Nuclear Fuel Cycle: Material Definitions 1


In this section, we’re going to start by just
going through some basic definitions for nuclear materials and to show you some pictures of
what some of the basic materials we would expect to see throughout the nuclear fuel
cycle are. So, let’s just start with, “What is nuclear
material?” Well, nuclear material is any source material
or special fissionable material that we might expect to see. What is source material; so what is meant
by that? Well, that includes natural uranium, depleted
uranium, or thorium, which are materials that we might find in Mother Nature, in the case
of natural uranium and thorium, or depleted uranium. All these are materials that we then could
use as a source material at the front end of the fuel cycle, which could then lead to
special fissionable material, which is plutonium-239, uranium, U-233, uranium enriched in U-235
or U-233. Special fissionable material is material that
would actually use for radiation, for instance in a reactor, or they could be used in the
manufacture of an explosive device. Also, there are radiological materials. Radiological materials are simply any radioactive
material that is not nuclear material, so that would include medical isotopes, industrial
isotopes, or waste products. This particular picture, which is a fairly
complicated picture, shows the basic buildup and decay chain you would expect to see for
uranium. What is displayed here is a set of boxes that
shows each of the individual isotopes that would exist for an element. So, for instance, for the element Uranium,
you could have U-234, U-235, 236, 237, 238, or 239. The only naturally occurring isotopes of uranium
are U-234, U-235, and U-238, and of those naturally-occurring isotopes, 99.3% of all
of that naturally occurring uranium, is U-238. Only a very small fraction, 0.7% is U-235,
and there’s a very small fraction of U-234, much less than that. Of those isotopes, when we want to use this
in a nuclear reactor, we would like to enrich in the isotope U-235. Typically, power reactors would enrich the
naturally-occurring material from 0.7% to somewhere around 3-5% U-235. And this material, then, this U-235 is what
then would cause fissions inside of the reactor and give you power. So most of the reactions then that would occur
in the U-235 isotope, for instance, will be fission reactions; however, on occasion, when
irradiated in a reactor, some of the neutrons will get absorbed in what we call radiative
capture reaction, which is a reaction then that would lead to the production of, let’s
say, the isotope U-236. That U-236 could also absorb a neutron and
produce U-237. U-237 then could beta decay, which is releasing
an electron to produce neptunium-237. Neptunium-237 is a different element than
uranium. Neptunium-237 is of the element Neptunium,
and so each of the rows on this chart are individual elements, so the elements uranium,
neptunium, americium, and then curium. As we irradiate this material, U-238, for
instance, would absorb neutrons and produce U-239. U-239 then would go through two subsequent
beta decays to plutonium-239. Plutonium-239 then is the material that is
principally used in most nuclear weapons. Plutonium-239, if it continues to be irradiated
in a reactor, would produce the higher mass plutonium isotopes of plutonium-240, plutonium-241,
and plutonium-242. And, of those isotope, plutonium-241, for
instance, could beta decay to americium-241, which could continue the chain. So, as we continue to irradiate a fuel in
a reactor, we would expect to see some of these other isotopes get produced. The elements neptunium, americium, and curium
and so forth are elements we refer to as minor actinides. They’re small players inside of the reactor
in that they are small absorbers, things like that. The principal elements of interest to us are
uranium and plutonium, both of which can serve as fuel sources inside the reactor.

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