  Simulation tools have become an integral tool in the analysis of neutronic
parameters of reactor units. These simulation tools are built to solve the
neutron transport equation. In this article, the MCU-PTR simulation tool is
used in the evaluation of the possibility of achieving extra-long fuel lifetime
in the RITM-200 reactor unit with consideration to the fuel composition. The
three dispersed fuel compositions (238U+235U)O2, (232Th+235U)O2 and
(232Th+233U)O2 are simulated at varying fuel element diameter of 4.9 mm to 8.9
mm with 1 mm interval. From the simulation, the dependence of reactivity margin
on the fuel burnup of the three dispersed fuels were established. The
dependence of the effective multiplication factor (keff) on the reactor
operation time for the three dispersed fuel composition were also determined.
From the results, it can be deduced that transition of the fuel from (238U +
235U)O2 to (232Th + 235U)O2 results in a 12% increase in the fuel burnup and no
change in fuel campaign. While the transition from (238U + 235U)O2 to (232Th +
233U)O2 leads to a 32.4% increase in the fuel campaign and 45.6% increase in
the fuel burnup. It has also been shown that the optimal fuel element diameter
is 7.9 mm for (232Th + 233U)O2 dispersed fuel. At this diameter, it is possible
to increase the duration of the fuel campaign by 85.3% and achieve a fuel
burnup of 51.9% in comparison to the design fuel diameter of 6.9 mm for (238U +
235U)O2 dispersed fuel.
