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

An extensive work was conducted in France related to the structural integri ty re-assessment of the French 900 and 1300 MW reactor pressure vessels in order to increase their Lifetime. Within the framework of this programme, n umerous 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-plas tic 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 spe cific plasticity correction (beta correction) taking into account the cladd ing yielded during a severe PTS transient - has been developed for the asse ssment of shallow subclad flaws, derived from 2D elastic and elastic-plasti c computations. This new approach is described and compared to results obta ined by reference 2D and 3D finite element elastic and elastic-plastic comp utations. This methodology is validated with reasonable margins and a suffi cient level of accuracy. A similar work has been conducted related to the assessment of embedded sha llow flaws located in the first layer of the stainless steel cladding. A ne w 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 a nd 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 sur face cracks) in a French RPV during a severe overcooling transient. Several through-clad surface cracks have been compared to shallow subclad flaws lo cated at the cladding-base metal interface, and the influence of cladding i n a PTS has been evaluated. The main results are presented. A RPV structura l integrity assessment based on through-clad surface cracks appears much mo re severe than the specific French approach based on more realistic shallow subclad flaws. The influence of the cladding is particularly significant o n the margins, regarding the risk of cleavage initiation along the crack fr ont in the base metal of the vessel, by significantly decreasing the level of the stress intensity factor and consequently increasing the maximum allo wable end-of-life RTNDT (C) 2001 Elsevier Science Ltd. All rights reserved.