Nuclear recoil as a means of fission product separation

J. Inorganic~,ld NuclearChemlloy.1956,eel. 2. pp. 180--183. ~ n

Prcu Ltd.. London

NUCLEAR RECOIL AS A MEANS OF FISSION PRODUCT SEPARATION* RICHARD WOLFGANG Chemistry Department, Brookhaven National Laboratory, Upton, Long Island, N.Y.

(Received 9 August 1955) A b s t r ~ - - I t has been demonstrated that,.by use of a suitable heterogeneous system, the nuclear recoil of fission products can be used to effect their complete separation from the parent fissile material. The potentialities of this simple technique are briefly discussed.

McMILLAN'S pioneering study Cll of the recoil of uranium fission products led to his 1939 discovery of neptunium activity. He had irradiated a thin film of uranium oxide with cyclotron neutrons and found that most of the fission products had recoiled out of it. This separation made it possible to recognize the strong 2.3-day activity of the Np 2s° that had been left behind. Since that time the momentum properties of fission products have been intensively investigated with a view to determining kinetic energies, angular distributions, and modes of energy loss. However, little further attention appears to have been given to the practical potentialitids inherent in fission recoil for the development of important separation techniques. ~2l If a fission reaction is carried out in particles of fissile materials with maximum dimensions smaller than the range of the fission products, these products will necessarily recoil out of the parent material. On the other hand, the heavier nuclei formed by neutron capture and beta decay will have a very much shorter recoil range and will remain within the original particle unless it is extremely small. This means that in a suitable heterogeneous system the fission products will completely separate themselves from the actinides, eliminating the need for complex chemical processing. This paper reports on a simple experiment carried out to test the practicability of such a procedure. EXPERIMENTAL The minimum r a ~ c of fission fragments from plutonium is 1-9 cm in air ~S~ (760 mm 15°C). From this it can readily be calculated that the equivalent minimum range in UsO8 is 8 microns. Assuming that the U aa~ fission recoils do not differ significantly from those from plutonitun, we can predict that all fission products produced in U308 will recoil out of any particle smaller than 8 microns. Fine UsOB particles were produced by grinding in a mortar and applying a simple sedimentation procedure. The particles thus produced were fairly uniform and somewhat less than 1 p in diameter. Despite this small size, the U3Oa still tended to settle out of a water suspension. In order to maintain a suspension without having to provide a means of agitation while irradiating, the particles were added to a h o t 3 per cent gelatin solution which was then chilled with stirring to produce a uniform distribution of UsO8 in the gel. About 10 ml of such a gel containing approximately 3 mg of UaOs were irradiated in the central portion of the Brookhaven reactor for about a minute. After irradiation the gel was liquefied by warming in a hot-water bath. The mixture was centrifuged, and portions of the supernate were removed to aluminium planchets for drying and counting. * Research performed under the auspices of the U.S. Atomic Energy Commission. ~1~ E. McMILLAN,Phys. Rev. 55, 510 (1939). t2~ F. JOUOTused a technique very similar to that of McMILLAN to obtain pure fission products which had recoiled into filter paper from layers of uranium oxide. Compt. Rend. 218, 733 (1944). ~a~S. KATCOFF,J. A. MlSKEL,and C. W. STANLEY,Phys. Rev. 74, 631 0948). 180

Nuclear recoil as a means of fission product separation

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The solid U=O8 in the bottom of the centrifuge cone was washed three times with water, then transferred to an aluminium planchet for drying and counting. The counters used were end-window gas-flow proportional beta counters. Pure 3 per cent gelatin solution containing no uranium was similarly irradiated and counted. The activity of this control sample never exceeded 3 per cent that of equivalent gelatin samples which had contained UsOs during the bombardment.

RESULTS Three separate runs were carried out. In two of these the suspension was neutral and in the third it had been made approximately 0.03N in HNOs. All these experiments gave essentially identical results and can thus be considered together. lOt

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]zIG. 1.--Decayof activityin supernateand uranium oxidefractions. The decay curves of the activity in the supernate and of the activity remaining in the UsOa are given in Fig. 1. The uranium oxide shows 25-rain U ~a9 activity decaying cleanly into its 2.3-day Np ~9 daughter. The decay of the Np z39 can then be followed for over fifteen half-lives before it finally dies out, leaving only a small amount of long-lived activity (apparently UX 1 and UXz in secular equilibrium with U~as). No contribution of fission products to the decay of these samples was detected. On the other hand, the decay of the supernate fractions is typical for gross fission products. As can be seen from Fig. 2, the function A --~ C t - v 1 6

(where A is activity, C a constant, and t time) is approximately followed. These results are exactly what would be expected if all the fission fragments left the uranium phase and entered and remained in the aqueous phase; while the U za9 and Np =a with the much smaller recoil energy imparted them by gamma and beta emission remained in the uranium particles. By comparing the maximum amount of fission

182

RICHARD WOLFGANG

product activity that could have been present in the uranium samples with the fission fragment activity of the supernate fractions, it can be concluded that a separation of at least 99 per cent of the fission product from the uranium particles has been effected. The fact that the fission fragments left the uranium particles is not surprising: it could hardly have been otherwise, in view of the known data on the recoil energy. It is more remarkable that the fission fragments remained separated, instead of becoming adsorbed on the surface of the uranium oxide particles. This was the true test of the practicability of this recoil method of separation. IO0,O00

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Fro. 2.--Log-log plot of decay of fission products in supernate fraction.

This experiment has demonstrated that, by making use of fission-product recoil, pure actinide activities such as Np ~39 and carrier-free gross fission products can be prepared with great ease. While this technique will be useful in the laboratory preparation of such materials, its application to reactor design may be much more important. Reactors may be fuelled with a dispersion of gaseous, liquid, or solid fissile material in a liquid or solid matrix. Such a system would automatically and continuously separate its fission products from the fissile into the other phase. Complex chemical processing is eliminated, and only a simple mechanical procedure such as filtration and centrifugation would be required to complete the elimination of the fision products. (4) Furthermore, the separated fission products are in a concentrated carrier-free form, suitable for disposal or for use as radiation sources.

Acknowledgement--The author

gratefully acknowledges the help and advice of MORRIS SLAV1Nof this laboratory. t4) In the case of a solid in solid suspension it may be convenient to use a simple chemical solution procedure to dissolve one o f the two phases o f the mixture.

Nuclear recoil as a means of fission product separation

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Note added in proof--At the International Conference on the Peaceful Uses of Atomic Energy, a Netherlands group (H. DE BRUYN et al., A/CONF. 8/P/936) disclosed that irradiation of a suspension of fine UO2 in pure water cauzed several fission products (Ba, Mo, Cs, and I) to appear at least partially in the aqueous phase. It is particularly interesting that readsorption of fission product on the surface of the fuel particles was observed, while in the work reported in this paper, this did not" occur. This suggests that the gelatin used in the present work efficiently sequestered the fission products, and thus prevented their readsorption. Undoubtedly, a large number of substances could be used as complexing agents, instead of the gelatin, and would also ensure complete separation.