The UK government is responsible for the world’s largest stockpile of civil plutonium (Pu). The intention is to manage the stockpile through the implementation of an appropriate recycling strategy, expected to centre around the use of Mixed OXide (MOX) fuelled Light Water Reactors (LWRs). Typically, MOX fuel involves the use of uranium (U) as a fertile carrier matrix for fissile Pu. However, the effect of aging and isotopic decay within the UK Pu stockpile impacts the feasibility of this approach. The build-up of americium-241 from decay of Pu-241 leads to increased fissile feed requirements which, in the case of U-Pu MOX fuels causes the Void Coefficient (VC) to become positive under transient conditions for a stockpile averaged Pu vector from the year 2055 onwards. This is unacceptable from a regulatory perspective. Replacing uranium with thorium (Th) significantly improves reactivity feedback coefficients such that, if UK Pu is to be recycled with Am-241 in-situ, Th-Pu MOX fuels provide a favourable alternative to U-Pu MOX. Analysing the effect of isotopic composition on reactivity feedback coefficients showed that the fissile isotopes provide the greatest contributions, regardless of the Am-241 content in the fuel. The main issue to note is that the use of Th-based MOX fuels results in Moderator Temperature Coefficient (MTC) trends which do not become less negative with burnup, meaning that batch averaging effects cannot be relied upon as a passive safety measure. Heterogeneous loading of Am and Pu where Am-241 is concentrated in approximately half of the peripheral fuel assembly pins has minimal effect on the overall Pu and Am destruction rates in the PWR and does not lead to significant improvements in the MTC trends. However, radial, and axial heterogeneous loading of Am and Pu in the ABWR offers fuel performance benefits in terms of increased Am-241 destruction and reduced curium (Cm) accumulation. From a fuel cycle perspective, the security burden associated with the UK Pu stockpile is better managed using Th-Pu MOX than U-Pu MOX. Th-Pu MOX fuelled PWRs require a great fissile feed than U-Pu MOX fuelled PWRs and can achieve significantly higher levels of Pu and minor actinide (MA) destruction leading to rapid and more complete stockpile depletion. The higher fissile loadings and greater Pu and MA destruction potential in the Th-Pu MOX case results in a lower mass of spent nuclear fuel (SNF) produced and marginally lower decay heat, radioactivity, and radiotoxicity - though the differences between Th-Pu and U-Pu MOX SNF are small enough that this will offer only limited benefits from a handling and disposal perspective. The potential profits associated with recycling the stockpile are comparable regardless of whether the recycling vehicle used is thorium or uranium. These profits may be marginally increased by removing Am-241 from the stockpile and recycling the purified plutonium. However, the difference in profits associated with removing the Am-241 from the stockpile versus leaving the Am-241 in-situ is minor. In addition, removing the Am (or “cleaning” the stockpile of Am-241) will complicate the overall UK Pu management strategy because an additional strategy would be needed to deal with the separated Am-241. Implementation timescales are important as delays in selecting a recycling strategy lead to greater fissile feed requirements needed to overcome the reactivity penalty associated with increased levels of Am-241. This further complicates the fuel manufacturing process, limits the income potential, and prolongs the security burden. A major difference now compared to fifteen years ago is that the need to design and build a MOX fuel fabrication facility (MFFF) means that Th-Pu MOX fuels have the opportunity to be ready for commercial use within the same timescale as U-Pu MOX fuels if research and development (R&D) into Th-UK-Pu MOX is conducted in parallel with MFFF construction and whilst R&D into UK Pu in general is ongoing. Three papers have been published as a result of the research presented in this thesis, with an additional paper currently in preparation:

1. Morrison, S. L., Lindley, B. A., Parks, G. T., Isotopic and spectral effects of Pu quality in Th-Pu fuelled PWRs, Annals of Nuclear Energy, Volume 117, 318–332 (2018).
2. Morrison, S. L., Parks, G. T., The effect of Am-241 on UK plutonium recycle options in thorium-plutonium fuelled LWRs – Part I: PWRs, Annals of Nuclear Energy, Volume 135, 106952 (2020).
3. Morrison, S. L., Parks, G. T., The effect of Am-241 on UK plutonium recycle options in thorium-plutonium fuelled LWRs – Part II: BWRs, Annals of Nuclear Energy, Volume 135, 106974 (2020).
4. Morrison, S. L., Worrall, A., Gregg, R., Parks, G. T., Recycle options for UK plutonium using MOX fuelled LWRs (in preparation).