Design and core performance analysis of a Lead-cooled Fast Reactor for plutonium re-qualification: development and verification of a new fuel transmutation code

Lead-cooled Fast Reactors represent a promising reality among the concepts proposed by the Generation IV Forum, allowing the production of cheap and safe electricity while exploiting the fast spectrum of neutrons to burn plutonium and minor actinides. Nowadays, the closure of the fuel cycle represents a key point for the discouragement of proliferation actions and the utilisation of nuclear fuel. Thus, one of the purposes of the future nuclear fission reactors based on liquid metals is to address the closure of the fuel cycle. In this framework, one possibility is to enhance the utilisation of wasted plutonium coming from the actual Light Water Reactors (LWRs) that is currently planned to be stored in geological repositories. This is indeed possible by using a Fast Reactor: a regeneration process can be exploited with this plutonium in order to make it burnable in LWRs. The aim of this thesis is the design of Lead Fast Reactor based of the LFR-AS-200 concept proposed by newcleo to re-qualify the exhausted Mixed Oxide Fuel (MOX) of current LWRs. By means of a hard neutron spectrum, the isotopic composition of plutonium can be modified to increase the quality of plutonium. Starting from the initial configuration of the LFR-AS-200, a Serpent-2 model has been developed and the core performances have been investigated considering as fuel a MOX of second generation. The initial characteristics, mainly the fuel inventory and the core geometry, turned out to be unsatisfactory in terms of criticality and plutonium quality. To address these issues, an improved design was proposed, with some geometry modifications, the insertion of an axial blanket (depleted uranium) and the change of fuel enrichment. Since full Monte Carlo burnup simulations are computationally expensive, a fuel transmutation code, namely TREMOR, was then developed in order to predict the fuel composition at the discharge and to speed up the investigations. TREMOR was firstly verified through a code-to-code comparison with a Serpent-2 calculation and then used to optimise the design phase. Once a satisfactory configuration was obtained, the corresponding operative cycle was analysed. The subsequent post-irradiation decay was studied with TREMOR, completing the fuel analysis. Finally, a coupled simulation with the FRENETIC code was carried out to preliminarily verify the core design by means of neutronic and thermal-hydraulic analyses. Although the thermo-mechanical aspects have not be assessed, the study hereby carried out has shown that it is possible to exploit the LFR-AS-200 concept for the plutonium re- qualification, respecting the neutronic and thermal-hydraulic constraints while maintaining the core criticality with an optimised fuel utilisation.