NEA report considers long-term management of severe accidents

Long-Term Management and Actions for a Severe Accident in a Nuclear Power Plant has three objectives: to review the existing regulations and guidance, practices, technical bases and issues considered in NEA member countries regarding long-term management of a nuclear power plant; to identify, describe and discuss the main challenges and issues to be tackled; and, to propose recommendations and areas for future investigation to improve long-term management of a plant.

With all three accidents, there was limited knowledge of long-term management entry state (i.e. the status of the core and the plant at the entry to the long-term management phase), of risks to evolve to a new unstable situation, and of risks related to long-term management and actions implementation.

The three accidents have resulted in distinctive damaged fuel distributions and characteristics, even among the three damaged reactors at Fukushima Daiichi, the report says.

It suggests possible approaches to long-term management and actions considering diverse scenarios of severe accidents and used fuel storage pool accidents, including methods for issues identification and risks ranking. It also provides a summary and a discussion of the main identified challenges and open issues faced.

The main long-term management actions following a severe accident aim at: evaluating the plant damaged state from a physical and radiological standpoint; maintaining a stabilised and controlled state of the damaged plant from a safety perspective; implementing provisions against further failures; cleaning up and decontaminating; managing accident wastes (conditioning, treatment, packaging and storage); preparing and achieving fuel and debris retrieval; and protecting plant personnel from exposure.

The report's primary focus is on-site long-term management and actions. It does not consider off-site long-term management and actions, nor radiation protection, waste disposal and decommissioning, which have been investigated in other NEA reports.

Based on a questionnaire that was circulated among NEA member countries it was concluded that most do not have specific regulations for the long-term phase of a severe accident. Instead it is commonly considered to be covered by existing regulation and severe accident management guidelines.

Recommendations

The NEA proposes recommendations and areas for future investigation to enhance long-term management of a nuclear power plant as regards necessary knowledge and provisions development, particularly for the optimisation of management of contaminated cooling waters.

It says knowledge should be developed or consolidated for: calculation tools and methods for analysis of severe accidents at reactors and used fuel pools; ascertaining the status of components, equipment, systems, including passive ones, and structures after a severe accident with emphasis on those that contribute to maintaining a stabilised state on the long-term; long-term phenomena that can affect the long-term management of the plant - such as corrosion-erosion reactions, fuel "dusting" and dispersion; and methods or expert systems for risk assessment for long-term management and actions optimisation.

Plant operators should also provide for: monitoring of the plant damaged state and its evolution; upgrading equipment, components, systems and structures for long-term management; and developing harmonised practices and technical means to limit workers' occupational exposure in long-term management.

"Common between the three accidents reviewed are the serious challenges associated with handling of contaminated and leaking cooling water," the NEA says.

It therefore also recommends that provisions are developed for the optimisation of management of cooling waters to facilitate long-term management. These include implementing closed-loop cooling as early as possible during the emergency phase.

It says strategies for flooding and cooling the corium should avoid transfer of contaminated waters outside the confinement. Also, the use of water with controlled chemistry should be further studied with respect to limitation of risk for re-criticality, fission products remobilisation, corrosion, clogging, and for facilitation of water management in the long term.