High energy radiography for inspection of the lid weld in disposal canisters

The copper canister is mainly designed to act as a corrosion barrier for the cast iron insert, which in turn contains the spent fuel bundles. The canister needs to be absolutely tight and the canister-to-lid weld is a critical part. Therefore this weld has to be inspected by non-destructive methods. The objective of this work is to investigate the application of high energy radiography to inspection of the sealing weld. The approach is based on available written material in the field. The canister is filled with radioactive waste when the inspection will be done. This fact sets the requirement that the inspection must be completely automated. A further challenge is the thickness of the copper wall. Due to an estimate based on a canister drawing the X-rays need to penetrate varying thicknesses of copper from about 40 mm to nearly 120 mm in order to cover the whole weld area. For these thicknesses X-ray energies of a few MeV are required. To get a high enough photon flux a linear accelerator is assumed to be needed. The scattered radiation from the working accelerator will itself cause an intense field of radiation in the inspection room. All these aspects make the high energy X-ray inspection of the canister sealing weld very special. SKB in Sweden has done initial tests with accelerator based X-ray inspections of this kind. The weld geometry, the X-ray attenuation and scattering in copper is presented in this study. The radiation field around the nuclear waste-filled canister is investigated. Some X-ray sources and digital detector concepts are presented. X-ray film is not well suited for this application. Also X-ray modelling tools available and their potential for this special problem are described. Most tools for simulation of X-ray inspections are designed for ordinary X-ray inspection and are not directly suited for accelerator based inspections.