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of the
NEW ENGLAND ASSOCIATION of CllEllSTRY TEACHERS
Synthesis of the New Elements Neptunium and Plutonium' LAURENCE S. FOSTER Massachusetts Institute of Technology, Cambridge, Massachusetts
B
Y THE release of information concerning the
atomlc . bomb and the Manhattan Project which was set up for its development, the attention of chemists has been called to the fact that two new elements, which do not exist in nature, have been synthesized. These transuranic elements were presaged by the work of Fermi in 1934, but the discovery of atomicfission by Hahn and Strasseman in 1938 seemed for a while to deny the possibility of their existence. However, it was possible to report in THISJOURNAL' in September, 1940, that, "On June 7, 1940, E. McMillan and P. H. Abelson were able to announce that elements 93 and 94 actually are produced, but instead of having properties thought to characterize eka-rhenium and eka-asmium, their chemical properties seem to correspond more closely to those of uranium itself. This opens the possibility that, as a consequence of inner-shell building, a nev series ofelements, analogous to, but chemically dissimilar to the rare earths, makes its appearance with element 93 as the first member." In their excellent book, "Applied Nuclear physic^,"^ published as late as 1942, Pollard and Davidson on the other hand expressed the opinion held by many scientists when they state, "It was finally proved by McMillan and Abelson that 93239 decays to 94239nria a 2.3 day beta activity. The fate of element 94'" is as yet unhown. The once-flourishing transuranic section of the periodic table has indeed fallen on evil times. ~~~t physicists seem content to let i t remain." There matters seemed to rest. Actually, what had happened was that a b l a c k - ~ had ~ t descended in 1940 on the work underway in the physics laboratories, and until the announcement by President Truman of the &t atomic bomb dropped on Hiroshima on August 6, 1945, no inkling of the feverish activity in the field of the transuranic elements was permitted to reach the public. Instead of having "fallen on evil times," by the
-
'Presented at the 232nd Meeting of the N.E.A.C.T., St. Paul's School, Concord. N. H., October 27. 1945. ' FOSTER, L. S., "What's been going on," THISJOURNAL, 17,448 (1940). a P o L L m , E., AND W. L. D~vmsoN, JR., "Applied Nuclear Physics," John Wiley and sons, Inc., New ~ o r k 1942, . chap. 10.
end of 1942 a t least a half milligram of plutonium had been isolated, and the plans of the Manhattan Project to produce it in quantity were well under way. This first lot had been formed by long exposure of uranium solutions to deutrons accelerated in cyclotrons. To place these developments in their proper sequence, i t is helpful to review what was known a t the time the veil of secrecy was drawn in 1940. Soon after the discovery of the neutron by Chadwick in 1932 and the equally important discovery of artificial radioactivity by Irene and Frederic Joliet-Curie in 1934, Fermi and co-workers a t the University of Rome exposed practically all the known elemen@to neutrons. Greatest interest wyaroused by the behavior of uranium. It became beta ray active. The only possible consequence of this type of activity is the production of elements of higher atomic number than element 92. The work was continued and repeated in other laboratories, and during the next five years transuranic elements up to element 97 (eka-gold) were announced: In late 1938 Hahn and Strassemann demonstrated that these transuranic elements were in reality radioactive isotopes of ordinary elements of atomic weight about half that of uranium. U
+ n 4 Ba + Kr + etc
The observation of fission of the heaviest elements was confirmed almost immediately in laboratories scattered throughout the world. It was noted very early that two atomic products of fission were ejected in opposite directions a t extremely high velocities, thus carrying enormous quantities of energy of the order of magnitude of 200 million electron-volts, equivalent to 4.6 X 1012 calories per mole (238 grams) of uranium. Fission of one pound of uranium produces the energy equivalent to burning 1500 tons of coal or 200,000 gallons of gasoline. In April, 1940, Nier of Minnesota with Booth, Dunning, and Grosse of Columbia announced that submicroscopic quantities of the natural isotopes of uranium, U-234, U-235, and U-238, had been separated by means of a mass spectrograph and individually exposed
619
