NUREG/IA-0446 

International 

 Agreement Report

Assessment of Channel Coolant Voiding in 

RD-14M Test Facility using TRACE 

Prepared by:

Andrew Oussoren

Aleksandar Delja

Canadian Nuclear Safety Commission 

280 Slater Street 

P.O. Box 1046 Station B 

Ottawa, Ontario, Canada 

K1P 5S9 

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: December 2013

Date Published: August 2014

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
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NUREG/IA-0446 

International 

 Agreement Report

Assessment of Channel Coolant Voiding in 

RD-14M Test Facility using TRACE 

Prepared by:

Andrew Oussoren

Aleksandar Delja

Canadian Nuclear Safety Commission 

280 Slater Street 

P.O. Box 1046 Station B 

Ottawa, Ontario, Canada 

K1P 5S9 

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: December 2013

Date Published: August 2014

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

Coolant voiding in fuel channels of CANDU reactors during a Loss of Coolant Accident (LOCA) is 

important to reactor behaviour due to the power pulse arising from positive void reactivity. The 

prediction of voiding in fuel channels is therefore of fundamental importance to LOCA safety 

analysis. The CNSC is evaluating the applicability of the TRACE thermal hydraulics code to CANDU 

reactor simulation.

In this study, inlet header LOCA test B0106 from AECL’s RD-14M thermal hydraulic test facility was 

modeled using TRACE. RD-14M is a scaled CANDU heat transport system containing 10 electrically 

heated fuel channels and full elevation feeders and boilers. In this test, a break in an inlet header is 

simulated using a fast opening valve. The size of the break is such that it creates near stagnation

flow in the affected loop. A neutron scatterometer installed on one of the test channels measures

void fraction in the channel during the transient. 

The TRACE code was capable of predicting channel voiding trends with good accuracy, while the 

extent of voiding was generally over predicted. System pressure is predicted very well over the first 

several seconds of the transient; however pressure is under-predicted later in the transient. This 

under-prediction can be corrected through modifying the multiplying coefficients in the TRACE 

critical flow model. Fuel sheath temperature was under predicted for most of the test; this is due to 

limitations in the ability to represent a horizontal fuel channel in TRACE. 

v

CONTENTS

Page

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

CONTENTS ..................................................................................................................................... v

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

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

ACKNOWLEDGMENTS.............................................................................................................. xi

ABBREVIATIONS ...................................................................................................................... xiii

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

2. Modeling Candu with trace .................................................................................................. 3

2.1 Unique CANDU Geometries............................................................................................................. 3

2.1.1 Horizontal Fuel Channels.......................................................................................................... 3

2.1.2 Headers...................................................................................................................................... 4

2.2 TRACE Modeling of CANDU To Date............................................................................................ 5

3. Experiment Description ........................................................................................................ 7

3.1 RD-14M Facility ............................................................................................................................... 7

3.2 B0106 Test......................................................................................................................................10

4. TRACE model development ............................................................................................... 11

4.1 Nodalization ....................................................................................................................................11

4.2 Model Options................................................................................................................................. 14

4.2.1 Critical Flow Model ................................................................................................................14

4.2.2 Maximum Time Step...............................................................................................................14

4.2.3 Wall Drag Coefficient Model..................................................................................................15

4.2.4 Off-take Model........................................................................................................................15

4.3 Model Calibration ...........................................................................................................................15

5. Steady state conditions ........................................................................................................ 17

6. Transient results .................................................................................................................. 19

6.1 Channel Voiding .............................................................................................................................19

6.2 Header Pressure...............................................................................................................................20

6.3 Differential Pressures......................................................................................................................22

6.4 Fuel Sheath Temperature.................................................................................................................23

7. Sensitivity Studies ................................................................................................................ 25
vi

7.1 Time Step Sensitivity ......................................................................................................................25

7.2 Critical Flow Model Sensitivity.......................................................................................................26

7.3 Header Nodalization Sensitivity......................................................................................................29

7.4 Fuel Channel Nodalization Study....................................................................................................30

7.5 Further Sensitivity Studies ..............................................................................................................33

7.5.1 Valve Opening Time ...............................................................................................................33

7.5.2 Off-take Model........................................................................................................................33

7.5.3 Drag Coefficient Model...........................................................................................................33

7.5.4 Fuel Element Thermal Diameter .............................................................................................33

7.5.5 Choking Plane Study...............................................................................................................34

7.5.6 Valve Internal Loss Model ......................................................................................................34

8. EXTENSION STUDY: CRITICAL FLOW MULTIPLIERS ......................................... 35

8.1 Changes to Nodalization ................................................................................................................. 35

8.2 Results.............................................................................................................................................36

8.3 Critical Flow Model Optimization...................................................................................................37

9. ConclusionS .......................................................................................................................... 39

10. REFERENCES .................................................................................................................... 41

APPENDIX A ................................................................................................A-1
vii

FIGURES

Page

Figure 1: Allowable side junction angles for TRACE off-take model, pipe cross-section . 5

Figure 2: RD-14M facility schematic ......................................................................................... 7

Figure 3: Diagram of Inlet Header 8 showing connections to pump and feeders............... 8

Figure 4: The Fuel Element Simulator (FES) used at RD-14M. .............................................. 9

Figure 5: Blowdown valve used to simulate LOCA in RD-14M.............................................. 9

Figure 6: SNAP visualization of RD-14M nodalization.......................................................... 11

Figure 7: SNAP nodalization of one loop of the below header piping................................ 12

Figure 8: SNAP visualization of one of the steam generators............................................. 13

Figure 9: Channel void fraction predictions at location of scatterometer ........................ 19

Figure 10: TRACE prediction of header 8 pressure ................................................................ 20

Figure 11: TRACE prediction of header 7 pressure ................................................................ 21

Figure 12: TRACE prediction of header 6 pressure ................................................................ 21

Figure 13: TRACE prediction of header 5 pressure ................................................................ 21

Figure 14: TRACE prediction of header 8 – header 5 differential pressure ......................... 22

Figure 15: TRACE prediction of header 6 – header 7 differential pressure ......................... 22

Figure 16: TRACE prediction of fuel sheath temperature in top pin of channel 14 

 bundle 10 .................................................................................................................. 23

Figure 17: Comparison of Header 8 pressure and Channel 14 void for timestep size 

 study.......................................................................................................................... 25

Figure 18: Header 8 pressure for CHM2 sensitivity study...................................................... 26

Figure 19: Header 5 pressure for CHM2 sensitivity study...................................................... 27

Figure 20: Header 8 – header 5 differential pressure for CHM2 sensitivity study............... 28

