publications in the NUREG series that are posted 

at NRC’s Web site address www.nrc.gov/reading-rm/

doc-collections/nuregs are updated periodically and may 

differ from the last printed version. Although references to 

material found on a Web site bear the date the material 

was accessed, the material available on the date cited 

may subsequently be removed from the site.

Non-NRC Reference Material

Documents available from public and special technical 

libraries include all open literature items, such as books, 

journal articles, transactions, Federal Register notices, 

Federal and State legislation, and congressional reports. 

Such documents as theses, dissertations, foreign reports 

and translations, and non-NRC conference proceedings 

may be purchased from their sponsoring organization.

Copies of industry codes and standards used in a

substantive manner in the NRC regulatory process are 

maintained at—

The NRC Technical Library

Two White Flint North

11545 Rockville Pike

Rockville, MD 20852-2738

These standards are available in the library for reference 

use by the public. Codes and standards are usually 

copyrighted and may be purchased from the originating 

organization or, if they are American National Standards, 

from—

American National Standards Institute

11 West 42nd Street

New York, NY 10036-8002

www.ansi.org

(212) 642-4900

Legally binding regulatory requirements are stated only in 

laws; NRC regulations; licenses, including technical specifications; or orders, not in NUREG-series publications. The 

views expressed in contractor prepared publications in this 

series are not necessarily those of the NRC.

The NUREG series comprises (1) technical and administrative reports and books prepared by the staff (NUREG–

XXXX) or agency contractors (NUREG/CR–XXXX), (2) 

proceedings of conferences (NUREG/CP–XXXX), (3) reports 

resulting from international agreements (NUREG/IA–XXXX), 

(4) brochures (NUREG/BR–XXXX), and (5) compilations of 

legal decisions and orders of the Commission and Atomic 

and Safety Licensing Boards and of Directors’ decisions 

under Section 2.206 of NRC’s regulations (NUREG–0750).

DISCLAIMER: This report was prepared under an international cooperative agreement for the exchange of technical 

information. Neither the U.S. Government nor any agency 

thereof, nor any employee, makes any warranty, expressed 

or implied, or assumes any legal liability or responsibility 

for any third party’s use, or the results of such use, of any 

information, apparatus, product or process disclosed in this 

publication, or represents that its use by such third party 

would not infringe privately owned rights.
NUREG/IA-0454

International 

 Agreement Report

Modelling of ROCOM Mixing Test 2.2 with 

TRACE v5.0 Patch 3

 Prepared by:

Joona Kurki

VTT – Technical Research Centre of Finland

Nuclear Power Plant Modelling

P.O.B. 1000

02044 VTT

Finland

K. Tien, NRC Project Manager

 

Division of Systems Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript Completed: April 2015

Date Published: August 2015

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

iii

ABSTRACT

Experimental data on non-uniform flows in reactor pressure vessel downcomer has been 

recently produced at the ROCOM test facility at Helmholtz-Zentrum Dresden-Rossendorf, 

within the framework of the OECD/PKL2 project. This data can be used for validation of 

three-dimensional thermal-hydraulic codes.

The transient ROCOM test 2.2 is calculated with the system-code TRACE v 5.0 Patch 3, 

using the new vessel junction component in order to produce local calculation grid refinement 

within the lower plenum. 

The simulation results obtained with TRACE are qualitatively in good agreement with the 

experimental measurements, but quantitatively there is more diffusion in the calculated 

temperature distributions and some bias from the outlet boundary condition utilized in the 

simulation.

v

CONTENTS

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

FIGURES.............................................................................................................................vii

TABLES................................................................................................................................ix

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

2. ROCOM TEST FACILITY.............................................................................................. 3

 2.1 ROCOM TEST 2.2................................................................................................. 4

3. TRACE MODEL OF THE ROCOM PRESSURE VESSEL............................................. 7

4. SIMULATION RESULTS..............................................................................................13

5. SUMMARY...................................................................................................................21

6. REFERENCES.............................................................................................................23

vii

FIGURES

Figure 1. Schematic view of the ROCOM test facility [4]. ....................................... 3

Figure 2. Measured loop flow rates in the ROCOM test 2.2 /8/. ............................. 5 

Figure 3. TRACE model of the ROCOM facility. ..................................................... 7 

Figure 4. Side view of the RPV nodalization in the TRACE model. The no-flow 

edges are displayed in red....................................................................... 9 

Figure 5. Top view of the RPV nodalization. The thick red lines represent 

the no-flow edges at the axial level of the RPV inputs and outputs. 

