Nuclear Fuel Cycle: Irradiation, part 2

Another type of reactor that you will not
see very many of, but that used to exist in large number in the world is graphite moderated
reactors. In this case, the reactor uses a solid graphite
moderator. That moderator then moderates the neutrons. Since graphite also absorbs very few neutrons,
compared to light water, this system also can use natural uranium in either oxide or
metallic form. What’s shown here is the MAGNOX reactor from
Great Britain. The MAGNOX reactor uses a metallic uranium
fuel usually in a uranium alloy, and then it uses a gas coolant. In some of these cases, it can use a gas coolant
in a Brayton cycle, or, in some cases, it might use a light water coolant in a Rankine
cycle or even a gas coolant in a Rankine cycle. There are a number of different types of graphite
moderated reactors. The picture that is shown here is actually
from a design reactor called an HGTR. This is actually from one of the ones that
was built in the United States. I believe that’s where that picture came from. This particular reactor, again, is a gas-cooled,
graphite moderated, natural uranium fueled reactor. There’s also the RBMK-style reactor, which
is the style the Russians designed. The most famous of those was the Chernobyl
plant, which was an RBMK-style reactor. There are not many, if any, RBMKs still operating
in the world today, which is probably a good thing. Then, of course, there’s also the HGTR-type
reactor, which is what is shown here in this picture. The last kind of reactor we are going to talk
about is the fast reactor. Fast reactor systems are actually a very complicated,
very advanced technology. Very few states have developed this type of
reactor system. They typically have three loops to it, so
there’s a primary loop, which is the one shown here, which flows through the reactor. That goes through a heat exchanger that goes
to an intermediate loop, which then goes through a heat exchanger that goes to a third loop. The third loop, of course, is the one that
actually then produces electricity. The intermediate stage is added in here because
of the fact that the coolant that is used in this reactor becomes very, very radioactive,
and it becomes so radioactive, in fact, that it can disassociate hydrogen out of water,
and so this intermediate loop is there to provide a buffer between this radioactive
coolant and the water loop, where this coolant that is in between here would be a non-radioactive
metallic coolant. The most common version of this uses liquid
metal as its coolant. The liquid metal can become very intensely
radioactive. The liquid metal also, typically, is sodium-cooled
liquid metal, or sodium liquid metal, and the sodium itself is fairly dangerous in that
it catches fire in air and water, and it can cause some other difficulties. One nice thing about these reactors, they’re
extremely small. Typical size of a fast reactor is about the
size of a trash can, so they’re very compact. They will often use highly enriched uranium
or plutonium as a driver fuel that is then surrounded by a depleted uranium or natural
uranium blanket, and that blanket then is used to breed plutonium. And these particular cores are designed in
such a way that they actually can produce more fuel than they consume, and so the typical
analogy of this is it’s similar to your car and that, with this type of reactor, as you
operate the reactor you actually make more fuel than you burn up, which is the same thing
as you filling up your gas tank halfway, operating your car for a while, and, when you stop,
having the entire tank full. The coolant that’s in here is usually, like
I said, liquid metal or sodium, but there are types of reactors that have been designed
of this type that use gas coolant instead of liquid metal coolant. The liquid metal has some very excellent heat
transfer properties, which is one of the really good reasons to use it. The gas coolant doesn’t have near as good
of heat transfer properties, but it does have some other advantages as well. Control characteristics of this reactor are
very complicated. The fuel manufacturing is difficult, so this
particular technology is very advanced, and only a few states really ever mastered it. If we then look at what the production of
material in an operating reactor is–this particular chart just shows the plutonium
buildup in a pressurized water reactor, where you have the mass of plutonium, or concentration
of plutonium, as a function of burnup. Burnup is the amount of energy liberated from
the fuel per unit mass of the fuel. So, if a reactor, or if a fuel, was operating
at a constant power, this x-axis would just translate to time. And so, if I was running the reactor for a
certain period of time, I would get a certain amount of plutonium, and, in this particular
case, you can see that plutonium-239 builds up linearly at first, and then, as I continue
to operate the reactor, at some point in time the plutonium then starts absorbing neutrons,
and then it’ll reach equilibrium inside the system, where the production of plutonium
is equal to the destruction of plutonium due to fission or due to absorption. This curve actually, if one could keep taking
this thing out, would actually start to come down. At some point, you would actually stop producing
plutonium-239 and actually start absorbing it, and it’ll start to decrease, but you have
to get to really high burnups to do that. As I continue to burn the fuel, while I stop
producing as much plutontium-239, I start producing more of the higher mass plutonium
isotope, like plutonium-241 or plutonium-240, plutonium-241, plutonium-242, etc. If I just look at the total plutonium content
of fuel as a function of time, I get this curve, which shows that, again, early on the
production of that is very, very linear, so up until about ten thousand megawatt days
per metric ton, but as I continue to burn the reactor, at some point, I’m producing
more energy but not producing as much plutonium per unit time that I’m operating the core. What that tells me is that a particular operative
reactor, if one wanted to operate a reactor for the purpose of producing plutonium, you
would operate the fuel down in this lower area, where you would be producing as much
plutonium per unit time of operation as you possibly can. If somebody’s operating a reactor for the
purpose of power, they typically operate the fuel out in this area, and so typically what
we expect to see is that if a fuel is operated for power, I would expect to see burnups in
the order of 30, 40, 50 gigawatt days per ton. If it’s operated in terms of plutonium production,
I would expect to see lower burnups, 10,000 megawatt days per metric ton or lower. If I look at different types of reactors,
you see similar curves. The amount of plutonium produced is different
for different reactor types, so this is a very similar curve; it just shows the concentration
of plutonium as a function of time, or a function of burnup for PWR, BWR, Candu, MAGNOX, etc.,
different types of reactors. And you see curves that are similar, but not
necessarily exactly the same, but essentially the same rule still applies. In the case of the MAGNOX and Candu reactors,
in those type of reactor systems you typically can’t operate the fuel beyond about 10,000-20,000
megawatt days per metric ton because there simply isn’t enough U-235 in the fuel to keep
it critical in the reactor. PWR and BWR reactors, on the other hand, can
operate to typically much higher burnups. So that goes through the basic types of nuclear
reactors and some of the plutonium production that you might see for those systems.

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