EP2160358A1

EP2160358A1 - Method for the production of spherical combustible or fertile material particles

Info

Publication number: EP2160358A1
Application number: EP08759783A
Authority: EP
Inventors: Werner Heit; Martin Kadner; Georg Braehler; Karl Froschauer; Guo Wenli; Liang Tonxiang; Tang Chunhe
Original assignee: Nukem Technologies GmbH
Current assignee: Nukem Technologies GmbH
Priority date: 2007-05-24
Filing date: 2008-05-20
Publication date: 2010-03-10
Also published as: US20100077657A1; ZA200909029B; US8535579B2; WO2008142072A1; RU2459766C2; JP2010527884A; DE102007037473A1; KR101378729B1; KR20100021486A; RU2009148048A; JP5657381B2

Abstract

The invention relates to a method for the production of spherical combustible or fertile material particles from an oxide of the group of the heavy metals uranium, thorium, plutonium, or mixtures thereof. For this purpose, the process steps of producing a base solution of the nitrates of the heavy metal(s), adding at least one first reagent in order to adjust the viscosity of the solution, dropping the solution to form microspheres, at least superficially solidifying the microspheres in an atmosphere containing ammonia, collecting the microspheres in a solution containing ammonia, and subsequent washing, drying and thermal treatment are carried out, wherein urea and/or ammonium carbonate and/or ammonium hydrogen carbonate and/or ammonium cyanate and/or biuret are added to the base solution before adding the first reagent, and the solution thus prepared is heated to a temperature T at 80° ≤ T < Ts where Ts = boiling temperature of the solution, and is maintained at said temperature over a time period t where 2 h ≤ t ≤ 8 h.

Description

description

Process for the preparation of spherical fuel or breeding particles

The invention relates to a process for the production of spherical fuel or Hutzstoffpartikeln from an oxide of the group of heavy metals uranium, thorium, plutonium or mixtures thereof comprising the steps of preparing a starting solution of the nitrates of at least one of the heavy metals, adding a first reagent from the group Urea and / or ammonium carbonate and / or ammonium bicarbonate and / or ammonium cyanate and / or biuret, adding at least one second reagent in the form of PVA and / or THFA to adjust the viscosity of the solution, dripping the solution into microspheres, at least superficial solidifying the Microspheres in an ammonia-containing atmosphere, collecting the microspheres in an ammonia-containing solution and then washing, drying and thermal treatment.

In high-temperature reactors, which are becoming increasingly interesting again because of their safety properties and because of the possibility of utilizing the high operating temperatures for generating process heat, graphitic fuel elements in different geometrical configurations are usually used. Common to them is that the actual fuel or breeding substance, uranium, thorium or plutonium, is present in it in the form of coated particles. These are spherical particles of the corresponding heavy metal oxides, with diameters between 0.1 and 1 mm, which are surrounded by layers of carbon and, for example, silicon carbide in order to make them suitable for use in the reactor. The pure oxide spheres are called kernels in nuclear technology or kernels in the English-speaking world. The cores are usually prepared from the corresponding nitric acid solutions: the low-viscosity nitrate solutions are converted into highly viscous solutions, which can be dripped into ideally round drops, and then converted into solid gel beads by chemical reaction. Called gel method. The reagent of choice for the reaction to the solid is ammonia: uranium reacts to form ADU (ammonium diuranate), thorium and plutonium to the hydroxide. So far, two methods have been used to make these cores:

The processes for "internal gelling" are based on relatively highly concentrated solutions, drop solutions of heavy metals into heated organic oils, the release of ammonia from the additives urotropin, urea or the like for the solidification.The problem here is the use of oils as a precipitation bath In further steps, the fresh cores must be washed with water to remove reaction products, thus inevitably producing water / oil mixtures which need to be regenerated in a costly manner, especially in view of the fact that they are radioactively contaminated liquids , significant technical problems.

