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International Atomic Energy Agency

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  1. International Atomic Energy Agency REGULATORY GUIDANCE ON REVIEW OF BEYOND DESIGN BASIS AND SEVERE ACCIDENTS WITHIN THE FRAMEWORK OF LICENSING Jozef Misak –SAS/NSNI Workshop on “Safety Analysis Report, Safety Analysis in Licensing, including Analysis of Severe Accidents, and EOPs/SAMGs Development and Review” 23 – 26 February 2004, Islamabad, Pakistan

  2. Contents • Categorization of analysis and steps in analysis • Objectives and scope of analysis • Computational tools (computer codes) • Selection of initiating events and scenarios • Interpretation of results • Review methodology

  3. Basis • Each NPP should have an AMP • AMP includes preventive (EOP domain) and mitigative parts (SAMG domain) • Safety Analysis Report should include BDBA and severe accident analyses to the extent needed for AMP and emergency planning • Role of analytical support for AMP is crucial since often it is the only way for predicting the course of accidents • More precise meaning of these requirements to be specified

  4. COVERAGE OF NPP ABNORMAL REGIMES AND ACCIDENTS BY PROCEDURES AND GUIDELINES Incidents Normal operation Design basis accidents Beyond design accidents Severe accidents Power operation EMERGENCY OPERATING PROCEDURES SEVERE ACCIDENT GUIDELINES ABNOR- MAL PROCE- DURES Hot shutdown Intermediate shutdown Cold shutdown RCS open

  5. Organization of activities in typical SAMG approach (Westinghouse) Reactor trip, ESFAS CSF CHALLE– NGED CORE DEPLE- TION CORE DEGRA- DATION RPV CHALLENGE NORMAL OPERA - TION AB- NORMAL OPERA - TION EVENT ORIEN – TED CONTAINMENT CHALLENGE earlylate CONTROL ROOM NOP AOP Actions directed by TSC EOP/ event EOP/ SF restor. SA CRG 1 SA CRG 2 TECHNICAL SUPPORT CENTER Information Instructions SAMG TSC activation EMERGENCY CENTER Communication EMERGENCY PLAN

  6. Steps in development and implementation of AM (EOP/SAMG) Team formation Selection of approach, boundary conditions Perform DBA and BDBA analysis Development of adequate EOPs Perform severe accident analysis Ensure availability of information Select suitable AM strategies Develop guidelines for selected strategies Set-up staff responsibilities Validate AM guidelines Organize appropriate training Incorporate new developments

  7. AMP Developer Analyst Define approach Identify Preliminary phase Challenges and analysis without vulnerabilites operator actions High level strategies Development phase analysis for strategies and guidelines Guidelines / detailed strategies Implementation Implementation / phase analysis validation Relation between AMP Developers and Supporting Analysts

  8. Important principles for EOPs and SAMGs

  9. General objectives of analytical support • Understanding of the behaviour of a specific plant during BDBA and severe accidents, to determine which accident phenomena are important for a specific design, and to understand and rank the challenges to FP boundaries (containment in particular) • Understanding the plant’s capabilities and vulnerabilities, and to provide a sound basis for subsequent investigations of preventive and mitigative AM measures

  10. Categories of plant specific analysis in support of AMP • preliminary analysis that are needed for evaluating basic strategies of EOPs and SAM guidelines • procedure and guideline development analysis that are needed for confirmation of strategies and set-point calculations • verification analyses for procedures and guidelines

  11. Preliminary phase analysis - purposes • Understand plant response to BDBA/severe accidents • Identify nature and importance of challenges to FP boundaries • Identify timing of challenges • Identify symptoms (plant parameters indicative of challenge) A good PSA Level 1+2 often contain adequate severe accident analyses for these needs. If there is no PSA, then new analyses are necessary

  12. Development phase analysis - purposes • Evaluate systems capabilities • Confirm choice of symptoms and strategies • Support set-point calculations • Support development of computational aids (for SAM) • Investigate potential plant modifications

  13. Implementation phase analysis - purposes • Demonstrate capabilities and choice of strategies • Optimize strategies • Pre-analysis of validation and exercise scenarios • Support to training

  14. BEFORE (EOPs) pressure waves inside RPV pressurized thermal shock mechanical impact of escaping coolant reaction forces on components direct releases due to containment by-pass containment pressurization limited radioactivity releases from the containment or due to containment by-pass AFTER (SAMGs) direct containment heating due to high pressure expulsion of the corium hydrogen or carbon monoxide combustion (deflagration /detonation), local or global core-concrete interactions (containment foundation melt-through) long term containment pressurisation (decomposition of concrete, steam) major radioactivity releases Comparison of important phenomena before and after core damage

