DE3109067A1 - Process for preparing nitric acid-soluble mixed-oxide nuclear fuel pellets - Google Patents

Abstract

In order to prepare nitric acid-soluble mixed-oxide nuclear fuel pellets with an adjustable mixing ratio, the starting powders are ground, using a halogen-free grinding aid, to a primary particle size of < 2 mu m and are then mixed, are granulated as a bed in a rotating container, are compacted into pellets and sintered.

Description

Process for the preparation of nitric acid soluble

Mixed oxide nuclear fuel tablets addition to patent. .. ... ..... ..... (Patent application P 30 10 547.6 - VPA 80 P 8517 BRD) The main patent concerns a Process for the production of nitric acid-soluble mixed oxide nuclear fuel tablets from uranium dioxide (U02 + x) and plutonium dioxide (PuO2) by grinding a powder mixture with a PuO2 content of up to 50% in mills inside glove boxes and then Granulating, pressing and sintering. The starting material becomes halogen-free before grinding Grinding aids, in particular polyalcohols, fatty acids, amines or water, supplied, the subsequent drying processes or temperature treatments (sintering) can be driven out again. The grinding process is controlled so that a primary grain size of <2 micrometers is achieved.

The mixed oxide nuclear fuel tablets obtained in this way can after Burn-up in a nuclear reactor during reprocessing for recovery more valuable Raw materials can easily be dissolved in nitric acid. Furthermore, the one for mechanical Process in itself typical accumulation of dust, which occurs during the manufacture of the nuclear fuel pellets during grinding or when emptying the mills on all surfaces in closed Glove boxes deposited, and so in the long run an increase in Radiation exposure for the operating personnel and therefore through decontamination would have to be eliminated again to comply with the permissible radiation dose largely avoided the process according to the main patent. This will not last achieved because the grinding aids provide the necessary grain fineness is achieved in a single operation. The grinding aid used ensures also ensure that the material to be ground does not stick to the walls of the grinding container or the grinding balls adheres. Therefore the mills practically do not need to be cleaned when the powder mixture used has a different plutonium or uranium content. Even the grinding aid facilitates the discharge of the ground material from the grinding containers and occasional cleaning of the same.

Particularly suitable grinding aids are polyalcohols and fatty acids or amines with surfactant-like behavior.

Water is also suitable as a grinding aid. In this case it is Note that due to the moderating properties of the water, the danger criticality of the powder mixture must be avoided. This means that the the amount of powder to be processed must be restricted accordingly and In addition, the vessels and mills used have volumes that are a priori exclude the possibility of the occurrence of a critical arrangement.

The primary grain size of <2 / um of the uranium-plutonium oxide mixed powder is achieved on the one hand by adding the grinding aid to the powder mixture and on the one hand others through process parameters such as grinding time, degree of filling of the mills and mill type achieved. This grain fineness is necessary for the formation of uranium-plutonium oxide mixed crystals in the subsequent sintering process and on the occasion of solubility in pure nitric acid later treatment of the spent nuclear fuel pellets is necessary.

Any uranium / plutonium mixing ratio can be used in the mill can be set. A powder mixture with a higher plutonium content than normally needed, produced and subsequently be stored. By the appropriate subsequent admixture of what is kept ready Uranium dioxide powder can later be short-term powder types with any lower Plutonium content are made available. Because here powder mixed together in this way the desired homogeneous distribution of the plutonium in the Mixed oxide nuclear fuel achieved.

For setting the fuel tablets for further processing in the press required flowability are ground and, if necessary, with uranium oxide Granulated mixed oxide powder faded down to the desired plutonium content. For this purpose, the powder can be pre-pressed into cylindrical bodies in a compacting press will. That has granules obtained but a wide grain distribution, it is therefore necessary to sieve the top grain. However, fine grain is also available is, these work steps, especially the sieving, lead to a considerable Dust pollution in the glove boxes in which this work is carried out.

