Department of Mechanical and Nuclear EngineeringDepartment of Mechanical and Nuclear EngineeringReactor Dynamics and Fuel Management GroupReactor Dynamics and Fuel Management Group

Comparative Analysis of PWR Core Comparative Analysis of PWR Core Wide Wide

and Hot Channel Calculationsand Hot Channel Calculations

ANS Winter Meeting, Washington DCANS Winter Meeting, Washington DCNovember 20, 2002November 20, 2002

M. AvramovaM. Avramova SS. Balzus . Balzus

K. IvanovK. Ivanov R. MuellerR. Mueller

L. HochreiterL. Hochreiter

The Pennsylvania State University Framatome ANP GmbH, Germany

2ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

OUTLINEOUTLINE

IntroductionIntroduction

COBRA-TF CodeCOBRA-TF Code

PWR Core ModelPWR Core Model

Code-to-Code ComparisonCode-to-Code Comparison

ConclusionsConclusions

3ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

In the framework of joint research program In the framework of joint research program between the Pennsylvania State University (PSU) between the Pennsylvania State University (PSU) and Framatome ANP the COBRA-TF best-estimate and Framatome ANP the COBRA-TF best-estimate thermal-hydraulic code is being validated for LWR thermal-hydraulic code is being validated for LWR core analysiscore analysis

As a part of this program a PWR core wide and As a part of this program a PWR core wide and hot channel analysis problem was modeled using hot channel analysis problem was modeled using COBRA-TF and compared with COBRA 3-CP COBRA-TF and compared with COBRA 3-CP

INTRODUCTIONINTRODUCTION

PSUPSU

COBRA-TF SimulationsCOBRA-TF Simulations

Framatome ANPFramatome ANP

COBRA 3-CP SimulationsCOBRA 3-CP Simulations

4ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

INTRODUCTIONINTRODUCTION

COBRA-TF CodeCOBRA-TF Code - developed to provide - developed to provide best-estimate thermal-hydraulic analysis of best-estimate thermal-hydraulic analysis of LWR vessel for design basis accidents and LWR vessel for design basis accidents and anticipated transientsanticipated transients

COBRA 3-CPCOBRA 3-CP - used at Framatome ANP - used at Framatome ANP as a thermal-hydraulic subchannel analysis as a thermal-hydraulic subchannel analysis and core design codeand core design code

5ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

COBRA-TF Thermal-Hydraulic CodeCOBRA-TF Thermal-Hydraulic Code

COBRA-TF Application AreasCOBRA-TF Application Areas

COBRA-TF Modeling FeaturesCOBRA-TF Modeling Features

Two-FluidsTwo-Fluids Three-DimensionsThree-DimensionsThree-FieldsThree-Fields

Continuous Continuous VaporVapor

Continuous Continuous LiquidLiquid

Entrained Entrained Liquid DropsLiquid Drops

PWR Primary System PWR Primary System LOCA AnalysisLOCA Analysis

LWR Rod Bundle LWR Rod Bundle Accident AnalysisAccident Analysis

6ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

COBRA-TF Thermal-Hydraulic CodeCOBRA-TF Thermal-Hydraulic Code

COBRA-TF Regimes MapsCOBRA-TF Regimes Maps

COBRA-TF VESSEL Structures ModelsCOBRA-TF VESSEL Structures Models

Normal Flow RegimeNormal Flow Regime Hot Wall RegimeHot Wall Regime

Heat-Generating StructuresHeat-Generating Structures Unheated StructuresUnheated Structures

Nuclear Nuclear Fuel RodsFuel Rods

Heated TubesHeated Tubes

Heated Heated Flat PlatesFlat Plates

Hollow Hollow TubesTubes

Solid CylindersSolid Cylinders

Flat PlatesFlat Plates

7ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

COBRA-TF PWR Core Modeling – BackgroundCOBRA-TF PWR Core Modeling – Background

COBRA-TF PWR Core Modeling – Stand Alone and CoupledCOBRA-TF PWR Core Modeling – Stand Alone and Coupled

Core Wide AnalysisCore Wide Analysis Steady StateSteady State

Anticipated Transients Anticipated Transients - Flow Reduction- Flow Reduction

- Power Rise- Power Rise

- Pressure Reduction- Pressure ReductionHot Channel AnalysisHot Channel Analysis

TRAC-PF1/NEM/COBRA-TFTRAC-PF1/NEM/COBRA-TFRod Ejection Accident (REA)

