NEWS | May 31, 2011

NRL History - Phil Abelson The Atomic Age

By Daniel Parry

In the latter half of the 1930s worldwide attention was focusing on the unraveling peril occurring in the Eastern Hemisphere. As the threat of a second World War was looming eminent, a young physicist, Philip Hauge Abelson, had been finishing doctorate studies in nuclear physics at the University of California, Berkley, where he graduated in 1939.

With a freshly minted doctorate, Abelson moved forward to perform post-doc research on new electron accelerators at the Lawrence Berkeley Laboratory. Aiding physicist Edwin McMillan with experiments in nuclear fission, the two were eventually able to prove the existence of neptunium (Np, 93), an isotope having an atomic number heavier than uranium and an element that set the stage for the development of the world's first atomic bomb. It was around this period (c. 1940) Abelson was selected to work as assistant physicist of cyclotron development at the Department of Terrestrial Magnetism at Carnegie Institute of Washington (CIW).

A World at War

As Europe became fully immersed in World War II, Abelson was very much aware of the potential of the uranium isotope 235 for not only the development of immensely powerful weapons, a testament by Albert Einstein, but as a vast source of energy. However, uranium-235, or simply U-235, does not occur naturally and must be isolated from common uranium to sustain the process of nuclear fission — scientists knew that fission would become a commercial reality only if a way could be found to enrich natural uranium.

With a greater interest in the prospects of uranium as a source of energy, the U. S. Navy, in the summer of 1940, provided $100,000 to fund isotope separation research. Several universities, including University of Virginia, Harvard University and Columbia University in the City of New York, started developing methods of U-235 isolation to include centrifuge and gaseous thermal diffusion. Abelson, looking at a more conservative means to isotope separation, developed a liquid thermal diffusion process based on a foundation of pressurized steam and long concentric columns of cylindrical piping.

To achieve separation using this method, Abelson preferred the relatively inexpensive yet highly volatile, uranium derivative, uranium haxafluoride (UF6). Other methods of separation being used to produce the enriched isotope were proving time consuming and expensive, utilizing a reaction of highly toxic fluorine gas and a scarcely found powdered uranium-nickel alloy. Abelson, requiring several kilograms of UF6 for his experiments, set out to independently produce the necessary volumes of uranium hexafluoride crystals using the more readily abundant salt of uranium.

The basic idea of liquid thermal diffusion is to dissolve UF6 crystals into a column whose ends are kept at very different temperatures. Since those uranium hexafluoride molecules harboring the lighter U-235 atoms diffuse slightly faster to the warmer side of the column than do those molecules with heavier uranium isotopes, mainly U-238, the lighter molecules collect at the warm side of the column. This U-235 enriched solution then becomes the starting fluid for the next iteration in a diffusion tube and so on.

Steam being the essential source of required heat and pressure for the liquid thermal diffusion process, Abelson, at CIW, was encouraged to seek the much needed space and ample steam production available at the nearby National Bureau of Standards (NBS). His stay, however, would prove short-lived as Abelson quickly realized in order to achieve the desired quantity of U-235, he would need even larger columns in greater numbers requiring higher steam pressures and abundance.

As Abelson's research had been progressing, Dr. Ross Gunn, a physicist and technical advisor to the director of the U.S. Naval Research Laboratory (NRL), had been directly involved in high-level Department of Defense meetings that sparked his interest in the power of nuclear energy. In one such meeting (March 1939), Enrico Fermi, an emigrant physicist from Italy and co-researcher on the first U.S. nuclear fission experiments at Columbia University, had lectured on the potential that nuclear fission held as an inexhaustible source of energy.

Enthralled by this news, Gunn, researching alternative sources of power for the Navy, quickly envisioned its potential as the answer to the Navy's submarine propulsion woes. At the time, concurrent research was investigating the use of fuel cells, hydrogen peroxide-alcohol steam turbine, and closed cycle diesel engines, however, each of these methods continued to offer no improvement to submarine stealthiness or crew safety as the need to surface, even if partially, maintained the vulnerability of the submarine.

