RU2479490C2 - Method of cleaning uranium hexafluoride from ruthenium fluorides - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the technology of recycling nuclear energy materials and specifically to methods of cleaning uranium hexafluoride from ruthenium fluorides, and can be used in returning uranium, extracted from spent nuclear fuel, into the fuel cycle of light water reactors. The method of cleaning uranium hexafluoride from ruthenium fluorides involves reaction of gaseous uranium hexafluoride with a sorbent, the sorbent used being porous granules of sintered metal powder of nickel or nickel which contains up to 10 wt % copper.

EFFECT: invention provides efficient cleaning of uranium hexafluoride from ruthenium fluorides at low temperatures, safety and simplification of the process, and also enables to reuse the sorbent after regeneration.

5 cl, 2 tbl, 5 ex

Description

The invention relates to a technology for recycling nuclear energy materials, in particular to methods for purifying uranium hexafluoride (HFC) from ruthenium fluorides, and can be used to return uranium extracted from spent nuclear fuel to the fuel cycle of light-water reactors.

Raw uranium regenerate contains the residual amount of nuclides formed as a result of nuclear transformations: transuranium elements (neptunium, plutonium), fission products (ruthenium, cerium, antimony, etc.), technetium. Temporary exposure, repeated radiochemical purification, and the conversion of regenerated uranium to hexafluoride make it possible to reduce the content of some radiation hazardous impurities to an insignificant level.

But even after the mentioned operations, the regenerated uranium hexafluoride remains noticeably contaminated with the residual amount of ruthenium fluorides ( ¹⁰⁶ RuF ₅ , ¹⁰⁶ RuF ₆ ), which is associated with the multivalence of this element.

A known method of purification of uranium hexafluoride from impurities in the form of fluorides on granular (pelletized) sorbents - sodium, calcium, magnesium and lithium fluorides (Chemical technology of irradiated nuclear fuel. Edited by VB Shevchenko. - M .: Atomizdat, 1971, p. .338). The process is carried out at a temperature of 100-500 ° C. The method indicates the possibility of the formation at high temperature of fusible complex compounds that clog the pores of granular sorbents and prevent the full use of their sorption capacity.

It is known to purify gaseous uranium hexafluoride from impurities, including RuF ₅ , on a CaCO ₃ sorbent (US Patent No. 4364906, C01G 43/06, publ. 1982). Cleaning is also carried out at high temperatures of 150-600 ° C.

The use of granular barium fluoride, selectively absorbing impurity fluorides and providing a purification coefficient of 3.2 to 6.9, is described in (Shatalov V.V. et al. Gas fluoride processing technology for spent oxide fuel. - Atomic energy, 2001, T.90 , issue 3, p. 215). It is also shown here that the use of granular NaF at 400 ° C reduces the content of fission products by 4.6 times. However, at elevated temperatures, along with the main process of sorption of impurities on sodium fluoride, a reaction of thermal decomposition of uranium hexafluoride occurs.

A common disadvantage of these methods is the high temperature of the process.

A known method of purification of uranium hexafluoride from ruthenium compounds at room temperature (Patent RU No. 2068287, IPC A62D 1/00, publ. 10.27.1996). The method includes contacting a gaseous uranium hexafluoride containing ruthenium fluorides at a pressure of 56.6 mm Hg. and a temperature of 24.72 ° C with an inorganic sorbent - ferric fluoride (FeF ₃ ). The method provides extraction on sorbent ¹⁰⁶ RuF ₅ from a stream of regenerated uranium hexafluoride by 36-37 wt.% With a sorbent layer height of about 0.40 m and a contact time of about 10 seconds. With an iron trifluoride layer height of 1.10 m and a contact time of 20 s, the extraction of ruthenium compounds was 73.2%. This method is selected for the prototype.

Ferric fluoride powder is difficult to compact. To increase the strength of FeF ₃ tablets, plasticizers are added to the initial powder, the choice of which is limited due to the need for chemical inertness to the HFC medium. Ferric fluoride granules are inconvenient to use - brittle, break when loaded into the adsorber, dust.

The objective of the invention is the expansion of the arsenal of methods for the effective purification of uranium hexafluoride from ruthenium fluorides at low temperatures.

