Nuclear fission

4.3KNOWLEDGE GAPS AND FUTURE NEEDS

Even today’s advanced L2 PSA and the related research encounter some important knowledge gaps. The following topics belong to this group where research is needed to improve the L2 PSA quality.
Fuel coolant interaction

In Water-cooled reactors, the liquid corium might come into contact with water at several occasions, during melt relocation phases, or reflooding. These phases are potentially marked by strong melt cooling through fragmentation, hydrogen production and produce new melt configurations. Additionally, under certain circumstances, FCI might lead to a so-called steam explosion.

Despite continuous progress, in particular through international collaboration, e.g. the OECD SERENA-2 program and work done in SARNET-2 network project [137], there are still a number of knowledge gaps on both understanding and modelling, particularly for the frame of PSAs where simple models are preferable.

Due to the fact that attention on steam explosion has been focused on the ex-vessel situation, e.g. if IVR strategy comes to fail, little attention has been put on the phase of relocation of melt in the lower head. Although there is no formal proof, it is admitted that a steam explosion is unlikely in this situation and that the structures will probably withstand an associated power peak. Nevertheless, when it is taken into account in models, the mixing itself does not take into account the oxidation and hydrogen production. Also, the particular and still unclear melt flow conditions themselves render the existing evaluations very uncertain.

The ex-vessel situation is of high interest for PWRs with failed IVR strategies and BWRs, particularly those with large pits. It was concluded from recent international coolaborations that a reasonable achievement of understanding and modelling has been reached for the following simplified configuration:

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  2D axisymmetric,
  
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  UO2/ZrO2 melts,
  
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  central gravity driven melt injection.
  

The application to more realistic situations is then regognized as uncertain. In particular, a probable situation is the one of vessel failure during inversion of melt stratification in the vessel. This would lead to the release at first of highly superheated metal, a situation which has not been investigated experimentally with sufficient attention.

It is noticed that steam explosion is the current subject of an OECD Technical Opinion Paper, which should be released by 2017.

Debris bed coolability.

The coolability of a corium debris bed is currently the subject of numerous investigations and a recent review can be found in [138]. The CFD codes have shown their overall capacity in modelling the phenomenon. Nevertheless, the applicability to PSA studies is quite complex due to the large number of potential geometrical configurations.

Moreover, for cases where corium falls into water, a directly coolable debris bed seems difficult to be created, unless with very large pits and small jet diameter. In a large number of situations, the fragmentation may not be complete and the melt could partly spread on the concrete or along the vessel. Such intermediate situations have not been investigated with sufficient details.
Steam explosion triggering during melt spreading.

Recent experiments conducted by KTH in the SES-PULIMS installation showed that, under certain circumstances that are to be clarified, melt spreding under water could lead to strong explosion triggering. This is in contradiction with previous conclusions on the stratified steam explosion. The mechanisms in these recent experiments needs clarification: the visualizations indicate that the spreading occurs with a very unstable melt interface. More research is needed for application to L2 PSA or reactor safety studies.

Molten corium-concrete interaction

The general comprehension of MCCI phenomena and of mechanism for corium stabilization has progressed significantly, even if some gaps are still identified (see the future OECD MCCI SOAR or [136]), for example on:

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  the effects of presence of metal within the melt or within the concrete (steel rebars);
  
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  the initial conditions for MCCI based on melt pour conditions into the reactor pit;
  
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  the impact of presence of impurities in cooling water (e.g., seawater or brackish water).
  

In the context of L2 PSA, the importance of uncertainties has be determined specifically for each plant design and should not be addressed in general.
Releases into the waters and ground

The ASAMPSA_E May 2014 meeting participants noted (among other issues) that most of L2 PSA exclusively addresses releases into the atmosphere. Quantitative analyses of releases into water (river, lake, sea – see the Fukushima Dai-chi experience) were considered as missing. This is rooted in historic developments which concentrated on (immediate) health effects, and which seem to be less significant for water and ground releases. Nevertheless the consequences of such releases may be very significant.

