Fission chain reaction

Uranium-235 is of even greater importance because it is the key to utilizing uranium. 23su while occuring in natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone. 

Criticality Precautions. The presence of a critical mass of Pu ia a container can result ia a fission chain reaction. Lethal amounts of gamma and neutron radiation are emitted, and a large amount of heat is produced. The assembly can simmer near critical or can make repeated critical excursions. The generation of heat results eventually ia an explosion which destroys the assembly. The quantity of Pu required for a critical mass depends on several factors the form and concentration of the Pu, the geometry of the system, the presence of moderators (water, hydrogen-rich compounds such as polyethylene, cadmium, etc), the proximity of neutron reflectors, the presence of nuclear poisons, and the potential iateraction with neighboring fissile systems (188). As Httle as 509 g of Pu(N02)4 solution at a concentration Pu of 33 g/L ia a spherical container, reflected by an infinite amount of water, is a critical mass (189,190). Evaluation of criticaUty controls is available (32,190). 

Commercially, there are only two nuclear species that will function in a self-sustaining nuclear fission chain reaction and Pu (plutonium). 

The fission ofor Tu liberates, on average, two to three neutrons. One neutron is required to sustain the nuclear fission chain reaction. In a nuclear breeder reactor, the extra neutrons are used to induce nuclear reactions that lead to the production of Tu. The sequence begins by arranging for... 

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,... 

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. 

In fast (neutron) reactors, the fission chain reaction is sustained by fast neutrons, unlike in thermal reactors. Thus, fast reactors require fuel that is relatively rich in fissile material highly enriched uranium (> 20%) or plutonium. As fast neutrons are desired, there is also the need to eliminate neutron moderators hence, certain liquid metals, such as sodium, are used for cooling instead of water. Fast reactors more deliberately use the 238U as well as the fissile 235U isotope used in most reactors. If designed to produce more plutonium than they consume, they are called fast-breeder reactors if they are net consumers of plutonium, they are called burners . 

The essence of a conventional nuclear reactor is the controlled fission chain reaction of U-235 and Pn-239. This produces heat, which is used to make steam which drives a turbine. The chain reaction depends on having a surplus of neutrons to keep it going. 

The idea is to use a particle accelerator producing neutrons by spallation to feed a fuel/moderator assembly where the neutrons multiply by fission chain reactions. 

Hafnium has a great affinity for absorbing slow neutrons. This attribute, along with its strength and resistance to corrosion, makes it superior to cadmium, which is also used for making control rods for nuclear reactors. This use is of particular importance for the type of nuclear reactors used aboard submarines. By moving the control rods in and out of a nuclear reactor, the fission chain reaction can be controlled as the neutrons are absorbed in the metal of the rods. The drawback to hafnium control rods is their expense it costs approximately one million dollars for several dozen rods for use in a single nuclear reactor. 

Because the isotope uranium-235 is fissionable, meaning that it produces free neutrons that cause other atoms to split, it generates enough free neutrons to make it unstable. When the unstable U-235 reaches a critical mass of a few pounds, it produces a self-sustaining fission chain reaction that results in a rapid explosion with tremendous energy and becomes a nuclear (atomic) bomb. The first nuclear bombs were made of uranium and plutonium. Today, both of these fuels are used in reactors to produce electrical power. Moderators (control rods) in nuclear power reactors absorb some of the neutrons, which prevents the mass... 

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. 

The most common use of plutonium is as a fuel in nuclear reactors to produce electricity or as a source for the critical mass required to sustain a fission chain reaction to produce nuclear weapons. Plutonium also is used to convert nonfissionable uranium-238 into the isotope capable of sustaining a controlled nuclear chain reaction in nuclear power plants. It takes only 10 pounds of plutonium-239 to reach a critical mass and cause a nuclear explosion, as compared with about 33 pounds of fissionable, but scarce, uranium-235. 

Fusion in H-bombs is ignited by a fission chain reaction of uranium or plutonium an atom bomb is used to set off the hydrogen bomb. The first hydrogen bomb test was conducted in 1952 on the Pacific... 

Nuclear Fuel Fissionable materials that have been enriched to such a composition that, when placed in a nuclear reactor, they will support a self-sustaining fission chain reaction, producing heat in a controlled manner for process use. 

The source of energy in a nuclear reactor is a fission reaction in which neutrons collide with nuelei of uranium-235 or plutonium-239 (the fuel), causing them to split apart. The products of a fission reaction include not only energy but also new elements (known as fission products) and free neutrons. A constant and reliable flow of neutrons is insured in the reactor by a moderator, which slows down the speed of neutrons, and by control rods, which limit the number of neutrons available in the reactor and, hence, the rate at which fission can occur. In a nuclear weapon, the fission chain reaction, once triggered, proceeds at an exponentially increasing rate, resulting in an explosion in a nuclear reactor, it proceeds at a steady, controlled rate. Most commercial nuclear power plants are incapable of undergoing an explosive nuclear chain reaction, even should their safety systems fail this is not true of all research reactors (e.g., some breeder reactors). 

The basic nuclear weapon is the fission bomb, or A-bomb (A for atomic) as it was first called. A fission chain reaction is used to produce a very large amount of energy in a very short time—roughly a millionth of a second—and therefore a very powerful explosion. 

The shorter half life of compared with that of meant that at this time, ca. 1.8 x 10 y ago, natural uranium contained some 3% rather than its current level of 0.7%. Thus, in regions of the ore body where uranium concentrations greater than 10% were present in seams over 0.5 m thick, there was sufficient fissile material for the moderating effect of percolating groundwater to lead to a fission chain reaction. When the heat released was sufficient to boil the water and expel it, the moderating effect would be lost, so that the reactor zone would cycle between critical and subcritical conditions depending upon its water content. It has been estimated that the principal reactor zone at Oklo must have operated in this way for at least 1.5 x lO years at a power level between 10 and 100 kW. 

A sample of fissionable material must have sufficient mass in order for a fission chain reaction to occur. If it does not, neutrons escape from the sample before they have the opportunity to strike other nuclei and continue the chain reaction— the chain reaction never begins. A sample that is not massive enough to sustain a chain reaction is said to have subcritical mass. A sample that is massive enough to sustain a chain reaction has critical mass. When a critical mass is present, the neutrons released in one fission cause other fissions to occur. If much more mass than the critical mass is present, the chain reaction rapidly escalates. This can lead to a violent nuclear explosion. A sample of fissionable material with a mass greater than the critical mass is said to have supercritical mass. Figure 25-18 shows the effect of mass on the initiation and progression of a fission reaction. 

This figure illustrates the ongoing reactions characteristic of a nuclear fission chain reaction. 

In a chain reaction, one reaction induces others to occur. A sufficient mass of a fissionable material must be present for a fission chain reaction to occur. 

With atomic number 92 and atomic weight 238.03, uranium is the heaviest naturally occurring element. There are eleven known isotopes, of which three — with atomic weights 234, 235 and 238 — occur in nature. They are all radioactive, with half-lives (in years) of 2.35 X 10 , 7 X 10 and 4.5 X 10 , respectively. The relative abundance of the isotopes varies depending on the age and geological history of the uranium occurrence a typical distribution is U 99.28% U 0.71% U 0.005%. A quite exceptional deviation from this distribution has been found at Oklo in Gabon as a result of spontaneous fission chain reactions in the remote past (Anon., 1975). At Oklo U concentrations as low as 0.29% have been found. 

---