The FuTure oF nuclear Fuel cycle - MIT Energy Initiative

As can be observed from Figure A.2, all common fissile isotopes exhibit a similar increase in
the number of neutrons available for breeding at high energies. However, for Uranium-233,
unlike U-235 and Pu-239, the number of neutrons released per absorption in the thermal
neutron energy region (below 1 eV) is sufficiently higher than 2.0 to enable breeding also
in thermal reactors. Thus, self sustainable Th-U fuel cycles can operate at any neutron spectrum,
while the U-Pu fuel cycle unavoidably requires a fast neutron spectrum.
In a thermal spectrum, Th-232 has higher neutron capture cross section than U-238 by
about a factor of three, which also makes the fertile to fissile material conversion more efficient.
It has the additional advantage of reducing the excess reactivity in an initial core,
which reduces the amount of needed control material to suppress this initial reactivity.
These unique features of Th – U-233 fuel combine to allow the proven light water reactor
technology to be used for achieving self sustainable reactor operation thus avoiding the
development of more complex fast reactors with large cost uncertainty.
It is also worth noting that even in fast breeder reactors, the thorium fuel cycle may offer
some advantages with respect to flexibility of the core design. One of the safety related
concerns common to all fast spectrum reactors is the positive reactivity feedback due to the
coolant thermal expansion. The use of Th fuel reduces the magnitude of this effect (or may
even eliminate it) because of the smaller increase in the number of neutrons released per
absorption in U-233 as the spectrum hardens as compared with other fissile nuclides, and
also due to the smaller fast fissions effect of Th-232 compared to U-238.
Considerable research has been conducted in the past to investigate feasibility of thorium
cycle [IAEA, 2005, Todosow et al., 2005, Kim and Downar, 2002]. Thorium fuel has been
irradiated and examined in a variety of reactors, including the US and German gas cooled
reactors featuring coated particle fuel, in addition to boiling and pressurized water reactors
at Elk River and Indian Point in the US. These studies showed very good performance
of Th fuel as a material [Belle and Berman, 1984], in both oxide form in LWRs as well as
in carbide form in gas cooled reactors. Economics favored the uranium fuel cycle and the
work was discontinued.
Most notably however, the feasibility of a closed Th – U-233 fuel cycle has been demonstrated
by the Light Water Breeder Reactor (LWBR) program in a pressurized water reactor
at Shippingport, Pa. The results of this program confirmed experimentally that net breeding
of U-233 (with a fissile conversion ratio of just over one) can be achieved using a heterogeneous
uranium-thorium core in a thermal spectrum light water reactor [Atherton, 1987].
In the near future perspective, a number of difficulties will have to be overcome in order for
the Th-U fuel cycle to be implemented in the current or advanced reactors.
proliFeration and SeCurity GroundruleS. Irradiating thorium produces weaponsuseable
material. Policy decisions on appropriate ground rules are required before devoting
significant resources toward such fuel cycles. U-233 can be treated two ways.
p Analogous to U-235. If the U-235 content of uranium is less than 20% U-235 or less
than 13% U-233 with the remainder being U-238, the uranium mixture is non-weapons
material. However, isotopic dilution in U-238 can significantly compromise many of the
benefits.
appendix a: Thorium Fuel cycle options 183