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Know It All: The Atomic Bomb -- Inside the Bomb

Published in Geare magazine, Issue #51, 2008

All seated class? Got your notebooks ready? Let's make ourselves an atomic bomb!

Making an atomic bomb is hard. Hard, for two quite separate reasons. The first is that the materials and equipment you need are difficult to come by, and trying to do so tends to attract the attention of hard-eyed men with big guns. So don't even think about it unless you're friends with Kim Jong-Il and have a large bank account.

The second reason making an atomic bomb is hard is that the work must be performed very precisely indeed.

All that aside, understanding what happens in an atomic bomb is not hard at all. Once, that is, you understand a little of mid-20th century physics. This is where your notebook comes in.

Bomb Building Blocks

Only kidding! In my first draft I wrote about the twelve-and-a-half year half-life of tritium and the 0.02% of hydrogen which is in the form of deuterium. Major league boring! We can skip that and still, I hope, make sense.

You already know, of course, that every physical object that you can pick up and hold is made out of atoms. An atom consists of a nucleus and electrons. When electrons move from atom to atom, we have electricity. Electrons can get around. An atom may have too few electrons, or too many, but the essential character of the atom remains the same.

The atom, as such, is not relevant to our bomb. Because with the poorly-named 'atomic' bomb, we are talking about doing stuff to the nucleus of the atom. A nucleus consists of protons and neutrons. Both are roughly the same weight (and nearly 2,000 times heavier than electrons). The type of atom -- which element it is -- is defined by the number of protons. If the nucleus has one proton, then that atom is hydrogen. If two, then it's helium.

Some elements have a lot of protons. An atom of gold has 79 protons and uranium has 92. The element formed by the atom determines its chemical properties. But not all gold is identical, nor all uranium, nor any other element. That's because their nuclei can have different numbers of neutrons. The same atom with a particular number of neutrons is called an isotope.

Things get a bit weird here. Some isotopes self-destruct. When any particular atom will self-destruct is unpredictable. However the proportion of atoms forming a particular isotope that will self-destruct in a particular time period is very predictable. We use a measure called the 'half-life' for that. Many isotopes have very short half lives. For example, you have probably never heard of the element Astatine. I know I hadn't until I was researching this piece. Its longest lasting naturally occurring isotope has a half life of 56 seconds. That is, if I handed you a test tube of the stuff, one minute later more than half of it would be gone. After two minutes, more than three quarters would be gone. And so on.

You wouldn't be around for too long yourself after holding a test tube of Astatine, since it is highly radioactive.

Chain Reactions

When an atom self-destructs, it generally splits into a new atom with a smaller number of protons, and frequently shoots off an 'alpha' particle (basically, a helium nucleus), one or more neutrons, one or more electrons (called 'beta' particles), and a blast of pure energy.

If you dig up some uranium ore from the ground and analyse it, you will find that more than 99% of it consists of the U-238 isotope (92 protons, 146 neutrons), and about three quarters of one per cent consists of U-235 (143 neutrons). Both isotopes are radioactive. That is, they self-destruct spontaneously. Neither is strongly radioactive. U-235 has a half-life of over 700 million years, and U-238 six times longer.

But U-235 is unique in naturally occurring isotopes: it is 'fissile'. That means that if a wandering neutron stumbles into it, it will split. You can split lots of atoms by firing neutrons at them at enormous speeds (and, consequently, energies). That isn't necessary for U-235. Roll a neutron in at a snail's pace and it will split. The same will happen with two different isotopes of plutonium, and another of uranium, but none of these naturally occur.

Now if you split a U-235 atom, it shoots off a couple of neutrons. If one of these hits another U-235 atom, it will in turn split, ahead of schedule, and shoot off some neutrons.

If the U-235 atoms are close enough together, you can get a domino effect -- the so-called chain reaction. These days U-235 atoms are too far apart in natural uranium for a chain reaction (since it decays faster than U-238, its concentration has been gradually diminishing). Most of the neutrons that are fired off miss other U-235 atoms, hitting U-238 atoms instead. These are too stable to be split by these low-energy neutrons.


To go further, the uranium needs to be 'enriched'. That is, the percentage of the U-235 in the uranium needs to be increased. How do you select atoms by the number of neutrons they contain? You can't just peer at them with an electron microscope and pick them out one by one.

