Chemistry

5.   Chemistry

Nuclear fission

I was about 12 years old when I learnt how to make an atomic bomb. In theory, you understand, not in practice. It’s really quite simple. All you have to do is gather together a sufficient amount of the right kind of Uranium and bang it together pretty quickly.

Uranium comes in two flavours – U-235 and U-238. U-238 is stable and fairly boring. It is also the most common kind – just as well, really. Actually, there is also a very small amount of U-234, but that’s not very significant.

U-235 is a different kettle of fish. It is unstable, and if it is hit by a passing neutron, it splits into two lighter atoms, and (here’s the trick), two or three new neutrons. According to the rules of atom construction, there is also a surplus of energy given off as radiation. This is where the famous e=mc² comes in; some of the mass of the atom (the ‘m’ bit) is converted into energy (the ‘e’ bit). And because the ‘c’ bit is the speed of light, which is very fast, it’s a big number and so the energy is a lot. The energy given off by all the atoms in a pound of U-235 is about the same as that contained in a million gallons of petrol. This all happens pretty quickly you understand, in fact in about one picosecond (that’s one millionth of a millionth of a second – I said it was fast).

Now those following closely will have noticed that we started with one neutron, and ended up with two or three. These extra neutrons wander off and, probably, hit some more U-235 atoms. Then even more neutrons are given off, and more energy. Sound familiar? This is positive feedback with a vengeance. The whole process is over before you can blink an eye, and a large mushroom cloud starts rising. This is, of course, an atomic bomb – or a fission device (because the atoms are split, as opposed to a Hydrogen or fusion bomb, where they are combined).

For this process to create a fission device, there must be enough U-235 to keep the chain reaction going; this amount is called the critical mass. There also must be a high percentage of U-235 - around 90%. Luckily it is quite difficult to make enriched Uranium at this level, partly because there is about 140 times as much U-238 as U-235. It is also actually quite tricky to bang together the pieces quickly enough to get a nuclear explosion. If they don’t come together fast enough they start to overheat too soon, and the whole mess sort of goes off half cocked. So I never got to actually make a nuclear bomb, but I did make some satisfactorily loud bangs with a mix of chemicals that I now shudder to recall. I also made a wide collection of fireworks, guided by some ‘do it yourself’ books that would surely not be published nowadays. Nor, I imagine, would my friendly local chemist sell me the lethal assortment of chemicals that I asked him for – I would probably receive a visit from the securities agencies instead.

Nuclear reactors

A nuclear reactor in a power station also uses enriched Uranium, but only around three percent U-235 is needed in this case. Typically, the Uranium is made into rods, and the rods are made into bundles. The bundles are then placed in a coolant, such as water, inside a pressure vessel. The U-235 does its thing, splitting up and releasing neutrons. If these hit a U-238 atom, they get absorbed. If they hit another U-235, it will probably split up and release more neutrons. But because the U-238 slows things down a lot, the reactor doesn’t explode. It just gets hot, and the heat is extracted to generate electric power. Now the neutrons in reactors are low energy or ‘slow’ neutrons, rather than their more energetic cousins in a nuclear bomb, but the principle is similar. Just a lot safer.

Actually, things are a bit more tricky. Water slows down neutrons, as well as being a coolant, and the slower neutrons are better absorbed by U-235, so the water increases the reaction rate.

Control rods are used to control the rate of the nuclear reaction, by raising or lowering them into the Uranium bundles. By absorbing neutrons, they can slow down the reaction.

That’s not quite the end of the story however. When U-238 absorbs a neutron, it changes into PU-239, otherwise known as Plutonium. Now Plutonium does split when hit by a neutron, so it can be used as fuel itself – this is the case in the so called ‘Breeder Reactors’.

So, as is often the case in messy real life, there are several layers of feedback going on here.

Meltdown

If something nasty goes wrong in a nuclear reactor it could start a process leading to meltdown. This is really another example of thermal runaway. Excessive heat generated in one part of the reactor could lock up the operation of  moderator elements, leading to increased reaction and heat generation. This could, in theory, lead to the whole mess getting so hot that the entire reactor melts. Now safety is something that the nuclear industry takes pretty seriously, so there are all kinds of mechanisms to make sure that this kind of thing does not happen.

But sometimes things go wrong, as in Three Mile Island, where a partial meltdown did occur. The exact details are complicated, but basically the safety mechanisms designed to cope with a component failure did not operate as planned.

Later at Chernobyl, there was a full meltdown. Again, there was a complex sequence of events, but ironically the main cause of the accident was a test of the safety systems that went wrong.

Meltdowns cannot cause nuclear explosions, because the geometry of the plant simply does not allow this to happen. But accidents at nuclear plants can cause explosions through excess pressure.

Explosions

A chemical explosion, using something like gunpowder, for example, is another form of thermal runaway. Some trigger causes part of the chemical to react and burn extremely quickly. This causes heat and a pressure shock wave to move through the rest of the chemical. The action of this increased energy causes other parts to react and burn and generate even more heat and pressure. This whole process takes place so quickly that it causes an explosion, with a consequent release of hot gases and a pressure wave. Another example of the bad effects of positive feedback (unless the explosion is being used for good purposes of course).

Auto Catalytic Reactions

Catalysts are chemicals that help speed up chemical reactions. They don’t actually get involved in the reaction except temporarily. The amount of a catalyst is unchanged after the reaction, but it sort of gets lent to the process, and returned afterwards. Think of it like a bank lending money to a company to help it improve its operations, and then getting it back. Only catalysts do not charge interest.

Some chemicals are auto catalysts; that is they catalyse a reaction that creates themselves. This should sound familiar by now, it is starting to sound like a loop. Chemical A takes other chemicals B and C and help them make more A, more A makes more A. Pretty obviously, this can rapidly make lots of A in rather the same way as an explosion. Auto catalysts are very interesting, because it is possible that life started in some similar manner. The point is that, given some raw chemical ingredients, we have a mechanism for producing lots of copies of something. And copying is akin to reproduction. Experiments with auto catalysts have shown that variation in the copying also takes place, which is one of the other prerequisites for evolution to take place. You can’t have evolution if all the copies are identical, it relies on having variation and selection.

Supercooling

This is not, sadly, a tale of people who are super cool. A supercool liquid is one that is colder than its normal freezing point, so for water that would be below zero Centigrade. How can that happen? Well, when a liquid freezes, it goes through quite an arduous process where the molecular structure completely changes to a lower energy state. Although falling temperature pushes the liquid down this path, it can need a little help to get things going.

This help can be in the form of impurities in the water that act as nuclei for the new structures to form around – a bit like catalysts. If the water is very pure (as in distilled or de ionised water), there aren’t enough impurities to get things started, especially if the water is quite still. Any change in the circumstance, like introducing some impurities, or disturbing the water, can trigger an avalanche type effect where the whole lot rapidly freezes – or at least forms ice crystals. It is positive feedback at work again as each freezing event triggers adjacent ones.

This actually happened to me once. When I was a teenager I had a large shed in our garden which I used as a chemistry laboratory, full of wondrous supplies of chemicals and apparatus (well, I though it was wonderful at the time). I entered one very cold morning and observed a glass beaker full of unfrozen distilled water. Poking the water with a glass rod immediately caused the whole beaker to become a mess of ice slush. So I know this works.

The same thing applies if a vapour is over saturated, it can stay a vapour when it ‘should’ really condense. Some newcomer moving in can cause the vapour to condense around it as it passes through. This mechanism is used in nuclear physics as particle detection ‘cloud chambers’, though bubble chambers are now more common. These again use a similar mechanism, where a liquid is caused to bubble into a gas by incoming particles.