DESIGN SUMMARY AND T-H R&D NEEDS OF THE GT-MHR Workshop on R&D in the Areas of Thermal Fluids and Reactor Safety C. B. Baxi General Atomics, San Diego,

DESIGN SUMMARY AND T-H R&D NEEDS OF THE GT-MHR

Workshop on

R&D in the Areas of Thermal Fluids and Reactor Safety

C. B. Baxi

General Atomics, San Diego, CA

Module Below Grade Provides Security and Sabotage Protection

• Electrical output 286 MW(e) per module

• Each module includes Reactor System and Power Conversion System

• Reactor System 600 MW(t), 102 column, annular core, hexagonal prismatic blocks, very similar to successful FSV tests

• Power Conversion System includes generator, turbine, compressors on single shaft, surrounded by recuperator, pre-cooler and inter-cooler

• Natural sabotage protectionReactor building

Gradelevel

35 M

GT-MHR MODLE

MELTDOWN-PROOF ADVANCED REACTOR

&HIGH EFFICENCY

GAS TURBINEPOWER CONVERSION

SYSTEM

POWER LEVEL600 MWt

MELTDOWN-PROOF ADVANCED REACTOR

&HIGH EFFICENCY

GAS TURBINEPOWER CONVERSION

SYSTEM

POWER LEVEL600 MWt

TESTTEST

GT-MHR / LWR COMPARISON

Item GT-MHR LWR

Moderator Graphite Water

Coolant Helium Water

Avg core coolant

exit temperature 850° - 1000 °C 310°C

Structural material Graphite Steel

Fuel clad Graphite & silicon Zircaloy

Fuel UCO or PuCO UO2

Fuel damage temperature >2000°C 1260°C

Power density, w/cc 6.5 58 - 105

Linear heat rate, kW/ft 1.6 19

Avg thermal neutron energy, eV0.22 0.17

Migration length, cms 57 6

GT-MHR EMPLOYS DIRECT BRAYTON CYCLE FOR ELECTRICITY GENERATION

   Reactor Power (MWt) 600Inlet Pressure to turbine (Mpa) 7Inlet temperature to turbine ( C ) 850 RPM 4400He Flow (kg/s) 320TC mass (T) 33Gen mass 35Max Load on TC Radial EMB (kN) 28Max Load on gen Radial EMB 34Max Load on TS axial EMB 326Max Load on gen axial EMB 350Turbine Stages 9HP Compressor Stages 13LP Compressor Stages 10

PCU PARAMETERS

NORMAL OPERATION

PARAMETERS DISTRIBUTION THROUGH PCS

Reactor System Design

REACTOR SYSTEM

ControlRodDriveAssemblies

ReactorMetallic Internals

ReplaceableReflector

Core

Reactor Vessel

Shutdown Cooling System

Hot GasDuct

ControlRodDriveAssemblies

ReactorMetallic Internals

ReplaceableReflector

Core

Reactor Vessel

Shutdown Cooling System

Hot GasDuct

GT-MHR CORE LAYOUT

REPLACEABLE CENTRAL& SIDE REFLECTORS

CORE BARREL

ACTIVE CORE102 COLUMNS10 BLOCKS HIGH

PERMANENTSIDEREFLECTOR

36 X OPERATINGCONTROL RODS

BORATED PINS (TYP)

REFUELINGPENETRATIONS

12 X START-UPCONTROL RODS

18 X RESERVE SHUTDOWNCHANNELS

FUEL ASSEMBLY IS BASIC STRUCTURAL UNIT OF CORE

• Fuel Particle SiC and PyC coatings retain fission products

• Fuel compact contains particles

• Graphite block supports fuel compacts in arrangement compatible with nuclear reaction and heat transfer to helium

• Dowels align coolant holes between blocks

0.8 m x 0.36 m

Requirement Limit Basis

Fuel 1250°C (steady state) Fuel Integrity1600°C (accident)

Control rods >2000°C Stress (structural integrity)

Graphite blocks Limit T/X, temp, Stress (structural integrity) fluence

Core array Limit P (~70 kPa) Flow-inducedVibrations

Hot duct 900°C -1000°C Stress (structural integrity)

CORE T/H REQUIREMENTS

• Maximize flow in coolant channels (limit Tfuel)– Adequate control rod flow– Minimize gap flows (1 mm gap needed for

refueling)

• Uniform coolant channel flows (limit T/X)– Minimize crossflows between coolant

channels and gaps– minimize crossflow between control rod

channels and gaps

CORE FLOW DISTRIBUTION

STEADY STATE CONSIDERATIONS

CORE T/H CHARACTERISTICS

• Core coolant temperature rise is large

• Temperature rise from coolant to fuel is small

Control of the coolant temperature rise is very important to reactor core performance

