To Prevent Excursions

Early nuclear reactors were easy to slap down. If one of them made what physicists euphemistically call an "excursion" -i.e., started to react too fast -it could be slowed down by pushing into it a simple rod of neutron-absorbing material. Control rods are still used, but the operators of big modern reactors dare not depend on them alone. Under some conditions, the fierce nuclear fire in the reactor's core can make a disastrous excursion in a fraction of a second.

In Nucleonics, Dr. Norman Earl Huston and Norman Carl Miller of North American Aviation, Inc. tell about quick-acting safety devices to prevent such calamities. They do not think much of gadgets that require a power source or an electrical "scram" signal to tell them the reactor is about to misbehave. Either power or signal might fail.

A really dependable safety device, say Huston and Miller, should tell on its own when the reactor is starting an excursion. The best way to trigger its action is to combine a pad of material containing uranium with a layer of high-melting solder. When the neutrons in the reactor rise above a critical level, showing that an excursion has started, the uranium fissions at a rate that creates enough heat to melt the solder. Then high-pressure gas will shoot neutron-absorbing poison into the reactor. Even if other controls have failed, this last-ditch nuclear fire extinguisher will keep the reactor from exploding or melting itself into radioactive ?lag.

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