Modelling and Simulation of Fission Product Release from Melt Pools

Abstract

During the early phase of a postulated severe reactor accident, predominantly volatile fission products (FP) are released into the cooling circuit. With progression of the accident, the FP move faster in the melting core material than in the crystal structures of the solid fuel, which also allows the release of less volatile FP. In addition, volatile species can be formed by certain low volatile FP in oxidizing or reducing conditions, which may significantly influence their release rates. An adequate simulation of the release of such FP species is therefore necessary to provide reliable initial conditions for source term analyses.

For the representation of the FP release from molten core material in the lower plenum, a new in-house model based on the diffusion equation is being developed, which is intended to be prospectively integrated into the system code AC² model basis developed by the German Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH. The thermodynamic quantities of the multicomponent system are described in an idealized way. To consider the varying volatilities of different FP species in different atmospheric conditions, the Antoine equation is applied. Convective mass transfer is considered by an approach based on the analogy of heat and mass transfer. Verification analyses are being conducted by calculations of experiments of the Late Phase Source Term Phenomena (LPP) project (5th euratom framework program). Preliminary results show that the release behaviour of relevant FP is plausibly represented by the new model.