Inspection findings in austenitic RPV internals of German BWR plants and BWRs built in other countries and resulting measures for Isar 1 nuclear power station

As visual examinations carried out in autumn 1994 detected cracks in a Germ an BWR plant due to intergranular stress corrosion cracking (IGSCC) in seve ral core shroud components manufactured from 1.4550 steel, precautionary ex aminations and assessments were performed for all other plants. In accordan ce with these analyses, it can be stated for Isar 1 that the heat treatment to which the components in question were subjected in the course of manufa cture cannot have caused sensitization of the material, and that crack form ation due to the damage mechanism primarily identified in the reactor vesse l internals at Wurgassen Nuclear Power Station need not be feared. Although the material and corrosion-chemical assessments performed to date did not give any indications for the other crack formation mechanisms that are theo retically relevant for reactor vessel internals (IGSCC due to weld sensitiz ation, IASCC (irradiation assisted stress corrosion cracking)), visual exam inations with a limited scope will be carried out with the independant expe rt's agreement during the scheduled inservice inspections. The fluid-dynami c and structure-mechanical analyses showed that the individual components a re subjected only to low loadings, even in the event of accidents, and that the safety objectives shutdown and residual heat removal can be fulfilled even in the case of large postulated cracks. The fracture-mechanics analyse s indicated critical through-wall crack lengths which, however. can be prom ptly and reliably detected during random inservice inspections even when as suming stress corrosion cracking and irradiation-induced low-toughness mate rial conditions. In addition, both the VGB and the Isar I plant are pursuin g further prophylactic measures such as alternative water chemistry modes a nd an appropriate repair and replacement concept. (C) 1999 Elsevier Science S.A. All rights reserved.