An interesting experiment in an alternative approach to nuclear reactors..
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The Aqueous Homogeneous Reactor, almost always referred to in Oak Ridge as the Homogeneous Reactor, was originally designed in Los Alamos during World War II. The advantages of the Homogeneous Reactor were numerous. Reactors were designed by physicists. In a way reactors are the descendants of a physics experiment conducted in Chicago in 1942. That experiment was so successful, that it had a profound influence on the future of reactor design. But chemist had a different take on reactor design. As the History of ORNL states, "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."
Alviin Weinberg for all his theoretical brilliance was not a physicist. Indeed his PhD was in biology. He was also open to what Scientist had to say, as long as they knew what they were talking about. The point the chemists made was simple, as long as you are stuck with the Stagg Field reactor model, you are separating the physics of the reactor from the chemistry of nuclear fuels. The Homogeneous Reactor was a way to get around that. Its design treated the operation of the reactor as a continuous chemical process instead of a physics experiment. The ORNL history points out the advantages of the chemists way of thinking:
"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."
It should be noted that the same advantages also apply to the Molten Salt Reactor. A homogeneous reactor was far simpler than than a light water reactor. The picture above illustrates the simplicity. There was no pile. The core of the reactor is a simple spherical metal pot. One pipe leads in the other out. The trick was simple. Less than 1 pound (454 grams) of U235 was dissolved in a water solution and pumped into the core. The shape of the core brought the fissionable material into criticality, and a chain reaction commenced, as Xenon built up in the reactor the original fluid was pumped out to be replaced by more fuel dissolved in water. As water left the reactor heat could be extracted via a secondary coolant loop, and a turbo generator driven, A further advantage was that fission byproducts like Xenon could be extracted from the reactor fluid.
There was a decided problem, with the homogeneous reactor. It boiled, not with hot water, but with hydrogen and oxygen gases. Radiation from the the chain reaction broke water molecules apart. This lead to the biggest problem with the design, with hydrogen and oxygen in a water filled core, combined with heat and radiation there was bound to be a big problem with metal corrosion, and in fact there was. It might be possible to solve this problem today with heat and radiation resistant carbon composites.
The biggest problem for the homogeneous reactor was the AEC's light water reactor bandwagon. The light water reactor was an advanced version of the Stagg Field Pile. It was something that people in Washington understood. Forgetting that Wienberg held the patent on the light water reactor, the people in Washington seemed to have thought of Weinberg as a wild eyed dreamer.
Weinberg was by training a biologist. He saw reactors in terms of evolution. Weinberg realized what few in his generation did, that there were simpler and potentially far better reactor designs. Indeed Weinberg understood the problems of reactors more deeply than almost anyone else during his time or since. Weinberg saw that rather reactors must be treated like processes rather than entities. That is why he called in chemist like my father. My father had been an industrial chemist before he came to Oak Ridge. My father deeply understood chemical processes. Indeed his own PhD research had been directed to turning chemical analysis into a continuous process. Had the homogeneous reactor been the only fish Weinberg had to fry, he might have pushed for it further.
Oak Ridge was not the only place interested in the homogeneous reactor. At the same time Oak Ridge was conducting its first homogeneous reactor experiment, Edward Teller had pushed Los Alamos into conducting an experiment with a Molten Salt Reactor as a Thorium Breeder. By using heavy water, Los Alamos was able to up the Neutron Economy of the reactor to a truly formidable level.
Except for its corrosion problem, the homogeneous reactor was extremely safe. Even if the core was breached, and hot radioactive water came spilling out, there would not have been a mess. There would have been no molten mass to contend with, no China Syndrome. The radioactive Xenon escape would have been limited because Xenon was being continuously drawn off the reactor fluid as part of the operating process. There would of course have been a mess to clean up, but a mess is a mess, not a disaster. The problem would have been contained in the reactor building.
As a breeder the Homogeneous reactor held big promise. Weinberg was able to get the AEC to approve a second Homogeneous Reactor experiment, this one a more complex model with a core and a blanket. The ORNL history observes:
"The aim was not only to produce economical electric power but also to irradiate a thorium slurry blanket surrounding the reactor, thereby producing fissionable uranium-233. If this pilot plant proved successful, the Laboratory hoped to accomplish two major goals: to build a full-scale homogeneous reactor as a thorium "breeder" and to supply cheap electric power to the K-25 plant to enrich uranium."
There appears little doubt that the homogeneous reactor represented a major breakthrough. But a breakthrough the politicians and AEC paper shufflers had little appreciation for. The ORNL history continues:
Initial success stimulated international and private industrial interest in homogeneous reactors, and in 1955 Westinghouse Corporation asked the Laboratory to study the feasibility of building a full-scale homogeneous power breeder. British and Dutch scientists studied similar reactors, and the Los Alamos Scientific Laboratory built a high-temperature homogeneous reactor using uranyl phosphate fluid fuel. If the Laboratory's pilot plant operated successfully, staff at Oak Ridge thought that homogeneous reactors could become the most sought-after prototype in the intense worldwide competition to develop an efficient commercial reactor. Proponents of solid-fuel reactors, the option of choice for many in the AEC, would find themselves in the unenviable position of playing catch-up. But this was not to be.
