Implementation of a cladding failure model for a Loss of Coolant Accident (LOCA)-analysis in Transuranus

To simulate the behaviour of the LWR-fuel rod in a LOCA-transient the entire lifetime of the fuel rod has to be considered. In the German fuel licensing process, the pre-transient life-time of the fuel rod is calculated with Transuranus and the transient part is analyzed by means of different codes that were developed specifically for LOCA conditions. Data transfer from Transuranus to the LOCA-specific code is limited to a few variables. Within these LOCA-specific codes the behaviour of the fuel e.g. fission gas release and thermal-mechanical properties are not treated or only considered in simplified models. In order to refine the calculation it was decided to use Transuranus for both, the pre-transient part and the subsequent LOCA-transient, in one consistent simulation. An additional advantage to integrate a LOCA-transient calculation in Transuranus is to reduce the data amount and efforts on data handling for full core analysis methods. To calculate the fuel rod behaviour during a LOCA with Transuranus one had to implement for the cladding materials Zircaloy and M5 a high temperature creep model, a high temperature corrosion model and a failure criterion model. The high temperature creep model considers the shift in phase transition temperatures and alpha-beta phase fraction due to heating rates. The creep and failure models for LOCA-conditions, the high temperature oxidation model as well as the material properties are fully integrated in the Transuranus code. Model change from the normal to the transient conditions is performed by using the Transuranus restart option. Several test runs have shown the numerical stability of the code. The ongoing validation process of the LOCA-creep models shows so far an excellent compliance with burst tests. (Author)