Figure 21: Void fraction for CHM2 sensitivity study ............................................................... 28

Figure 22: 15 cell, 3 cell and 27 cell header nodalizations used in study ............................ 29

Figure 23: Header 8 pressure for header nodalization study ................................................ 30

Figure 24: Parallel pipe nodalization of channel 14................................................................ 31

Figure 25: Void fraction for parallel pipes model.................................................................... 31

Figure 26: Void fraction in the three parallel pipes................................................................. 32

Figure 27: Header pressures for B0108 CHM2 study.............................................................. 36

Figure 28: Header pressures for B0104 CHM2 study.............................................................. 37

Figure 29: Header pressures for B0105 CHM2 study.............................................................. 37

ix

TABLES

Page

Table 1: Maximum time step size............................................................................................. 14

Table 2: Steady state header conditions for experiment and TRACE model...................... 17

Table 3: Error in fuel channel parameters for TRACE model................................................ 17

Table 4: Break Size of B01XX test series ................................................................................ 35

xi

ACKNOWLEDGMENTS 

The authors would like to acknowledge the Canadian Nuclear Safety Commission for supporting 

involvement in the Code Applications and Maintenance Program (CAMP), Atomic Energy of Canada 

Limited for providing the experimental data necessary to perform this analysis, and the US NRC for 

making the TRACE code available to the international community.

xiii

ABBREVIATIONS

AECL Atomic Energy of Canada Limited

BWR Boiling Water Reactor

CANDU Canadian Deuterium Uranium reactor

CNSC Canadian Nuclear Safety Commission 

ECC Emergency Core Cooling

FES Fuel Element Simulator

LOCA Loss Of Coolant Accident

PHTS Primary Heat Transport System

PHWR Pressurized Heavy Water Reactor

PWR Pressurized Water Reactor

TRACE TRAC/RELAP Advanced Computational Engine

US NRC United States Nuclear Regulatory Commission

1 

1. INTRODUCTION

The Canadian nuclear power industry is based on CANDU technology. CANDU is a Pressurized 

Heavy Water Reactors (PHWR) design, using heavy water to moderate and cool natural uranium 

fuel. In 2013 there were 31 CANDU reactors operating in 7 countries around the world [1]. 

One of the design basis events for CANDU safety analysis is a postulated Loss of Coolant Accident 

(LOCA). The use of a heavy water moderator and natural uranium fuel results in a positive void 

reactivity coefficient. Coolant voiding in the fuel channels during a LOCA will result in an increase in 

reactor power prior to initiation of shutdown systems. For this reason prediction of channel voiding 

during LOCA is a critical parameter in CANDU safety analysis. 

RD-14M is a thermal hydraulic test facility operated by Atomic Energy of Canada Ltd. (AECL) which 

reproduces most of the key components of a CANDU Primary Heat Transport System (PHTS). RD14M is used to investigate PHTS behavior under different postulated operating and accident 

scenarios. Experiment B0106 was conducted at RD-14M in 2001 as part of a series of experiments 

investigating channel coolant voiding rates during a simulated LOCA. These tests were performed in 

partial fulfillment of the requirements of CNSC Generic Action Item 00G01: “Channel Coolant 

Voiding During LOCA”, [2]. Experiment B0106 simulates an inlet header break which creates nearstagnation conditions in the fuel channels downstream of the break.

TRACE is a thermal hydraulics system code which has been developed by the US NRC to provide 

best estimate analysis of thermal hydraulic transients in Pressurized Light Water Reactors (PWR) 

and Boiling Water Reactors (BWR). The CNSC is evaluating the applicability of TRACE to model 

transient behavior in CANDU reactors. To date there is limited validation of TRACE for CANDU-type 

geometries.

In this study, RD-14M test B0106 is modeled using TRACE V5.0 Patch 3. Modeling results are 

compared to experimental measurements to assess the suitability of TRACE to model CANDUtypical LOCA phenomena. 

3 

2. MODELING CANDU WITH TRACE

The TRACE thermal hydraulics code was developed for analysis of PWR and BWR behavior. There 

are some features which are unique to CANDU reactor design which are not easily reproducible in a 

TRACE nodalization. These limitations are outlined in this section. 

2.1 Unique CANDU Geometries

Two CANDU-specific components which are of particular importance for thermal hydraulics 

modeling are the horizontal fuel channels and the headers.

2.1.1 Horizontal Fuel Channels

The fuel channels in a CANDU reactor are horizontal pipes called pressure tubes which contain 

twelve or thirteen fuel bundles, each comprised of 28 or 37 individual fuel elements. Under LOCA 

conditions flow through the fuel channels can become stratified; this has important implications for 

heat transfer, as lower fuel elements which are surrounded by liquid will transfer much more heat to 

the coolant than upper elements which are surrounded by vapour. Consequently, fuel sheath 

temperatures in upper elements will be greater than in the lower elements.

TRACE predicts flow in horizontal pipes to be stratified if the relative velocity (gas velocity minus 

liquid velocity) is below a critical velocity defined by the Taitel-Dukler criterion. This criterion was 

developed for flow through unobstructed, circular pipes [3, p154], and is therefore not directly 

applicable to flow through a CANDU fuel channel. The obstruction of the fuel bundles would allow 

stratification to occur at higher relative velocities than in an open pipe [4], therefore TRACE would 

be expected to under predict the length of time during which stratified flow conditions exists in a fuel 

channel. 

The TRACE stratified flow model includes a stratified wall drag correlation, however there is no

special heat transfer model for the stratified flow regime; instead TRACE uses the average fluid 

properties across each cell. Therefore the differential heat transfer between upper and lower 

elements cannot be captured in a TRACE model.
4 

For modeling fuel assemblies in PWR and BWR, TRACE provides a VESSEL component which is 

capable of 3-D modeling of the core. However the VESSEL component is restricted to a vertical 

orientation, thus it cannot be used to model CANDU fuel channels. 

2.1.2 Headers

The headers in a CANDU reactor are large horizontal pipes: coolant from the main heat transport 

pumps enters the top of an inlet header and divides into many different feeder outlets, which deliver 

flow to the fuel channels through feeder pipes. The feeder outlets are arranged in rows along the 

bottom half of a header and enter at various angles. The process is reversed in the outlet header, 

with hot coolant entering from the feeders and exiting the top of the header to the steam generators. 

Headers represent complex flow geometries for modeling.

The feeder connections to the headers are best modeled in TRACE using side junctions; however 

the TRACE Theory Manual notes that: “A rigorous formulation for momentum transfer terms has not 

been developed for the case where flow is out through the side [junctions]”, [3, p20]. The modeling 

of an inlet header may therefore present some difficulties in correctly predicting momentum transfer. 

