Gallium removal during pyrochemical reprocessing of weapons plutonium into oxide fuel

Abstract

It is well known that plutonium is a long-lived and renewable source of energy. In France, power plutonium obtained mainly after regeneration of irradiated oxide fuel is used as a mixed fuel. However, the use of such fuel in thermal reactors is too inefficient [1]. Ultimately, fast reactors which will make it possible to use plutonium cost-effectively and in an ecologically well-substantiated manner as well as weapons and tailings uranium in the form of mixed fuel will play the dominant role in nuclear power.Weapons plutonium is a plutonium-gallium alloy [2][3][4][5]. The addition of several percent gallium to plutonium is necessary to stabilize the δ modification of plutonium and to impart the required mechanical properties to weapons plutonium. The utilization of weapons plutonium in nuclear fuel and, specifically, its conversion into oxide fuel is a topical problem [6]. The reprocessing of weapons plutonium into a disperse PuO 2 powder by a water-chemical method involves a large number of operations and a large amount of wastes. A more efficient method of reprocessing weapons plutonium is the pyrochemical process, which contains an intermediate operation where weapons plutonium is hydrided by hydrogen and the plutonium hydride is oxidized by the oxygen present in the mixture together with inert gases and nitrogen. The gallium present in weapons plutonium is oxidized to Ga 2 O 3 .The effect of gallium oxide impurities on the fuel-element cladding materials (radiation-and corrosion-resistant steel and zirconium alloys) when mixed uranium-plutonium fuel is used has not been adequately studied. Consequently, it is recommended that its content in mixed fuel not exceed 0.001 mass% to prevent cladding corrosion inside fuel elements [4]. In this connection it useful to evaluate the possibility of removing gallium during pyrochemical treatment of PuO 2 -Ga 2 O 3 powder and sintering of mixed uranium-plutonium oxide fuel kernels in a hydrogen atmosphere.Thermodynamic Analysis of Hydrogen Reduction of Gallium Oxides. A thermodynamic analysis of the reduction of gallium oxide was performed to evaluate the possible removal of gallium from plutonium oxide by heat-treatment in a hydrogen atmosphere. Figure 1 shows two variants of the pyrochemical reprocessing of weapons plutonium into plutonium dioxide powders. One variant consists in hydriding weapons plutonium by hydrogen (at comparatively low temperature to 500 K) and oxidizing the products so formed in a mixture with inert gases to PuO 2 and Ga 2 O 3 . In the other variant, nitriding plutonium by nitrogen, which yields plutonium nitride, followed by oxidation can be added to increase safety. Thermodynamic modeling was performed using the IVTANTERMO (Russia) program system [7]. The data published in the handbook [8] were used for the thermodynamic calculations.

Pyrochemical method of salvaging weapons plutonium in oxide for fabricating mixed fuel for fast reactors

“…It was determined that when fuel kernels consisting of plutonium oxide of mixed uranium oxide with plutonium, which are fabricated using weapons plutonium, are sintered in an atmosphere of hydrogen or a mixture of hydrogen and an inert gas at temperatures to 1650°C, gallium oxide Ga 2 O 3 is reduced to Ga 2 O [4], which sublimates. It was determined that inert gas need not be added, since there was no hydrogen inside the volume of the apparatus.…”

“…Gallium can be removed in this variant of fuel fabrication [3,4].Americium is removed from plutonium oxide in order to bring excess plutonium into the fuel cycle and use it to fabricate mixed fuel for power reactors, first and foremost, thermal reactors, provided that the process is radiologically safe [2,5]. Weapons plutonium was hydrogenized by hydrogen at 220°C, after which the plutonium hydride obtained was acidified at 550-560°C with the formation of PuO 2 .…”

“…Gallium can be removed in this variant of fuel fabrication [3,4]. The initial plutonium and uranium oxide powders, from which after mixing, granulation, and pressing blanks kernels were sintered in a hydrogen-containing atmosphere at 1600-1750°C.…”