  In this paper we simulated the behavior of a simple ADS model, based on MOX
fuel embedded in solid lead, in terms of multiplication coefficient keff,
thermal power and absolute neutron spectra. In the first part of the paper, we
report on the results obtained when modifying the reflector surrounding the
fission core, by replacing pure lead with a layered graphite/lead structure. We
found that, by appropriately choosing position and thickness of the graphite
and lead layers, it is possible to obtain a hybrid system where the neutron
spectrum in the core still exhibits a fast character, while the spectrum in the
graphite layer is considerably softer, becoming thermal in the most peripheral
positions. In order to obtain such a modulation of the neutron spectra from the
center of the system to the periphery, a careful choice of the materials has to
be made in order to avoid large variations of the local power at the core
boundary. However, the smoothness of the power distribution is obtained at the
expense of lower values of keff and the total power of the system. In the
second part of the paper, we explored the option of adopting light water as
coolant, instead of the helium gas assumed in the initial design. We found that
this produces an increase in keff and thermal power, without significantly
perturbing the fast character of the system and without introducing spatial
power excursions in any place within the core. The characteristics obtained may
allow to design a system where fast, mixed and thermal spectra can be used to
expand the use of the ADS as an irradiation facility.
