NUCLEAR FUELS 



469 



known for most of thorium's uses, especially in gas 

 mantles and in alloys (Shortt, 1970, p. 209). Being 

 radioactive, thorium has considerable potential as a 

 nuclear fuel. It cannot, however, be used directly in 

 a nuclear reaction, because it does not contain fission- 

 able isotopes to start the reaction ; bombardment by 

 neutrons from fissionable U^^^ converts thorium to 

 fissionable U^^^ Five types of thorium-cycle reactor 

 systems have been investigated by the U.S. Atomic 

 Energy Commission (Stevens, 1969, p. 1072). The 

 farthest advanced of these is the HTGR (high- 

 temperature gas-cooled reactor). A 40-MWE (mega- 

 watts electrical) prototype of the HTGR was put 

 into operation by Philadelphia Electric at Peachbot- 

 tom. Pa., in 1966. The first reactor designed specially 

 for commercial use is near Platteville, Colo., where 

 Public Service Co. of Colorado has a 330 MWE 

 HTGR that is expected to start producing electricity 

 in April 1973. Four other HTGR reactors have been 

 ordered — two of 770 MWE by Delmarva Power and 

 Light Co. and two of 1,100 MWE by Philadelphia 

 Electric. The HTGR has a number of technical ad- 

 vantages over the uranium reactors now in use — it 

 is more efficient to operate, runs on cheaper fuel, 

 and produces less thermal pollution. The Public 

 Service Co. of Colorado's HTGR will have an effi- 

 ciency of 39 percent, which is on a par with the 

 best coal-operated electric generators and is better 

 than the best uranium generators in use in 1971, 

 which have 34 percent efficiency. 



EXPLOITATION 



Thorium was discovered by Berzelius in 1828, but 

 the first real demand for the element was created 

 after Carl Auer von Welsbach invented the thoriated 

 incandescent gas mantle in 1891. The only common 

 thorium mineral known at that time was monazite. 

 Although monazite is also a principal source of rare 

 earths, monazite production from 1895 to 1921 (fig. 

 56) reflects the demand for ThOz and minor amounts 

 of CeOz used in the gas mantle industry (Olson and 

 Overstreet, 1964, p. 3). Monazite production in the 

 United States prior to 1909 came principally from 

 stream placers in North and South Carolina, and 

 about equalled the country's demand. The rest of 

 the world production came chiefly from beach placers 

 in Brazil. From 1912 to 1945 the large beach placers 

 in India produced a major part of the world's mona- 

 zite. After 1920 the use of ThOz in gas mantles de- 

 clined (fig. 56), and the little monazite produced be- 

 tween 1921 and 1934 was used largely for its rare 

 earth content. Before World War II, uses and pro- 

 duction of both rare earths and thorium increased. 

 Because thorium is a radioactive material, India and 



Brazil nationalized their monazite deposits in 1946 

 and 1951, respectively, and restricted monazite ex- 

 ports during the period in which the United States 

 was building up its stockpiles. The difference in 

 demand was taken up by monazite from beach 

 placers, chiefly in Australia, Malaysia, South Korea, 

 Indonesia, United States, and Malagasy, and from a 

 unique vein deposit in South Africa. Both India and 

 Brazil resumed monazite mining, but separated the 

 thorium and rare earths from the monazite in local 

 plants. The thorium was not exported, but foreign 

 markets were sought for the rare earths (Over- 

 street, 1967, p. 10). During the 1950's and 1960'3 

 monazite was mainly a byproduct of titanium (il- 

 menite or rutile) mining in Australia, India, and 

 United States and of tin in Malaysia and Indonesia. 

 The only nonplacer monazite mined came from a 

 vein 13 feet thick exposed for 900 feet on the Steen- 

 kampskraal farm in Cape Province, South Africa 

 (Pike, 1958, p. 92). This vein consists of 80 percent 

 monazite and apatite, and veins of this type have 

 not been reported elsewhere. About 51,100 tons of 

 monazite were shipped from this property between 

 1952 and 1959 to Lindsay Chemical Co. in the United 

 States and 8,000 tons in 1962 and 1963 to American 

 Potash and Chemical Corp. Because of production 

 from this mine. South Africa largely dominated the 

 monazite market during these periods (fig. 56) , but 

 at present it is shut down. 



Data on the amount of ThOs actually obtained 

 from monazite production during various years are 

 not always obtainable, for two principal reasons. 

 One is that the amount of ThOz ranges from to 

 31.5 percent in various monazites (Overstreet, 1967, 

 p. 1), or from about 3.5 to 9.0 percent in commercial 

 monazite concentrates (Parker and Baroch, 1971, 

 p. 67). The ThOz content of monazite from each 

 geographical area, however, commonly falls within 

 relatively short ranges. The other principal reason 

 is that in some years, such as the late 1960's, mona- 

 zite is sold principally for its rare earth content, and 

 the amount of ThOz actually produced is not known. 

 Monazite was practically the only source of tho- 

 rium prior to 1953. Since that time thorium has also 

 been recovered as a byproduct from the following 

 two types of uranium ores : 



1. In Malagasy, from 1953 to 1968, relatively small, 

 rich uranothorianite deposits yielded 4,170 

 short tons of concentrates that averaged about 

 60 percent ThOz (Parker and Baroch, 1971, p. 

 68). These concentrates were produced prin- 

 cipally by the French Atomic Energy Commis- 

 sion and shipped to France. This amount of 

 ThOz is about twice the amount used in the 