captured by an impurity, such as boron. Thus the question of whether a chain reaction does or does not go depends on the result of a competition among four processes: (1) escape, (2) non-fission capture by U-238, (3) non-fission capture by impyrities, and (4) capture resulting in fission. If the loss of neutrons by the lirst three processes is less than the surplus produced by the fourth, the chain reaction occurs; otherwise i t does not. Evidently, any one of the first three processes may have such a high probability in a given arrangement that the extra neutrons created by fission will be insufficient to keep the reaction going." This is true in ordinary massive uranium, and the solution of the problem of the utilization of atomic energy resolves itself into a reduction of the losses of the extra neutrons by such processes. To set up a self-sustaining reaction, one may proceed in any of several ways. (1) The isotope, U-235, may be separated from U-238 and from impurities which absorb neutrons, so that most of the extra neutrons (fast) resulting from fission of one atom of U-235 are absorbed by other atoms of U-235. This has actually been accomplished and the violence of the result is testified to by the devastation of Hiroshima. (2) The neutrons resulting from fission may be slowed down by means of non-absorbers of neutrons (so-called "moderators") such as heavy water, beryllium, graphite, etc., until their energies are in the range of ordinary gas molecules under laboratory conditions. U-235 has a much greater tendency to absorb slow neutrons than fast, and a larger percentage of the fission-neutrons can thus be made to enter into new fission reactions before being absorbed by atoms pf U-238. In the presence of moderators the chances for continued fission of U-23.5 are markedly en258 U gamma ray U :n --+ hanced. If the amount of uranium is made large enough 239 and pure enough so that losses of neutrons by escape Np + c + gamma rays 23 min. and by reaction with impurities are minimized, the re239 action does become self-sustaining in the presence of a 93 Np 239 94 PU -7 6 gamma rays 2.3 days moderator. The assembly of -uranium and moderator 259 PU Y ~ u :He gamma rays is called a "pile." In a pile U-235 is consumed and is long the source of energy, and due to absorption of fission It is this series of reactions with which we are con- neutrons, much U-238 is ultimately converted to plutoncerned in a discussion of the production of the new ele- ium, '$ Pu. This process is discussed in greater detaiI ments, neptunium 2t: Np and plutonium :2 Pu. in a later section. (3) Under bombardment of neutrons, Possibilities of Chain Reaction. In the mixture of iso- the susceptibility of Pu-239 to fission is similar io that of topes which constitute ordinary uranium, there is evi- U-235 and i t also can be made chain reacting. It was dently a competition between U-235 and U-238 for realized quite early that if sufficient plutonium could be neutrons of any velocity. Because of its predominance separated from the uranium in a self-sustaining pile, i t and its greater abiity to absorb fast neutrons, U-238 is also could be used as the basis of an atomic bomb. The much more likely to be converted to transuranic spe- trial bomb set off in New Mexico on July 16, 1945, and cies than is U-235to undergo fission. Smyth has ~ t a t e d : ~the bomb released over Nagasaki on August 9, 1945, "If oneneutron causes a fission that produces more than utilized the newly synthesized element in just this way. one neutron, the number of fissions may increase tre- (4) Other elements, namely, thorium and protoactinium, mendously with the release of enormous amounts of also undergo fission when bombarded by fast neutrons. energy. It is a question of probabilities. Neutrons Thorium is not a scarce element and may eventually be produced in the fission process may escape entirely from applied, but protoactinium is too rare to be considered. the uranium, may be captured by uranium in a process Because of the great susceptibility of the uranium isonot resulting in fission (production of Z3U), or may be topes to slow neutrons and the promise of success of SMITH, H. D., " ~ t o m i cEnergy for Military Purposes," inducing a chain reaction among them, little attention has yet been given to the possibilities of thorium. Princeton University Press, Princeton 1945, paragraph2.3. to neutrons. It was found, as had been predicted by Bohr earlier, that of the three isotopes only U-235 underwent fission and only with slow neutrons. U-238, contrariwise, was capable of absorbing both fast and slow neutrons and in both actions was converted to the unstable isotope U which was rapidly converted to elements 93 and 94 by emission of negative electrons. U-234 is so rare that i t may be ignored. U-235 occurs to an extent of only 0.7 per cent in ordinary uranium, and to be utilized for atomic power or in atomic bombs would have to be separated from the main component U-238. In 1940 i t was estimated that to separate a single pound of U-235, a t least 75,000 years would be required. By improvement in instruments and by the use of large numbers of them, as well as by development of new methods of separation, this pessimism was demonstrated to have been unwarranted. Action of Fast and Slow Neutrons on Uranium Isotopes. Fission of U-235 is accomplished most effectively by slow or thermal neutrons, that is, neutrons which have about the same kinetic energy as ordinary molecules a t usual laboratory temperatures. It is now known, on the other. hand, that i t is also rendered fissionable by fast neutrons, a condition of prime importance in producing fission in pure U-235 a t an explosive rate. Slow reactions are not suitable for bombs which must explode extremely rapidly, in microseconds. U-238, as has been pointed out, can absorb both fast and slow neutrons without undergoing fission. The sequence of reactions which is set in motion by neutron bombardment of U-238, together with the half-lives of the various atomic speries resulting, is given by the following equations:
2z
,, +
'' -,, ,,
-
