  Neutron production and transport in spallation targets made of uranium and
americium are studied with a Geant4-based code MCADS (Monte Carlo model for
Accelerator Driven Systems). A good agreement of MCADS results with
experimental data on neutron- and proton-induced reactions on $^{241}$Am and
$^{243}$Am nuclei allows to use this model for simulations with extended Am
targets. It was demonstrated that MCADS model can be used for calculating the
values of critical mass for $^{233,235}$U, $^{237}$Np, $^{239}$Pu and
$^{241}$Am. Several geometry options and material compositions (U, U+Am, Am,
Am$_2$O$_3$) are considered for spallation targets to be used in Accelerator
Driven Systems. All considered options operate as deep subcritical targets
having neutron multiplication factor of $k \sim 0.5$. It is found that more
than 4 kg of Am can be burned in one spallation target during the first year of
operation.
