POD Evaluation of NDE Techniques for Canister-Components for Risk Assessment of Nuclear Waste Encapsulation

In order to handle long living radioactive waste Sweden is planning to build a deep repository that requires no monitoring by future generations. The spent nuclear fuel will be encapsulated in copper canisters consisting of a graphite cast iron insert shielded by an outer 30-50 mm thick copper cylinder for corrosion protection. A subpart of the final risk assessment is to determine the risk of premature canister leak caused by defects in the sealing weld and the enveloping components. Therefore the quality of the production process and the reliability of the NDT system must be satisfactorily determined and combined to derive assumptions regarding the frequency of undetected welding defects for the ensemble of canisters. The reliability of the NDT systems can be derived from POD curves which are investigated for X-ray and mainly for ultrasonic techniques applied by SKB. These complex canister are not only a challenge for the NDT techniques but also for the reliability investigation requiring a further development of the common “â versus a” approach due to the complex multidimensional situation and risk assessment. The reliability study started in 2003 for the welds and was in 2006 extended to the iron cast insert and the copper shield. The evaluation technique is developed in applying a multi parameter and data field-POD.