Fission Product Boundary Challenges

Transcript

Lecture 5a/5b Severe Accident Phenomena Fission Product Boundary Challenges Module 2 Chapter 2 IAEA Training Workshop on Severe Accident Management Guideline Development using the IAEA SAMG-D Toolkit Jeff Gabor IAEA International Atomic Energy Agency Summary of Module 5a/5b • Fission Product Barrier 1. Large releases 2. Bypass events 3. High Pressure Melt Ejection 4. RPV melt-through 5. Hydrogen production and 6. 7. 8. 9. 10. 11. 12. 13. combustion Molten Core Concrete Interaction Containment pressurization Sub-atmospheric Release of FP to environment Long term Spent Fuel Pool Plant damage states Summary IAEA 2 Fission Product Barriers • For AM development, it is important to understand the challenges to Fission Product (FP) barriers • Mitigating strategies my compete for resources, therefore, it is important to establish priorities An understanding of severe accident phenomena is critical to AM IAEA 3 Stages of Core Damage OX BD EX • • • • Degraded fuel conditions Cladding oxidation significant Fuel degradation sufficient to lead to appreciable fuel debris relocation Potential for critical fuel configurations • Degraded fuel conditions with RCS/RPV challenged • Significant fuel relocation • Coolability of the fuel geometry degraded • Degraded fuel conditions with RCS/RPV lower head breached • Core debris relocation into containment occurred • Direct attack of the concrete containment can occur Ref: EPRI Technical Basis Report, 2012 IAEA 4 Stages of Spent Fuel Pool Damage SFP-OX SFP-BD • Degraded conditions • Cladding oxidation significant • Fuel degradation sufficient to lead to appreciable fuel debris relocation • Potential for critical fuel configurations • Degraded conditions with challenge to SFP structure • Significant material relocation • Coolability of the fuel assembly geometry degraded Ref: EPRI Technical Basis Report, 2012 IAEA 5 Stages of Containment Damage CC CH B I • Containment intact and cooled • Containment challenged • Appreciable buildup of energy • Presence of flammable gases in containment • Containment bypass • Direct pathway from RCS/RPV out of containment (e.g. SGTR, ISLOCA) • Containment impaired • Containment isolation failure or some other breach • Direct pathway out of containment exists Ref: EPRI Technical Basis Report, 2012 IAEA 6 Challenges - Summary • RPV melt-through not included, however, strategies exist to prevent this challenge • AM development needs to consider that not all strategies may be successful, due to equipment and human failures, and plan on mitigation of the releases. Strategies often referred to as “Candidate High Level Actions” Ref: EPRI Technical Basis Report, 2012 IAEA 7 Fission Products • Types of releases possible • • • • • • Noble gases – Xe, Kr Iodine – I2 Iodides – CsI Oxides – BaO, SrO, TeO2 Hydroxides – CsOH Metals - Sb IAEA 8 Fission Products (cont’d) • Isotopes can be: • Gaseous – I2, CH3I • Volatile – Cs, I • Medium volatile – Ba, Ce • Non-volatile – Sr, Ru • Fission products released from fuel matrix condense in cooler regions to form solid aerosols IAEA Ref: NUREG/CR-6533, Code Manual for CONTAIN 2.0, 1997 9 Potential for Large Releases • Initiating event (examples) • Reactivity event • Large external event • Aircraft impact • Accident progression • Steam explosions • Hydrogen deflagration/detonation • Direct Containment Heating Large releases require immediate attention to protect staff and public IAEA 10 Large Releases Strategies • Habitability challenges • Desire to locate the breach and isolate • Use of internal and external sprays • External filters • Operation of ventilation systems • Requires close integration with Emergency Preparedness (EP) plan IAEA 11 Containment Bypass • Possible confusion with containment impairment or pre-existing failure of containment • Impairment and pre-existing failure • Opening in containment occurs prior to core damage • Bypass – normally refers to the creation of a direct release path from the RCS/RPV to the environment • Induced SGTR (post-core damage) • Interfacing system LOCA (pre-core damage) IAEA 12 Strategies for SGTR • Prevention and Mitigation of SGTR • Flood secondary side with water • Depressurize the RCS IAEA 13 High Pressure Melt Ejection • Requires elevated RCS/RPV pressure (e.g. > 2 Mpa) • Spread of molten debris over large