Nuclear Fission

Somewhere inside a working reactor a single neutron is drifting between atoms at a few kilometres a second. It is not aimed and nobody launched it. In a moment it will wander close enough to one particular nucleus to be swallowed, and that nucleus will tear itself in half and fling out two or three more neutrons of its own. Each of those can do the same thing to a neighbour. Left to itself the count goes 1, 2, 4, 8, 16, and twenty steps later it is over a million. Held in check, it goes 1, 1, 1, 1 for years, and a country keeps its lights on.

The difference between those two sequences is a set of metal rods that can be lowered a few centimetres. This lesson takes you through what fission actually is, how the arithmetic of one generation feeding the next turns a single split into a power station or into a disaster, and where the two examination tiers part company on this topic. You will learn to draw the chain-reaction diagram the specification asks for, count neutrons in and out of a reactor core, judge from a set of figures whether a reaction is growing, steady or dying, and finish by weighing the one problem nobody has fully solved: what to do with the waste.