Thorium

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. Rev. 0 Page 7 MS-05