Neutron Production with Thorium Fuel in Accelerator Driven Subcritical Reactors

ADSRs (Accelerator-Driven Subcritical Reactors) incorporate a spallation technique which is an efficient way to produce high neutron flux by externally supplying neutrons into the reactor.

In this thesis, a review of spallation is given explaining the spallation reaction process, describing spallation reactions in dense metal and analysing the resulting neutron energy spectra. The thesis also discusses current spallation sources around the world. Studies involving proton-induced neutron production in spallation target are demonstrated. Spallation reactions on a lead target have been simulated using a Monte Carlo transport code called GEANT4, and the benchmarking of these simulations against experimental neutron spectra produced from a thick lead target bombarded with 0.5 and 1.5 GeV protons is discussed. This is followed by discussion of the angular distribution of neutrons of different energies in order to understand the emission of neutrons from a spallation target.

Lead and Lead Bismuth Eutectic (LBE) are both widely utilised to produce neutrons; this is due to the fact that lead is a high Z, heavy metal element which is relatively cheap to use. This thesis provides a comparison between lead and LBE in terms of their effect on neutron energy spectra at various projecting angles.

Given the confidence in the GEANT4 simulation provided by the benchmarking studies, the thesis goes on to discuss neutron production and behaviour in the environment of thorium-fuelled ADSRs. With a spallation target composed of LBE, the design of the MYRRHA reactor developed by SCKCEN has been configured to explore neutron production created from each layer filled with thorium fuel. This is then followed by a focus on neutrons escaping from the LBE-composed reflector, and the thesis provides an analysis of the effect of several different materials used for inner and outer shielding in the reactor core.

By using the latest nuclear data library and numerical techniques provided in the GEANT4 code, the author has been able to simulate the actual usage of thorium in an ADSR reactor set-up which has not yet been fully demonstrated. The thesis concludes with the idea that thorium could well be utilised as an actual fuel source in an ADSR for the purpose of transmutation, and possibly for energy production as well.