DD207862A1 - PROCESS FOR PRODUCING NUCLEAR MICROBALLS - Google Patents

Abstract

The invention relates to a process for the production of nuclear fuel microspheres from aqueous solutions of the nitrates of nuclear fuels for the production of vibration-sealed fuel thermal and fast reactors and for the production of pellets. The object of the invention is to develop a process of low number of stages which can be carried out in a simple manner for the production of high-quality nuclear fuel microspheres. The invention has for its object to develop a process for the production of spherical nuclear fuel, which avoids the use of high levels NH NH deep 4 NO deep 3 to set favorable conditions Faellungsbedingungen. This object is achieved according to the invention by a process in which the nitrate content of the highly concentrated solution of nuclear fuels is reduced by strong reducing agents which are introduced into the solution in a gaseous form by means of tracer gas.

Description

Field of application of the invention

The invention relates to a process for the production of nuclear fuel microspheres from aqueous solutions of the nitrates of nuclear fuels for the production of vibration-compressed fuel elements of thermal and fast reactors and for the production of pellets.

Characteristic of the known technical solutions

Numerous wet-chemical processes for producing spherical nuclear fuel particles, so-called microspheres, are described in the literature. Ϊ́ΑΕΑ Panel Meeting, Report IAEA-161 (1974), Report IAEA-CH-36 (1977); H, V / .H. Lahr, Nucl. Technol. 31 (1976) 183]. They can all be classified in 4 basic methods

1. Sol-gel method

2. Hydrolysis process

3. Gel precipitation method. '~

4. Polycondensation process ".

A further developed gel precipitation method is the EGU method. Here, the casting solution is prepared by mixing uranyl nitrate, urea and Ämraoniumnitratlösung, The final solution contains 1.5 mol / l UO ₂ (NOo) ₂ > further 3-4 moles of urea and 2-3 mol of NHJTO ₃ per mole of uranium. This solution is preneutralized by addition of NH- to 25-30 % . This, as well as the relatively high content of NH.NOt are necessary for setting favorable precipitation conditions and for rapid curing of the droplet. The casting solution is injected horizontally into the gelling column. By the action of an electromagnetic vibration system, the liquid jets are divided into uniform droplets. The drops are precured by reaction with NH 2 gas. After a flight distance of 10 - 20 cm

-äJU! .19e2 * 02l67C

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^ the microspheres fall into a 5-7 mol WH.OH solution in which they are cured. Washing out in. WH, WO- contained in the microspheres. and urea takes place in .a separate step, with 7 moles of WH, OH solution. In a further step, the microspheres at 60 ⁰ C for about 10 min. Age in the same solution to age. The drying of the microspheres must be done in a monolayer; ' The U (VI) is reduced at temperatures of 600 - 800 ⁰ C with hydrogen to UOp. The sintering of the UO ^ microspheres is carried out at 1400 ⁰ C (3h) in H ₂ or Ar / Hg. The final density of the UOg microspheres reached is 98 % of the th. Density of UOp. '

 The known methods contain the following deficiency:

1. Preparation of colloidal solutions of oxides-the. Nuclear fuel e '

2. Use of larger amounts of thermally unstable organic compounds (e.g. 'HMTA)

3. Denitrification of the solution by extraction processes, which are difficult to control, especially in the presence of plutonium.

4. Use of thickeners to increase the viscosity of solutions of relatively low concentration.

5. High levels of WH »HO, in the solution for realizing favorable precipitation conditions for uranium, lead to very unfavorable precipitation conditions for plutonium in the presence of plutonium. ,

6. High Witratgehält in the microspheres forces to several stages of the process for curing, washing and aging of the microspheres.

The above-mentioned EGU procedure bypasses the deficiencies according to point 1 - 4. Existence or new occurrence but the deficiency according to point 5 u. 6. Therefore, the method is little or not suitable for the production of U / Pu mixed oxide fuel, for example.

Object of the invention

The object of the invention is to develop a process of low process steps that can be carried out in a simple manner to produce high-quality nuclear fuel microspheres.

Explanation of the essence of the invention

The object of the invention is to develop a process for the production of spherical nuclear fuels which avoids the use of "high contents of NH.NO.sub.2 to set favorable conditions for palletizing." This object is achieved according to the invention by a process in which the nitrate content of the highly concentrated Solution of nuclear fuels by strong reducing agents, which are introduced in gaseous form by means of carrier gas into the solution is reduced.

Nitrate reduction produces a stable solution of increased viscosity.

