SDC and SDG discussions related to Design Extension Condition [DEC] GIF SDC Task Force Member Yasushi OKANO

SDC and SDG discussions related to Design Extension Condition [DEC] GIF SDC Task Force Member Yasushi OKANO

Contents 1. SDC descriptions related to DEC 2. Identification of DEC postulated events 3. Design requirements for DEC Guideline for ATWS Guideline for LOHRS Application of SFR Safety Features 4. Summary Slide 2

SDC descriptions related to DEC Section 2.2.2 Relationship among plant states, probabilistic and deterministic approaches Criteria Criterion 20: Design extension condition Criterion 44: Structural capability of the reactor core Criterion 45: Control of the reactor core Criterion 46: Reactor shutdown Criterion 47: Design of reactor coolant systems Criterion 49: Level of reactor coolant Criterion 51: Decay heat removal system Appendix B: Guide to Design Extension Conditions Slide 3

Safety for design extension condition [DEC]* Providing practical measures for managing DEC is important In order to prevent their occurrences (if possible) and/or mitigate their consequences This will enhance the robustness of the system and will permit reaching the safety level required for Gen-IV reactors Due consideration of the potential for common cause failures shall be taken into account in the safety design. Due consideration for applying passive design measures, by utilizing/enhancing favourable safety features specific to the Gen- IV SFR system, will also be required for DEC. Feedback from past experience in this field will be used to improve reliability. The reactor should be designed such that accident progresses slow enough to allow time for systems to respond and appropriate actions needed to mitigate the consequences to proceed. * In the SDC Phase1 Report Slide 4

Utilisation of passive safety features* Provisions of well-balanced design measures are necessary Appropriate combination of active & passive safety systems to enhance safety against wide-ranging events (incl. DBA and DEC) For DBA, it is important to well characterize the safety features of SSCs, including inherent characteristics. And the reliability of the safety systems should be enhanced based on proven technologies (safety systems with adequate redundancy and diversity) that have been conventionally and widely used. For DEC. it is possible to ensure diversity with different operation principles, without further multiplexing the measures already applied for DBAs. Using passive and inherent safety features of the design should allow termination of accidents or mitigation of consequences of a design extension conditions, even in postulated failure of active safety systems. * In the SDC Phase1 Report Slide 5

Prevention of cliff edge effect* Severe accidents that could lead to a significant and sudden radioactive release due to a possible cliff edge effect, not reasonably manageable by design improvement, shall be practically eliminated by appropriate provisions. The severe accidents that are determined to be practical eliminated should be restricted to those that are not deemed physically impossible as determined by deterministic and probabilistic considerations. Safety demonstrations of practically eliminated situations shall be robust and based on deterministic and probabilistic analyses that address uncertainties and covers a large spectrum of events. * In the SDC Phase1 Report Slide 6

Containment function* The containment should be designed so that it can withstand postulated severe accidents with core degradation. Safety provisions required to mitigate consequences of core degradation and to retain the degraded core materials should be built-in. For radiological confinement, design provisions related to the confinement function should be enhanced, as far as reasonably achievable, confinement measures must take into account a source term whatever the origin of the radioactive material in the plant (e.g. core, spent fuel storage ) * In the SDC Phase1 Report Slide 7

Postulated events, Safety Systems & Prevention/Mitigation of DEC Defence-in-Depth (IAEA) Level 1: Prevention of abnormal operation and failures postulated events Coolant leakage Design Abnormal control rod withdrawal Primary pump seizure Design Basis Level 2 : Control of abnormal condition and detection of failures Level 3 : Control of accidents within the design basis Abnormal condition detection & Plant and reactor control system Reactor shutdown system Safety Systems Coolant level retention system Decay heat removal system Containment system Design Extension Condition Prevention prevention of core damage Level 4 :Control of severe plant conditions including prevention of accident progression and mitigation of the consequences of severe accidents Mitigation Ensuring containment function Extension of capability of above safety systems or additional features Mitigation of challenging factors (additional features) Enhancement of robustness for containment system ( Extension of capability of containment system ) Off-site activity Level 5 : Mitigation of radiological consequences of significant releases of radioactive materials * In the SDC Phase1 Report Slide 8

SDC Appendix (B): Guide to Design Extension Conditions Identification of design extension conditions in SFRs The design measures are those for» prevention of core damage and» mitigation to ensure containment function Need to consider time margin to core damage» Short time - additional rapid safety measures for DEC (in addition to those for DBA) must be provided.» Long time - various options can be provided/prepared, e.g. additional safety measures for DEC, and/or Accident management procedures to recover the safety systems for DBA * In the SDC Phase1 Report Slide 9

