Nuclear fission

4.5Safety Research areas identified in the NEA-SAREF-Project

The NEA/CSNI undertook the “Safety Research Opportunities Post-Fukushima” (SAREF) initiative to provide guidance on potential examinations of the damaged reactors at Fukushima Daiichi, with the twofold goal of addressing safety research gaps and of supporting Japan in achieving safe and timely decommissioning. The following sections essentially are an excerpt of the SAREF draft report which will probably be competed in 2017, after finalization of the present ASAMPSA_E document. Since the following text is based on a draft only, the reader is encouraged to address CSNI for obtaining the final document.

A group of senior experts in nuclear safety research as well as decommissioning was assembled to identify areas of common interest and recommend to the CSNI safety research activities including their priority. The group reviewed research areas relevant to three categories: 1) severe accident progression, 2) SSC performance and condition, and 3) the recovery phase of an accident – and included a fourth category – 4) general or already addressed. For each research area consideration was given to the international safety research interest, the decommissioning interest, potential examinations, challenges (e.g. cost, timing, dose, etc.), feasibility and ongoing R&D activities. A research area is identified as having high safety research interest if information that can be gained during Fukushima Daiichi decommissioning addresses significant knowledge gaps. If the knowledge gaps are not significant, or the information could be better gained another way, the safety research interest is only medium or low. For example, the safety research interest in information on hydrogen combustion is rated medium even though the combustion events at Fukushima Daiichi were the first for a nuclear plant during a severe accident. This rating was chosen because it is difficult to retroactively determine how the hydrogen behaved, and because the effects of the combustion events cannot be easily separated from other sources of pressurisation and damage.

Similarly, decommissioning interest in a research area is high if the information gained is necessary to support safe and timely decommissioning. For example, information on the survivability of instruments during and following the accident is of low interest to decommissioning as any malfunctioning equipment is either not required to ensure ongoing safety or can be replaced without determining how it survived the accident.
With these factors in mind, the Senior Expert Group on SAREF is recommending four long-term considerations. There are two long-term considerations on fuel debris retrieval and characterisation from inside and outside reactor pressure vessels that directly address the areas of in-vessel and ex-vessel phenomena. The ex-vessel proposal also addresses in part containment failure and venting. There is a long-term consideration to compile a database on fission product measurements that addresses fission product behaviour and source term. The last long-term consideration is based on opportunities for examinations of reactor components that may arise as decommissioning proceeds to address areas of high safety interest under mission time and system survivability. Recognising that these long-term considerations require additional information and development of decommissioning planning and processes, two near-term proposals are also recommended, which are of less significance from a PSA point of view.
Issues which have been associated with high safety research interest are:

In-vessel phenomena: knowledge gaps in BWR severe accident progression, corium formation and relocation within RPV, key to improve understanding and modelling of late severe accident progression.

Primary system and RPV failure: addresses gaps in reactor system, RPV or component failure mechanisms of BWRs; key to improve understanding and modelling of late SA progression.

Ex-vessel phenomena: knowledge gaps in melt relocation from RPV, in interaction with BWR typical ex-vessel structures, in molten core concrete interaction and melt coolability by water; is important to SAM strategies and mitigation technologies (e.g. melt flooding); key to improve understanding and modelling of ex-vessel severe accident progression; focus is on melt relocation and MCCI and not on fuel coolant interaction or direct containment heating phenomena.

Containment failure and venting: knowledge gaps in containment failure mechanisms under SA conditions and high containment pressure and temperature loads, could provide information to SAM and modelling

FP behaviour and source term: Supports public dose assessment, EQ, Reducing uncertainty in dose significant FPs.

Pool scrubbing: Fission product (FP) transport through the reactor coolant system and into the containment (especially through the suppression pool), as well as potential FP release from the containment, are of major importance when assessing the release of activity to the environment.

Cable and Sealing: Ability of containment seals and electrical systems to continue to perform its function as designed is key to SAM.

Instrumentation: Key instruments are required for SAM.

RCIC: RCIC system performance had a major influence on accident progression at Fukushima.

Relief valves and piping: Performance of valves and / or piping failure key to RPV depressurisation

These issues of high interest mentioned above are not specifically linked to PSA topics, but they contribute to the completeness and quality of PSA in general. One specific PSA topic has been assigned a medium research interest: External effects and multi-unit risk and loss of ultimate heat sink. According to SAREF, many countries have interest in estimating these issues.