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"Irradiated enriched sample intended for you being removed from Clinton (Oak Ridge) pile today...,"cclix
            Samuel Allison
            From a cable to Robert J. Oppenheimer
            March 17, 1944
            (The traditional history asserts plutonium
            was bred in reactor piles fueled with raw
            uranium, not enriched uranium - author's note)

  Because the long road to a valid uranium enrichment program from the beginning was thought to be a longshot, the discovery of plutonium in December 1940 was a godsend to the bomb makers. More than a year after Glenn T. Seaborg, Joseph W. Kennedy and Arthur C. Wahl confirmed they had re-created an elementcclx heavier than uranium that had long ago disappeared from earth, Seaborg and his team, along with Italian physicist Emilio Segre, proved that the new substance would fission. The cleaving of this first man-made element allowed the great American nuclear braintrust a second, more sensible option than trying to pluck a small minority of nearly identical atoms from an otherwise homogenous body of matter, as was the requirement for enriching uranium.
  Plutonium was an element unto itself, with characteristics all its own.cclxi  The difference meant that instead of devising methods to differentiate and take advantage of infinitesimal weight discrepancies between sub-microscopic atomic particles, as was the case with separating uranium isotopes, the plutonium created by bombarding raw uranium with neutrons, which absorbs U238 and thus metamorphs into plutonium, could simply be separated from the uranium by dissolving the mass and rinsing the solution with a chemical found to bind with plutonium but not with uranium.  As the "binder" later was separated away, the plutonium would be exposed for the taking.  Such an explanation is a vast oversimplification but suitable for a basic understanding.
  The process was substantially simpler, nonetheless, than that of enriching uranium. There still existed significant barriers to overcome; like, how could uranium be bombarded with enough neutrons to transmute into plutonium, as would be required to reach production-level outputs? The cyclotron that Seaborg's team used to create plutonium was far too small and neutron-anemic to produce anything but microscopic amounts of plutonium.  And once the irradiated, plutonium-carrying slugs of uranium were ready to be dissolved, how could the task be accomplished without radiation poisoning the people assigned the task of working with the highly radioactive material?  Plutonium, in theory, was a great solution for a bomb but its practical application would prove to be a prickly challenge in and of itself.
  The chemical differences, however, were not the only advantages plutonium held over enriched uranium. With U238 being 139 times more common in natural uranium than U235, and plutonium being a product of neutron bombardment of U238, it was possible to create much more plutonium out of an equal amount of uranium than would ever be possible to separate U235 from the mother substance.cclxii  And conversely, even while more plutonium fissile material could be made faster and cheaper than enriched uranium, only one-third as much plutonium was needed for a bomb than enriched uranium because plutonium is more radioactive.cclxiii More nuclear fuel, at higher quality, for less time and money - the advantages were obvious.  Despite all of the time and effort and money being poured into uranium enrichment, pursuit of plutonium quickly became the primary objective of the Manhattan Project.
  The Manhattan Project's scientific community rallied around the proposal.  In fact, Ernest O. Lawrence, the father of the calutrons, plutonium's "competitor," led the charge in favor of plutonium with Oppenheimer's blessing.cclxiv  Arthur Compton, Nobel laureate in physics and one of the original movers and shakers that made the bomb project possible, thought in 1941, before isotope separation had been proven, that the plutonium alternative saved American bomb research altogether.cclxv  Compton's committee, in fact, recommended the creation of a central lab just to handle the development of a plutonium bomb.cclxvi  Jewish-German war refugee Hans Bethe, whom one would have thought would jump at the slightest chance of developing a successful bomb to be used against the Nazis, who had driven him from his home, had refused to join an atomic bomb research group. Bethe considered the creation of a bomb impossible; until the plutonium option became available, at which time he jumped into the project with both feet.cclxvii General Groves, who received his assignment to lead the Manhattan Project in the midst of the plutonium option development, put his best hope in creating a plutonium-fueled bombcclxviii and made it the number one priority.
  All of this was well and good but plutonium research, though an excellent prospect, was "getting out of the blocks" late.  Assessing a plutonium bomb's legitimacy took time. An answer for the weak neutron bombardment problem caused by the cyclotron's limitations was not found until almost the end of 1942. On 2 December of that year, Enrico Fermi's research group successfully sustained the first man-made nuclear chain reaction during their famous experiment in a squash court under the bleachers of the University of Chicago's football stadium.  The astounding success meant neutrons could be released in unimaginable numbers, to be absorbed by U238 and thus transmute the uranium into plutonium.
