Methodology for the pressurized thermal shock evaluation: recent improvements in French RPV PTS assessment

Abstract

An extensive work was conducted in France related to the structural integrity re-assessment of the French 900 and 1300 MW reactor pressure vessels in order to increase their lifetime. Within the framework of this programme, numerous developments have been implemented related to the methodology used for the assessment of flaws during a pressurized thermal shock (PTS) event. This extensive work includes the development of new simplified approaches, with their validations by two-dimensional (2D) and three-dimensional (3D) finite element computations, the development of 3D elastic and elastic–plastic analyses, and finally the evaluation of the influence of the stainless steel cladding in a PTS analysis. A new simplified methodology — based on an elastic approach including a specific plasticity correction ( β correction) taking into account the cladding yielded during a severe PTS transient — has been developed for the assessment of shallow subclad flaws, derived from 2D elastic and elastic–plastic computations. This new approach is described and compared to results obtained by reference 2D and 3D finite element elastic and elastic–plastic computations. This methodology is validated with reasonable margins and a sufficient level of accuracy. A similar work has been conducted related to the assessment of embedded shallow flaws located in the first layer of the stainless steel cladding. A new approach has been proposed, more realistic and less conservative than the methodology previously used for this kind of flaw. These two methodologies have been accepted by the French Safety Authority and are now used in the RPV structural integrity assessment in progress. Some 3D finite element analyses have been also performed in order to study the behaviour of several cracks (shallow subclad flaws and through-clad surface cracks) in a French RPV during a severe overcooling transient. Several through-clad surface cracks have been compared to shallow subclad flaws located at the cladding–base metal interface, and the influence of cladding in a PTS has been evaluated. The main results are presented. A RPV structural integrity assessment based on through-clad surface cracks appears much more severe than the specific French approach based on more realistic shallow subclad flaws. The influence of the cladding is particularly significant on the margins, regarding the risk of cleavage initiation along the crack front in the base metal of the vessel, by significantly decreasing the level of the stress intensity factor and consequently increasing the maximum allowable end-of-life RT NDT.