ASTEC and ICARE/CATHARE modelling improvement for VVERs

ASTEC and ICARE/CATHARE computer codes, developed by IRSN (France) (the former with GRS, Germany), are used in RRC KI (Russia) for the analyses of accident transients on VVER-type NPPs. The latest versions of the codes were continuously improved and validated to provide a better understanding of the main processes during hypothetical severe accidents on VVERs.This paper describes modelling improvements for VVERs carried out recently in the ICARE common part of the above codes. These actions concern the important models of fuel rod cladding mechanical behaviour and oxidation in steam at high and very high temperatures. The existing models were improved basing on the experience in the field and latest literature data sources for Zr + 1%Nb material used for manufacture of VVERs fuel rod claddings.Best-fitted correlations for the Zr alloy oxidation through a broad temperature range were established, along with recommendations on model application in clad geometry and starvation conditions. A model for the creep velocity was chosen for the clad mechanical model and some cladding burst criteria were established as a function of temperature.After verification of modelling improvements on Separate Effect Tests, validation was carried out on integral bundle tests such as QUENCH, CODEX-CT, PARAMETER-SF (the application to the CORA-VVER experiments is not described in the present paper) and on the Paks-2 cleaning tank incident. The comparison of updated code results with experimental data demonstrated very good numerical predictions, which increases the level of code applicability to VVER-type materials.