Nuclear Fuel Cycle: Irradiation, part 1

Let’s take a look at your basic nuclear reactors
and give you just some basic descriptions of those reactor systems. There typically are three different reasons
why somebody produces a nuclear reactor. The most common reason is to produce electricity
or power. Most reactors in the world, by far most reactors
in the world, are of that type. There also are reactors that are produced
for research purposes, to produce radiation it can be used for research, as well as to
produce medical isotopes and industrial isotopes. Then they’re also reactors that are produced
for the production of plutonium for nuclear weapons, and there are only a small number
of those, but they do, of course, exist. Reactors generally break into two main categories:
thermal reactors and fast reactors. What is meant by that is that thermal reactors
operate off of, principally operate off of, neutrons that are moving slowly inside the
system. Fast reactors operate off of faster moving
neutrons. In order to slow neutrons down we use what’s
called a moderator. The most common moderator used in the world
is light water, so neutrons actually bounce off of the water in the reactor, and every
time they bounce off the water they lose energy, they slow down. Of the light water reactors, there are principally
two types: what is called the pressurized water reactor and the boiling water reactor. The pressurized water reactor, or PWR, is
what we principally have here in the United States, though there are also boiling water
reactors in the United States as well. The Russians also have a type of pressurized
water reactor called a VVR, which is generally very similar to the US PWR. There are also heavy water moderated versions
of thermal reactors, which use heavy water, which is a type of water that contains deuterium
instead of regular hydrogen. Deuterium actually has a mass about twice
that of hydrogen. The most common kind of that, kind of reactor
of that, is called the Candu reactor, which is a Canadian designed reactor that uses a
heavy water moderator and a natural uranium fuel. There are also thermal reactors that use graphite
as their moderator, in that case, they have a solid moderator of graphite, and then they
either have a gas coolant or a light water coolant to cool the fuel. Of the fast reactor types, there is really
only one type the truly exists, which is the liquid metal type reactors. There have been several liquid metal cooled
reactors: EBR-I and EBR-II were ones in the United States, Phenix in France, and then
Monju in Japan, and then, of course, the Russians also have a set of liquid metal cooled reactors
called the BN series reactors. The gas-cooled fast reactor is a type that
has been designed but really hasn’t seen much in the way of production in the world. The BWR reactor, or boiling water reactor,
is a very simple reactor type. In this case, the reactor produces steam;
that steam then drives a turbine; the steam then gets condensed in the condenser and pumped
back up in the throes of the reactor. So, the cycle for it, the thermodynamic loop
for it is very simple. This reactor operates off of a low pressure
light water coolant. It has a low enriched uranium or LEU oxide
fuel, and the coolant is just ordinary light water. The coolant itself serves really three purposes,
two of which we’ve already talked about. It’s the coolant to cool the fuel and clad,
so that keeps the fuel from melting. It’s also the moderator that slows the neutrons
down so that they can then get absorbed in the fuel and cause fission, and then it is
also the working fluid, which then spins the turbine and produces electricity. This type of reactor, the BWR reactors have
to use enriched uranium fuel. That’s because the coolant that used here,
which is a light water coolant, absorbs a lot of neutrons. It absorbs so many neutrons that you can’t
use natural uranium. You have to enrich the fuel. This is just internal structures of the reactor
itself, where this area here is the actual fuel. The rest of this is all control structures
and then areas that are used for the coolant system. The pressurized water reactor, or PWR reactor,
which is probably the most common reactor system in the world, is slightly more complicated. Actually, it has a dual loop structure, where
it has a primary loop and then a secondary loop. The primary loop is kept at a very high pressure,
around 2200psi. It still uses a light water coolant, and it
uses a low enriched uranium oxide fuel, like the BWR, but this coolant, since it’s at a
higher pressure, this coolant does not boil. It stays as a liquid form the entire time,
and then the secondary loop is kept at a lower pressure, and that coolant then boils in this
thing called steam generator, and then that steam is used to drive a turbine. The dual loop system allows you keep any radiation
from contaminating the turbine; there’s no radioactive materials that will go over and
contaminate the turbine, because it’s separated into these two separate loops. The PWR reactor also uses a coolant that serves
as the moderator but that same coolant, that same coolant for the fuel, does not serve
as the working fluid. This reactor, since it uses light water, also
has to use an enriched uranium fuel, and, again, this is just a picture showing the
typical Westinghouse-style PWR reactor. This is the structures of the of the pressurized
water reactor, which is the containment structure that you would expect to see. The reactor is the small little thing right
there in the center. The large structures here are actually the
steam generators for the reactor and then containment around the outside, which is there
to provide safety, so that if anything bad happened inside the reactor that this would
all get contained inside this large concrete and steel structure. Also shown here in this other building is
the spent fuel pool, where spent fuel from the reactor is stored in a separate building
outside of containment. Another type of reactor we see in the world
is the Candu reactor. The Candu reactor was designed by the Canadians. It’s based again on dual loop Rankine cycle,
similar in many ways to PWR. The coolant itself is a either light water
or heavy water coolant that flows through a set of pressure tubes in the center of the
reactors, so what’s shown here is each of these little tubes here is a pressure tube
running through the core, and each tube is an individual tube. Inside of that tube, then, is a natural uranium
fuel, usually in oxide form, but it technically could be metallic form as well. And then in this larger tank around the outside
is this heavy water moderator, which is kept at a fairly low pressure, and the heavy water
moderator then serves to moderate the neutrons. Like I said, the system is in a dual loop
system, so this is your primary loop here, and then that primary loop then produces a
steam in a steam generator, which goes through a secondary loop, including flow through a
turbine to produce electricity. So, in that case, in that respect, it’s fairly
similar to a PWR. One different so you see is that this reactor
is actually on its side. The PWR reactor, the cylinder was up and down. In this case, the cylinder is on its side,
and the water flows through the reactor sideways This picture shows the refueling machine of
a Candu reactor. One of its more interesting characteristics
is that the reactor can be online refueled, so while the reactor is still operating they
can turn off individual pressure tubes and then pull fuel out of those pressure tubes
and put new fuel in. The coolant here only serves the purpose of
keeping the fuel and clad from melting. The moderator itself, which is in a separate
vehicle called the Calandria, is a heavy water moderator. Since it has a heavy water moderator, it can
use a natural uranium fuel. Heavy water absorbs a lot less neutrons than
uranium, sorry, than LIGHT WATER, and so, therefore, the fuel that is in the reactor
does not need to be enriched.

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1 Response

  1. developercm says:

    Very informative. Thank you.

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