New Harness for Atoms

Ever since the dawn of atomic power, scientists have dreamed of converting nuclear energy directly into electricity. The potential is clear from a simple statistic: a single pound of uranium 235 has the same fuel energy as 1,500 Ibs. of coal. But present atomic power plants must go through costly intermediate steps: nuclear fission produces heat, the heat is used to generate steam, the steam drives a turbine, the turbine generates the electricity.

Last week the AEC's Los Alamos Scientific Laboratory announced a major step toward direct conversion: an experimental "plasma thermocouple" no bigger than a can of frozen orange juice. Placed inside the core of a research reactor, the device produced 40 watts of electricity --enough to light a household light bulb.

A plasma thermocouple differs little in principle from the well-known bimetallic thermocouple that has been in use for a century to measure temperatures. In a bimetallic thermocouple, two pieces of unlike metals are joined. When the joint is heated, a feeble electric current is generated, flows through a wire connecting the cold ends. Chief obstacle to commercial use is the difficulty in finding metals that can operate efficiently at the high temperature required for large-scale power production (TIME, April 13).

In Los Alamos' plasma thermocouple, the solid metals of a bimetallic thermocouple are replaced by a tiny (finch long) rod of uranium suspended inside a vacuum-sealed can that contains liquid cesium. The uranium is enriched with U-235. Around the cesium is a circulating coolant (see diagram). When the device is lowered inside a reactor, the uranium is bombarded by the neutrons generated by the reactor, causing the U-235 to fission and give off intense heat.

Under vacuum, cesium turns partly to gas. As the uranium heats up. it ionizes the cesium gas to a plasma of charged particles (electrons wrenched from their atoms). As in conventional thermocouples, there is a flow of electric current between hot and cold: from the hot (2,000DEG C.) junction of the uranium and ionized cesium to the cold (300DEG C.) junction of the cesium and the oil coolant.

After twelve hours, the device had to be shut down because the uranium fission produces gas as a byproduct that dilutes the plasma and dangerously raises the pressure inside the can. In future plasma thermocouples, this can be solved by bleeding off the gas. But the cesium plasma proved to have a thermoelectric efficiency much higher than any combination of solid metals.

Significance of last week's experiment is not just that it generated a little electricity--but that a series of such plasma cells placed in a large nuclear reactor could produce power in major quantities.

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