Beyond Design Basis Seismic Analysis for Atucha II Nuclear Plant

The Atucha II nuclear power plant is a pressurized heavy water reactor being constructed in Argentina. Nuclear power plants must be designed to maintain their integrity and performance of safety functions for a bounding set of normal operational events as well as abnormal events that might occur during the lifetime of the plant. Seismic fracture mechanics evaluations for the Atucha II plant showed that even with a seismic event with the amplitudes corresponding to an event with a probability of 10−6 per year, that a double-ended guillotine break (DEGB) was pragmatically impossible due to the incredibly high leakage rates and total loss of make-up water inventory. The critical circumferential through-wall flaw size for this case is 94-percent of the circumference. These analyses are performed by placing cracked-pipe-elements into a complete model of the primary cooling system including the reactor pressure vessel, pumps, and steam generators as summarized in the paper.This paper summarizes these results and further shows how much higher the applied accelerations would have to be to cause a DEGB for an initial circumferential through-wall crack that was 33 percent (about 120°) around the circumference. This flaw length would also be easily detected by leakage and loss of make-up water inventory. These analyses showed that the applied seismic peak-ground accelerations had to exceed 25 g’s for the case of this through-wall-crack to become a DEGB during a single seismic loading event. This is a factor of 80 times higher than the 10−6 seismic event accelerations, or 240 times higher than the SSE accelerations. This suggests there is a huge safety margin for beyond design basis seismic events and Atucha II plant rupture is pragmatically impossible. These surprising results are discussed and could be potentially applicable to other nuclear power plants as well.Copyright © 2012 by ASME