In this study the model had to be calibrated to achieve accurate flow division between the channels, 

described in Section 4.3. 

In stratified flow conditions the void fraction of flow through a side junction will depend on the 

connection location relative to the water level in the pipe. TRACE accounts for the void fraction in 

side junction outflow, including liquid entrainment and vapour pull-through, using an off-take model. 

The off-take model requires side junctions to be at 90 degrees to the main flow path of a pipe and 

be oriented vertically up, vertically down or horizontal (see Figure 1). CANDU headers contain feeder 

connections at a variety of angles in between horizontal and vertically down; these connection 

angles cannot be accounted for with the TRACE off-take model. The connection to the break valve 

is horizontally oriented but is in line with the main flow path of the header pipe, and therefore the offtake model is not applicable.
5 

Figure 1: Allowable side junction angles for TRACE off-take model, pipe cross-section

2.2 TRACE Modeling of CANDU To Date

There is limited validation of TRACE for CANDU-type geometries; there are 3 notable reports on 

TRACE modeling in the CANDU community which are summarized briefly below.

A. Mysen (2008) modeled the RD-14M facility test B0104, a LOCA simulation with measured 

channel voiding (part of the same test series as the test described in this report). Mysen found the 

TRACE predicted the general trends of the experiment well, noting that both header pressure and 

fuel clad temperature were under predicted [5]. 

A master’s thesis by D. Hummel (2010) modeled RD-14M test B9401, a LOCA simulation with ECC. 

This test was notable for being used for an IAEA benchmark study on code inter-comparison and 

validation. Hummel found that TRACE “predicted the important test parameters as well as or better 

than some of the established codes used in the IAEA exercise” [9]. It is notable that Hummel found 

the best results were obtained with the TRACE critical flow model disabled, a finding not shared by 

this study. 

A. Delja (2011) modeled RD-14M test B0113, a LOCA experiment with a simulated power pulse. 

Delja found that the general trends of the experiment were reproduced well compared to the 

CANDU-specific thermal hydraulics codes CATHENA and TUF, [6].

7 

3. EXPERIMENT DESCRIPTION

This section describes the RD-14M test facility and the details of experiment B0106. 

3.1 RD-14M Facility 

RD-14M is a thermal hydraulic test loop at AECL’s Whiteshell Laboratories. The facility contains 

scaled representations of most of the key components of a CANDU reactor. Figure 2 shows a 

simplified schematic of the facility. The heat transport system is scaled to produce CANDU-typical 

conditions including pressure, mass flux, temperatures and transit times under forced and natural 

circulation.

Figure 2: RD-14M facility schematic
8 

The primary coolant loop consists of ten fuel channels of typical CANDU length, connected by end 

fitting simulators to full elevation feeder pipes. The end fitting simulators represent a metal mass and 

stagnant coolant volume similar to the end fittings in a CANDU reactor. The ten fuel channels are 

connected to two full elevation U-tube steam generators in a figure of eight arrangement via four 

headers, one of which is shown in Figure 3. Circulation is provided by two bottom suction centrifugal 

pumps scaled for reactor-typical head and flow rate. A surge tank provides pressure control.

Figure 3: Diagram of Inlet Header 8 showing connections to pump and feeders

The secondary side of the steam generators includes an integral preheater and external downcomer

pipe. Steam is directed from the steam separators to a jet condenser and cooling heat exchangers.

Feedwater pumps return the cool feedwater to the steam generators. The jet condenser and 

feedwater pumps are treated as boundary conditions for modeling purposes. 

The Fuel Element Simulators (FES) are 7 element, electrically heated assemblies designed for a 

power density, heat flux and heat capacity similar to that of 37 element CANDU fuel bundles. Figure 

4 shows an RD-14M FES. Each FES is divided into twelve 495 mm sections representing 12 fuel 

bundles. Each section is separated by an unheated section and a bundle restrain clip which 

simulates the endplates between adjacent fuel bundles. 
9 

Figure 4: The Fuel Element Simulator (FES) used at RD-14M.

Loss of Coolant Accidents (LOCA) are simulated at RD-14M using a fast-acting valve (the break 

valve) connected to the end of either an inlet or outlet header. For this experiment the valve is 

connected to inlet header 8, shown in Figure 5. An interchangeable orifice plate installed between 

the header and the valve is used to control break size. Opening the valve connects the header to a 

20 inch blowdown line which vents to the roof of the facility, simulating the break of a header into 

containment. It should be noted that flow through the blowdown line is not measured in RD-14M.

Figure 5: Blowdown valve used to simulate LOCA in RD-14M

An Emergency Core Cooling (ECC) system with high pressure and low pressure stages is proved in

the RD-14M facility. This system was not used in the experiment described in this report and 

therefore was not modeled.
10

The RD-14M facility is extensively instrumented; during the test over 200 measurements were 

recorded at 20 ms intervals for pressures, temperature and flow rate. Void is measured in the inlet 

and outlet feeders, steam generator inlet and outlet and pump discharge using gamma 

densitometers. To measure channel voiding, a high sensitivity neutron scatterometer was installed 

on one of the simulated fuel channels (channel 14). This device utilizes a neutron source and eight 

detectors; it determines void fraction by measuring thermal neutrons scattered by the coolant in the 

channel. 

Flow is measured during steady state conditions with turbine flow meters which are calibrated for 

single phase flow. During transient two-phase conditions flow direction and magnitude may be 

inferred from differential pressure measurements, [7]. 

3.2 B0106 Test

RD-14M experiment B0106 was conducted as part of a series of tests to investigate the rate of 

coolant voiding in fuel channels during a LOCA [8]. For these tests, orifice plates with various sized 

openings were installed between inlet header 8 and the break valve to simulate different break 

sizes. For experiment B0106 an orifice plate was used to simulate a critical break, where flow in the 

downstream channels reaches near-stagnation conditions following break opening. 

After warming the loop and achieving the desired steady state conditions, data collection began. 

Shortly after isolation of the surge tank (within 5 seconds) the break valve was opened. Within 2 

seconds the power was run down to decay heat levels and a rotation-controlled pump rundown 

began. Data collection lasted for approximately 3 minutes after break valve opening.
11

4. TRACE MODEL DEVELOPMENT

A TRACE nodalization of the RD-14M facility was developed using the Symbolic Nuclear Analysis 

Package (SNAP). 