  15. BEFORE (EOPs) power excursions or recriticality boiling crisis overheating, damage of cladding limited exothermic Zr-water reaction, limited H2 production primary/secondary system pressurization AFTER (SAMGs) formation of eutectics melting of the cladding, fuel and core materials downward relocation of the corium massive Zr-water reaction and production of hydrogen interaction of corium with residual water, potential steam explosions heating of the RPV by corium Comparison of important phenomena before and after core damage

  16. Challenges to barriers resulting from severe accidents • cladding damage;excessive heat-up in combination with pressure difference acting on cladding leads to loss of cladding integrity with gap release • fuel melting and core degradation;FPs accumulated in the fuel matrix are released • fuel-coolant interaction in the RPV;steam explosion with potential generation of missiles and additional dynamic loading of the reactor coolant system (RCS) • high-pressure melt ejection (HPME) from the reactor vessel;with direct containment heating leading to rapid increase of containment temperature and pressure

  17. Challenges to barriers resulting from severe accidents • slow RPV melt through;with a possibility of ex-vessel steam explosion, generation of missiles, dynamic loads of the containment and ex-vessel containment phenomena • hydrogen combustion;(deflagration/detonation) leading to fast loading with possible early containment failure • containment overpressurization;due to generation of steam or non-condensable gases from decomposition of the containment concrete and combustion of combustible gases • core-concrete interaction;possible loss of containment integrity due to basemat melt-through • containment by-pass;e.g. steam generator (SG) tube rupture or damage or interface systems and direct release of reactor coolant and FPs to outside containment

  18. Two categories of analytical support • Level of understanding of phenomena is different for preventive (EOP) and mitigative (SAM) domain • Due to different level of understanding the phenomena and different objectives, analytical support should be separated into support of EOPs and support of SAMGs • Because of the need to analyze integrity of barriers, not only system TH codes, but also other detailed codes (e.g. coolant mixing, structural analysis) should be used

  19. Computer codes for analytical support of AM Prevention Mitigation (EOP domain) (SAM domain) System BE Special codes for analysis of indi- vidual phenomena (mixing, PTS) Integral fast run. Detailed codes Detailed codes Special codes Ther.-hydraulic codes for analys. for analysis for analysis for analysis of codes both in- and ex- of in-vessel of ex-vessel individual vessel phenom. phenomena phenomena phenomena RELAP5/MOD3 MAAP 4 SCDAP/RELAP5 CONTAIN ATHLET MELCOR 1.8. ATHLET-CD COCOSYS CATHARE V1 ASTEC ICARE/CATHARE GOTHIC

  20. Examples: symptoms and AM entry point • SG water level • RCS or secondary system pressure • Core exit temperature • Containment water level • Containment pressure • Containment hydrogen concentration • Site radioactivity releases

  21. Examples: preventive strategies for PWRs • Manual restoration of` systems aimed at restoration of efficient heat removal • Primary circuit feed and bleed to restore primary side cooling • Secondary circuit feed and bleed to restore secondary side cooling • Primary circuit depressurization to allow for injection from low o pressure water sources • Secondary circuit depressurization to allow feeding the stem generators (SGs); • Restart of the main circulation pumps to transport residual coolant to the core

  22. Basic features of EOP supporting analysis • Analysis to be done using an up-to-date best estimate approach, including BE codes and models, BE data and BE assumptions • Considerations of the most probable response of both plant systems and operators need to form the base case of the analysis; reasonable agreement of calculations with reality is most important, while the speed of calculations is less important • If the strategy applied allows the operator to choose among various systems which have similar safety functions, analysis considering various possibilities and combinations of such systems needs to be performed

  23. Basic features of EOP supporting analysis • If the value of certain parameters can affect the necessary actions significantly, sensitivity studies need to be performed • The available plant instrumentation needs to be modelled in order to confirm that the event can be diagnosed and to check the steps in the procedure • The performance of systems (e.g. instrumentation) not included in the model, but potentially affecting the course of accident, needs to be considered; in some cases, this can be accounted for by changing the input sequence of events in the model or assuming a range of variation for relevant parameter • QA including independent assessment of the analysis results should be arranged

  24. Analytical support for critical safety functions restoration - examples • Subcriticality: • ATWS - capability to borate the RCS under high pressure conditions with existing boration systems • FW flow control and its impact on reactor power (due to coolant temperature reactivity feedback) • recriticality aspects - loss of shutdown margin due to RCS cooldown (resulting from excessive secondary side heat removal) or RCS boron dilution • Core cooling • RCS depressurisation strategies based on heat removal into the secondary side (i.e. dumping of steam from SGs to available sinks - turbine condenser, atmosphere) and primary side (i.e. pressurizer SVs and RVs) for different RCS cooling rates (depending on the severity of challenge to the core cooling CSF) • effectiveness of the RCPs restart for delay of core degradation; • FRGs entry conditions/set-points (i.e. 650°C).

