Neutronic and burnup studies of accelerator-driven systems dedicated to nuclear waste transmutation

Partitioning and transmutation of plutonium, americium, and curium is inevitable if the radiotoxic inventory of spent nuclear fuel is to be reduced by more than a factor of 100. But, admixing minor actinides into the fuel severely degrades system safety parameters, particularly coolant void reactivity, Doppler effect, and (effective) delayed neutron fractions. The incineration process is therefore envisioned to be carried out in dedicated, accelerator-driven sub-critical reactors (ADS). However, ADS cores operating in concert with light-water reactors (two-component scenario) also exhibit high burnup reactivity swing with penalty on the system performance/economy. In the frame of this design work, we attempted, by choice of coolant and optimisation of fuel concept and core design, to achieve favourable neutronic, burnup and safety characteristics of the transuranium ADS burner. Key thermal hydraulic and material-related constraints were respected. A novel fuel matrix material, hafnium nitride, was identified as an attractive diluent option for highly reactive transuranics. (TRU,Hf)N fuels appeared to have a good combination of neutronic, burnup and thermal characteristics: maintaining hard neutron spectra, yielding acceptable values of coolant void reactivity and source efficiency, and providing small burnup reactivity loss. A conceptual design of a (TRU,Hf)N fuelled, lead/bismuth eutectic cooled ADS was developed. The average discharge burnup of 20% fissions per initial metal atom could be reached even without fuel reshuffling. The fission fraction ratios of even-neutron number americium nuclides are increased by a factor of two in comparison to burners with inert matrix based fuels. Hence, thanks to the reduced production of higher actinides and helium, fuel cycle economy is improved. The coolant void worth proved to be a strong function of the fuel composition - reactor cores with high content of fertile material or minor actinides in fuel exhibit larger void reactivities than systems with plutonium-rich, inert matrix fuels. In reactor systems cooled by lead/bismuth eutectic, a radial steel pin reflector significantly lowered coolant void reactivity. For transuranic fuel, fertile and strongly absorbing matrices exhibited increasing void worth with increasing pitch, while the opposite was valid for the coolant void worth of inert matrix fuels. Large pitches also appeared to be beneficial for limiting the reactivity worth of the cladding material and improving source efficiency. The economy of the source neutrons was investigated as a function of core and target design. An incentive to design the core with as low target radius as allowable by the thermal constraints posed by the ability to dissipate accelerator beam power was identified.