Marked also are the cell numbers within the azimuthal direction 

for the inlet and outlet cells. ................................................................... 10

Figure 6. Inlet flow velocities for the TRACE calculation. ..................................... 12

Figure 7. Snapshots of the temperature distributions in the unwrapped 

downcomer during the initial phase of the transient, with one 

second (1 s) interval. Experimental data on left and TRACE 

calculations on right. Approximate locations of the four RPV 

inlets are marked by arrows................................................................... 15

Figure 8. Snapshots of the temperature distributions in the unwrapped 

donwncomer at time instants 30 s, 60 s, 90 s, 120 s and 150 s. 

Experimental data on left and TRACE calculations on right. 

Approximate locations of the four RPV inlets are marked by arrows. .... 16

Figure 9. Snapshots of the core inlet distributions from time instants 10, 11, 

12, 13, 14, 15 (in the left column) and 16, 30, 60, 90, 120, 150 

(in the right column). Experimental data on left and TRACE 

calculations on right. For the TRACE results, the temperatures 

values have been sampled to match the location of each core 

inlet passage from the cylindrical calculation grid used 

in the simulation..................................................................................... 17

Figure 10. Average temperatures at the core inlet plane, the inner downcomer 

and the outer downcomer. ..................................................................... 18

Figure 11. Minimum temperatures at the core inlet plane, the inner downcomer and 

the outer downcomer. ............................................................................ 19

ix

TABLES

Table 1. Initial conditions of the ROCOM test 2.2................................................... 4

Table 2. Axial nodalization of the downcomer and core region............................. 10

Table 3. Radial nodalization of the downcomer and core region. ......................... 11

Table 4. Axial nodalization of the lower plenum.................................................... 11

Table 5. Radial nodalization of the lower plenum. ................................................ 11

Table 6. Volumetric porosities in the lower plenum. ............................................. 11

Table 7. Initial conditions used in the TRACE calculation..................................... 12

1

1. INTRODUCTION

In general, thermal-hydraulic safety analyses of nuclear power plants have to be conducted 

using system-scale analysis tools due to the limited available computational power and 

computer memory. The system-scale codes are typically based on one-dimensional 

description of the two-phase flow, which makes the solution of the flows in the nuclear 

reactor core and the coolant loops much faster and less memory consuming in comparison to 

proper computational fluid dynamics (CFD) codes. However, certain asymmetric events, such 

as the non-uniformity of the flow field in the reactor pressure vessel (RPV) downcomer when 

for example the flow rates, temperatures or boron concentrations in the cold legs are not 

equal, cannot be properly described with one-dimensional models. In order to respond to this 

need, specific three-dimensional component models have been incorporated into such widelyused analysis codes as RELAP5 [1], CATHARE [2] and TRACE [3]. 

The three-dimensional models in the system-scale tools are intended to capture only the largescale three-dimensional effects, and they are typically used with very coarse nodalizations 

utilizing the porous-medium approach. Details of the flow field such as small-scale 

fluctuations and pressure distribution in the vicinity of flow obstructions cannot be captured 

by these tools. 

When making predictions with tools based on such simplified methodologies, one has to 

assure that the used models and methods work as intended, i.e. that they capture all the 

relevant phenomena they are supposed to capture, and also to assess how well the simplified 

model corresponds to the physical reality in the end. These tasks can be achieved through 

validation against experimental data, or alternatively through code-to-code comparison if 

another, already-validated code, is available.

