SE441438B - SET TO MAKE AMMONIUM URANIUM CARBONATE - Google Patents

Description

carbon dioxide from such a solution, as well as removing uranium salts and fluorides from it.

In the production of sintered nuclear fuel bodies (pellets) of uranium dioxide, the particle size distribution of the powder of uranium dioxide, as well as the shape of the particles, is of great importance for the properties of the pellets. The aim is a powder which has a large sintering activity and which gives a pellet with a small porosity. Furthermore, the powder should preferably not need to be ground as grinding gives an unfavorable irregular particle shape. Grinding also results in a reduced flowability of the powder, so that the powder may need to be pre-pressed and granulated before it can be pressed into green bodies. By producing ammonium uranyl carbonate according to the present invention, it is possible to achieve a particularly advantageous and reproducible particle size distribution produced by reducing uranium dioxide powder therefrom, so that the powder has a high sintering activity and a small porosity sintered pellet produced therefrom. In particular, it is possible to produce a powder with a higher content of coarser and higher content of finer particles and thus with a lower content of particles of intermediate sizes than with previously used methods. At the same time, the need for grinding can be reduced or completely eliminated.

Another extremely important feature of the invention is that it enables a maximum recovery of ammonia in the production of the ammonium uranyl carbonate. This is due to the fact that the uranium compound can be added to the precipitation vessel without the use of carbon dioxide which is salvaged. Supply of uranium hexafluoride, which, as mentioned, has been used as a uranium compound in the AUC process, is conventionally done by means of carbon dioxide in order to avoid clogging of pipes with sublimed fluoride.

According to the present invention, the ammonium uranyl carbonate is prepared by adding to a first aqueous solution containing ammonium ions and carbonate ions a) an aqueous solution of uranyl fluoride to precipitate a first amount of ammonium uranyl carbonate and then b) further aqueous solution of uranyl fluoride with at least substantially simultaneous addition of a second water containing ammonium ions and carbonate ions in higher concentrations of these ions than in the first aqueous solution for precipitating the principal amount of ammonium uranyl carbonate.

The first aqueous solution preferably contains 1-7% by weight and preferably--6% by weight of ammonium ions, calculated as NH3, and preferably 1-6% by weight of 8-401057-8% and preferably 3-5% by weight of carbonate ions, calculated as C02.

It is possible to influence the size distribution of the particles in the finally obtained powder of the ammonium uranyl carbonate by choosing the concentrations of ammonium and carbonate ions in the first aqueous solution. If said concentrations are kept low, a higher average particle size is obtained than if the concentrations are kept high within the specified ranges.

The aqueous solution of uranyl fluoride suitably contains 35-60% by weight and preferably 45-55% by weight of uranyl fluoride. The aqueous solution is suitably prepared in a separate plant by passing uranium hexafluoride down to 1 water. Heat generated thereby can be dissipated by circulating the aqueous solution in a circuit with a heat exchanger in connection with the supply of the uranium hexafluoride. Because the exothermic dissolution in water takes place outside the vessel 10, ie separately from the formation of the ammonium uranyl carbonate, the precipitation temperature can be better regulated and the reproducibility increases.

The second aqueous solution with ammonium ions and carbonate ions suitably contains 25-30% by weight and preferably 25-28% by weight of ammonium ions, calculated as NH 3, and preferably 25-32% by weight and preferably 28-30% by weight of carbonate ions, calculated as CO 2. The solution may advantageously consist of a solution prepared during the recovery of ammonia and carbon dioxide from the solution obtained as filtrate in the preparation of the ammonium uranyl carbonate. The recovery of ammonia and carbon dioxide takes place while removing the fluoride contained in the solution with calcium carbonate, which provides the carbonate ions.

The precipitation of the ammonium uranyl carbonate preferably takes place at a temperature of 60-65 ° C. A portion of the required amount of ammonium ions may be added to the solution in the reaction vessel in the form of gaseous ammonia. Ammonium ion fed in the form of gaseous NH3 suitably constitutes 5-15 per cent and preferably 8-12 per cent of the total amount of ammonium ion required, calculated as NH3, for the reaction.

The invention will be explained in more detail by describing an exemplary embodiment with reference to the accompanying drawing, in which Fig. 1 schematically shows a plant for the production of ammonium uranyl carbonate according to the present invention and for its conversion to uranium dioxide, and Fig. 2 a part of the plant according to Fig. 1 more in detail. 8 8 201057- 8 The device according to Fig. 1 includes a reaction vessel 10 for precipitating ammonium uranyl carbonate. The reaction vessel is fed via line 11b 250 20 k 3, 8 carbonate ions, calculated as CO 2, and some other constituents such as per liter of water containing 25 kg of ammonium ions, calculated as NH goes from the following text, and via line 70 250 liters of water. The vessel thus contains a first solution 71 with ammonium ions and carbonate ions. Then, for 30 minutes, 75 liters of an aqueous solution containing 50% by weight of uranyl fluoride is fed to the reaction vessel via line 12, resulting in the precipitation of a first amount of ammonium uranyl carbonate.

