Beznau nuclearUnit 1 at Switzerland’s Beznau nuclear plant, the world’s oldest power reactor, has been permitted to restart after a three-year shutdown for repairs, the Swiss Federal Nuclear Safety Inspectorate (Ensi) said on 6 March.  

The two 365MWe Westinghouse units at Beznau, which began operating in 1969 and 1972, provide district heating as well as power. Beznau 1 has been out of service since March 2015, when anomalies were found in its reactor pressure vessel (RPV).  Ensi cleared it for restart after evidence submitted by utility company Axpo last December proved aluminium oxide inclusions in the RPV do not affect the unit's safety. The RPV was manufactured in the Creusot Forge in France in the 1960s. In line with production guidelines at that time, aluminium was added during the casting process to facilitate oxygen bonding.

Following Ensi’s announcement, Axpo said it hoped to have the reactor fully operational again by the end of March. However, the news sparked immediate protest from environmental groups. Ensi said: "This marks the successful completion of an extensive testing and analysis process. With the supervisory authority's approval of the safety case, Axpo can now demonstrate that the safety of the RPV of unit 1 is ensured in compliance with the state of the art as well as national and international regulations."

In  2015, irregularities were detected in the RPV base material during ultra-sonic testing, and ENSI called for further investigations. Manufacturing documentation gave indications for the cause of the irregularities. Ultrasonic testing and analyses were able to prove that the irregularities were non-metallic inclusion of aluminium oxide that occurred during the manufacturing process.
ENSI said that Axpo looked for comparable base material to carry out strength tests as a basis for the structural-mechanical analyses on the integrity of the RPV, but that no such material was available in sufficient quantities. As a result, Axpo decided to have a replica of RPV ring C made by a forge in the UK.

"As anticipated, subsequent analyses found that this ring exhibited the same material properties as the existing ring," ENSI said. "With the replica, it was possible to experimentally verify the source of the ultra-sonic detections." In addition, the replica was also used as a basis to create material samples, several hundred of which were used to perform destructive strength tests (tensile and impact strength). Metallographic analyses and tests confirmed that the inclusions had no negative impacts on the material properties of the reactor vessel.

ENSI said integrity tests and calculations were carried out in a further step according to valid international requirements under the ASME Code, and national provisions. The analyses establish that the strength of the reactor pressure vessel is not compromised by the aluminium-oxide inclusions. "The safety case corroborates earlier assessments and investigations, and once again validates the existing safety margin for the safe operation of the plant for 60 years."

In the course of the analyses, which had taken nearly three years, KKB had "set international standards with the safety case", Ensi said. KKB will now launch a phased return to service for Beznau 1, with extensive safety tests to be carried out during the process.

Switzerland has five nuclear power reactors which generate about 34.5% of its energy needs. In May 2017, Swiss voters endorsed a new energy law that aims to promote renewable energy by banning new reactors and reducing energy consumption. The Mühleberg nuclear plant near Bern, built in 1972, will shut down in 2019, according to the power station operators.

Photo: Beznau nuclear plant (Credit: Axpo)