The production of a replica part of the reactor pressure vessel (RPV) used at unit 1 of Switzerland's Beznau NPP has shown that aluminium oxide inclusions in the vessel were formed during its manufacture, power company Axpo said. The safety of the RPV has been undergoing investigation since the unit was taken offline in May 2015 for a routine maintenance outage. The RPV inspections, made 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".
The two 365MWe Westinghouse units at the Beznau plant are the oldest power reactors in Switzerland. Unit 1 began operating in 1969, and unit 2 in 1972. In addition to providing power, both units also produce district heating.
Axpo has since conducted "sophisticated ultrasound measurements, extensively reviewed the fabrication process, and carried out a root cause analysis of the detected irregularities". Investigations using material samples confirmed that the irregularities are not due to hydrogen flaking, given their location, distribution and size, as well as the composition of the steel.
Earlier this year, Axpo had confirmed the presence of aluminium oxide inclusions in the Beznau 1 RPV and noted that test material used in earlier investigations "did not fully meet the high quality standards needed to demonstrate physical integrity".
The company then instructed an "internationally renowned forge in England" to produce a replica of Ring C of the RPV - the ring with the most inclusions - according to the documented manufacturing conditions that applied to the Le Creusot forge in the 1960s, where the RPV was made.
Axpo said that tests carried out so far have confirmed the replica Ring C has practically the same chemical and mechanical features as the original part fitted in Beznau 1. Ultrasound measurements on the replica have also identified inclusions almost identical to those in the original part. This, it claims, "validates the theory that the irregularities were introduced during the manufacturing process rather than during operation".
Further extensive investigations and assessments will be conducted in the coming months. Axpo said it will have proved the safety of Beznau 1's RPV when "all tests and analyses have concluded with positive results in accordance with the road map accepted by the Federal Nuclear Safety Inspectorate (ENSI)". The company said: "There continue to be no safety concerns about the continued operation of Beznau 1," Axpo said. "Just the opposite in fact: Axpo is more convinced than ever that it can provide the proof and safety case required for restarting the power plant."
The company expects to submit its safety case for restarting the unit to ENSI later this year, which will then study the safety case. In May, Axpo said it expects to be given regulatory approval to restart the unit by the end of this year.