Integrity assessment of loss of coolant accidents in VVER type reactor pressure vessel

One of the most severe loading conditions of a reactor pressure vessel (RPV) under operation is the loss of coolant accident (LOCA). Cold water is injected into the cold legs and falls into the downcomer of the RPV forming colder plums. Complex thermal hydraulics situations occur and the fluid temperature adjacent to inner RPV wall as well as the RPV wall-to fluid heat transfer coefficient show two-dimensional distributions varying with time. Fluid-fluid mixing of ECC-water with ambient hotter water occurs in the cold legs, in the downcomer and in the lower plenum. Therefore numerical thermal hydraulics calculations are performed to quantify the mixing in order to obtain the distributions of fluid temperature and wall to fluid heat transfer coefficients to inner RPV surface. Because of this non-symmetric temperature and heat transfer conditions, the following fracture mechanics investigations are carried out by three-dimensional linear and non-linear finite element analyses, which allow for an accurate assessment of the postulated flaws. Plume cooling during a LOCA of the RPV leads to locally increased axial stresses in the plume region compared to axisymmetric cooling. The higher axial stress could make the circumferential crack more relevant for the safety assessment. The thermal shock effects are in this case calculated for different LOCA transients. For the most severe transient a full elastic plastic finite element analysis is performed assuming a sub-surface (underclad) semi-elliptical flaw. By a fast fracture analysis the fracture parameters are evaluated and the allowable transition temperature is determined (author)