Development of beta-voltaic power cells via neutron techniques for the reuse of nuclear waste

Abstract

This work presents an overview of the approach to nuclear waste management, particularly for irradiated graphite. Within which ¹⁴C and ³H are highlighted as commercially useful and valuable if extracted.
The agreed consensus on the dominant neutron activation production routes for ¹⁴C is challenged, where ¹⁴N is understood as the main source, and the lack of emphasis on ¹³C is highlighted. High uncertainty in previous ¹³C neutron capture cross section measurements contribute to this assessment, and only a single recent experiment by A. Wallner et al. in 2016 has revealed a previously unseen resonance with a high degree of certainty. In light of this, the breadth of variance in the agreed neutron reaction cross-sections for ¹³C is outlined through the use of Monte-Carlo simulation software; GEANT4 and MCNP6.2. Bringing into sharp focus the need for more comprehensive measurements of the neutron reaction cross-sections of ¹³C.
A new methodology is developed which allows for the production of more massive and more dense solid samples using an ethanol slurry compaction technique. Where the influence of any hydrogen impurities is minimised through the use of ethanol as a binder which dries off quickly and easily in a vacuum furnace. The ¹³C material is characterised through a variety of techniques, to better understand the structure and composition of the powder. Pelletron Accelerator neutron capture cross-section measurements are performed at the Tokyo Institute of Technology at 50 keV, where the influence of neutron scattering is quantified through analysis and concurrent MCNP6.2 simulations highlight discrepancies between experimental and simulated data. Initial data indicates a possible second resonance at 50 keV and a factor 31 enhancement in the neutron capture cross section at this energy, having implications for the dominant production route for ¹⁴C in much of the UK nuclear waste graphite inventory as well as a potential underestimation of its total ¹⁴C content. Further experiments at the Tokyo Institute of Technology aim to reduce uncertainty in the data and probe the original 150 keV resonance.
Several potential uses of ¹⁴C material from nuclear waste are highlighted, and beta- voltaics are chosen as an area ripe for research and commercial potential. A novel design is proposed and explored with the radioisotope situated within the semiconductor bulk to improve efficiency and lower cost. Wherein diamond is chosen as an ideal semiconductor for this purpose. A range of techniques to incorporate radioisotope into the semiconductor is outlined and neutron activation is highlighted as a rapid prototyping route in a research setting. A swathe of MCNP6.2 calculations are run to assess different diamond compositions and incorporation methods which are suitable for neutron activation. Several designs stand head and shoulders above the rest as viable routes not only for prototyping such devices but also potentially commercially viable. Additionally, MCNP6.2 beta particle simulations provide ideal thicknesses for ¹⁴C and ³H diamond devices. Where initial reactor irradiation experiments at Kyoto University demonstrate the effectiveness in using diamond as an ideal semiconductor to accommodate beta emitting radioisotopes. Furthermore, it is shown that ion implantation and high temperature, high pressure infusion have potential in becoming methods for incorporating fertile precursor material to be activated. Where, given the infrastructure required, direct incorporation of radioactive ¹⁴C and ³H may also be a viable route for production.

Date of Award	3 Oct 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Thomas Bligh Scott (Supervisor) & Neil A Fox (Supervisor)