Availability of a pressurized water reactor pressure vessel after a small LOCA

The integrity of a pressure vessel after a loss-of-coolant accident is investigated. The pressure and temperature in the pressure vessel is found by RELAP-4 for different break sizes. The effect of the pressure and thermal shock on the pressure vessel is evaluated by a two-dimensional finite element method (code SAP). By assuming axisymmetry, stress resulting from the shocks is calculated in the nozzle shell and in the beltline region of the pressure vessel. Similar calculations are carried out for the closure head and the lower head regions of the pressure vessel. Fracture mechanics is used to evaluate the stress intensity factor as a function of crack depth, and the critical crack depths are then obtained at various locations in the pressure vessel. The post-LOCA usability of the pressure vessel is predicted by comparing the stress intensity factor with the fracture toughness of the steel. (Auth.)