Replica vessel component used to verify Beznau 1 safety

14 September 2016

The production of a replica part of the reactor pressure vessel (RPV) used in unit 1 of Switzerland's Beznau nuclear power plant has shown that aluminium oxide inclusions in the vessel formed during its manufacture, utility Axpo said. The unit has been offline for more than a year whilst the safety of its vessel is investigated.

Beznau 1 replica Ring C - 460  (Axpo)
The replica Ring C being forged in the UK (Image: Axpo)

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 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, 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 ruled out hydrogen flaking.

However, Axpo confirmed earlier this year the presence of aluminium oxide inclusions in the RPV of Beznau 1. It noted that the test material used in earlier investigations "did not fully meet the high quality standards needed to demonstrate physical integrity".

The company said today it 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.

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".

The replica allows Axpo to run the full program to test the physical integrity of the RPV of Beznau 1, including facture-mechanical tests, the company said.

Axpo will carry out further extensive investigations and assessments over the coming months. It 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 additional testing will enable it to: identify the material-specific effect of the inclusions on the base material; prove the fracture toughness of the material; and, answer "the question regarding the remaining safety buffer".

"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 and make a decision on restarting Beznau 1. In May, Axpo said it expects to be given regulatory approval to restart the unit by the end of this year.

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