Reactor Pressure Vessel Internals: Research on the Effects of Aging Caused by Operating Conditions

The operating conditions of a nuclear reactor vessel are characterized by high values of pressure and temperature of the fluid along the primary circuit, variable loads, intense neutron fluxes and a corrosively aggressive environment. All these factors cause aging of the Reactor Pressure Vessel (RPV) Internals metal. The mechanisms of degradation of the mechanical properties are neutron and thermal brittleness, corrosion, erosion, fatigue and wear. Corrosion of the metal is caused by the fluid along the primary circuit of the nuclear power plant. The high values of the reactor temperatures and the operating stresses cause stress corrosion and corrosion fatigue. Neutron flux causes the most damaging effect on the metal structure. After prolonged periods of exposure to high-energy neutrons (several decades), the metal of the reactor vessel becomes brittle and strengthened. Temperature and dose load gradients can cause swelling of the metal of the reactor internals. Swelling is an increase in the external dimension of solid materials after irradiation and is an extremely dangerous condition of the metal structure, because it reduces the bearing capacity of structures. In addition, the change in the geometric dimensions of the RPV Internals may disturb the movement of the heat carrier and the heat balance, to obtain jamming and blocking of units of the reactor control and protection system. This article discusses the reasons for the degradation of the mechanical properties of RPV Internals. Two methods for studying the effects of aging on objects are presented—visual method for monitoring the condition of the metal and method for measuring geometric dimensions, as well as the results obtained.