Swiss utility Axpo has received regulatory approval to bring unit 1 of the Beznau nuclear power plant (KKB) back into service this month after evidence it submitted last December proved aluminium oxide inclusions in the reactor pressure vessel do not affect the unit’s safety.
The RPV was manufactured in the Creusot Forge in France in the 1960s. According to the production guidelines at that time, aluminium was added during the casting process to facilitate oxygen bonding.
Switzerland’s Federal Nuclear Safety Inspectorate (ENSI) said today: “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.”
It added: “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. KKB will now launch the return to service process.”
KKB has “set international standards with the safety case”, the regulator said, adding that the analyses had taken nearly three years to complete.
In mid-2015, irregularities were detected in the RPV base material in the course of ultra-sonic testing, and ENSI called for further investigations. Manufacturing documentation had already provided the first indications for the cause of the detected irregularities, ENSI said.
“In the course of refined ultra-sonic testing and analyses regarding the source and cause of the irregularities, it was possible to prove that these were non-metallic inclusions (aluminium-oxide) that occurred during the manufacturing process, and that can also form larger conglomerations,” it said.
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 no comparable material in sufficient quantity was available anywhere in the world, ENSI said. 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.”
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 material tests confirmed that the inclusions had no negative impacts on the material properties of the RPV, 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, ENSI said.
KKB will now launch the phase-wise return to service process for unit 1, with extensive safety tests to be carried out during the process. The unit is expected to be operating at full capacity by the end of this month.
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.
Source: World Nuclear News