  25. Analytical support for critical safety functions restoration - examples • Heat removal: • Entry condition/set-point for application of the feed&bleed strategy (SG level, RCS temperature, etc.) • Time window for successful feed&bleed for plant specific feeding and bleeding capabilities (i.e. coolant delivery versus RCS relief capacities) • Strategy how to reduce number of HP SI pumps/open PRZR SVs in order to depressurise the RCS while maintaining adequate cooldown rate (avoiding PTS) and transfer from HP SI to LP SI pumps • Ultimate RCS temperature achievable by feed&bleed strategy - possibility to transfer to RHR system for stable long term residual heat removal • Thermal stress analysis/assessment for recovery of FW into a hot SG, limits • Reactor Pressure Vessel (RPV) integrity • PTS • cold overpressurisation

  26. Analytical support for critical safety functions restoration - examples • Containment integrity: • containment pressure response to pipeline breaks • strategies for operator control of containment pressure by operation of sprays • containment underpressure concern • Station blackout: • effectiveness of RCS depressurisation by heat removal into the secondary side (dumping of SG steam – entry condition) • effectiveness of RCS depressurisation by the pressurizer SVs/RV - entry condition • shutdown margin provided by HA injection • preferred sequencing of depressurisation steps (primary versus secondary depressurisation)

  27. Some Important Issues in BDBA Preventive AM Typically Requiring Thermal Hydraulic Analysis (examples)

  28. Some Important Issues in BDBA Preventive AM Typically Requiring Thermal Hydraulic Analysis (examples)

  29. Some Important Issues in BDBA Preventive AM Typically Requiring Thermal Hydraulic Analysis (examples)

  30. Examples of analyses for development of preventive (Critical Safety Function restoration part) and mitigative AM

  31. Examples of analyses for development of preventive (Critical Safety Function restoration part) and mitigative AM (cont’d)

  32. Specific tasks for AM supporting analysis • choice of key symptoms; confirmation of choice of symptoms for long-term processes • specification of set-points to initiate and to exit a strategy • prioritisation and optimisation of strategies; positive effects and possible negative effects of the strategy • evaluation of capability of systems to perform intended functions; expected trends in the accident progression (projections of the timing) • conditions for leaving SAM domain • specifications of environmental conditions for operation of instrumentation and NPP systems; recommendations for equipment/instrumentation upgrades • computational aid development

  33. Examples: mitigative strategies for PWRs • Coolant injection to the degraded core (from any source) • Primary circuit depressurization to prevent HPME • External RPV cooling to avoid ex-vessel effects • Operation of hydrogen recombiners/igniters • Secondary circuit feeding to protect SG tube integrity and scrubbing of radioactive releases • Spraying of the containment to remove FPs from containment atmosphere, to reduce the containment pressure • Containment filtered venting to protect containment integrity • Operation of containment fan coolers • Containment injection to submerge RPV and to cool ex-vessel core debris • Containment inertization

  34. Examples: contradictory effects for mitigative AM actions • Water added to the core cools the core but at the same time it may increase production of hydrogen from cladding-steam reaction • Spraying of the containment reduces the containment pressure but may increase volumetric concentration of hydrogen resulting in concentration inside the flammability limit • Injecting water into the SGs contributes to removal of the residual heat but may lead to damage of SG tubes and to the containment by-pass Uncertainty in the NPP response to mitigation measures is the main reason why guidelines are used in the severe accident domain instead of structured EOPs

  35. Level of understanding of phenomena for in-vessel analysis • Well understood phenomena • Majority of phenomena in early phase of core degradation (boil-off, recriticality, reflooding before significant oxidation, cladding balooning, dissolution of fuel and other materials, …) • Low level of knowledge of phenomena • Hydrogen production during flooding of degraded core • Recriticality of degraded core • Steam flow through the degraded core • Formation of debris be • Formation of molten pool, formation of crust, its stability, break-through • Molten core relocation

