Source efficiency and high-energy 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. For this reason, the physics of sub-critical systems are being studied in several different experimental programs across the world. Three of these experiments have been studied within the scope of the present thesis; the MUSE experiments in France, the Yalina experiments in Belarus and the SAD experiments in Russia. The investigations of the MUSE experiments have focused on three different neutronic parameters; the neutron energy spectrum, the external neutron source efficiency and the dynamic neutron source response. It has been shown that the choice of external neutron source has negligible effect on the neutron energy spectrum in the core. Therefore, from this point of view, the MUSE experiments can be considered representative of an ADS. From the analyses of different reactivity determination methods in the Yalina experiments, it can be concluded that the slope fit method gives results in good agreement with the results obtained by the Monte Carlo method MCNP. Moreover, it was found that the Sjöstrand method underestimates keff slightly, in comparison with MCNP and the other investigated methods. In the radiation shielding studies of the SAD experiments, it was shown that the entire part of the effective dose detected at the top of the biological shielding originates from the proton-induced spallation reactions in the target. Thus, it can be concluded that the effective dose is directly proportional to the proton beam power, but independent of the reactivity of the sub-critical core. In order to study the energy gain of an ADS, i.e., the core power divided by the proton beam power, the proton source efficiency, ?*, has been studied for various ADS models. ?* is defined in analogy with the neutron source efficiency, ?*, but relates the core power directly to the source protons instead of to the source neutrons. ?* 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, ?* has been defined only for ADS studies, where the system is driven by a proton-induced spallation source. The main advantages of using ?* instead of ?* are that the way of defining the external source is unique and that ?* is proportional to the energy gain. An important part of this thesis has been devoted to studies of ?* as a function of different system parameters, thereby providing a basis for an ADS design with optimal properties for obtaining a high core power over beam power ratio. For instance, ?* was found to decrease considerably with increasing spallation target radius.
School:Kungliga Tekniska högskolan
Source Type:Doctoral Dissertation
Keywords:NATURAL SCIENCES; Physics; Atomic and molecular physics; Atomic physics
Date of Publication:01/01/2005