High Polymer Conlposites for Containers for the Long-Term Storage of Spent Nuclear Fuel and High Level Radioactive Waste

Abstract

The feasibility of using polymeric composite materials as an alternative to metals in the design of a nuclear waste disposal container was examined. The disposal containers would be stored in deep underground vaults in plutonic rock formations within the Canadian Shield for several thousands of years. The conditions of disposal considered in the evaluation of the polymeric composite materials were based on the long-term disposal concept proposed by Atomic Energy of Canada Limited. Four different composites were considered for this work, all based on boron fibre as reinforcing material, imbedded in polymeric matrices made of polystyrene(PS), polymethyl methacrylate (PMMA), Devcon 10210 epoxy, and polyetheretherketone (PEEK).

Both PS and PMMA were determined as unsuitable for use in the fabrication of the storage container because of thermal failure. This was determined following thermal analysis of the materials in which heat transfer calculations yielded the temperature of the container wall and of the surroundings resulting from heat generated by the spent nuclear fuel stored inside the container. In the case of the PS, the temperature of the container, the buffer and the backfill would exceed the 100°C imposed in the AECL's proposal as the maximum allowable. In the case of the PMMA, the 100°C temperature is too close to the glass transition temperature of this material(105°C) and would cause structural degradation of the container wall. The other two materials present acceptable thermal characteristics for this application.

An important concern for polymeric materials in such use is their resistance to radiations. The Devcon 10210 epoxy has been the object of research at the Royal Military College in the past years and fair, but limited, resistance to both neutrons and gamma radiation has been demonstrated, with the evidence of increased mechanical strength when subjected to moderate doses. Provided that the container wall could be sufficiently shielded from the radiations emitted by the spent nuclear fuel or other high level radioactive waste, this material may well be an interesting candidate for this application. More recent work at RMC on the effects of radiations on PEEK has demonstrated that this high polymer thermoplastic material was even superior to epoxies under radiation environments. Part of this research concentrated on the estimation of the doses accumulated in the container wall over the years using three basic models for the container: one without filling material, one with glass beads as proposed by AECL, and one using thorium dioxide (ThO₂)as filling material. This choice is based on the excellent physical and chemical properties of this compound (resistance to corrosion in particular) and to the expected low cost since thorium is usually discarded in the tailings of uranium mine concentrating plants. The dose calculations were carried out using the MicroshieldTM software and showed that both the epoxy and the PEEK could maintain structural integrity provided that they are shielded sufficiently against the radiations emitted by the high level radioactive waste. This research investigated also the resistance to the mechanical forces to which the container walls would be submitted in the underground vaults and it was concluded that these materials displayed sufficient mechanical strength for such application. It also permitted the identification of several aspects of the design of the storage containers that need closer investigation.