Fissionable materials

The breeder reactor, which would produce and bum plutonium and gradually increase the inventory of fissionable material, requires reprocessing of nuclear fuel. As of 1995 only limited research and development was in progress on breeder reactors, mainly in France and Japan. 

Weapons materials from production reactors were accumulated during the Cold War period as a part of the U.S. defense program. Prominent were tritium, ie, hydrogen-3, having a of 12.3 yr, and plutonium-239, 1/2 = 2.4 X lO" yr. The latter constitutes a waste both as a by-product of weapons fabrication in a waste material called transuranic waste (TRU), and as an excess fissionable material if not used for power production in a reactor. 

One feature of reprocessing plants which poses potential risks of a different nature from those ia a power plant is the need to handle highly radioactive and fissionable material ia Hquid form. This is necessary to carry out the chemical separations process. The Hquid materials and the equipment with which it comes ia contact need to be surrounded by 1.5—1.8-m thick high density concrete shielding and enclosures to protect the workers both from direct radiation exposure and from inhalation of airborne radioisotopes. Rigid controls must also be provided to assure that an iaadvertent criticahty does not occur. 

American National Standardfor Nuclear Criticality in Operations with Fissionable Materials Outside Reactors, ANS Standard N 16.1-1975, American Nuclear Society, LaGrange Park, lU., 1975. 

The basic requirements of a reactor are 1) fissionable material in a geometry that inhibits the escape of neutrons, 2) a high likelihood that neutron capture causes fission, 3) control of the neutron production to prevent a runaway reaction, and 4) removal of the heat generated in operation and after shutdown. The inability to completely turnoff the heat evolution when the chain reaction stops is a safety problem that distinguishes a nuclear reactor from a fossil-fuel burning power plant. 

A critical assembly is a split bed on which fissionable material used to mock up up a separated reactor core that is stacked half on each half. One half is on roller guides so that the two halves may be quickly pulled apart if the neutron multiplication gets too high. Use the Preliminary Hazards Analysis method described in section 3,2.1 to identify the possible accidents that may occur and the qualitative probabilities and consequences. List the initiators in a matrix to systematically investigate the whole process. Don t forget human error. 

Nuclear power production involves bringing fissionable material together to react nuclearly, removing the heat, converting the heat to steam to drive a turbogenerator. and managing the wastes. 

In addition to these are studies prepared before President Carter stopped the GESMO (Generic Environmental Statement for Mixed Oxide) that addressed the chemical processing of fissionable material for the nuclear fuel cycle. Some references are Cohen (1975), Schneider (1982), Erdmann (1979), Fuliwood (1980), and Fullwood (1983). 

A critical assembly is a split bed on which fissionable material used to mockup up a... 

HLW comprises most of the radioactivity associated with nuclear waste. Because that designation can cover radioactive waste from more than one source, the term spent nuclear fuel (SNF) will be used to discuss HLW originating from commercial nuclear reactors. LLW comprises nearly 90 percent of the volume of nuclear waste but little of the radioactivity. Nuclear power reactors produce SNF and most of the nation s LLW, although there are approximately 20,000 different sources of LLW. The name SNF is a bit of a misnomer because it implies that there is no useful material left in the fuel, when in fact some fissionable material is left in it. 

Several alternative technologies that were heavily supported failed to become commercially viable. The most obvious case was the fast breeder reactor. Such reactors are designed to produce more fissionable material from nonfissionable uranium than is consumed. The effort was justified by fears of uranium exhaustion made moot by massive discoveries in Australia and Canada. Prior to these discoveries extensive programs to develop breeder reactors were government-supported. In addition, several different conventional reactor technologies were aided. The main ongoing nuclear effort is research to develop a means to effect controlled fusion of atoms. 

Neutrons produced in a chain reaction are moving very fast, and most escape into the surroundings without colliding with another fissionable nucleus. However, if a large enough number of uranium nuclei are present in the sample, enough neutrons can be captured to sustain the chain reaction. In that case, there is a critical mass, a mass of fissionable material above which so few neutrons escape from the sample that the fission chain reaction is sustained. If a sample is supercritical,... 

Nuclear energy can be extracted by arranging for a nuclear chain reaction to take place in a critical mass of fissionable material. with neutrons as the chain carriers. A moderator is used to reduce the speeds of the neutrons in a reactor that uses fissile material. 

CH3(CH2)3CH3 + CH3CH=CH2. critical mass The mass of fissionable material above which so few neutrons escape from a sample of nuclear fuel that the fission chain reaction is sustained a greater mass is supercritical and a smaller mass is subcritical. 