TMI-1 Rod Ejection

Main-Steam-Line-Break (MSLB)

TMI-1 MSLB (Exercise 2)

8ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

PWR Core ModelPWR Core Model

The Simulated PWR Core Contains 121 14x14 FAThe Simulated PWR Core Contains 121 14x14 FA

The hot assembly is located at the center of the core The hot assembly is located at the center of the core

A quarter core model was chosen for the COBRA-TF model A quarter core model was chosen for the COBRA-TF model similar to the COBRA 3-CP modelsimilar to the COBRA 3-CP model

Parameter

Fuel rod outside diameter (in) 0.424Guide tube outside diameter (in) 0.539Instrumentation tube outside diameter (in) 0.424Fuel rod pitch (in) 0.556Fuel assembly pitch (in) 7.803Fuel assembly dimensions 14x14Gap between fuel assemblies (in) 0.020Number of fuel rods 179Number of guide tubes 16Number of instrumentation rods 1Fuel active length (in) 95.00

The sub-channels The sub-channels surrounding the surrounding the limiting rod were limiting rod were represented on a sub-represented on a sub-channel basischannel basis

The remaining part of The remaining part of the quarter-core was the quarter-core was modeled as lumped modeled as lumped channelschannels

9ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

PWR Core ModelPWR Core Model

1 2

4 3

5

6

3

4

6

7

9

10

5

11

12

2

8

1

13

14

15

16

17

7

Guide Tube

Subchannel layout of the macro-cell

The macro-cell is comprised of The macro-cell is comprised of subchannels 1 through 7subchannels 1 through 7

The subchannels surrounding The subchannels surrounding the limiting rod have been the limiting rod have been modeled exactly as subchannels modeled exactly as subchannels 1 through 4 1 through 4

Surrounding this area are lumped Surrounding this area are lumped in channels 5, 6, and 7in channels 5, 6, and 7

10 ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

PWR Core ModelPWR Core Model

-

Macro-cell

(Subchannels 1-7)

Subchannel 8 Instrumentation Tubes

Subchannel 9

Layout of the ¼ core model The remaining parts of the four The remaining parts of the four fuel assemblies are modeled as fuel assemblies are modeled as channel 8 channel 8

The rest of the quarter core is The rest of the quarter core is modeled as channel 9modeled as channel 9

5 Spacer Grids (5 Spacer Grids (4 mixing spacers 4 mixing spacers and 1 structural spacerand 1 structural spacer ) )

Chopped cosine with a peak value Chopped cosine with a peak value of of 1.551.55 Axial Power ProfileAxial Power Profile

Non-uniform Radial Power ProfileNon-uniform Radial Power Profile

Inlet BC - Inlet Flow Rate andInlet BC - Inlet Flow Rate and

Inlet EnthalpyInlet Enthalpy

Outlet BC - Outlet PressureOutlet BC - Outlet Pressure

11 ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

COBRA-TF ModificationsCOBRA-TF Modifications

In order to define an identical basis for the In order to define an identical basis for the comparative analysis two modifications were made comparative analysis two modifications were made to COBRA-TF as code features:to COBRA-TF as code features:

1.1. The same correlation for the rod friction factor The same correlation for the rod friction factor used in the COBRA 3-CP code was introduced in used in the COBRA 3-CP code was introduced in COBRA-TFCOBRA-TF

2.2. The W3 Critical Heat Flux correlation was also The W3 Critical Heat Flux correlation was also added to the codeadded to the code

12 ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

Code-to-Code ComparisonsCode-to-Code Comparisons

STEADY STATESTEADY STATE

The codes demonstrate steady-state results with The codes demonstrate steady-state results with excellent agreementexcellent agreement

The axial distributions of the mass flow rate, calculated The axial distributions of the mass flow rate, calculated by the two codes differ by only about 1% (on average)by the two codes differ by only about 1% (on average)

Liquid EnthalpySteady-State

520

540

560

580

600

620

640

660

0 20 40 60 80 100

Axial Location (in)

En

thal

py

(Btu

/lbm

)

COBRA 3-CP

COBRA-TF

Channel # 3

Liquid Mass FlowrateSteady-State

0.620

0.630

0.640

0.650

0.660

0.670

0 20 40 60 80 100

Axial Location (in)

Flo

wra

te (

lbm

/s)

