Nuclear fission

4.4SUMMARY FOR L2 PSA CONSIDERING RECENT R&D

The present section concentrates on fields of research which affect L2 PSA. In principle, the present section is updating the pertinent ASAMPSA2 documentation [1], [2], [3]. Recent R&D and the ongoing research with relevance on extended L2 PSA are evaluated. The survey concentrated on ASAMPSA_E, SARNET (Severe Accident Research Network), SARNET-2, OECD and European projects (public results only), NUGENIA roadmap and ASAMPSA2.
The following Table 4.4. summarizes some R&D activities which are considered relevant for taking them into account in the discussion of further L2 PSA guidance. This list is preliminary, and since it is at least partly covering projects which are not yet finalized, no references are given. Where appropriate, the relevant leading organization(s) responsible for the topic is provided. Most of the issues mentioned below have links to one of the previous sections in the present document.
Table 4.4. Recent R&D related to L2 PSA and associated guidance suggestions for extended PSA

#	Recent R&D issues related to L2 PSA	Suggestion for improvement of guidance in ASAMPSA_E
	Analysis of fuel melt process in spent fuel pool with integral code (e.g. MELCOR)	There is no specific guidance for fuel melting in spent fuel pools; however existing guidance for core melting is largely applicable. Heat transfer from SFP to structures above and large scale MCCI need particular consideration, but no specific R&D or guidance is needed on this topic (see section 3.3.6).
	Deterministic and probabilistic analysis of accidents caused by external hazards in full power state	There is no specific guidance for accident analysis caused by external hazards; however existing guidance for internal initiators is generally applicable. According to D40.4 [50] L2 PSA for external events needs no specific guidance.
	Conditional probability of containment loss of tightness after an earthquake	Seismic fragility of the Containment Vessel for overall structure failure and local failure can be evaluated based on the seismic design where response analyses for design basis earthquakes are conducted. In the context of an extended PSA also internal and external hazards could be taken into account which may affect the possibility to close the containment. Definition of mechanical criteria for loss of tightness is beyond the common practices of mechanical and seismic engineering practices. The definition of mechanical criteria for loss of containment tightness is not elaborated in this report. This issue shall be addressed in L1 PSA. Regarding L2 PSA, this issue is already discussed in D40.4 [50].
	Source term assessment for L2 PSA	Source term research remains of high priority for evaluation and reduction of radioactive releases during accidents in NPP. Despite the recent achievement of major experimental programs and significant advances in understanding of source term issues, additional research is still required. A short synthesis of acquired knowledge and remaining gaps, e.g. fission product release, its behaviour focusing on iodine and its chemistry is provided in this report. On-going research programs (STEM2, BIP3, THAI3, PASSAM) and the implementation of their outcomes in Severe Accident codes are expected by 2020.
	Analysis of the complexity of severe accident phenomenology by code simulation, ASTEC and MELCOR	Existing guidance for L2 PSA is generally applicable. In the framework of 2013-2015 MELCOR development different tasks, i.e. mechanistic fan cooler model, new debris cooling models in the CAV package (water-ingression and melt eruption through crust) is completed. Other model development is in progress, i.e. CONTAIN/LMR model for liquid metals reactors, multiple fuel rod types in a COR cell.
	Analysis of heavy load drops into the SFP	There is no specific guidance. Existing guidance for core melting is generally applicable.
	Investigation of the IVR by external cooling of reactor vessel for VVER-1000 type reactors	Guidance is under development The European project IVMR (grant no. 662157) – H2020 is started with the main objective to review, from an analytical point of view, the possibility to retain the corium inside the vessel due to external cooling, for several kinds of reactors in Europe (existing or under project).
	Accident progression and possible off-site consequences	On-going research activities in FASTNET. Development of two tools is summarised in section 4.1.4.9. These tools (developed by LRC and GRS) facilitate diagnosis and decision-making by prognosis of source terms for nuclear emergency management.
	Spent Fuel Pool Rupture Characterization Based on Water Level Monitoring	There is no specific guidance on critically important instrumentation and measurements. In case of SFP LOCA the location and size of SFP rupture is critical. Therefore pertinent instrumentation availability deserves consideration in L2 PSA and related guidance.