Experimental data on non-uniform flows in reactor pressure vessel downcomer has been 

recently produced at the ROCOM (Rossendorf Coolant Mixing) test facility at HelmholtzZentrum Dresden-Rossendorf (HZDR) [4], within the framework of the OECD/PKL2 project 

[5]. The ROCOM test facility, which is a model of the German KONVOI type Pressurized 

Water Reactor (PWR), is specifically designed for coolant mixing studies in the RPV 

downcomer. 

The purpose of this work is to examine the three-dimensional modelling capabilities of the 

system code TRACE for temperature distribution behaviour in the reactor pressure 

downcomer and at the core inlet, by calculating the recent ROCOM Test 2.2 [9, 10]. 

3

2. ROCOM TEST FACILITY

The ROCOM test facility at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is a model of a 

German KONVOI-type PWR, and it is intended for coolant mixing studies in the reactor 

pressure vessel downcomer. The facility includes all the important details of the reference 

plant at linear scale of 1:5, including the pressure vessel and the four coolant loops with fully 

controllable coolant circulation pumps. The pressure vessel at the test facility is made from 

acrylic glass, and the experiments are conducted at ambient temperature and near atmospheric 

pressures. The reactor core at the test facility is represented by a core basket, which simulates 

the hydraulic resistance provoked by the fuel rods in the real reactor core, and no core heating 

is provided. A schematic view of the facility is presented in Figure 1. 

Figure 1. Schematic view of the ROCOM test facility [4]. 

Sugar and salt are used to alter the density and electric conductivity of fluid injected to a 

specified cold leg, so that its mixing can be measured by means of electric conductivity 

sensors. This can be used for simulating overcooled or under-borated slugs of the coolant. 

Special wire-mesh sensors that allow high-resolution measurement of the transient salt or 

brine concentration with regard to space and time are installed into the whole of the pressure 

vessel downcomer and the core inlet plane.

The measurements obtained from the test facility are described by a dimensionless mixing 

scalar, which characterises the local instantaneous share of fluid originating from the specified 

cold leg. The mixing scalar Θx,y,z(t) is calculated by relating the local instantaneous 
4

conductivity σx,y,z(t) to the amplitude of the conductivity change at a reference point (σ1) 

according to the formula 

01

,, 0

,,

)( )(

σσ

σσ

−

− =Θt

t zyx

zyx (1)

The lower reference value σ2 is the initial conductivity of the water in the test facility before 

starting the experiment. For comparison to the PKL experiments, the corresponding 

temperature distribution can be reproduced from the mixing scalar using weighted 

interpolation between the average temperature of the intact coolant loops T2-4 and the 

temperature of the broken loop T1: 

( ) T ,, zyxzyx − +−Θ= TTT −42421,, (2)

2.1 ROCOM test 2.2

The ROCOM test 2.2 was performed under transient conditions. The main objective was the 

assessment of influence of changing mass flow rate in the non-affected loops on the mixing 

pattern inside the reactor pressure vessel; earlier tests at the ROCOM facility had showed that 

varying the boundary conditions caused a transition from a sector-shaped temperature 

distribution to a nearly-homogeneous temperature distribution. 

The initial conditions of the test are summarized in Table 1. The experiment was carried out 

so that the equal flow rates were first established in loops 2 through 4, and then, at time 

instant zero, injection was started to loop 1 and at the same time the decrease of flow rates in 

the other loops was commenced. The measured loop flow rates during the experiment are 

presented in Figure 2. 

Table 1. Initial conditions of the ROCOM test 2.2. 

Loop 1 2 3 4

Normalized volume flow rate [-] 0.0 12.2 12.2 12.2

Volume flow rate [l/s] 0.0 6.27 6.27 6.27

Relative density [-] 1.12 1.00 1.00 1.00

Temperature [°C] (PKL) 153 236.1 236.1 236.1
5

Figure 2. Measured loop flow rates in the ROCOM test 2.2 /8/. 