External gelation processes drop relatively low concentration (about 100 g U / l) solutions in an ammonia atmosphere, catch the resulting beads in an ammonia solution, allow them to age (to react with ADU or hydroxides), wash first with ammonia-containing water, but then with a water-miscible organic solvent such as isopropanol or another alcohol The first wash, with water, serves to remove by-products of the reaction, such as ammonium nitrate and also the added THFA (tetrahydrofurfuryl) The second wash, with isopropanol, is to remove the water from the cores, and the wash must even be carried out with absolute isopropanol, which is particularly laborious since the reprocessing of the isopropanol is beyond the azeotrope for recrystallization Only anhydrous cores can then be further processed without being damaged dry Cores are calcined to remove the contained organic matter, reduced and finally sintered to the UO ₂ core.

The disadvantage of both methods lies in the necessity of using organic liquids for (after) -treatment of the freshly formed fuel / plastic droplets:

- The production areas in which they are used must meet strict explosion protection requirements. The use of some from a process engineering point of view advantageous apparatus such as continuous dryers is even impossible.

- Their emission is limited from an environmental point of view, they must be removed from the exhaust air streams.

- Their disposal or recycling is complex, especially since the recycling presupposes the absolutization.

Overall, the use of organic substances contributes significantly to the still substantial costs of producing fuels for high-temperature reactors.

DE-B-20 37 232, DE-B-15 92 477, DE-B-18 17 092 are known as prior art in which corresponding methods are described. DE-B-24 59 445 provides in that an aqueous suspension of uranyl nitrate, polyvinyl alcohol (PVA), urea and carbon black is dripped. DE-A-19 60 289 uses an aqueous solution of uranyl nitrate and urea, the hexamethylenetetramine in a temperature range between 0 ⁰ C and 10 ⁰ C is added to obtain a stable solution. The stable solution is dripped by means of a cooled to 5 ⁰ C nozzle in paraffin oil. From the Journal of Nuclear Science and Technology, Vol. 41, no. 9, p. 943-948 (September 2004) "Preparation Of UO ₂ Kernels For HTR-10 Fuel Elements" discloses a method of producing fuel particles after which urea / oxide solution is added prior to adding PVA and THFA urea.

The object of the present invention is to develop a process which retains the known good properties of fuel or hull cores, which are produced by the solution / gel process, but dispenses with the use of problematic organic substances. Also, a concentrated solution of water-soluble complex cations of heavy metals is to be provided to allow easy further processing while avoiding the disadvantages inherent in the prior art. It should easily be made a stable solution that is to drip.

The task is essentially solved according to the invention, the first reagent is added to the starting solution at room temperature and the solution thus prepared is heated to a temperature T of 80 ° <T <T _s with T _s = boiling temperature of the solution and over a time t with 2 h <t <8 h, the solution is then cooled to a temperature T _A with T _A ≥ room temperature and finally the second reagent is added. In particular, the second reagent is added to the solution at room temperature, the second reagent should be dissolved in solid form to avoid dilution of the solution.

Surprisingly, it has been found that when the starting solution provided with the dissolved first reagent is heated at an elevated temperature in the range preferably between 80 ^° C. and 100 ^° C., in particular in the range from 90 ^° C., and over several hours, preferably 3 to 6 Hours kept at this temperature, the solution remains stable, but at the same time the decomposition of the first Reagenzie and the removal of the excess of water takes place and at the same time a concentrated solution of water-soluble complex cations of heavy metals is formed, which can then be solidified after the addition of the second Reagenzie after the gel precipitation s process in an ammonia-containing atmosphere into beads, wherein in the subsequent treatment of the washed Globules the use of an alcohol such as isopropanol is not required.

The starting solution itself should be a 1 to 2.5 molar nitrate solution.

In order to remove the remaining water after washing in the microspheres, a further treatment in a vacuum atmosphere. It is envisaged that the further treatment for removing water at a pressure p with 0.07 MPa <p <0.09 MPa is performed.

According to the invention, urea or an equivalent first reagent is added to the starting solution at room temperature, in particular as a solid, in order to avoid dilution of the solution. The solution thus prepared is then heated to a temperature below the boiling point, preferably in the range between 80 ⁰ C and 100 ⁰ C, in particular of about 90 ⁰ C over a period of between 3 and 6 hours, preferably in about 4 hours, the Decomposition of the urea or the equivalent first reagent and the removal of the excess of water leads to a concentrated solution of water-soluble complex cations of heavy metals. With the addition of PVA and / or THFA as the second reagent beads are then prepared by the gel precipitation method, which are post-treated in a known manner while avoiding alcohol.