-
+ 4 + +
+
+
Construction and Operation of a Pile. In order to increase the probability that a fission neutron will be absorbed by an atom of U-235 rather than one of U-238, the neutron has to make many collisions with the atoms of a moderator before it is returned to the uranium mass. The first design of a pile to accomplish this was to imbed cubic lumps of pure uranium in a lattice or matrix of pure graphite. A small unit of this type was set up a t the University of Chicago, and on December 2, 1943, the first self-sustaining fission chain reaction ever produced was initiated. The pile contained over six tons of metallic uranium and additional cubes of uranium dioxide, separated by blocks of graphite. It was provided with safety shields to protect the operators from radiation and with control rods of cadmium to absorb excess neutrons and thus keep their concentration a t a safe level. Because of its location under the west stands of the football stadium, once i t was established that such a pile would be self-sustaining, it was kept in operation only for a short period. Too many neutrons were escaping and made walking by the stadium potentially dangerous. By November, 1943, a larger air-cooled pile was in operation a t the Clinton Laboratories a t Oak Ridge, Tennessee. Construction of still larger water-cooled piles was underway a t Hanford, Washington, in 1943, and the first large unit went into operation in September, 1944. Countless complex problems had to be solved by the research groups and a gigantic engineering program had to be set in operation to carry the prodnction of plutonium from a microgram scale in 1942 to a kilogram scale in 1945. The details of these developments constitute the major part of the Smyth Report, referred to above. When U-235 undergoes fission, in addition to creating the neutrons used in producing new fissions and converting U-238 to plutonium, numerous fission products are simultaneously accumulated. Becyse these are all highly radioactive, they render operation of a pile an extremely hazardous task and require"adoption of proper safeguards, the most obvious of dchich are the thick walls of concrete which surround one. As a result of the constant neutron bombardment in the pile, these fission products, add a difficulty of the first order to the separation of plutonium from the uranium in which it is now located, not only from the standpoint of the hazard but also by the intricate chemical operations necessitated by their interferences. The dissolving of the uranium, the separation and concentration of the plutonium, and its final purification are operations which have to be camed out by remote control. In order to design the plants for doing these on a large scale, a tremendous effort was expended in establishing the optimum operating conditions, using only the micro quantities of plutonium available a t the time this had t o be done. The chemistry of the new element, of which less than a milligram was available, was better known than that of many of the natural elements. This could be true, of course, only because of its radioactivity by which it is readily detected and traced
through a series of chemical transfomations. Because of the military urgency, it was necessary to design and build large operating plants without the customary intermediate experience of pilot plant operation, often on the basis of only radio-tracer experiments. So certain were the data, however, that with few exceptions the efficiency of the plants was higher than the estimates, and few alterations in plans were required. Relationship Between Pourer Output of a Pile and Rate of Formation of Plutonium. The overall equation for the fission of U-235 is :
In a stable self-sustaining pile, as many neutrons are being used up as are being evolved. Parallel with the production of neutrons, there is a corresponding amount of energy produced of the order of magnitude of 200 Mev per fission. This may be divided into two parts, Q the amount of energy which is evolved a t the time of fission, 160 Mev, and the energy contained in the fission fragments A and B which undergo radioactive decomposition to form stable isotopes with the gradual evolution of an additional amount of energy, 40 Mev. Dependent upon the efficiency of the design of a pile, the production of a gram of plutonium per day would be accompanied by the release of energy a t a rate up to 1500 kilowatts. The power level of the Clinton Pile is of the order of 2000 kilowatts, but even a t the high power level of the Hanford pile, only a few grams of U-238 and U-235 are used up per day and the small amount of plutonium formed must be separated from about 1,000,000 grams of residual uranium. It is obvious from these figures that a pile of this type operating a t a low temperature cannot yet compete economically with coal and other types of fuel as a sotrce of power. This problem has been discussed in an interesting summary by the McGraw-Hill editorial staff." Chemistry of Plutonium and Methods of Separating it from Uranium. In the early work on elements 93 and 94, the first species identified is now known to have been ,'3! Np which was isolated through the use of tracer methods by McMiian and Abelson from uranium which had been bombarded for months by deuterons in the Berkeley cyclotron.
