Further inspections for Beznau reactor vessels

The restart of unit 1 of Switzerland's Beznau nuclear power plant, currently offline for an annual overhaul, has been postponed while flaw indications detected in its reactor pressure vessel (RPV) are investigated further. A maintenance outage at unit 2 has been brought forward so that its RPV can also be checked.

The restart of unit 1 of Switzerland's Beznau nuclear power plant, currently offline for an annual overhaul, has been postponed while flaw indications detected in its reactor pressure vessel (RPV) are investigated further. A maintenance outage at unit 2 has been brought forward so that its RPV can also be checked.

The two-unit Beznau plant (Image: Axpo)

Plant operator Axpo said the return to service of Beznau 1 following the scheduled outage - which included the replacement of the unit's RPV head and control rod drive - is expected to be postponed until the end of October. Its original restart date had earlier been put back due to delays in installing the new vessel head.

Axpo said that during the outage, scheduled inspections were made of the unit's RPV using state-of-the-art ultrasonic equipment. The company said these measurements "registered findings at some points in the base material of the block 1 RPV indicating minimal irregularities in the fabrication process". It said it conducted further measurements and analyses "in order to assess these in compliance with current regulations" and has submitted a report to Switzerland's Federal Nuclear Safety Inspectorate (ENSI).

"First assessments do not lead to any misgivings concerning the continued safe operation of unit 1," Axpo stressed. "This preliminary result must now be verified in the upcoming weeks based on measurement analyses with detailed calculations and findings."

ENSI deputy director Georg Schwarz said there is "no time pressure" at Beznau 1 as the unit is offline and the fuel has been unloaded from its core. However, he added, "Of course we will only issue the permission to restart Beznau 1 once we are certain that the findings do not represent any impairment of safety."

Following the detection in 2012 of material defects in the RPVs of the Doel 2 and Tihange 2 units in Belgium, ENSI ordered an inspection of the manufacturing documents for Swiss nuclear power plants. The documents for the RPV of Beznau 1 did not indicate any faults in the material, ENSI noted.

Axpo said that it plans to conduct similar checks of the vessel of Beznau unit 2 "as soon as possible". It has therefore decided to bring forward the scheduled maintenance and upgrade outage at the unit, planned to begin in mid-August, and will "initiate the revision shutdown process for block 2 in the upcoming days". The company said that other scheduled work at unit 2 - including the integration of a new emergency power supply, the replacement of the vessel head and the commissioning of a new plant information system - will take place as planned.

The additional measurements and analyses of the RPV of unit 1 together with the earlier shutdown of unit 2 "will result in substantial additional expenditure", Axpo noted.

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