Vessel integrity tests extend Beznau 1 outage

04 May 2016

Proving the integrity of the reactor pressure vessel (RPV) of unit 1 of Switzerland's Beznau nuclear power plant is taking longer than originally expected, owner Axpo said. It now expects to be given regulatory approval to restart the unit by the end of this year.  

Beznau 1 - 460 (ENSI)  
Beznau unit 1 (Image: ENSI)  

During a scheduled outage that began in May 2015, inspections were made of the unit's vessel using state-of-the-art ultrasonic equipment. These measurements "registered findings at some points in the base material of unit 1's RPV indicating minimal irregularities in the fabrication process".

Axpo said it has since conducted "sophisticated ultrasound measurements, extensively reviewed the fabrication process, and carried out a root cause analysis of the detected irregularities". The 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, Axpo said.

They are "non-metallic inclusions in the raw material that occurred during the fabrication process and, therefore, were not induced during operation," Beznau plant director Mike Dost said. The findings indicate "there are no safety-related concerns for the continued safe operation of the plant", Axpo said. However, the utility must still confirm the reactor vessel integrity to the Federal Nuclear Safety Inspectorate (ENSI) before it will be granted permission to restart Beznau 1.

"Proving that the integrity of unit 1's reactor pressure vessel has not been compromised has turned out to be more time consuming than Axpo originally anticipated," the company said yesterday. 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.

"We will have fulfilled the proof of safety requirements when the tests and analyses have been completed with results that we consider positive," he added. "To do so, the impacts of the detected irregularities on the RPV have to be clarified. We have to investigate the influence of the irregularities on the material properties, provide proof of fracture toughness, and rule out any remaining safety reservations. The proof of integrity for the so-called 'safety case' can be provided on this basis."

Axpo said it now expects to receive approval from ENSI to restart Beznau 1 "towards the end of 2016". The company had earlier expected the unit to restart before August at the latest.

"Depending on the issues that will have to be clarified in the course of the on-going process, investigation expenses and the loss of revenues from electricity sales since August 2015 will cost Axpo about CHF200 million ($209 million)," it said.

The two 365 MWe Westinghouse units at the Beznau plant are the oldest power reactors in Switzerland. Unit 1 began operating in 1969, while unit 2 entered operation in 1972. In addition to providing power, both units also produce district heating.

Researched and written
by World Nuclear News