  The success of Fermi's plutonium breeding pile resulted in a major change of plans. While the original purchase of the property at Oak Ridge included plans to house plutonium development facilities, General Groves soon realized the risks of building production-size breeder reactors were too great for a highly populated area like Knoxville, which was close to Oak Ridge.  A new reservation had to be found, far from a large population center and prying eyes.  A site team was dispatched to locate such a location, visiting sites in California, Oregon, Idaho and Washington, and eventually returning to Groves with a recommendation - Hanford, on the barren, eastern plains of the state of Washington.cclxix  Groves soon flew out to Washington and approved the site.  But in February 1943, with barely two and one-half years left to successfully fulfill the future time objective (as yet unknown, since Russia was not showing any signs of declaring war on Japan) the property at Hanford was still in the process of being purchased.cclxx
  Construction on the site was officially begun March 22, with a multitude of development, construction and research projects running concurrently, not only at Hanford, but at Oak Ridge, Chicago, and elsewhere. By the end of 1943, however, the building of the first reactor pile - so named because a reactor was simply a sophisticated pile of graphite blocks with uranium slugs and control devises inserted in holes drilled through the graphite - had not been begun.  Eighteen months to what would be the future objective, and counting, and still no production reactors were under construction.
  Which is not to say no work was being accomplished.  A small pilot reactor at Oak Ridge had been assembled and was beginning to provide milligram quantities of plutonium for experimentation and metallurgical research.cclxxi  Progress in the chemical process of plutonium separation was being made, with the proposal and eventual validation of bismuth phosphate as a plutonium carrier to separate plutonium from uranium. Innovative methods in miniaturization and robotics, and to some degree television, which would lay the groundwork for the future high-tech industry that would burst forth a quarter-century later, were being developed to perform the dirty, dangerous work of separating plutonium from its mother raw uranium without irradiating the people performing the work.
  And at Hanford, although reactor piles had not been started, great strides were already being made toward the construction of the mechanical aspects of the chemical separation facilities.cclxxii  The separation team had devised a semi-automated system where irradiated slugs mechanically were dropped into a huge "trough" that contained the equipment and substances required to run the slugs through the series of steps necessary to dissolve the slugs and then separate the different elements according to requirements.  The trough was buried almost completely in the ground and lined with huge cement walls and 20,000 tons of steel plate and cellulose, as well as 7,500,000 square feet of Masonite,cclxxiii all forms of biological shielding to protect operators from the dangers of radioactivity.
  At its peak, 42,400 construction workers plied their trades building the Hanford reservation.cclxxiv  Even more than in the uranium enrichment program, everything was being thrown into the endeavor to make the plutonium bomb succeed.  Still, the chances of producing more than just a few grams of plutonium in 1943, and not much more in 1944, even under the best of circumstances, was all they could hope for, according to General Groves.cclxxv  Groves did not expect production levels of plutonium until 1945, and there were many doubts about that.
  The doubts were well-founded.  A year earlier, in the beginning of 1942, Seaborg had written that bombs were planned to be in production around the beginning of 1944.cclxxvi  Obviously, that had not occurred. No plutonium was produced in 1943 at all, at Hanford or at the scaled-down experimental pilot reactor at Oak Ridge, which had been built as a working model to develop the Hanford technology.  The Oak Ridge plant had been loaded with uranium fuel in early November, however, and went critical soon afterward.  As a result, the first day of 1944 saw the inaugural delivery of milligram quantities of plutonium sent to Chicago for experimentation.cclxxvii The Oak Ridge reactor continued to send experimental amounts of plutonium to the metallurgical laboratory in Chicago and to the nuclear laboratory at Los Alamos.  But bomb-production quantities from Hanford would not be produced for almost another full year, beginning on 24 November, 1944 (B reactor, the first to be fueled at Hanford, went critical 26 September, 1944).cclxxviii Only eight months were left on the countdown to August 1945 when the first small quantity of production plutonium was created.
  Like the uranium enrichment effort, continual dilemmas and delays had slowed the plutonium program. A most serious problem, realized before production even started, was the low concentration of plutonium the initial pile design would produce.cclxxix  The difficulty, simply put, was that raw uranium contains so few U235 atoms, only one out of every 140 uranium atoms.  These U235 atoms fission and release neutrons that in turn either fission more U235 - continuing the chain reaction - or are absorbed into U238 atoms and thus transmute the uranium to plutonium, which is the desired end-product.  But even after the maximum amount of fission occurred, after long weeks in the reactor when the U235 was finally spent, much more U238 remained that could have been transmuted to plutonium.  Plutonium production, while better than enriched uranium output, was still woefully lean.  Available records of the time appear to indicate the plutonium content of the initial Hanford discharge was so low that the chemical separation process had to be further refined to optimize the product yield to an acceptable level.cclxxx
