The COVASTOL program

The COVASTOL program has been developed jointly by CEA, Euratom and Framatome. The resulting code is based on fracture mechanics with propagation expressed in a probabilistic form. The spatial and size distributions for the defects have been obtained from 3 European manufacturers, a total of 338 meters of LWR pressure vessel welds being analysed. The final sizing takes into account the NDE reliability and efficiency. Loading of the vessels has been determined on the basis of statistical data; resulting stresses and stress factors are computed at different depth. Paris law coefficients are also determined probabilistically with toughness for which irradiation embrittlement is considered taking into account P and Cu concentrations. Calculations performed using the COVASTOL code and French PWR specifications show that the main rupture is due to a LOCA (large or small break). The rupture probability at 40 years varies from 2x10/sup -8/ to 3x10/sup -9/ depending on the safety injection water temperature (respectively 10 deg C and 20 deg C). The most dangerous defects are located within the first millimeters of the internal layer and the most sensitive weld is located at the belt line. (author)