The addition of the solution of the uranyl fluoride is then continued, but then with the simultaneous addition of a second solution of ammonium ions and carbonate ions, in which the content of ammonium ions amounts to 26.5% by weight and the content of carbonate ions to 30% by weight. The latter solution is fed via line 13. The total amount of uranyl fluoride solution added to the reaction vessel is 300 liters and the total time for its supply is 2 hours. The volume of the second solution with ammonium ions and carbonate ions amounts to 750 liters and it is supplied for a period of 2 hours, ie the supply of this is continued 1/2 hour after the supply of the uranyl fluoride has ceased. During the entire time that the supply of uranyl fluoride and the supply of the second solution with ammonium ions and carbonate ions takes place, NH3 is also supplied via a line 14 in a total amount of 28 kg. The precipitation takes place at a temperature of 60-65 OC. After completion of the uranyl fluoride solution and another 1/2 hour of the second solution of ammonium ions and carbonate ions remaining, the reaction vessel is momentarily fed via a line 15b 250 liters of an aqueous solution containing 23 kg of ammonium ions, calculated as NH3, ooh 22 kg of carbonate ions , calculated as CO2, and the temperature is kept at the above value below 1 / Zftime. The reaction mixture is then cooled to room temperature. By supplying the solution via line 15b, the contents of this solution (from washing of the previous precipitate) are used to obtain as complete a precipitation of uranyl ions as possible in the reaction vessel 10.

The reaction mixture is then transferred from the vessel 10 to a filter 16, where the precipitate of the ammonium uranyl carbonate is filtered off. The precipitate is first washed with water containing ammonium ions and carbonate ions from line 11a, whereby mainly ammonium fluoride is washed away and a minor dissolution of the ammonium uranyl carbonate takes place. The wash water is passed via line 11b to the reaction vessel 10 for use in the manner previously described. Thereafter, the precipitate is washed with a solution from line 15a, which is of the same kind as the solution in line 13. Thereby, additional ammonium fluoride is washed away from the precipitate. This occurs during negligible dissolution of the ammonium uranyl carbonate. (The precipitate is also washed with methanol.) The washing liquid from line 15a is led via line 15b to the reaction vessel 10 for use in the manner previously described. The line 15b suitably contains a collecting vessel (not shown), where the washing liquid can be stored temporarily before it is supplied to the reaction vessel 10.

After drying, the precipitate is transferred from the filter 16 to a fluidized bed furnace 17, where it is subjected to a reduction in a known manner and transferred to uranium dioxide. In this reaction, 68 kg of HHS and 131 kg of CO2 are emitted as gases via the line 18 to a gas washing plant 19. The gases are dissolved in the water, which is supplied via a line 20 with a spray device. Loss gases, mainly ammonia, from the reaction vessel are also led via a line 21 to the plant 19, which is provided with a drain 22 for gases that are not dissolved in the plant. The resulting solution, which contains U% by weight of ammonium ions, calculated as NH3, and H% by weight of carbonate ions, calculated as CO2, is collected via line 23 in a storage tank 24. Part of the solution is passed via lines 25 and 11a to the filter and used on previously described manner. Another part is led via the line 25 through a filter 26 for separating any solid particles, such as particles of uranium dioxide, to a tank 27.

The filtrate passing the filter 16 is collected via a conduit 28 in a tank 29 and passed therefrom via a conduit 30 to a carbon dioxide stripper 31.

The evaporated gas, containing NH 3 and CO 2, is passed via a line 32 to a recycling plant 33. From the stripper, the solution is led via a line 3H to a device 35 for precipitating uranium with hydrogen peroxide. The solution, whose uranium content is reduced from 300 ppm to less than 5 ppm in the device 35, is led via line 36 to a container 37 containing a bed of grains of calcium carbonate or other alkaline earth carbonate. In this the fluoride ions of the solution react to form calcium fluoride, which is retained in the container. This reduces the fluorine content of the solution from about 50-100 g / l to about 1 g / l. At the same time, carbonate ions are added to the solution in an amount which stoichiometrically corresponds to the amount of fluorine fed to the reaction vessel 10 in the uranium hexafluoride, i.e. 3 moles of CO 2 are released in the form of carbonate per 84010å = 7 ~ = 8 moles of uranium fed to the reaction vessel 10. The solution goes via a line 38 to the previously mentioned tank 27, which serves as a storage tank for solutions containing ammonium ions and carbonate ions. From this tank the solution is led via a line 39 to the recycling plant 33, which is described in Fig. 2.

