Isotopes do not just differ in reactivity, however.
Which isotope of an
element you are dealing with in
nuclear physics is very important. For instance,
deuterons in
heavy water are used in
particle accelerators. They are an isotope of
hydrogen, and regular
hydrogen nuclei simply will not do the same job.
Another example is in preparing
fuel for
nuclear reactors. Before
purified uranium is made into
fuel pellets, it must be
enriched - a process that calls for increasing the percentage of the isotope
U-235 in the fuel from less than one percent, up to about five percent.
The reason for this is the major constituent of
uranium, the isotope
U-238.
Neutrons are a scarce resource in a
nuclear reactor, and the feasibility of the
fission process depends on getting a
chain reaction to occur. When a
U-235 nucleus
fissions, it releases one or more
neutrons. When one of these
neutrons hits another
U-235 nucleus, it causes that
nucleus to
fission also, releasing more
neutrons, and so on.
U-238 nuclei, on the other hand, will simply 'gobble up' any
neutron that hits them without
fissioning. (They later decay to
plutonium (?), but not fast enough for our purposes, and they do not emit
neutrons when they do.) As a result, if there is too much
U-238 present, the
chain reaction simply will not proceed.
If the percentage of
U-235 is increased, the
chain reaction becomes
sustainable, and generating power from
fission becomes possible.
(Incidentally,
breeder reactors make more
fuel than they burn, by altering the process so that a lot of
plutonium gets formed during the
fission of
enriched uranium fuel. After
reprocessing, they can then use the
plutonium as fuel.)