Most nuclear power plants use uranium-235 as fuel. Under geological conditions, the thermodynamically stable species of uranium is the uranyl cation (UO2+), which is fairly soluble in water. As a result, uranium is present everywhere in the Earth’s crust, its concentration is relatively low, and the average concentration is about 3—5 ppm. Uranium can be extracted economically from rocks that have a concentration of uranium that is at least a couple of thousand parts per million. The most important uranium ore is uranium pitchblende, with mean uranium content about 0.5—0.8%. About 40% of the uranium of the Earth is in Australia.

The uranium is produced from the ore by crashing the ore into smaller pieces, and then concentrated the uranium containing ores by flotation. If uranium is pres­ent as U(IV), it is oxidized to U(VI) by air, or sometimes in a microbiological way. Then, the substance is leached by sulfuric acid. The formed uranyl sulfate complex, [UO2(SO4)2]2_, is separated by ion exchange resins or by extraction using an

238 235

organic solvent. Since natural uranium contains 99.3% U and only 0.7% 235U, and because 235U-enriched uranium compound is needed as fuel for nuclear reac­tors, the uranyl sulfate must be converted into a species that is appropriate for iso­tope enrichment. This species of uranium is UF6, the gas diffusion of which can be used for isotope enrichment. However, even uranium with a natural isotopic ratio can initiate fission chain reaction.

In addition to uranium, artificial fissile material such as Pu, Pu, and U (see Eqs. (6.22) and (6.23)) can be used as fuels. The fuel has to have a high cross section for thermal neutrons. Since the released energy is huge, the heat resistance of the compound of the fissile isotope is also important. The activation of the other atoms in the compound has to be avoided. These conditions are fulfilled by oxides. Usually, uranium dioxide (UO2), plutonium dioxide (PuO2), and thorium dioxide (ThO2) are used. In some reactors, uranium carbide (UC) has been tested. Mixed oxides, the so-called MOXs, are also produced from plutonium oxide and uranium oxide. For homogeneity, the oxides are co-precipitated from oxalate and then calcinated to oxide. MOX is used in light water reactors (as discussed in Section 2.1.1.2). One advantage of MOX fuel is that it provides a way to dispose of the surplus of weapons-grade plutonium, which otherwise would have to be disposed of as nuclear waste and would remain a nuclear proliferation risk. The characteristic properties of the most important fuels are shown in Figure 7.4.

Uranium dioxide is prepared as pellets and placed into rods made of zircon, zircalloy (zirconium with 1% niobium), or another metal. The rods are hermetically sealed and placed into the active zone of the reactor with the moderator. The seal should ideally be hermetic, but in reality, the fuel rods often have micro — and macro-ruptures through which the gaseous and soluble fission products can escape. A part of the gaseous fission products (Kr-85, Xe-133, and Xe-135 isotopes) is

emitted into the atmosphere. The gaseous molecules and compounds of iodine are filtered, usually by coal filters. The relative activities of the iodine isotopes with different half-lives give information on the size of the ruptures. The presence of iodine isotopes with short and long half-lives indicates the existence of macro — or micro-ruptures, respectively. Besides the gaseous fission products, the gaseous compounds of tritium and C-14 (see Section 7.3) are also released into the atmo­sphere. To decrease the emitted radioactivity, the gas emission is delayed or ignited and the products are condensed.

If the moderator (Section 7.1.1.2) and/or the coolant (Section 7.1.1.5) is water or heavy water, the soluble fission products (e. g., Cs-137, Cs-134, strontium, and iodine ions) can dissolve in them. For this reason, water is continuously purified by ion exchangers.

During the operation of nuclear reactors, the quantity of 235U continuously decreases; the fissile material is burning up. Most of the 235U present in the reactor undergoes fission reaction; however, a small part converts to 236U in an (n, Y) reaction. Similarly, the (n, Y) reaction of 238U produces transuranium elements,

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including 9Pu and Pu (Eq. (6.23)), which also undergoes fission reaction, increasing the power of the nuclear reactors. There are nuclear reactors specifically made to produce fissile plutonium isotopes, which are called “breeder reactors.”

The operation of the nuclear reactors is influenced by the fission products. Some of them (e. g., 135Xe and 149Sm) strongly absorb neutrons, decreasing the number of the neutrons and the reactivity. These fission products are called “reactor poisons.”