| 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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