Plant Materials
DOE-HDBK-1017/2-93
FUEL MATERIALS
Thorium
Natural thorium consists of one isotope, 232Th, with only trace quantities of other much more
radioactive thorium isotopes. The only ore mineral of thorium, that is found in useful amounts
is monazite. Monazite-bearing sands provide most commercial supplies. The extraction and
purification of thorium is carried out in much the same manner as for uranium. Thorium dioxide
(ThO2) is used as the fuel of some reactors. Thorium dioxide can be prepared by heating thorium
metal or a wide variety of other thorium compounds in air. It occurs typically as a fine white
powder and is extremely refractory (hard to melt or work) and resistant to chemical attack.
The sole reason for using thorium in nuclear reactors is the fact that thorium (232Th) is not fissile,
but can be converted to uranium-233 (fissile) via neutron capture. Uranium-233 is an isotope of
uranium that does not occur in nature. When a thermal neutron is absorbed by this isotope, the
number of neutrons produced is sufficiently larger than two, which permits breeding in a thermal
nuclear reactor. No other fuel can be used for thermal breeding applications. It has the superior
nuclear properties of the thorium fuel cycle when applied in thermal reactors that motivated the
development of thorium-based fuels. The development of the uranium fuel cycle preceded that
of thorium because of the natural occurrence of a fissile isotope in natural uranium, uranium-235,
which was capable of sustaining a nuclear chain reaction. Once the utilization of uranium
dioxide nuclear fuels had been established, development of the compound thorium dioxide
logically followed.
As stated above, thorium dioxide is known to be one of the most refractory and chemically
nonreactive solid substances available. This material has many advantages over uranium dioxide.
Its melting point is higher; it is among the highest measured. It is not subject to oxidation
beyond stoichiometric (elements entering into and resulting from combination) ThO2. At
comparable temperatures over most of the expected operating range its thermal conductivity is
higher than that of UO2. One disadvantage is that the thorium cycle produces more fission gas
per fission, although experience has shown that thorium dioxide is superior to uranium dioxide
in retaining these gases. Another disadvantage is the cost of recycling thoria-base fuels, or the
"spiking" of initial-load fuels with 233U. It is more difficult because 233U always contains 232U
as a contaminant. 232U alpha decays to 228Th with a 1.9 year half-life. The decay chain of 228Th
produces strong gamma and alpha emitters. All handling of such material must be done under
remote conditions with containment.
Investigation and utilization of thorium dioxide and thorium dioxide-uranium dioxide
(thoria-urania) solid solutions as nuclear fuel materials have been conducted at the Shipping port
Light Water Breeder Reactor (LWBR). After a history of successful operation, the reactor was
shut down on October 1, 1982. Other reactor experience with ThO2 and ThO2-UO2 fuels have
been conducted at the Elk River (Minnesota) Reactor, the Indian Point (N.Y.) No. 1 Reactor, and
the HTGR (High-temperature Gas-cooled Reactor) at Peach Bottom, Pennsylvania, and at Fort
St. Vrain, a commercial HTGR in Colorado.
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