GB2025685A

GB2025685A - A process for solidifying radioactive fission products

Info

Publication number: GB2025685A
Application number: GB7924837A
Authority: GB
Original assignee: Nukem GmbH
Current assignee: Nukem GmbH
Priority date: 1978-07-18
Filing date: 1979-07-17
Publication date: 1980-01-23
Also published as: DE2831429A1; JPS5517491A; FR2431756A1

Abstract

A highly radioactive fission product solution accumulated during the reprocessing of nuclear fuels is converted into a solid, non-crust-forming, free-flowing and pourable product by the addition of a surface-rich, capillary- active, glass-forming inorganic carrier substance so that the solid product can in turn be converted into a permanently storable form by embedding in a glass mass or other material. The surface- fich, capillary-active, glass-forming substance may be silicon dioxide, aluminium silicate and/or aluminium oxide, preferably in the form of a free-flowing and pourable, evenly formed granulate having a grain size of from 1 to 5 mm.

Description

SPECIFICATION A process for solidfying radioactive fission products This invention relates to a process for solidifying radioactive fission products.

More particularly the invention relates to a process for solidifying radioactive fission products by converting highly radioactive fission product solutions accumulating during the reprocessing of nuclear fuels into a solid, non-crust-forming, free-flowing and pourable product which can in turn be converted into a permanently storable form by embedding in a glass mass or other material.

A suitable method of solidifying the radioactive, in particular highly radioactive, fission products accumulating during the reprocessing of burnt-out nuclear fuels is in general to introduce these products into glass, i.e. vitrification. To this end, the nitric acid fission product nitrate solution which is obtained after dissolution of the nuclear fuel in nitric acid and extraction of the uranium and plutonium and which essentially contains radioactive isotopes of Rb, Cs, Sr, Ba, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Zr, Mo, Tc, Ru, Rh, Pd, Te and residues of uranium and Pu, is concentrated to form a solid product which is then melted together with added glass-forming substances to form a glass.

Various processes have been proposed for concentrating the fission product solution, including for example spray drying, spray calcination, thickening in a revolving tubular furnace, in a wiper-blade evaporator or in a cylinder dryer.

The major disadvantage of these known processes lies in the fact that, during thickening, the fission products tend to form crusts which adhere firmly to the walls of the apparatus and lead to blockages of the installation. On the one hand, this makes it very difficult to control the reaction in the exact manner required and to obtain a continuous, uniform throughput, on the other hand it also interferes seriously with melting to form a homogeneous glass having a controlled composition.

In addition, it has been proposed directly to introduce the fission product nitrate solution dropwise into a liquid glass melt. However, this leads to large quantities of waste gas containing fission products which are difficult and expensive to purify.

Under the conditions of a hot cell regime, the basically a!ready difficult technology of solidifying these materials poses additional problems.

Accordingly, an object of the present invention is to provide a process for solidifying radioactive fission products by converting the highly radioactive fission product solutions which accumulate during the reprocessing of nuclearfuels into a solid product which does not have any of the disadvantages referred to above and which, in particular, gives a product which is free-flowing and pourable, does not form any crusts during concentration and does not cake in the concentration vessels. In addition, this product is intended to be able to be readily converted into a permanently storable form by embedding in a glass mass or other material.

According to the invention, the fission product solution is converted into a solid product by the addition of a surface-rich, capillary-active, glassforming inorganic carrier substance. In this way, the fission product solution is converted by absorption into the preferably spherical, bead-form or other evenly particulate capillary-active, glass-forming substances into a free-flowing conveyable product which remains free-flowing, does not form any crusts and does not cake on the vessel walls during the subsequent drying and calcination process.

Active silicon dioxide, aluminium silicate and/or aluminium oxide is/are advantageously used as the capillary-active glass-forming substance. It is also of advantage to use the active substance in the form of a free-flowing and pourable, evenly formed granulate, particularly in sphere or bead form, having a particle size of from 1 to 5 mm.

The capiliary-active substances are preferably added in such a quantity that up to about 90% of their respective saturated values are charged with the fission product solution.

The product thus obtained has the further advantage that it may be temporarily stored and processed into a permanently storabe product by various, in some cases already known processes. To this end, the product charged with the fission product solution is drived, for example at 1 300C, and subsequently calcined at a temperature of from 450 to 800"C to decompose the nitrates. It is then converted into a permanently storable product by embedding in a glass mass or other material.

