Safety Issues for High Temperature Gas Reactors Andrew C. Kadak Professor of the Practice
Major Questions That Need Good Technical Answers Fuel Performance Normal operational performance Transient performance Ejected Rod (maximum energy insertion capability) Reactivity insertions (seismic, water) Accident Performance Weak fuel issues Mechanistic source term for high burn-up fuel Fuel fabrication quality assurance
Risk Dominant Accident Sequences Establish risk informed design to identify risk dominant accident sequences to be analyzed. Use either IAEA 1 or NRC 2 risk informed approach to establish safety requirements of plant. Use of safety goal as a design guide Application of risk informed Defense in Depth Scope of risk analysis may be easier due to inherent robustness of basic design. 1. Development of Technology Neutral Safety Requirements for Innovative Reactors, IAEA TECDOC Draft Dec. 2004 2. Regulatory Structure for New Plant Licensing, Part 1: Technology Neutral Framework, Dec. 2004, Draft, US NRC.
F Initiating Event/Y Challenges Risk Informed Safety Profile Failure of Level 1 Initiating Event Lev. 1 Lev. 2 Lev. 3 10-2 AOO AC Lev. 4 10-6 10-7 SPC Lev. 5 DESIGN BASIS Consequences Plant conditions Consequences Normal Operations (NO) Anticipated Operational Occurrences (AOO) Accident conditions (AC) Severe plant conditions* (SSPC) Doses to Operators 50 msv/a ALARA (5 msv/a target) 50 msv/a 20 msv/a (average 5 y) (5 msv/a target) 50mSv/a (Could be exceeded for rear recovery events) 500 msv (limit) (This value derived from Finnish regulation) NOTE 1: Doses for NO, AOO, AC are derived from IAEA-SS No 115 Doses to the public 1 msv/a (10 µ Sv/a target) 5 msv/a 1 msv/a (average 5 y) 5 msv/a (For 1 year period following the accident) 5 msv/a (For 1 year period following the accident) NOTE 2: Doses are derived from IAEA-SS No 115 Off-site Actions No off-site actions beyond defined distance from the plant Minimal emergency actions beyond defined distance from the plant * Severe challenge to the Fission Products Confinement Function
LEVELS OF DEFENCE IN DEPTH (From INSAG-10) Levels of defence Objective Essential means Acceptable failures of the Level of Defence Level 1 Prevention of deviations from normal operation and failures Conservative design and high quality in construction and operation (events/year) Level 2 Control of deviations from normal operation and detection of failures Control, limiting and protection systems and other surveillance features < 10-2 Level 3 Control of accident conditions within the design basis Engineered safety features and accident procedures 10-2 - 10-6 Level 4 Control of severe plant conditions Complementary measures and accident management 10-6 - 10-7 Level 5 Mitigation of radiological consequences of significant releases of radioactive materials Off-site emergency response < 10-7
Expected Significant Accident Sequences Air Ingress Water Ingress (reactivity insertion) Seismic Events (reactivity insertion) Loss of Load Rod Ejection (more significant in block reactors) Failure of reactor cavity cooling system Recuperator By-pass events (overcooling) Graphite dust, plate-out, lift off Impact of Terrorism Identification of cliff edge effects
Knowledge Required Improved understanding of core behavior Improved understanding of heat transfer in core and vessel - pebble and block - bypass flows Materials behavior at high temperature in helium (plus contaminants) including radiation effects and chemical attack on graphite Blow down loads and timing of accident event sequences. Behavior of fuel, fission product release behavior in reactor building and structures under accident conditions. Development and validation of computer codes used in the analysis Validation of passive performance of safety systems - natural circulation - heat conduction and convection.
Issues Fuel Temperature limits (1600 C?) Regulatory Credit for Basic Design Strengths Need new risk informed licensing process to allow credit for innovative systems.
Containment Based on design and accident analysis of source term and sequences - a containment of radioactive materials strategy is developed to assure that safety goals are met. Full pressure containment Confinement - low pressure - not pressure tight Dynamic containment/confinement (time dependent) Performance is quite different than water reactors.
Classification of Safety Systems Ideally safety system classification should be done on importance to safety function in a risk informed manner. Some systems are not components but parameters in analysis for passive performance (ex. emissivity of reactor vessel).
Expectations Water Ingress - generally understood and can be limited by amount of water ingress - some German experience at AVR Seismic - reactivity simulations can assess reactivity impact. Rod ejection - more significant for block reactors but fuel energy limits like for LWRs can be established for rod worths. Testing on heat transfer and flow can be verified by South African tests and Chinese pebble bed reactor including reactor cavity cooling systems. Fuel behavior data to be provided by past German and focused South African and US testing programs
Challenges Verification of high temperature material behavior (fuel, graphite, metals, carbides) Validation of analysis tools Air ingress Most visible concern among the public Most significant in terms of potential offsite consequences Can not be eliminated by design
Air Ingress Status Most eliminate connecting vessel failure as too low a probability event (10-8 ). Break sizes limited to largest connecting pipe. Two breaks (top and bottom) considered unlikely but are analyzed (chimney effect) Graphite corrosion behavior not well modeled in existing codes. CFD analysis and confirmatory experiments needed.