  As early as 1941, however, Philip Abelson, a physicist for the United States Navy, had realized that using enriched uranium to fuel a reactor would make the reactor rich in free neutrons. The reactor would not only be more powerful, with a greatly reduced size requirement,cclxxxi but, most importantly, the modification would produce significantly more plutonium.
  From the beginning and throughout the Manhattan Project, all avenues to improve success were pursued.  So it was with efforts to increase plutonium yield. Plutonium was the top priority for a bomb; and with a growing arsenal of newly developed technologies from which to draw, Groves and his advisory board appear to have made a logical and obvious, but very fateful, decision.  Unknown to history up to today, they appear to have used the invaluable enriched uranium from Oak Ridge - which was fat in U235 that would provide the neutron flood needed to create significantly more plutonium per production run - to fuel the reactors at Hanford.  The decision was not without risk and potential political fallout, however, and so it was vigilantly guarded at the time; and following later dubious developments, it appears to have been carefully buried ever since.
  The traditional history simply tells us that the Hanford reactors' design was modified from helium-cooled piles to water-cooled piles.  Purportedly this was done to increase the power of the reactors, which would proportionately increase plutonium production - and which would require water's better cooling characteristics - and for ease of design and cost savings in construction.cclxxxii  The modification itself, however, almost certainly implies the piles were actually modified to be uranium enriched.  Three keys provide evidence of this fact.  First, according to Dr. Bernard Wehring, Director of the J.J. Pickle Research Center for Nuclear Engineering at the University of Texas, cclxxxiii and Dr. Delmar Bergen, a retired physicist from the Los Alamos National Laboratories,cclxxxiv water-cooling a pile would be used only to cool a uranium-enriched reactor, not one fueled by raw uranium. Both scientists agree that water absorbs neutrons voraciously and therefore is in competition for neutrons with U235 - which, as mentioned, needs them to maintain the chain reaction - and with U238, which needs to absorb the neutrons to transmute to plutonium.
  A raw uranium reactor cooled by water would produce even less plutonium than would a helium-cooled pile, not more.  The neutron-hungry water in the pile would consume the very neutrons needed to make plutonium.
  Fueling the reactor pile with uranium significantly enriched in U235, on the other hand, would increase the neutrons to a level that supports a high rate of fission.  At the same time, sufficiently enriched uranium would provide more neutrons for transmuting much greater quantities of U238 to plutonium, all the time feeding the cooling water's hungry appetite for neutrons as well. The water would be required to cool the more powerful enriched reactor, for which helium would be insufficient.  The end result, depending on the level of uranium enrichment utilized, would be more plutonium produced at a faster rate.
  Drs. Wehring and Bergen both admit to not being historians of
nuclear physics and that without knowing the full background of the
Hanford reactors they could not declare with certainty that the reactors
were fueled by enriched uranium.  But on theoretical grounds alone, neither of them could conceive of a case in which a raw uranium reactor would be cooled by water.
  Second, according to Dr. Wehring, there are only two alternatives for increasing the plutonium-producing capacity of a reactor pile; either add more raw uranium, forcing the pile to be larger, or fuel the pile with a more fissile material - either enriched uranium or plutonium. Since the first Hanford pile, at least, was first fueled by natural uranium and was housed in a facility built for such, there seems to have been limitations on the size of the pile that could have been installed in the building.  The author, following extensive research, could find no reference specifically to alteration of the size of the Hanford piles, effectively eliminating the addition of more raw uranium to increase the power of the reactor.  No such event having taken place suggests a second proof that the enriched uranium alternative was adopted as the method to increase reactor power and therefore increase plutonium production at Hanford.
  A third and compelling proof that the Hanford reactor piles were fueled by enriched uranium lies in the uses of, and changes made to, their forebearer and model, the pilot reactor at Oak Ridge. Communications beginning in March 1944 between Samuel K Allison,cclxxxv who worked at the University of Chicago metallurgy laboratory - called the Met Lab - specifically solving plutonium problems, and Robert J.Oppenheimer clearly show that the Oak Ridge reactor was being used to explore enriched uranium as a reactor fuel.  Apparently Phillip Abelson's recommendations three years earlier were being followed.
  "Irradiated enriched sample intended for you being removed from Clinton (Oak Ridge) pile today...,"cclxxxvi states the first communiqué from Allison to Oppenheimer matter-of-factly.  A portion of a letter sent from Allison to Oppy the following day to provide more details said,

