Applicability of polymer and composite inner linings in containers for borehole disposal of sealed radioactive sources − A simulation-based study of radiation effects

Abstract Borehole disposal concept is a widely applicable, standardized solution for disposal of sealed radioactive sources, developed jointly by IAEA and South African NECSA with the goal of offering a safe and secure disposal solution, based on readily accessible gear and materials. Previous research has considered use of polymer and composite materials in place of standard materials such as concrete and steel in various phases of radioactive waste management. Use of alternatives such as these can be advantageous in environments that have adverse effects on commonly utilized materials, in terms of chemical aggressiveness or radiation damage. The present study examines the suitability of polymers and polymer-based composites for potential use as inner lining in borehole disposal containers, from the perspective of radiation effects. Investigated materials, not envisioned by the original concept, include HDPE, PS, PEEK, PEI and PSU, as well as composite materials having one of these polymers as a matrix reinforced by boron fibers. Such containers would be simpler and safer to manipulate, owing to the lower mass of these materials. Applicability of the investigated materials is analyzed by using Monte Carlo simulations to determine the energy deposited or absorbed dose in various container linings. These results are then used for calculating the overpressure within the container, caused by radiation-induced (i.e. radiolytic) gas generation, as well as the changes in polymer molecular weights which may influence their mechanical characteristics. Radioactive sources used in simulation models, positioned inside disposal containers of known standard dimensions, include Co-60 and Cs-137 gamma sources and the americium-beryllium (AmBe) neutron source. The obtained results indicate that the examined materials do not produce any significant increase in container gas pressure or experience notable changes in molecular weights, and are hence a viable option for linings in borehole disposal containers.