Nuclear waste is the material that nuclear fuel
becomes after it is used in a reactor .

It looks exactly like the fuel that was loaded into the
reactor assemblies of metal rods enclosing
stacked-up ceramic pellets.

But since nuclear reactions have occurred, the contents aren?t quite the same. Before producing power, the fuel
was mostly Uranium (or Thorium ), oxygen, and
steel. Afterwards, many Uranium atoms have
split into various isotopes of almost all of the
transition metals on your periodic table of the
elements.

The waste, sometimes called spent fuel, is
dangerously radioactive , and remains so for
thousands of years. When it first comes out of
the reactor, it is so toxic that if you stood within
a few meters of it while it was unshielded, you
would receive a lethal radioactive dose within a
few seconds and would die of acute radiation
sickness within a few daysHence all
the worry about it.

In practice, the spent fuel is never unshielded. It
is kept underwater (water is an excellent shield)
for a few years until the radiation decays to
levels that can be shielded by concrete in large
storage casks.

The final disposal of this spent fuel is a hot topic, and is often an argument against the use of nuclear reactors. Options include deep geologic storage and recycling. The sun would consume it nicely if we could get into
space, but since rockets are so unreliable, we
can?t afford to risk atmospheric dispersal on
lift-off.

Nuclear reactors are typically loaded with
Uranium Oxide fuel, UO .

Neutrons are introduced to the system, and many of them are absorbed by uranium atoms, causing them to
become unstable and split, or fission, into two
smaller atoms known as fission products.

Sometimes, the uranium absorbs a neutron and
does not fission, but rather transforms to a
heavier isotope of uranium, such as U-239.

U-239 beta-decays to Np-239, which in turn
beta-decays to Pu-239. The heavier nuclide may
then absorb another neutron to become an even
heavier element.

These heavier atoms are known
as transuranics. Nuclear waste, with regard to
nuclear reactors, is the collection of nuclides left
over after a reactor has extracted some energy
out of nuclear fuel.

Many of the isotopes are very radioactive for a very long time before they decay to stability.

The radioactivity causes the spent nuclear fuel to continue emitting heat long after it has been removed from the reactor.

A few of the radioactive isotopes in the mix of
spent fuel are gaseous and need to be carefully
contained so that they do not escape to the
environment and cause radiation damage to
living things. Other types of nuclear waste exist,
such as low level waste from other applications.
This discussion will focus on high-level waste
(HLW), the spent nuclear fuel from nuclear
power reactors.