# Chapter 4.4 Chain reactions

Atomic bombs are dangerous because they are so simple. As far as we know, every nation that has attempted to build and explode them has succeeded. Relatively crude technology will do. The first bombs were built in the early 1940s before transistors and computers were invented. Think of an old radio or automobile from that time: the same technology was used to build the first bombs.

The central idea is this. Suppose that we have small but strong steel springs that can be compressed and latched. Since they store energy when latched, they are slightly heavier. Suppose, however, that the latches are fragile and barely manage to keep the spring from extending out again. With the slightest jar or bang, the latches may break and release the spring. Thus the compressed and latched spring is unstable.

Suppose that we collect hundreds of such latched springs, pack them tightly in a barrel and screw the lid down. They may sit there peacefully for a while. But suppose that someone bumps the barrel, or suppose that a latch somewhere spontaneously breaks apart. If just one spring is released the commotion may give its neighbours a knock. They too will burst their latches, and knock their neighbours in turn. Soon the barrel will be rocking and bouncing with uncoiling springs. Perhaps all of them will expand and explode the barrel.

This is an atomic bomb: unstable units packed together tightly and then disturbed. Each unit releases energy as it breaks apart, and this energy disturbs its neighbours, releasing even more energy. If each exploding unit causes more than one of it neighbours to burst apart, then the numbers of bursting units will rise rapidly. This is the famous chain reaction. Atoms are used instead of steel springs because they are very small. Enormous numbers of them can therefore be packed into a bomb small enough to be carried by an aeroplane or truck. Not all kinds of atoms are appropriate “fuel” for an atom bomb.

The core or nucleus of each atom contains particles called protons.

These particles all powerfully repel each other and are always struggling to escape and run off in all directions, but they are held in place and bound together by strong forces, which usually keep the atom quite stable. However, in very large atoms, there are so many protons that their repulsion from each other is almost as powerful as the attractive forces that knit them together. These large atoms are unstable. A nudge or shock may break them apart and liberate the protons.
If many large unstable atoms make up a hunk of matter and a single one disintegrates, the liberated particles may knock a neighbouring atom and cause its disintegration. If each atom destabilizes and destroys more than one of its neighbours, the chain reaction would soon “avalanche” and cause much of the hunk of matter to explode.

Unluckily for us, atoms just right for making atomic bombs are lying around in nature, and can be mined in the deserts of South Africa or the western United States. The most well-known is uranium, a yellowy heavy metal. Its nucleus can contain 238 large particles, which makes it very large and very fragile. Even sitting in the desert, some of its atoms decay spontaneously and send particles shooting outwards. But they usually escape without hitting and destroying a neighbouring atom (the ordinary matter around us and in our bodies is 99.99 per cent empty space). In a bomb, largish hunks of uranium are put together so that escaping particles have a high probability of hitting a neighbouring uranium atom and starting a chain reaction.

There is one important trick needed to make an atomic bomb.

Suppose a chain reaction starts. One atom breaks apart and particles shoot outwards and break two neighbouring atoms apart. The particles expelled from these two atoms break four more apart, and the chain reaction proceeds, affecting 1, 2, 4, 8, 16, 32, 64 atoms, and so on. This series of mini-explosions will release heat and energy, and this will cause the metal to melt and ooze down into a puddle. As it does so, the atoms will separate from each other: heat causes expansion. In turn, this will make it less likely that the escaping particles will bump into a neighbouring atom. Thus the chain reaction will fizzle, affecting, say, 64, 32, 27, 16, 5, and eventually 0 atoms.

The uranium will melt and become white hot but nothing more.

This is where the physicists call in the engineers. To sustain the chain reaction, the uranium atoms must be held together tightly for just long enough for the chain reaction to proceed. This is a delicate feat of engineering. In one design, unstable atoms are formed into a ball and surrounded by dynamite. Just as the chain reaction is sparked off, the surrounding dynamite is exploded. The inward compressing force of the ordinary explosion holds the uranium together for just long enough (only a small fraction of a second) for the chain reaction to race through the entire core. Even though the metal becomes incredibly hot, the atoms remain close enough to sustain the chain reaction. Suddenly, so much energy is released that the compressing force is brushed aside and a huge explosion is unleashed.

The recipe for an atom bomb is thus simple physics and delicate engineering. First, obtain and purify a few pounds of uranium or plutonium. Keep the material in small samples so that no chain reaction begins spontaneously. Place them gently together in a bomb.

At the desired instant, compress the samples together and trigger a chain reaction, say by sharply striking the metal. Hold the compressed sample together for long enough for the chain reaction to consume large numbers of unstable atoms.
In a way, we cannot comprehend the horror of these bombs. The city of Hiroshima has erected a museum and left a few shattered buildings untouched since 1945. This is a moving reminder that science threatens us, and continues to threaten us with annihilation.

It is not clear how we will cope in the long run. Our hopes depend in part on the same intelligence that has endangered us, and on a vigilant understanding of these weapons and the physics behind them. Against the horrors of the Second World War, we can now weigh one shining, collective achievement. For more than half a century, no one has dropped an atomic bomb on another human being. Every year that ticks by adds to this fragile miracle.