Cladding in RPV Integrity and Lifetime Evaluation

Abstract

According to all design codes (ASME, RCC-M, KTA, JARC, PNAEG,…) austenitic cladding on inner surface of RPVs is usually taken only as a protection against corrosion. Its tensile properties as well as its existence is not taken into account in design of the RPVs, i.e. in the determination of their dimensions as well as during stress analysis and comparison with allowable stresses/stress intensities.Austenitic cladding due to its manufacturing history – mostly by strip welding without subsequent austenitization – has not very high toughness that can be important in evaluation of some regimes like during emergency cooling when “PTS – pressurized thermal shock” is present.Existence of cladding, its properties, defectness and system of in-service NDT inspection determine the type, size and shape of so-called “postulated defect” in evaluation of RPV resistance against non-ductile failure. According to some codes (e.g. IAEA VERLIFE etc.) fracture properties must be taken into account in these calculations. Unfortunately, each of cladding layers has different properties and their fracture properties are changing during operation – some embrittlement can be found as a result of neutron irradiation.This paper will describe problems connected with the effect of cladding on RPV resistance against non-ductile failure during regimes of PTS type and also gives some examples and criteria for their evaluation.