Uranium, a heavy metal, is found widely spread in nature, always in combination with other elements. In January 1997, world resources reasonably assured at a production cost of $80 or less a kilo amounted to 2.53 million tons [OCDE 97]. The uranium needed for the world’s reactors as of the year 2010 is estimated to be 67,000 tons a year [CEAUi 99].

Uranium has sixteen isotopes, all radioactive. The isotopes of a given element contain the same number of protons but a different number of neutrons. They present almost the same chemical and physical properties, but different masses and nuclear characteristics. In this section we treat only the three isotopes of uranium found in natural uranium: uranium 235 (0.71%), uranium 238 (99.28%) and uranium 234 (0.0058%).

Uranium 235

Uranium 235 is fissionable and fissile. When a fast neutron bombards an element said to be fissionable, the element divides into lighter elements and, at the same time, releases energy. Many heavy elements are fissionable. On the other hand, the only substances characterized by the term fissile are uranium 235, uranium 233, plutonium 239, and plutonium 241. These substances can be split by slow neutrons as well as by fast neutrons, a property necessary to sustain a chain reaction.

Nuclear bombs and nuclear reactors are both based on chain reactions. A fission chain reaction occurs when neutrons freed by the splitting of nuclei split other nuclei and free other neutrons and so on. When a nuclear reactor operates, the chain reaction suddenly liberates an enormous amount of energy. In a reactor, the reaction is controlled and the energy is liberated progressively.

The critical mass of a fissile substance is the smallest mass that can maintain a chain reaction. For a given substance, the quantity varies according to such factors such as geometry, the density, and the presence of moderators or neutron reflectors.

Uranium 238

Uranium 238 is fissionable and fertile. Fertile atoms create new fissile substances by the absorption of neutrons. An atom of uranium 238 which absorbs a neutron becomes, by means of intermediate stages, an atom of plutonium 239, a fissile substance. The transformation of uranium 238 into plutonium 239 normally takes place in a reactor, where uranium 238 is the majority constituent (about 96.5%) of the fuel and/or in the targets or covers located near the core (in the case of breeder reactors). Reactors designated as military reactors are optimized for the creation of plutonium 239, but electricity-generating reactors create plutonium as a byproduct of the generation of electricity, if, as is usual, uranium 238 is present in the fuel.

Uranium ore directly extracted from the mine contains in general less than one percent uranium mixed with the descendents of uranium, with various metals and sulphides and other substances. The uranium has to be concentrated before undergoing physical-chemical transformations. One of these transformations can be enrichment, that is the increase in the percentage of uranium 235.

For a given quantity of natural uranium, enrichment generates a relatively small quantity of uranium at more than 0.71% uranium 235 (enriched uranium) and a great quantity of uranium at more than 99.28% uranium 238 (depleted uranium).

The categories of uranium depend on the percentage of uranium 235 contained. The ministry of industry requires of the holders of uranium a separate accounting for each of the following categories:

--uranium enriched to 20% or more in uranium 235;

--uranium enriched to 10% or more but less than 20%;

--uranium enriched to less than 10% uranium 235;

--natural uranium;

--depleted uranium [JO 8.vi.94]

All categories are of interest from the military and civilian point of view.

Very highly enriched uranium, more than 90% uranium 235, is the uranium chosen for the military warheads, which ordinarily use uranium enriched to 93.5% [NRDC 84]. According to the International Atomic Energy Agency (IAEA), about 25 kg of uranium enriched to 20% or more (highly enriched uranium; HEU), to fabricate a nuclear device. Nevertheless, it is possible to fabricate a device with a power of one kiloton, by using only 2.5 to 8 kg of uranium enriched to 20% or more, the exact quantity depending on the technological capacity of the manufacturer [Cochran 94].

The less the uranium is enriched, the more is necessary. Twenty percent enrichment is generally considered to be the lowest level of enrichment usable for making a bomb. The theoretical absolute limit is 5-6% enrichment and, at this level, an enormous quantity would be necessary [NRDC 84]. At levels of enrichment below 20%, the greatest military utility of the uranium to potential manufacturers of bombs would be providing a start on enrichment to higher levels.

Another military use of enriched uranium is submarine fuel. In the fifties, the French tried to construct a submarine with fuel of natural uranium, the Q244, but had to abandon this project because it proved to be impractical. For this purpose, the enrichment varies between 7.5% and 90% or more.

Depleted uranium and natural uranium are both used for their high density. The military manufacture kinetic projectiles and armor for tanks with depleted uranium.

Each category of uranium that is counted by the ministry of industry corresponds to the fuel in certain types of reactors. Uranium enriched to more than 20% is used in fuel for research reactors. For purposes of nonproliferation, that is to say to avoid potential use for military purposes, fuel of uranium silicide enriched to about 19.5% or Caramel fuel with uranium enriched to about 7% are used today. Pressurized water reactors (PWRs)-the electricity-generating reactors in use today in France-and boiling water reactors (BWRs) use uranium enriched to about 3.5%. The UNGG reactors (uranium-natural-graphite-gas) and the Canadian line Candu are fueled with natural uranium. Depleted uranium or natural uranium are used in Mox fuel (mixed uranium oxide--plutonium oxide) and the covers of fast neutron reactors (RNR) and in Mox fuel for light water reactors (pressurized water or boiling water). Among the other civilian uses of depleted uranium are the compact shielding of some irradiators and alloys for aeronautics.

The isotopes of uranium in natural uranium pose chemical and radiological problems.

