Development of waste packages for TRU-disposal. 7. Development of the long-term confinement container (titanium-carbon steel composite container) for TRU waste disposal

An innovative TRU titanium (Ti-Pd alloy) clad steel waste package has been developed to confine mainly C-14, one of the dominant nuclides involved in a performance assessment of TRU waste geological disposal. From the results of the performance assessments, the target containment period of this waste package is set at about 60,000 years such that the dose rate of this case is approximately two orders smaller than that of the reference case, where the containment function is not taken into consideration. Consequently, the focal point of development is enhancing the reliability of long-term integrity against corrosion of candidate titanium alloys in the TRU waste repository environment. The corrosion modes assumed in this environment are as follows: 1) depassivation and 2) crevice corrosion in the early stage of disposal, and 3) hydrogen-induced stress corrosion cracking (SCC) in the later stage. Based on these results, the greater part of a corrosion resistant layer with a thickness of seven millimeters was thought to remain stable for approximately 60,000 years. As a result, the above target confinement period was found to be feasible with respect to TRU waste repository, with the use of a Ti-Pd alloy as the corrosion resistance layer of the composite container. (author)