A tiny bit on plutonium production

Sometimes Mother Nature plays funny. Sometimes she is offering something cheaply and so we take it – but not always to our benefit.

This is a short story on plutonium. Well, plutonium is a popular chemical element because it is used in The Device. Nevertheless, in the story below you will find something other that is really fascinating – not The Device.

Uranium story

As you know some heavy atoms are not stable. That is, their nucleuses spontaneously split. Usually you can detect some radioactivity while this is happening. That is, some particles are generated and radiated away at the moment of such split. Further, during such decay some atom types radiate particles that are able to split other not-jet-decayed atoms of the same kind. In fact, under right conditions a chain reaction may occur where each split atom will, in average, cause more than one other atom to split.

Well, the only chain reaction capable element we found in nature in appropriate quantities is uranium isotope U235. The uranium isotope U235 is mined – this is how we are getting it.

If only it is so simple. The real problem is that the uranium isotope U235 is always mixed with uranium isotope U238. In fact, out of all the uranium only about 0.7% is the valuable U235. Almost everything else is the U238. Sadly enough, U238 is the one that is no good for a chain reaction (U238 is not even radioactive).

Separating U235 out of U238 is a challenging task and an intense labor. These are both the same chemical element, having the same chemical properties. The only difference is that U235 is very slightly lighter than U238. You separate them in centrifuges over and over again. In each step you get uranium that has more percentage share of U235 (they like to call this process enrichment, and uranium having higher-than-natural share of U235 they like to call enriched uranium).

To make a sustainable chain reaction, fortunately, we don’t have to make 100% pure U235. We only need to enrich uranium to, say, 4% of U235. This is a quite affordable task. In fact, nuclear power plants generate much cheaper energy than most other plants do.

If we still want to produce more pure U235 we will soon get into troubles. Each further percentage point becomes a harder to reach milestone. There are not many countries in the world that are wealthy enough to produce only a few kilograms of 95% pure U235.

Okay, but why would anyone want to spend all that money to produce high-purity U235 considering that electric energy can be effectively produced with even cheap 3%-4% U235? You know the answer – The Device.

And where is the plutonium in this story. Don’t worry - the plutonium stuff follows.

Plutonium story

Plutonium isotope Pu239 is also very good in making chain reactions. Unfortunately there are no plutonium mines around. How are we getting plutonium then? The answer is – we are creating it. We make plutonium (specifically plutonium isotope Pu239) in nuclear reactors.

Now follows the amazing part… Do you remember U238 – the one that is so abundant but no good for a chain reaction? Well, if you expose the U238 to the same radiation that is generated during U235 chain reaction, the U238 gets transmuted into Pu239! Somehow the Mother Nature was very lovely toward people that are working on The Device.

I said lovely because now you created the plutonium Pu239 isotope that is completely different chemical element – quite diverse than uranium. It means that it is much, much easier now to extract it out of uranium using only cheap chemical methods. In turn it means that it is pretty easy to get the high purity Pu239. That is why they love it so much – cheap, pure and available in large quantities. Used only for a single purpose.

So… you run your nuclear rector using relatively cheap 4% U235 that you can buy on open market. As reactor runs, the inactive U238 gets transmuted into Pu239. You then take the uranium out of the reactor and simply and effectively extract almost pure Pu239.

That is why are different committees of strong nations always visiting nuclear power plants in weaker nations - to check if they are secretly producing plutonium.

And the Breeder

One type of nuclear reactor, called fast breeder reactor (FBR), is particularly effective in producing plutonium. A typical thermal reactor uses moderator to slow-down neutrons because slow neutrons are doing better job in breaking U235 atoms during a chain reaction. But to transmute U238 to Pu239 fast neutrons are favored. The fast breeder reactor is specifically designed to operate using fast neutrons (to make it possible it must, however, use high-grade U235 fuel that is much more expensive). This type of reactor is almost banned because of its ability to produce large quantities of potentially dangerous Pu239.

But now I want to bring this article to happy ending.

As I said Pu239 is quite good in making chain reactions. It can also be used for electric energy production, and not only to energize The Device. What if we don’t take the Pu239 out of the reactor? What if we leave it there to engage into chain reaction and start producing energy? And yes, this can be done, and already was done. Such a reactor produces more fuel than it wastes! Plus it powers a power plant. The breeder reactor really is an ingenious machine – only I wish the plutonium is not so needed for other purposes.

Danijel Gorupec, 2006