LWR Pressure Vessel Embrittlement and Conditions of Validity of Current Predictive Methodologies

Because of the irradiation coming from the core, the beltline region of LWR vessels experiences a slow degradation of its mechanical properties, which is known as Reactor Pressure Vessel (RPV) embrittlement. An accurate prediction of the severity of this phenomenon, at each time during the operating life of the RPV, is fundamental in order to prevent the vessel from experiencing stress states that may lead to a non-ductile propagation of a critical crack that may be present in the vessel walls, thus yielding vessel failure. Most of the literature, as well as current NRC regulations, relate the severity of embrittlement to fast neutron fluence only, neglecting the irradiation time rate as well as the effect of gamma-rays. In some circumstances, however, this procedure leads to an underestimation of the severity of embrittlement. Irradiation rate plays an important role when RPV walls experience a low fast neutron flux, like in BWRs. Gamma-rays give a significant contribution to embrittlement when a thick water gap separates the core from RPV walls, like in the ABWR. The current design tendency of decreasing fast neutron flux at the RPV wall, coupled with the introduction of new and innovative reactor features, is making the mentioned circumstances more and more frequent, requiring a revision of the current fast-fluence-based regulations. Scope of the paper The present paper is focused on RPV embrittlement in LWRs. It is aimed at providing practical information concerning (in the order they are presented): the microscopic causes of embrittlement; the macroscopic effects of embrittlement and its indicator parameters; the methodologies used to quantitatively estimate the indicator parameters; the process used to recover the mechanical properties degradated by an excessive level of embrittlement; the regulations aimed at preventing severe consequences that may be made possible by an excessive level of embrittlement; the limitations of the current regulations and their proven inadequacy in some circumstances. The paper has a practical nature, in that it was written with the purpose of highlighting ready-to-use information and “rules of thumb”, whose importance is often underrated.