Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (III)

Abstract

Overpack, a high-level radioactive waste package for Japan's geological disposal program, is required to isolate the sealed vitrified waste from contact with groundwater for 1,000 years. In this study, EBW (Electron Beam Welding), a typical application of high energy density beams widely used in various industries, was examined for its applicability to sealing a carbon steel overpack lid with a thickness of 190 mm. Welding conditions and welding parameters were examined for single-pass welding in a narrow area for three different penetration depths. Weld joint tests including macro-and microstructure, and mechanical properties were conducted and assessed within tentatively applied criteria for weld joints. Measurement and numerical calculation for residual stress were also conducted and the tendency of residual stress distribution was discussed. These test results were compared with the basic requirements of the welding method for overpack, which were pointed out in our first report. The induced void and cold shut inside the weld joint and surface roughness were also discussed for their improvement and evaluation, which need to be established to assure the long-term integrity of overpack lid closure.

Propagation behaviour of general and localised corrosion of carbon steel in simulated groundwater under aerobic conditions

“…Immersion tests for SF340 were performed using welded coupons cut from a full scale model of an overpack lid welded by different methods: TIG, MAG and EBW methods. [2][3][4] In TIG and MAG welding, filler wires are required for the welding. Therefore, the chemical composition of the welded metal is different from that of the base metal.…”

“…The as-welded areas mainly consist of acicular ferrite and grain boundary ferrite. [2][3][4] Huang et al 11 pointed out that the ratio of acicular ferrite and grain boundary ferrite affects the corrosion rate of welded carbon steel. In HCC solutions as an example of localised corrosion, not only crevice corrosion around the crevice former was observed, but also pitting corrosion away from the crevice former was observed and corrosion on the surface was not uniform.…”

“…Some of the immersion tests were performed using welded coupons cut from samples of a full scale protype of an overpack lid welded by different methods: tungsten inert gas (TIG), metal inert gas (MAG) and electron beam welding (EBW). [2][3][4] Extreme value statistical analysis using the Gumbel distribution function was applied to the maximum corrosion depth data. Based on the results of the analysis, an empirical model for the assessment of maximum corrosion depth of carbon steel overpack was derived.…”

Long term integrity of overpack closure weld for HLW geological disposal Part 2 – corrosion properties under anaerobic conditions

“…Corrosion tests in the present study were conducted with weld specimens made from full scale welding test specimens using three kinds of welding methods: gas 1 Radioactive Waste Management Funding and Research Center (RWMC), 1-15-7, Tsukishima, Chuo-ku, Tokyo, Japan 2 Mitsubishi Heavy Industries, Ltd (MHI), 1-1, Arai-cho Shinhama 2-chome, Takasago, Hyogo, Japan 3 Japan Atomic Energy Agency (JAEA), 4-33 Muramatsu, Tokai-mura, Ibaraki, Japan tungsten arc welding (TIG), 3 gas metal arc welding (MAG) 4 and electron beam welding (EBW). 5 Table 1 shows the chemical compositions of the base metal, weld metal and filler material for each welding method. The chemical compositions of the base and weld metals do not differ significantly.…”

“…The likelihood of hydrogen embrittlement for steel depends on the hydrogen concentration and tensile strength. 9 From the results of mechanical property testing [3][4][5] and C H_diff. analysis, there appears to be little possibility of weld joint hydrogen embrittlement without TIG weld metal.…”