Investigation of criticality of different debris bed configurations in case of a severe accident in LWRs - KEK. Final report

The general aim of this project was to evaluate the criticality potential of debris beds after severe accidents (SA) in Light Water Reactors (LWRs). Debris beds are formed after the corium solidification and may be located at the bottom of the reactor vessel or at the reactor cavity in more severe cases. Both scenarios should be investigated since they are linked to specific phenomena and the debris characteristics may vary significantly. For this purpose, it is necessary a computing tool able to simulate the reactivity behaviour of the debris bed taking into account the thermohydraulic and thermomechanical conditions. The neutronic calculations were performed with Monte Carlo codes (MCNP, Serpent) that simulate the neutron transport; the thermohydraulic characteristics of the debris were simulated with COCOMO-3D code. For the computation of criticality in debris beds, it is first of all necessary to find an adequate neutronic model of the system. Conservative assumptions and simplifications should be applied in order to overcome the high uncertainties regarding the debris conditions and to achieve a feasible model from the computational point of view. The influence of the particle shape, particle size distribution and particle arrangement on the neutron multiplication factor was investigated as well as the impact of possible simplifications. In order to assess the adequacy of the simplified models and their applicability ranges, a near-to-reality model (reference model) was developed, whose higher fidelity results were used as a reference. Taking the debris beds in Fukushima as reference, some conservative neutronic calculations were performed and safety parameter ranges, i.e. zones where the recriticality can be totally excluded, were identified. Parameters as debris porosity, fuel composition (enrichment and burnup), presence of structural materials, water void fraction, water boration and debris size were considered for the sensitivity calculations. Additionally, a more realistic case was assessed for a typical ex-vessel debris bed. The tool for uncertainty and sensitivity analyses SUSA was used to identify those parameters with a highest influence on the neutron multiplication factor. This will allow in the future to focus on the relevant cases optimizing the computing efforts. The thermohydraulic and neutronic analyses were limited to stationary calculations. This implies that only subcritical configurations can be considered for the coupled calculations. For these cases and considering the Fukushima debris bed conditions, the feedback effect of the neutronics on the thermohydraulics was estimated and the necessity of further source term as well as coupled analyses was assessed.