The FuTure oF nuclear Fuel cycle - MIT Energy Initiative

the core are limited for the MOX fuel because neutron captures in U-238 generate new Pu
as the originally loaded Pu is burnt by fission.
Thorium, on the other hand, would not generate any new Pu if used as a matrix for Pu disposition.
The use of Th fuel matrix instead of natural or depleted uranium would roughly
triple the rate of Pu destruction and improve the fractional Pu burnup from about 20% for
the conventional MOX case to over 60% for the Th MOX case [Shwageraus et al.,2004].
Even deeper burndown is possible if Weapons Grade (WG) Pu is used as fissile driver instead
of Reactor Grade (RG) Pu. However, using thorium as the exclusive inert material
creates a proliferation issue since the resulting uranium would be weapons-useable and
recoverable by chemical separation.
Isotopic composition of the Pu vector changes dramatically with irradiation in both Uranium
and Thorium based MOX fuels because the fissile isotopes are preferentially depleted
in the thermal LWR spectrum leaving mostly “even” Pu isotopes in the spent fuel. Such an
isotopic mix is generally considered more proliferation resistant.
Production of minor actinides in the Pu-Th MOX fuel will also be less than in Pu-U MOX,
reducing the long-term environmental impact of the spent fuel in the repository [Gruppelaar
and Schapira, 2000].
Results of several independent studies showed that LWR core physics of mixed oxide PuO 2 -
ThO 2 fuel is very similar to the conventional PuO 2 -UO 2 MOX fuel leading to the conclusion
that Th MOX fuel can be used in the existing LWRs with minimal impact on the core design
and operation [IAEA, 2003].
Although all of the studies so far focused on once-through Pu burndown, multiple Pu recycling
may be feasible with ThO 2 matrix in LWRs because of the more favorable void coefficient
of reactivity.
Thorium that is used as a ThO 2 matrix has very favorable thermo-physical properties such
as higher than UO 2 thermal conductivity and lower coefficient of thermal expansion. It has
also been shown that ThO 2 has higher radiation stability and retention of fission gases when
irradiated to the same burnup at the same temperature [Belle and Berman, 1984].
The once-through Pu disposition scenario assumes direct disposal of the spent MOX fuel in
a geological repository. In this case, the high chemical stability of ThO 2 becomes beneficial.
Thorium has only one oxidation state unlike Uranium which oxidizes from UO 2 to U 3 O 8
and thus tends to be more mobile in the repository environment such as Yucca Mountain.
Smaller amounts of long lived minor actinides also help to reduce the long term radiotoxicity
of the spent fuel. However, decay of Th transmutation chain nuclides such as Pa-231
and U-233 make the spent Th MOX fuel in-situ radiotoxicity even higher than that of the
conventional MOX for the time period between about 10 4 and 10 6 years [Gruppelaar and
Schapira, 2000, IAEA, 2003].
Although initially loaded Pu is effectively destroyed using the Th MOX, the presence of
U-233 in the spent fuel, by itself, raises proliferation concerns because, theoretically, it can
be chemically separated and used in weapons. As mentioned earlier, one approach would be
to argue that trace quantities of U-232 will be carried together with U-233 and therefore provide
sufficient barrier against diversion of fissile material due to the high radiation dose rate.
appendix a: Thorium Fuel cycle options 185