Analysis of specific design aspects of a thorium-uranium fuelled European Pressurised Reactor

Abstract

The global nuclear industry is an established industry, however, should governments decide to move forward with more nuclear power. Enough resources are required to succeed in this endeavour of generating the electric power for the years to come. The nuclear power technology has received increased attention in South Africa, especially after the publication of the IRP2010. The IAEA reported that the available uranium resources are enough to provide nuclear energy for about 100 more years at the current rate of use (NEA & IAEA, 2012). This will however not be the case should nuclear power demand increase worldwide. This would necessitate the utilization of other resources to supply the growing global energy market. Uranium alone cannot carry this load and it also produces dangerous plutonium. Therefore, a need for an alternative nuclear fuel source exists. The current pressure on governments has forced researchers to investigate alternative fuel technologies that can burn more efficiently in order to increase fuel lifetime and therefore fuel-cycle length and minimise plutonium production. The majority of thorium fuel research on PWRs is limited to reactor physics investigations and therefore require further R&D in core design and fuel-cycle optimisation in order to achieve practical and commercial implementation (IAEA, 2012). This thesis focuses on contributing research in terms of core design and fuel-cycle optimisation to help close the gap of reaching commercial readiness for thorium-uranium fuel. The study focussed on developing a full-core reference 3D model of the EPR for neutron transport simulations using MCNP6. This is unique in the field of study, since most studies model only fuel assemblies using Monte Carlo methods Gen 2 PWRs. The reference model was compared with the Final Safety Analysis Report of the EPR. Coupling was introduced between MCNP6 and RELAP5 to take into account the feedback from the thermal hydraulic network of the core in support of the research activities of the reactor analysis group at the School of Mechanical and Nuclear Engineering. The coupling methodology was correctly implemented in NWURCS; however, it is recommended to repeat the process by reducing the relative errors in MCNP6. The results of the reference EPR model were evaluated with no major differences. This also gave confidence in the verification of NWURCS in generating the input decks. The EPR reference model can now be used as the basis for the thorium-uranium fuel development. The systematic literature review was integral in understanding reactor physics when analysing thorium-based fuel in a standard PWR. The literature provided a solid foundation for the new fuel design, which formed a starting point for thorium-uranium fuel in the EPR. The design goals were that the fuel should be compatible with the compact EPR core design, while running 24-month fuel cycles and still adhering to the neutronics requirements and limits.

New thorium-uranium fuel for the EPR was developed and evaluated. The initial fissile content was changed to produce similar reactivity as compared with the uranium EPR. Different fuel compositions and combinations were tested. The newly designed thorium-uranium fuel was evaluated and the final fuel design had an equivalent atom % initial fissile content as the EPR. This was with the exception for fuel-pin sections where pure ThO2 replaced the (U-Gd)O2 sections in the original EPR design. In this way there was no need to increase the enrichment as predicted by (Herring, et al., 2001; Saglam, et al., 2003; Galperin, et al., 2001; Joo, et al., 2003) due to the effective fuel design of the EPR. This design followed the developed methodology by reducing the burnable poison (Gadolinium) requirements, optimising the initial fissile content and still achieving a 24-month fuel-cycle. Due to the fact that the initial fissile content was not increased, the reactivity coefficients and design limits for a fresh full-core Th-EPR are all within acceptable limits and there was no need to increase the soluble boron enrichment. The burnt EOL Th-EPR FAB1 properties were shown to be satisfactory. The newly designed thorium-uranium fuel for the EPR is therefore feasible and moderation control was applied to further enhance breeding. The novel idea of utilising moderation control using existing control-rod positions in available fuel assemblies was tested on the Th-EPR FAB1. Results showed an increase in the breeding of fissile content when helium filled moderation rods were inserted (to change the neutron spectrum to be slightly more epithermal). The breeding of 233U increased. Also an increase in 239Pu caused the Xe and Sm concentrations to increase, which offset the addition of excess reactivity due to higher fissile content. However, the initial Th-EPR FAB1 design without the addition of moderation rods proved to be the best choice. The project succeeded in designing thorium-uranium fuel for a new Gen 3+ PWR that reached 24-month fuel cycles without altering the geometry and disregarding any design limits. The thesis should contribute to research in terms of core design and fuel-cycle optimisation, which will support the fuel licencing and commercialisation of thorium-based PWR designs. The proposed fuel design can be investigated further as suggested in recommendations to continue the process of fuel commercialisation.