Geological disposal of Japan is based on the concept of a multi-barrier system constituted of the engineered and natural barriers. For the disposal system, a vitrified high-level radioactive waste will be contained in a metal overpack emplaced together with a bentonite buffer. The overpack is aiming to prevent the contact of groundwater to vitrified waste during the high heat generation period of the first 1,000 years after emplacement. In addition, the overpack requires corrosion resistivity/prevention and structural integrity against mechanical loading for at least 1,000 years.
Within the Japanese program, consideration for overpack candidate materials has included carbon steel, copper-steel composite and titanium-steel composite. Within the extensive safety assessment conducted in 2000, steel was selected. This selection was partly based on the manufacturability of carbon steel, as well as its well-understood corrosion behaviour. However, the understanding of copper corrosion in deep geological environments and welding/manufacturing technologies have greatly progressed over the past two decades: for example, SKB succeeded in applying friction stir welds to thick copper containers, while NWMO developed the copper coating container with a few millimeters of copper layer. These progresses lead us to re-evaluate the applicability of copper containers to the Japanese geological disposal concept, based on state-of-the-art technologies.
In this study, we focus on the copper coating container developed by NWMO, because this technology is seemingly effective to maintain the long-term containment with the low-cost of manufacturing in the Canadian program. We are investigating the applicability of the copper coating technologies which have been developed by NWMO, from the view points as follows:
- the corrosion allowance in Japanese geological environments,
- the design of container which is effective for minimizing the risk of stress corrosion cracking,
- and the manufacturability of copper coating container technology.
As it does not form a protective oxide layer like stainless steel or titanium, copper is classified as a corrosion allowance metal. Owing to its stability in water as a metal, it generally shows a corrosion rate that is much lower than that of carbon steel in an anoxic, wet environment; however, the corrosion rate does increase with increasing hydrosulphide concentration. In the previous Japanese safety assessment, , the corrosion allowance of copper was determined to be 39 mm for 1,000 years of the target period of containment on the basis of the diffusion-limiting corrosion by pessimistically assuming the hydrosulfide concentration of 0.03 mol/L (or ~1000 ppm). While such a concentration is possible for rock systems containing extensive sulphide species, this is not consistent mineralogy for any geological disposal site, either in Japan or elsewhere. For one reason, in iron bearing minerals, mackiwanite and to a lesser extent, pyrite, will sequester sulphide as precipitates and reduce soluble hydrogen sulphide in groundwater to less than 1×10-7 mol/L. According to many reports which measured the hydrosulfide concentration in groundwater, the measured value distributed from “not detected” to the order of 10-4 mol/L for hydrosulfide. These observations are indicative of the existence of the low-sulfide groundwater in Japan at the pre-siting stage with no specific disposal site. The anaerobic corrosion depth for 1,000 years assuming the hydrosulfide concentration of 1 x 10-7 mol/L is less than 1 mm. The allowance for aerobic corrosion due to residual oxygen in the disposal tunnel can be determined to be 0.25 mm via mass balance calculations; this assumes all oxygen interacts with copper and does not get consumed by other processes such as steel corrosion or microbial reactions. Taking a very conservative roughness of the surface (a factor of 3) into consideration, the required corrosion allowance was determined to be only 0.75 mm. Thus the copper coating layer with 3 mm thick is sufficient for the geological repository concept of Japan.
To further advance the copper coating concept in Japan, we designed two different steel inner vessels with a flat head and a hemi-spherical head to maintain the structural integrity under the hydrostatic pressure up to 15 MPa. To minimize the risk of stress corrosion cracking, a container with the latter head is a little more suitable because the compression stress occurs all over the surface of container.
On the manufacturability, as the inner vessel tube for the Japanese vitrified waste was the almost same as that of the Mark-II used fuel container of NWMO, we can apply the same manufacturing technology of copper coating container of NWMO to NUMO’s overpack. This is a great advantage for the efficient and rational R&D by collaboration between NWMO and NUMO. The steel inner vessel is coated with 3 mm of electrodeposited copper and the welding part is also coated by thick copper spray coating. These manufacturing technologies have been well demonstrated in mock-up scale.
Thus, we can conclude that the copper coating overpack is applicable for the Japanese geological disposal concept, and a promising alternative of the carbon steel overpack which is considered as the current reference design.
Vitrified waste, geological disposal, overpack, copper coating
