Plutonium a Wartime Nightmare but a Metallurgist’S Dream
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Plutonium A Wartime Nightmare but a Metallurgist’s Dream by Richard D. Baker, Siegfried S. Hecker, and Delbert R. Harbur n 1942 the theoretical outline for an Research from 1943 to 1946 electron-volt deuterons in the cyclotron at atomic bomb was clear: compress Berkeley. Within about a month it was enough fissionable material long The Los Alamos work on plutonium and shown that plutonium-239 fissioned when I enough to properly ignite a chain enriched uranium, the so-called special nu- bombarded with slow neutrons, and a de- reaction. Construction of an actual weapon, clear materials, was extensive, covering a cision was made to build large reactors at however, required translation of “fissionable variety of research problems ranging from Hanford for the production of pluto- material” into real pieces of plutonium or purification of material received from reac- nium—this before the uranium-graphite pile uranium metal. These metals had to be free tors to the prevention of oxidation of the at Chicago had demonstrated that a sus- of impurities that would adversely affect the final product. Further, because many chemi- tained and controlled chain reaction was neutron flux during the chain reaction and cal processes and physics experiments re- even possible! That demonstration soon fol- yet be fabricable enough that precise shapes quired very pure materials, such as gold, lowed, proving that large quantities of pluto- could be formed. Whether this would even be beryllium oxide, graphite, and many plastics, nium could be produced, although no pluto- possible with plutonium was not then known, considerable general materials research was nium was extracted from the Chicago reac- however, because plutonium was a new, also carried out. tor. manmade element and the metal had not Much of the chemistry and metallurgy of At this point only microgram amounts of been produced. uranium was already known from the pro- plutonium had been separated from the Accounts of the Manhattan Project have duction of uranium metal for the uranium- targets used in the cyclotrons. Remarkably, neglected (for security reasons, initially) the graphite reactor pile at Chicago in 1942. The the basic chemistry of plutonium was important metallurgical work that preceded work remaining on enriched uranium in- worked out at Berkeley and Chicago on this fabrication of these materials into integral cluded preparation of high-purity metal, fab- microgram scale, and it formed the basis for parts of real weapons. For example, the rication of components, and recycling of the scale-up—by a factor of a billion— Smyth Report* devotes one short paragraph residues. However, the most challenging re- needed for plants that would eventually to the wartime work of the entire Chemistry search and development was carried out on separate plutonium from spent reactor fuel. and Metallurgy Division at Los Alamos—a the new element plutonium. At the same time the first micrograms of the division that in 1945 numbered 400 scientists Table I gives the important dates in the metal were produced at Chicago by the and technicians. Our article will attempt to early history of plutonium and shows the fill part of this gap for plutonium by high- short time—four years—that elapsed be- *Henry DeWolf Smyth, Atomic Energy for Mili- lighting key developments of the wartime tween its discovery and its use in the first tary Purposes: The Official Report on the Devel- research and will continue with some of the atomic device at Trinity. The discovery oc- opment of the Atomic Bomb under the Auspices of the United States Government, 1940-1945 exciting research that has occurred since the curred, as predicted by nuclear theory, when (Princeton University Press, Princeton, 1945), war. uranium was bombarded with 16-million- pp. 221-222. 142 Winter/Spring 1983 LOS ALAMOS SCIENCE TABLE I EARLY HISTORY OF PLUTONIUM reduction of fluorides, and preliminary Plutonium discovered February 23,1941 metallurgical properties were determined. Neutron-induced fission of plutonium-239, proved March 25, 1941 However, the influence of impurities on such Decision reached for large, full-scale plutonium production December 6, 1941 First controlled fission chain reaction achieved, December 2, 1942 tiny samples distorted many of the results; proving method for full-scale production of plutonium for example, the melting point of plutonium Preparation of plutonium metal from microgram November, 1943 was first thought to be about 1800 degrees quantities produced with cyclotron Gram quantities of plutonium nitrate from March, 1944 Celsius, considerably above the true melting experimental reactor received at Los Alamos point of 641 degrees. Ultimately, the Plutonium nitrate