A probability of rupture for WWER-1000 main piping was calculated based on the Failure Assessment Diagram (FAD), treating material properties of welds (the most likely zone for crack growing and its nucleation) and crack morphology parameters as stochastic values. In order to perform probabilistic calculations, Critical Temperature of Brittleness (CTB, WWER’s analogue of PWRs transition temperature which used to index the Fracture Toughness curve) and Yield (Ultimate) Strength as well were fitted by normal distribution, based on experimental data taken from the manufacture documentation of Ukrainian Nuclear Power Plants (NPPs). A set of calculations were conducted for Normal Operating conditions (NOC), Safe Shutdown Earthquake (SSE) and several emergency situations like: shaft jamming of a reactor cooling pump and break of the piping’s, connected to the considered ones (a set of LOCA events). Based on static and dynamic calculations, the most loaded zones were selected, where the cracks were postulated. Crack opening area was calculated according to original developed procedure, which accounts for membrane and linear stress components through the wall thickness. The Henry-Fauske flow model is used with modified parameters accounting for crack morphology as a normally distributed random variable. It is an important part of analysis, since different crack types have great differences in friction, bend protrusion and flow length parameters. The rupture probabilities for Main Circulating Piping were calculated with accounting for thermal aging effect. It was proven, that crack morphology parameters highly affect the leak rate and its distribution becomes more scattered. Among the mechanical characteristic, a Fracture toughness has more influence rather than Ultimate of Yield strength.