Air Ingress Tests Japanese series on prismatic configuration Diffusion Natural Circulation Corrosion (multi-component) German NACOK tests - pebble bed Natural circulation Corrosion MIT CFD (Fluent Methodology Development)
Experimental Apparatus - Japanese C4 H4 C3 H3 C2 H2 C1 Helium H1 Valves 2 7 0 Nitrogen Figure 16: Apparatus for Isothermal and Non-Isothermal experiments Figure 17: Structured mesh
Isothermal Experiment 0.80 0.60 Mole fraction 0.40 0.20 0.00 H-1 & C-1(Calculation) H-2 & C2 (Calculation) H-3 & C3 (Calculation) H-4 & C4 (Calculation) H-1 & C-1(Experiment) H-2 & C2 (Experiment) H-3 & C3 (Experiment) H-4 & C4 (Experiment) 0 50 100 150 200 250 300 Time (min) Figure 18: Mole fraction of N 2 for the isothermal experiment
Thermal Experiment Pure Helium in top pipe, pure Nitrogen in the bottom tank N 2 Mole fractions are monitored in 8 points Hot leg heated Diffusion Coefficients as a function of temperature Figure 19: The contour of the temperature bound4ary condition
Thermal Experiment 1 0.8 H-1(FLUENT) C-1(FLUENT) H-1(Experiment) C-1(Experiment) 1 0.8 H2(Experiment) C2(Experiment) H-2(FLUENT) C-2(FLUENT) Mole fraction of N2 0.6 0.4 Mole Fraction 0.6 0.4 0.2 0.2 0 0 50 100 150 200 0 0 50 100 150 200 Time (min) Time(min) Figure 20: Comparison of mole fraction of N 2 at Positions H-1 and C-1 Figure 21: Comparison of mole fraction of N 2 at Positions H-2 and C-2
Thermal Experiment (Cont.) 1 H4(Exp) 0.25 Mole Fraction of N2 0.8 0.6 0.4 0.2 C4(Exp) H-4(Calc) C-4(Calc) Velocity (m/second) 0.20 0.15 0.10 0.05 0.00-0.05 0 2 4 6 0 0 50 100 150 200 250 Time(min) -0.10-0.15 Time (Second) Figure 22: Comparison of mole fraction of N 2 at Positions H-1 and C-1 Figure 23: The vibration after the opening of the valves.
Multi-Component Experiment Graphite Inserted Multiple gases: O 2, CO, CO 2, N 2, He, H 2 O Mole fraction at 3 points are measured Much higher calculation requirements Diffusion Coefficients 4 3 Heated Graphite 2 1 Helium Air Figure 34: Apparatus for multi- Component experiment of JAERI
Multi-Component Experiment(Cont.) Mole Fraction 0.21 0.18 0.15 0.12 0.09 0.06 O2(Experiment) O2(Calculation) CO(Experiment) CO(Calculation) CO2(Experiment) CO2(Calculation) 0.03 0.00 0 20 40 60 80 100 120 140 Time(min) Figure 36: Mole Fraction at Point-1 (80% Diffusion Coff.)
Multi-Component Experiment(Cont.) 0.24 Mole Fraction 0.20 0.16 0.12 0.08 O2(Experiment) O2(Calculation) CO(Experiment) CO(Calculation) CO2(Experiment) CO2(Calculation) 0.04 0.00 0 20 40 60 80 100 120 140 Time(min) Figure 37: Mole Fraction at Point-3
Mole Fraction 0.25 0.20 0.15 0.10 Multi-Component Experiment(Cont.) O2(Experiment) O2(Calculation) CO(Experiment) CO(Calculation) CO2(Experiment) CO2(Calculation) 0.05 0.00 0 20 40 60 80 100 120 140 Time (min) Figure 38: Mole Fraction at Point-4
NACOK Natural Convection Experiments Figure 39: NACOK Experiment
Boundary Conditions Figure 41: Temperature Profile for one experiment
The Mass Flow Rates 5.0E-03 4.0E-03 Mass Flow Rate (kg/s ) 3.0E-03 2.0E-03 1.0E-03 0.0E+00 T_R=200 DC(Exp.) T_R=400 DC(Exp.) T_R=600 DC(Exp.) T_R=800 DC(Exp.) T_R=200 DC(FLUENT) T_R=400 DC(FLUENT) T_R=600 DC(FLUENT) T_R=800 DC(FLUENT) 100 300 500 700 900 1100 Temperature of the Pebble Bed (C) Figure 42: Mass Flow Rates for the NACOK Experiment
Future NACOK Tests Blind Benchmark using MIT methodology to reproduce recent tests. Update models Expectation to have a validated model to be used with system codes such as RELAP and INL Melcor.
Air Ingress Mitigation Air ingress mitigation strategies need to be developed Realistic understanding of failures and repairs Must be integrated with containment strategy to limit air ingress Short and long term solution needed
Overall Safety Performance Demonstration and Validation China s HTR-10 provides an excellent test bed for validation of fundamentals of reactor performance and safety. Japan s HTTR provides a similar platform for block reactors. Germany s NACOK facility vital for understanding of air ingress events for both types. PBMR s Helium Test Facility, Heat Transfer Test Facility, Fuel Irradiation Tests, PCU Test Model. Needed - open sharing of important technical details to allow for validation and common understanding.
Chinese HTR-10 Safety Loss of flow test Shut off circulator Demonstration Restrict Control Rods from Shutting down reactor Isolate Steam Generator - no direct core heat removal only but vessel conduction to reactor cavity
Video of Similar Test
Loss of Cooling Test Power
Loss of Cooling Test Power
Summary Safety advantages of High Temperature Reactors are a significant advantage. Air ingress most challenging to address Fuel performance needs to be demonstrated in operational, transient and accident conditions. Validation of analysis codes is important Materials issues may limit maximum operating temperatures and lifetimes of some components. International cooperation is essential on key safety issues.