Waste packages expected to long-term containment are sealed by welding after the waste is placed inside. The chemical composition and metal structure of weld zone are different from these of base metal due to heat input and filler material used. It is important to evaluate the influence of changes in mechanical strength and corrosion behavior on ling-term integrity of the container. In the characteristic evaluation test of the welded portion, mechanical strength and corrosion behavior are obtained by the test pieces cut out from weld specimen and reflect the result on the design and production method. Changes in welding methods and welding materials are one of the quality improvements without changes in container shape and base material. It is important to pay attention to not only quality improvement but also the manufacturability of the package itself and various restrictions.
A heat input is considered as a factor that has a large influence on production. Especially, it is considered that temperature increase during welding and post-weld heat treatment affect not only the characteristics of the metal used for container but also deterioration of internal components. The increase of temperature of inner components is due to the influence of heat transfer between every component of waste package, and dominated by thermal conduction, emission and convection. These heat transfer strongly depend on the arrangement, contacts and gaps between each component. In addition, they can be controlled by the design of waste package containers, the manufacturing technology of each component and the assembling method and procedure at the ground facility.
For example, high-level radioactive waste is a vitrified waste solution from reprocessing, and it is sealed in an overpack made of carbon steel with welding to prevent contact with groundwater for 1,000 years after site closure. Full scale weld testing has been conducted and temperature increase of vitrified waste has been estimated by a numerical analysis about the post heat treatment for the purpose of reduction of welding residual stress. It has been confirmed by analysis that the temperature is 500 oC. or less at which there is a concern that the glass may deteriorate. TRU waste is classified into five groups in Japan, and the forms of the waste are a canister made of stainless steel, a 200 L drum, etc. The shape of package is designed as square for each waste. The components of TRU waste package are waste, filling material filled inner gap and steel container. The most stringent of waste is a temperature of the upper-limit estimated 195oC to prevent bitumen solidification from self-thermal runaway.
The present study describes the relationship between each component, the feasibility of package production, and the extraction of quality control items, from the consideration of production process of waste packages for geological disposal in Japan.
Acknowledgment
This research includes portions of the results of the “Development of Geological Disposal Technologies of TRU Disposal FY2018” under a grant from the Agency for Natural Resources and Energy (ANRE) in the Ministry of Economy, Trade and Industry (METI) of Japan
Souhaitez-vous présenter une affiche ?Weld closure, Heat input, production process