Summary of Investigations on Technical Feasibility of Direct Disposal of Dual-Purpose Canisters

Abstract

This report is a revision of the Fiscal Year (FY)-15 summary report on the investigations on technical feasibility of direct disposal of dual-purpose canisters (DPCs) in a geological repository. Notable progress has been made in the postclosure criticality area supporting direct disposal of DPCs in As such, this revision updates the postclosure criticality related studies to capture the advances made over the last two FYs.This study has evaluated the technical feasibility of direct disposal in a geologic repository, of commercial spent nuclear fuel (SNF) in dual-purpose canisters (DPCs) of existing designs. The authors, representing several national laboratories, considered waste isolation safety, engineering feasibility, thermal management, and postclosure criticality control. The 5-year study concludes that direct disposal is technically feasible for most DPCs, depending on the repository host geology. Postclosure criticality control, and thermal management strategies that allow permanent disposal within 150 years, are two of the most challenging aspects. This document summarizes technical results from a series of previous reports, and describes additional studies that can be done especially if site-specific information becomes available from one or more prospective repository sites.Generic (non-site specific) performance assessments have been conducted for generalized disposal concepts in different host media as part of the used fuel disposal and Spent Fuel and Waste Science and Technology research and development (R&D) programs. These show how regulatory performance objectives on individual protection and groundwater protection could be met; however, they are not detailed enough to discern differences in performance between DPC direct disposal and disposal of the same SNF in purpose-designed packaging. Such differences could arise because of the quantity of waste in each package, the duration of elevated temperature, and/or the internal design of the canisters. More detailed simulation of postclosure waste isolation is an important area that can be advanced when sitespecific information becomes available. Another area is the R&D needed to incorporate cementitious materials into repository design, which could be beneficial for any disposal concept but more important for disposal of DPC-based packages because of their size and weight. Postclosure criticality control is essentially also a safety question but is addressed below in a separate discussion.Handling and packaging of DPCs are within the state-of-the-practice in the U.S. nuclear industry, so engineering feasibility and preclosure safety can be assured. The means of transporting DPC-based waste packages underground and emplacing them in disposal tunnels are more developmental, but the equipment would be similar to existing designs for shielded DPC handling equipment. Designs and relevant experience exist for shafts, ramps, and funicular options to transport waste packages underground. In most cases, such systems would be largest of their kinds and could...