  I am sending you in a separate package 57 milligrams of enriched T3O8 ("T" stood for "Tubealloy," the code name for uranium, "O" for oxide; thus the material was enriched uranium oxide - author's note). This is part of the sample which was exposed at X ("X" was the code name for Oak Ridge).
  You should receive the irradiated material directly from X in the next shipment of product within about a week, and material I am sending you will serve as a control. (emphasis the author's)

  Allison's plainly written communications reveal with certainty that experimentation using enriched uranium as a reactor fuel, in at least some of the Oak Ridge pile's fuel, was underway.  It is difficult to believe that the plutonium-producing enhancement of using enriched uranium to fuel the reactors, which proved completely successful - virtually all later reactors were fueled by enriched uranium or plutonium -was ignored at such a critical moment in history when its need was so great.
  While Allison's references to irradiating enriched uranium in the Oak Ridge pile are the only direct documentation the author has been able to uncover of enriched fuel in reactors during the war, the implication that the change was covered up is seen in how this modification was later recorded for official history.  H.D. Smyth, who wrote the first history of the Manhattan Project, Atomic Energy For Military Purposes,cclxxxvii writes that in the spring of 1944, "a change was made in the distribution of uranium" within the reactor.  Without mentioning enriched uranium, he goes on to describe how the uranium fuel cells were reconfigured with fewer uranium slugs in the middle so power could be increased without overheating the reactor pile.  The result was that reactor performance in June 1944 "considerably exceeded expectations."  To produce more plutonium with less uranium would have been impossible - unless the uranium was enriched.  And thus, it appears, is purposely hidden the real reason for the increased output - enriched uranium had apparently worked its magic.
  The timing of the reconfiguration of the pile in the spring not only coincides with Allison's enriched uranium experiments, but the description coincides with a statement made by Dr. Wehring when such a reconfiguration was described to him.  Dr. Wehring theorized that the core realignment would have been required to increase the size and/or number of cooling passages in order to control the additional heat created by the introduction of at least some enriched-uranium cells to the pile.  In addition, although Smyth states flatly that the pile was run at higher power levels as a result of the reconfiguration, he never suggests that the pile was expanded in size to achieve that increase. In fact, as noted previously, research shows no increase in the size of the reactor, leaving enrichment the only option.
  The apparent cloaking of material information about the use of enriched uranium in the Oak Ridge reactor suggests a similar subterfuge was used when the Hanford reactor designs were described as having been converted from helium-cooled to water-cooled.  As has been articulated, water cooling is used to cool enriched uranium reactors and not to cool raw uranium reactors, so, in essence, saying a reactor is water cooled is saying it is enriched uranium fueled - or plutonium-fueled if plutonium was available, which it was not.
  All of this information: the knowledge that water-cooling and an increase in power without increasing pile size both denote enriched fueling, and the revelation that Oak Ridge had already performed research on irradiated enriched slugs may suggest that other, more minor, details of the Hanford pile development support the enriched fuel theory as well.  For example, the management of Hanford went to great expense and effort after the redesign of the reactor piles to remove an existing system designed to store the radioactive waste by-products of the pile, and installed in its place a system for extracting uranium from the effluent.cclxxxviii  The reason given was to reduce the waste of uranium.  But the cost of the modification probably far exceeded the value of the "raw" uranium salvaged - unless the uranium still contained residual amounts enriched in U235.  In that case, the inestimable value of the reclaimed uranium would have justified the expense of the reclamation project many times over.  A similar reclamation process was already being used in the calutrons for the same reason.  And shortly after the war ended, the system was discontinued, suggesting the successful plutonium bomb negated the need for the expense of reclaiming the entrained enriched uranium.  Perhaps the reclamation system is yet another evidence the Hanford piles were fueled by enriched uranium.
  In April 1944 - again, note the time frame relative to Allison's experiments in March of the same year - a purification system for Hanford's reactors required redesigning when "criticality requirements" were eased.cclxxxix  The essence of using enriched uranium instead of raw uranium in the pile was to increase criticality within the slugs to above the marginal level of critical activity provided by raw uranium. Might this modification to the system herald a change in the types of fuel used, also?
  And lastly, the chemical laboratory at Los Alamos experienced an unexpected increase in the output of product received from Hanford, straining the department's resources.ccxc  Estimates of plutonium product coming from Hanford should have been easy to make and reliable not to have drastically increased - unless something drastic had been done to increase plutonium output - something drastic, like fueling the reactors with enriched uranium.
  The evidence seems powerful if not incontrovertible that enriched uranium was used to fuel the plutonium breeding reactor piles at Hanford and Oak Ridge.  The enriched uranium could have come from no other source than the hard-earned but negligibly growing cache of U235 from Y-12.
 