With a clear sight set on nuclear propulsion, Gunn knew that any practical system would entail having a plentiful source of uranium fuel enriched in the U-235 isotope. Shortly after the Fermi lecture, Gunn and NRL director, Captain Hollis Cooley successfully pleaded the potential benefits of this technology to Admiral Harold Bowen, Chief of the Bureau of Engineering, resulting in an initial investment of approximately $1,500.

Working with the modest funds, Gunn and his colleagues immediately began to develop their own methods of uranium enrichment choosing fluorine gas and a powdered uranium-nickel alloy, a method that Gunn and others were discovering proved to be extremely dangerous, expensive and time consuming. By year's end, the team was only producing a few grams of UF6, insurmountable to the scale Abelson could achieve through his thermal diffusion method.

Abelson Transfers to NRL

Learning of Abelson's dilemma at NBS and having intently followed developments in his success to acquire substantial quantities of UF6, Gunn took advantage of the opportunity to invite Abelson to NRL to see if the facility could accommodate the additional growth and steam production to meet his needs. Abelson was immediately sold, and in June 1941 — six months prior to the Japanese attack at Pearl Harbor and the coercion of the United States to fully enter the World War — started work at the NRL with the addition of a small pilot plant, with 36-foot separation columns, situated next to the laboratory's boiler house.

Despite Abelson's successes, it became popular consensus that thermal diffusion was not the best method for enriching uranium. The Army's Manhattan Project, started in October 1939 to research nuclear reactions and weaponry, concentrated on gaseous diffusion as a means of producing highly enriched uranium, and although not formally associated with the atom bomb project, Abelson's method of liquid thermal diffusion would later prove to be a critical step in creating sufficient fuel toward the effort.

With the Army project underway, the Navy remained committed to the use of uranium for the sole purpose of submarine propulsion. Maintaining considerable interest in an economically feasible enrichment process that would provide quantity over quality, the Navy chose to further support Abelson's research, adding an additional $100,000 to fund the endeavor.

Proving successfully that each 36-foot column could produce substantial quantities of the uranium isotope, the additional funding led to an expanded effort to build and operate fourteen, 48-foot tall columns. It also led to the procurement of a propane-fired boiler capable of delivering 1,000 pounds per square inch of steam, and for a time, made the facility at the Naval Research Laboratory the world's most successful separator of uranium isotopes, producing nearly 40-pounds of UF6 a month by June 1943.

Several months earlier, a Naval Research Laboratory report submitted to the Bureau of Ships by Gunn (January 1943) pointed to the advantages of using enriched uranium in nuclear reactors, a critical step toward the development of nuclear powered submarines. Gunn's report continued by exclaiming that if the necessary facility was funded, it could produce the required separated isotopes necessary for the success of the project and included a survey of naval establishments showing that large-scale sources of high-pressure steam could be made available at the Naval Boiler and Turbine Laboratory at the Philadelphia naval base. A response received November 27, 1943, gave authorization to build a 306-unit plant at Philadelphia.

In June 1944, the Philadelphia plant was nearing completion, but just as the plant was ready to go on-line, Gunn and Abelson learned that the War Production Board had turned down their request for the necessary catalysts. With the Manhattan Project fully implemented, but lagging in the production of enriched uranium, the Army eventually took over all nuclear energy research in the United States, putting a halt to all uranium enrichment efforts undertaken by the Navy.

Not to be entirely discouraged, Abelson had become aware that the Manhattan Project's gaseous diffusion plant at Oak Ridge Tennessee was badly behind schedule and became concerned that the entire bomb project might be in jeopardy. With this in mind, he wanted Robert Oppenheimer, scientific director of the Manhattan Project at Los Alamos, N.M., to know what NRL had to offer at the Philadelphia plant — the pilot plant produced 236 pounds of UF6 in a six-month period, a quantity greater than had been obtained by the gaseous diffusion method at that time.

Oppenheimer, recognizing that a supply of partially separated uranium would increase the production of the electromagnetic plant at Oak Ridge, communicated this to Brig. Gen. Leslie Groves, Army Corps of Engineers and director of the Manhattan Project, who sent a reviewing committee to Philadelphia. The review resulted in a favorable report and the decision to build a 2,142-column thermal diffusion plant (S-50) at Oak Ridge, with supplemental production being provided by the Philadelphia plant. This modestly U-235 enriched material became feedstock for yet another separation process at Oak Ridge, whose more thoroughly enriched U-235 became part of the radioactive heart of the first atomic bombs. By the time the Philadelphia facility closed in early 1946, it had produced just over two-and-a-half tons of enriched uranium.

