230
neutrons during the fission process, and when concentrated into enough mass, it can
be made to explode in a “chain reaction” as the neutrons hit other 235 U atoms, causing them to fission and release additional neutrons. The explosive threshold is called
the critical mass, and this is how an atomic bomb works.
There is a lot more 238 U than 235 U in the world, so the lighter isotope must be
painstakingly separated and concentrated with centrifuges, gas diffusion systems,
and other sophisticated technology for use in nuclear weapons. This is difficult
because the two isotopes are chemically identical, and only have about a 1% difference in weight. Power plant fuel is made from the leftover 238 U mixed with about
3.5–5% 235 U in what is called low-enriched uranium. It is supplied to nuclear power
plants as ceramic pellets of uranium oxide for making electricity. These are stacked
in tubular metal jackets to create fuel rods for running the reactor. The fission rate
inside the reactor is controlled by a moderator such as water or graphite that absorbs
excess neutrons and keeps the reactor core at an optimal temperature. Low-enriched
uranium won’t explode into a mushroom cloud, but it does have enough fission
energy to boil water, create steam and turn a turbine (Source: https://www.worldnuclear.org/; World Nuclear Association websites).
The 238 U in a fuel rod is exposed to a flux of neutrons in the reactor core, where
it eventually adds some weight and becomes the slightly heavier isotope 239 U. This
doesn’t last long, however, as the unstable atom grabs onto an electron and becomes
a new element: 239-plutonium, which is both a weapons-grade fissionable material
and easy to separate from the uranium chemically because it is a completely different element. About half of the 239 Pu created by the neutron flux in the reactor is
broken down into fission products known as daughter isotopes during the life cycle
of the fuel rod.
An individual fuel rod lasts anywhere from 18 months to 3 years in a commercial
reactor before it becomes “spent” or inefficient from the accumulation of daughter
isotopes and has to be replaced. Uranium is a mined resource and therefore it is not
considered to be renewable energy. Nevertheless, the life of nuclear fuel rods can be
extended almost indefinitely by chemically removing the impurities from spent fuel
and re-concentrating the fissionable materials, primarily U and Pu into “mixed
oxide” or MOX pellets that can be re-used as fuel.
Such reprocessing of nuclear materials is a much more efficient use of the mined
uranium resource rather than having it pass through the reactor one time only and
then become nuclear waste. However, the presence of 239 Pu in spent fuel rods
sparked all sorts of fears about nuclear proliferation in the 1970s. Each time a fuel
rod is reprocessed, the concentration of 239 Pu increases in the MOX and the more
desirable the rod supposedly becomes to someone who wants to use it to build an
illicit atomic weapon. As such, the Carter Administration banned the U.S. from
reprocessing nuclear fuel. Despite these fears, the U.K., France and Russia have
been successfully reprocessing spent reactor fuel and using MOX for decades without any mishaps, and Japan is commissioning a facility to do the same. If nuclear
power is to have a future in the United States, the reprocessing of spent fuel needs
to be seriously reconsidered.
12 Moving into the Energy Future
neutrons during the fission process, and when concentrated into enough mass, it can
be made to explode in a “chain reaction” as the neutrons hit other 235 U atoms, causing them to fission and release additional neutrons. The explosive threshold is called
the critical mass, and this is how an atomic bomb works.
There is a lot more 238 U than 235 U in the world, so the lighter isotope must be
painstakingly separated and concentrated with centrifuges, gas diffusion systems,
and other sophisticated technology for use in nuclear weapons. This is difficult
because the two isotopes are chemically identical, and only have about a 1% difference in weight. Power plant fuel is made from the leftover 238 U mixed with about
3.5–5% 235 U in what is called low-enriched uranium. It is supplied to nuclear power
plants as ceramic pellets of uranium oxide for making electricity. These are stacked
in tubular metal jackets to create fuel rods for running the reactor. The fission rate
inside the reactor is controlled by a moderator such as water or graphite that absorbs
excess neutrons and keeps the reactor core at an optimal temperature. Low-enriched
uranium won’t explode into a mushroom cloud, but it does have enough fission
energy to boil water, create steam and turn a turbine (Source: https://www.worldnuclear.org/; World Nuclear Association websites).
The 238 U in a fuel rod is exposed to a flux of neutrons in the reactor core, where
it eventually adds some weight and becomes the slightly heavier isotope 239 U. This
doesn’t last long, however, as the unstable atom grabs onto an electron and becomes
a new element: 239-plutonium, which is both a weapons-grade fissionable material
and easy to separate from the uranium chemically because it is a completely different element. About half of the 239 Pu created by the neutron flux in the reactor is
broken down into fission products known as daughter isotopes during the life cycle
of the fuel rod.
An individual fuel rod lasts anywhere from 18 months to 3 years in a commercial
reactor before it becomes “spent” or inefficient from the accumulation of daughter
isotopes and has to be replaced. Uranium is a mined resource and therefore it is not
considered to be renewable energy. Nevertheless, the life of nuclear fuel rods can be
extended almost indefinitely by chemically removing the impurities from spent fuel
and re-concentrating the fissionable materials, primarily U and Pu into “mixed
oxide” or MOX pellets that can be re-used as fuel.
Such reprocessing of nuclear materials is a much more efficient use of the mined
uranium resource rather than having it pass through the reactor one time only and
then become nuclear waste. However, the presence of 239 Pu in spent fuel rods
sparked all sorts of fears about nuclear proliferation in the 1970s. Each time a fuel
rod is reprocessed, the concentration of 239 Pu increases in the MOX and the more
desirable the rod supposedly becomes to someone who wants to use it to build an
illicit atomic weapon. As such, the Carter Administration banned the U.S. from
reprocessing nuclear fuel. Despite these fears, the U.K., France and Russia have
been successfully reprocessing spent reactor fuel and using MOX for decades without any mishaps, and Japan is commissioning a facility to do the same. If nuclear
power is to have a future in the United States, the reprocessing of spent fuel needs
to be seriously reconsidered.
12 Moving into the Energy Future