Conducting Assessments for Fast Fracture of Nuclear Reactor Metal

Abstract

Galya Dimova*

Nuclear power plants must be safe, reliable and ensure the planned supply of electricity. The metal of the equipment operates in conditions of high values of hydraulic pressure of the fluid, high temperatures and radiation exposure, fatigue, corrosion and wear. The metal is subjected to mechanisms of degradation of mechanical properties. It is known that each of these mechanisms was studied separately from the others, in laboratory conditions. There is no data on the synergistic interaction of the various mechanisms of degradation in real operating conditions of a nuclear power plant. Defects appear in the structure and this leads to a decrease in the operability of the equipment and to compromise the NPP safety. By visual, capillary and ultrasonic control, defects are recorded and dimensioned, but their development cannot be predicted until the next periodic operational control. In practice, those mechanical characteristics of the metal, which can lead to the danger of brittle fracture, are not measured (calculated). Nuclear power plant maintenance regulations do not include such a requirement. In the “worst case”, the facilities could be torn apart and a nuclear accident could occur. The topic in this article is the study of stress intensity factor for defects on the inner surface of the reactor vessel. The metals on the inner surfaces of the vessels of two nuclear reactors were studied by a visual method. Stress intensity coefficients have been calculated at the site of defects, taking into account the influence of the embrittlement factors. The results are compared with the requirements of the strength standards. The purpose of the investigation is to check whether the facility can continue to be operated safely. The algorithm can be used as a methodology for periodically examining defects detected during the operation of the unit.

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