  36. Level of understanding of phenomena for ex-vessel analysis • Well understood phenomena • Both local and global containment pressurization • Low level of knowledge of phenomena • Long lasting processes, including late phase of in-vessel phenomena as a boundary condition • Natural convention in the containment • Heat exchange with structures • Temperature stratification (typically underpredicted) • Hydrogen distribution • Material interactions, mainly molten corium concrete interaction

  37. IN-VESSEL : Natural circulation in RPV, RCS Radiation heat transfer Oxidation during heat up and quenching Formation of eutectics, dissolution of UO2 Formation of flow blockages in the core, core coolability Melt interactions with supports Melt fragmentation coolant Coolability of the reactor lower head EX-VESSEL : Prediction of gas flows (steam, non-condensable gases) Hydrogen production, distribution, deflagration Melt release Corium-concrete interactions, effect of water injection, coolability Relocation, spreading of melt Most important aspects in modelling

  38. Example for severe accident phenomena significant for potential challenges to fission product boundaries

  39. Example of the development of severe accident management insights for a NPP

  40. Identification of Suitable Accident Sequences for the Investigation of Hydrogen Deflagration Challenges

  41. Components of safety analysis for AM Selection of analytical tools Collection of information needed for analysis Selection of accident sequences Analysis without operator action Evaluation of capabilities and limitations of existing equipment Identification and analysis of preventive measures Identification and analysis of mitigative measures Consideration of uncertainties

  42. Regulatory review of safety analysis for AM – issues to be reviewed Selection of analytical tools • Select computer code reasonably covering the most important phenomena (MAAP, MELCOR, ASTEC • Consider how specific design features will be addressed by the code • Using code documentation, check validity of models and correlations for a given application • Estimate main uncertainties related to use of the code for a specific purpose • Specify other computational/experimental sources to complement the code calculation

  43. Regulatory review of safety analysis for AM – issues to be reviewed (cont’d) Information needed for analysis • Collect plant specific data with clear references to data sources; attention to data with largest effect on results • Provide for verification of the data • Develop plant database and engineering handbook • Verify engineering handbook

  44. Regulatory review of safety analysis for AM – issues to be reviewed (cont’d) Selection of accident sequences • Select list of possible plant specific damage states (based on PSA or any other source of information - design specifications, operational experience, accident precursors, design specific experimental results, severe accident research, information from similar plants) • Specify groups of sequences to be analyzed (typically LB LOCA +LOECC, SB LOCA +LOECC, station blackout, loss of feedwater) • Select representative sequences for analysis (typically several tens of sequences)

  45. Regulatory review of safety analysis for AM – issues to be reviewed (cont’d) Analysis without operator action • Define important phenomena to be modeled • Select reasonable number of representative scenarios • Perform analyses of scenarios without any accident management action • Determine challenges to fission product boundaries, their timing and expected mode of a failure • Summarize results of analysis in a practicable format

  46. Regulatory review of safety analysis for AM – issues to be reviewed (cont’d) Identification and analysis of preventive measures • Specify possible preventive measures • Select symptoms to be used to initiate preventive strategy (core exit temperature, SG level, containment pressure, radioactivity level, …) • Define proper timing (time window) to initiate the action • Confirm effectiveness of individual preventive measures • Define order of priority for different preventive actions (design specific) • Consider uncertainties in specifying preventive measures • Prepare complete and concise report with results of analysis (large number of calculations, user-friendly format)

  47. Regulatory review of safety analysis for AM – issues to be reviewed (cont’d) Identification and analysis of mitigative measures (similar as for preventive measures) • Specify possible mitigative measures • Select symptoms to be used to initiate preventive strategy (containment pressure, hydrogen concentration, ra- level, …) • Define proper timing (time window) to initiate the action • Confirm effectiveness of individual mitigative measures • Define order of priority for different mitigative actions (design specific) • Consider uncertainties in specifying mitigative measures • Prepare complete and concise report with results of analysis (large number of calculations, user-friendly format)

  48. Regulatory review of safety analysis for AM – issues to be reviewed (cont’d) Evaluation of capabilities and limitations of existing instrumentation and control • Confirm availability of measurements for selected symptoms • Specify environmental conditions for operation of I&C • Evaluate reliability of measurements, also outside their design range • Specify alternate measurements • Develop computational aids to replace/complement measurements

  49. Regulatory review of safety analysis for AM – issues to be reviewed (cont’d) Evaluation of capabilities and limitations of existing equipment • Specify environmental conditions for operation of the equipment • Evaluate functioning of the equipment, also outside its design range • Evaluate availability of power/media supply for operation of equipment • Define priorities in operation of different kinds of equipment • Specify alternatives for equipment and evaluate their performance