4.1 Nodalization

The entire RD-14M facility nodalization as represented by SNAP is show in Figure 6

Figure 6: SNAP visualization of RD-14M nodalization

The below header section of the model, comprising the feeder pipes, end fitting simulators, and test 

sections is shown in Figure 7. Feeders are modeled as multi-cell PIPE components connecting to 

the headers via 90 degree side junctions. Bends and elevations in the feeders are represented as 

accurately as practicable. The three cells at the lower end of each feeder pipe represent the main 
12

flow path through the end fitting simulators. The dead space behind the end fitting shield plug is 

simulated by a separate, 1 cell PIPE connected by a side junction at an angle of 0 degrees. A heat 

structure representing the liner tube thermally connects the dead space to the main flow path. 

Figure 7: SNAP nodalization of one loop of the below header piping

Each fuel channel test section is modeled as a 12 cell PIPE. A heat structure with 9 radial nodes 

represents the different materials in the FES cross section. A unique POWER component is 

connected to each heat structure between radial nodes 3 and 4 which represents the Inconel-600 

heater portion of the FES. The heat generated by the POWER component is controlled as a function 

of time to simulate initial power and subsequent reduction to decay power levels. 

The headers are nodalized as a 15 cell PIPE component. Header nodalization is finer around the 

feeder connections to conform with the TRACE guideline that adjacent cells should have a volume 

ratio of 10:1 or less. Whereas in the physical design the connections to the above header piping are 

at 90 degrees to the header long axis, in the model these connections are made at the ends of the 
13

headers; this is because TRACE does not allow for PIPE components with only side junction 

connections. To work around this limitation a nodalization of the header as multiple connected pipes

was trialed; however this model encountered significant convergence difficulties. 

The surge tank is connected to header 5. The surge tank was isolated during this experiment, 

therefore it was modeled simply as a BREAK component set to the steady state pressure in header 

5. Surge tank isolation is modeled with a time-controlled VALVE component.

The break valve is modeled as a 2 cell VALVE component attached to inlet header 8. The valve is 

set to open at problem time 60.1s with an opening time of 0.25s. A PIPE component models the

blowdown stack, connecting the break valve to a BREAK component set to atmospheric pressure 

and temperature.

The above header section of the model consists to the Steam Generator primary side, HT pumps, 

associated piping, and the entire secondary side, shown in Figure 8

Figure 8: SNAP visualization of one of the steam generators
14

The U-tubes in the steam generators are modeled as a single PIPE component representing the 

combined flow area of all the tubes. Inlet and outlet plenums are also represented by PIPE 

components. The primary side pumps are modeled as 2 cell PUMP components with rated values 

and pump curves representing the RD-14M pumps. Rotation is controlled as a function of time, 

allowing the model to recreate the rotation controlled pump speed in the experiment.

The feedwater pumps are represented as a FILL boundary condition, which is time controlled to 

reproduce the flow rates in the experiment. The FILL is connected to an angled PIPE component 

representing the integral preheater. The steam separators are modeled as SEPERATOR 

components. The jet condenser is modeled as a BREAK component with pressure set to the jet 

condenser pressure as measured in the experiment. Four heat structures thermally connect the 

steam generator primary and secondary sides. 

4.2 Model Options

4.2.1 Critical Flow Model

Prediction of choking or critical flow is very important for modeling LOCA because of the rapid 

depressurization in the early stages of the transient. The TRACE critical flow model is enabled using 

default multipliers at the junction between inlet header 8 and the break valve, which is the location of 

the orifice plate. 

4.2.2 Maximum Time Step

To increase computational efficiency the maximum allowed time step size is varied over the course 

of the transient based on the expected rate of change of parameter. Allowed maximum time step 

size is shown in Table 1. 

Table 1: Maximum time step size

Time Range Max Allowed Time Step Size Notes

0 – 54.0 s 1.0 s

54.0 – 57.0 s 0.1 s Surge tank isolated at 55.0 s

57.0 – 62.0 s 0.01 s Break opens at 60.1 s

62.0 – 92.0 s 0.1 s

92 – 296.0 s 0.5 s
15

4.2.3 Wall Drag Coefficient Model

The fuel channels and headers are expected to experience stratified flow at some point during the 

transient. Therefore the wall drag dispersed/stratified model is activated. With this option TRACE 

determines if flow in horizontal components is stratified based on a critical relative velocity; if 

stratified conditions are predicted a special stratified flow wall drag coefficient is implemented. The 

limitations of this model with respect of modeling CANDU fuel channels are discussed in section 

2.1. 

4.2.4 Off-take Model

The TRACE off-take accounts for liquid entrainment and vapour pull-through in stratified flow 

through junctions in horizontal pipes. As discussed in Section 2.1.2 the off-take model is not 

applicable to the geometry of the RD-14M headers. Therefore the off-take model is not used in the 

reference nodalization of RD-14M. A separate study using the off-take model was performed which 

is detailed in Section 7.5. 

4.3 Model Calibration

The TRACE nodalization was initially designed with K-factors given in the RD-14M facility 

description. When steady state was reached flow division between the test sections was found to 

deviate from the values measured at the beginning of the test. To correct channel flow rates the Kfactors in each test section were calibrated in a trial and error process to give a correct channel 

mass flows and pressures, with particular attention paid to correct prediction of flows through heated 

section 14 where channel voiding is measured.

17

5. STEADY STATE CONDITIONS

Steady state was obtained by running the model for a period of 2000 seconds to check for

oscillations. Model parameters at the end of the test were then saved as the initial conditions for 

subsequent transient calculations. 

Pressure and temperature measurements in the headers are a good indication of the state of the

heat transport system. Table 2 shows steady state pressure and temperature values in the headers, 

all of which are within 1% of experimental values.

Table 2: Steady state header conditions for experiment and TRACE model

Pressure (kPa) Experiment TRACE % Difference

Header 5 10009.0 9999.8 -0.09

Header 6 11527.4 11535.5 0.07

Header 7 9971.5 10045.3 0.74

Header 8 11508.7 11531.3 0.20

Temperature (oC)

Header 5 298.1 295.6 -0.83

Header 6 265.7 264.1 -0.60

Header 7 298.1 298.4 0.11

Header 8 265.0 264.2 -0.32

Table 3 shows a summary of error in inlet and outlet feeder pressure, fuel channel outlet

temperature and mass flow rate. As mentioned in Section 4.3, special attention was paid in 

calibration of the model to ensure channel 14 error was minimized, as this is the location of channel 

voiding measurements.

Table 3: Error in fuel channel parameters for TRACE model

Parameter Average % Error Max Error Channel 14 Error

Pressures 0.48% 1.42% 0.30%

Outlet Temperature -0.89% 1.40% 0.61%

Mass Flow 2.22% 5.49% 0.13%

19

6. TRANSIENT RESULTS

This section presents the results of the TRACE model in comparison with experimental values. Each 

data set is presented on two times scales: a 5 second scale to show the rapid changes occurring 

immediately after break opening, and a 60 second scale to show behavior as the system 

depressurizes. A discussion on measurement selection appears in Appendix A. 

