- Historic Sites
A Demonstration At Shippingport
Coming on Line
June/july 1981 | Volume 32, Issue 4
From the beginning it was clear—in this case the beginning was December 2, 1942, the day the first man-made nuclear reactor was nudged to criticality in a squash court beneath the west stands of the University of Chicago’s Stagg Field and incidentally the first day of wartime gasoline rationing—that the fissioning atom radiated heat energy and that such energy might, in the fullness of time, be applied to make electricity for power. Fifteen years would pass before nuclear electricity was generated in any quantity in the United States. That is rapid development or surprising delay, depending upon one’s perspective, but the fact is that despite its imposing technical lead in nuclear matters, the United States did not arrive first at the production of commercial nuclear power. Great Britain did. On a smaller scale, even the Soviet Union preceded us. The reasons are intriguing. How the United States contrived to back into the nuclear power business is instructive. “There are overtones in this development,” wrote the physicist and statesman J. Robert Oppenheimer in 1957, “that have been absent in power developments in other respects not wholly beyond comparison, such as the diesel engine and the gas turbine: overtones of pride and terror, of mystery and hope.” There are still such overtones today.
Enrico Fermi’s first “atomic pile,” literally a flattened spherical pile of graphite blocks plugged with cylinders of purified natural uranium, radiated heat equivalent to some two hundred watts of electricity, no more. The dark, dirty mechanism, its emergency quenching system three young men crouched on top the last layer of graphite, up under the ceiling, balancing buckets of cadmium solution, was designed to prove that a reactor would work, and it barely did—so barely that it required no cooling system and no shielding. It was simplicity itself. The graphite served as a sort of physical catalyst; the uranium did the work.
Uranium purified from ore consists of two isotopes—variant physical forms—in the proportions in which they are found today in nature: U 235, bomb material, an unstable substance continually undergoing radioactive decay, to the extent of seven parts per thousand; and U 238, stable “ordinary” uranium, the preponderant balance. U 235 atoms spontaneously eject neutrons from their nuclei as they decay; collisions with atoms of a suitable moderator can slow some of those neutrons sufficiently to allow other uranium atoms to capture them and, in so doing, to fission; in fissioning—splitting—some of the matter of the uranium is converted into energy in the awesome proportions of Einstein’s famous formula. The fissioning of one uranium atom, minuscule though it is, produces enough energy to make a grain of sand visibly jump.
It happens that the average number of neutrons emitted by a decaying atom of U 235 is slightly more than one. This happenstance suggested, to Fermi and his colleagues, that in a sufficiently “massive assembly of natural uranium and moderator, each decaying atom might fission at least one other atom, and some decaying atoms might fission two. The result then might be a “chain reaction” of fission events that would be self-sustaining and controllable. A control mechanism—rods of some neutron-absorbing material—might be raised out of the pile to allow the chain reaction to begin and to continue and be lowered into the pile to slow or stop it. In Chicago, in 1942, Fermi proved such speculations correct.
The goal of Fermi’s reactor work was not to produce energy. It was, immediately, to prove the chain reaction, and subsequently to devise a machine that could make bomb material. U 235 is excellent bomb material, but it is extremely difficult to separate from U 238 because the two isotopes are chemically identical. Theory indicated that a reactor could be used to transmute ordinary U 238 into an entirely new, man-made element that would also serve for bombs: plutonium. And plutonium, chemically different, might be efficiently separated chemically from its parent, uranium.
Theory proved, again, correct. Fermi’s modest Stagg Field pile was the immediate forerunner of the truly massive uranium-graphite reactors built at Hanf ord, Washington, in the midst of the Second World War, for the production of plutonium. The Hanford reactors radiated so much heat that they were cooled by diverting a considerable portion of the Columbia River through them, raising the river’s temperature by measurable degrees downstream. But they were no more designed for power production than was Fermi’s first pile. They wasted copious quantities of heat, but their temperature was inefficiently low. In power production, the greater the temperature differential, the greater the efficiency.
The peaceful atom got short shrift in wartime. Every effort of the secret Manhattan Project was bent to making bombs. Yet some took thought. In 1944, anticipating the end of the war, General Leslie R. Groves of the U.S. Corps of Engineers, the man who commanded the bomb building, convened a committee of experts chaired by Dean R. C. Tolman of the California Institute of Technology to assess the atom’s postwar prospects. The Tolman Committee thought atomic energy would lend itself to three areas of development: power, weapons, and scientific tools. It recommended to the War Department that the United States pursue atomic power “for the propulsion of naval vessels.” It was pessimistic about the prospects of commercial power. “The development of fission piles solely for the production of power for ordinary commercial use,” it found, “does not appear economically sound nor advisable from the point of view of preserving national resources.”