The Swiss Federal Nuclear Safety Inspectorate (ENSI) has cleared the Beznau power plant's Unit 1 reactor for a restart after utility company Axpo proved that aluminum oxide inclusions discovered in the reactor pressure vessel (RPV) did not sacrifice safety.
The unit, which is the world's oldest operating reactor, was idled in March 2015 when the anomalies in the reactor pressure vessel base material were found during ultra-sonic testing. According to Nuclear Engineering International, the questionable material was forged at Areva's Creusot Forge in France in the 1960s, when it was standard practice for aluminum to be incorporated into the casting process to assist with oxygen bonding.
This left it up to Axpo to test similar material to prove to the regulator that the RPV was safe. However, sufficient quantity of the material could not be located, forcing Axpo to re-create the original casting and test the new material. This was done at a forge in Britain.
ENSI was satisfied with the results. “As anticipated, subsequent analysis found that this ring exhibited the same material properties as the existing ring. With the replica, it was possible to experimentally verify the source of the ultra-sonic detections,” ENSI said. The testing of the new material found there were no safety compromises to concern the operator or the public.
“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,” ENSI said.
The restart is expected to take place within the next two weeks.