20 Mar (NucNet): The Beznau-1 nuclear unit in Switzerland has been reconnected to the grid after the nuclear regulator approved the safety case presented by owner Axpo following the discovery in 2015 of flaw indications in the reactor pressure vessel (RPV).
Axpo said today that the 365-MW pressurised water reactor unit, which began commercial operation in 1969 and is the world’s oldest commercial nuclear plant still in operation, had undergone “extensive tests and inspections” during the commissioning process.
“In addition to providing the safety case for the RPV of Unit 1, extensive modernisation and maintenance work that would normally have been performed during plant operation was carried out during the shutdown,” Axpo said.
Major projects completed during the shutdown included an additional earthquake- and flood-proof emergency power supply system, the replacement of the RPV head, and the commissioning of a new plant information system.
The Federal Nuclear Safety Inspectorate, ENSI, said on 6 March 2018 that Axpo had carried out “extensive investigations and analyses” to demonstrate that the RPV is safe.
Materials testing showed that agglomerates in the RPV wall material do not affect its key properties, and structural integrity analyses showed that the RPV does not contain any flaws that could lead to its failure. “ENSI is satisfied that work has been done to all appropriate national and international standards,” a statement said.
Axpo said the safety case for Beznau-1 corroborated earlier assessments and investigations, and validated the existing safety margin for the safe operation of the plant for 60 years. Axpo said the plant should be operating at full fuel load by the end of March.
In December 2015 Axpo submitted a roadmap to ENSI detailing plans for further investigations of flaw indications in the RPV. During a scheduled outage that began in May 2015, inspections of the RPV registered findings at some points in the base material of the RPV indicating “minimal irregularities in the fabrication process”, Axpo said. The company carried out further measurements and analyses and submitted a report to ENSI.
In July 2015, Axpo announced that the restart of Beznau-1 had been postponed while the flaw indications were investigated further. Then in August, ENSI called for additional investigations.
Beznau-2 was not affected by the flaw indications and was returned to service after its scheduled outage in 2015.