6.1 Channel Voiding 

Void fraction in channel 14 is the key parameter in this experiment. Figure 9 shows channel voiding

results. The model predicts the timing of the onset of voiding correctly. Void predictions follow the 

trend of the experiment and are generally over predicted by 0.1 to 0.2. It should be noted that the 

maximum measurement error for the neutron scatterometer is 0.1. 

Channel 14 Scatterometer Void Fraction

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

60 61 62 63 64 65

Time (s)

Void Fraction

Experiment

TRACE

Channel 14 Scatterometer Void Fraction

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

60 70 80 90 100 110 120

Time (s)

Void Fraction

Experiment

TRACE

Figure 9: Channel void fraction predictions at location of scatterometer

As discussed in Section 2.1.1 there are inherent difficulties in modeling CANDU fuel channel heat 

transfer in TRACE. A likely contributor to the over prediction of voiding is that TRACE does not 

account for the reduction in heat transfer from the fuel to the liquid phase which occurs with 

stratification. The influence of stratification is also suggested by fuel sheath temperature predictions 

detailed in Section 6.4. 
20

6.2 Header Pressure

Pressure in the headers is a key indication of the state of the heat transport system. Pressure in 

Header 8, the location of the break, is of particular significance as it is indicative of break mass flow. 

Note that mass flow from the break valve is not measured at the RD-14M facility. 

Figure 10 through Figure 13 show pressures in the headers during the transient. 

Inlet Header 8 Pressure

5000

6000

7000

8000

9000

10000

11000

12000

60 61 62 63 64 65

Time (s)

Pressure (kPa)

Experiment

TRACE

Inlet Header 8 Pressure

0

2000

4000

6000

8000

10000

12000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

TRACE

Figure 10: TRACE prediction of header 8 pressure

Pressure predictions in the headers over the first 5 seconds are in very good agreement with the 

experimental results. The maximum error during this period is a 0.5 MPa under prediction in header 

8.

Beyond 5 seconds pressure predictions begin to diverge from the experimental values and 

pressures in all headers are under-predicted. This indicates that the predicted two-phase critical 

velocity is higher than in the experiment, resulting in a more rapid depressurization. 

TRACE allows for manipulation of the critical flow velocity through user-defined multipliers, which 

are defined separately for the single-phase and two-phase models. This test used the default 

multiplier values of 1.0. Calibration of the critical flow model is explored in Section 7.2. 
21

Outlet Header 7 Pressure

6000

7000

8000

9000

10000

11000

12000

60 61 62 63 64 65

Time (s)

Pressure (kPa)

Experiment

TRACE

Outlet Header 7 Pressure

0

2000

4000

6000

8000

10000

12000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

TRACE

Figure 11: TRACE prediction of header 7 pressure

Inlet Header 6 Pressure

6000

7000

8000

9000

10000

11000

12000

60 61 62 63 64 65

Time (s)

Pressure (kPa)

Experiment

TRACE

Inlet Header 6 Pressure

0

2000

4000

6000

8000

10000

12000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

TRACE

Figure 12: TRACE prediction of header 6 pressure

Outlet Header 5 Pressure

6000

7000

8000

9000

10000

11000

12000

60 61 62 63 64 65

Time (s)

Pressure (pa)

Experiment

TRACE

Outlet Header 5 Pressure

0

2000

4000

6000

8000

10000

12000

60 70 80 90 100 110 120

Time (s)

Pressure (pa)

Experiment

TRACE

Figure 13: TRACE prediction of header 5 pressure
22

6.3 Differential Pressures 

The turbine flow meters in the RD-14M facility are calibrated for single phase flow conditions [7] and 

therefore their precision degrades during the two-phase transient. Header to header pressure 

difference gives insight into the direction and magnitude of channel flow, which has a strong effect 

on channel voiding. Figure 14 and Figure 15 show differential pressures between inlet and outlet 

headers. 

Header 8 - Header 5 Delta P

-5000

-4000

-3000

-2000

-1000

0

1000

2000

60 61 62 63 64 65

Time (s)

Pressure Differential (kPa)

Experiment

TRACE

Header 8 - Header 5 Delta P

-5000

-4000

-3000

-2000

-1000

0

1000

2000

60 70 80 90 100 110 120

Time (s)

Pressure Differential (kPa)

Experiment

TRACE

Figure 14: TRACE prediction of header 8 – header 5 differential pressure 

Header 6 - Header 7 Delta P

0

500

1000

1500

2000

2500

3000

3500

60 61 62 63 64 65

Time (s)

Pressure Differential (kPa)

Experiment

TRACE

Header 6 - Header 7 Delta P

0

500

1000

1500

2000

2500

3000

3500

60 70 80 90 100 110 120

Time (s)

Pressure Differential (kPa)

Experiment

TRACE

Figure 15: TRACE prediction of header 6 – header 7 differential pressure

As with absolute pressure, TRACE predicts differential pressure with very good accuracy over the 

first 5 seconds, beyond which predicted trends occur earlier than observed in the experiment. The 

low predicted differential pressure between header 8 and header 5 at around 90 seconds likely 

contributes to complete channel voiding occurring earlier in the TRACE results than in the 

experiment. 
23

6.4 Fuel Sheath Temperature

Fuel sheath temperature is a critical parameter in safety analysis for prediction of fuel sheath 

integrity and potential fission product release. 

Figure 16 shows sheath temperature at the location of the void fraction measurement. The 

thermocouple is located in the sheath of one of the upper fuel elements. The experimental data is 

corrected to give sheath surface temperature.

Channel 14 Bundle 10 Surface Temperature

280

290

300

310

320

330

340

60 61 62 63 64 65

Time (s)

Temperature (C)

Experiment

TRACE

Channel 14 Bundle 10 Surface Temperature

200

220

240

260

280

300

320

340

60 70 80 90 100 110 120

Time (s)

Temperature (C)

Experiment

TRACE

Figure 16: TRACE prediction of fuel sheath temperature in top pin of channel 14 bundle 10

TRACE captures the initial peak in temperature at 61.5 seconds but fails to capture the larger peak

between 64 and 70 seconds. This second spike does not appear in measurements of elements at 

the bottom of the fuel channel, therefore it is likely due to stratification in the channel where the top 

pin is surrounded by vapour. As mentioned in section 2.1.1, the TRACE heat transfer model does 

not account for stratification effects. 

