Submission of the Safety Case
Axpo submitted the safety case for Beznau Unit 1 to the Federal Nuclear Safety Inspectorate ENSI on 14 November 2016. In the safety case, Axpo provides the verifiable proof that the safety of the power plant is ensured in keeping with the state of the art of science and technology. From Axpo’s viewpoint, there are no safety-technical concerns regarding the continued operation of the plant up until 2030. ENSI will now review the safety case, which is the basis for issuing permission to restart the plant.
What is the safety case?
The safety case provides the results of all the tests that we carried out on the RPV in the past half-year in order to produce the proof of physical integrity and safety case. We were able to point out the source of the detected inclusions, and thanks to the replica, clarify the impact of these inclusions on the material properties, provide verification of fracture toughness and answer the question of available safety buffers. The safety case establishes that we were able to successfully provide verification for the safety and physical integrity of the reactor pressure vessel according to national and international standards. The legal provisions for the safe operation of Beznau 1 have been fulfilled. Therefore, from Axpo’s viewpoint nothing stands in the way of restarting Beznau Unit 1. ENSI will now review the submitted documentation together with international experts.
What were the criteria for determining the physical integrity of the RPV?
Verification of physical integrity is based on ultrasonic tests, the root cause analysis, as well as fracture-mechanical material tests according to the ASME Code. The detected irregularities were also assessed according to international standards. An adequate safety margin is sustained. Our tests indicate that the inclusions do not have an impact on the physical integrity of the RPV, and that safety is ensured for a service life of 60 years.
Critics maintain that the replica is not representative and that it cannot be used for the tests. What do you say?
No, the exact opposite is the case. The replica has the same chemical and mechanical properties as ring C of the RPV, and the existing non-metallic inclusions in the replica exhibit a nearly identical fingerprint to those in the RPV. Internationally recognised companies and independent experts have certified the representative qualities of the replica. The replica fulfilled all the suitability tests and quality requirements.
How did Axpo proceed with the material tests?
In the course of the metal analyses, we cut apart sections of the ring to make test bodies from which samples were created for the required mechanical structural tests such as tension or brittleness properties. These tests were carried out in sufficient number in order to be able to determine the mechanical properties. The metallographic and material tests prove that the detected inclusions have no impact on the mechanical material properties, and that they are within the normal tolerances.
Did the material tests confirm previous findings?
Yes. The material tests also verified and validated the inclusion formation modalities found in the course of the root cause analysis. The strength tests that we were able to perform thanks to the replica were carried out according to international standards (ASME) and the valid ENSI provisions. We made our calculations based on very conservative assumptions. Accordingly, we also interpreted the ultrasonic findings conservatively and still come to the conclusion that the physical integrity of the RPV is given. Verification of physical integrity was carried out according to the roadmap accepted by ENSI.
So Beznau 1 is fit for 60 years of operation?
From Axpo’s viewpoint there are no safety-technical concerns for the continued operation of the plant up until 2030. All our investigations indicate that the inclusions occurred during manufacturing and were not induced through operation. With the material tests, we were able to show that the inclusions have no influence on the material properties and no impacts on the radiation-induced brittleness of the base material. The physical integrity verifications confirm analyses that were performed in 2011/2012 in connection with long-term operation. Based on the safety case, we expect that ENSI will issue the permit for restarting.