Nuclear fission

3.4SUMMARY FOR L2 PSA FOR SPENT FUEL POOLS

The ASAMPSA2 [1], [2], [3] guidelines provide the best practice guidelines for the performance and application of L2 PSA development for the Gen II PWR, Gen II BWR L2 PSAs and extension to Gen III and Gen IV reactors, however discussion on SFP guidance is not included in the scope of ASAMPSA2, so the SFP L2 PSA discussion is complemented in this report.
In the past, the SFP has not been considered with a high safety risk for operating plants. Studies, such as the one conducted by Idaho National Engineering Laboratory in 1996 [12], generally showed that the frequency for an accident involving the SFP was low compared to the contribution of the core to the fuel damage frequency.

Nevertheless, the anxiety during the Fukushima Dai-chi accident for the SFP N°4 was extremely high and has increased the interest of the nuclear safety community for the SFP issues.

There are some challenges in considering SFP PSA, for instance reactor-SFP interactions, radioactive and hydrogen release, shared support system between reactor and SFP, maintaining SFP cooling and human actions/responses in these scenarios.
Table 3.4. contains a list of the issues which have been compiled within deliverable [D40.3] [4]. This deliverable was some kind of road map to be followed in the subsequent ASAMPSA_E activities. It can be seen that the issue list is covered to a large extent by the assessments and statements given in the previous sections.

Table 3.4. Specific L2 PSA issues for spent fuel pool and associated guidance suggestions

#	Specific L2 PSA issue for spent fuel pool	Suggestion for improvement of guidance in ASAMPSA_E
	Fuel degradation process, including energy and fission product release from melting spent fuel into containment	There is concern about the impact of air on the fuel degradation process. However, this may not be relevant for loss of heat removal scenarios. Several analyses performed with MELCOR show that the previous evaporation of the large amount of water from the SFP would almost completely generate a steam atmosphere with little air having access to the degrading fuel. It is recommended to further substantiate this statement by performing additional analyses. During fuel degradation in the SFP (before MCCI begins) the fuel temperatures in some of the sequences are lower than in RPV accidents during normal operation. Therefore less radionuclide are released initially. However, after MCCI has started, the release fractions from fuel reach levels which are known from accidents in the RPV.
	Hydrogen generation in spent fuel pool and its distribution in containment	As mentioned earlier for loss of heat removal sequences, above the SFP there is a steam atmosphere with little air having access to the degrading fuel. Consequently, hydrogen generation by steam in a melting SFP is an issue. In addition, large amounts of hydrogen will be generated when concrete erosion occurs. Hydrogen generated in a SFP inside the containment is in principle covered by the arrangements foreseen for core melt accidents. If the SFP is located outside the containment in the reactor building or in specific buildings, in general no provisions against hydrogen challenge are available. Consequently, a significant risk of deflagration or even detonation exists. Furthermore, there are less reliable barriers between the SFP and the environment. Altogether, there is a high probability for catastrophic releases if a SFP outside the containment begins to melt.
	Heat load from the melting spent fuel to structures above (e.g. to the containment roof)	Several analyses show that the heat load from the SFP upwards to structures above (containment dome, or roof of reactor hall) is significant. Analytical models should include thermal radiation and apply a suitable nodalization to model convection. Consequences of the high thermal load should be considered (e.g. reduction of containment pressure bearing capacity, impact of hot gas on venting system, induced fires).
	Release pathway for radionuclides from degrading spent fuel to environment	If the SFP is located inside the containment, the potential release paths to the environment are almost the same as for core melt accidents in the RPV. If the SFP is located outside the containment, the potential release paths to the environment depend very much on plant specific properties, e.g. ventilation systems, building doors, roof under thermal impact, size of rooms on the path etc. In any case the impact of very hot gas and of hydrogen has to be considered.
	Concurrent accident progression in spent fuel pool and reactor system	Fuel melt occurs only if the plant status is in severe disorder. It seems difficult to prove that not both the reactor and the SFP would be affected by such disorder. This is especially the case for external hazards. There are a large number of analyses for various containments to cope with the consequences of core melt accidents. However, additional loadings due to SFP steam generation and melting processes will add an additional challenge for containments which house the SPF. This could be considered as a cliff-edge effect for containment performance. It is conceivable that melt-through of the SFP bottom or wall could affect systems and components which are important for reactor safety, e.g. molten material from the SFP could enter the sump and damage ECCS components.
	Core concrete interactions for spent fuel pool accidents	The melt level in the SFP can become rather thick. Such a thick melt layer would probably develop convection patterns which predominantly transfer the heat to the upper edge of the melt. In addition, a metal layer could float on top of the melt and also create local high lateral heat fluxes. On the other hand, vigorous bubbling due to fuel-concrete interaction would tend to equalize heat fluxes. In summary, it has to be taken into account that local peak heat fluxes at the upper edge of the melt pool in the SFP can exist.
	Criticality	A qualitative analysis can be performed to demonstrate that SFP criticality is not likely in case of PWR spent fuel pool as it has sufficient fixed neutron absorber plates to mitigate any reactivity increase.
	Safety assessment of spent fuel pool during decommissioning	All phenomena of SFP accidents which are relevant in operating reactors are relevant for the decommissioning phase as well. An interesting additional issue still to be solved is whether after a certain extended time the decay heat is so low that even without water no significant fuel damage and radioactive release would occur.