Aside from stratification effects, TRACE predicts fuel sheath temperature with reasonable accuracy 

over the first 20 seconds of the transient. The timing of minimum fuel temperature corresponds to 

where TRACE predicts complete channel voiding. This occurs earlier in the TRACE predictions than 

in the experiment. After the minimum, TRACE predicts more rapid heat up of the fuel, indicating 

under prediction of heat transfer in the single phase vapour region. 

25

7. SENSITIVITY STUDIES

Several analyses were performed to assess model sensitivity to nodalization and various input 

parameters. For brevity, studies which did not produce meaningful changes in results are 

summarized at the end of this section.

7.1 Time Step Sensitivity

TRACE internally calculates time step size based on the rate of change of system parameters. The 

user may control time step size by defining a maximum allowable size. To test time step sensitivity, 

the model was run with maximum time step sizes of 1.0, 0.1, 0.01 and 0.001s. By comparison, the 

reference maximum time step size was varied during the run with 0.01s immediately before and after 

break opening (see section 4.2.2) 

Figure 17 compares header 8 pressure and channel 14 void fraction for the reference case and the 

various time step sizes. The results for the reference case and finest time step size are almost

identical, indicating that the reference time step size is sufficiently small. 

Inlet Header 8 Pressure

4000

5000

6000

7000

8000

9000

10000

11000

12000

60 60.5 61 61.5 62

Time (s)

Pressure (kPa)

Reference

dT = 1.0s

dT = 0.1s

dT = 0.001s

Channel 14 Void Fraction

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

60 61 62 63 64 65

Time (s)

Void Fraction

Reference

dT = 1.0s

dt = 0.1s

dT = 0.001s

Figure 17: Comparison of Header 8 pressure and Channel 14 void for timestep size study
26

7.2 Critical Flow Model Sensitivity 

The prediction of critical flow is important for determining the rate of depressurization during a

LOCA. As seen in Section 6.2 system pressure is under predicted after the first 5 seconds of the 

transient using the default critical flow model, indicating an over prediction in break discharge. The 

critical flow model may be adjusted by modifying multiplying coefficients CHM1 and CHM2, which 

are applied to the predicted critical velocity for single phase and two phase flow, respectively. 

Reference results use the default value of 1.0 for both terms.

Reference model results diverge from experimental results after the first 5 seconds of the transient, 

where break discharge is two phase. Therefore the two phase critical flow multiplier CHM2 was 

adjusted to values less than 1.0 to lower critical velocity and reduce the rate of depressurization of 

the system. 

Figure 18 shows header 8 pressures for different values of CHM2. Study results diverge from the 

reference case at about 5 seconds, with CHM2 = 0.7 showing much closer correlation with the 

experimental results. 

Inlet Header 8 Pressure

0

1000

2000

3000

4000

5000

6000

7000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

Reference

CHM2 = 0.5

CHM2 = 0.6

CHM2 = 0.7

Figure 18: Header 8 pressure for CHM2 sensitivity study

Figure 19 shows study results for pressure in header 5. CHM2 = 0.6 provides the most accurate 

header 5 pressure results. The difference between header 5 and header 8 optimal CHM2 values
27

may be impacted by the modeling of flow resistance through the fuel channels, which would appear 

to be too low in the early portion of the transient. This could possibly be addressed using Reynolds 

number dependent friction factors to more accurately predict resistance for the low flow portion of 

the transient, however this was not explored.

Outlet Header 5 Pressure

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

60 70 80 90 100 110 120

Time (s)

Pressure (Pa)

Experiment

Reference

CHM2 = 0.5

CHM2 = 0.6

CHM2 = 0.7

Figure 19: Header 5 pressure for CHM2 sensitivity study

Figure 20 shows that lower values of CHM2 results in lower differential pressure which would 

correspond to lower flow rates. This lower flow causes increased voiding predictions as shown in 

Figure 21. 
28

Header 8 - Header 5 Delta P

-4000

-3000

-2000

-1000

0

1000

2000

60 70 80 90 100 110 120

Time (s)

Pressure Differential (kPa)

Experiment

Reference

CHM2 = 0.5

CHM2 = 0.6

CHM2 = 0.7

Figure 20: Header 8 – header 5 differential pressure for CHM2 sensitivity study

Channel 14 Scatterometer Void Fraction

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

60 65 70 75 80 85 90 95 100

Time (s)

Void Fraction

Experiment

Reference

CHM2 = 0.5

CHM2 = 0.6

CHM2 = 0.7

Figure 21: Void fraction for CHM2 sensitivity study

Selection of the correct critical flow multiplier values is clearly important to the prediction of system 

behavior. To investigate this effect further, a study extension modeling three additional RD-14M 

tests is detailed in Section 8. 
29

7.3 Header Nodalization Sensitivity

There are 3 planes of feeder connections in the RD-14M headers, therefore a minimum of 3 cells is 

required to properly represent them. However, TRACE modeling guidelines recommend a maximum 

ratio of adjacent cell volumes of 10:1, therefore the header for the reference model was made finer 

at the feeder connections, resulting in a 15 cell header. To determine sensitivity to header 

nodalization, the model was run using the 3 cell and 27 cell header nodalizations shown in Figure 

22. 

Figure 22: 15 cell, 3 cell and 27 cell header nodalizations used in study

Header 8 pressure for the 3 nodalizations is shown in Figure 23. The finer 27 cell nodalization 

shows no difference from the reference 15 cell model. 

The 3 cell header shows a much slower rate of depressurization than the reference model. The 

notable difference in this nodalization is that the cell at the break location is also connected to two

feeder outlet side junctions. This results in side junction flow being included in the momentum 

calculation for the break, which seems to have a detrimental impact on the evaluation of critical flow 

velocity. Based on these results, separation of break location from feeder connection cells is an

important aspect of header nodalization in TRACE. 
30

Inlet Header 8 Pressure

0

2000

4000

6000

8000

10000

12000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

3 Cells

15 Cells (Reference)

27 Cells

Figure 23: Header 8 pressure for header nodalization study

7.4 Fuel Channel Nodalization Study

As discussed in section 2.1, the TRACE heat transfer equations do not account for the effects of 

flow stratification in horizontal geometries, which is important to analysis of CANDU fuel channels. 

To try to capture the effects of stratification, a “quasi-2D” representation of fuel channel 14 was 

constructed, as proposed by Hummel [9]. 

