Atom-Poisoned Metals

The structural materials in an atomic bomb need not be long-lasting; they haven't much future, anyway. But one of the biggest problems in designing an atomic power plant is finding materials that will maintain their strength indefinitely against atomic bombardment. In the current Journal of Applied Physics, Dr. J. C. Slater of Massachusetts Institute of Technology describes the damage that fast-moving atomic particles do to solid substances, especially metals and alloys.

Nearly all solids, he explains, are made of atoms bound together in regular, crystalline "lattices"--something like pool balls set up in a neat triangle at the beginning of a game. Into this orderly pattern, the radiation particles smash like cue balls, knocking the peaceful atoms every which way.

Neutrons penetrate deepest, slipping between the atoms of the material, hitting one here, one there. Each atom they hit is knocked out of its position in the crystalline lattice. These collisions slow the neutron down. When it is moving slowly enough, it usually joins the nucleus of an atom, changing it into a different, and often radioactive, element. So a material that has been bombarded by neutrons becomes eventually a mass of displaced atoms mingled intimately with atomic changelings.

Fast-moving protons and "fission fragments" (from uranium fission) penetrate only a few thousandths of an inch. They expend their energy suddenly, raising a narrow "track" of the bombarded material to a temperature far above its melting point, or even its boiling point. Some of the hot atoms evaporate, leaving the sur face permanently. The rest of the heated stuff solidifies so suddenly that its atoms cannot rearrange themselves in a crystalline pattern. Soon the material's skin is riddled with these radiation wormholes. If the bombardment is heavy enough, considerable portions of the material may be kept molten all the time--and dangerously weak.

No one knows how much harm the atomic "poisoning" does, and it is not easy to find out. When a material has been left in a reactor long enough to be affected, it is usually so radioactive that it cannot be handled safely. At Arco, Idaho, the Atomic Energy Commission is building a special reactor for testing structural materials. It hopes to develop metals alloys and ceramics that can be trusted not to weaken too much under atomic bombardment. Until this is done, atomic power plants will remain as treacherous as primitive iron boilers in the early days of steam.

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