MELCOR Modeling of HTGR’s · MELCOR Model of PBMR Modeling of the Gas Coolant • Water is the coolant used by default in MELCOR – Uses a two-phase flow model (pool and atmosphere)

MELCOR Modeling of HTGR’s

Program Review Meeting College Station TX February 25 2009College Station, TX, February 25, 2009

K. VierowDepartment of Nuclear Engineering, Texas A&M UniversityCollege Station, TX USA

Laboratory for Nuclear Heat Transfer Systems

Outline of Presentation

• MELCOR Model of PBMR • MELCOR Model of Prismatic Core Reactor• MELCOR Application in NRC-OSU Project Task 4

Laboratory for Nuclear Heat Transfer Systems2

MELCOR Model of PBMR

• PBMR, Ltd. Design chosen as reference designg g– 268 MWe– Pressure vessel at 7.1 MPa– Fueled and unfueled “pebbles”– Fueled and unfueled pebbles

• 6 cm diameter spheres• Approximately 440,000 total spheres in the core, 330,000 fuel

spheresspheres• Fuel particle coatings serve as the containment

– Helium coolant• Downflow reactor• Downflow reactor• Helium passes high temperature heat for electricity and/or

hydrogen production• T inlet = 500oC T outlet = 900oC


T,inlet 500 C, T,outlet 900 C


Graphite Fuel spheres spheres

From: A.C. Kadak and M.Z. Bazant 2nd InternationalBazant, 2nd International Topical Meeting on High-Temperature Reactor Technology, 2004.

Aerial view of PBMR core



TRISO Fuel Details



• Technical Challenges for MELCOR• Technical Challenges for MELCOR– Fuel geometry differs from that of LWR– Gas coolant replaces light water coolant– Core modeling includes packed bed flow and other

considerations– Severe accident models needed for phenomena peculiar to

high-temperature gas-cooled reactors



Gas Inlet

Gas Upper

PlPlenum

Pebble Bed Core

Gas Gas Outlet

Lower Plenum

} } } } }} } } } }

R1 Unfueled

R2 50% Fueled

R3 Fueled

R4 Fueled

R5 Fueled


• Fuel ModelingA th t f l h b d l d ti l t d b i– Assume that fuel spheres can be modeled as particulate debris

• Particulate Debris model within COR package– After LWR fuel pin collapses, “Particulate Debris” (PD) formste ue p co apses, a t cu ate eb s ( ) o s

• Computationally defined as a packed bed with a user-specified radius and porosity

– Triggers a flow blockage model in the flow path (FL) package gg g p ( ) p gthat calculates pressure drop using the Ergun equation

• Alters cell flow area• Calculates pressure drop using the generalized Ergun equation (for p p g g g q (

packed beds)– Calculates convective heat transfer over pebbles



• Modeling of Fueled and Nonfueled Spheres as g pParticulate Debris– Lumped parameter modeling

• Spheres are isothermalSpheres are isothermal• Assumes fuel is a homogeneous mixture of materials• Properties can be averaged together to approximate a “pebble”

material– Spheres with fuel contain UO2 and are otherwise the same

as unfueled spheres



Approximate Solution for Pebble Surface TemperatureH t ti i h i l di tHeat equation in spherical coordinates:

B d diti I iti l ditiBoundary condition: Initial condition:

First term approximate numerical solution [1]:


[1] Frank P. Incropera and David P.DeWitt,Fundamentals of Heat and Mass Transfer,2006.


Modeling of the Gas Coolantg• Water is the coolant used by default in MELCOR

– Uses a two-phase flow model (pool and atmosphere)• A gaseous coolant may be specified instead of steam/waterA gaseous coolant may be specified instead of steam/water

– Gas properties embedded in code as formulas, particularly, the ideal gas law

• High velocities not a problem for the nodalizations usedHigh velocities not a problem for the nodalizations used– Courant number is not limiting even at high velocities (> 100 m/s)

• Ideal gas behavior under PBMR conditions was confirmed



• Further considerations for core modelingg– Neutronic feedback may be needed– Core porosity is not constant throughout pebble bed– Hot spot development within fuel should be evaluated– Hot spot development within fuel should be evaluated– Hot spot development within coolant should be evaluated– Fuel element relocation occurs in dynamic cores– More accurate pressure drop calculations throughout reactor

pressure vessel depend on unknown design details



MELCOR Nodalization for


MELCOR Nodalization for DLOFC Accident Simulation


Case 1: No decay heat

Case 2: with decay heat


Pressure Vessel Depressurization


Case 2: with decay heat and no oxidation.

Case 1: No decay heat and no oxidation.

Case 3: with decay heat and with oxidation Fuel temperature

comparisoncomparisonfor different cases


MELCOR Model of Prismatic Core Reactor

GT-MHR Reactor System [INL Point Design Report]


Cross sectional view of the GT-MHR and NGNP cores [INL Point Design Report]



Cross sectional view of the Graphite Fuel Block [GA Conceptual Design Report]


