“Extensive test program shows no evidence of a detrimental effect of the inclusions on the brittle fracture reference temperature”

An international panel of experts supported the Swiss Federal Nuclear Safety Inspectorate ENSI in assessing Axpo’s safety case for the Beznau 1 reactor pressure vessel. In an interview, the chair of the review panel Tim Williams explains why they support the conclusion that Beznau 1 may be restarted.

The IRP has been following Axpo’s safety assessment process of Beznau for two years and has independently assessed Axpo’s safety case within a scope defined by ENSI. What was the conclusion of the experts?

Tim Williams: The IRP considers that the safety case is acceptable.

The IRP therefore supports ENSI’s decision that the reactor pressure vessel is safe. What gives you this confidence?

Axpo Power AG has done extensive investigations and analyses to demonstrate that the reactor pressure vessel is safe. The cause of the great majority of the ultrasonic test indications has been convincingly established, by the manufacture of a replica RPV shell, to be alumina agglomerates. The materials testing has shown that these agglomerates do not affect the key materials properties. The structural integrity analysis shows that the RPV does not contain any flaws that could lead to RPV failure. The IRP is satisfied that work has been done to all appropriate national and international standards.

Are the available data sufficient to make such a decision?

Yes, Axpo has inspected the RPV very thoroughly for flaws and validated the flaw data used in the structural integrity assessment. It has carried out a large amount of materials testing to provide the materials properties information required. It has performed extensive metallurgical investigations and analyses to ensure correct interpretation of the results. The IRP is satisfied that there is no need to generate more data.

When the work started there were concerns that, in view of their unusual size and large numbers, the inclusions might have a negative influence on the brittle fracture reference temperature. What do the IRP experts say about this?

The extensive test program shows no experimental evidence of a detrimental effect of the inclusions on the brittle fracture reference temperature. This result is consistent with expectations based on theoretical models.

The determination of the brittle fracture reference temperature based on the master curve concept is also criticized. How do the IRP experts assess this approach?

The use of the Master Curve to set the brittle fracture reference temperature has been established by extensive international research over many years and is accepted in international codes and standards. However, it is known to have limitations that depend on material, and there is strong evidence that these limitations apply to the Beznau 1 materials. The IRP has therefore recommended that the more conservative of the methods allowed by ENSI to determine end of life brittle fracture reference temperatures be used (method II-b). This does not affect the acceptability of the safety case.