The present invention is based on the object, also during granulation the powder mixture to avoid dust exposure as much as possible.

To solve this problem, a method is that mentioned at the beginning Type according to the invention characterized in that the ground powder mixture as Granulated bulk in a rotating container.

Rotating containers are e.g. those shown in Fig. 1 on page 149 of the Zeitschrift für Werkstofftechnik / Journal of Materials Technologie, 4th year, booklet 3 (1973) are suitable, e.g. hollow granulating cylinders or granulating plates, one iotation movement with a horizontal, inclined, tumbling or circling movement Carry out rotation axis and which are operated inside a glove box can. Since the shape of the granulating container has not proven to be important, Other K-shaped containers can also be used, e.g. hollow spheres or conical containers can be used for granulating.

When using such a granulating container, which is advantageous for the Granulate are closed, the dust generation is low compared to the Dust generation, those in the usual granulation by precompacting the powder mixture in a press and subsequent breaking of the pressed material occur and the excessive radiation exposure in the glove boxes over a long period of time can lead.

The invention and its advantages are based on an exemplary embodiment explained in more detail: 7 parts by weight of uranium oxide powder and 3 parts by weight of plutonium oxide powder are weighed into a ball mill, with a weight fraction of 0.1 organic Grinding aids (e.g. 0.5 g polyalcohol, e.g. propanediol) within the The glove box and ground for 12 hours. Then the grist becomes separated from the grinding balls of the ball mill and poured into the container of a Granulating device filled.

After a granulation time of approx. 120 minutes in the rotating granulating container the ground, optionally dried powder mixture becomes a good Free-flowing, homogeneous, easily processable and sinterable build-up granulate. The ground powder mixture can be used to facilitate the formation of these granules (Grist) fine agglomeration nuclei are added before granulating, which from sintered uranium-plutonium mixed oxide. These germs can come from return powder be made from previous manufacture of mixed oxide nuclear fuel pellets originates. The addition of a binder to the ground powder mixture before granulating not necessary.

The speed of the granulating container can be 10 - 70% of the critical one Speed at which the bulk of the powder due to the centrifugal force begins to lie against the inner wall of the rotating granulating container. The degree of filling of the granulating container is less than or at most equal to 80 5 'of the volume of the Granulating container.

Leave starting from any agglomeration nuclei that may have been mixed in spherical granules with an average diameter of less than 300 micrometers produce. The maximum diameter of the granules is about 1 mm. The one for dust formation The fine fraction of the granulate tends to be considerably lower than that of granulates, which are produced by compacting and subsequent crushing. Granules of The latter type of production have a wide range of grains with a high dusty fine grain content.

The build-up granulate produced by the process according to the invention Can be used directly on a press without pressing aids, e.g. zinc stearate, to make fuel tablets be compacted with given dimensions. These tablets are final sintered in a reducing atmosphere at approx. 17000C.

2 claims

Claims (2)

Process for the production of nitric acid-soluble mixed oxide nuclear fuel tablets from uranium dioxide (U02 + x) and plutonium dioxide (Pu02) by grinding a powder mixture with a PuO2 content of up to 50% in mills inside glove boxes and then Granulating, pressing and sintering, the starting material being halogen-free before grinding Grinding aids, in particular polyalcohols, fatty acids, amines or water supplied that by subsequent drying processes or temperature treatments (sintering) are driven out again and in which the grinding process is controlled so that a Primary grain size of (2 micrometers is achieved, according to patent. .. .. ... (patent application P 30 10 547.6 = VPA 80 P 8517 BRD), d u r c h g e k e n n -z e i c h n e t, that the ground powder mixture is granulated as a bulk in a rotating container will. 2. The method according to claim 1, d a d u r c h g e -k e n n z e i c h n e t that agglomeration nuclei are added to the ground powder mixture, which consist of sintered uranium-plutonium mixed oxide.