Patent Number: 046769358
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing mixed-oxide nuclear fuel pellets of uranium dioxide (UO.sub.2+x) and plutonium dioxide (PuO.sub.2) soluble in nitric acid. 2. Background of the Invention It was known to make fuel pellets by milling the powder mixture with a PuO.sub.2 -content of up to 50% in mills wihin glove boxes and by subsequent granulating, pressing and sintering. The requirement of solubility of nuclear fuel pellets results from the necessity of reprocessing burned-up nuclear fuels and the recovery, connected therewith, of most valuable raw materials. This requirement in itself is not new: reference is made to German Published Non-Prosecuted Application No. 28 33 054. According to the method proposed there, the formation of the mixed crystals, as a prerequisite for the solubility, is achieved by repeated milling of the radiotoxic material. Apart from the increased radiation exposure, connected therewith, of the operating personnel, which cannot be avoided completely even when working with so-called shielded glove boxes, this method is relatively cumbersome. The dust production in milling or when emptying the mills is also a disadvantage, since the dust deposits in the closed-off glove boxes on all surfaces and thus must be removed again by decontamination. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing mixed-oxide nuclear fuels which are soluble in nitric acid, which method is substantially simpler and with which the disadvantages connected with the dust accumulation and deposition can be avoided to a very large extent. With the foregoing and other objects in view, there is provided in accordance with the invention, a method for producing mixedoxide fuel pellets of uranium dioxide and plutonium dioxide by milling a powder mixture of uranium dioxide with up to 50% by weight plutonium dioxide and by subsequent granulation, pressing and sintering, the improvement comprising increasing the solubility of the plutonium component in the pellet in nitric acid by effecting the milling of the powder mixture in the presence of a halogenfree milling aid which can be expelled from the mixture by heating to a temperature up to said sintering, and wherein milling is conducted to produce a primary grain size of less than 2 micrometers, and wherein the mixture is heated to expel the milling aid. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a method for producing mixed-oxide nuclear fuel pellets soluble in nitric acid, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The invention, however, together with additional objects and advantages thereof will be best understood from the following description. DETAILED DESCRIPTION OF THE INVENTION According to the invention, there is added to the starting materials, prior to the milling, halogen-free milling aids which can be expelled by subsequent drying processes or heat treatments (sintering). Also the milling operation is controlled to obtain a primary grain size of under 2 .mu.m. With such a milling aid, the required fine grain size can be achieved in one operation, so that the formation of dust and thereby, the above-described annoyances can be reduced to a large degree. The materials must be chosen so that no undesirable residual materials remain in the nuclear fuel pellets; this relates particularly to fluorine. For the development of the mixed crystals, which are soluble in nitric acid, a primary grain size of the uranium dioxide and plutonium dioxide powders of less than 2 .mu.m is required. This is set by process parameters such as milling time, degree of filling of the mills as well as the type of mill. In this manner, any desired mixing ratios Pu/U can be adjusted. The further processing into nuclear fuel pellets takes place in the customary manner by pressing, granulating and sintering at about 1700.degree. C. A further advantage is to first prepare a powder mixture with a plutonium content higher than normally required and to store it and to keep at the same time a supply of uranium dioxide powder in readiness. By appropriately mixing the two kinds of powder, mixedoxide nuclear fuels with the desired plutonium content can then be prepared on short notice. Since the startng powders are mixed with each other, the desired homogeneous distribution of plutonium in the nuclear fuel is achieved in this manner. This new method can be carried out, largely avoiding the formation of dust under plutonium glove box conditions. The mills need not necessarily be cleaned if the plutonium or fission material content of the powder mixture is changed. The exact adjustment of the plutonium or uranium content need be made only shortly before the nuclear fuel pellets are pressed. The milling aid used also takes care that the material to be milled does not stick to the walls of the milling vessel or the balls. This facilitates the removal of the milled material from the milling vessels and the cleaning of same. It is also very important here that by means of these milling aids, very high grain fineness can be achieved, which is necessary in view of the development of the desired mixed crystals, the solubility of which in nitric acid is of decisive importance. Proven milling aids are organic substances with tenside-like behavior such as polyalcohols or amines. Also water could be used. In that case, it should be noted, however, that due to the moderating properties, the danger that the powder mixture might become critical, must be taken into consideration. This means that the amount of powder must be limited accordingly and also that the vessels and mills must have a volume which precludes the possibility of the occurrence of a critical configuration from the start.