21. HOW A NUCLEAR POWER STATION WORKS

by Sir John Cockroft

(from a television talk arranged by Granada TV)

Nuclear power stations differ from ordinary power stations only in the source of heat. In place of coal or oil-fired boilers, they use a nuclear reactor to provide the heat. The reactor is contained in a giant pressure vessel, a spherical vessel about seventy feet in diameter. Inside the pressure-drum are thousands of graphite blocks; standing in channels between the blocks are bars of uranium metal an inch or so in diameter, sheathed in an alloy of magnesium'. Heat is developed in the uranium metal by the fission or splitting of some of the atomic nuclei and is carried away by carbon dioxide gas under pressure which is blown through the channels past the uranium bars. The hot carbon dioxide is carried through pipes to tall towers called heat exchangers which contain hundreds of tubes filled with water. The carbon dioxide is cooled and the water heated so that it turns to steam. The steam is carried through pipes to steam turbines which drive the electricity generators. The cool carbon dioxide from the heat exchanger is pumped back to the reactor to be heated once more.

Much of the complication of the nuclear power station reactor is, as you see, due to the problems of carrying the heat away and raising steam with it. The simplest sort of reactor is no more than a pile of graphite bricks, each about eight inches square, with channels cut in them in which the uranium metal bars sit. The bars are, in fact, sheathed in

aluminium' to prevent oxidisation of the uranium and to prevent escape of the radio-active waste products. A simple "pile" of uranium bars and graphite bricks, if you make it large enough, will "go critical" and start to produce power. This comes about because a chain reaction starts in the uranium bars. The uranium obtained from ores consists of mainly heavy uranium, uranium-238 mixed with about one part in 140 of a slightly lighter kind of uranium - uranium-235. The nuclei of the heavy uranium occasionally split up spontaneously into two fragments of roughly equal size - the process is called fission and the fragments fission products. These fragments include a wide variety of atomic nuclei - they are actually radio-active and in their early life they can throw out neutrons. So there are always a few neutrons coming out of the uranium bars. The neutrons, because they have no electric charge, are able to enter the positively charged atomic nuclei easily and they can induce fission in other uranium nuclei. They can produce fission in the heavy uranium only when they are moving fast. They produce fission in the light uranium more effectively when they are moving slowly.

The neutrons produced by spontaneous fission come out at speeds of about 10⁹ cm/second; a few of these fast neutrons will produce further fissions in uranium-238. The remainder will enter the graphite and there they are slowed down by collision with the carbon atoms. After about thirty collisions their speed is reduced by a factor of about 10,000 and they are now pretty effective in producing fission in uranium-235. So more fissions are produced and these produce more neutrons and these produce still more fissions.

(from Discovery, published for Granada TV by Methuen, London, 1961)