Lead is widely used as a shielding material in nuclear power plants (nuclear and radiation facilities), and lead and lead-containing materials contaminated with radioactive materials are generated as waste during decommissioning. The operator must secure a disposal site for waste generated during decommissioning. Therefore, operators reduce waste by treating low-level radioactive waste below the clearance level through a clearance system. This study aimed to establish a method to precisely cut only the contaminated parts with the aim of minimizing the amount of contaminated lead waste. The method involves reacting lead with Bi and Sn to turn the lead into a low-melting-point alloy and then cutting it. For this low-melting-point alloying of lead, a method was examined in which a Bi-Sn alloy was produced and supplied to the lead. As a result of the study, experiments were conducted to confirm the effectiveness of a method in which the Bi-Sn alloy was dripped in liquid form on the lead cutting part and a method in which it was poured in paste form. As a result of the experiment, the method in which the Bi-Sn alloy was poured in paste form on the lead cutting part was superior in precisely cutting the targeted part.

  In nuclide migration analyses for the safety assessments of geological disposal, when there is not necessarily sufficient confidence knowledge and data required for modeling phenomena, analyses are conducted using sufficiently conservative depending on the reliability of the technical basis, such as selecting data settings and modeling that yield severe results. There is a high level of uncertainty for setting a parameter value of dispersion length used for the nuclide migration analysis, due to the difficulty in measuring field scale data. However, the effects of fluctuation of dispersion length on maximum nuclide release rate has not yet been fully understood. In this study, the effect of fluctuation of dispersion length on maximum nuclide release rate was investigated. Specifically, a series of the sensitivity analysis focusing on not only the effect of fluctuation of dispersion length but also the effect of parameters related to the retardation effect such as distribution coefficient and half-life of nuclides was performed for fracture medium. As a result, it was suggested the maximum nuclide release rate may increase due to the shortened nuclide transit time as the dispersion length increases. When peak of the nuclide release rate is less than the half-life of the nuclide, the increase of the maximum release rate due to an increase in the dispersion length remains limited to a few times. On the other hand, in case of exceeding half-life, the maximum nuclide release rate increases ten times or more as the maximum release rate occurrence time of the release rate decreases. These findings would contribute to setting the conservative value of dispersion length taking the uncertainty into consideration.