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 protection Reactor building Grade level 35 M

GT-MHR MODLE MELTDOWN-PROOF ADVANCED REACTOR & HIGH EFFICENCY GAS TURBINE POWER CONVERSION SYSTEM POWER LEVEL 600 MWt

TEST GT-MHR / LWR COMPARISON ItemGT-MHRLWR ModeratorGraphiteWater CoolantHeliumWater Avg core coolant exit temperature 850° °C 310°C Structural materialGraphiteSteel Fuel cladGraphite & siliconZircaloy FuelUCO or PuCOUO 2 Fuel damage temperature>2000°C1260°C Power density, w/cc Linear heat rate, kW/ft1.619 Avg thermal neutron energy, eV Migration length, cms576

GT-MHR EMPLOYS DIRECT BRAYTON CYCLE FOR ELECTRICITY GENERATION

Reactor Power (MWt)600 Inlet Pressure to turbine (Mpa)7 Inlet temperature to turbine ( C )850 RPM4400 He Flow (kg/s)320 TC mass (T)33 Gen mass35 Max Load on TC Radial EMB (kN)28 Max Load on gen Radial EMB34 Max Load on TS axial EMB326 Max Load on gen axial EMB350 Turbine Stages9 HP Compressor Stages13 LP Compressor Stages10 PCU PARAMETERS

NORMAL OPERATION PARAMETERS DISTRIBUTION THROUGH PCS

Reactor System Design REACTOR SYSTEM

GT-MHR CORE LAYOUT REPLACEABLE CENTRAL & SIDE REFLECTORS CORE BARREL ACTIVE CORE 102 COLUMNS 10 BLOCKS HIGH PERMANENT SIDE REFLECTOR 36 X OPERATING CONTROL RODS BORATED PINS (TYP) REFUELING PENETRATIONS 12 X START-UP CONTROL RODS 18 X RESERVE SHUTDOWN CHANNELS

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

RequirementLimitBasis Fuel1250°C (steady state)Fuel Integrity 1600°C (accident) Control rods>2000°C Stress (structural integrity) Graphite blocksLimit  T/  X, temp,Stress (structural integrity) fluence Core arrayLimit  P (~70 kPa)Flow-induced Vibrations Hot duct900°C -1000°C Stress (structural integrity) CORE T/H REQUIREMENTS

Maximize flow in coolant channels (limit T fuel ) –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  T cool is small but  T fuel is large

COOLANT TEMPERATURE RISE IS IMPORTANT IN HTGR CORES

FUEL COLUMN SCHEMATIC CORE CROSSFLOW

T-H ACCIDENT CONDITIONS

MHTGR SAFETY RELIES ON THREE BASIC FUNCTIONS Retain Radionuclides in Coated Particles Remove Core Heat Remove Core Heat Control Heat Generation Control Heat Generation Control Chemical Attack Control Chemical Attack

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... DEFENSE-IN-DEPTH BUTTRESSED BY INHERENT CHARACTERISTICS... DEFENSE-IN-DEPTH BUTTRESSED BY INHERENT CHARACTERISTICS A)Active Shutdown Cooling System B)Passive Reactor Cavity Cooling System C)Passive Radiation and Conduction of Afterheat to Silo Containment (Beyond Design Basis Event) Air Blast Heat Exchanger Relief Valve Reactor Cavity Cooling System Panels Natural Draft, Air Cooled Passive System Surge Tank Shutdown Cooling System Heat Exchanger and 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

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

CONCLUSIONS 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 flow ACCIDENT 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