containment volume • Debris stored heat transferred to containment atmosphere • Short time scale IAEA Ref: NUREG/CR-6533, Code Manual for CONTAIN 2.0, 1997 14 Direct Containment Heating • As a result of HPME, rapid heat-up of the containment atmosphere could occur with a consequential pressure spike • DCH important factors: • Cavity geometry • De-entrainment of debris • Re-entrainment of debris • HPME can also impact the final debris distribution and success of debris cooling IAEA 15 Debris Transport Ref: EPRI Technical Basis Report, 2012 IAEA 16 Strategies for HPME/DCH • Depressurize the RCS/RPV • • • • • Restore secondary side cooling (PWR) Operate isolation condenser (BWR) Depressurize the steam generator(s) (PWR) Open primary safety valves (BWR & PWR) Open main steam drain lines (BWR) RCS depressurization also allows injection from low pressure sources IAEA 17 Core cooling, ultimate heat sink and RPV melt-through • Challenges • Heat-up, melting and relocation of core material • Potential for melt/oxide stratification in lower plenum and “focusing effect” • Attack of lower plenum penetrations Primary strategy is to prevent or delay vessel failure IAEA 18 Challenges to Containment • Restoration of core cooling will then pose a potential challenge to the containment • Depressurization of the RCS/RPV challenges the containment • In-vessel retention of core material by in-vessel or ex-vessel flooding transfers additional heat to containment Strategies to prevent or delay vessel breach require the availability and identification of an Ultimate Heat Sink IAEA 19 Hydrogen Generation • Steam oxidation of zirconium fuel cladding • CO also generated ex-vessel due to core-concrete interactions • Hydrogen flammable • Ignition at 4% • Deflagration at 8% • Detonation at > 14% • Steam inerting at 55% IAEA Ref. www.world-nucler.org 20 Hydrogen Strategies 1. No strategy – pressure rise well within containment capability 2. Mixing containment atmosphere to prevent locally high concentrations • May be a consequence of containment design • Active systems to promote mixing 3. Inerting containment atmosphere • Employed in BWR Mark I and II design • Dilution from purging systems IAEA 21 Hydrogen Strategies (cont’d) 4. Purging containment by vent and purge 5. Intentional consumption of hydrogen • Passive autocatalytic recombiners • Hydrogen igniters • Combination of two above IAEA 22 Implementing H2 Strategies • Hydrogen monitoring system • Sampling • Computational aids • Core Damage Assessment Guide • H2 flammability curve Ref. NUREG/CR-3468 IAEA 23 Molten Core Concrete Interaction • Ex-vessel challenge • Basemat erosion • Sidewall erosion • H2, CO, CO2 • Occurs in dry cavity conditions • No debris cooling • Wet cavity • May still occur for deep core debris pools (e.g. > 10 cm) IAEA Ref: EPRI Technical Basis Report, 2012 24 Cavity Flooded Prior to Vessel Breach • Strategy for AM Positive Negative Break up core material to enhance coolability Ex-vessel steam explosion Protect containment boundary Rapid steam generation Reduce radiation heat transfer from surface of debris Containment pressurization IAEA 25 GE HITACHI Ex-vessel Core Cooling IAEA Ref: Risk-Informing ESBWR Design with Probabilistic Safety Assessments, INPRO Dialogue Forum, Nov. 2013 26 AREVA Ex-vessel Core Cooling IAEA Ref: INPRO User Requirement 1.4 ‘Release into the Containment’ Position of the EPR reactor, INPRO dialogue forum, Nov. 2013 27 EPR Spreading Configuration Ref: www.tvo.fi IAEA 28 Containment Pressurization • Sources of mass and energy • • • • • • • • Accident initiator – LOCA Discharge from RCS prior to core damage – SRVs Heat from reactor vessel Steam generation ex-vessel H2, CO, CO2 due to MCCI H2 and CO combustion or recombination Direct Containment Heating Containment flooding (reduces gas volume) IAEA 29 Containment Capability • Typical PSA includes structural analysis of the containment • Considers several potential failure locations • Includes both pressure and temperature challenges • Looks at static and dynamic loads • Addresses penetrations and seals in addition to structural components Containment capability assessment is critical to planning AM strategies IAEA 30 Strategies for Containment Pressure Control • Vent • Could use ventilation system – may be limited capacity • Containment coolers • Sprays IAEA 31 Cautions to be