The Aqueous Homogeneous Reactor

At the dawning of the nuclear era there were a number of different ideas about how to build nuclear reactors. The earliest attempt was maid at the Cavendish Laboratory in England. Two French Scientists, Hans Von Halban and Lew Kowarski, both of whom were Jewish and recent refugees from Nazi occupied France designed and built the first nuclear reactor experiment. Von Halban and Kowarski had brought with them from Paris two things. The first was the knowledge they had acquired working with Frédéric Joliot-Curie, Madam Marie Curie's son-in-law, who was a Nobel Prize winning physicist, and the leading French nuclear researcher during the 1930's. The second thing Von Halban and Kowarski had brought from Paris to the United Kingdom was a rare supply of a substance known as heavy water, 40 gallons of it loaned by Norway to France, and entrusted by Joliot-Curie to the British for safe keeping from the Nazis.

In the fall of 1940 Von Halban and Kowarski set out to rework a earlier failed experiment. in 1939, they had attempted to initiate a chain reaction in a sphere of water which contained U3O8 mixed with ordinary water, that is a form of uranium mud called a slurry. The experiment had failed, but Von Halban and Kowarski knew that if the water were replaced with heavy water, they could produce neutrons from U-235 fission, if not an outright chain reaction. in order to produce a full chain reaction, Von Halban and Kowarski needed 25 times more uranium than they used. Later it emerged that the Von Halban and Kowarski heavy water and Uranium slurry experiment had produced more neutrons that expected by theory. The question then arose whether theory of the Von Halban and Kowarski observations were correct.

That question was explored by Enrico Fermi, who along with Harold Urey was interested in the discrepancy between the the Neutron multiplication data obtained by Von Halban andKowarski in their Cavendish Laboratory experiment, and the results predicted by theory. James Lane noted

If the results of Halban and Kowarski were correct, then a homogeneous system containing a few tons of heavy water would be chain reacting . On the other hand, if the theoretical estimates were correct, the order of 100 tons of D2O would be required. Urey and Fermi recommended [5] that the earlier U3O8-D2O experiments be repeated with the improved techniques then known, and that consideration be given to incorporating a mixture of uranium and heavy water into the pile at Chicago to determine its effect on the pile reactivity .

Problems with the uranium slurry lead Manhattan Project scientists to consider uranyl nitrate dissolved in heavy water. By 1943 active research on homogeneous reactors was underway at Columbia university, "the Metallurgical Laboratory" at the University of Chicago, and in LosAlamos. In addition to Fermi and Urey, Eugene Wigner was interested. The availability of enriched uranium from Oak Ridge, made possible the design and construction of a small, low powered homogeneous reactor at Los Alamos. The reactor used UO25O4 dissolved in heavy water as fuel. The "LOPO" reactor was built and proved successful. By the end of the year, a more powerful "HYPO" reactor followed, and began to offer Los Alamos scientist a new research tool. Enrico Fermi was involved in the World War II development of homogeneous reactors at Los Alamos, but he saw them as research tools rather than practical energy sources.

Other Manhattan Project researchers saw more promise in the aqueous homogeneous reactor (AHR). The ORNL 50 year Laboratory history observed,

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, however, presented daunting design and materials challenges.

What is often forgotten is that the great physicist Eugene Wigner, was by training a chemical engineer. the ORNL 50 year history continued,

A homogeneous (liquid-fuel) reactor had two major advantages over heterogeneous (solid-fuel and liquid-coolant) reactors. Its fuel solution would circulate continuously between the reactor core and a processing plant that would remove unwanted fissionable products. Thus, unlike a solid-fuel reactor, a homogeneous reactor would not have to be taken off-line periodically to discard spent fuel.
Equally important, a homogeneous reactor's fuel and the solution in which it was dissolved served as the source of power generation. For this reason, a homogeneous reactor held the promise of simplifying nuclear reactor designs.

Eugene Wigner believed that the aqueous homogeneous reactor had practical potential. He proposed that it be used as an energy source for nuclear powered aircraft. Other proposals were more promising and the idea was quickly dropped, but Wigner's protegee, Alvin Weinberg as research director and later Laboratory Director at ORNL, was interested in the homogeneous reactor as a thorium breeder and an electrical generation source. Thus ORNLbegan a serious development program for a thorium breeding homogeneous reactor program that ORNL pursued during the 1950s.