Consequently, according to the prior art, the casting solution, that is, the solution adjusted to a desired viscosity, dripped into microspheres in vibration nozzles, the microspheres solidified in an ammonia atmosphere on the surface, then collected in aqueous ammonia solution, in which they are aged. At- Finally, the microspheres are washed, dried, calcined and finally reduced to the desired cores and sintered. In that regard, however, reference is made to method steps that can be found in the prior art. Deviating from these process steps, however, a post-treatment without the use of alcohol.

Due to the teaching according to the invention, the solution is prepared without the use of hexamethylenetetramine and thus no cooling below room temperature is required.

According to the invention, a casting solution is prepared, as is known per se for the known "external gelation", but the first reagent is added for the internal release of ammonia upon increasing the temperature The first reagent, such as urea, allows the uranium concentration to be increased to levels, for example, up to 400 g / l (conventional methods with external gelation operate at concentrations around 100 g / l). This solution can then be dripped into relatively small droplets, the droplets are gelled in an ammonia atmosphere, then aged at elevated temperature in ammonia solution, washed with water, dried, calcined and sintered to specification-compliant UO ₂ cores.

For the preparation of spherical fuel particles or particles of uranium, thorium or plutonium oxide or the corresponding mixed oxides in the diameter range, preferably from about 0.1 mm to 1 mm, a solution of the corresponding heavy metal nitrates with urea or equivalent first reagent, the or ., Which dissociates ammonia in the heat and thereby reacts as precipitant, and mix additives for adjusting the viscosity, dribble this solution, superficially solidify the resulting beads in an ammonia atmosphere, further solidify the beads in an ammonia solution under elevated temperature, they only with water. Do not wash with water, dry, calcine and finally sinter to the final product with organic solvents.

In particular, it is envisaged that the solution PVA and THFA in the ratio of about 1:10 is added, in particular about 50 g / kgU PVA and about 500 g / kgU of THFA.

The invention is explained in more detail below by means of examples, from which further details, advantages and features of the invention result. The explained amounts or ratios and parameters are to be evaluated in such a way that they are also isolated from the other information for themselves.

Example 1:

For a core batch 20 kg of uranium in the form of U ₃ Os are dissolved in nitric acid, the resulting solution of uranyl nitrate is added after cooling and filtration with 8.8 kg of urea and held for 4 h at about 9O ⁰ C. After cooling, 1 kg of PVA and 9.5 kg of THFA are added and the solution is homogenized.

This solution is now dripped via nozzles in vibrating plates to droplets of the required size, in the example for a target diameter of the finished cores of 500 microns, the droplets fall through an approximately 50 mm high ammonia gas route before entering the precipitation 7-12 Immerse molar NH ₃ solution and gel here.

The fresh cores are transferred with the Fälllösungsstrom in the container for aging, washing, drying and aged here for several hours at about 60 ⁰ C, then washed at about 6O ⁰ C several times with water, and finally up to 8O ⁰ C dried under vacuum. The dried cores are first calcined in corresponding ovens under air for a special temperature program between 100 ⁰ C and 500 ⁰ C to UO ₃ , then under hydrogen at 600 ⁰ C - 700 ⁰ C and then at 165O ⁰ C to UO ₂ cores reduced and sintered.

Following the usual quality control methods such as screening and sorting, UO ₂ cores can be provided with over 90% yield for further processing with the properties shown in the following table:

Properties of the UO ₂ cores

Example 2:

For a UO ₂ core batch with a diameter of 500 μm, 4 kg of uranium in the form of U ₃ θ ₈ powder are dissolved in nitric acid. The solution is prepared by heating to 90 ⁰ C within 4 hours, wherein a stoichiometric uranyl nitrate solution of composition UO ₂ (NOs) ₂ is formed. After cooling and filtration 8 l of solution with 500 g U / l are obtained.

In this solution, 0.44 kg urea / kgU, so a total of 1.76 kg of urea are dissolved at room temperature.

Subsequently, this solution is heated to about 90 ⁰ C and kept at this temperature for 4 hours. Subsequently, the hot solution is cooled to room temperature and used for the production of casting solution. The volume is 8.1 1/4 kgU, corresponding to a U content of 494 g / l and a density of approximately 1.6 g / ml. For the production of casting solution, polyvinyl alcohol (PVA) is used, among other things, to increase the viscosity. By dissolving PVA in ultrapure water, a 10 weight percent PVA solution is prepared, with a density of 1.022 g / ml.

