Uranium reactors

The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. 

Uranium-235 and U-238 behave differently in the presence of a controlled nuclear reaction. Uranium-235 is naturally fissile. A fissile element is one that splits when bombarded by a neutron during a controlled process of nuclear fission (like that which occurs in a nuclear reactor). Uranium-235 is the only naturally fissile isotope of uranium. Uranium-238 is fertile. A fertile element is one that is not itself fissile, but one that can produce a fissile element. When a U-238 atom is struck by a neutron, it likely will absorb the neutron to form U-239. Through spontaneous radioactive decay, the U-239 will turn into plutonium (Pu-239). This new isotope of plutonium is fissile, and if struck by a neutron, will likely split. 

The uranium is consumed in the fission reaction. Energy generation by burning fossil fuels consumes fossil fuel chemicals and converts them to harmful combustion products. Nuclear reactors "bum" uranium and convert it to harmful fission products. Unlike fossil fuel materials, uranium has little other use than for the production of energy, like fossil fuels, there is a finite supply of the minerals used to produce the uranium for the fuel cycle. The worldwide amount of potential energy available by use of the burner reactor cycle is similar to that available from oil. If used at a high level the supplies of burner reactor uranium could be depleted in the middle of the next century. 

The nuclear fuel consists of uranium, usually in the form of its oxide, U3O8 (Figure 23.12). Naturally occurring uranium contains about 0.7 percent of the uranium-235 isotope, which is too low a concentration to sustain a small-scale chain reaction. For effective operation of a light water reactor, uranium-235 must be enriched to a concentration of 3 or 4 percent. In principle, the main difference between an atomic bomb and a nuclear reactor is that the chain reaction that takes place in a nuclear reactor is kept under control at all times. The factor limiting the rate of the reaction is the number of neutrons present. This can be controlled by lowering cadmium or boron rods between the fuel elements. These rods capture neutrons according to the equations... 

Water attacks massive uranium slowly at room temperature and rapidly at higher temperatures. UO2 and UH3 are formed, heat is evolved, and the metal swells and disintegrates. In waterstainless steel, or zirconium. Nitric acid dissolves uranium readily. 

Uranium. For its use in nuclear reactors, uranium must be exceedingly pure and free from elements such as B or Cd which have high absorption capacities for thermal neutrons. 

There are three essential nuclear materials for the CANDU reactor uranium, heavy water, and zirconium. The latter material has involved little chemical engineering activity in Canada and will not be considered further in this review. 

Boulyga, S. F., Becker, J. S., Matusevitch, J. L., and Dietze, H. J. 2000. Isotope ratio measurements of spent reactor uranium in environmental samples by using inductively coupled plasma mass spectrometry. Int J Mass Spectrom 203(1-3), 143-154. 

The other commonly used fissionable isotope, plutonium-239 (Pu-239), is very rare in nature. But there s a way to make Pu-239 from U-238 in a specif fission reactor called a breeder reactor. Uranium-238 is first bombarded with a neutron to produce U-239, which decays to Pu-239. The process is shown in Figure 5-4. 

In production of fuel for a fast reactor, uranium — natural or depleted — is only needed to make up for the fission products separated during reprocessing of irradiated nuclear fuel (INF), which accounts for about 10% of the fresh fuel mass. For a thermal reactor, natural uranium is enriched to a required level, with roughly 10% of the mined uranium going into fuel, while its remaining 90% ends up in the tails of enrichment processes and is not involved in energy generation. 

The occurrence of U and trace amounts of Pu and T4p in the DU penetrator (ammunition tips) recovered from target sites in Kosovo, Serbia, and Bosnia reflects the previous existence of neutron-related processes and points to a possible presence of spent reactor uranium in munitions (Table 10). This is in agreement with the information reported in the US DOE News in September 1999 [103] that some DOE (Paducah uranium enrichment plant) and military facilities in the USA... 

Transition from tetragonal to cubic at 1765 °C. Decomposed in H2O, slightly soluble in alcohol. Used in microsphere pellets to fuel nuclear reactors. Uranium dicarbide... 

Both fissile materials from the spent fuel of light water reactors (uranium or mixed) and plutonium produced in other breeder reactors can be used for the reactor makeup. In the distant future when cheap uranium reserves are exhausted, the reactor can be provided with an option of fuel breeding to change thermal reactors to the Th-U cycle. 

Another application of sintered oxides is in nuclear reactors—uranium oxide is used as a nuclear reactor fuel material. A large fraction of uranium atoms can be fissioned without degradation of the structure. Other oxides that are used in pure form after sintering are BeO, MgO, Th02, Zr02, and MgAl204. 

Limited uranium concentralion. In solution reactors, uranium concentration is limited b solubility or corrosion effects, and in slurries, by the effective iscosity and settling characteristics. In H20-moderated reactors, in particular, a high uranium or thorium concentration is necessary for a high convcr.sion ratio. Concentrations up to 1000 g/liter, however, may be con.sidcred for solutions and up to 4000 g/liter for fluidized beds. 

Chemical stability of uranium oxides. Both the early Manhattan Project work [9] and the ORNL work [10] indicated that uranium trioxide would be the probable stable form of uranium oxide under the radiolytic gas formed by the radiation-induced decomposition of water in a reactor. Uranium dioxide in an aqueous slurry at 250°C was oxidized to uranium trioxide in the presence of oxygen overpressure and even in the presence of excess hydrogen gas. The extent of this oxidation depended on the oxygen pressure, and seemed to be independent of the partial pressure of hydrogen (Table 4-1). The extent of oxidation of U3O8 to uranium trioxide depended on both temperature and oxygen pressure. The presence... 

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