Uranium and Thorium

Abstract

Uranium is a heavy, radioactive metal, the 92nd element in the periodic table, and a member of the actinide series. Its name and chemical symbol U are derived from the planet Uranus, discovered (1781) a few years before the element. A compound of uranium (uranium oxide) was discovered in the uranium ore pitchblende by M. H. Klaproth in 1789. Klaproth believed that he had isolated the element, but this was not achieved until 1841 when a French chemist, E. M. Peligot, reduced uranium tetrachloride with potassium in a platinum crucible to obtain elemental uranium. Uranium is not as rare as once believed. Widely distributed in the earth's crust, uranium occurs to the extent of about 0.0004%, making the metal more plentiful than mercury, antimony, or silver. Before World War II, uranium was of interest only to the chemists and physicists who studied the element as they would any other substance. With the advent of the nuclear age, uranium now occupies a key position in nuclear weapons and energy. The physical and chemical properties of uranium and some of its compounds are listed. To enhance its use in reactors and nuclear weapons, uranium undergoes an industrial enrichment process that increases the 235U content from 0.7% found naturally to a content between 2 and 90%. 235U is the only natural uranium isotope that can sustain the nuclear chain reaction required for reactors and weapons processes. No deposits of concentrated uranium ore have been discovered. As a result, uranium must be extracted from ores containing less than 0.1% U. Because it is necessary to use low-grade ores, substantial and complex processing of these ores is required to obtain pure uranium. Usually it is necessary to preconcentrate the ore by grinding and flotation or similar processes. Hazardous exposures in the uranium industry begin in the mining process. Hazards are of two types, chemical and radiological; of the two, radiation is the more dangerous. Effective ventilation control measures have reduced the radiation exposures in the larger mines, but far less satisfactory radiation-exposure conditions exist in small mines without the benefit of ventilation. In addition to the alpha-particle radiation hazard from uranium in the ore, the most hazardous elements are radon gas and its particulate daughters, RaA and RaC, all alpha emitters. Some mine waters are high in radon and thus are an additional exposure source and should not be used for wet drilling. In the mines some beta and gamma exposures from RaB, RaC, and Ra also occur but are of relatively minor importance. The chemical toxicity of uranium is similar to other heavy metals. Storage in the skeleton and excretion via the urine are accompanied by renal toxicity and are discussed. Hazards in milling uranium to produce a concentrate were thought to be relatively minor because a wet process was used. However, some chronic health effects, including nonmalignant respiratory disease and renal tubular biochemical abnormalities, have been documented in these workers and are discussed. A variety of both mandatory and voluntary health-based exposure limits for uranium are derived from both its chemical and radiological toxicity. Regulating bodies include international, national, and state organizations. Some of the pertinent regulations and guidelines on exposure limits are summarized here, but the reader is cautioned to consult other sources to ensure health protection and regulatory compliance. Uranium is unusual among the elements because it presents both chemical and radiological hazards. Thorium, the second element in the actinide series, exists in the earth's crust as an unstable, radioactive element that undergoes decay by alpha emission and gives rise to a series of short-lived daughter products that ends in a stable isotope of lead. Thorium is used as a source of atomic fuel, in the production of incandescent mantles, as an alloying element with magnesium, tungsten and nickel, and in the past was used as a diagnostic agent for systemic radiological studies. Thorium is primarily a radioactive hazard in humans; however, its chemical toxicity must also be considered.