Abstract The determination of extremely low probability of the reactor pressure vessel and the primary main coolant pipe fracture is one of the prerequisites for further operation of the first generation NPPs in Czechoslovakia. The project for this task includes experimental and analytical work on the reactor pressure vessel and on the main piping components. Review of the project and preliminary results are presented. The results show extremely low probability of failure. Future work is focussed on annealing studies and tests for the pressure vessel and the application of the Leak-Before-Break technology on primary piping circuit.