Fission recoil impregnation of graphite powder with Xenon-133

Carbon

1963, Vol. 1, pp. 85-87.

Pergamon

Press Ltd.

Printed in Great Britain

LETTERS TO THE EDITOR microns. The density of the graphite is 2.1. Each capsule contained about 2 g of the hexahydrate. The capsules were irradiated in the Penn State Reactor for 1 hr at a thermal neutron flux of 2~ 10” n/cm*/sec. In this neutron flux 9 W of fission heat is generated in the capsule which is low enough to avoid melting of the nitrate during irradiation. Graphite will settle if melting occurs. Our study was restricted to graphite volume fractions up to 15 per cent. There are two reasons for this restriction. One is the physical difficulty of dispersing the graphite in molten hexahydrate when the volume fraction exceeds 15 per cent. High viscosity and poor wetting of the graphite prevent good mixing. The second reason concerns (1) the probability of surrounding all the particles with a zone of the hexahydrate, and (2) the thickness of this zone. A low volume fraction favors a high probability and a zone thick relative to the recoil range in hexahydrate. It is estimated that the ranges in graphite and the hexahydrate are roughly the same (15 FL). The graphite is separated from the irradiated casting by heating the latter in warm 6 N nitric acid for 1 hr followed by filtration of the suspension. This acid treatment is necessary to remove all but a trace of uranium from the graphite. Although hot water alone will readily dissolve the casting, removal of uranium is far from complete in hot water even after prolonged heating. A separate study showed that the graphite does not lose a significant amount of xenon at hot water temperatures or under ambient conditions in the open air. For the xenon-133 determinations reported below, a lo-50 mg sample of the separated graphite is burned in oxygen and the combustion gases in a sweep gas are

Fission Recoil Impregnation of Graphite Powder with Xenon-133 (Received29 October 1962) DIFFUSIONof xenon from graphite powders is currently under investigation in this laboratory. We wish to report a convenient method of impregnating the graphite powder with radioactive fission-product xenon-133. The method involves the recoil impregnation of graphite with this species during thermal neutron irradiation of a mixture of graphite and water-soluble uranium nitrate hexahydrate. The hexahydrate has a density of 2.8 and contains 47 weight per cent uranium of normal enrichment. After irradiating the mixture, the graphite is separated by dissolving the uranyl nitrate. When the mixture is made by simply mixing the powdered components, the bulk density is low and the local distribution of uranium around the particles is random. A more compact and easily handled mixture with better uranium distribution around the graphite particles is achieved by casting a mixture of molten hexahydrate and graphite. The casting method and its evaluation are reported below. The casting method is very simple. Weighed amounts of graphite powder and hexahydrate crystals, in the desired proportion, are placed in a test tube in a water bath at a temperature slightly above the melting point of the hexahydrate (60°C). On melting, the uranyl nitrate dissolves almost completely in its water of crystallization to give a freely-flowing liquid for suspension of the graphite powder. The graphite is readily suspended in the liquid after brief stirring. The graphite does not settle rapidly. An aliquoit of the suspension is pipetted into a capsule of 8 mm Pyrex glass immersed in the water bath. The capsule is removed from the water bath, cooled for several minutes in air, and finally chilled rapidly under running tap water to crystallize the hexadydrate. After sealing the capsule with a torch, it is ready for irradiation. This procedure produces a one-piece, .dense casting with few voids. A void extending along the entire length of the casting centerline is sometimes observed. Voids are not important as long as they are not accompanied by gross separation of the graphite from the nitrate matrix. Usually, the concentration of graphite is visibly higher at the bottom of the casting than at the top but, as the data below suggest, this separation is not serious. For an evaluation of the method four castings 6 mm dia. and 25 mm long containing different volume fractions of Madagascar natural graphite were made, irradiated, and the xenon-133 content of the graphite determined. Particle size distribution of the graphite is shown in Fig. 1 where F is the weight fraction of the powder having a particle diameter greater than D

FIG. 1. Particle size distribution. 85

LETTERS

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EDITOR

powder will depend on the details of particle shape and particle-size distribution. This work was supported under AEC Contract AT(30-l)-2792. Nuclear Engineering Department, Pennsylvania State University, University Park, Pennsylvania

