Think Fast!!!

In my last article on the thorium fuel cycle I threw the bait out hoping that you might bite...now it is time for me to sink the hook.  I made a lot of statements about how using thorium would lower the risk of proliferation, would reduce the waste issues involved with current reactors, and would even make nuclear power more of a long term energy solution.  How would all this come into play simply from changing the fuel we use?  Well, I explained some of the basics in the last post, but many of the answers come from a reactor design that was taken off the table almost 30 years ago.

The fast breeder reactors were so exciting that it seems nuclear engineers were getting high over the idea of them back in the early 80s.  Let me first say that I wanted to find a cool picture to put with this post...my advice is that you should think before typing "breeding" into Google images.  I will spare you guys the cool images this time.  Anyway, some of you guys may have heard the term breeder reactor before and wondered what all the buzz was about.  You see, the idea of the breeder reactors made power generation of fission based nuclear reactors almost limitless.  Why?  Because when operating a breeder reactor, you create more fissile material than you use to run the reactor.  Not impressed?  Well, remember back to my first post on how a nuclear reactor works?  It is the fissile material that we need in the fuel so that we get heat out of it.  This is the material that will capture neutrons and spontaneously undergo fission.  When we take natural uranium out of the ground, less than 1% of it is fissile (actually about 0.71%).  When it comes out of the ground, we can't really use it to power a nuclear reactor.  Some of the non-fissile U-238 must be removed by refinement so as to raise the percentage of fissile U-235 in the fuel to between 3% and 5%.  Phew, tired of numbers yet?  My point is that the amount of fissile material we have on Earth is pretty small, and considering that this is what we need to run nuclear reactors, this is kind of a big deal.

Fortunately, this is not a killer blow to the nuclear industry.  You see, there are reactions which certain materials can undergo by which they become a fissile isotope.  What do I mean?  I mean that certain reactions create more fuel for our nuclear reactors.  This is going on in every nuclear reactor around the world as we speak.  The material is known as fertile, which just means that it can be "bred" to become fissile so that it will undergo fission and power the reactor.  Uranium-238 is one of these fertile substances.  Like a fissile material, it also has the ability to absorb a neutron.  When it absorbs a neutron though, it does not undergo fission but instead transforms into the infamous plutonium-239.  This is a fissile isotope and is the isotope well known as the material for nuclear weapons.  As you can probably see, we are now starting to run into the problem that they had with breeder reactors back in the 80s, but I will get to that in a second.  The plutonium-239 that is produced while the reactor is running is responsible for the fuel being able to last as long as it does in a nuclear reactor.

Unfortunately, current reactors are not very efficient for turning fertile materials into fissile materials.  This is a result of modern reactors being what are known as "thermal" reactors.  This simply refers to the energy at which the neutrons are causing fission.  Today's reactors largely depend on the fact that U-235 has a much higher probability of capturing lower energy neutrons.  This is what "thermal" refers to...the neutrons are at low energies meaning that they are moving slower.  Thus, we use materials in our reactors that slow down the neutrons so to make most of the neutrons in the reactor to be what are considered slow neutrons.

As it turns out, slower neutrons might be ideal for inducing fission which is great for making heat, but they don't take advantage of the large amounts of fertile material we have in the reactor.  You see, we can create more fissile material and thus more fuel if we speed up the neutrons and operate the reactor in what is known as the "fast" range.  Yeah, I know, really creative...  Anyway, with fast neutrons, the uranium-238 in the fuel has a better chance of capturing a neutron and turning into plutonium-239, which is fissile!  Just by operating reactors in the fast regime, we can create more fuel than we burn!  Amazed yet?  Well you shouldn't be because there is still a bit of a problem with this.

For good reason, people were afraid of operating types of reactors that bred plutonium-239.  We are again running into the idea of nuclear proliferation, and here in a very big way.  The thermal reactors that we are running today operate off lowly enriched fuel you see.  This means that only about 5% of the fuel is fissile.  In order for a reactor to run under the fast conditions that I talked about above, they need to have fuel that is enriched so that about 20% of the fuel is fissile.  So now we are talking about 20% of the reactor fuel being plutonium-239.  How easy do we want to make it for people to get their hands on nuclear weapons material?  Not very would be the right answer, so the industry was forced to abandon this idea.  Now though, the idea is coming back in a big, big way!

Remember back to that thorium stuff I was talking about at the beginning of all this?  Well, it seems to hold the key to making nuclear a long term energy solution.  Thorium in its natural state does not actually contain any fissile material at all.  That means that we must enrich it, meaning we must add fissile material to it before we can even think about putting the stuff in a nuclear reactor.  As it turns out though, the natural isotope of thorium (Th-232) is a fertile material.  Now is when your eyebrows should be raising and that smile of complete amazement should be starting to creep across your face.  When Th-232 captures a neutron, it becomes uranium-233, which is fissile!  And even better, it isn't a weapons material!  Hallelujah!  We are saved!

The bottom line is that by putting thorium-232 into a breeding type reactor, we would create more fissile material than would be consumed by the reactor.  Another method of creating fissile material is by simply created a sort of blanket made of thorium-232 which would just be placed over the reactor.  The fast neutrons escaping the operating reactor would be sent into the blanket where they would be captured by the thorium-232 and turned into fissile fuel.  Doing this, reactors would basically be able to power themselves indefinitely.

What limits our current power production from our fuel is the amount of fissile material present.  Currently, the fuel is being run in the reactors until the amount of fissile material gets too small to keep the chain reaction going.  The fertile material is not being used, meaning that we are throwing away more than 95% of the energy left in the fuel when we consider the fuel "spent."  Breeder reactors would change this.  All the material in the fuel could be caused to undergo fission!  This is a powerful idea in the way that it gives us the ability to access virtually limitless energy as thorium is quite abundant in the Earth's crust.  About 5 times more abundant than even uranium.  This should be more than enough fuel to power us until the time where monkeys give birth to another race smarter than ourselves who can solve the problem of fusion power.  I might be missing the concept of evolution here...anybody?

So is the energy crisis solved?  Well, not quite.  There are several industries which need to be better developed before we are ready to launch into breeder type reactors.  First is that we will need to reprocess the fuel as to remove the fission fragments from the fuel.  This is just part of the process, but currently reprocessing of nuclear fuel is not allowed in the United States.  The tide seems to be changing on that though.  I mean with much lower risk of proliferation, there is really no reason not to reprocess the fuel.

The idea of geological storage must also be solved.  Currently we don't have a place to put the high level waste, and though the amount of high level waste coming from breeder reactors is much lower than that from our current reactors, there is still waste that we must store.  I will talk more about this in the future, but I just want you to see some of the issues that must be confronted before we can really depend on this energy source.  Are these challenges worth overcoming?  I would argue that if we want a viable energy solution in the near future, we must overcome these challenges.  Believe it or not, we are a lot closer than one might think to solving all of these problems.  It is largely a matter of politics now, but I am going to venture to guess that nuclear power just might become the first viable renewable energy resource.  Nuclear power is going to solve the energy crisis like anybody would want to solve a problem: by breeding!  Sorry, couldn't resist.  And yes, I said renewable.