For a casting solution formulation of 4 kgU, corresponding to 4.5 kg UCv cores of diameter 500 μm, the following components are mixed to form a homogeneous casting solution:

8,1 1 of the urea heated to 90 ⁰ C for 4 h and again cooled to room temperature Uranylnitratlösung

1.92 kg of the 10% by weight PVA solution

2.0 1 tetrahydrofurfuryl alcohol of density 1.05 g / ml (Kp 177 ⁰ C).

The volume of this casting solution is 12.0 1 and the weight 17.3 kg. The U content is 231 g / kg solution and the viscosity in the range 55 to 80 mPa · s.

The casting solution is divided in a known manner by means of 5 flow meters and a 5-nozzle vibrator system at a frequency of 100 hertz in uniform drops, which harden after formation of the spherical shape in ammonia gas and then in 5 to 12 molar ammonia solution as a spherical Particles are collected from ammonium diuranate.

With a U content per 500 μm U0 ₂ core of 0.622 mg, at f = 100 Hertz and a U content of the casting solution of 231 g / kg results per nozzle a flow rate Q of

Q = 0.622 ^• f ^• 60/231 = 16.15 g / min.

For a 5-nozzle vibrator system, there is a throughput of 80.75 g casting solution / min or 4.845 kg / h. In an aging, washing and drying container in criticality-safe geometry, the cores are aged in ammonia solution at 60 ⁰ C, then washed three times with deionized water at 60 ⁰ C per 20 min and then in a vacuum of 0.07 to 0.09 MPa Dried at 80 to 90 ⁰ C for 6 h.

After calcination at 300 ⁰ C in air, followed by reduction and sintering under H ₂ ZAr gas to 1600 ⁰ C, the UO ₂ cores corresponding in diameter 500 + 50 microns and a density 10.8 g / cm ³ to the requirements.

Claims

A process for the production of spherical fuel and / or breeding particles

1. A process for the production of spherical fuel or Hutzstoffpartikeln from an oxide of the group of heavy metals uranium, thorium, plutonium or mixtures thereof comprising the steps of preparing a starting solution of the nitrates of at least one heavy metal of heavy metals, adding a first reagent from the group urea and / or ammonium carbonate and / or ammonium bicarbonate and / or ammonium cyanate and / or biuret, adding at least one second reagent in the form of PVA and / or THFA to adjust the viscosity of the solution, dripping the solution into microspheres, at least superficial solidifying the microspheres in an ammonia-containing Atmosphere, collecting the microspheres in an ammonia-containing solution and then washing, drying and thermal treatment, characterized in that the first reagent of the starting solution was added at room temperature and the solution thus prepared to a temperature T is heated with T _s = boiling temperature of the solution at 80 ° <T <T _s and held for a time t 2 h <t <8 h, the solution is then cooled to a temperature T _A with T _A ≥ room temperature and finally the second reagent is added.

2. The method according to claim 1, characterized in that the solution is cooled to room temperature prior to addition of the second reagent.

3. The method according to claim 1, characterized in that the first reagent from the group urea and / or ammonium carbonate and / or ammonium bicarbonate and / or ammonium cyanate and / or biuret is dissolved in solid form in the starting solution.

4. The method according to claim 1, characterized in that the solution is maintained at a temperature T with T ~ 90 ⁰ C over time t.

5. The method according to at least claim 1, characterized in that the solution over a time t with 3 h <t <6 h at the temperature T is maintained.

6. The method according to claim 1, characterized in that a 1 to 2.5 molar nitrate solution is used as the starting solution.

7. The method according to claim 1, characterized in that the washed microspheres are aftertreated without the use of alcohol.

8. The method according to claim 1, characterized in that after washing the microspheres is removed in this existing water by further treatment in a vacuum atmosphere.

9. The method according to at least claim 8, characterized in that the further treatment for removing water at a pressure p with

0.07 MPa <p <0.09 MPa.

10. The method according to claim 1, characterized in that PVA and THFA in the amount ratio of about 1:10 of the solution is added.