Paul D. Bayless - RELAP5-3D

Idaho National Engineering and Environmental Laboratory

Prismatic Core VHTR Analysis usingRELAP5-3D/ATHENAPaul D. Bayless

August 28, 2003


2

Outline• Analysis objectives• VHTR description• RELAP5-3D/ATHENA input model description• Benchmarking results• Scoping calculation results• Code implications


3

VHTR Analysis Objectives• Near term

– develop a representative model of the reactor vessel

– perform scoping analyses to establish basic operating parameters

• Longer term– develop an independent analysis capability for

DOE to use during the plant design and licensing phases


4

What is the VHTR?• One of the six selected Generation IV reactor

concepts• Helium cooled, graphite moderated, thermal neutron

spectrum reactor• Passively safe• Reactor vessel coolant outlet temperature of 1000°C• Will be used for generating both electricity and

hydrogen


5

VHTR Plant Schematic


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Reactor Vessel Cutaway

Control Rod Drive Assembly

Refueling Stand Pipe

Control Rod Guide tubes

Cold leg Core Coolant Upper Plenum

Central Reflector Graphite

Annular shaped Active Core

Outer Side Reflector Graphite

Core Exit Hot Gas Plenum

Graphite Core Support Columns

Reactor Vessel

Upper Plenum Shroud

Shutdown Cooling System Module Hot Duct

Insulation Module

Cross Vessel Nipple

Hot Duct Structural Element

Metallic Core Support Structure

Core Inlet Flow

Core Outlet Flow

Insulation Layer for Metallic Core Support Plate

Upper Core Restraint Structure

Control Rods

7m(23 ft)

23.7m(78ft)

2.2m(7ft)

8.2m(27ft) Dia Vessel Flange


7

Reactor Vessel Cross Section


8

Fuel Element Cross Section

Fuel rods

Coolant channels

Block handling hole

Location for burnablepoison rod


9

Model Overview• A simplified RELAP5-3D/ATHENA system model is

being used, in which the balance of plant has been neglected thus far.

• Reactor vessel with helium coolant• Reactor cavity with water coolant and dry

noncondensible air• Reactor cavity cooling system with water coolant and

dry noncondensible air


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Reactor Vessel Model• Coolant active and stagnant volumes• Structures in the core region

– inner and outer reflectors– upper and lower reflectors– core barrel– upper plenum shield– reactor vessel wall and upper head

• Structures below the core are being ignored• Boundary conditions

– coolant inlet temperature– coolant outlet pressure– inlet flow rate adjusted during steady state to provide desired outlet

temperature


11

VHTR Vessel Hydraulic Nodalization

120

130 140

152

110

170100 105

160

154 156


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Ex-vessel Model• Containment air volume• Reactor cavity cooling system (RCCS)

– Inlet plenum/downcomer piping– Lower distribution plenum– Riser/outlet plenum– Riser, downcomer, and outer metal walls

• Containment concrete wall and surrounding soil (behind RCCS downcomer)

• Other structures/walls neglected


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VHTR Reactor Cavity Nodalization

905 975 955

965

900 900

970 960

950980


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Heat Transfer Modeling with Original RCCS Model

Core

Inner reflector

conductionconduction

Outer reflector

Reactor vessel

RCCS inner wall

RCCS interior wall

Containment wall

radiation

radiation

radiation

radiation

convection

convection

convection

convection

He coolant

Axial conduction incore and reflectors

convection


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Heat Transfer Modeling with Revised RCCS Model

Core

Inner reflector

conductionconduction

Outer reflector

Reactor vessel

RCCS riser wall RCCS downcomer wall

Containment wall

radiation

radiation

radiation

convection,radiation

convection

convection

convection

He coolant

Axial conduction incore and reflectors

convection


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Reactor Cavity Radiation ModeledRisers DowncomerReactor Vessel


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GT-MHR Benchmarking• The model is being benchmarked against

calculations performed for the gas turbine-modular helium reactor (GT-MHR) by General Atomics.

• The steady state conditions for the VHTR model are adjusted to match the GT-MHR values (outlet temperature of 850°C, lower inlet temperature, higher flow rate).