addressed in AM • Aerosols can clog ventilation system filters • H2 in ventilation system • Sprays can de-inert containment atmosphere • High radiation in proximity of vent path IAEA 32 Sub-atmospheric Pressure - Challenge • Leakage or venting of non-condensable gas may later lead to sub-atmospheric conditions if steam is removed • Containment structures not designed for significant negative pressure force IAEA 33 Sub-atmospheric Pressure - Strategies • Containment vacuum breakers • Confirm operation during a severe accident • Termination of sprays and coolers at low pressure • Purge containment atmosphere IAEA 34 Release of Fission Products to Environment IAEA Ref: The National Diet of Japan, The official report of The Fukushima Nuclear Accident Independent Investigation Commission, 2012 35 Release Consequences • Habitability constraints • • • • • • • • • Containment leakage Bypass Failure Venting Steam generator tube rupture Isolation condenser tube failure Drywell liner failure Basemat failure Spent fuel pool release IAEA 36 Release Mitigation Strategies • Discussed in previous sections • Venting, sprays, etc. • Preparations need to be taken for site access, lodging, food, water, fuel, medical, communications IAEA 37 Contaminated Water Management • pH control of water pools • Continued makeup to prevent pool dryout and revaporization of fission products • Capture and storage of contaminated run-off water IAEA 38 Long Term Provisions • SAMGs are typically developed for short (days) term response • Place plant into a safe stable state • Can be supplemented with longer term (weeks, months, years) provisions • Repair of failed systems • Staff change Exit conditions can be identified and tracked using logic diagram or similar techniques IAEA 39 Examples of Safe, Stable Conditions • Site release terminated or small and • • • • decreasing Core debris covered and cooled RCS pressure low and stable Containment pressure low and stable • Combustible gasses under control Water management under control IAEA 40 Spent Fuel Pool Challenges • Damage due to: • Initiating event (e.g. seismic event) • Pool drain can create rapidly developing challenge • Loss of pool cooling • Slower evolving challenge due to heat-up and boil-off • Typically Spent Fuel Pool not inside containment, therefore, potential for unscrubbed release IAEA 41 Spent Fuel Pool Strategies • Water makeup • Fire water, hoses, portable pumps • Spraying (mitigate pool drain event • Ventilation • Opening of panels and doors • Active fans IAEA 42 Example for Reactor Building Ventilation Ref: http://www.tepco.co.jp/en/nu/kk-np/safety/mitigation-e.html IAEA 43 Plant Damage Conditions (PDC) • EPRI TBR • Characterization of severe accident progression • Extent of fuel damage • Containment status • PDC helps identify available fission product barrier to be protected • Barriers include fuel, RCS, spent fuel, primary, and secondary containment IAEA 44 Candidate High Level Actions Limit Potential for Releases Recover core IAEA Minimize Maintain releases containment from integrity containment Minimize off-site releases 45 Overview of Candidate High Level Actions No. Candidate High Level Action 1. Inject into (makeup to) reactor pressure vessel/reactor coolant system (RPV/RCS) 2. Depressurize the RPV/RCS 3. Spray within the RPV (BWR) 4. Restart reactor coolant pump (RCP) (PWR) 5. Depressurize steam generators (PWR) 6. Inject into (feed) the steam generators (PWR) 7. Operate isolation condenser (IC) (BWR) 8. Spray into containment 9. Inject into containment 10. Operate fan coolers 11. Operate recombiners 12. Operate igniters IAEA 46 Overview of Candidate High Level Actions No. Candidate High Level Action 13. Inert the containment with noncondensable gases (BWR) 14. Vent the primary containment 15. Spray the secondary containment 16. Flood the secondary containment 17. Inject into the spent fuel pool 18. Spray the spent fuel pool 19. Vent/ventilate the reactor building or auxiliary building 20. Scrub releases by external spraying of buildings IAEA 47 Severe Accident Phenomenology Fission Product Barrier/Issue Phenomenological Challenge Fuel reactivity Recriticality Fuel cladding Ballooning and rupture Over temperature and oxidation RCS Hot leg creep rupture (PWR) Steam generator tube rupture (PWR) Overpressure RPV Main steam line creep failure (BWR) Stuck-open