As was pointed out in the introduction, this isotope of neptunium was known to be a beta emitter, but element 94 was not detected. .Later Seaborg, Kennedy, and Wahl a t the University of California were able to establish the existence of 8,': PUand by tracer methods to study its chemical behavior. This isotope of plutonium has a shorter half life than '2 Pu and because of its
'
"The Atom-New Source of Energy,'' printed in the September issue of all McGraw-Hill magazines; cf., Chw. &Met. Eng.. 52,93 (1945).
ford were designed for a precipitation process. Quoting Smytb? "Most of the precipitation processes which have received serious consideration made use of an alternation between the (IV) and (VI) oxidation states of plutonium. Such processes involve a precipitation of Pu(IV) with a certain compound as a carrier, then dissolution of the precipitate, oxidation of the Pu to the (VI) state, and reprecipitation of the carrier compound while the Pu(V1) remains in solution. Fission products which are not carried by these compounds remain in solution when Pu(V1) is precipitated. The fission products which carry are removed from the Pu when i t is in the (VI) state. Successive oxidation-reduction cycles are carried out until the desired decontamination is achieved. The process of elimination of the fission products is called decontamination, and the degree of elimination is tested by measuring the change in radioactivity of the material." "A much greater and more uncertain step [than the operation of the pile] was required in the case of the separation process, for the Clinton separation plant was designed on the basis of experiments using only microgram amounts of plutonium. . . . Nevertheless the separation process worked! The lirst batch of slugs from the pile entered the separation plant on December 20, 1943. By the end of January, 1944, metal from the pile was going to the separation plant a t the rate of ton per day. By February 1, 1944, 190 mg. of plutonium had been delivered and by March 1, 1944, several grams had been delivered. Furthermore, the efficiency of recovery a t the very start was about 50 per cent and by June, 1944, it was between 80 and 90 per cent." "The success of the separation process a t Hanford has exceeded all expectations. The high yields and decontamination factors and the relative ease of operation have amply demonstrated the wisdom of its choice as a process. This choice was based upon a knowledge of plutonium chemistry which had been gleaned from less than a milligram of plutonium. Further developments may make the present Hanford process obsolete, but the principal goal, which was to have a workable and efficient process for use as soon as the Hanford piles were delivering plutonium, has been attained." Future Applications of UraniumPiles. In addition to the heterogeneous pile discusSed in the foregoing, other types have been designed and a t least one bas been constructed. This is one in which heavy water is used as a moderator. This has been explored most actively by the Canadians. The most striking feature is the smaller size which is permitted by the change in technique. Types of piles to be used as power sources, which must be operated a t higher temperatures for good efficiency, are under consideration. Radio elements are bv-oroducts of the fission of ura' There seems to be no experimental evidence for the sug- nium. Moreover, the uranium pile is the richest known gested revision of the seventh period recommended in the fola suggested revision of the seventh Source of neutrons, and nuclear reactions which can be lowing: G.E, VELm, period of the periodic table," THIS JOURNAL, 19, 286 (1942); accomplished on a small scale in the beam of a cyclotron "A suggested revision of the position of thorium in the fourth can be carried out more efficaciously in a pile. The period of t h e periodic table," ibid., 19,329 (1942); 1.A. BABOR, greater radioactivity is easier to detect. s' a result of tracer scale experiments, by 1942 methods of handling micro amounts of plutonium had been developed to the point that 0.5 milligram of ' ~ P uhad been obtained in the form of pure salts by separation from uranyl nitrate which had been bombarded by deuterons. The study of the chemistry of plutonium on a tracer and micro scale was continued a t the Metallurgical Laboratory of the University of Chicago and elsewhere and provided the basis for the plant methods adopted for pile operations. The position of a new