SDC Appendix (B): Guide to Design Extension Conditions Postulated Design Extension Conditions for SFRs Information from safety studies based on deterministic and probabilistic approaches Severe accident sequences - two major types» Anticipated Transient Without Scram (ATWS) -- Rapid accident sequences Unprotected Loss-of-Flow (ULOF), Unprotected Transient Overpower (UTOP), Unprotected Loss of Heat Sink (ULOHS)» Loss Of Heat Removal System (LOHRS) -- Long-term accident sequences Loss Of Reactor Level (LORL) Loss Of Heat Sink (LOHS) * In the SDC Phase1 Report Slide 10

Ex-Vessel Events (Containment Response) In Vessel Events DEC Events Sequences ATWS LOCAL FAULT LOHRS LOHRS ULOF UTOP ULOHS LF TIB LOHS LORL ~min Plant Response Phase Reactor Vessel Failure or leakage from loop ~hr~10hr Plant Response Phase Reactor Vessel Failure or coolant boil off ~min Initiating Phase Transition Phase R R R:recriticality, or prompt criticality Post Disassembly Expansion R Core uncovery ~10min ~hr Post Accident Material Relocation (PAMR) Post Accident Heat Removal (PAHR) Structural Response / Sodium Ejection Protected Melt Dow n ~hr T:thermal influence T Reactor Vessel Failure M M:mechanical influence M/T Na-concrete reaction / Hydrogen burning Debris-concrete interaction / Hydrogen burning Re-criticality Na spray fire Fuel-Coolant interaction ~hr~10hr M/T M/T M M M Containment vessel response M/T Radioactive materials Transport Containment Vessel Failure *Discussed in GIF SDC-TF Slide 11

Design requirements for DEC ATWS Postulated from off-normal in-balance between power and cooling without scram» i.e. active reactor shutdown failure, or coolant temperature increase without scram. Proceed the accident to core disruptive accident (i.e. significant mechanical energy release derived from prompt criticality) should be practically eliminated Measures for core damage prevention» Passive or inherent reactor shutdown Measures for core damage mitigation» Retention and cooling of degraded core materials» Prevention of significant energy release in the course of core disruptive accident Slide 12

Design requirements for DEC LORL [Loss Of Reactor coolant Level] Core uncoverage should be practically eliminated by design measures Reactor Vessel (RV) & Guard Vessel (GV) to prevent double leakage» Prevention of dependent failure of RV and GV GV should withstand mechanical loads from earthquakes whilst retaining leaked sodium for a long time. GV should withstand any interference with a failed RV (even considering thermal expansion, vibration by earthquake and other aspects).» Prevention of common cause failure Separate the support structures of the RV and GV to the extent practicable, or prevent the failure of common parts of the support structures. Prevent common cause defects in manufacturing Ensure sufficient margins against earthquakes Slide 13

Design requirements for DEC LOHRS [Loss of Heat Removal System] Reactor vessel melt through should be practically eliminated Design measures for enhancing/recovering core cooling capability, in order to prevent core damage and to prevent reactor coolant boundary failure due to overheating.» Extend the design of decay heat removal system which is designed for utilization for Design Basis Accidents,» And/or, Alternative cooling measures Slide 14

Application of SFR Safety Features Passive/Inherence System Reactor shutdown for accident termination» Magnetic, Hydraulic, Gas Expansion Decay heat removal [DHR] by only Natural Circulation and with final heat sink of air» DHR systems in RV, Primary-/Secondary-circuits In-Vessel Retention Ensuring long term coolability of the core materials inside the reactor vessel SFR has favorable characteristics for achieving IVR» i.e., low pressure, high temperature boiling point, capability of natural circulation decay heat removal Slide 15

Summary ATWS ULOF; Unprotected Loss Of Flow UTOP; Unprotected Transient Over Power ULOHS; Unprotected Loss Of Heat Sink Time margin leading to core damage is relatively small in comparison with LOHRS. Design measures for DEC: Rapid means] for passive or inherent reactor shutdown Means for long-term] for prevention of significant energy release and for retention and cooling of degraded core materials LOHRS LOHS; Loss Of Heat Sink LORL; Loss Of Reactor Level Time scale to core degradation is in the range of e.g. several-to-tens of hours Design measures for DEC: Means for long-term] to maintain coolant level in reactor vessel for avoiding significant core damage with melt. avoiding creep failure of primary coolant boundary (e.g. reactor vessel) Means for long-term] to maintain heat sink by design and to recover/add the heat sink path for emergency Slide 16