Every fission reaction releases some neutrons, and these neutrons can be recaptured by other nuclei, causing more fission reactions. When the amount of fissionable material is small, most neutrons escape from the sample, and only a few neutrons are recaptured. Increasing the amount of material increases the likelihood that neutrons will be recaptured and cause additional fission reactions. The critical mass is defined as the amount of material that is just large enough to recapture one neutron, on average, for every fission reaction. 

As long as the amount of fissionable material is less than the critical mass, the rate of fission events does not grow, and the rate of energy release remains low. In contrast, a sample behaves quite differently when the amount of fissionable material is larger than the critical mass. Above the critical mass, more than one neutron, on average, is recaptured for every fission that occurs. Now the number of fission reactions grows rapidly. As an illustration, consider what happens when two neutrons are recaptured from each fission reaction. As shown in figure 22-11 on... 

A hydrogen bomb, which uses nuclear fusion for its destructive power, is three bombs in one. A conventional explosive charge triggers a fission bomb, which in turn triggers a fusion reaction. Such bombs can be considerably more powerful than fission bombs because they can incorporate larger masses of nuclear fuel. In a fission bomb, no component of fissionable material can exceed the critical mass. In fusion, there is no critical mass because fusion begins at a threshold temperature and is independent of the amount of nuclear fuel present. Thus, there is no theoretical limit on how much nuclear fiiel can be squeezed into a fusion bomb. 

THE NEW VISION INCLUDES VIABLE PROGRAMS FOR MANAGEMENT OF THE BY-PRODUCTS OF THE NUCLEAR FISSION PROCESS, INCLUDING RECYCLEOF ALL FISSIONABLE MATERIALS AND LONG-TERMISOLATION OF UNWANTED FISSION PRODUCTS FROM THE BIOSPHERE. 

Viable programs for management of the by-products of the nuclear fission process, including recycle of all fissionable materials and long-term isolation of unwanted fission products from the biosphere... 

The critical mass of a fissionable material is the minimum mass of a particular fissionable nuclide in a set volume that is necessary to sustain a nuclear chain reaction. 

Weapons-grade fissionable material (U-233) is harder to retrieve safely and clandestinely from the thorium reactor than plutonium is from the uranium breeder reactor. 

A single kilogram of radioactive metallic plutonium-238 produces as much as 22 million kilowatt-hours of heat energy. Larger amounts of Pu-238 produce more heat. However, Pu-238 is not fissionable, and thus it cannot sustain a chain reaction. However, plutonium-239 is fissionable, and a 10-pound ball can reach a critical mass sufficient to sustain a fission chain reaction, resulting in an explosion, releasing the equivalent of over 20,000 tons of TNT. This 10-pound ball of Pu-239 is only about one-third the size of fissionable uranium-235 required to reach a critical mass. This makes plutonium-239 the preferred fissionable material for nuclear weapons and some nuclear reactors that produce electricity. 

Uses. To implode fissionable material in nuclear devices to achieve critical mass as a component of plastic-bonded explosives and solid fuel rocket propellants and as burster charges in military munitions. 

Neutron bombardment converts thorium-232 to its isotope of mass 233. The thorium-233 formed undergoes two successive beta decays to form uranium-233, a fissionable material, similar to uranium-235 and plutonium-239. 

Fuel Rods The fuel or fissionable material used in enriched uranium-235 (in the form of U30g). 

The three neutrons produced when uranium splits have the ability to split other U-235 nuclei and start a self-sustaining chain reaction. Whether a chain reaction takes place depends on the amount of fissionable material present. The more fissionable material that is present, the greater the probability that a neutron will interact with another U-235 nucleus. The reason for this involves the basic relationship between surface area and volume as mass increases. If a cube with a length of 1 unit is compared to a cube of 2 units, it is found that the surface area to volume ratio of the 1 unit cube is twice that of the 2 unit cube (Figure 17.6). This shows that volume increases at a greater rate than surface area as size increases. The probability that neutrons escape rather than react also depends on the surface area to volume ratio. The higher this ratio is the more likely neutrons escape. When a U-235 nucleus contained in a small mass of fissionable uranium is bombarded by a neutron, the... 

Cracking process by which a compound is broken down into simpler substances, typically employed in petroleum industry to break carbon-carbon bonds Crenation condition that results when cells lose water and shrivel up Critical Mass quantity of fissionable material necessary to give a self-sustaining nuclear reaction... 

Precisely controlled detonations are used to create an implosion of fissionable material to achieve critical mass in the explosion of an atomic bomb (See Vol 1, pp A499-504)... 

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