Topsy and Godiva

The payoff in nuclear energy comes when a mass of fissionable material "goes critical," i.e., when it begins to support a nuclear chain reaction. In the early days of the Los Alamos atom-bomb laboratory, critical points were determined by hand, by physicists who felt a little jumpy. The start of a chain reaction cannot be predicted dependably. Even a human hand moving near a mass that is barely subcritical can reflect enough neutrons into it to start the reaction and loose a cold and silent flood of death-dealing radiation.

The technique is less hair-raising now. This week Los Alamos told of two machines that assemble critical masses by remote control. Called Topsy and Godiva, the two are housed at a safe distance from the laboratory workers.

Topsy (she "just growed") is the simpler of the two. Her essential part is a vertical hydraulic ram that carries a subcritical mass of fissionable material surrounded by a casing of neutron-reflecting material. When the ram rises, it slowly pushes its dangerous load against the base of another subcritical mass. When the two are joined, they are still barely subcritical. Then the machine pushes rods of natural uranium into holes in the neutron reflector. The reaction starts and gathers momentum until the distant operators, or the automatic safety devices, make Topsy "scram", i.e., pull the masses apart.

"The name of Godiva" says Los Alamos, deadpan, "has a logical basis." She is a "bare reactor" (no neutron reflectors), and she can assemble three or more pieces of fissionable material into a critical mass. Like Topsy, she will scram if a reaction gets out of hand.

Watching both Topsy and Godiva are electronic Peeping Toms: TV cameras connected to screens in the control building a quarter of a mile away. If anything goes wrong, only metal and glass, rather than flesh, will take the blast of radiation.

This file is automatically generated by a robot program, so reader's discretion is required.