Therefore, the related source term characteristics should be explored by L2 PSA. The WP40 partners note that relevant research and guidance in this field is missing.
Long-time effects inside plants

The ASAMPSA_E May 2014 meeting participants noted (among other issues) that long time effects – in particular related to the long term resilience of containments against fuel degradation accidents – should be addressed by L2 PSA. There may be some activities going on in this field, but the state of the art seems unfit for producing guidance.

Iodine and Ruthenium chemistry

L2 PSA typically considers iodine releases to the environment in the chemical form of CsI. However, in the presence of intense radiation fields, as would be expected in severe accident conditions, complex iodine chemistry can develop over time resulting in the formation of additional gaseous molecular iodine (I2) via a number of routes, as well as stimulating other reactions with containment surfaces and aerosols that consume the gaseous iodine. In long-term accident sequences which do not develop early catastrophic source terms, the release of various iodine compounds may dominate over the CsI release.

With regard to Ruthenium, the amount of ruthenium produced by nuclear fission is important and increases with the fuel burn-up. Ruthenium has a high specific activity and high radio-toxicity compared to the other released fission products. The formation of volatile Ruthenium compounds is a significant concern.

Much basic research has been done in the radiochemistry field, but the existing models are not yet suitable for routine application in L2 PSA. Therefore, guidance is needed how to introduce the existing information in L2 PSA, and practically usable methods should be developed.

Source term R&D programmes conducted in the last two decades have shown that iodine oxide particles, gaseous organic iodides and gaseous ruthenium tetroxide may contribute significantly to the environment source term in case of venting. The filtration efficiency review and update of the filtered containment venting systems is the scope in European ongoing projects (MIRE and PASSAM). Furthermore the potential revolatization of the various deposited iodine and ruthenium species has to be further assessed for conditions representative of a SA.

Combustible gases outside the containment

Hydrogen and carbon monoxide issues within the containment are routinely taken into account in PSA. However, related issues outside the containment seem to require more attention. As an example, still today there is no conclusive interpretation of the combustion events in the Fukushima Dai-chi accident sequences, notably within block four. It seems that PSA need to focus more on the related issues. The following topics belong to that field:

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  distribution and transport of combustible gas in containment venting systems, in particular connected to steam condensation processes;
  
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  leak of combustible gases out of the containment into adjacent rooms, and related distribution of these gases;
  
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  distribution and transport in ventilation systems, taking into account the disturbed plant conditions after core melt;
  
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  probabilities of ignition for potentially ignitable atmosphere in different parts of the disturbed plant.
  

Detailed CFD models or lumped-parameter containment models may in principle be available for precise evaluations, but given the multitude of potential accident sequences, their routine application in PSA is not practical. Additional guidance seems to be needed for adequately addressing these issues.

Treatment of uncertainties

Assessment of uncertainties should provide among other things a measure of the confidence that the results provided by PSA represent “real life” (what used to be called “robustness of results”). If the confidence is found to be low, the uncertainty analysis in L2 PSA shall provide information on the possible deviation in accident progression on the NPP and impact on the accident consequences.

The IAEA [10] provides a discussion of the sources of uncertainties (and some methodologies applied to Design Basis Accidents safety demonstrations that can be extrapolated also to severe accidents), and the USNRC [16] provides some guidance on the treatment of uncertainties for decision making. The ASAMPSA2 guidelines [2] in Vol. 2 provide discussions on this subject.
Solutions to this issue with respect to L2 PSA have been investigated within the EU project BEEJT (Benchmark Exercise on Expert Judgment Techniques) summarized in [135]. However, several sources of uncertainties cannot be easily addressed or quantified (see [87]). Some may actually be the biggest sources of uncertainties (Fukushima Dai-chi may be the best example in terms of modelling uncertainties and completeness). Nevertheless, advances in this area have not been forthcoming since issuance of the ASAMPSA2 guidelines; hence ASAMPSA_E needs not address or repeat what has been already discussed at length in the past in these areas.
Since the Fukushima accidents sometimes doubts were raised whether PSA truly represent the accidental risk of NPPs. In this discussion it seems prudent to distinguish between L1 PSA issues and L2 PSA issues which are subject of the present document. With regard to L2 PSA the available experience in TMI, Chernobyl and Fukushima is not at all surprising. If L2 PSA had been performed based on the status of these NPPs at the onset of core damage, L2 PSA would have probably provided results not far from the actual experience.