The FES can be divided into three horizontal planes of elements, therefore the quasi-2D fuel 

channel model consists of 3 parallel pipes. Channel flow area is divided into 3 segments: one each 

for the flow area around the top 2, bottom 2 and middle 3 FES elements. Separate heat structures 

define the FES surface area in each pipe. The top and bottom pipe elevations are adjusted to 

account for the vertical distance between the fuel pin centerlines. Side junctions connect the pipes 

along their length allowing for fluid transfer between pipes. The quasi-2D representation is shown in 

Figure 24. 
31

Figure 24: Parallel pipe nodalization of channel 14

 

Figure 25 shows the average void fraction across the three pipes compared with the reference case. 

The void fraction for the individual pipes is show in Figure 26. 

Channel 14 Scatterometer Void Fraction

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

60 65 70 75 80 85 90 95 100

Time (s)

Void Fraction

Experiment

Reference

Parallel Pipes Model

Figure 25: Void fraction for parallel pipes model
32

3 Channel Void Distribution

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

60 60.5 61 61.5 62 62.5 63 63.5 64 64.5 65

Time (s)

Voif Fraction

Top

Middle

Bottom

Figure 26: Void fraction in the three parallel pipes 

Contrary to expectations, stratification did not occur between the three pipes. It appears that the 

TRACE momentum equations do not allow the vapour phase to rise to the top pipe through the 

junction connections. Small differences were observed between the conditions in the top/bottom

pipes and the central pipe; these appear to be mostly due to differences in geometry: the 3 fuel 

elements in the central pipe have a smaller associated flow area relative to the reference case and

thus produce a higher void fraction. 

System pressure is unaffected by using the parallel pipes model. Prediction of the fuel sheath 

temperature is slightly less accurate. 

Based on these results a quasi-2D representation of CANDU fuel channels is not able to resolve 

modeling difficulties related to fuel channel heat transfer and flow stratification in TRACE.
33

7.5 Further Sensitivity Studies

Several sensitivity studies were conducted which produced negligible changes in results. For 

completeness they summarized here.

7.5.1 Valve Opening Time

Adjusting the valve opening time between 0.15s and 1.0s showed minor pressure differences over 

the first 0.2s of the transient, after which results were identical to the reference case of 0.25s 

opening time. 

7.5.2 Off-take Model

To test the impact of liquid entrainment and vapour pull-through on break flow the reference 

nodalization was modified so that the break valve was connected to the header as a side junction.

This allowed the use of the off-take model. Model results are mostly unchanged, except for small

oscillations in pressure between 63 and 70 seconds, corresponding to the period of low differential 

pressure between the headers. 

7.5.3 Drag Coefficient Model

The reference model allows TRACE to apply a stratified flow drag coefficient if flow is predicted to 

be stratified. Disabling this option, TRACE only applies a dispersed drag coefficient. Disabling the 

stratified drag coefficient resulted in only minor changes in pressure. This indicates that TRACE 

predicts a very small period of flow stratification during the transient. 

7.5.4 Fuel Element Thermal Diameter

By default, TRACE uses cell hydraulic diameters in wall heat transfer calculations. The user is able 

to define a separate thermal diameter for these calculations. A studied was performed to assess if 

manipulation of this value could improve fuel sheath temperature predictions. It was found that 

unrealistically large thermal diameter values were required to achieve small changes in sheath 

temperature, and overall trends could not be predicted more accurately. 
34

7.5.5 Choking Plane Study

Choking is expected to occur at flow restrictions with large pressure differentials. TRACE requires 

the user to define cell edges were choking is expected to occur. A model was run with the choking 

model enabled for all three cell edges in the break valve. In this test TRACE did not predict the 

correct choking plane, which resulted in over prediction of pressure and voiding in the early 

transient. 

7.5.6 Valve Internal Loss Model

No data was available for the minor loss through the break valve at intermediate stages of opening; 

therefore these losses were ignored in the reference model. The TRACE internal loss model 

automatically calculates valve internal losses; however enabling this option had a minimal impact on 

results.
35

8. EXTENSION STUDY: CRITICAL FLOW MULTIPLIERS

The selection of critical flow multiplier values was shown to have a strong impact on the rate of 

system depressurization. While no specific guidelines for selecting multiplier values were found in 

TRACE documentation, the User’s Manual notes that:

“the choked-flow model allows [the user] to input subcooled and two-phase choked-flow multipliers 

... these multipliers allow [the user] to adjust the predicted liquid, steam/gas-mixture, or both, 

choking velocities to account for break or nozzle geometry effects.” [10] 

To further investigate optimization of the TRACE critical flow model, RD-14M tests B0104, B0105

and B0108 were modeled. These tests simulate different sized inlet header breaks using an 

experimental setup nearly identical to the previously analyzed B0106. 

8.1 Changes to Nodalization

For the B01XX test series different orifice plates were installed between inlet header 8 and the break 

valve to simulate different sized breaks, as shown in Table 4. In the TRACE nodalization, the flow 

area and hydraulic diameter of the junction between header 8 and the break valve were modified 

accordingly.

Table 4: Break Size of B01XX test series

Test Break Orifice Plate Area 

Relative to Reference Case

Surge Tank Isolation

B0105 0.327 Header 7 < 5.5 MPa

B0104 0.462 Header 7 < 5.5 MPa

B0106 (reference case) 1.000 Before transient

B0108 1.173 Header 7 < 5.5 MPa

The surge tank was valved in for the start of these tests and isolated when header 7 pressure 

dropped to 5.5 MPa. Therefore a more detailed nodalization of the surge tank was added to the 

model. 

Surge tank nodalization consists of 2 PIPE components: one representing the surge line and the 

other representing the volume of the surge tank itself. A control block closes the isolation valve 

when header 7 pressure drops to 5.5 MPa.
36

All boundary conditions were adjusted to the values recorded for each experiment, and new steady 

state calculations were performed prior to running transient cases.

8.2 Results

Figure 27 through Figure 29 show pressures for headers 5 and 8 for the various tests with a range 

of values of CHM2. 

Test B0108 had the largest break size in the series and consequently the most rapid 

depressurization. For this test, optimal CHM2 values were similar to the reference test B0106: 

CHM2 = 0.7 gives better predictions for header 8 pressure, while CHM2 = 0.6 gives better 

predictions for header 5 pressure.

