Light water reactor safety : the development of advanced models and codes for light water reactor safety analysis

1 INTRODUCTION 1 1 . 1 Nuclear Power 1 1 .2 Light Water Reactors 2 1 .3 Prominant Safety Studies 2 1 .4 Major Technical Areas 3 1 .5 Nuclear Safety Research Objectives 4 1 .6 Modelling Requirements and Capabilities 5 2 BASIC FEATURES OF LIGHT WATER REACTORS 9 2. 1 Introduction 9 2.2 Pressurised Water Reactors 9 2.2. 1 Reactor Coolant System 9 2.2.2 Reactor Core and Pressure Vessel 1 1 2.2.3 Containment 1 3 2.2.4 Safety Systems 1 3 2.3 Boiling Water Reactors 14 2.3 . 1 BWR S team Cycle 1 6 2.3.2 Reactor Core and Vessel 1 6 2.3.3 Containment 1 6 2.3 .4 Safety Systems 1 7 3 ACCIDENT SCENARIOS 2 1 3.1 Introduction 2 1 3.2 Accident Classification 2 1 3.3 Intact Circuit Faults 2 1 3 .3 . 1 Pressurised Water Reactors 2 1 3.3.2 Boiling Water Reactors 23 3.4 Loss of Coolant Accidents (LOCAs) 24 3.4. 1 Pressurised Water Reactors 24 3.4 . 1 . 1 Large Break LOCAs 24 3 .4. 1 .2 Small Break LOCAs 24 3.4.2 Boiling Water Reactors 25 3 .4.2. 1 Large Break LOCAs 25 3 .4.2.2 Small Break LOCAs 26 3 .5 Severe Accidents 26 3.5 . 1 Pressurised Water Reactors 26 3.5. 1 . 1 Intact Circuit 26 3.5.1 .2 LOCAs 28 3 .5. 1 .3 Containment Bypass Sequences 30 3.5 .2 Boiling Water Reactors 3 13.5 .2. 1 Intact Circuit Faults 3 1 3.5.2.2 LOCAs 3 1 4 INTEGRAL EXPERIMENTS 33 4. 1 Introduction 33 4.2 Thermal Hydraulics 33 4.3 Fuel and Cladding Behaviour 35 4.4 Materials and Structural Behaviour 36 4.5 Core Melt Programmes 36 4.6 Natural Circulation 39 4.7 Fission Product Release and Transport 39 4.8 Debris Beds 40 4.9 Melt/W ater Interactions 4 1 4 . 1 0 Core Concrete Interaction 4 1 4 . 1 1 High Pressure Melt Ejection and Direct Containment Heating Experiments 42 4 . 1 2 Containment Phenomena 43 S THERMAL HYDRAULIC MODELS 47 5.1 Introduction 47 5.2 Code Categories 47 5.3 Detailed Physical Phenomena 48 5.4 Classification of Two-Phase Flow Models 49 (a) Homogeneous Flow 49 (b) Homogeneous Model with Slip 49 (c) Drift Flux Model 49 (d) Disequilibrium Two-Phase Fluid Models 49 (e) Disequilibrium Fluid Models including Incondensable Gases 49 5 .5 Derivation of Two-Phase Flow Equations 50 5.5 . 1 Field Equations 5 1 5.5.2 Additional Field Equations 53 5 .6 Constitutive Relations 54 5.6. 1 Flow Regime Maps 54 (a) Vertical Flow Maps 54 (b) Horizontal Flow Maps 54 (c) Other Flow Maps 54 5.6.2 Interphase Drag 54 5.6.3 Wall Friction 56 5.6.4 Wall Heat Transfer 56 5.6.5 Interphase Mass Transfer 57 5.6.6 Reflood Heat Transfer 57 5.6.7 Turbulence Modelling 58 5 .6.8 Choked Flow 60 (a) Subcooled Flow 60 (b) Two-Phase Flow 60 (c) Single-Phase Vapour 60 (d) Transition Regime 609.5 Core Fragmentation and Blockage Formation 1 0 1 9.6 Molten Pools 103 9.7 B lockage Melt Release Models 103 9.8 Melt Relocation from Blockages and Melt Progression Paths 1 04 10 STEAM EXPLOSIONS 107 10. 1 Introduction 107 10.2 Phases of a Steam Explosion 1 09 10.3 Boundary Conditions 1 1 1 10.4 Mixing of Molten Debris with Water 1 1 2 10.5 Triggering 1 1 5 10.6 Mechanical Energy from Steam Explosions 1 16 10.7 Damage Potential 1 1 9 10.8 Experiments 1 20 1 0.9 Calculational Models 1 2 1 10.9. 1 Heat Transfer from Particles Following a Steam Explosion 1 2 1 10.9.2 Relief and Safety Valves 1 22 11 DEBRIS COOLABILITY MODELS 1 27 1 1 . 1 Introduction 1 27 1 1 .2 Phenomena 1 28 1 1 .3 Experimental Programmes 1 28 1 1 .4 Bottom Reflood Models 1 3 1 1 1 .4.1 Lumped Parameter Models 1 3 1 1 1 .4.2 Quenching Models 1 3 1 1 1 .4.3 Models for Fluidisation 1 32 1 1 .4.4 More Sophisticated Models 1 32 1 1 .5 Top Reflood Models 1 32 1 1 .5 . 1 Critical Heat Flux Models 1 34 1 1 .5.2 Upper Bed Quenching Rate Limitation Models 1 34 1 1 .5.3 Quench Front Limitation Models 1 3 5 1 1 .6 Status of Modelling 1 36 1 1 .6. 