• High and low pressure conduction cooldown (loss of forced flow) transients are modeled.


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The 3-structure riser model has improved the RCCS modeling.

Case Concretemax T

(K)

RV outerwall T

(K)

RCCS maxwall T

(K)

RCCSoutlet air T

(K)

RCCS flowrate

(kg/s)

RCCSpower(MW)

Target (GA) 322 719 596 547 14.3 3.30Scopinganalysismodel

347 706 527 450 19.1 2.90

Currentmodel

321 698 621 545 14.2 3.31


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Reduced decay heat has improved the transient benchmark.

Peak Fuel T(°C) Peak Vessel T (°C)CaseSteady HPCC LPCC Steady HPCC LPCC

Target (GA) 1238 1521 480 497 490

Scopinganalysis model

964 1504 1730 458 571 619

Lower decayheat model

971 1285 1514 455 471 520

Lower pressuretransient

1369 490


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Scoping Transient Calculations• Objectives

– Provide feedback to the neutronics development on the effects of different core geometries

– Determine modeling sensitivities• High pressure conduction cooldown

– 60-s flow coastdown– Steady state operating pressure maintained

• Low pressure conduction cooldown– 10-s blowdown to atmospheric pressure– Air ingress precluded


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Core Configuration Calculations

Peak Fuel T (°C) Peak Vessel T (°C)Fuelrings

Coreheight(blocks)

ReactorvesseldP (kPa)

Steady HPCC LPCC Steady HPCC LPCC

6-8 10 71 1119 1596 1807 551 597 643

6-8 11 75 1112 1535 1728 552 583 627

6-8 12 79 1107 1481 1659 552 572 611

5-7 11 92 1064 1707 1937 551 597 644

7-9 10 56 1113 1457 1622 552 595 634

7-9 12 62 1102 1360 1502 553 571 606


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HPCC Peak Fuel Temperatures

0 20 40 60 80 100Time (h)

1000

1200

1400

1600

1800

2000

Tem

pera

ture

(C)

rings 6-8, 10 blocksrings 5-7, 11 blocksrings 6-8, 12 blocksrings 7-9, 10 blocksrings 7-9, 12 blocks


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HPCC Peak Vessel Temperatures

0 20 40 60 80 100Time (h)

450

500

550

600

650

Tem

pera

ture

(C)



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LPCC Peak Fuel Temperatures

0 20 40 60 80 100Time (h)

1000

1200

1400

1600

1800

2000

Tem

pera

ture

(C)



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LPCC Peak Vessel Temperatures

0 20 40 60 80 100Time (h)

450

500

550

600

650

Tem

pera

ture

(C)



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Modeling Sensitivity CalculationsPeak Fuel T (°C) Peak Vessel T (°C)

Case

ReactorvesseldP (kPa) Steady HPCC LPCC Steady HPCC LPCC

Base 71 1119 1596 1807 551 597 643Coolant channeldiameter reduced

79 1125 1628 1805 551 603 643

Flat axial and radialpower profiles

71 1094 1530 1684 551 585 618

Inner reflector heatcapacity increased

71 1119 1522 1694 551 583 622

0.1 mm He gaparound fuel

71 1133 1604 1813 551 598 645

Bypass channel ininner reflector

61 1170 1547 1749 551 587 626

Bypass channel inouter reflector

61 1180 1549 1784 550 582 634

Decay powerreduced 15%

71 1119 1483 1682 551 566 610

New RCCS model 71 1119 1574 1792 538 538 567


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Code Results Summary• The code and model appear to be able to provide

reasonable results for the VHTR loss of flow transients.

• Thermal-hydraulic analyses indicate that changes in the core configuration may be helpful.

• Decay power and power distribution may have large impacts on the calculated transient fuel temperatures.


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Code Improvement Possibilities• Axial conduction capability (outside of reflood) for the

heat structures• Air ingress modeling (molecular diffusion)• Extend/improve the decay heat model to account for

the epithermal neutron spectrum in gas reactors• Extend the material property definition options

available to the user• Allow SCDAP structures to participate in RELAP5

radiation/conduction enclosures

Documents

Paul D. Bayless - RELAP5-3D