Gallium Removal from Weapons-Grade Plutonium and Cerium Oxide Surrogate by a Thermal Technique
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as a reasonably good surrogate for PuO 2. ,[1-11]. In addition, a well-characterized surrogate outside of a glove box environment aids evaluations for pilot or full-scale production equipment, thus preventing unnecessary radiation exposure to personnel, unnecessary generation of radioactive waste, and costly errors that could be associated with setting up a large-scale system in a glove box without the advantage of prior operational checks. Thus, the primary focus of this paper is understanding the fundamentals of Ga removal in the PuO2 feedstock through the use of a CeO2 surrogate. EXPERIMENT CeO 2-based MOX surrogate feedstock material was prepared by blending, sintering, and milling CeO 2 plus 2 wt % Ga 20 3 powders. The overall process is summarized as follows. The desired quantities of CeO 2 and Ga 2 0 3 powder were measured and blended, and vibration-milled in a small SPEX mixer 8000 for 20 minutes. The powder was pressed into green pellets to ý35% of theoretical density. The green pellets were milled again, sifted through a -150 pin screen, and then pressed into 1.1 cm x 0.65 cm diameter, -65% dense pellets. The pellets were then fired at 450'C for 4 h to remove the binder and were immediately sintered at I650'C for 2 to 6 h in air. After sintering, the pellet density was measured by the Archimedes method. The sintered pellet was then re-crushed and sieved through a 150prm screen. This powder was used as a starting 129 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
material. Weapons-grade plutonium, Pu-1 wt% Ga, was converted to oxide using a three-step process; metal to hydride, hydride to nitride, and nitride to oxide. The average particle size of plutonia was 40 pim. Gallium removal studies were performed by exposing different powder mass sizes of the surrogate feedstock as well as complete pellets to ultra-high purity Ar and Ar-6% H2. In this paper we report the results of experiments between plutonia and ceria performed in the temperature range of 600'C to 1200'C as a function of time (0.5 to 12 h), gas flow rate (1.5, 3.0, and 6.0 cmns), and sample size (0.3, 0.9, 2.5g powder and pellet). Samples were placed in high purity alumina crucibles, which is in turn were placed in alumina tube furnace. The samples were heated to temperature of interest at 20°C/min with controlled flow rate. RESULTS AND DISCUSSION The amount of Ga in the CeO 2 -2 wxt% Ca20
3
surrogate has been reduced about 50% from
14500 wppm to 7800 wppm during sintering in an air atmosphere[12]. It is apparent that the amount of Ga after sintering provided a surrogate with a Ga content comparable to that in weapons-grade PuO 2 (8700 ppm). Figure 1 shows the sintering effects on theoretical density for CeO 2 and the CeO 2-2 wt % Ga 2 0 3 surrogate. The densities of surrogates are larger than those of pure CeO,, which implies that Ga plays a role as a sintering aid. Figure 2 shows the corresponding morphology from the cross section of the sintered surrogate pellet. The micrograph shows agglomerated grains and pores d
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