Bold Reactor

When Russia's top nuclear engineers visited Oak Ridge National Laboratory last fall, the thing that impressed them most was a cylindrical, tanklike object 55 ft. long. They sat in rows of chairs while short, slender Dr. Alvin M. Weinberg, the 44-year-old physicist who is the lab's director, told them what was inside the tank: an experimental reactor in which liquid fuel replaces the troublesome solid-fuel elements of conventional power reactors. "A very bold idea," conceded Professor Vasily S. Emelyanov, chief of the Russian group. Last week Dr. Weinberg cautiously told his laboratory mates that the reactor has now run long enough to be considered "a tenable engineering device."

Largely a brainchild of Dr. Weinberg, the reactor HRE-2 (for Homogeneous Reactor Experiment No. 2) is an attempt to avoid some of the worst disadvantages of solid-fuel reactors. Since solid uranium is quickly corroded at high temperature, it must be enclosed in a more resistant metal such as zirconium or stainless steel. As the uranium fissions, it generates gases that tend to burst the container. Other fission products absorb neutrons, and when too much of this "poison" has accumulated, it makes the nuclear reaction slow down or stop. At intervals, the fuel elements must be removed and their unburned uranium re-purified by a difficult and expensive chemical process.

HRE-2's fuel is uranyl sulphate dissolved in heavy water (which does not absorb as many neutrons as ordinary water). When this solution is flowing in a small-bore pipe, it does not react, because the fissionable uranium atoms are too strung out to form a critical mass. But when the fuel solution flows into a spherical reaction chamber, the compact mass becomes critical. A nuclear chain reaction starts, and heats the solution. Before the reaction goes too far, the solution is sucked away by pumps and forced through a heat exchanger, where it heats ordinary water to produce high-pressure steam (see diagram), which in turn can be harnessed to an electric generator. Since the fuel is liquid, it can be renewed by periodically passing it through a special purifier.

Worst problem was how to deal with the hot, corrosive, high-pressure fuel, which is fiercely radioactive as it comes from the spherical cell and cannot be handled or even observed except by special, remotely controlled devices. By ingenuity and careful engineering, Dr. Weinberg's staff managed to tame this lethal brew. His report proudly announced that "the reactor cell has been sealed with the circulating pumps running uninterruptedly for 1,600 hours (67 days), a feat which begins to approach the longest uninterrupted runs of any power reactor."

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