SRV (BWR) Overpressure In-vessel steam explosion Molten jet attack Creep failure of the lower head Penetration failure IAEA 48 Severe Accident Phenomenology Fission Product Barrier/Issue Phenomenological Challenge Containment basemat Core-concrete interaction Containment Static overpressure Overtemperature Containment bypass Containment isolation failure Flammable gas combustion Ex-vessel steam explosion Direct containment heating Melt-liner attack (BWR/Mark I) Reactor/auxiliary building Static overpressure Overtemperature Flammable gas combustion IAEA 49 Candidate High Level Actions Recover Fuel • Injection needed defined by decay heat • Must remove at least all decay heat to begin recovery • Time to recover influenced by • Exothermic energy addition from metal oxidation • Stored energy in core materials • Two injection thresholds • Decay heat converted into latent energy (vaporization) Wvap • Decay heat converted into sensible energy IAEA Wsat 50 Stages of Core Damage OX BD EX • • • • Degraded fuel conditions Cladding oxidation significant Fuel degradation sufficient to lead to appreciable fuel debris relocation Potential for critical fuel configurations • Degraded fuel conditions with RCS/RPV challenged • Significant fuel relocation • Coolability of the fuel geometry degraded • Degraded fuel conditions with RCS/RPV lower head breached • Core debris relocation into containment occurred • Direct attack of the concrete containment can occur Ref: EPRI Technical Basis Report, 2012 IAEA 51 Candidate High Level Actions Recover Core Recover Core OX Operate Isolation Condenser OX/BD EX OX/BD/EX (BWR) Inject into RPV/RCS Spray within the RPV (BWR) Spray into containment Inject into SFP Restart the RCPs (PWR) Inject into (Feed) the SGs Inject into or flood containment Spray into SFP Operate the containment fan coolers IAEA 52 Stages of Containment Damage CC CH B I • Containment intact and cooled • Containment challenged • Appreciable buildup of energy • Presence of flammable gases in containment • Containment bypass • Direct pathway from RCS/RPV out of containment (e.g. SGTR, ISLOCA) • Containment impaired • Containment isolation failure or some other breach • Direct pathway out of containment exists Ref: EPRI Technical Basis Report, 2012 IAEA 53 Candidate High Level Actions Maintain Containment Integrity Containment Integrity OX/BD/EX CC/CH CH Operate Isolation Condenser Inject into RPV/RCS Spray containment Inject into or flood containment Operate recombiners Spray within the RPV (BWR) Restart the RCPs (PWR) Operate the containment fan coolers Vent containment Operate igniters Inject into (Feed) the SGs IAEA Inert containment 54 Candidate High Level Actions Minimize Radiological Release from Containment Minimize release from containment OX/BD/EX B(ypass) I(mpaired) Inject into RPV/RCS Inject into (feed) SGs (PWR) Operate Isolation Condenser (BWR) Spray containment Inject into or flood containment Spray within RPV (BWR) Spray reactor/auxiliary building Flood reactor/auxiliary building Operate fan coolers Vent containment Vent/ventilate reactor/auxiliary building IAEA 55 Stages of Spent Fuel Pool Damage SFP-OX SFP-BD • Degraded conditions • Cladding oxidation significant • Fuel degradation sufficient to lead to appreciable fuel debris relocation • Potential for critical fuel configurations • Degraded conditions with challenge to SFP structure • Significant material relocation • Coolability of the fuel assembly geometry degraded Ref: EPRI Technical Basis Report, 2012 IAEA 56 Candidate High Level Actions Recover Spent Fuel Recover Spent Fuel SFP-OX/SFPBD OX/BD Inject into RPV/RCS IAEA Spray into RPV/RCS (BWR) Spray into SFP Inject into SFP 57 Reactor/Auxiliary Building Damage Conditions • Reactor/auxiliary building intact and cooled SC-CC SC-CH SC-I • Reactor/auxiliary building challenged • Appreciable buildup of energy • Presence of flammable gases in building atmosphere • Reactor/auxiliary building impaired • Direct pathway to environment exists IAEA 58 Candidate High Level Actions Minimize Off-site Radiological Release Minimize off-site release OX/BD/EX SFP-OX/SFPBD Inject into RPV/RCS Inject into SFP IAEA Spray within RPV (BWR) Spray into SFP SC-CH Vent/ventilate reactor/auxiliary building SC-I Spray into reactor/auxiliary building External spray of building to scrub releases 59 Questions? IAEA 60