from production reactor received mid 1944 properties of plutonium were found to be at Los Alamos Plutonium weapon demonstrated with Trinity test July 15, 1945 incredibly sensitive to impurities. It had been agreed that Los Alamos would not work on batches of plutonium of less than about 1 gram, and the microgram-scale extensive effort to obtain pure and nonreac- work continued at Chicago. Finally, in early tive refractories to contain molten phuto- 1944 Los Alamos received plutonium nitrate nium. The high purity requirements also samples containing half-gram amounts of the necessitated the development of new meth- element from the “Clinton” reactor and pilot ods for analysis of all materials, including extraction plant at Oak Ridge. Later, larger plutonium. amounts were received from the production The potential health problem associated facility at Hanford. with the handling of plutonium had been The plutonium nitrate arrived in relatively recognized at Chicago, and work on the impure form, and techniques and equipment subject began with receipt of the first small had to be developed for a number of amounts of plutonium. A Health Group was processes, including purification, preparation formed to monitor plutonium work areas, of plutonium tetrafluoride and other com- and, within the Chemistry and Metallurgy pounds, reduction to metal, and metal fabri- Division itself, committees were established cation. Also, because plutonium was in very to design suitable radiation detectors and short supply, it was imperative to develop apparatus for handling plutonium and to processes to recycle all residues. formulate safe handling procedures. Because Initially, the purity requirements for the alpha counters then lacked either sensitivity metal were very stringent because some or portability and were in short supply, oiled elements, if present, would emit neutrons filter paper was swiped over surfaces to pick upon absorbing alpha particles from the up possible stray bits of plutonium and then radioactive plutonium, These extra neutrons measured at stationary counters. Similar were undesirable in the gun-type plutonium procedures were used to detect suspected weapon then envisioned: they would initiate contamination of hands and nostrils. The air- a chain reaction before the material had conditioning system in the plutonium labora- properly assembled into its supercritical con- tory (D Building), which was installed in- figuration, and this “pre-initiation” would itially to help maintain high purity by filter- decrease the explosive force of the weapon. ing out dust, ultimately served the more Two early discs of plutonium metal after The purity requirement for certain elements important function of confining the pluto- reduction from the tetrafluoride. Pluto- was a few parts per million and for some, nium. The building was equipped with hoods nium generally arrived at the Labora- less than one part per million. As a result, all with minimal ventilation and with the fore- tory from the Hanford reactors in the the materials used in the preparation of the runner of the modern glove box—plywood form of a relatively impure nitrate solu- plutonium metal, everything from the proc- “dry boxes.” The successful handling of tion. Techniques were developed at Los ess chemicals to the containers, had to be of large quantities of plutonium without serious Alamos for purification, preparation of very high purity. This necessitated develop- problems was at that time an outstanding various compounds, reduction to the ment work on many materials, including an achievement. metal, and metal fabrication. LOS ALAMOS SCIENCE Winter/Spring 1983 143 At first plutonium metal was prepared at Los Alamos either by lithium reduction of plutonium tetrafluoride in a centrifuge, the metal settling out as the closed reaction vessel rotated, or by the electrolysis of fused salts containing plutonium. Soon, however, calcium reduction of the tetrafluoride was perfected. The vessel used to contain this reaction was called a stationary “bomb” because the reaction was highly exothermic and the metal product settled out in the closed, nonrotating vessel simply by gravity. This technique became the preferred method and was used to prepare plutonium for almost all metallurgical studies and for the nuclear devices. The microgram metallurgy at Chicago had provided values for the density of the metal that clustered about either 16 or 20 grams per cubic centimeter. This bimodal spread, due surely in part to impurities, nevertheless pointed toward interesting metallurgy by hinting that the element had more than one phase. Working with larger amounts, Los Alamos refined these measure- ments and by the middle of 1944 had discovered that plutonium was a nightmare: no less than five allotropic phases existed between room temperature