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

Abstract

The corrosion behaviour of a carbon steel weld joint under anaerobic conditions was investigated to estimate the long term integrity of the carbon steel overpack. The welded specimens in this study were prepared using three different welding methods: gas tungsten arc welding, gas metal arc welding and electron beam welding. General corrosion was observed for each immersion specimen, and the corrosion rate at the weld joint was the same as or less than that of the base metal. The concentration of hydrogen that absorbed during immersion testing was ,2?48610 25 mol kg[Fe] 21 (0?05 ppm) after 3 years, a value regarded as having little influence on hydrogen embrittlement. The susceptibility to hydrogen embrittlement was the highest in the base metal, suggesting that there was little adverse effect on the weld joint from welding. The welded carbon steel overpack is assumed to maintain its resistance to corrosion as a disposal container for the expected lifetime under anaerobic underground conditions.

Corrosion resistance of a cast steel overpack for high‐level radioactive waste disposal in Japan

“…The literature data for other types or lots of forged steels and rolled steels for up to 1 year are also shown in the same bar graph. [ 4,6,7 ] Although differences can be observed among the mean corrosion rates for different steel grades and production lots, the differences were within a factor of two. The relationship between the magnitudes of the mean corrosion rates was not necessarily the same over time.…”

“…The chemical compositions of all specimens used for the immersion tests are listed in Table 7, together with that of the forged steel and the rolled steel specimens used for similar immersion tests. [ 4,6,7 ] No significant differences in chemical composition were found among the examined materials.…”

“…The test solution was synthetic seawater, according to ASTM D1141, which is often used as simulated saline groundwater in the Japanese corrosion tests for geological disposal. [ 4–8 ] Table 6 shows the composition of the synthetic seawater used for the preliminary anodic and cathodic potentiodynamic polarisation tests. Argon gas was bubbled into the solution during the experiment.…”

“…Although it is not clear whether this treatment can remove oxygen from bentonite completely, even if some oxygen remains, its effect on corrosion is considered to be small, because it has been shown that the contribution of oxygen to corrosion in compacted bentonite saturated with water promptly (within 10 −3 years) becomes negligible due to the effect of suppression of the supply of oxygen by bentonite. [ 10 ] For the test solution used for the immersion tests, synthetic seawater prepared on the basis of ASTM D1141, as shown in Table 6, was used as simulated saline groundwater to compare with literature data [ 4,6,7 ] obtained in the same environment. The dissolved oxygen in the test solution was removed by purging with nitrogen gas in the same glove box (<1 ppm oxygen) for at least 24 h. The titanium cells were placed in polytetrafluoroethylene (PTFE) containers having a capacity of 500 ml and the test solution was added.…”

Long term integrity of overpack closure weld for HLW geological disposal Part 1 - prediction and evaluation method for structural integrity of weld joint

“…They also showed that no significant effects of hydrogen absorption were expected on the basis of measurements of diffusive hydrogen concentrations in the weld zone (see LongTermCor2010-20). 15 Detectability of NDE for weld flaws As described in the section 'Structural integrity evaluation model for overpack weld joint', the presence of weld flaws is connected directly to the driving force for the crack growth. In particular, the weld flaw height is recognised as a critical dimensional index in the proposed evaluation model.…”