AREVA NP EUROTRANS WP1.5 Technical Meeting Task – ETD Safety approach Safety approach for XT-ADS: Deliverable 1.20 Lyon, October Sophie EHSTER

Task Safety approach for XT-ADS – October AREVA NP Contents  Progress in activities associated with task  Main safety objectives  Safety functions  "Dealt with" events  "Excluded" events  Conclusion and discussion

Task Safety approach for XT-ADS – October AREVA NP Progress in activities associated with task  Task 1.5.1: Safety approach  Coordination: FANP (AREVA NP)  Participants: FZK, CEA, EA, SCK, KTH  Deliverables:  D1.20: Report on the approach and acceptance criteria for safety design of XT-ADS Meeting in May with designers and June regarding safety analyses Issued in summer 2006  D1.21: Report on the approach and acceptance criteria for safety design of EFIT First draft: To be issued by the end of October 2006  Participation to the safety studies (definition, assessment of results + Design check & review/Safety)

Task Safety approach for XT-ADS – October AREVA NP Main safety objectives  Application of defense in depth principle: prevention and mitigation of severe core damage  Elimination of the necessity of off site emergency response (Generation IV objective)  Probabilistic design targets:  Cumulative severe core damage frequency: per reactor year for is a minimum objective (due to lack of experience feedback) Enhancement of prevention assessed with ALARP  Severe core damage is studied as a Design Extension Condition  Severe core damage situations which cannot be mitigated: they must result from a limited number of sequences for which a higher level of prevention is required. Their exclusion has to be justified: they have to be "practically eliminated" (i.e. implementation of adequate prevention provisions)

Task Safety approach for XT-ADS – October AREVA NP Safety functions  Reactivity control function:  Definition of sub-criticality level (from WP1.2 and checked further by WP1.5): Consideration of most defavorable core configuration (possible adaptation) Consideration of reactivity insertion:Keff to be justified through reactivity insertion studies Consideration of hot to cold state transient Consideration of uncertainties Consideration of experimental devices  XT-ADS assumption: Use of aborber rods (design in WP1.2): during shutdown conditions to be moved preferentially by dedicated mechanisms (in case of critical core configuration)  Measurement of sub-criticality level To be performed before start-up with accelerator, target and absorbers inserted

Task Safety approach for XT-ADS – October AREVA NP Safety functions  Power control function:  Power control by the accelerator  Proton beam must be shut down in case of abnormal variation of core parameters, in particular in case of failure of heat removal means  High reliable proton beam trip is requested: at least 2 a LOD (seems achievable with 2 independent and diverse I&C) to prevent "excluded" situations, 2a+b LOD are requested: b must be diversified (passive devices (target coupling) and operator action (large grace time needed))  Implementation of core instrumentation: Neutron flux Temperature at core outlet (each fuel assembly if efficient for flow blockage) DND (very efficient in the detection of local accidents for SFR) Flowrate  Implementation of target instrumentation

Task Safety approach for XT-ADS – October AREVA NP Safety functions  Decay heat removal function:  Performed by SCS: Primary Heat exchangers (PHX): forced and natural convection ECS: Emergency Heat Exchangers (EHX): natural convection  A high reliability of the function is requested e.g. number of systems, redundancy, diversity, duty of the cavity walls cooling system Consideration of common modes (e.g. freezing, corrosion) to be prevented by design Definition of safe shutdown state/mission duration  Emergency core unloading (yes: independent core storage within the vessel)  DHR function needs optimization  Review of systems and architecture (ECS, RVACS, SCS):  MYRRHA draft2 :ECS trains unsufficient to reach reliability targets (EFR: 3 trains with diversification, PDS-XADS LBE concept)  need to be confirmed by a reliability study ?  Feedback from transient studies: Dhmini (2m), Dpmaxi (<1 bar)  Design optimization to meet performances underway

Task Safety approach for XT-ADS – October AREVA NP Safety functions  Confinement function:  Performed by three barriers Fuel cladding Reactor vessel and reactor roof Reactor building  Design must accommodate The radiological releases The pressure if any (cooling system lekage)  Specific issues: Coupling of the reactor, spallation target and the accelerator needs to be assessed Generation of polonium 210 due to the activation of bismuth under irradiation =>In the current Draft 2 design, the reactor hall is oxygen free and with a slight overpressure/ atmospheric conditions to avoid oxygen intrusion. Within the primary system, the cover gas pressure is below the pressure hall to avoid contamination of the reactor hall area. =>If all the reactor building is in overpressure, control of release to atmosphere will not be possible and a double shell building would be required. It therefore recommended to limit the overpressure zone to the above roof and components maintenance/storage area within an underpressure building.

Task Safety approach for XT-ADS – October AREVA NP Safety functions  Core support function:  Performed by The reactor internals The reactor vessel and its supports  Exclusion of large failure? Is the demonstration credible? Checking of the capability of severe core damage mitigation provisions on this scenario  Specific issues: ISIR of in-vessel structures under a metal coolant. For core support line, favourable option taken for MYRRHA. Possibility of storage of a full core and removal of core support barrel for inspection outside the reactor. Case of vessel and internal storage damage?) Consideration of oxide formation (design, monitoring, mitigation provisions)

Task Safety approach for XT-ADS – October AREVA NP "Dealt with" events  "Dealt with" events: their consequences are considered in the design  Initiating faults list has been established and a preliminary categorisation has been provided  Practical analysis rules have been proposed  A preliminary list of sequences to be analysed have been proposed  Radiological consequences: use of national method (i.e. Belgium)  Determination of barriers (e.g. fuel, cladding, structures) criteria: qualitative criteria are defined. The definition of quantitative values is underway. They have to be confirmed by R&D about the knowledge of material behaviour for higher temperatures.

Task Safety approach for XT-ADS – October AREVA NP "Excluded" events  "Excluded" events: their consequences are not considered in the design  Their non consideration had to be justified  Preliminary list:  Large reactivity insertions  Core support failure  Complete loss of proton beam trip function  Complete loss of decay heat removal function

Task Safety approach for XT-ADS – October AREVA NP Conclusion and discussion  For XT-ADS, safety objectives with regard to design and analyses are established in D1.20  Feedback on their consideration in the design?  Feedback on their consideration in the analyses?