Nuclear Fission


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


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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