Instead, you use their difference in mass. Remember, in just about all chemical and physical ways U-235 and U-238 are indistinguishable. They react the same way chemically. They melt at the same temperature (1,132C). But U-235 has three fewer neutrons than U-238, so each U-235 atom weighs about 1.3% less than a U-238 atom.

Now as it happens, the international bureaucrats who concern themselves with nuclear proliferation pay attention not just to where uranium ore gets shipped, but also certain categories of equipment. One such category is the centrifuge. This is a gadget that spins around, allowing you to separate stuff by weight. The uranium is combined with other elements into a gaseous form. It is spun around. The U-238 content, being heavier, moves to the outside while the U-235 part stays a bit closer to the centre. The two can be separated out and the uranium chemically processed back out of the gas.

This is not a highly efficient process. Late last year Iran was boasting that it had some 3,000 uranium enrichment centrifuges up and running, yet proliferation experts still talk about months, or even years, as being the timescale required for that nation to produce sufficient enriched uranium for a single atomic bomb.

How 'enriched' does the uranium need to be? If you're planning on generating power with a nuclear reactor, you need to bump up the proportion of U-235 in the uranium from the natural 0.72% to between three and five per cent. If you want to make a crude, inefficient atomic bomb, then you need uranium enriched to 20% U-235. If you want a really cool, highly efficient atomic bomb, then you'll be shooting for 85% U-235.

Incidentally, you've probably heard of 'depleted uranium'. They make armour on tanks and artillery shells out of this stuff. Depleted uranium is the opposite of enriched uranium. It is the uranium left over after you have extracted all the U-235 you can economically manage out of natural uranium. Typically the 0.72% U-235 concentration is reduced by two thirds. The reason depleted uranium is used is the same reason that lead is used in firearm bullets. Lead is heavy. Uranium is about 70% heavier. It's good at resisting penetration, thus its use as armour plating. And it is good at penetrating, thus its use in artillery.

But we are looking at enriched uranium. In a nuclear reactor, you put a bunch of the 3-5% stuff in a particular area. You have damping materials (which absorb neutrons harmlessly) to control the extent of the chain reaction, and you use the heat given off by the process to boil water, driving steam turbines and, subsequently, electrical generators. In a nuclear reactor, it's all about control.

Building the Bomb

In an atomic bomb, it's all about acceleration. You want the chain reaction to run out of control as quickly as possible. There are a number of different ways of doing this, so we'll look at the 'Little Boy' way. That is the name given to the first atomic bomb used in anger, the bomb dropped on Hiroshima.

This used a so-called 'shot gun' design. There was one bundle of highly enriched uranium (ie. with a high proportion of U-235) suspended above another bundle. Below this second bundle was a set of 'initiators' made of polonium and beryllium. These emitted a constant stream of neutrons.

When the bomb was dropped, on achieving the desired altitude of 580 metres a chemical explosive was used to propel the first bundle of uranium into the second. This combined mass of highly compressed uranium resulted in a 'critical mass' -- a mass in which a neutron could hardly fail to hit a U-235 atom, causing it to split and fire off more neutrons, hitting more U-235 atoms and ... The polonium/beryllium initiators provided the triggering neutrons.

Einstein provided the equation: energy equal the mass times the square of the speed of light. Light travels fast (300,000 kilometres per second). So a little bit of mass makes an awful lot of energy.

Now, grab yourself a sheet of standard 80gsm laser printer paper. Fold it in half. Fold that in half again. And then once more. Unfold it. You will have eight equal sized sections of the sheet marked with creases. Cut one of them out. Write on that piece 'Hiroshima'.

That small piece of paper weighs about sixth tenths of one gram. When 'Little Boy' was detonated around 8:16 on the morning of 6 August 1945, six tenths of one gram of it was converted to energy, thanks to the fission of U-235. Zero point six of a gram.

That energy was the same as that produced by the detonation of twelve thousand tons of TNT.

A moment later 70,000 people were dead. The 'radius of total destruction' was 1.6 kilometres.

All from six tenths of a gram of mass.

Uranium is much heavier than paper, so let us re-envisage those six tenths of a gram. Consider a tiny cube that measures just one third of a millimetre on each side. That cube would hold the entire amount of U-235 that was converted to energy that day.

That's smaller than a pinhead.

© 2002-2009, Stephen Dawson