While carrying out ultrasonic measurements of the reactor pressure vessel of the Beznau 1 nuclear power plant in Switzerland, the operator detected flaw indications that require evaluation. The measurments were carried out as a result of the 2013 WENRA Recommendation in connection with flaw indications found in Belgian reactors.

The operator Axpo submitted a corresponding report to the Swiss Federal Nuclear Safety Inspectorate ENSI on 15 July. Beznau 1 is presently inactive because of the main annual overhaul. ENSI is now awaiting an assessment of these findings by Axpo. Operation of Beznau 1 may only be started again if the safety of the reactor pressure vessel is confirmed. 

ENSI has notified all WENRA members. 

"There is no time pressure at Beznau 1", explains Georg Schwarz, Deputy Director of ENSI and Director of the Division for Nuclear Power Plants, because the nuclear power plant is currently switched off for the annual overhaul and the reactor core is unloaded. "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", he stressed further.

ENSI has now instructed the operator to examine and assess the ultrasonic indications in more detail. This examination has to include the following steps: 

-  A detailed evaluation of the ultrasound results is the precondition for the fracture mechanics analysis of the indications  and  the safety case of RPV Beznau 1. The aim is to gain the most comprehensive and reliable information possible from the measurement results. In particular the size and location of the ultrasound indications must now be examined more closely.

- In the second phase of the examination, the aim is to characterise the type and cause of the findings.

- Finally one has to examine how the reactor pressure vessel may have been weakened because of the findings, and whether it still meets the regulatory requirements.

Following the discovery in 2012 of material defects in the reactor pressure vessels at the Belgian nuclear power plants Doel 3 and Tihange 2, ENSI ordered  in January 2013 that the manufacturing documents for the Swiss nuclear power plants should be inspected. The documents for the reactor pressure vessel of Beznau 1 did not indicate that there were any faults in the material.

Inspection work following WENRA Recommendation on Doel and Tihange

In summer 2012 numerous material defects in the base material of the forged reactor pressure vessels were detected in the Belgian nuclear power plants Doel 3 and Tihange 2.

In response to these material defects, in summer 2013  WENRA recommended that all forged reactor pressure vessels in Europe should be inspected as part of the normal weld seam inspections. Therefore ENSI directed that during the next recurring inspection of the weld seams of the reactor pressure vessels, the Beznau and Gösgen nuclear power plants should also inspect the base material.

The Mühleberg nuclear power plant had already carried out this inspection in summer 2012 following a request by ENSI. No reportable ultrasound indications were found in the reactor pressure vessels during this inspection.

The Gösgen nuclear power plant carried out this inspection during the annual overhaul in 2015, and did not find any relevant indications either.

The examination of the findings at Doel 3 and Tihange 2 is of little significance for the reactor pressure vessel of the Leibstadt nuclear power plant. This reactor pressure vessel differs from the Belgian reactors both in manufacturer and in its manufacturing process, and is made of rolled sheets rather than forged rings.

Beznau 2 is to be inspected as well

The condition of the reactor pressure vessel in the Beznau 2 nuclear power plant will also be examined during the main annual overhaul in 2015, which starts at the beginning of August. The planning of the shut-down of Beznau 2 for the overhaul makes provision for the same measurements of the reactor pressure vessel to be carried out as those for Beznau 1.

Together with the report about the ultrasonic indications for the pressure vessel in the Beznau 1 reactor, yesterday Axpo also submitted a substantiated statement to ENSI which is intended to show that it is not necessary to take Beznau 2 off the grid earlier than was planned. "We will examine this evidence over the coming week and state our opinion about it", says Georg Schwarz. "Should there be any reason to doubt the safety of the reactor pressure vessel  we will order that Block 2 be switched off even before the middle of August."

For more information please visit the ENSI website (www.ensi.ch)