A joint OECD/NEA – SARNET workshop on In-vessel Coolability was organised at the NEA (Nuclear Energy Agency) Headquarter (Issy-les-Moulineaux, France) in 12-14 October 2009. ON SARNET side, the preparation and the organisation were shared by KIT, IRSN, PSI and GRS representatives. Even though Severe Accident Management Guidelines (SAMGs) increase focus on containment integrity after some progression in the course of a severe accident, trying to cool the degrading fuel and/or the corium within the Reactor Pressure Vessel (RPV) is a way to slowdown or stop the progression of an accident.
The workshop was organised in 4 technical sessions:
general studies, experimental work, phenomenological and modelling work, and specific reactor studies. Twenty-two papers were presented, authors being members of research organisations, industry and technical safety organisations. Sixty-six people attended the workshop, coming from Belgium, Bulgaria, Canada, Czech Republic, Finland, France, Germany, Hungary, Italy, Korea, Slovak Republic, Spain, Sweden, Switzerland, United Kingdom, United States and OECD/NEA Secretariat.
As a result of discussions, there was a general agreement on the importance of the In-vessel coolability issue. The likelihood to stop the progression of a core melt-down accident by water injection is generally considered as high in the early phase of core degradation and depends on reactor specific features, nevertheless, even in later stages e.g. during the corium relocation in the lower head, cooling still can be achieved but depends on reactor specific features and the accident scenario.
Ongoing, starting and planned experimental programmes address the coolability issues in the different relevant configurations, i.e. reflooding of bundles, debris beds, molten pools and RPV external cooling. The code developments are promisingly directed towards a more mechanistic approach using a porous medium modelling to treat the different configurations of a degraded core. Nevertheless, the models to describe adequately the relocation of parts of the molten core to the lower head and the debris bed formation still need further development and validation against the results of ongoing experimental programmes. The transposition of results to the reactor scale where multi-dimensional effects are expected needs to be evaluated, all the more as larger scale experiments are probably not feasible.
Another way to cope with the uncertainties is to implement specific engineered features and/or management procedures to act on influential parameters such as an increase of the available water mass flow rate. Specific examples were given: good prospects for external RPV cooling for VVER- 440/213 reactors; use of spray found efficient for Sizewell B PWR; potential of Control Rods Guide Tubes flow to cool molten pools in BWRs.
The following recommendations were given at the final panel discussion: although it was concluded that the present efforts to solve the issue are welloriented, feedback experience from the analysis of safety cases of Nuclear Power Plants having, planning and/or contemplating the implementation of specific engineered features to solve this issue would be of great benefit. It is expected that ongoing experimental programmes and analytical efforts will help making progress in the coming years. Finally, the suggestion of organising a followup workshop on in-vessel coolability was given.
Contact: Walter Tromm