• This is opposite from LWR cores, where Tcool is small but Tfuel is large

COOLANT TEMPERATURE RISE IS IMPORTANT IN HTGR CORES

FUEL COLUMN SCHEMATIC

CORE CROSSFLOW

T-H ACCIDENT CONDITIONS

MHTGR SAFETY RELIES ON THREEBASIC FUNCTIONS

MHTGR SAFETY RELIES ON THREEBASIC FUNCTIONS

RetainRadionuclides inCoated Particles

RetainRadionuclides inCoated Particles

RemoveCore HeatRemove

Core HeatControl

Heat GenerationControl

Heat GenerationControl

Chemical AttackControl

Chemical Attack

APPROACH:PASSIVE SAFETY BY DESIGN

APPROACH:PASSIVE SAFETY BY DESIGN

• Fission Products Retained in Coated Particles – High temperature stability materials

• Refractory coated fuel• Graphite moderator

– Worst case fuel temperature limited by design features• Low power density• Low thermal rating per module• Annular core• Passive heat removal

 ... CORE CAN’T MELT• Core Shuts Down Without Rod Motion

– Large negative temperature coefficient• Coolant Not a Safety Problem

– Neutronically and chemically inert: no energy reactions– Single phase– Low stored energy

• Operator Not in the Safety Equation– Insensitive to operator error (commission or omission)– Long response times for recovery

DECAY HEAT REMOVAL PATHS WHEN NORMAL POWER CONVERSION SYSTEM IS UNAVAILABLE DECAY HEAT REMOVAL PATHS WHEN NORMAL POWER CONVERSION SYSTEM IS UNAVAILABLE

. . . DEFENSE-IN-DEPTH BUTTRESSED BYINHERENT CHARACTERISTICS

. . . DEFENSE-IN-DEPTH BUTTRESSED BYINHERENT CHARACTERISTICS

A) Active ShutdownCooling System

B) Passive Reactor CavityCooling System

C) Passive Radiationand Conduction of Afterheat to Silo Containment(Beyond DesignBasis Event)

Air BlastHeat Exchanger

ReliefValve

ReactorCavityCoolingSystemPanels

Natural Draft,Air CooledPassive System

SurgeTank

ShutdownCooling SystemHeat Exchangerand Circulator

HEAT REMOVAL BY PASSIVE MEANS DURING PRESSURIZED CONDUCTION COOLDOWN

Heat removed by:

• Core Convection

• Core Conduction

• Core Internal Radiation

• Vessel Radiation

• RCCS Convection

HEAT REMOVAL BY PASSIVE MEANS DURING DEPRESSURIZED CONDUCTION COOLDOWN

Heat removed by:

• Core Conduction

• Core Internal Radiation

• Vessel Radiation

• RCCS Convection

GT-MHR FUEL TEMPERATURES REMAIN BELOW DESIGN LIMITS DURING CONDUCTION COOLDOWN EVENTS

. . . passive design features ensure fuel remains below 1600°C. . . passive design features ensure fuel remains below 1600°C

TOTAL CORE FLOW RATE DURING CONDUCTION COOLDOWN AT VARIOUS HELIUM INVENTORIES

NEW TEST T-H TEST RESULTS REQUIRED

STEADTY STATE

• A NUMBER OF TESTS HAVE BEEN PERFORMED

ACCIDENT CONDITIONS

• DATA REQUIRED ON CONDUCTION COOLING

CONCLUSIONSCONCLUSIONS

• Coupling Modular Helium Reactor with Gas Turbine (turbomachine, magnetic bearings, recuperator) results in unique passively safe reactor

• GT-MHR has safety characteristics similar to MHTGR– Similar conduction cooldown transient results– Similar reactivity event transient results– Reduced frequency of water ingress events

• GT-MHR maintains high level of safety eliminating core melt without operator action

SUMMARY OF R&D REQUIRED

• Validate engineering assumptions

• Assess mixing and flow distribution

• Assess gap and cross flows

• Assess natural circulation

GT-MHR T-H R&D TOPICS

STEADY STATE• Lower plenum mixing during normal operation• Turbine outlet mixing during loss of load or rapid load

change• Flow distribution from cold duct to upper plenum• Core gap flow and cross flowACCIDENT• Natural circulation in reactor cavity• Natural circulation in RCCS• Natural circulation within reactor vessel• SCS startup and transition from natural circulation to

forced convection cooling• Air ingress