A wad in Keswick

 

The window of the Graphite Gallery in Keswick has a wonderful display. Wad was the local name for the graphite that was mined at the far end of Borrowdale. It was incredibly valuable at the time for military reasons. It has had many uses in Physics. One is as a moderator for nuclear fission reactions. For uranium-235, fission is more likely with slower neutrons but those released tend to be fast. A moderator slows them down. This article explains why graphite is a good moderator. It explains that if you throw a tennis ball at a wall, it bounces back at roughly the same speed. The large mass of the wall means that it stays still. It would be modelled as an elastic collision where both momentum and kinetic energy are conserved. But for two snooker balls, the one moving stops on collision and the stationary ball moves on. So carbon works as a moderator because it is not too much bigger than a neutron. The cited article explains brilliantly about carbon not being good at absorbing neutrons.