1 Bottom Reflood 1 36 1 1 .6.2 Top Reflood 1 37 12 DEBRIS INTERACTIONS WITHIN THE VESSEL 14 1 1 2. 1 In trod uction 14 1 1 2.2 Release of Material from the Core Region 142 1 2.3 Debris Interactions with the Lower Vessel Internals 142 1 2.4 Debris Interactions with Water 144 1 2.5 Debris Interactions with the Vessel 145 12.6 Vessel Failure Potential 146 1 2.7 Vessel Response at Elevated Temperatures 146 12.8 Behaviour of Penetrations 147 12.9 Debris Behaviour at Vessel Failure 14916.5 Aerosol Production via Bubble Collapse 195 16.6 Vaporisation 195 16.7 Condensation 195 16.8 Aerosol Particle Size 196 16.9 Gas Composition at Equilibrium Over the Melt 196 16.10 Bubble Rise Phenomena 197 16.11 Mass Transport 198 16.12 Departure from Equilibrium Conditions 200 16.13 Mechanical Aerosol Production 200 17 CONTAINMENT THERMAL-HYDRAULICS 203 17.1 Introduction 203 17.2 Major Phenomena Affecting Thennal Hydraulics 204 17.3 Flow Between Compartments 204 17.4 Fundamental Thennal-Hydraulics 205 17.5 BWR Specific Processes 207 17.6 Material Properties 208 17.7 Gas Burning Models 208 17.8 Energy and Mass Transfer 209 17.9 Heat Conduction in the Structures 211 17.10 Engineered Safety Features 211 17.11 Summary 213 18 CONTAINMENT AEROSOL AND FISSION PRODUCT MODELS 215 18.1 Introduction 215 18.2 Size Distribution 216 18.3 Agglomeration 216 18.4 Aerosol Condensation and Evaporation 218 18.5 Deposition 218 18.6 Aerosol Sources 219 18.7 Scrubbing 219 18.8 Radionuclide Behaviour Modelling 220 18.9 Decay Chains 221 18.10 Decay Heating Models 221 18.11 Transfer Rates in the Atmosphere 222 18.12 Transfer in the Liquid 222 18.13 Engineered Safety Features 222 18.14 Containment Sprays 224 19 THERMOPHYSICAL PROPERTIES 227 19.1 Introduction 227 19.2 Fuel Requirements 227 19.3 Fuel Pin Cladding 231 19.4 Control Rod Material 234 19.5 Control Rod Cladding 235 19.6 Fuel Cladding Eutectic 237 19.7 Control Rod Eutectic 23823 PLANT STUDIES 297 23 . 1 Introduction 297 23.2 Intact Circuit Faults 298 23 .2. 1 Loss of Feed Water 298 23 .2.2 Steam Line Break 300 23 .2.3 Reactivity Transients 300 23.2.4 Anticipated Transient Without Scram (A TWS) 301 23 .3 Loss of Coolant Accidents 301 23 .3 . 1 Steam Generation Tube Rupture (SGlR) 301 23 .3.2 Large Break LOCA 303 23.4 Core Degradation and Thermal Hydraulics 304 23.4 . 1 Station Blackout 305 23 .4.2 Small Break Loss of Cooling Severe Accident 308 23.4 .3 Containment Bypass: Interfacing LOCA 309 23 .4.4 Shutdown Accidents 309 23.5 Fission Product Transport under Severe Accident Conditions 3 10 23.5. 1 Station Blackout 3 10 23.6 Containment Behaviour Under Severe Accident Conditions 3 1 2 23 .6. 1 Station Blackout 3 1 2 24 ACCIDENT MANAGEMENT 3 1 7 24. 1 Introduction 3 1 7 24.2 Preventative Accident Management 3 1 8 24.2. 1 Total Loss of Feed Water 3 19 24.3 Computer Code Requirements 320 24.4 Code Assessments 321 24.5 Summary - Preventative Accident Management 322 24.6 Mitigative Accident Management 322 24.7 Water Addition to a Degraded Core 324 24.8 Primary Circuit Integrity 324 24.9 Late Phase Melt Progression 325 24. 10 Melt-Water Interactions in the Reactor Vessel 325 24. 1 1 Steam Explosions 326 24. 1 2 Failure of the Reactor Vessel 326 24. 1 3 Threat to the Containment 326 24. 14 Ex-Vessel Melt/Water Interactions 327 24. 1 5 Molten Core/Concrete Interactions 327 24. 16 Summary - Mitigative Accident Management 327 2S ADV ANCED REACTORS 33 1 25 . 1 Introduction 33 1 25.2 Design Concepts 33 1 25.2. 1 Evolutionary Designs 33 1 25.2.2 Passi ve Designs 333 25.2.3 Innovative Designs 335 25.3 New Phenomena 33625.4 Experiments 25.5 Accident Assessments 25.6 Computer Code Requirements