External Source Effects and Neutronics in Accelerator-driven Systems

Abstract: Transmutation of plutonium and minor actinides inaccelerator-driven systems (ADS) is being envisaged for thepurpose of reducing the long-term radiotoxic inventory of spentnuclear reactor fuel. Consequently, the physics of sub-criticalsystems are being studied in several different experimentalprograms across the world, one of them being the MUSE(MUltiplication of External Source) program. In theseexperiments, an intense external neutron source is coupled to asub-critical core. In order to investigate the neutronics andsourceeffects in a sub-critical system, Monte Carlosimulations have in this thesis been performed for a modelrepresentative of the MUSE-4 experiments. The investigationshave focused on three different neutronic parameters; theneutron energy spectrum, the external neutron source efficiency(?') and the dynamic neutron source response.In order to study the beam power amplification of an ADS, wehave introduced a new parameter, the proton source efficiency(?'). ?' represents the average importance of theexternal proton source, relative to the average importance ofthe eigenmode neutron production. It is defined in analogy withthe neutron source efficiency ?', but relates the corepower to the source protons instead of to the source neutrons.?' is commonly used in the physics of sub-criticalsystems, driven by any external neutron source (spallationsource, (d,d), (d,t),252Cf spontaneous fission etc.). On the contrary,?' has been defined only for ADS studies, where the systemis driven by a proton-induced spallation source. The mainadvantages with using ?' instead of ?' are that theway of defining the external source is unique and that ?'is proportional to the core power divided by the proton beampower, independently of the neutron source distribution. Thesecond part of this thesis has focused on studying ?' as afunction of different system parameters, thereby providing abasis for an ADS design with optimal proton beamamplification.