Uranium is the element having 92 protons in its nucleus with
typically 146 neutrons. It is the
largest naturally occurring element.
Like naturally occurring Thorium, uranium is radioactive and eventually
decays into radium and radon which are likewise radioactive. Both uranium and thorium decay through many series
of radioactive elements until they eventually become lead.
The process of radioactive decay follows a common yet generally
unfamiliar law known as exponential decay.
It is from this decay law that we get the term,
"half-life". The way this
works is that when you start with a fixed amount of a radioactive material, if
you wait through its half life, half of the material will have undergone radioactive
decay. That remaining material which had
not undergone radioactive decay will again have half of that amount undergo decay
in the amount of time representing the next following half life. So with each half life, half of what is left
will decay.
In this sense, if a radioactive isotope has a half life of 1
year, the amount left at the end of the first year is 1/2, the amount after the
second year is 1/4 the original and the amount left after 3 years is only 1/8
and so on. After 10 years, the remaining
amount of radioactive material which has not decayed would then be less than 1
tenth of 1 percent of the original (or more exactly, one half to the tenth
power).
This system can get much more complicated when a radioactive
isotope decays into another different radioactive isotope with a different half
life. In the case of uranium and
thorium, these have up to 12 sequential radioactive decay products in a series
(uranium has 12 and thorium has 9).
These decay products include different isotopes of radium and radon.
An isotope is an atom with the same number of protons which
define it as an element but different amounts of neutrons. So although all uranium atoms have 92
protons, some have only 143 neutrons.
Adding up the numbers of neutrons and protons in this case give 235
total and so this isotope of uranium is known as U235. The more common form of uranium described
earlier has a sum of 238 neutrons and protons and so is known as U238.
The U235 isotope is only found at very low concentrations in
nature, less than 1%. Furthermore, U235
is the only isotope which is also fissile.
To be fissile, an isotope must be able to absorb a slow neutron and then
split into multiple nuclides giving off one or more neutrons in the
process. These free neutrons are then
able to be absorbed by other fissile nuclides (i.e., U235) carrying on the
process.
This means that U235 can give off more than enough neutrons such
that under carefully controlled conditions, it can sustain a chain
reaction. In chemistry, a chain reaction
would simply be an ongoing fire or a rusting metal bar in water. Basically a chain reaction is one that is able
to maintain itself under specific conditions such as sufficient fuel. A nuclear chain reaction on the other hand is
much more difficult to create and sustain and requires higher than current
natural U235 concentrations.
So in order for nuclear fuel to support commercial
electricity generation, the naturally low enrichment of U235 has to be
increased to allow a nuclear chain reaction to be engineered. The heat from the nuclear chain reaction is
able to boil water to create steam in the same way that a natural gas or coal
fired power plant creates steam to run through a turbine and create
electricity.
The half life of U235 is much smaller than that of U238 so
that far enough back in time, natural uranium would have been enriched more
than current commercial nuclear fuel. Basically
the U235 decays much faster than U238 and so today, there is much less U235
than there was in the distant past. Modern
scientific methods have found evidence that a natural nuclear reactor once did
occur on earth (when there was much more U235 around) and actually went
critical similar to that in modern nuclear power plants.
The only known natural nuclear reactor in earth's history
was found in the Oklo uranium mine of Gabon, Liberia on the continent of
Africa. Some of the U235 was missing and
the stable fission decay products present in the mine gave evidence that a
natural fission reaction process occurred a very long time ago.
Many have argued that this is evidence that radioactive
waste can be safely disposed of in deep geologic formations. The idea being that as Oklo kept the fission
products long enough to fully decay into stable elements, duplication of this
design would then be technically feasible.
