Spallation Neutron Source for an Accelerator Driven Subcritical Reactor

The progressive improvement in the operating performance of existing nuclear reactors has brightened the outlook for nuclear power around the globe. In order to use less carbon based power, the use of nuclear reactors provides an attractive solution. However, questions are raised by people on the safety and long term radioactive waste disposal in conventional nuclear reactor. Also a need for uranium enrichment replacement arises due to security and proliferation issues. Accelerator driven sub critical reactors (ADSR) are the subject of international research and development due to their enhanced safety and the potential to deal with nuclear waste. To sustain fission, an ADSR utilizes the neutron produced by spallation which has become an established technique for the production of high intensity neutron ux. Development of MYRRHA (Multipurpose hYbrid Research Reactor for High-tech Applications) is one of the promising designs in this area.

The focus of this thesis is to examine the potential of MYRRHA, with thorium fuel, as an actinide burner using the GEANT4 simulation toolkit. It compares the neutron uxes and spectra in the reactor for thorium based fuel with those for a standard uranium-plutonium mixture. The uxes and spectra that would be useful for transmutation studies are examined at designated locations of the reactor: the fuel cells, In-Pile Section (IPS) regions and isotope production cells. From this, the feasibility of the thorium mixture is demonstrated and the burn up rates are calculated. Fuel evolution studies are performed by solving the Bateman equations.

The Geant4 simulation toolkit is used for the investigations and the results are compared to MCNPX predictions. Before implementing the detailed geometry of MYRRHA in Geant4, a simple model is studied to predict the numbers and properties of spallation neutrons produced by proton beams on simple lead targets for proton energies between 100 and 1400 MeV. This is relevant for ADSR systems and other neutron sources. The results agree well with the limited experimental data for the spallation neutron yields. Results of MCNPX and Geant4 are compared and show good agreement between the two programs for the overall numbers, energy spectra, spatial and radial distributions. A parametrized form is presented which can be used for neutronics studies in reactor cores.

The use of thorium as an alternative to uranium fuel has advantages due to its proliferation resistance, abundance in nature and nuclear waste manage-ment. Only small quantities of plutonium and minor actinides are generated in the thorium fuel cycle, which reduces the long term radiotoxicity of the spent fuel. However, few detailed numerical studies have been performed on this. Geant4 modeling of a reactor is not supported by the default toolkit as this does not provide data for isotopes having atomic number Z >92. The JEFF 3.1 library was installed to import the data for transuranic elements and changes were needed to the program.

The results reveal that MYRRHA can be utilized as a prototype for industrial transmutation system as it can convert a measurable amount of minor actinide waste into short lived products. Specifically, the amount of americium produced is much smaller than the amount incinerated, if thorium fuel is used.