Aqueous Homogeneous Reactor - History

History

Initial studies of homogeneous reactors took place toward the close of World War II. It pained chemists to see precisely fabricated solid-fuel elements of heterogeneous reactors eventually dissolved in acids to remove fission products—the "ashes" of a nuclear reaction. Chemical engineers hoped to design liquid-fuel reactors that would dispense with the costly destruction and processing of solid fuel elements. The formation of gas bubbles in liquid fuels and the corrosive attack on materials (in uranyl sulfate base solutions), however, presented daunting design and materials challenges.

Enrico Fermi advocated construction at Los Alamos of what was to become the world’s third reactor, the first homogeneous liquid-fuel reactor, and the first reactor to be fueled by uranium enriched in uranium-235. Eventually three versions were built, all based on the same concept. For security purposes these reactors were given the code name "water boilers". The name was appropriate because in the higher power versions the fuel solution appeared to boil as hydrogen and oxygen bubbles were formed through decomposition of the water solvent by the energetic fission products.

The reactor was called LOPO (for low power) because its power output was virtually zero. LOPO served the purposes for which it had been intended: determination of the critical mass of a simple fuel configuration and testing of a new reactor concept. LOPO achieved criticality, in May 1944 after one final addition of enriched uranium. Enrico Fermi himself was at the controls. LOPO was dismantled to make way for a second Water Boiler that could be operated at power levels up to 5.5 kilowatts. Named HYPO (for high power), this version used solution of uranyl nitrate as fuel whereas the earlier device had used enriched uranyl sulfate. This reactor became operative in December 1944. Many of the key neutron measurements needed in the design of the early atomic bombs were made with HYPO. By 1950 higher neutron fluxes were desirable, consequently, extensive modifications were made to HYPO to permit operation at power levels up to 35 kilowatts this reactor was, of course, named SUPO. SUPO was operated almost daily until its deactivation in 1974.

In 1952, two sets of critical experiments with heavy water solutions of enriched uranium as uranyl fluoride were carried out at Los Alamos to support an idea of Edward Teller about weapon design. By the time the experiments were completed, Teller had lost interest, however the results were then applied to improve the earlier reactors. In one set of experiments the solution was in 25-and-30-inch-diameter (630 and 760 mm) tanks without a surrounding reflector. Solution heights were adjusted to criticality with D₂O solutions at D/235U atomic ratios of 1:230 and 1:419 in the smaller tank and 1:856 to 1:2081 in the larger tank. In the other set of experiments solution spheres were centered in a 35-inch-diameter (890 mm) spherical container into which D₂O was pumped from a reservoir at the base. Criticality was attained in six solution spheres from 13.5- to 18.5-inch diameter at D/235U atomic ratios from 1:34 to 1:431. On completion of the experiment that equipment too was retired.