Upper plenum volume 280

HS34

Nodalization of the GT-MHR/NGNP Model

Upper reflector CV 166

CV 116

CV

CV 126

CV

CV 136

CV

CV 146

CV

CV 156

CV

HS32272625

24

272625

2423

HS3334001

HS34

34002 34003 34004 34005Core barrel RPV wall

CV 165

CV 164

CV 114

CV 115

CV 124

CV 125

CV 134

CV 135

CV 144

CV 145

CV 154

CV 155

232221

20191817

16

232221

20191817

164CV 163

CV 162

CV 113

CV 112

CV 123

CV 122

CV 133

CV 132

CV 143

CV 142

CV 153

CV 152

16151413241211109

16151413241211109

Ring 1: inner reflector

Ring2, Ring3, Ring4: active core

Ring 5: outer reflector

CV 161

CV 160

Lower reflector

CV 111

CV 110

CV 121

CV 120

CV 131

CV 130

CV 141

CV 140

CV 151

CV 150

876543210

8765

4321

Core exit plenum 054 CV 190

CV 200

Cavity 050

0

Helium riser


GT-MHR RPV Model Results

[INL Point Design Report] MELCOR

Coolant Inlet Temperature (°C) 491 491Average Coolant Outlet Temperature (°C) 850 844.7Bypass Flow Fraction 0.2 0.18Maximum Fuel Temperature (°C) 1218 1514 4Maximum Fuel Temperature ( C) 1218 1514.4Maximum Graphite Temperature (°C) 1142 1334.3Temperature Difference Within the Same Block (°C)

50 – 100(from GA) 269.8

Maximum Coolant Outlet Temperature (°C) 1021 1044.1



ConclusionsConclusionsMELCOR results:MELCOR results:•• Predicting mass flow rate and coolant temperatures Predicting mass flow rate and coolant temperatures

ith hi hith hi hwith high accuracywith high accuracy•• Reasonable bypass fraction tooReasonable bypass fraction too•• Fuel temperatures higher than the references but stillFuel temperatures higher than the references but still•• Fuel temperatures higher than the references but still Fuel temperatures higher than the references but still

within design limits (1600within design limits (1600°°C)C)•• Peak graphite temperature and graphite temperature Peak graphite temperature and graphite temperature

difference within the same block higher than the difference within the same block higher than the referencesreferences



Hypothetical PCC accident:Hypothetical PCC accident:ypypSequence of events:Sequence of events:•• Turbine trip Turbine trip

B th th PCS d th SCS f il t f tiB th th PCS d th SCS f il t f ti•• Both the PCS and the SCS fail to function Both the PCS and the SCS fail to function •• Core residual heat (decay heat and sensible heat stored in Core residual heat (decay heat and sensible heat stored in

graphite) removed by the RCCS from the RPV through thermal graphite) removed by the RCCS from the RPV through thermal di ti t l ti d d tidi ti t l ti d d tiradiation, natural convection and conductionradiation, natural convection and conduction



Assumptions Assumptions pp•• The reactor is assumed to be shutdown within 10 s.The reactor is assumed to be shutdown within 10 s.•• The detection of turbine trip is automatic. The detection of turbine trip is automatic. •• The initiation of SCS is automatic and the detection of its failure toThe initiation of SCS is automatic and the detection of its failure to•• The initiation of SCS is automatic and the detection of its failure to The initiation of SCS is automatic and the detection of its failure to

start is automatic.start is automatic.•• The PCC begins right after the end of normal operation. The PCC begins right after the end of normal operation.

When PCC begins the inlet and outlet boundary have theWhen PCC begins the inlet and outlet boundary have the•• When PCC begins, the inlet and outlet boundary have the When PCC begins, the inlet and outlet boundary have the equilibrium pressure of 5.03 MPa and the helium inlet equilibrium pressure of 5.03 MPa and the helium inlet temperature at normal operating conditions. temperature at normal operating conditions. These two boundaries are linked to the PCSThese two boundaries are linked to the PCS•• These two boundaries are linked to the PCS. These two boundaries are linked to the PCS.

•• The whole PCS has a pressure of 5.03 MPa and the helium inlet The whole PCS has a pressure of 5.03 MPa and the helium inlet temperature at normal operating conditions.temperature at normal operating conditions.


MELCOR Application in NRC-OSU Project Task 4

ConclusionsConclusions•• The inner reflector next to the hottest fuel should have The inner reflector next to the hottest fuel should have

a temperature of 1100a temperature of 1100°°C during steady state C during steady state according to GA. according to GA. gg–– MELCOR predicts a temperature of 812MELCOR predicts a temperature of 812°°C. C.

•• MELCOR underpredicts fuel temperaturesMELCOR underpredicts fuel temperatures–– The MELCOR results show a much greater heat The MELCOR results show a much greater heat

removal from the hottest fuel block by the inner removal from the hottest fuel block by the inner reflector graphite through conduction and radiationreflector graphite through conduction and radiationreflector graphite through conduction and radiation.reflector graphite through conduction and radiation.

–– Since the heat removal is significantly bigger than Since the heat removal is significantly bigger than the generation from decay power, fuel temperature the generation from decay power, fuel temperature


drops.drops.


• Task 4– Development of Instrumentation Plan (OSU,

TAMU, NRC)MELCOR i t ill b d d t l• MELCOR user input will be needed to place instrumentation appropriately for obtaining code validation dataMELCOR ill i t i d t i ti f t• MELCOR users will assist in determination of parameters to measure



• Task 4– Development of Test Plan (OSU, TAMU, NRC)

• MELCOR input models of HTTF facility will be developedP bbl b d d i– Pebble bed design

– Prismatic core design• Pre-test predictions will be performed• Simulations will guide selection of initial and boundary

conditions



• Task 4– Phenomenological Test Data Analysis (OSU,

TAMU)HTTF d t ill b l d• HTTF data will be analyzed

• New models based on the test data may be compared against existing models via MELCOR simulations



• Task 4– Code Validation and Benchmarks (OSU, TAMU)

• MELCOR will be validated against test dataT i t diti– Transient conditions

– Accident conditions• Any identified input modeling deficiencies will be

addressed• Any identified code deficiencies will be documented• Test matrix will be re-analyzed with modified input modelsy p


Documents

MELCOR Modeling of HTGR’s · MELCOR Model of PBMR Modeling of the Gas Coolant • Water is the coolant used by default in MELCOR – Uses a two-phase flow model (pool and atmosphere)