As noted above, interest in thorium as a contributor to the world's useful energy supply is based
on its transmutability into the fissile isotope 233U. The ease with which this property can be
utilized depends on the impact of the nuclear characteristics of thorium on the various reactor
systems in which it might be placed and also on the ability to fabricate thorium into suitable fuel
elements and, after irradiation, to separate chemically the resultant uranium. The nuclear
characteristics of thorium are briefly discussed below by comparing them with 238U as a point of
First, a higher fissile material loading requirement exists for initial criticality for a given reactor
system and fissile fuel when thorium is used than is the case for an otherwise comparable system
Second, on the basis of nuclear performance, the interval between refueling for comparable
thermal reactor systems can be longer when thorium is the fertile fuel. However, for a given
reactor system, fuel element integrity may be the limiting factor in the depletion levels that can
Third, 233Pa (protactinium), which occurs in the transmutation chain for the conversion of thorium
to 233U, acts as a power history dependent neutron poison in a thorium-fueled nuclear reactor.
There is no isotope with comparable properties present in a 238U fuel system.
Fourth, for comparable reactor systems, the one using a thorium-base fuel will have a larger
negative feedback on neutron multiplication with increased fuel temperature (Doppler coefficient)
than will a 238U-fueled reactor.
Fifth, for comparable reactor configurations, a 232Th/233U fuel system will have a greater stability
relative to xenon-induced power oscillations than will a 238U/235U fuel system. The stability is
also enhanced by the larger Doppler coefficient for the 232Th/233U fuel system.
And sixth, the effective value of b for 232Th/233U systems is about half that of 235U-fueled reactors
and about the same as for plutonium-fueled reactors. A small value of b means that the reactor
is more responsive to reactivity changes.
In conclusion, the nuclear properties of thorium can be a source of vast energy production. As
demonstrated by the Light Water Breeder Reactor Program, this production can be achieved in
nuclear reactors utilizing proven light water reactor technology.
Nuclear Fuel Selection
The nuclear properties of a material must be the first consideration in the selection of a suitable
nuclear fuel. Principle properties are those bearing on neutron economy: absorption and fission
cross sections, the reactions and products that result, neutron production, and the energy released.
These are properties of a specific nuclide, such as 232Th, and its product during breeding, 233U.
To assess these properties in the performance of the bulk fuel, the density value, or frequency
of occurrence per unit volume, of the specific nuclide must be used.