7

3. TRACE MODEL OF THE ROCOM PRESSURE VESSEL

The TRACE model of the ROCOM test facility used in this work consists of the reactor 

pressure vessel starting from the cold legs up until the beginning of the hot legs. The pressure 

vessel is modelled using two VESSEL components, the cold and hot legs using PIPE 

components, and FILL and BREAK components are used to define the boundary conditions 

on the inlet and outlet sides respectively. The RPV is the divided into two VESSEL 

components in order to achieve local mesh refinement in the lower plenum.

The TRACE model of the facility as seen through the graphical interface SNAP is presented 

in Figure 3.

Figure 3. TRACE model of the ROCOM facility.

Since TRACE is not capable of connecting a one dimensional pipe component to multiple 

cells of a three dimensional vessel component, the inner diameter of the cold legs determines 

the minimum mesh scale that can be used for the pressure vessel — connecting a cold leg to a 

too small vessel node would cause unrealistic throttling of the inlet flow. On the other hand, 

validation calculations such as the one under discussion have to be performed using similar 

meshings to those that would be used in real analysis applications. Since the condition 

imposed by the inlet diameter results in a maximum of 24 calculation cells in the azimuthal 

direction, which is already a somewhat large number for a system-scale analysis, this 

limitation is hardly of concern.

The axial meshing is set up so that:

• The inlet (and outlet) cells have centre-point at the correct elevation, and the cell 

height is in reasonable proportion to the length in the azimuthal direction

• 10 axial cells cover the area of the downcomer wire-mesh sensor measurements

• All the cell heights are in mutually reasonable proportions

This leads to 15 axial cells in the downcomer region.

Within the radial direction, one cell suffices for the downcomer, and also only one cell is used 

for the core region because the core flow distributions are not of interest in the present work
8

(there are no measurements in core region of the ROCOM facility). The lower plenum is 

modelled with another VESSEL component, with 4 cells in the axial direction, 4 cells in the 

radial direction, and 24 cells in the azimuthal direction. The additional radial cells compared 

to the upper part of the RPV are needed to properly model the sieve drum. Volumetric and 

surface porosities were set as so to roughly emulate the half-spherical shape of the lower 

plenum with the cylindrical grid. 

The nodalization of the reactor pressure vessel is illustrated from side in Figure 4 and from 

top in Figure 5. The node sizes are listed in Tables 2 through 5. The volumetric porosities in 

the lower plenum are listed in Table 6.

The loss coefficients of the sieve drum and the core support plate are very difficult to estimate 

without measurements. In the absence of a better estimate, we use the formula presented by 

Idel’Chik [11] for perforated plates. This gives the loss coefficient in the form of 

2

h

0

1

D f

l

















+= λζζ , (3)

with the constant ζ0 calculated as

( ) 2

0 τζ −+−−+= fff )1()1(15.0 , (4)

where τ is a coefficient dependent on l/Dh. This gives the loss coefficients ζ = 36 for the sieve 

drum, and ζ = 5 for the core support plate1. 

1 The values used in calculation of these calculations are:

Drum thickness l = 8 mm, drum hole diameter Dh = 15 mm, drum porosity f = 21%, friction coefficient λ = 0.02

Core support plate thickness l = 106 mm, hole diameter Dh = 38 mm, drum porosity f = 43%, friction coefficient 

λ = 0.02
9

Figure 4. Side view of the RPV nodalization in the TRACE model. The no-flow edges 

are displayed in red. 
10

Figure 5. Top view of the RPV nodalization. The thick red lines represent the no-flow 

edges at the axial level of the RPV inputs and outputs. Marked also are the 

cell numbers within the azimuthal direction for the inlet and outlet cells.

Table 2. Axial nodalization of the downcomer and core region. 

