In order to handle the long living radioactive waste (spent nuclear fuel) SKB 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. The most critical part of the encapsulation process is the sealing of the canister, which is done by welding the copper lid to the cylindrical part of the copper shell using -radiographic and ultrasonic testing -must be satisfactorily determined and combined to derive assumptions regarding the frequency of undetected welding defects for the ensemble of canisters as input for the risk assessment. This is done using the POD method according to the "Reliability Handbook MIL 1823" and its generalization to more complex defect situations in welds.friction stir welding and electron beam welding. The quality of the welding process and the reliability of the NDT system