The Evening Star (September 4, 1945)
Slow neutrons, useless in bomb, hold key to commercial studies
By Howard W. Blakeslee, Associated Press science editor
NEW YORK (AP) – The British official report on the atomic bomb in details received today reveals that there are two kinds of atomic explosions, one the city-destroying type, and the other only about as powerful as ordinary explosives.
The low-power explosion has important implications for world safety, in the promise that in producing atomic power for peace the engineers will deal with a force that is not much greater, in destructive capacity, than the accidents in steam and gasoline power plants.
The two kinds of atomic explosions depend respectively, on fast neutrons for an atomic bomb and slow neutrons for atomic power. The American official report by Dr. H. D. Smyth made it clear that the bomb depended on fast neutrons. The British report tells more exactly why this is so.
More effective
Past neutrons are uncharged particles that travel at thousands of miles a second and with energies of millions of volts each. Slow neutrons drop to energies of only a few volts or even tractions of volts, and also to a few miles per second in speed.
But these slow neutrons are about 10 times more effective than the fast kind in producing the splits of cores of Uranium or Plutonium atoms.
Their very slowness prevents their use, in an atomic bomb. They would start the explosion without difficulty but before they could spread to detonate the entire atomic charge the expansion would separate portions of the atomic explosive. This separation would end the reaction. There would be an ordinary explosion.
A peculiarity of the atomic explosives, Uranium 235 and Plutonium is the fact that they are safe, until the instant when more than two pounds are brought together into a single closely-packed mass. The slow neutron explosion would separate this mass too soon.
Slow process in manufacture
The fast neutrons, however, work in millionths of a second and spread so fast that they ignite all, or nearly all of the atomic explosive.
The huge plant in the State of Washington that converts common non-explosive Uranium 238 into Plutonium, uses slow neutrons to produce its transmutation reactions. It is possible to control the slow neutrons reactions, so that they are safe from explosions.
Fast neutrons won’t even start the reactions of the Washington plant, even when a ton of pure common Uranium is concentrated. The fast neutron explosive effect becomes possible only when the rare Uranium 235 is extracted from common Uranium and concentrated into masses.