Life Estimation Strategy for a Nuclear Reactor Pressure Vessel

Reactor pressure vessel (RPV) of a pressurized water reactor (PWR) is an important pressure retaining equipment which decides the life of a nuclear power plant. RPV is constructed of special quality nuclear grade low-alloy steel having sufficient fracture toughness at the beginning of life and a good resistance to neutron embrittlement. However, the steel is not completely free from embrittlement phenomenon due to the presence of small amounts of impurities like copper, phosphorus, and sulfur. RPV houses the nuclear fuel core and thereby it is subjected to neutron flux which causes irradiation embrittlement of the material over a period of operation time. This results in loss of fracture toughness of the pressure boundary material leading to a possibility of brittle fracture toward the end of life. This governs the safe operating life of the RPV. In this paper, the strategy for life estimation of a RPV using LEFM approach is described and a sample calculation for a typical RPV is presented. A semi-elliptical surface crack is postulated as per ASME B & PV code, Section-III, Appendix-G, at critical locations like core belt region and nozzle-vessel junction, and the flawed structure is analyzed under the severe most loading, i.e., a rapid cooldown transient, using the finite element method. 3D FEM model of the respective portions of the RPV is made using brick elements for analyzing the behavior of the postulated cracks under the load. The model includes the 3D crack geometry as an unconstrained boundary within the structure, with quarter-point elements near the crack front for simulation of square root singularity. Fracture calculations are done to evaluate the stress intensity factors for the postulated cracks. The extent of irradiation embrittlement of the material (measured as the shift in nil-ductility-transition temperature) with respect to neutron fluence (and hence total operation time) is assessed by using the empirical correlations as per the USNRC regulatory guide. The safe operating life of the RPV is estimated from the criticality condition of the postulated cracks.