Long-term management and reliability of reactor core and containment cooling systems

Reactor core and containment cooling systems, Long-term management and reliability (RCCS-2021), 18-20 October 2021

Material degradation under the combined effects of temperature, pressure and the chemical environment following an accident at a nuclear power plant may lead to cooling systems failure and leakage outside containment. Based on the lessons learnt from the Three Mile Island, Chernobyl and Fukushima Daiichi nuclear accidents, efforts continue to build further knowledge on the behaviour of material, components and systems that are critical for maintaining cooling of the reactor core and containment in the long term after an accident.

To identify remaining knowledge gaps in this field, the NEA Working Group on Analysis and Management of Accidents (WGAMA) organised a specialist workshop entitled “Reactor core and containment cooling systems: Long-term management and reliability (RCCS-2021)”. Co-hosted by the French Institute for Radiological Protection and Nuclear Safety (IRSN) and VÚEZ, the workshop was held virtually on 18-20 October 2021 with nearly 150 specialists from 28 countries participating.

Building on the outcomes of previous NEA activities on Post-Fukushima safety research and the long-term management of a severe accident in a nuclear power plant, the workshop provided a comprehensive survey of the existing safety standards and rules related to cooling reliability in the long term. It addressed management strategies for core and containment cooling in different reactor designs, as well as the differences and similarities in this respect observed in design-basis accidents vs beyond-design-basis events.

Cooling systems can become impaired in the aftermath of nuclear accidents due to clogging issues resulting from debris and suspensions or dirt accumulation that are by-products of material and components degradation. The robustness of cooling systems can also be challenged by erosion corrosion on piping and other sensitive reactor components.

As severe accident chemical conditions are extremely complex to represent in detail, a key question is how to prioritise R&D efforts to investigate possible chemical effects on long-term cooling after a severe accident. Gaps in this field can be addressed with additional experimental research or by consolidating existing assessment methods and tools.

Regarding the issue of sump clogging, a remaining challenge is the scaling of the experiments and the definition of the experimental conditions (e.g. debris size). Chemical conditions in cooling waters, debris formation and long-term behaviour appear challenging and further assessment of the long-term management of an accident site would be needed to prioritise the needed R&D in the field. A Phenomena Identification and Ranking Table could help identify the remaining R&D required on those topics. Furthermore, sharing best practices and experience would be useful to establish guidance and recommendations; this is where the co-operation within the WGAMA would be most beneficial.

See also