# Δz z bottom z top

15 0.1440 1.3761 1.5201

14 0.1440 1.2321 1.3761

13 0.0987 1.1334 1.2321

12 0.0987 1.0347 1.1334

11 0.0964 0.9383 1.0347

10 0.0964 0.8419 0.9383

9 0.0964 0.7455 0.8419

8 0.0964 0.6491 0.7455

7 0.0964 0.5527 0.6491

6 0.0964 0.4563 0.5527

5 0.0964 0.3599 0.4563

4 0.0964 0.2635 0.3599
11

3 0.0964 0.1671 0.2635

2 0.0972 0.0699 0.1671

1 0.0699 0.0000 0.0699

Table 3. Radial nodalization of the downcomer and core region. 

# Δr r in r out

1 0.4370 0.0000 0.4370

2 0.0630 0.4370 0.5000

Table 4. Axial nodalization of the lower plenum. 

# Δz z bottom z top

4 0.076 0.2800 0.3560

3 0.095 0.1850 0.2800

2 0.095 0.0900 0.1850

1 0.090 0.0000 0.0900

Table 5. Radial nodalization of the lower plenum. 

# Δr r in r out

1 0.1430 0.0000 0.1430

2 0.1430 0.1430 0.2860

3 0.1155 0.2860

4 0.0985 0.4015 0.5000

Table 6. Volumetric porosities in the lower plenum. 

#Z / #r 4 3 2 1

4 0.6 0.5 0.5 0.5

3 0.3 1.0 1.0 1.0

2 0.0 0.5 1.0 1.0

1 0.0 0.0 0.4 0.9

Because TRACE obviously doesn’t contain fluid material properties for the sugar/water 

solution used in the real test, the simulation was carried out in thermodynamic conditions 

corresponding to the PKL tests rather than the atmospheric conditions of the ROCOM tests. 

This means that the geometry and flow rates are taken from the actual ROCOM test, but 

pressure is considerably higher than the pressure at the ROCOM facility (3.8 MPa), and the 

density differences are achieved through heating of the fluid rather than through altering its 

chemical composition. Since thermal conductivity of the fluid plays insignificant role in the 

simulated scenario, the results are expected to correspond to the measured “temperatures” 

reasonably well.

The time-dependent flow velocities and PKL temperatures were used as inflow boundary 

conditions, and constant PKL pressure as the outflow boundary condition. The initial 
12

boundary conditions are listed in Table 7, and the time-dependent inflow velocities based on 

the measured flow rates are presented in Figure 6.

The assumption of the constant pressure at the outlet is not completely realistic, since the 

pressure is produced by the circulation pumps, which in the transient scenario are not working 

at constant power. However, keeping in mind that the reactor pressure vessel is filled with 

(essentially incompressible) liquid for the whole duration of the transient, the outlet flow 

should at all times correspond to the inlet flow rate, and thus the small errors in the outlet 

pressure should cause no visible effect in the simulation results. 

Table 7. Initial conditions used in the TRACE calculation. 

Loop 1 2 3 4

Mass flow rate [kg/s] 0.0000 5.7436 5.7436 5.7436

Temperature [K] 426.15 509.24 509.24 509.24

Outlet pressure [MPa] 3.8 3.8 3.8 3.8

Relative density 1.12 1.00 1.00 1.00

Figure 6. Inlet flow velocities for the TRACE calculation. 
13

4. SIMULATION RESULTS

Snapshots of the simulated temperature distributions in the downcomer are presented 

alongside the experimental results in Figure 7 and in Figure 8. Likewise, snapshots of the 

simulate temperature distributions at the core inlet plane are presented together with the 

experimental distributions in Figure 9. The simulated distributions have been sampled from 

the cylindrical calculation grid to match the locations of the core inlet passages for easy 

comparison to the experimental data; there is still only 3 radial rings x 24 azimuthal sectors = 

72 different data points from the simulation results.

The initial phase of the transient, i.e. penetration of the cold injection water jet from loop 

number 1 into the pressure vessel is presented in Figure 7. In the experimental results, the 

shape of the jet, and especially its tip, is fluctuating while it progresses through the height of 

the downcomer. This effect cannot be captured by TRACE due to the relatively large cell 

sizes and time-step sized used in the simulation.