The solution in line 39 (Fig. 2) contains 3-10% by weight of ammonium ions, calculated as NH3, 3.5-12% by weight of carbonate ions, calculated as CO2, and various impurities such as calcium, silicon, manganese and aluminum compounds from the calcium carbonate in the container 37 (Fig. 1). The solution is heated in a heat exchanger H1 before it is led into the upper part of a column H2. the liquid is heated to a temperature of at least 90 OC, preferably to 95-100 OC, indirectly with steam in the device 73 at the bottom of the column. In the column, ammonia and carbon dioxide emit together as gases and water vapor. The gases pass through a second column 43 located above the first column, where it meets a solution, which is supplied via the line ÄH and which contains 22% by weight of ammonium ions, calculated as NH3, and 25% by weight of carbonate ions, calculated as CO2. The latter solution emits ammonia and carbon dioxide as gases. Column 43 is maintained at a temperature of at least 80 ° C, preferably at 85-92 ° C. The solution leaving the bottom of the column 42 via a line H5 contains all impurities entering via the line 39, residual fluorine from the container 37 and a stoichiometric amount of ammonium ions. The solution is otherwise free of carbonate ions and ammonium ions. The solution is led with the pump 46 through the heat exchanger H1 to heat the solution in the line 39 'to a drain, possibly after being subjected to a further purification to lower the fluorine content. (Line H5 is also excellent in Fig. 1.) The ammonia and carbon dioxide go from the top of column H3 via a line 47 to the lower part of a column 48. Line 32 (also in Fig. 1) is connected to line 47. The gas in line 32 contains 15-30% by weight of ammonium ions, calculated as NH3, and 1H-28% by weight of carbonate ions, calculated as CO2. In the upper part of the column H8 an aqueous solution is led in from a line H9 and sprinkled down over the filling bodies of the column. This solution, which is cooled in a condenser 50, contains slightly lower concentrations of ammonium and carbonate ions than the finished solution in the bottom part of the column H8, since solution with a lower concentration is supplied from the column 56 via the line 49.

In solution H8, the solution absorbs the mixture of ammonia and carbon dioxide initiated at the bottom and gives the finished product recovered in the recovery plant in the form of an aqueous solution containing 26.5% by weight of ammonium ions, calculated as NH3, and 3% by weight of carbonate ions, calculated as C02. The finished product was discharged via line 51 with pump 52 and collected in a storage tank 53 (Fig. 1). As previously mentioned, part of the finished product was added via line HU to the upper part of column H3.

The main flow from the pump 52 goes via line 54 to line H9 with the heat exchanger 50 and back to column M8.

Small amounts of ammonia and carbon dioxide depart from the top of the column 48 and are led via a line 55 into the lower part of a column 56. Over this column another column 57 is arranged. In the upper part of this, cold deionized water was introduced via a line 58, which dissolves any ammonia and carbon dioxide, which passed the column 56. The water flows down into the column 56. The excess solution at the bottom of this column was added via the line H9 with the pump 59 to the upper part of the column H8 while the main part is returned via a line 60 with a cooler 61 to the upper part of the column 56, where it is sprinkled down over the filling bodies of the column. In the upper part of the column 57 there is a drain 72 (Fig. 2) for gas, which is practically free of ammonia and carbon dioxide. All columns 42, H3, H8, 56 and 57 have stainless steel filler bodies. The numbers 62 ~ 69 denote valves. Valves 66, 68 and 69 are automatically regulated and maintain a constant level at the bottom of columns 56, 48 and H2 respectively.

According to the present invention, it is possible to achieve almost 100% recovery of ammonia, while with previously known methods of producing ammonium uranyl carbonate using uranium hexafluoride, a maximum of 85% recovery can be achieved. Using the invention, the reproducibility in the production of uranium dioxide powder with predetermined sintering activity can be increased so that the density after sintering will be in the range 10.5ü-10.55 g / omg compared to previously in the range 10, Ä8- -10.56 for 95 % of manufactured powder kits of uranium dioxide.

Claims (2)

8-401 @ 57 ~ 8 PATENT REQUIREMENTS A method of preparing ammonium uranyl carbonate for use in the production of uranium dioxide for nuclear reactor fuel, by reacting a uranium compound with ammonium ions and carbonate ions in aqueous solution, characterized in that to a first aqueous solution (71) containing ammonium ions and carbonate ions are added a ) an aqueous solution (12) of uranyl fluoride for precipitating a first amount of ammonium uranyl carbonate and then b) further aqueous solution (12) of uranyl fluoride while at least substantially simultaneously supplying a second aqueous solution (13) containing ammonium ions and carbonate ions in higher concentrations of these ions than in the first aqueous solution for precipitating the principal amount of ammonium uranyl carbonate. A method according to claim 1, characterized in that; that the first aqueous solution (71) of ammonium ions and carbonate ions contains 1-7% by weight of ammonium ions, calculated as NH3, and 1-6% by weight of carbonate ions, calculated as CO that the aqueous solution (12) of uranyl fluoride contains 2) 35-60% by weight of uranyl fluoride and the second aqueous solution (13) of ammonium ions and carbonate ions contains 25-30% by weight of ammonium ions and 25-32% by weight of carbonate ions.