РефератыИностранный языкNuNuclear Fission Essay Research Paper nuclear fissionFission

Nuclear Fission Essay Research Paper nuclear fissionFission

Nuclear Fission Essay, Research Paper


nuclear fission


Fission chain reactions and their control


The emission of several neutrons in the fission process leads to the


possibility of a chain reaction if at least one of the fission neutrons


induces fission in another fissile nucleus, which in turn fissions and


emits neutrons to continue the chain. If more than one neutron is


effective in inducing fission in other nuclei, the chain multiplies more


rapidly. The condition for a chain reaction is usually expressed in


terms of a multiplication factor, k, which is defined as the ratio of the


number of fissions produced in one step (or neutron generation) in


the chain to the number of fissions in the preceding generation. If k


is less than unity, a chain reaction cannot be sustained. If k = 1, a


steady-state chain reaction can be maintained; and if k is greater


than 1, the number of fissions increases at each step, resulting in a


divergent chain reaction. The term critical assembly is applied to a


configuration of fissionable material for which k = 1; if k > 1, the


assembly is said to be supercritical. A critical assembly might consist


of the fissile material in the form of a metal or oxide, a moderator to


slow the fission neutrons, and a reflector to scatter neutrons that


would otherwise be lost back into the assembly core.


In a fission bomb it is desirable to have k as large as possible and


the time between steps in the chain as short as possible so that


many fissions occur and a large amount of energy is generated


within a brief period (10-7 second) to produce a devastating


explosion. If one kilogram of uranium-235 were to fission, the energy


released would be equivalent to the explosion of 20,000 tons of the


chemical explosive trinitrotoluene (TNT). In a controlled nuclear


reactor, k is kept equal to unity for steady-state operation. A


practical reactor, however, must be designed with k somewhat


greater than unity. This permits power levels to be increased if


desired, as well as allowing for the following: the gradual loss of fuel


by the fission process; the buildup of “poisons” among the fission


products being formed that absorb neutrons and lower the k value;


and the use of some of the neutrons produced for research studies


or the preparation of radioactive species for various applications (see


below). The value of k is controlled during the operation of a reactor


by the positioning of movable rods made of a material that readily


absorbs neutrons (i.e., one with a high neutron-capture cross


section), such as boron, cadmium, or hafnium. The delayed-neutron


emitters among the fission products increase the time between


successive neutron generations in the chain reaction and make the


control of the reaction easier to accomplish by the mechanical


movement of the control rods.


Fission reactors can be classified by the energy of the neutrons that


propagate the chain reaction. The most common type, called a


thermal reactor, operates with thermal neutrons (those having the


same energy distribution as gas molecules at ordinary room


temperatures). In such a reactor the fission neutrons produced (with


an average kinetic energy of more than 1 MeV) must be slowed down


to thermal energy by scattering from a moderator, usually consisting


of ordinary water, heavy water (D2O), or graphite. In another type


termed an intermediate reactor the chain reaction is maintained by


neutrons of intermediate energy, and a beryllium moderator may be


used. In a fast reactor fast fission neutrons maintain the chain


reaction, and no moderator is needed. All of the reactor types require


a coolant to remove the heat generated; water, a gas, or a liquid


metal may be used for this purpose, depending on the design needs.


For details about reactor types, see nuclear reactor: Nuclear fission


reactors.


Uses of fission reactors and fission products


A nuclear reactor is essentially a furnace used to produce steam or


hot gases that can provide heat directly or drive turbines to generate


electricity. Nuclear reactors are employed for commercial


electric-power generation throughout much of the world and as a


power source for propelling submarines and certain kinds of surface


vessels. Another important use for reactors is the utilization of their


high neutron fluxes for studying the structure and properties of


materials and for producing a broad range of radionuclides, which,


along with a number of fission products, have found many different


applications. Heat generated by radioactive decay can be converted


into electricity through the thermoelectric effect in semiconductor


materials and thereby produce what is termed an atomic battery.


When powered by either a long-lived, beta-emitting fission product


(e.g., strontium-90, calcium-144, or promethium-147) or one that


emits alpha particles (plutonium-238 or curium-244), these batteries


are a particularly useful source of energy for cardiac pacemakers and


for instruments employed in remote, unmanned facilities, such as


those in outer space, the polar regions of the Earth, or the open


seas. There are many practical uses for other radionuclides, as discussed in


radioactivity: Applications of radioactivity.

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