Determination

of Degree of Crystallite Orientation in Graphite Products (Received

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FIG. 2. Specific activity and capture efficiency passed through a NaI(T1) scintillation flow detector. A recording, single-channel y-ray spectrometer set on the full 80 keV photopeak of xenon-133 permits the absolute assay of the xenon by the total count method. Detector efficiency is determined by irradiating small crystals of the hexahydrate with cobalt flux wires, dissolving the crystals in water, and sweeping the known amount of xenon-133 through the detector. This combustion method was adopted after finding that direct counting of the graphite in a well crystal gave very poor precision. The 80 keV photopeak is readily observed in a well crystal, but the other fission product activities present make quantitative measurements difficult. In Fig. 2, the specific activity S and the capture efficiency E are plotted versus the volume fraction of graphite I’ in the capsule. S is based on the combustion results and is expressed in atoms of xenon-133 present in 1 g of capsule graphite 5.2 days after fission. Selection of this latier capsule age for the comparison of S values is arbitrary but convenient. E is the amount of xenon captured by all graphite in the capsule divided by the total amount of xenon present in the capsule, and is independent of capsule age. 5, the weight of the casting, the number of fissions, and the fission yield permit the calculation of E. The casting is assumed to have the same composition as the solution from which it is prepared. Number of fissions is based on the radioassay of cobalt wires attached to the capsules. Flux depression in castings of this size is negligible and no correction of the flux wire data is required. The familiar parentdaughter relationship for the iodine-xenon pair is used to calculate the time dependent xenon content of the capsule following irradiation. Fig. 2 shows that within experimental error S is independent of and E is directly proportional to the volume fraction, in agreement with theory for particles well dispersed in a fissioning matrix. For a powder consisting of extremely small particles the slope of E vs. V should obviously approach unity (broken line). The observed slope is about 0.14, reflecting the effects on E of particle size and shape, and the range of fission recoils in graphite and the hexahydrate. Accordingly, the slope observed with a different graphite

J. 0. SCHIFFCENS W. S. DIETHORN

4 January

1963)

IN ORDER to find the orientation of crystallites in graphite products, BACON(~) developed a technique for determination of a corresponding “orientation function” and by simple geometrical considerations derived an “anisotropy factor” which is a measure of the degree of crystallite orientation. The present authors@) have shown subsequently that, using a diffractometer, the orientation function can be determined more easily and more accurately. This approach was fundamentally the same as the one used later by GUENTERT@), BLACKMAN and UBBELOHDE@)and BRAGG@)for pyrolytic graphite. The procedures proposed by these investigators are easily applicable to such products as fine-grain or pyrolytic graphites but unfortunately, they require use of complicated devices in case of coarse grain graphite products. A technique proposed by ALI et al.@) can be used for coarse grain products but is liable to produce errors since a deviation in the inclination of specimen from an intended inclination can easily occur in the process of preparation of specimen slabs. In this note the authors propose the use of a new anisotropy factor y: the “orientation ratio”. Although as it will be seen, this is an approximate expression, this factor shows a good correlation with the anisotropy of physical properties of graphite in case of coarse grain products. If the intensities Isor and Ino of the (004) and (110) diffractions are determined for a slab by the usual reflection method and if E is defined as E = Iaooc/ (1osp+ crlrlo) its value will be approximately proportional to the number of crystallites with layer planes parallel to the surface of specimen slab. Here C! stands for the ratio of the products of structure factors, Lorentz-polarization factors, absorption factors and relative multiplicity of the (004) to (110) diffraction. A slab with its plane perpendicular to the extrusion or compression (molding) direction is designated as F, and slabs with their planes parallel to this direction and under right angle to each other-tangential plane and radial plane in the case of a cylindrical product-are designated as T and N respectively. The values of E for slabs F, T or N are represented by up, &r and Ed respectively. In the case of an extruded product, a term ye defined as ye = ET/EF (or ye = &N/EF, since &T = EN) is the ratio of the number of the crystallites with layerplanes parallel to the extrusion direction, to the number of those perpendicular to the same direction. In the case