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Extension of Transuranus Code Applicability with Niobium Containing Cladding Models

Funded under: FP5-EAECTP C


The EXTRA Project of the EURATOM 5th Framework Programme focused on the simulation of the behaviour of the nuclear fuel in Russian-design VVER reactors under accident conditions. The analysis of the state of the art showed that the available information and computer tools to predict the performance, especially the off-normal performance of the niobium containing cladding, had been scarce. The main objectives of the project were the systematisation of the knowledge on experimental data of the Zr1%Nb alloy, the incorporation of newly developed correlations into the TRANSURANUS fuel performance code, code validation computations and the improvement of the safety analyses� methodology through the application of the extended TRANSURANUS code. These objectives were achieved in four main work packages.
In the first phase of the project a new database was compiled on the basis of the VVER-specific separate effect tests of the AEKI. The database of nearly 400 tests provides detailed information on the Zr1%Nb cladding behaviour (oxidation ballooning and rupture) under LOCA conditions and gives a solid background for code extension and validation.
The main task of the project was carried out in the second work package. The TRANSURANUS code was improved with new models for the high temperature steam oxidation, plastic deformation and failure of the cladding tube. New VVER specific empirical correlations for the mass gain rate, the zirconium-dioxide layer growth rate, the deformation rate and the cladding failure were evolved and adapted to the models. An advanced multidimensional minimization procedure was applied to fit the material constants of the correlations on the compiled experimental data. Outstandingly, the recent mechanical model takes into account the oxygen concentration as a parameter of the deformation rate and the failure stress, as well. In this manner, the effect of the oxidation on the cladding mechanical performance can be simulated in an accurate way.

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