B0108 Header 8 Pressure

0

1000

2000

3000

4000

5000

6000

7000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

Reference

CHM2 = 0.8

CHM2 = 0.7

CHM2 = 0.6

B0108 Header 5 Pressure

0

1000

2000

3000

4000

5000

6000

7000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

Reference

CHM2 = 0.8

CHM2 = 0.7

CHM2 = 0.6

Figure 27: Header pressures for B0108 CHM2 study

Test B0104 had a break size with slightly less than half the area of the reference case. For this test 

CHM = 0.8 provides the best pressure results for both header 8 and header 5. 
37

B0104 Header 8 Pressure

2000

2500

3000

3500

4000

4500

5000

5500

6000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

CHM2 = 1.0

CHM2 = 0.9

CHM2 = 0.8

CHM2 = 0.7

B0104 Header 5 Pressure

2000

2500

3000

3500

4000

4500

5000

5500

6000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

CHM2 = 1.0

CHM2 = 0.9

CHM2 = 0.8

CHM2 = 0.7

Figure 28: Header pressures for B0104 CHM2 study

Test B0105 had the smallest break size in the series. The default value of CHM2 = 1.0 provided the 

best results for this test, however in this test the pressures errors were larger in the early portion of 

the transient compared to other tests. The break is small enough that there is a significant period of 

single phase discharge, therefore full critical flow optimization for this test would involve modifying 

the single phase multiplier CHM1 as well. 

B0105 Header 8 Pressure

2000

2500

3000

3500

4000

4500

5000

5500

6000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

CHM2 = 1.0

CHM2 = 0.9

CHM2 = 1.1

CHM2 = 1.2

B0105 Header 5 Pressure

2000

2500

3000

3500

4000

4500

5000

5500

6000

60 70 80 90 100 110 120

Time (s)

Pressure (kPa)

Experiment

CHM2 = 1.0

CHM2 = 0.9

CHM2 = 1.1

CHM2 = 1.2

Figure 29: Header pressures for B0105 CHM2 study

8.3 Critical Flow Model Optimization

The results of this critical flow study show that, even between different breaks of the same shape, 

optimal critical flow multiplier values are geometry specific, and that larger break sizes tend to 

require lower multiplier values.

39

9. CONCLUSIONS 

The TRACE model predicted experimental results with good accuracy. The timing of the onset of 

fuel channel voiding matches experimental observations, while the extent of voiding is generally 

over predicted. System pressure is very accurate over the first 5 seconds of the transient, while later 

in the transient pressure is under predicted. Fuel sheath temperature predictions followed the 

general trend of the experiment but failed to capture temperature peaks resulting from flow 

stratification. 

The difference in results can mostly be attributed to the critical flow model and heat transfer in the

fuel channels. The default critical flow model over predicts flow velocity at the break location for two 

phase break discharge. This can be corrected using the two-phase critical flow multiplier; further 

study modeling additional RD-14M tests indicates that the appropriate value is dependent on break 

size, with larger breaks tending to require a lower multiplier value. Error in heat transfer from fuel to 

coolant arises from modeling horizontal fuel channels using correlations developed for unobstructed 

pipes, particularly the inability to represent fuel pin location within a stratified flow regime.

To improve CANDU modeling capabilities in TRACE, modifications to the code could be made to 

better account for CANDU specific geometries. These could include implementing a special PIPE 

model which uses heat transfer and flow stratification correlations developed specifically for CANDU

channels and a heat structure which accounts for fuel element position within a stratified flow 

regime. Alternatively the VESSEL component could be modified to allow 3D modeling of horizontal 

components. Improvements in header modeling could be made by expanding the off-take model to 

allow a range of connection angles to better account for feeder port location. Very similar 

modifications were previously made to the RELAP5 code by researchers in Korea as detailed in [4] 

and [11] 

Overall, the TRACE model of the RD-14M facility predicts system response with good accuracy. 

TRACE appears to be able to account for most of the phenomena important to LOCA analysis in 

CANDU geometries. 

41

10. REFERENCES

1. Morrison, Robert C., “Performance Improvement through Collaboration”, Workshop on Best 

Practice in Heavy Water Reactor Operation Buenos Aires, Nov 2013. 

<http://www.iaea.org/NuclearPower/Downloadable/Meetings/2013/2013-11-06-11-08-WSNPTD/p1.pdf> 

2. Canadian Nuclear Safety Commission, “Annual Canadian Nuclear Safety Commission Staff 

Report for 2005 on the Safety Performance of the Canadian Nuclear Power Industry”. 

September 2006. <http://publications.gc.ca/collections/collection_2007/cnsc-ccsn/CC171-1-

2005E.pdf> 

3. U.S. Nuclear Regulatory Commission, “TRACE V5.435 Theory Manual, Volume 1”, Division 

of Risk Assessment and Special Projects, Office of Nuclear Regulatory Research, U.S NRC, 

Washington DC, 2007.

4. Chung, B.D., Lee, W.J. and Bang, Y.S. “Improvements of RELAP5/MOD3.2.2 Models for the 

CANDU Plant Analysis” NUREG/IA-0189, October 2000. 

5. Mysen, A., “TRACE Simulation of RD-14M B0104 LOCA Test”, Fall CAMP Meeting,

Rockville, Maryland November 5-6, 2008.

6. Delja, A and Hawley, P., “Computer Codes Validation for Conditions of Core Voiding”, 14th

International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-14),

Toronto, Canada, Sept 25-29 2011, 

7. Swartz, R.S. (ed.), “RD-14M Facility Description and Characterization” COG Report COG-00-

034, January 2001.

8. Buell, J.R., Ingham, P.J. and Swartz, R.S. “RD-14M LOCA Tests with Channel Void 

Measurement: Phase I Tests. B0102-B0110”. COG-01-168, AECL Whiteshell Laboratories. 

March 2002.

9. Hummel, D.W., “Modeling an RD-14M LOCA Experiment with the TRACE Code”, Master of 

Applied Science Thesis, McMaster University, 2010.

10. U.S. Nuclear Regulatory Commission, “TRACE V5.435 User’s Manual, Volume 2: Modeling 

Guidelines”, Division of Safety Analysis, Office of Nuclear Regulatory Research, U.S NRC, 

Washington DC, Jan 20, 2011.

11. Chung, B.D., Lee, W.J., Hwang, M. “RELAP5/MOD3 Horizontal Off-Take Model for 

Application to Reactor Headers of CANDU Type Reactors” NUREG/IA 0238, June 2010.

APPENDIX A

Each header in RD-14M has two pressure measurement devices: a strain gauge transducer and a 

capacitance-type transducer. Differential pressures between headers are measured directly by two 

capacitance-type transducers for each set of measurements.

Examining the data, it is clear that the strain gauge transducers have a much higher response rate 

than the capacitance-type transducers. While pressure values after 1 second are consistent 

between devices, the timing of the initial pressure drop is recorded by the strain gauge transducers 

much closer to valve opening than the other devices. Therefore the strain gauge results are used for 

comparison in this report, with header differential pressure being calculated based on the difference 

between strain gauge measurements.




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NUCLEAR REGULATORY COMMISSION

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NUREG/IA-0446 Assessment of Channel Coolant Voiding in RD-14M Test Facility using TRACE