In: Physics
Nuclear reactors generate energy through fission, the process by
which an atomic nucleus splits into two or more smaller nuclei.
During fission, a small amount of mass is converted into energy,
which can be used to power a generator to create electricity. In
order to harness this energy, a controlled chain reaction is
required for fission to take place. When a uranium nucleus in a
reactor splits, it produces two or more neutrons that can then be
absorbed by other nuclei, causing them to undergo fission as well.
More neutrons are released in turn and continuous fission is
achieved.
Neutrons produced by fission have high energies and move extremely
quickly. These so-called fast neutrons do not cause fission as
efficiently as slower-moving ones so they are slowed down in most
reactors by the process of moderation. A liquid or gas moderator,
commonly water or helium, cools the neutrons to optimum energies
for causing fission. These slower neutrons are also called thermal
neutrons because they are brought to the same temperature as the
surrounding coolant.
In contrast to most normal nuclear reactors, however, a fast
reactor uses a coolant that is not an efficient moderator, such as
liquid sodium, so its neutrons remain high-energy. Although these
fast neutrons are not as good at causing fission, they are readily
captured by an isotope of uranium (U238), which then becomes
plutonium (Pu239). This plutonium isotope can be reprocessed and
used as more reactor fuel or in the production of nuclear weapons.
Reactors can be designed to maximize plutonium production, and in
some cases they actually produce more fuel than they consume. These
reactors are called breeder reactors.
Breeder reactors are possible because of the proportion of uranium isotopes that exist in nature. Natural uranium consists primarily of U238, which does not fission readily, and U235, which does. Natural uranium is unsuitable for use in a nuclear reactor, however, because it is only 0.72 percent U235, which is not enough to sustain a chain reaction. Commercial nuclear reactors normally use uranium fuel that has had its U235 content enriched to somewhere between 3 and 8 percent by weight. Although the U235 does most of the fissioning, more than 90 percent of the atoms in the fuel are U238--potential neutron capture targets and future plutonium atoms.
The Future of Nuclear Power
Pu239, which is created when U238 captures a neutron, forms U239
and then undergoes two beta decays, happens to be even better at
fissioning than U235. Pu239 is formed in every reactor and also
fissions as the reactor operates. In fact, a nuclear reactor can
derive a significant amount of energy from such plutonium fission.
But because this plutonium fissions, it reduces the amount that is
left in the fuel. To maximize plutonium production, therefore, a
reactor must create as much plutonium as possible while minimizing
the amount that splits.
This is why many breeder reactors are also fast reactors. Fast
neutrons are ideal for plutonium production because they are easily
absorbed by U238 to create Pu239, and they cause less fission than
thermal neutrons. Some fast breeder reactors can generate up to 30
percent more fuel than they use.