Improved models for the simulation of severe LWR accidents - processes during quenching and chemical interactions

In the Core Degradation Project, the contributions of the IKE mainly concerns the improvements and extensions of models, basic versions of which have been developed in the frame of the national BMBF - project KESS. In this project detailed models have been developed to simulate the main processes in the core during a severe accident in light water reactors. The first part of this report is focused on the interacting processes during quenching, like the embrittlement of the fuel rod cladding and of the refreezed melt, the oxidation of the cladding and the crust as well as the cooling effect due to the rapid vaporization. The improved and extended models have been implemented in the code system KESS and as a frist step of the validation the integral bundle experiment CORA-13 has been used. The second part of this report is directed to the chemical interaction between the fuel rod cladding and the Inconel grid spacer. Hereby, a basic diffusion model has been developed and applied to specific bundle conditions to take into account the time of failure of the grid spacer. (orig.)