Reactor in Orbit

When space talk turns to far-out exploration, to manned voyages far beyond the moon or Mars, most plans call for a nuclear reactor capable of providing abundant power without paying too much of a penalty for weight, and an ion engine capable of turning that power into thrust for months or years without paying too high a price in fuel consumption. Last week the first of such combinations, featuring SNAP-10A,* the world's first spaceworthy reactor, went into operation as it orbited the earth.

Bomb Fuel. The very conception of a nuclear reactor that can work by itself in space required new and imaginative technology. And scientists at California's Atomics International, who built SNAP-10A for the Atomic Energy Commission, produced a machine like nothing now working on earth. Its fuel is 4.75 kilograms (10.5 lbs.) of uranium 235, the nuclear explosive used in the first atomic bomb. Packed into 37 tubes of heat-resistant nickel alloy, the fuel is mixed with zirconium hydride, which acts as a moderator, slowing down the high-energy neutrons released by fissioning atoms of U 235. The heat of the reactor is carried away by a sodium-potassium alloy (NaK) that turns to liquid at 48DEGF. A beryllium reflector 2 1/2 in. thick bounces escaping neutrons back into the uranium and keeps the reactor operating. When four openings in the reflector are uncovered, neutrons leak away, slowing or stopping the nuclear reaction.

This weird and dangerous gadget, weighing 250 lbs., was gingerly set on the nose of an Air Force Atlas-Agena rocket at Vandenberg Air Force Base, Calif. The reflector ports were open to keep the nuclear action from starting, and a conical windscreen covered the reactor to protect it from buffeting as it climbed swiftly through dense, low-altitude air.

When the rocket cleared the atmosphere, the windscreen was jettisoned; the reactor and its conical support section went into orbit 800 miles above the earth. As soon as SNAP's scientists were convinced that the proper orbit had been attained, they sent a signal that told the reflector mechanism to reduce neutron leakage. Slowly the nuclear reaction started; heat built up in the core, and a magnetic pump circulated the metallic coolant at 1020DEGF. through tubes in the skin of the support structure. The inner ends of 2880 pellets of a germanium-silicon material were heated while their outer ends were kept comparatively cool by heat radiation into space. The germanium-silicon combination is "thermoelectric," it changes heat to electricity, and the difference between the two temperatures caused a faint current to flow. That current added up to about 650 watts--hardly enough to run a household toaster--but it was the first fission energy to be generated in space. Hitched to a more efficient converter, said the AEC, the same reactor could generate "some tens of kilowatts."

Ion Push. Part of SNAP's power went into the operation of "housekeeping" parts, including instruments, coolant pump and radio apparatus. The rest of the electricity charged a storage battery. When the scientists were satisfied that the nuclear generator was working well, they sent a signal that shot current from the battery into a 2.2-lb. ion engine made by Electro-Optical Systems, Inc. of Pasadena. The current turned metallic cesium to a tiny trickle of vapor that passed through a plug of hot, porous tungsten and emerged as a stream of positively charged ions. A negatively charged copper plate attracted the ions, which shot through holes in the plate and squirted into space at 176,000 m.p.h. Their swift departure produced thrust, exactly like hot gases shooting out of a rocket. To be sure, that thrust was only .002 lb. (an aspirin tablet weighs about .001 lb.) but so little cesium was used that the 3.5 oz. in the engine would last 300 hrs.

The light push of the tiny ion engine was not expected to have a measurable effect on the trajectory of the hefty reactor assembly. But when bigger ion engines are built and hitched to bigger generators, their gentle, continuous thrust, acting for hundreds, or thousands of hours, may push a spacecraft up to speeds that will carry it clear of the solar system itself.

*From Systems for Nuclear Auxiliary Power.

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