This paper presents a results of brittle fracture (BF) assessment of WWER-1000 reactor pressure vessel (RPV) subjected to pressurized thermal shook (PTS).
Work is focused on the two factors affected on the BF safety margin. The first one is the definition of the heat transfer conditions from outer surface of RPV, which is commonly not considered due to the assumption of conservatism. The second one is the warm pre-stressing (WPS) of RPV metal during PTS.
The procedure for the formation of thermal boundary conditions (BC) for the RPV (i.e. lower nozzle forging, cylindrical part and elliptical bottom), reactor supports, as well as for the reactor thermal insulation, is developed. This procedure takes into account the heat transfer due to radiation in the GA-301 room and the presence of forced supply of “cold” air by the ventilation system.
A brief history of WPS implementation in Ukraine is presented. Also, the comparative review of the WPS experience in PWR countries and WPS approaches is made.
The results of fracture mechanics calculations for the three most severe PTS scenarios of the Unit 3 RPV of South-Ukrainian NPP are presented. Wherein, both type of results is demonstrated: with and without consideration of the RPV outer surface heat transfer due to the air-cooling ventilation system.
It is shown that the RPV air cooling consideration: leads to decreasing of the stress intensity factor; provides to satisfaction of the cladding integrity criterion according to VERLIFE-2008 methodology; can justify lifetime of the RPV nozzle region in terms of not to exceed the upper shelf of fracture toughness 200 MPa·m0.5.
At the same time, consideration of the RPV air cooling alongside with the WPS approach, depending on WPS approach considered, can lead as to the decreasing the conservatism of RPV BF assessment, as to the increasing it.
Also, concluded that Ukrainian WPS approach needs to be refined more carefully with consideration of experimental nature of pre-stressing or harmonized with some of the modern ones.