2. AREVA NP EUROTRANS WP1.5 Technical Meeting Task 1.5.1 – Safety approach Madrid, November 13-14 2007 Sophie EHSTER

3. Task 1.5.1 Safety approach for EFIT-He – November 13-14 200733 AREVA NP Contents  Task T1.5.1 progress  Main features of the gas-cooled EFIT safety approach  "Dealt with" events  "Excluded" events

4. Task 1.5.1 Safety approach for EFIT-He – November 13-14 200744 AREVA NP Task T1.5.1 Progress  D1.20:Approach and acceptance criteria for safety design of XT-ADS  Issued in August 2006  Update when design is confirmed  D1.21:Approach and acceptance criteria for safety design of EFIT  Issued in August 2007  Addresses generic ETD (LBE cooled)  T1.5.1 covers also gas-cooled EFIT back up option  Task to be performed in 2008  AREVA contribution to be included in gas-cooled EFIT specific report  Main features of the gas-cooled EFIT safety approach are provided hereafter

5. Task 1.5.1 Safety approach for EFIT-He – November 13-14 200755 AREVA NP Main features of gas-cooled EFIT safety approach-1  General safety objectives and principles are the same as the ones defined for lead-cooled EFIT, in particular:  Application of defense in depth principle: prevention and mitigation of severe core damage (i.e. large degradation of the core) are considered,  Severe core damage is considered since it reveals specific risks of the technology which have to be dealt with.  EFIT is provided with a core loaded with minor actinides:  Potential consequences of severe core damage are expected larger as the amount of minor actinides in the core increases (e.g., lower fraction of delayed neutrons, lower Doppler effect, lower critical mass).

6. Task 1.5.1 Safety approach for EFIT-He – November 13-14 200766 AREVA NP Main features of gas-cooled EFIT safety approach-2  Definition of the sub-criticality level: core shall remain sub- critical in any event. Concerning severe core damage, criticality could occur if consequences are demonstrated acceptable.  Prevention of severe core damage: sequences leading to severe core damage shall be extremely rare. The confidence in the prevention provisions must be very high. Issues/gas-cooled design:  Limited operational feedback available (HTR technology),  High reliability requested for beam trip system and DHR system (mainly active means),  Very short grace periods. Advantages/gas cooled design:  No significant void effect,  ISI facilitated by He environment.

7. Task 1.5.1 Safety approach for EFIT-He – November 13-14 200777 AREVA NP Main features of gas-cooled EFIT safety approach-3  Severe core damage mitigation: has to be considered. In particular, prompt criticality has to be excluded. This could lead to the limitation of content of minor actinides and/or to a lowering of the sub-criticality level. At the pre-conceptual design phase of EFIT, studies associated with severe core damage should focus on the determination of the main phenomena, relevant risks and possible mitigating provisions (core design and dedicated systems).  Regarding severe situations which cannot mitigated by plant design (not technically possible a reasonable cost or with an insufficient confidence level), a specific demonstration showing that the associated risk is acceptable has to be provided:  Practices similar as the ones implemented for future nuclear plants such as EPR at least can be considered,  If the situations are not physically impossible, their occurrence has to be made sufficiently rare in order to not consider the consequences of the situation in the design. This will mainly rely on the implementation of a sufficient number of diverse practical prevention provisions. The adequacy of provisions can be justified by probabilistic insights.

8. Task 1.5.1 Safety approach for EFIT-He – November 13-14 200788 AREVA NP Main features of gas-cooled EFIT safety approach-4  The demonstration that the objectives related to severe core damage prevention are met can be performed by means of Probabilistic Risk Assessment. The cumulative severe core damage frequency should be lower than 10 -6 per reactor year.  At the early stages of EFIT, the Line Of Defense (LOD) method can be used to provide adequate prevention of severe core damage : at least two "strong" lines of defense plus one "medium" LOD are requested for each sequence.  Unique EFIT safety issues have to be considered such as the potential radiological impact on the public due to minor actinides and spallation products and such as the protection of workers with respect to target and accelerator.

9. Task 1.5.1 Safety approach for EFIT-He – November 13-14 200799 AREVA NP "Dealt with" events  "Dealt with" events:  Their consequences are considered in the design  A list of initiating faults and associated sequences to be studied has been determined in PDS-XADS contract for a similar design (except for Power Conversion System: water-steam, super-critical CO2 cycles). This list has to be updated.  Preliminary list of limiting events: Bounding reactivity insertion: core compaction, large water ingress Total Instantaneous Blockage

10. Task 1.5.1 Safety approach for EFIT-He – November 13-14 200710 AREVA NP "Excluded" events  "Excluded" events: their consequences are not mitigated by design provisions  A list of severe situations beyond those considered in the design is established and the associated consequences are assessed. If the consequences cannot be made reasonably and confidently mitigated, the situation is “excluded” when it is:  Physically impossible  “Practically eliminated” by adequate design and operating prevention provisions  Preliminary list:  Large reactivity insertion due to: Core compaction capable to approach criticality Moderator effect capable to approach criticality (e.g., caused by steam ingress) Large fuel loading error  Core support failure due to challenges on internals, primary circuit and primary circuit support, in particular: Large load drop (e.g. during handling operations) Rotating machinery failure