Sintering parameters for uranium-gadolinium oxide fuel pellets

Abstract

Consumable neutron absorbers are used in the cores of water moderated and cooled reactors to adjust power and compensate reactivity as well as to maintain the temperature coefficient of reactivity at a prescribed level [1]. The main drawbacks of the absorbers used as individual structural elements are that they occupy useful space in the fuel assemblies and generate additional expenses associated with production, transport, and storage. An integrated absorber in the form of gadolinium oxide introduced into a fuel pellet makes it possible to partially compensate for these drawbacks. Fuel with an integrated neutron absorber is widely used in the form of (U, Gd)O 2 pellets with different mass fractions of Gd 2 O 3 (from 4 to 12%) [2, 3]. The production of (U, Gd)O 2 pellets is similar to the technological process of manufacturing pelleted uranium oxide fuel. However, the gadolinium must be uniformly distributed over the pellet volume [4, 5]. An important step in the production of (U, Gd)O 2 pellets is sintering, which can be longer than 60‐70 h depending on the equipment implementation. For this reason, the aims of the present work were to determine the main parameters of the sintering and removal of the binding components from (U, Gd)O 2 pellets as well as to assess the possibilities of decreasing the total sintering time. A DIL 402 C high-temperature dilatometer and a STA 409 CD synchronous thermal analyzer (both instruments manufactured by the Netzsch Company, Germany) were used. (U, Gd)O 2 pellets were sintered in a reducing medium Ar‐8%H 2 with heating at a constant rate and step-wise. This procedure makes it possible to determine the main parameters of sintering (shrinkage onset, temperature, the corresponding maximum rate of shrinkage) and to simulate the sintering of pellets in through-type production furnaces. The kinetics of sintering and binder removal were studied on pressed (U, Gd)O 2 pellets fabricated from uranium oxide powder obtained by precipitation of ammonium urinates. The pellets were fabricated by the classic scheme of mixing UO 2 and Gd 2 O 3 powders with aluminum stearate and then pressing. Study of the Sintering Kinetics of (U, Gd)O 2 Pellets with Constant Heating Rate. Three heating rates were