Patent Number: 055815898
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method and an apparatus for the production by the sol-gel process of microspherical ceramic particles such as particulate ceramic fuels for use in nuclear reactors. 2. Prior Art Particulate ceramic fuels for use in nuclear reactors are commonly produced by the sol-gel process, in which a stock solution or sol containing uranium and other fuel elements is treated with a second fluid (gas or organic liquid) to form small droplets under the resulting surface tension and a gelling agent such as ammonia is allowed to act on the droplets to gel (solidify) them, which are then heat treated for conversion to ceramic particles. This sol-gel process may be implemented by external or internal gelation. In external gelation, the gelling agent is allowed to act on the small droplets from the outside; in internal gelation, hexamethylenetetramine (HMTA) is mixed preliminarily with the stock solution or sol and the small droplets formed by interaction with the second fluid are heated to decompose the contained HMTA thermally and the decomposition product, ammonia, is used as the gelling agent. According to a common method of internal gelation, heated silicone oil or the like is used as the second fluid and the small droplets of the stock solution or sol which are formed in that second fluid are heated up by about 80K so that HMTA is decomposed thermally, followed by gelation of the small droplets (see J. Nucl. Mater., 148 (1987) 324-331). Another approach that can be taken is the Ledergerber method, in which the small droplets of the stock solution or sol as formed within air are allowed to fall in the electric field of microwaves that have been generated within a cavity resonator (hereunder simply referred to as "resonator") so that they are heated dielectrically to decompose HMTA while gelling the small droplets (see Trans. Am. Nucl. Soc., 40 (1982) 55-56). A problem with the Ledergerber process is that the frequency specified for engineering purposes (0.915 or 2.45 GHz) is insufficient to provide a temperature elevation of about 80K that is held necessary to gel the small droplets of the stock solution or sol and the intended heating can only be accomplished by applying frequencies about ten times the engineering level (8.2-12.4 GHz). SUMMARY OF THE INVENTION An object of the invention is to provide a method by which dielectric heating that provides the temperature increase of about 80K necessary for causing internal gelation of the small droplets of a stock solution or sol can be accomplished using an inexpensive commercial microwave power source operating at an engineering frequency (2.45 GHz) in place of the high-output power source which was especially constructed by Ledergerber in order to apply the microwaves in the communication frequency band (X-band) to dielectric heating. Another object of the invention is to provide an apparatus for implementing this method.