Extraction of Uranium Nitride Pellets During Extraction of Lead from Brest Reactor Fuel-Element Simulators by Melting

Abstract

Spent fuel from the BREST-OD-300 fast reactor, which is now under development, is supposed to be reprocessed directly at the nuclear power plant by a water-free technology. The first operation is separation of the fuel (uranium and plutonium mononitride pellets) from the jacket. In BREST-OD-300 reactor fuel elements, the gap between the fuel pellets and the steel jacket is filled with lead. There are several known methods for separating the stainless-steel jacket from ceramic pellets: mechanical cutting [1], melting steel at 1500 °C [2], and dissolving steel jackets in low-melting metal melts [3]. Mechanical cutting of fuel elements filled with congealed lead will not separate the fuel and the jacket. Melting a steel jacket requires high temperatures and, aside from large amounts of energy and technical difficulties, it will lead to vaporization of the lead and premature destruction of the fuel. Dissolving in zinc will complicate the technology for opening fuel elements and will result in additional wastes.A method of separating the jacket from the fuel, including opening up of the fuel elements from the ends and extracting the pellets together with lead to be melted out by heating to 500°C, has been checked experimentally. Fuel-element simulators have been developed for check this method (Fig. 1). Fragments of simulators with different fill height were smelted. The extraction conditions, the completeness of the separation of the liquid lead and the pellets on filtering materials, and the degree to which the lead adheres to the surface of the steel tube and uranium mononitride pellets, were determined. Jackets, 9.4 × 0.4 in diameter, 7 × 0.3 mm thick, and 1101 mm long, made of ÉI-847 austenitic steel and ÉP-823 class ferrite martensite steel, were used for the experimental samples. The uranium (natural enrichment) mononitride pellet diameter and height were 7.9 × 12 and 5.8 × 10 mm. The gap between the fuel-element jacket and the fuel column was filled, on a special stand, with melted lead after pre-evacuation. This method gave a dense sublayer, and there was no discontinuity of the column of pellets and lead between them at the level of rsolution of the methods of analysis used. The filling of the fuel-element samples with lead was monitored by x-ray defectoscopy and eddy currents, whose sensitivity in detecting defects has been confirmed by investigations of the kernel by a destructive method. The samples were cut into fragments on screw-cutting lathe with a hard-allow cutter.A laboratory setup was designed, built, and assembled to perform investigations of the smelting. The setup contains a furnace-heated receptacle for the lead melted out and the pellets with a filtering barrier to separate them (nonrusting wire grid with 3.5 × 3.5 mm cell size), a tube heated with a separate furnace for the fuel-element simulator setup, a tube with coaxially installed rod for pushing the pellets out with a massive weight secured at the end, and an apparatus for signalling that