Soon after the bombing of the Japanese cities of Hiroshima, using an enriched U-235 bomb, and Nagasaki, a plutonium-239 detonation, ended the war (August 1945), Abelson, under the guidance of Ross Gunn, again turned his attention to applying nuclear power to naval propulsion. Granted permission to reconvene with his research, Abelson, that fall, returned to Oak Ridge where he participated in experiments of enriched uranium moderated by 'heavy-water' (D2O) to develop critical assemblies (an assembly of sufficient fissionable and moderator material to sustain a fission chain reaction at a low power level). However, after a fatal accident involving similar research at the Los Alamos facility the work was brought to a halt.

Navy Goes Nuclear

Having earlier sold the National Defense Research Committee on the idea of nuclear powered submarines during a visit to NRL and a meeting with NRL director Rear Adm. Alexander H. Van Keuren, Abelson, now back in Washington, focused more generally on the engineering aspects of fitting submarines with a nuclear power plant.

With blueprints of a German Type XXVI Walter submarine, considered the most advanced design of the time, obtained from Cmdr. Robert J. Olsen, a naval submariner attached to the laboratory during this period, Abelson, along with mechanical engineers Robert Ruskin and Chad Raseman, worked to retrofit the diesel and battery power system with an atomic reactor.

Thermal energy generated in the atomic 'pile' would be transferred to liquid sodium-potassium (KNa) alloy recirculated through the pile, states Abelson. This heat would drive a steam turbine...and the pile, together with its shielding and the KNa heat exchanger, would be located outside the pressure hull along the keel of the submarine. It would be necessary for the pile to be a cube that could conform to the streamline shape of the hull. This arrangement would allow for convenient maintenance and replacement in drydock.

With this concept, Abelson wrote the first ever report detailing how a nuclear reactor could be installed in a submarine, and while not written at an engineering-design level it provided detail of both propulsion and electrical power. The results were compiled and circulated in a report entitled Atomic Energy Submarine dated March 28, 1946, and followed by briefings with naval officials and the submarine community.

During briefings, Abelson, lining the walls with blueprints, equations and other diagrams, vigorously presented the importance of extended submerged operations and the need for nuclear power. In attendance at one such briefing, Vice Adm. Charles Lockwood, a veteran World War II submarine commander, likened what he heard to something out of Jules Verne's Twenty Thousand Leagues Under the Sea, an analogy that may have inadvertently inspired the naming of the first nuclear submarine.

After submitting the report and subsequent briefs, activities at NRL related to isotope separation and submarine propulsion were gradually curtailed, but surely not remiss. In January 1947, Chief of Naval Operations, Fleet Adm. Chester W. Nimitz, approved a program for the design and development of nuclear power plants in submarines, a concept later moved forward by Admiral Hyman G. Rickover.

Under the relentless leadership of Rickover, who received training in nuclear power at Oak Ridge and experience at the Bureau of Ships exploring the possibility of nuclear ship propulsion, the concept was brought to reality. In February 1949, Rickover was assigned to the Division of Reactor Development at the U.S. Atomic Energy Commission and then assumed control of the Navy's effort as director of the Naval Reactors Branch in the Bureau of Ships. In this capacity, Rickover was poised to finally bring Abelson's vision to fruition.

Echoing a namesake coined by Vice Adm. Lockwood in the early briefings by Abelson, the world's first nuclear-powered submarine, the USS Nautilus — from the classic 1870 novel — was authorized in 1951 and in 1954 was launched into Connecticut's Thames River. Serving the U.S. Navy for nearly a quarter century the Nautilus went on to not only log a record number of hours but to shatter many previous submarine records. The most momentous of these was a four-day trip in 1958 when the Nautilus traveled submerged and traversed 1,830 miles under the Arctic polar ice cap. With continued vindication to the compelling visions of Abelson and Gunn, nuclear remains the sole propulsion and power source to the more than 70 commissioned submarines in today's U.S. Navy fleet.