Particularly suitable reducing agents are those which themselves give only gaseous reaction products. This avoids dilution. By eliminating the high HH.NO., - Addition and additional reduction of the nitrate content in the solution, the content of impurities to be washed out of the microspheres is lowered so that hardening, V / a see and aging of the microspheres can be done in one step.

The process according to the invention is also suitable for elements which are difficult or impossible to process at high nitrate contents (especially true for plutonium). Depending on the reducing agent at a higher temperature (50-105 ⁰ C) to be performed results in reducing Pu-solutions containing a defined value, the Pu (almost completely (III) Pu), causes the beneficial Pallungsbedingungen. In

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simple manner can according to the described Ver. drive C- and N-containing nuclear fuels (eg UC, (U / Pu) C, UK.ua). For this purpose, the starting solutions according to 'finely divided carbon (for UC, for example, 3.Mol C per Möl U) add. The materials prepared as above, but only calcined or reduced, are subjected to a carbon reduction or a treatment in a BV) r-containing atmosphere at high temperatures (1200 '1600 °). , ^; , , , '.'

It also creates the possibility by 'simple admixture of soluble compounds' or colloidal powders according to this method. Starting solution with non-nuclear materials to produce doped nuclear fuel, the only prerequisite is the insolubility or low solubility of the materials in aqueous HK ^ OH Lögüngen. Compounds which are capable of solid solution formation form ideal Misbir crystals (eg (U / Pu) O ₂ ~ mixed crystal) when prepared by this process. .,

Exemplary embodiment>. - ...

1 .. Example ·

Formaldehyde is introduced into a UO ₂ (M) ₃ ) ₂ solution (2 ", 3 mol U / l) heated to 90 ^° C. by means of lip as the carrier gas until a 10, / U = 1.5 is reached. The cooled solution is mixed with urea (urea / U = - 1.8) and sprayed to drips (diameter = about 200 / Utn.) The drops fly through a MiL-containing gas space (flight distance about 20 cm) and then fall into a tube The HH ^ OH solution (6 rio 1/1 ML x) at a temperature of GO ⁰ C. High in a residence time of 5 min, the microspheres are separated from the solution and saturated with water at 70 ° C E ^ stream dried (heating rate = 150 ° / h).

At 300 C, the water vapor saturation is set and the

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Np flow 10% by volume H ₀ added. After reaching a temperature of 75O ⁰ C is heated at a heating rate of 800 ° C / h to 1250 C and maintained at this temperature for 3 h. The formed UO ₂ microspheres have 95-99 % of the thermal density of UO ₀ .

2nd example.

In a demitratisierte. as in Example 1 solution finely divided carbon black is added after addition of urea and dispersed by stirring (C / U = 3). The microspheres prepared as in Example 1 are cooled after reaching a temperature of 700 ⁰ C and after insertion into a vacuum oven (preferably 10 "Torr) using a heating rate of 1000 ° C / h to a temperature in the Karboreduktion (about 1100 ⁰ C) was heated in a high vacuum. Until the completion of the Karboreduktion the temperature is slowly increased to about 145 ° ⁰ C, sintering UC-microspheres at 175O ⁰ C gives densities 80-92% of th density..

3rd example ^;

In a heated to 85 ° C UO ₂ (NO ₃ ) ₂ solution (2.3 mol / lU, containing 0.2 mol / l zirconium, using Np as the carrier gas formaldehyde up to a NO, / Meto11 = 1 6. The urea (urea / metal = 1, 8) is added to the cooled solution, and the further processing of microspheres is carried out as in Example 1. The U0 _o / Zr0 _o microspheres that are formed bring the Zr homogeneously into the UO ₀ . Motrix distributed.

4th example

UOO microspheres were prepared as in Example 1, wherein the temperature of treatment was extended only up to 700 ⁰ C, and then cooled to be Pellets (diameter mm, height = 8 mm) is pressed. These pellets are in

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heated Ar / 10 % EL with heating rates of 500 ° C / h to 16OO ° C and at this temperature for 5 h voriiritert. The sintered pellets reach densities bia 95 % of th. Density.

Claims (2)

invention claim 1. A process for the production of nuclear fuel micro. Mulls from a Mtratlösung of nuclear fuels,> in which from the nitrate solution by admixture of V additives and auxiliaries a casting solution is prepared, which is divided into droplets and solidified by chemical j reaction and the like The resulting microspheres are subsequently washed, dried, calcined and sintered, characterized in that the nitrate content of the nitrate solution of the nuclear fuel is reduced by strong reducing agents which are introduced into the solution in gaseous form by means of carrier gas ... 2. Method according to point T, characterized in that reducing agents are used, which form only gaseous reaction products.