Figure 8 shows five snapshots covering the rest of the duration of the transient. TRACE is 

able to predict the filling of the downcomer with cold water reasonably well, i.e. an interface 

between the lower region of cold water and the upper region of hot water can be recognized, 

and this interface rises roughly at the same velocity as in the experiment. However, the 

interface becomes more and more blurry as the simulation proceeds and heat diffuses from the 

hot upper part to the cold lower part, decreasing the temperature in the upper part and rising 

the temperature at the lower part ahead of time. In the experimental results a strict division 

into the lower cold part and the upper hot part is clearly visible throughout the experiment, 

unlike in the TRACE simulation.

The simulated core inlet distributions, presented in Figure 9 are also in a reasonable 

agreement with the experimental data. Although the core inlet flow distribution stays 

practically homogeneous throughout the simulation, the benefit of the more detailed 

modelling of the lower plenum and the sieve drum is still evident from the snapshots of the 

earlier stages of the transient: the sieve drum causes the cold water flow to initially spread in 

the outer part of the lower plenum instead of being directed straight towards the core inlet. 

For a more quantitative assessment, characteristic integral variables can be selected. For the 

present purpose, such variables are the average and minimum temperature within different 

parts of the pressure vessel. While the average temperatures should correspond fairly well to 

the measurements since they are governed by the mass and energy balances in a rather 

straight-forward way, the minimum temperature is more prone to be biased by both physical 

and numerical diffusion in the simulation code. 

In Figure 10 and in Figure 11 the average temperature and the minimum temperature in the 

downcomer and at core inlet plane, as calculated by TRACE, are compared to the 

measurements from the ROCOM facility. The qualitative behaviour of the curves is again 

similar between the experimental and calculated results, but on a quantitative level, the 

simulated temperatures remain much higher than the experimental ones.

The deviations between the simulated and experimental curves in Figure 10 and in Figure 11 

are mainly caused by the coarse modelling — that is typical for system-scale analyses — of 

the downcomer and the lower plenum, which enhances mixing through the fact that constant 

properties (i.e. perfect mixing) are assumed within a calculation cell, and also partly the 

numeric diffusion from the semi-implicit method that TRACE uses for the solution of the 

fluid flow field equations. This enhanced mixing then results in averaging-out the local 
14

temperature minima, and also causes the average temperatures to stay above the experimental 

values; in the simulation the whole contents of the downcomer — even above the region of 

the wire mesh sensors — become mixed at least to a degree, while in the experiment the 

downcomer remains thermally stratified for the duration of the transient. As a consequence of 

this, in the simulation there is more heat in the downcomer wire mesh sensor region (i.e. the 

heat that in the experiment remains accumulated in the top part of the downcomer) at each 

instant, explaining why the simulated average temperatures stay well above the experimental 

values. To support this conclusion, the following verification calculation can be made. 

In the experiment, the cold water level reaches the upper end of the downcomer wire mesh 

sensor region at 120 seconds (see Figure 8). At this point the average temperature in the 

downcomer is about 170 °C (Figure 10), corresponding approximately to value 0.80 of the 

mixing scalar defined by Equation (2). If at this point the mixing scalar (i.e. salinity in the 

experiment) would be evenly distributed into the whole downcomer area, including also the 

top part above the wire mesh sensor, the mixing scalar would get value of around 0.50, 

corresponding to temperature ≈ 200 °C. This demonstrates that the heat accumulated above 

the wire mesh sensor can account for the whole of the discrepancy between the simulated and 

the experimental average downcomer temperature.
15

Figure 7. Snapshots of the temperature distributions in the unwrapped downcomer 

during the initial phase of the transient, with one second (1 s) interval. 

Experimental data on left and TRACE calculations on right. Approximate 

locations of the four RPV inlets are marked by arrows.

153.0 Temperature [°C] 236.1 
16

Figure 8. Snapshots of the temperature distributions in the unwrapped donwncomer at 

time instants 30 s, 60 s, 90 s, 120 s and 150 s. Experimental data on left and 

TRACE calculations on right. Approximate locations of the four RPV inlets 

are marked by arrows.

