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In this paper we simulated the behavior of a simple ADS model, based on MOXfuel embedded in solid lead, in terms of multiplication coefficient keff,thermal power and absolute neutron spectra. In the first part of the paper, wereport on the results obtained when modifying the reflector surrounding thefission core, by replacing pure lead with a layered graphite/lead structure. Wefound that, by appropriately choosing position and thickness of the graphiteand lead layers, it is possible to obtain a hybrid system where the neutronspectrum in the core still exhibits a fast character, while the spectrum in thegraphite layer is considerably softer, becoming thermal in the most peripheralpositions. In order to obtain such a modulation of the neutron spectra from thecenter of the system to the periphery, a careful choice of the materials has tobe made in order to avoid large variations of the local power at the coreboundary. However, the smoothness of the power distribution is obtained at theexpense of lower values of keff and the total power of the system. In thesecond part of the paper, we explored the option of adopting light water ascoolant, instead of the helium gas assumed in the initial design. We found thatthis produces an increase in keff and thermal power, without significantlyperturbing the fast character of the system and without introducing spatialpower excursions in any place within the core. The characteristics obtained mayallow to design a system where fast, mixed and thermal spectra can be used toexpand the use of the ADS as an irradiation facility.

Influence of reflector materials and core coolant on the characteristics of accelerator driven systems