External Source Effects and Neutronics in Accelerator-driven Systems

Transmutation of plutonium and minor actinides in accelerator-driven systems (ADS) is being envisaged for the purpose of reducing the long-term radiotoxic inventory of spent nuclear reactor fuel. Consequently, the physics of sub-critical systems are being studied in several different experimental programs across the world, one of them being the MUSE (MUltiplication of External Source) program. In these experiments, an intense external neutron source is coupled to a sub-critical core. In order to investigate the neutronics and source effects in a sub-critical system, Monte Carlo simulations have in this thesis been performed for a model representative of the MUSE-4 experiments. The investigations have focused on three different neutronic parameters; the neutron energy spectrum, the external neutron source efficiency (φ*) and the dynamic neutron source response. In order to study the beam power amplification of an ADS, we have introduced a new parameter, the proton source efficiency (ψ*). ψ* represents the average importance of the external proton source, relative to the average importance of the eigenmode neutron production. It is defined in analogy with the neutron source efficiency φ*, but relates the core power to the source protons instead of to the source neutrons. φ* is commonly used in the physics of sub-critical systems, driven by any external neutron source (spallation source, (d,d), (d,t), 252Cf spontaneous fission etc.). On the contrary, ψ* has been defined only for ADS studies, where the system is driven by a proton-induced spallation source. The main advantages with using ψ* instead of φ* are that the way of defining the external source is unique and that ψ* is proportional to the core power divided by the proton beam power, independently of the neutron source distribution. The second part of this thesis has focused on studying ψ* as a function of different system parameters, thereby providing a basis for an ADS design with optimal proton beam amplification