The ORNL history relates the rest of the story:
After successful operation of the first aqueous homogeneous reactor
in 1954, the Laboratory proceeded with design of a larger
homogeneous reactor on a pilot-plant scale. Whereas the first
reactor had been a one-time experiment to prove yet unproven
theoretical principles, the second reactor, sometimes identified as
the Homogeneous Reactor Test, was designed to operate routinely for
lengthy periods.
The second homogeneous reactor was fueled by a uranyl sulfate
solution containing 10 grams of enriched uranium per kilogram of
heavy water, which circulated through its core at the rate of 400
gallons (1450 liters) per minute. Its fuel loop included the
central core, a pressurizer, separator, steam generator,
circulating pump, and inter-connected piping. Its core vessel was
approximately a meter in diameter and centered inside a 60-inch
(152-centimeter) spherical pressure vessel made of stainless steel.
A reflector blanket of heavy water filled the space between the two
vessels.
Perhaps the most exotic nuclear reactor ever built, it gave
Laboratory staffers trouble from the start. During its shakedown
run with pressurized water, chloride ions contaminated the
leak-detector lines, forcing replacement of that system and
delaying the power test six months.
In January 1958, the Laboratory brought this reactor to critical
mass and operated it for many hours into February 1958, when it
became apparent that its outside stainless steel tank was corroding
too rapidly. In April the reactor reached its design power of 5 MW.
Then, in September, a hole suddenly formed in the interior zircaloy
tank. Viewing the hole through a jury-rigged periscope and mirrors,
operators determined that it had been melted into the tank--that
is, the uranium had settled out of the fuel solution and lodged on
the tank's side.
By the end of 1958, the AEC considered abandoning the Homogeneous
Reactor Test, and Eugene Wigner came to the Laboratory to inspect
it personally. "The trouble seems to be that the rich phase adsorbs
to the walls and forms a solid layer there," Wigner reported to the
AEC staff, relaying the findings of the Laboratory staff. He
thought altering the flow of fluid through the core would provide
the velocity needed to prevent the uranium from settling on the
tank walls. "It is my opinion that abandoning the program would be
a monumental mistake," he warned, pointing out that the reactor
could convert thorium into uranium-233 to supplement a dwindling
supply of uranium-235.
The AEC allowed the Laboratory to alter the reactor flow and
continue its testing in 1959. These activities were accomplished by
interchanging the inlet and outlet to reverse the fluid flow
through the reactor. Several lengthy test runs followed in 1959,
and the reactor operated continuously for 105 days--at the time, a
record for uninterrupted operation of reactors. The lengthy test
run demonstrated the advantages of a homogeneous system in which
new fuel could be added and fission products removed during reactor
operation.
Near the end of the year, a second hole burned in the core tank.
Laboratory staff again patched the hole using some difficult remote
repairs and started another test run. Because of these
difficulties, Pennsylvania Power and Light Company and Westinghouse
Corporation abandoned their proposal to build a homogeneous reactor
as a central power station.
During the shutdown and repairs, Congress viewed the aqueous
homogeneous reactor troubles unfavorably, and in December 1960, the
AEC directed the Laboratory to end testing and turn its attention
to developing a molten-salt reactor and thorium breeder. The last
aqueous homogeneous reactor test run continued until early 1961.
For months, the reactor operated at full power until a plug
installed earlier to patch one of the uranium holes disintegrated.
Although the homogeneous reactor never found direct commercial
applications, the Laboratory's efforts to test its long-term
usefulness ultimately strengthened its capabilities for maintaining
and repairing highly radioactive systems.
The story then was of a successful experiment that demonstrated teething problems with a new technology. There were metallurgical problems, but the experiment was a success.
Two questions remain. Should the experiment have been continued, and should the Homogeneous Reactor be revived. My answers are yes and yes. The ORNL and Los Alamos experiments showed the Homogeneous reactor to have outstanding promise. The only reactor design that was more promising was the Molten Salt Reactor, but the Molten Salt Reactor did not hold the breeding potential that the Homogeneous Reactor did.
The Liquid Sodium Fast Breeder is still the "official" candidate for breeder of the year. Yet no one has yet mastered sodium technology, and the last Liquid Sodium Breeding experiment, the French Superphoenix, has to be seen as a failure. Alvin Weinberg continued to view the Homogeneous reactor as having a place as a breeder, even after design work on the Molten Salt Reactor had been launched. The Homogeneous Reactor thus is a legitimate candidate for "Breeder of the Year," or the generation or even the century. It can breed thorium with high neutron efficiency. The resulting U233, can fuel Uranium burning Molten Salt Reactors that will in tern breed enough U233 from Thorium to sustain themselves. The Homogeneous Reactor is safe, far safer than the safest water cooled pile, it is simple, it solves many of the problems that critics of nuclear power raise, it can breed like a rabbit, it require modest amounts of materials, can be operated without refueling shutdowns, produces hydrogen as a by-product, produces, can produce electricity as efficiently as a light water reactor, and outside the corrosion problem has few vices. It also would be far cheeper to build. The Homogeneous Reactor is very flexible in output. It is said that the output can increase from 100 watts to 1,000,000,000 watts with no problems. For that reason it could be used to balance, and back up intermittent renewable energy technologies. Given a modest research input, the Homogeneous Reactor could be making a significant contribution to fighting global warming within 20 years.
Posted by Charles Barton at 5:44 AM
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