rare earth type group in Period 7 of the Periodic Table has long been a subject for conjectnre.6 In 1940, as has already been mentioned, i t was evident that neptunium and plutonium were more similar to uranium than to the third group elements (elements 57 to 71 and 89), and this early tracer observation has been strengthened as the chemistry of these elements has been elaborated on a larger and larger scale. It is now known that it possesses four states of oxidation, 3+, 4+, 5+, and 6 + . In the highest state it forms plutonyl derivatives with the positive radical, Pu02++, corresponding to the uranyl derivatives of uranium of which uranyl nitrate, U02(NO&, is a familiar example. Because of differences in the oxidation potentials of uranium and plutonium between the valence states, by selection of proper conditions it is possible to oxidize or reduce one and not the other, so that easy methods of separating them could be worked out. Recovery of plutonium from uranium which has served its function in a pile is complicated, however, by the presence of the radioactive fission products which interfere with the separation procedures. The radio elements formed are isotopes of stable elements lying between mass numbers 127 and 154 and between 83 and 115, wbich decay by beta emission accompanied by gamma rays. The longer the pile has run, the larger is the concentration of plutonium and the larger the concentration of fission products. Afcter the pile has been shut down, the radioactivity continues, but a t a diminishing rate until only the more stable species remain. Neptunium is converted to plutonium fairly rapidly. It is from such a complicated system that plutonium must be separated in pure form. The first successful method of separation was one of co-precipitation with a carrier which permitted the removal of most of the plutonium from the uranium and many of the fission products in a single step. Methods based on adsorption and solvent extraction have been developed wbich may supplant the precipitation method either in the main process or in recovery of valuable byproducts and residual uranium. The semi-works a t the Clinton Laboratories and the large scale plant a t Han-
A .
"A perlodlc table based on atomic number and electronic conlipration." ibid.. 21, 25 (1944).
' SMYTH, JOG. cit., pp. 138, 144, 193.
material to be altered is inserted (in a suitable container) intothe pile and removed after the desired time of ex. posure to the neutron flux has elapsed. By this method large amounts of radio elements will be made available for tracer work in all fields of science. William L. Laurence has written: "With this power a t his dispos81, man for the first time stands close to 'remold his world nearer to his heart's desire.' The chemist, the physicist, the biologist, and the engineer are on the threshold of new worlds: Instead of being circumscribed by the basic elements found in nature, they can now create new elements to order, elements that could be used for better, richer, healthier, and more abundant life." Nnu York Times, October 9, 1945, p. 6. ADDITIONAL REFERENCES Prewar erticles DARROW, K . K . , "Nuclear fission,"Bell System Tech. J., 19, 267 (1940). POTTER, R . D., "Is atomic power at hand?" Sci. Monthly, 50,570 (1940). TmER, L. -Nuclear fission,xsReo. ModerlZ Phys., 12, (1940).
Recent popular books based upon the Smyth report GEDDES.D. P.,ET u,"The atomic age opens," Pocket Books, New York, 1945. DIETZ,D., "Atomic energy in the coming era," Dodd. Mead & Company, New York, l945. O'NEILL,J. J., "Almighty atom." Ives Washburn, Inc., New v -t. z
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lose
YATES,R. F.. "Atom smashers," Dedier Publishing Company, York,
Notes The N.E.A.C.T. will meet on February 2 a t the Rhode Island College of Education, Providence. The 1946 Summer Conference will be held at Middlebury College, Middlebury, Vermont. Word has been received of the death, on September 27, 1945, of Mrs. Elwin Damon, wife of the late 19th president of the N.E.A.C.T. Dr. Andrew J. Scarlett went to France on July 4 to teach in the American University a t Biarritz, which has 4000 students. He reports having an interesting and busy time designing, building, and equipping the chemistry laboratory.
[Editor's Note: Since this papwwas prepared, two important developments have been released: the fact that plutonium occurs in nature in small amounts, and that two additional elements, 95 and 96, have been synthetized.]
