Modified Procedure for PTS Evaluation of Cladded WWER Reactor Pressure Vessels

Abstract

Assessment of integrity of reactor pressure vessels during Pressurized Thermal Shocks (PTS) is usually determining whole reactor pressure vessel lifetime. WWER (PWR type reactors of Russian design) reactor pressure vessels are characterized by relatively thick austenitic cladding on their inner surface. Existence of cladding results in high residual stresses in inner surface area and thus in high stress intensity coefficients of postulated/calculated under clad type defects. The principal objective was to prepare and validate a upgrading procedure for the integrity evaluation of WWER reactor pressure vessels within the VERLIFE procedure, taking into account the inside austenitic cladding, with respect to pressurized thermal shock (PTS) events. In order to reach this objective, a material characterization program was set up to determine the material properties needed for modeling the behavior of the cladding in PTS type loading conditions. One of the main activities of the Project was the performance of the large-scale specimen tests with specimens that consisted of both the base metal and cladding materials. This type of test configuration was considered necessary to determine the fracture behavior of cracks potentially occurring in the RPV with cladding. In order to reach this objective, the following specific targets were identified: • Critical analysis of cladding tensile and fracture toughness properties in un-irradiated and irradiated conditions. • Realization of standard and semi-large scale specimen tests with cladding for determining the fracture properties of RPVs with cladding and for choosing proper failure criteria to be used in RPV integrity evaluation. • Proposal of a procedure for the integrity and lifetime evaluation of RPVs with cladding and experimental validation of this procedure. • Assessment of the effect of the RPV cladding on the vessel integrity and lifetime. The work was planned to focus on the following three PTS issues which were known to be significant, but which have not been fully considered in the PTS models used in the Czech Republic, for evaluating the behavior of RPVs: Consideration of the effect of RPV cladding material in PTS type loading simulations. Assessment of the behavior of under-clad defects besides the surface cracks. Consideration of true cladding material properties in the numerical models. Attention was paid to the material properties of both the initial and irradiated material conditions that should be known and taken into account in the integrity evaluation. For additional validation of the experimental and numerical methods, the following measures were performed: • Non-destructive measurements using the ball indentation method (ABIT) for characterizing irradiated materials. • Acoustic emission and potential drop measurements for detecting crack growth. • Residual stress measurements of the cladding layer for estimating the true stress state. The main technical result of the Project has been a reactor pressure vessel integrity evaluation procedure (extension to VERLIFE – Unified Procedure for Lifetime Evaluation of Components and Piping in WWER NPPs during Operation, as prepared within the EU 5FP), which is conservative and principally applicable for both WWER 440 and WWER 1000 type RPVs.