The effect of flaw orientation on the integrity of PWR pressure vessel at the events of pressurized thermal shock

摘要

The reactor pressure vessel (RPV) is the most critical component in nuclear power plants, housing the reactor core and serving as a part of the primary system pressure boundary. Because of its proximity to the reactor core, the RPV is subjected to high fast neutron flux, losing ductility and fracture toughness. At the events of pressurized thermal shock (PTS), highly embrittled RPV may not have a sufficient safety margin for fast fracture. The US NRC PTS rule requires that the reference temperature (RT_(PTS)) should be limited to ensure sufficient safety margins against PTS. RT_(PTS) = 270℉ was defined as the screening criterion for axial welds based on extensive quantitative evaluation of associated risks. For circumferential welds, a technical margin of 30℉ was added to account for the effects of flaw orientation without same level of quantitative analysis. In this paper, the validity of the technical margin for circumferential welds is examined by comparing the quantitative risks depending on the flaw orientation. First, the result of the original work on axial welds was reproduced. Then, the risk associated with circumferential flaws was evaluated at the identical condition except for flaw orientation. The difference in screening criteria due to flaw orientation was at least 55℉, suggesting that current PTS screening criteria for circumferential flaws do not represent the same level of associated risks as that for axial flaws.