Correlation Approach to Evaluate Critical Brittleness Temperature for WWER-1000 Reactor Pressure Vessel Materials in Unirradiated Condition

A statistical data analysis for ten WWER-1000 units of Ukrainian nuclear power plants has been performed to derive the relationship between the critical brittleness temperatures T_k0 (technical data on reactor pressure vessel) and T_kt (surveillance test data) for base and weld metal in unirradiated condition. According to standard PNAE G-7-002-86, there are two methods to evaluate the critical brittleness temperature of RPV materials in unirradiated (initial) condition. One method is deterministic, in which specified criteria are to be fulfilled to determine the critical brittleness temperature (T_k0). This temperature is evaluated within RPV material qualification tests. The other method employs a regression analysis of the temperature dependence of impact toughness applying a hyperbolic tangent function. In the latter case, the brittleness temperature (T_kt) is determined using the surveillance test data. The analysis has shown that T_kt increases with higher T_k0, and this correlation is linear for the examined materials. The set temperature T_k0 is 15^deg C higher than T_kt on average. The standard deviation for the linear correlation is 6^deg C. An equation to evaluate T_k0 has been proposed within this correlation analysis and can be applied to determine the critical brittleness temperature with a required safety margin for justification of WWER-1000 RPV safe operation.