COBRA 3-CP

COBRA-TF

Channel # 3

13 ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

Code-to-Code ComparisonsCode-to-Code Comparisons

STEADY STATE STEADY STATE

The codes predict a similar DNBRThe codes predict a similar DNBR

COBRA 3-CP tends to predict a MDNBR at higher COBRA 3-CP tends to predict a MDNBR at higher elevationelevation

COBRA-TF - constant “F” COBRA-TF - constant “F” factorfactor

COBRA 3-CP - COBRA 3-CP - dynamically computeddynamically computed “F” “F” factor factor

DNBR Steady State

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100Axial Location (in)

DN

BR

COBRA 3-CP

COBRA-TF

Channel # 3

14 ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

Transient ModelsTransient Models

Main differencesMain differences

COBRA 3-CPCOBRA 3-CP - the wall heat flux time history is specified - the wall heat flux time history is specified as a boundary condition as a boundary condition

COBRA-TFCOBRA-TF - the wall heat flux was calculated from the - the wall heat flux was calculated from the rod heat conduction solution in the rod heat conduction solution in the

codecode

Therefore in COBRA-TF the rod power was specified and Therefore in COBRA-TF the rod power was specified and during a transient the heat flux took into account the stored during a transient the heat flux took into account the stored heat releaseheat release

15 ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

Transient ModelsTransient Models

SolutionSolution

These differences between the two transient models for the These differences between the two transient models for the wall heat flux are eliminated in the following way:wall heat flux are eliminated in the following way:

In the COBRA-TF input deck the fuel rods are In the COBRA-TF input deck the fuel rods are modeled as tubes with very small thickness of the modeled as tubes with very small thickness of the wallwall

In this case the generated heat in the fuel rods is In this case the generated heat in the fuel rods is neglectedneglected

Wall heat flux time history is specified as a Wall heat flux time history is specified as a boundary condition (in a similar way as in the boundary condition (in a similar way as in the COBRA 3-CP code)COBRA 3-CP code)

16 ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

Code-to-Code ComparisonsCode-to-Code Comparisons

50% Loss of Flow Transient 50% Loss of Flow Transient

The maximum heat flux to flow ratio is predicted at two The maximum heat flux to flow ratio is predicted at two seconds into the transient by both codes and as a result the seconds into the transient by both codes and as a result the minimum DNBR is reached at about two seconds into the minimum DNBR is reached at about two seconds into the transient for both code simulationstransient for both code simulations

Minimum DNBR

2

4

6

8

10

0 2 4 6 8 10Time (seconds)

MD

NB

R

COBRA 3-CP

COBRA-TF

Channel # 3

17 ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

CONCLUSIONSCONCLUSIONS

The PWR core-wide and hot channel analysis problem The PWR core-wide and hot channel analysis problem was modeled with both COBRA 3-CP and COBRA-TF was modeled with both COBRA 3-CP and COBRA-TF computer codescomputer codes

Identical modeling basis for rod friction has been defined Identical modeling basis for rod friction has been defined and the COBRA 3-CP correlation has been implemented and the COBRA 3-CP correlation has been implemented into the COBRA-TF sourceinto the COBRA-TF source

In COBRA 3-CP the Critical Heat Flux is calculated using In COBRA 3-CP the Critical Heat Flux is calculated using the W3 correlation and this correlation was added to the the W3 correlation and this correlation was added to the current version of COBRA-TFcurrent version of COBRA-TF

Consistent transient surface heat flux boundary Consistent transient surface heat flux boundary conditions were used such that more exact comparisons conditions were used such that more exact comparisons can be made between the two different code calculationscan be made between the two different code calculations

18 ANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel CalculationsANS Winter Meeting, Washington DC, November 20, 2002, Comparative Analysis of PWR Core Wide and Hot Channel Calculations

CONCLUSIONS – cont.CONCLUSIONS – cont.

Results from the codes show a very good agreement for Results from the codes show a very good agreement for the initial steady-state conditions as well as for the the initial steady-state conditions as well as for the simulated loss of flow transient simulated loss of flow transient

The only difference in the two calculations is the location The only difference in the two calculations is the location of the minimum DNBR of the minimum DNBR

This is explained by the fact that in COBRA-TF a This is explained by the fact that in COBRA-TF a constant Tong “F” factor (which accounts for a non-constant Tong “F” factor (which accounts for a non-uniform axial power shape) is used while in COBRA 3-CP uniform axial power shape) is used while in COBRA 3-CP this “F” factor is dynamically computedthis “F” factor is dynamically computed