Coolability of particulate beds in severe accidents: Status and remaining uncertainties

Particulate debris beds may form during different stages of a severe accident; e.g. in the degrading hot core, due to thermal stresses during reflooding, in the lower plenum, by melt flow from the core into water in the lower head, and in the cavity by melt flow ou of a failing RPV into a wet cavity. Deep water pools in the cavity are used in Nordic BWRs as an accident management measure aiming at particulate debris formation and coolability. Data from experiments included in the SARNET network (DEBRIS at IKE and STYX at VTT) and earlier ones (e.g. POMECO at KTH) have been used to validate key constitutive laws in 2D code as WABE (IKE) and ICARE/CATHARE (IRSN), especially concerning flow friction and heat transfer. Major questions concern the need of the explicit use of interfacial friction to adequately treat the various flow situations in a unified approach, as well as the adequate characterization of realistic debris composed of irregularly shaped particles of different sizes. Joint work has been supported by transfer of WABE to KTH and VTT. The codes have been applied to reactor conditions for analyzing the potential for coolability in the different phases of a severe accident.Calculations have been performed with WABE(MEWA) implemented in ATHLET-CD and with ICARE/CATHARE for degraded cores and debris beds in the lower plenum, under reflooding and boil-off. Ex-vessel situations have also been analysed. Strong effects of lateral water inflow and cooling by steam in hot areas have been demonstrated. Melt pool formation or coolability of particulate debris is a major issue concerning melt retention in the core and the lower head. Experimental and analysis efforts and respective continued joint actions are discussed, which are needed to reach resolution of the coolability issue.