153.0 Temperature [°C] 236.1 
17

Figure 9. Snapshots of the core inlet distributions from time instants 10, 11, 12, 13, 14, 

15 (in the left column) and 16, 30, 60, 90, 120, 150 (in the right column). 

Experimental data on left and TRACE calculations on right. For the TRACE 

results, the temperatures values have been sampled to match the location of 

each core inlet passage from the cylindrical calculation grid used in the 

simulation. 
18

Figure 10. Average temperatures at the core inlet plane, the inner downcomer and the 

outer downcomer. 
19

Figure 11. Minimum temperatures at the core inlet plane, the inner downcomer and the 

outer downcomer.

21

5. SUMMARY

The transient ROCOM test 2.2 has been calculated with the system-code TRACE v 5.0 Patch 

3, using the new Vessel junction component to divide the reactor pressure vessel into two 

distinct cylindrical grids and thus obtaining local grid refinement within the lower plenum. 

The simulation results obtained with TRACE are qualitatively in a fairly good agreement with 

the experimental measurements, but quantitatively there is clearly more diffusion in the 

calculated temperature distributions. This diffusion seems to arise mainly from the coarse 

nodalization used in the simulation, and also partly from the numerical scheme used to solve 

the fluid flow field equations. However, despite the over-prediction of the average and 

minimum temperatures that are caused by this increased diffusion, the simulation results 

obtained here are in such an agreement with the experimental data that can be expected from a 

system-scale analysis.

The grid refinement within the lower plenum made it possible to model the internals of the 

lower plenum, such as the sieve drum, more reasonably. These enhancements in the 

modelling should manifest themselves in the core inlet distribution, which is actually the main 

motivation for three-dimensional modelling of the reactor pressure vessel in the first place; in 

a real-life safety analysis application we are interested in any non-uniformities in the core 

inlet distributions of temperature or boron concentration as they have a direct effect on the 

neutronic behaviour of the nuclear reactor core. Even though the transient case simulated here 

is such that the core inlet distribution remains largely uniform, the benefit of the grid 

refinement within the lower plenum became evident.

23

6. REFERENCES

1 The RELAP5-3D Code Development Team: RELAP5-3D Code Manual 

Volume I: Code Structure, System Models and Solution Methods, Idaho 

National Laboratory, INEEL-EXT-98-00834, Revision 4.0 (2012) 

2 G. Geffraye et al.: CATHARE 2 V2.5_2: A single version for various 

applications, Nuclear Engineering and Design, 241, pp. 4456 – 4463 (2011). 

3 U.S. Nuclear Regulatory Commission, TRACE V5.0 User’s Manual (2012). 

4 S. Kliem et al.: Experiments at the mixing test facility ROCOM for 

benchmarking of CFD codes, Nuclear Engineering and Design, 238, pp. 566 – 

576 (2008) 

5 OECD/NEA PKL-2 Project, http://www.oecd-nea.org/jointproj/pkl.html, 

(Retrieved 9th October 2012). 

9 S. Kliem & R. Franz, Quick-look report of the ROCOM Tests 1.3, 2.1 and 2.2 

conducted within the OECD PKL2 Project, Institute Report 

HZDR\FWO\2012\02, Helmholtz Zentrum Dresden Rossendorf (2012) 

10 S. Kliem & R. Franz, OECD PKL2 Project — Final Report on the ROCOM 

Tests, Institute Report HZDR\FWO\2012\03, Helmholtz-Zentrum 

Dresden-Rossendorf (2012). 

11 I. E. Idel’Chik: Handbook of Hydraulic Resistance – Coefficients of Local 

Resistance and of Friction, AEC-TR-6630, U.S. Department of Commerce, 

National Technical Information Service (1960). 





NUREG/IA-0454 Modelling of ROCOM Mixing Test 2.2 with TRACE v5.0 Patch 3