| I. MATERIALS > THE SIX NUCLEAR MATERIALS > Thorium |
Description: a natural metal
Isotopes: natural thorium includes thorium 232, thorium 228, and other
isotopes in small quantitiesProduction: by extraction and treatment of minerals such as
uranothorianite and monaziteUse: production of uranium 233 by irradiation of thorium 232
Radioactivity: thorium 232 is a very radiotoxic alpha emitter
Comments: thorium 232 is a fertile substance: descendants include
radon 220 (thoron), bismuth 212, and thallium 208
Thorium, a metal, is found in nature in various minerals, including
monazite, bastnaesite, and uranothorianite. In industry, it is most often obtained as a by-product in the treatment of monazite.
In the natural state, thorium is a mixture of thorium 232, of thorium
228, and, in small but variable quantities, of four other isotopes. Thorium 232, with a period of 14 billion years, is by far the most abundant thorium isotope. Thorium 228, with a period of 1.9 years, is a descendant of thorium 232.
Production
The principal stages in the production of purified thorium are similar to those in the production of uranium: extraction and concentration of the mineral; separation and purification of thorium, normally by means of the solvent TBP, producing either an aqueous solution of thorium nitrate or nitrate crystals of hydrated thorium; and finally the conversion into metal or a compound of thorium [Benedict 81].
Uses
Thorium 232, like uranium 238, is fertile. If bombarded by neutrons,
thorium 232 is transformed into uranium 233, a fissile substance. We present the military use in the section on U 233. On the civilian side, certain reactors can use thorium 232 as a fertile substance and uranium 235 as a fissile substance. One of the principal options, and the most interesting one, for fueling High Temperature Reactors (HTR) is the thorium cycle, using 90 % (or higher) enriched uranium 235 and thorium 232, both mixed in the same particle of fuel or in two different particles. Prototypes of HTR have been constructed in England, Germany, and the U.S. The French companies Sfec and Cerca have produced, or have participated in the production of, thorium fuel. However, France abandoned the HTR in 1979 without having built a reactor.
On the military side, the only French use known to us is the
addition of thorium to the metal of the crankcase [or gearbox? or oil pan] of the ATAR jets of the Etendard and Mirage airplanes in order to reinforce its resistance to heat.
Scientists are presently studying the possibility of producing energy
from thorium 232 by means of sub-critical reactors coupled with accelerators.
Health
Thorium is a very radiotoxic alpha emitter. For this isotope, the
annual limit of intake by inhalation is 90 bequerels per year, compared to 300 bq/year for plutonium 239. Thus, thorium 232 is considered almost 3.3 times more radiotoxic than plutonium [CIPR 90].
Thorium is equally dangerous because of its degeneration chain.
The descendants of thorium 228, which have short periods, accumulate rapidly. They include radon 220 (thoron), a gas that releases alpha particles, then bismuth 212 and thallium 208, which emit high energy gamma rays. Forty years after the separation of thorium from its parent ore, thorium 232 and its descendants are five times more active than thorium 232 and 238 at the moment of their separation [81].
French sources
From around 1910 [Andra 96] to 1975, the Société des Terres Rares (which ran the Le Bouchet factory from 1946 to 1948), and, from 1975 to 1994, Rhône-Poulenc produced thorium as a by-product of the treatment of monazite. Moreover, in the 50s and 60s, the CEA imported thorium from Madagascar in the form of monazite with a high content of thorium oxide and uranothorianite. The annual reports of the CEA do not indicate the destination of the monazite coming from Madagascar, but they note that the uranothorianite was treated at Le Bouchet.
The CEA experimented with thorium irradiation and extracted uranium
233 from it. Thorium was irradiated especially at Marcoule. A laboratory at Saclay reprocessed radiated thorium by means of the Thorex process to obtain uranium 233. This laboratory operated beginning in 1958 and was dismantled in 1963, "no important production or use of this isotope being foreseen for the following years"[CEARa 63]. However, in 1968, the pilot shop at Marcoule (Atelier pilote de Marcoule, APM) extracted 2 kilograms of thorium irradiated at Marcoule.
The CEA has sold abroad part of the thorium nitrate produced at Le
Bouchet [CEARa 62]; another part of it, never used, is stored at Cadarache. Waste originating in the production of thorium is stored at various places in France.
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