Uranium, like other heavy metals, is very toxic. Its incorporation in the human body manifests itself, as with other heavy metals, by kidney problems, which are often irreversible and by lesions on the arteries [Belbéoch 93, Gillet 92].

Uranium isotopes, like other radioactive material, emit ionizing radiation, that is to say radiation strong enough to tear electrons from the peripheral layer of atoms in the substance they strike; therefore, strong enough to damage or destroy living cells. The ionizing radiation of a given radioactive substance takes one or several of these four forms: gamma radiation, beta particles (electrons or positrons), alpha particles (small nuclei of helium, formed of two neutrons and two protons), and neutrons.

Gamma radiation is the most penetrating. The alpha particles are the least penetrating and can, theoretically, be stopped by a thin screen. However, alpha radiation rarely appears without being accompanied by other emitters (X in particular) and because they penetrate a very short distance, these particles concentrate on a few micrometers of tissue and are therefore very harmful. Natural uranium emits gamma radiation and alpha particles.

The half-life (likewise called “period”)-the length of time necessary for the disintegration of half of a substance-differs from one isotope to another. For uranium 234, 235, and 238, the half lives are respectively 247,000 years, 0.71 billion years, and 4.51 billion years. The longer the half life, the slower the disintegration of the isotope, and the less radiation is emitted during a given time period. Thus, uranium 238 emits fewer alpha particles per second than does uranium 235.

The number of disintegrations of a given mass during a given period of time is the specific activity, which can be measured in becquerels (Bq). A becquerel is equal to one disintegration per second. Because of the different percentages of uranium 235 in the different categories of uranium, each category presents a different specific activity. That of natural uranium is 25,000 Bq/g; that of uranium enriched to 5% is 100,000 Bq/g; that of uranium enriched to 100%, 1,800,000 Bq/g [Gillet 92]. The specific activity is one of the characteristics that determines the intensity of the damage that an isotope can cause. The other characteristics are the energy of the emitted particles, the trajectory and, the length of stay in the body.

Uranium compounds

During conversion processes, uranium passes through various forms. The health risk created by a given form depends on the type of exposure and on the degree of solubility of that form in the body as well as on the composition of the uranium.

As long as uranium stays outside the body, it causes relatively little harm, principally the gamma radiation emitted by the uranium 234. Moreover, several uranium compounds, including UO2 and U308, are very little or not at all absorbed by the gastro-intestinal tract. Therefore the risk of inhalation is often greater than the risk of ingestion. The inhalation of particles of less than a micron in diameter is more dangerous than the inhalation of bigger particles because these particles penetrate into the depth of the lung.

When uranium is inhaled in the form of soluble compounds, it passes rapidly into the blood. A part is eliminated in the urine. The remainder is deposited in the kidneys and bones, and it can cause very severe kidney blockages.

When uranium is inhaled in the form of insoluble compounds, it is in part retained by the lungs, which it irradiates. This may cause cancer. Uranium is evacuated very slowly from the lungs of animals.

The characteristics of the most common uranium compounds are as follows:

Uranium hexafluoride (UF6): colorless, soluble

Uranyl nitrate (UO2(NO3)2): yellow, soluble

Uranium oxide (UO2): brown powder, very insoluble, in particular after frittage

Uranium oxide (U3O8): black, insoluble

Uranium oxyfluoride (UO2F2): yellow, soluble

Uranium tetrafluoride (UF4): green salt, relatively soluble

UF6 poses a particular problem: at ambient temperature it is a solid (uncolored crystals); it becomes a liquid at 65 degrees Centigrade with a pressure of 1.5 bars and becomes a gas if it is no longer under pressure. In a humid atmosphere or in the presence of water, it is transformed into uranyl fluoride (UO2F2) and hydrogen fluoride (HF). The transformation is immediate and violent and is accompanied by the emission of an abundance of opaque, irritating, and suffocating fumes of hydrogen fluoride.

Decay chains

Atoms of uranium 235 and 238 generate descendents that pose a health problem. An atom of uranium 238 is transformed into thorium 234, which becomes protactinium 234 and so on, until lead 206, which is stable, is reached. The series is called a decay chain. The uranium 238 chain includes thorium 230 (half life 80,000 years), which decays slowly producing radium 226 (half life of 1600 years). The radium 226 decays slowly producing radon 222 (half life of 3.8 days), an alpha emitter which, inhaled can cause lung cancer.

Pyrophoric character (spontaneous combustion). This phenomenon occurs, in particular, when uranium is divided, for example in the form of metal powder, metal shavings, or oxide powder. Conservation in a liquid (water, oil) to avoid combustion may lead to the formation of a critical mass. A fire, or even the slow oxidation of undivided uranium, produces fine and insoluble particles of uranium oxide, which are very dangerous to inhale.

Criticality accidents. This phenomenon only concerns uranium enriched to more than 1% [Gillet 92]. Such an accident comes from an undetected accumulation of fissile material that creates a critical mass. The chain reaction, which follows, produces intense radiation capable of seriously harming or killing, after the event, the people present. It can even disperse the fissile material itself and fission products around the area where the accident occurs. Because of the dispersion of the fissile material, the chain reaction does not release the energy that a nuclear bomb would release. “The chain reaction is favored by the confinement of the uranium and the slowing of the neutrons by the light nuclei, in particular the hydrogen contained in the water, the organic solvents, the plastic materials . . .” Uranium 234, which emits gamma radiation, can migrate to the surface, for example during melting [Gillet 92].

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