Moderator nuclear fission

In summary, a heavy-water-moderated nuclear fission reactor fueled with UO3 or U3O8 can be operated with natural uranium that is produced by means of a standard uranium wet chemistry solvent extraction purification process. Additionally, given the neutronic properties of other elements, it is expected that similar reactors can be operated with uranium carbide, uranium tetrafluoride, and uranium-beryllium alloys. 

Economic Aspects. The principal market for deuterium has been as a moderator for nuclear fission reactors fueled by unenriched uranium. The decline in nuclear reactor constmetion has sharply reduced the demand for heavy water. The United States has stopped large-scale production of D2O, and Canada is the only suppHer of heavy water at this time. Heavy water is priced as a fine chemical, and its price is not subject to market forces. 

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

Canada, are examples. These reactors do not use ordinai y water for the moderator. Most nuclear fission reactors use ordinaiy water for a moderator which requires that the fuel he about 3 percent and about 97 percent U. Achieving this enrichment requires that the solid uranium compounds in the yellow cake be converted to gaseous uranium hexafluoride (UF,). Following enrichment, gaseous UF is converted to solid uranium oxide (UO,) for fabrication of fuel elements for a nuclear reactor. 

To slow down and control the rate of reaction, a moderator is also required. Typically, the moderator is boric acid, graphite, or heavy water (D20) and is present in the high-purity water, which also serves as a primary coolant for the fuel and the reactor vessel. The tremendous heat generated by nuclear fission is transferred to this closed-loop coolant, which is contained within a reactor primary-coolant circulation system. The high-purity water coolant also contains a suitable pH buffer such as lithium hydroxide, which has the additional effect of limiting the corrosion of fuel-cladding and other components. 

The phrase "nuclear power" covers a number of technologies for producing electric power other than by burning a fossil fuel. Nuclear fission in pressurized water-moderated reactors—light water reactors— represents the enrrent teehnology for nuclear power. Down the line are fast breeder reactors. On the distant horizon is nnclear fusion. 

Unenriched uranium—which contains more than 99 percent of the nonfissionable isotope U-238—undergoes a chain reaction only if it is mixed with a moderator to slow down the neutrons. Uranium in ore is mixed with other substances that impede the reaction and has no moderator to slow down the neutrons, so no chain reaction occurs. 75- Nuclear fission is a poor prospect for powering automobiles primarily because of the massive shielding that would be required to protect the occupants and others from the radioactivity and the problem of radioactive waste disposal. 

Nuclear fission reactors ( nuclear power reactors ) are devices that use controlled neutron-induced fission to generate energy. While a complete description of the design of these devices is beyond the scope of this book, there are certain basic principles related to nuclear reactors that are worth studying and that can be described and understood with a moderate effort. 

Thus, energy policy makers face an exceptionally severe challenge. They must find politically acceptable ways to produce and market the remaining oil and gas resources in quantities and at prices which do not impair the capabilities of the industrialized world to manage the transition to inexhaustible energy forms. They must determine the path of the transition Is it to rely primarily on the still abundant coal, bitumen and marginal hydrocarbon resources in conjunction with synthetic fuels and a moderate increase in electrification Or, is it to follow a high electrification scenario based on coal and/or nuclear fission ... 

When controlled carefully, fission can be used to produce electricity. In nuclear power plants, moderators are used to slow down the neutrons produced during nuclear fission. It is common to have moderators made from graphite or with heavy water, 2H20. Control rods are also placed in the nuclear reactor to absorb neutrons and slow down the rate at which the fission takes place. 

A nuclear reactor containing only fuel elements would be unusable because a chain reaction could probably not be sustained within it. The reason is that nuclear fission occurs best with neutrons that move at relatively modest speeds, called thermal neutrons. But the neutrcns released from fission reactions tend to be moving very rapidly, at about 1/15 the speed of light. In order to maintain a chain reaction, therefore, it is necessary to irrtroduce some material that will slow down the neutrcns released during lissicn. Such a material is known as a moderator. 

The physical properties of isotopes differ slightly because of differences in atomic mass. For example, water that contains deuterium is called heavy water because the neutrons in deuterium add mass to the water molecule. Some nuclear reactors use heavy water to help keep the chain reaction going. The heavy water slows down (or moderates) the neutrons produced during nuclear fission so that they can be absorbed by the uranium fuel. You will learn more about nuclear reactions in Chapter 25. 

The moderator in this particular plant is ordinary water. Heat from the nuclear fission of uranium causes the water to boil. The resulting steam is carried to conventional steam... 

The logical explanation for the low percentages of U-235 was that a nuclear fission reaction at the mine must have consumed some of the U-235 isotopes. But how did this happen There are several conditions under which such a nuclear fission reaction could take place. In the presence of heavy water, for example, a chain reaction is possible with unenriched uranium. Without heavy water, such a fission reaction could still occur if the uranium ore and the moderator were arranged according to some specific geometric constraints at the site of the reaction. Both of the possibilities seem rather farfetched. The most plausible explanation is that the uranium ore originally present in the mine was enriched with U-235 and that a nuclear fission reaction took place with light water, as in a conventional nuclear reactor. 

The production of energy by nuclear fission in a nuclear reactor must be a controlled process. Neutrons released from the fission of lose most of their kinetic energy by passage through a moderator (graphite or D2O). They then undergo one of two nuclear reactions. The first is capture by leading to further fission the second... 

When fabrication and handling of metal nuclear fuels is contemplated, a different type of design curve is required. Moderation of fission neutrons is absent and conditions for fast-neutron fission exists. Critical values are larger under these conditions (Fig. 10-11). 

The following month, on April 22, Joliot, von Halban and Kowarski published a second paper in Nature concerning secondary neutrons. This one, Number of neutrons liberated in the nuclear fission of uranium, rang bells. Calculating on the basis of the experiment previously reported, the French team found 3.5 secondary neutrons per fissioa The interest of the phenomenon discussed here as a means of producing a chain of nuclear reactions, the three men wrote, was already mentioned in our previous letter. Now they concluded that if a sufficient amount of uranium were immersed in a suitable moderator, the fission chain will perpetuate itself and break up only after reaching the walls limiting the medium. Our experimental results show that this condition will most probably be satisfied. That is, uranium would most probably chain-react. 

Nuclear power plants use nuclear fission to generate energy. The core of a typical nuclear reactor consists of four principal components fuel elements, control rods, a moderator, and a primary coolant ( FIGURE 21.18). The fuel is a fissionable substance, such as uranium-235. The natural isotopic abundance of uranium-235 is only 0.7%, too low to sustain a chain reaction in most reactors. Therefore, the content of... 

The nuclear fission reactions operate most readily with neutrons whose energies have been reduced from the high energies at their formation. The material which accomplishes this is a moderator which can be graphite or ordinary or heavy water. The role of coolant and moderator can be combined when water is used. The assembly of fuel elements and moderator is called the reactor core . 

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