Evaluation of Metal Ageing of Reactor Pressure Vessels in a Nuclear Power Plant under the Influence of Ageing Mechanisms and Environmental Conditions

Abstract

The reactor metal in NPP is subjected to ageing mechanisms – corrosion, erosion, fatigue and neutron fluence embrittlement. These mechanisms cause degradation of the mechanical properties and may lead to the occurrence of defects. Environmental conditions are high pressure and fluid temperature values along the primary circuit. Where defects occur, it is possible to observe fracture of the metal under the environmental conditions. The RPV metal was studied by non-destructive testing methods. The article contains the results of the methods applied. In the defects site the values of fluence, circular stresses and temperatures were defined for a reactor operating under accident conditions. The impact of these factors in creating zones in the metal with risk of destruction has been considered. The results show, that corrosion and erosion have not caused defects in the metal. Neutron fluence and its thermal impact on the metal have caused embrittlement which is manifested by indications of defects. Two main factors were observed that lead to the creation of sites with a potential risk of fracture: 1) the neutron and thermal embrittlement of the metal, and 2) the hydraulic stresses from the fluid. The stress intensity factors of some defects, as well as the critical values of these factors have been calculated. The comparison of these values demonstrates that the metal in the danger zones will not be damaged.