Notes:
cclix Samuel Allison classified cable to J.R. Oppenheimer, 17 March
1944, National Archives, Southeast Region, East Point, Georgia, A-
84-019-16-8

cclx Harry Thayer, Management of the Hanford Engineer Works In World War II, p. 133

cclxi Leona Libby, The Uranium People, p. 77

cclxii David Irving, The German Atomic Bomb, p. 93

cclxiii Robert Serber, The Los Alamos Primer, pp. xv, xvi

cclxiv Herbert Childs, An American Genius, p. 325

cclxv Richard Rhodes, The Making of the Atomic Bomb, p. 368

cclxvi Leona Libby, The Uranium People, p. 79; Richard Rhodes, The
Making of the Atomic Bomb, pp. 388,389

cclxvii Richard Rhodes, The Making of the Atomic Bomb, p. 416

cclxviii Richard Rhodes, The Making of the Atomic Bomb, p. 431

cclxix Harry Thayer, Management of the Hanford Engineer Works In World War II, p. 136

cclxx Leslie Groves, Now It Can Be Told, p. 76

cclxxi Harry Thayer, Management of the Hanford Engineer Works In World War II, p. 139

cclxxii Harry Thayer, Management of the Hanford Engineer Works In World War II, pp. 138, 139

cclxxiii Harry Thayer, Management of the Hanford Engineer Works In World War II, pp. 138, 139

cclxxiv Richard Rhodes, The Making of the Atomic Bomb, p. 557

cclxxv Leslie Groves, Now It Can Be Told, p. 51

cclxxvi Richard Rhodes, The Making of the Atomic Bomb, p. 412

cclxxvii Harry Thayer, Management of the Hanford Engineer Works In World War II, p. 139

cclxxviii Harry Thayer, Management of the Hanford Engineer Works In
World War II, p. 141

cclxxix Harry Thayer, Management of the Hanford Engineer Works In World War II, p. 140

cclxxx Harry Thayer, Management of the Hanford Engineer Works In World War II, p. 140

cclxxxi Richard Rhodes, The Making of the Atomic Bomb, p. 549

cclxxxii Harry Thayer, Management of the Hanford Engineer Works In World War II, pp. 136-139; Mystery Book, pp. 113, 114; Leland Johnson and Daniel Schaffer, Oak Ridge National Laboratory: The First Fifty Years, p. 20; H.D. Smyth, Atomic Energy For Military Purposes, p. 129

cclxxxiii Dr. Bernard Wehring, personal telephone conversations with
author, August 6, 1997 and October 10, 1997

cclxxxiv Dr. Delmar Bergen, personal telephone conversation with the
author, March 24, 1998.

cclxxxv Samuel K. Allison, classified cables to Robert J. Oppenheimer of March 17 and 18, 1944, and May 22, 26 and 27, 1944; also cable from Robert J. Oppenheimer to General Leslie Groves, May 27, 1944; all documents located at National Archives Southeast Region, East Point, GA; A-84-019-16-8

cclxxxvi Samuel Allison classified cable to J.R. Oppenheimer, 17 March 1944, National Archives, Southeast Region, East Point, Georgia, A- 84-019-16-8

cclxxxvii H.D. Smyth, Atomic Energy For Military Purposes, p. 143, 144

cclxxxviii John F. Hogerton, The Atomic Energy Desk Book, p.220

cclxxxix Harry Thayer, Management of the Hanford Engineer Works In
World War II, p. 140

ccxc Anthony Cave Brown and Charles B. MacDonald, Secret History of the Atomic Bomb, p. 507