Ed Bettis, an Oak Ridge engineer was employed at the Oak Ridge uranium separation plant, K-25. K-25 had its own nuclear powered aircraft program , and Bettis, assigned to it, was dissatisfied with the reactor design he had been assigned to work on. instead, Bettis and his associates developed a daring and radical approach to reactor design, a homogeneous reactor that used fluoride salts as a coolant. Eventually, ORNL began its own aircraft reactor project, using some of the people and ideas left over from the K-25 project. Ed Bettis was one of the people, and his Molten Salt Reactor was one of the ideas. The Molten Salt Reactor concept appealed to ORNL's scientific leadership, and it want on to be developed as a potential air craft reactor during the early to mid 1950's.

Thus Oak Ridge National Laboratory had during the 1950's two separate fluid fueled reactor development programs. Eventually at the end of the decade one, the aqueous homogeneous reactor was to fall by the wayside. While the Homogeneous reactor was promising. its development was faced by more obstacles, and its promises were not nearly as impressive as those of the Molten Salt Reactor. In 1959 the US AEC concluded that research on fluid fuel reactors and proceeded far enough that it was possible to evaluate the possible outcomes of further research focusing on the technologies. The AEC commissioned a task force of 15 engineers and scientists, to evaluate three fluid fueled reactor concepts, including the two concepts from Oak Ridge. The third concept was a liquid metal fuel concept which was somewhat handicapped by the fact that no prototype Liquid Metal fuel reactor had been built, and thus assertions about it were less certain.

It soon became apparent to the task force, that the aqueous homogeneous reactor had several disadvantages, especially when compared to the Molten Salt Reactor.

The aqueous homogeneous system requires more equipment, due to the necessity of having three reactor systems to achieve the same power production as one reactor in the other two systems, It also has the necessity of doing more frequent reprocessing of the fuel, with associated on-plant reprocessing facilities. . . .

In conclusion, based on today's knowledge and the experience of EIBE-2, a properly designed aqueous homogeneous system can be maintained by the use of wet maintenance, although at high cost and considerable down time,

The maintenance picture for the Molten Salt Reactor was less certain. The AHR also had some significant safety hazards,

The AHR is potentially the most hazardous of the three fluid fuel concepts. The primary reason for this is that it is a high pressure system. Additional contributing factors are the radiolytic gas explosion hazard, and the fact that its fuel stability is probably the most precarious of the three concepts.

Research on the AHR had demonstrated some problems with operational stability, a tendency of core power output to oscillate. The task force observed,

Of the three concepts, the AHR is probably the most susceptible to nuclear instability. This is a consequence of its large negative temperature coefficient which, due to circulation effects, has a delayed component which may be a contributing factor to instability, Also, hydrodynamic fluctuations are most likely in the AHRbecause the flow pattern encompasses the entire core.

Thus it became clear to the fluid fuel reactor task force, that although more experienced had been gained from AHR prototype development than for the other systems, that the AHR would be the most challenging to develop.

Disadvantages of these reactors are also related to the characteristics of water and the fuel compounds. Fuels which are now in use have major faults. The uranylsulphate solution fuel is corrosive and its stability decreases with increasing temperature. The thoria fuel is erosive, and settles unless it is agitated continuously; no methods are presently envisioned which show promise of separating bred uranium from the thorium without dissolving the thoria particles. Corrosion of materials in the reactor core is increased by radiation. Special precautionary measures must be taken in the design and operation of the reactors to prevent the moderator decomposition products, D2 and O2, from introducing a serious explosion hazard. Aqueous systems operate at relatively low temperature so the thermodynamic efficiency of the power cycle is low. The vapor pressure of the water is high at the operating temperature of the reactor so the radio- activity is more difficult to contain. Care must be taken to prevent the expensive heavy water from being contaminated by light water.

The task force concluded that although less was known about the Molten Salt Reactor, it appeared more promising. than the other two concepts, and thus shortly after the AEC issued the task force report, ORNL shut down is AHR research, less than 20 years after Von Halban and Kowarski Cavendish Laboratory experiment. In those 20 years a new world had commenced to emerge, a world which many people were to feel uncomfortable with, but one which unlike the AHR could not be made to disappear.

While the ORNL AHR research project had fallen short of expectation, it had conceptually laid the groundwork for a more advanced and more promising fluid fuel reactor system, the Molten Salt Reactor.