Nuclear Reactor’s fuel

Yesterday I saw a commercial of Chernobyl HBO TV series and it inspired me to write this article on nuclear power reactors. The idea is to briefly explain the physics behind the nuclear fission and what happen to the uranium inside the reactor. So lets start.

You may have heard that common nuclear reactors works by bombarding uranium atoms with neutrons and splitting them into “smaller pieces”. During the splitting process, more neutrons are created that hit and split other uranium atoms, creating a chain reaction. This process is called Nuclear Fission (not to be confused with nuclear fusion that is the one that happen in stars). Because the sum of the “pieces” weight less than the original atom, that missing mass is converted directly into energy according with the famous Einstein formula (remember that photons with no mass also have energy!):

E2 = m02c4 + (p c)2

Usually this is more or less what the people knows about “nuclear power”. Also they know that is somehow dangerous. So well, if you want to know why is dangerous, continue to read!

First things that you will need to know is that there are actually two type of neutrons: the fast ones and the slow ones (also called Thermal neutrons). The difference is pretty easy: the slow one are slower! When a Fast neutron pass trough some materials (for example water), the neutron can “bump” into the material atoms (hydrogen in case of the water) and lose some of its momentum. The result is a slow neutrons.  Materials capable of slowing down neutrons are called “moderators”.  So we can just slow down fast neutron by submerging them into water. Other example of moderators are Graphite (we will see why is a bad idea to use it in the next article), heavy water or beryllium.

Another concept needed is the sustainable reaction principle. In order to make a nuclear reaction self sustainable (chain reaction), each uranium atom that get split must in average split at least another uranium atom. If this is not the case the reaction dies out. On the other hands, if each split uranium atom release enough neutron to split many more uranium atoms, then the reaction became quickly uncontrollable and you created a nice nuclear mushroom.

Ok now we can go back to our uranium stuff. Uranium has many isotopes but the two most important are the 235 and 238. Uranium in nature is mostly present as uranium-238 (99%) and only 1% is uranium-235. This two isotopes behave very differently, when bombarded with neutrons.  In particular:

  • If you bombard uranium 238 with a slow neutron, most likely it is gonna absorb that neutron and became a plutonium-239 atom.
  • if you bombard uranium 238 with fast neutrons, either is gonna break into smaller parts and release energy or is gonna absorb that neutron and became a plutonium-239 atom.
  • if you bombard uranium 235 with slow neutron, mostly likely is gonna break into smaller parts and release energy with a great neutron efficiency (good)
  • if you bombard uranium 235 with fast neutron, they still split into smaller pieces, but is not so efficient, because the probability of splitting up is much much smaller than slow neutrons case.

More in general, the efficiency of fast neutrons of splitting atoms is much lower than the efficiency of slow neutrons. So, because enriching uranium is very very expensive, it turns out that the cheapest way for now to produce nuclear energy is to use uranium 235 together with slow neutrons.

Unfortunately, even with the slow neutrons, the 1%  concentration of uranium 235 present in nature is not enough to sustain a chain reaction, so it must be enriched by reaching at least 3% to 5%. This basically means that before we can use natural uranium as fuel, we need to increase the concentration of uranium 235 by a factor or 5 or so. This process is called “Enrichment”.

So once that we have this enriched uranium bars, we can then put them in a moderator (water)  and start a chain reaction by bombarding it with neutrons. Once started, the reaction will self sustain (if not, please adjust the neutron speed ). The reaction heat is also transferred to water that is used to power up turbines etc.

This is basically how all the current nuclear power station work. However this approach has few drawbacks.

  1. You may have noticed that we need to enrich the uranium… very expensive process to do.
  2. We are only using the uranium 235 to create energy. This means that more than 95% of the uranium is not used!!!
  3. Because that 95% of uranium is actually uranium 238 and we are bombarding it with slow neutrons, it happen that some percentage of it is converted in heavier elements called “Transuranium” elements (reported below).

The problem with these elements is that are highly radioactive, non fissile (except plutonium 239 that is actually fissile) and their half life is thousands of years.

So for resuming, we extract uranium from  earth, we only use 1% of it and the 99% left is highly dangerous radioactive waste that will last few geologic eras.

The funny fact is that before we put it in a reactor we can handle bars of enriched uranium with bare hands or sleep in a bed made of uranium without any problem (as far as we do not ingest or inhale it). But after “burning” it in the reactor, it would kill you within few second of exposure!

So would be cool to be able to burn everything in the reactor and get rid of all those toxic shit! Luckily,  there is a solution, and is the Fast Neutron Reactors.  This reactors instead of using slow neutrons, they use the fast neutrons. As we saw before, when the uranium 238 is hit by fast neutrons, either is split, producing energy, or is absorbed and transformed into plutonium 239. When the plutonium 239 is hit by a fast neutron, it splits into lighter elements, producing energy. So how you can see, when hit by a fast neutron, the uranium 238 either produce energy directly (good) or gets converted is something that can produce energy directly (also good!)

Furthermore, When using fast neutron reactors, we do not need to use any moderator, so we can reduce the size of the reactor and use it for propelling a submarine for example (and that is what they do!).

So, we can make the reactor smaller, cheaper, 99 times more efficient and with almost no dangerous waste that last thousand of years. Sound great, lets do it!… Well not so fast… There is one little problem…

Because the efficiency of the fast neutrons is not so high, we need to start from fuel that has uranium 235 (or plutonium-239) enriched to 25%! That is very expensive to produce.. and  is a weapon grade uranium (in the meaning that you can actually use it to create Nukes!)…

The good news is that, once the reaction starts, because the fast neutron hit release many more neutrons, some of those neutrons will convert fertile uranium 238 into fissile plutonium 239. So it kind of create its own fuel. This process is used in breeder reactors.

So at the end, we will have much less waste, and because it has burned all the Transuranium materials, their half life will be hundred years, and not thousand of years!

However there is problem. Because fast neutron reactors need high enriched fuel, the same fuel can be used for create nuclear weapons. Also, breeder reactors itselfs can be used to create high enriched plutonium239 material that can also be used for bombs.

So how you can see the choice was between, use normal nuclear reactors that uses only few % of the uranium and create dangerous long lasting radioactive waste or use the fast neutrons reactors that produce much less wastes but uses fuel that can be used for bombs…

What you would have choose?

Anyway, next article i will discuss how the current nuclear power plants manage depleted uranium.


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