Swiss power company Axpo expects to be given regulatory approval to restart unit 1 of the Beznau NPP by the end of this year. The unit has been offline since May 2015 when it was shut down for a routine outage and had been expected to restart before August. However, inspections of the unit's reactor pressure vessel (RPV) using state-of-the-art ultrasonic equipment "registered findings at some points in the base material of unit 1's RPV indicating minimal irregularities in the fabrication process", Axpo said. Although the company has since conducted "sophisticated ultrasound measurements, extensively reviewed the fabrication process, and carried out a root cause analysis of the detected irregularities", proving the integrity of the RPV is taking longer than originally anticipated.
Axpo said investigations have so far confirmed that, due to their location and distribution, as well as their size, the irregularities are not due to hydrogen flaking. The composition of the steel also rules out hydrogen flaking. They are "non-metallic inclusions in the raw material that occurred during the fabrication process and, therefore, were not induced during operation," according to Beznau plant director Mike Dost. The findings indicate "there are no safety-related concerns for the continued safe operation of the plant", Axpo stressed. However, the utility must still confirm the RPV integrity to the Federal Nuclear Safety Inspectorate (ENSI) in order to be granted permission to restart the unit.
The delays have been caused by the procurement and testing of suitable additional material samples and by the complexity of the testing and analysis processes. Axpo said there have been numerous legal and commercial issues related to ownership and utilization rights of the samples. "We are currently examining the accelerated irradiation of specimens that were fabricated from original reactor pressure vessel forgings, and that were used in the area with the highest neutron irradiation," Dost said.
Beznau NPP's two 365MWe Westinghouse are the oldest power reactors in Switzerland. Unit 1 began operating in 1969, and unit 2 in 1972. They both supply district heating as well as electricity. Axpo estimates that Investigation expenses and the loss of revenues from electricity sales will cost about CHF200m ($209m).