For example, the free-flowing product obtained in accordance with the invention may be embedded in a glass mass by filling up a dense pile of the granules containing the fission products with liquid lowmelting boron silicate glass so that the granules are surrounded by this glass mass on all sides without being melted. In addition, it is possible to melt the granules consisting of glass-forming substances with more glass-forming materials to form a homogeneous glass and to pour the glass thus formed into suitable containers. Alternatively, the granules may even be directly embedded in metal or metal alloys.

The process according to the invention is illustrated by the following Examples.

1. Afission product solution containing approximately 140 g of fission product element per litre is absorbed by spherical or granular active SiO2, between 0.6 and 0.9 g of fission product solution being taken up per gram of SiO2, depending on the physical quality of the commercial SiO2-beads.

This material is then dried in a drum furnace, revolving tubular furnace,.belt dryer, fluidised bed or, preferably, in a simple inclined and vibrating tubular coil heated by the direct passage of current and subsequently calcined in a revolving tubular furnace or drum furnace or likewise in a heated, inclined and vibrating tubular coil at a temperature above 405"C in order to remove the nitric acid.

2. 1 litre of a 2 molar nitric acid fission product nitrate solution containing 146 g of fission product per litre (made up of earths and rare earths = 52 g/1.

Ba= 8 g/l; Sr =6g/l; Rb = 3 g/l; Cs = 12g/1;ZR= iPlg/l; Mo = 20 g/l; Te = 3 g/l; Ru = 10 g/l; Pd = 7 g/l; Rh = 4 g/1 ;), which was slightly clouded through the precipitation of molybdic acid and noble metal compounds, was dried in 5 minutes on 1.4 kg of commercial active SiO2-spheres 3 mm in diameter in a rotating drying drum at a temperature of 1300C.

The dried free-flowing product with molybdic acid adhering to its surface was subsequently heated in 30 minutes to 500"C in a heated, rotating steel drum through which a stream of air was passed ( 1 litre per minute) and kept at that temperature for 10 minutes.

The substantially nitrate-free product was freeflowing and contained only about 0.5% of fine particles less than 1 mm in diameter.

3. The procedure was as in Example 2, except that calcination was carried out at 800"C. The material was again free-flowing. No crust formation or caking on the wall of the drum was observed.

4. Following the procedure of Example 2, 1 litre of the above-described solution was poured onto 1.6 kg of spherical active At203. As in Example 2, this product was dried at 130 C, heated in 45 minutes to 800"C in a heated, rotating steel drum while air was passed over and kept at that temperature for 10 minutes. The product was satisfactorily free-flowing.

No depositions on the wall of the drum were observed.

5. 1 litre of the fission product solution described in Example 2 was poured onto 2.2 kg of bead-form aluminium silicate with a dimeter of 3 mm. The product was dried at 1 30 C and, again as described in Example 2, calcined at 500 C. A free-flowing non-caking product was obtained.

6. Liquid boron silicate glass was added to 200 g of the material produced in accordance with Example 3 in a heat-resistant steel cylinder heated to 800 C and accommodated in an evacuated vessel. The mixture was cooled to room temperature over a period of 10 hours. A satisfactory, crack-free glass body was obtained.

Claims

1. A process for solidifying radioactive fission produces which comprises coverting a highly radioactive fission product solution accumulating during the reprocessing of nuclear fuels into a solid, non-crust-forming, free-flowing and pourable product by the addition of a surface-rich capillary-active, glass-forming inorganic carrier substance so that the solid product can in turn be converted into a permanently storable form by embedding in a glass mass or other material.

2. A process as claimed in claim 1, wherein silicon dioxide, aluminium silicate and/or aluminium oxide is/are used as the surface-rich, capillary-active glass-forming substance.

3. A process as claimed in claim 1 or 2, wherein the surface-rich, capillary-active carrier substance is used in the form of a free-flowing and pourable, evenly formed granulate having a grain size of from 1 to 5 mm

4. A process as claimed in any of claims 1 to 3, wherein the capillary-active substances are added in such quantities that up to 90% of their, respective saturation values are charged with the fission product solution.

5. A process for solidifying radioactive fission products substantially as described with particular reference to any of the Examples.