This object is achieved in that in a method for purifying uranium hexafluoride from ruthenium fluorides, including the interaction of gaseous uranium hexafluoride with a sorbent, uranium hexafluoride is passed through porous granules of sintered metal powder of nickel or nickel containing up to 10 wt.% Copper.

Use granules of sintered metal powder, which are plates or pieces of tape with a thickness of (0.05-0.15) × 10 ^-3 m, the pore size of which is (0.5-5.0) × 10 ^-6 m.

As granules, scrap of porous filtering nozzles from uranium diffusion enrichment machines is used.

The interaction of gaseous uranium hexafluoride with granules is carried out at a temperature of 22-25 ° C and a pressure of (3.9-40.0) × 10 ³ Pa for 10-150 seconds with a density of filling granules (0.28-0.70) kg / dm ³ .

Granules after loading into the adsorber are treated (passivated) with fluorine or inorganic fluoroxides.

The proposed method is based on the ability of the surface of metallic nickel and a nickel fluoride film on the surface of metallic nickel to actively bind volatile ruthenium fluorides from the gas phase of raw uranium regenerate hexafluoride. A nickel fluoride film on the surface of metallic nickel is formed upon passivation of the nickel surface before contact with uranium hexafluoride medium.

The absorber is a granule in the form of plates or pieces (segments, fragments) of tapes, rammed in the filter-adsorber to a given value of the density of the backfill. Tapes or plates are made of sintered metal nickel powder, nickel can be alloyed with copper (copper content up to 10 wt.%), The size of the initial powder particles is (0.1-10) × 10 ^-6 m. Tapes or plates have a thickness (0, 05-0.15) × 10 ^-3 m and pore size (0.5-5.0) × 10 ^-6 m. The pores are through. The absorption of impurities by the sorbent occurs when a cleaned medium (HFC) is passed through the pores of the sorbent (in fact, filter material). The surface area of the sintered metal powder is 1.3-1.4 m ² / g.

A method of manufacturing a filter material for highly effective cleaning of process media having similar thicknesses and pores through which the medium to be cleaned and which absorb impurities includes forming a substrate of nickel powder, sintering, filling the pores of the substrate with ultrafine nickel powder, pressing and subsequent sintering of the ultrafine powder to the substrate while in the coarse-porous layer of the substrate creates a selective finely porous layer. The manufacturing technology of such a filter material (absorbent microimpurities) is described, for example, in patent RU No. 2055694, IPC B22F 3/10, publ. 03/10/1996.

To achieve the goal of the claimed invention, it is most expedient to use scrap of porous filter nozzles from uranium diffusion enrichment machines as the granules described above. Mentioned porous filtering nozzles are made of tapes with a thickness of (0.05-0.15) × 10 ^-3 m from sintered metal nickel powder; Nickel can be partially alloyed with copper (up to 10 wt.%). The equivalent pore diameter in the sintered nickel powder is (0.5-5.0) × 10 ^-6 m. The tapes in the nozzles are folded into sleeves. Scrap is pieces (fragments, segments) of ribbons that are obtained by cutting nozzles. Pieces of tapes are rammed in an adsorber to achieve the required density of the sorbent backfill (0.28-0.70) kg / dm ³ .

The thickness of the tape and the pore size in it, on the one hand, are most favorable for the diffusion of HFCs into the porous structure of the tape and adsorption of ruthenium fluorides in it, and on the other hand, provide the mechanical strength of the granules (tape fragments).

Before use, the adsorber backfill is treated with fluorine or inorganic fluoroxidants, for example, chlorine trifluoride or iodine heptafluoride, by contacting the granules with a fluorine-containing medium for 15-60 minutes.

The following are examples of the method.

Example 1

The gas phase of uranium hexafluoride containing ruthenium fluorides in the amount of 1.94 × 10 ² BqRu-106 / gU was passed at a pressure of 3.9 × 10 ³ Pa and a temperature of 23.0 ° C through two series-connected adsorption columns filled with porous granules metallic nickel. Granules are scrap of filtering porous nozzles from uranium diffusion enrichment machines in the form of fragments of tapes from which the nozzles are made, fragments have a length of (40-80) × 10 ^-3 m. Tapes have a pore size of (0.5-5.0) × 10 ^-6 m, the thickness of the tapes (0.05-0.15) × 10 ^-3 m. The total volume of backfill of scrap is 40 dm ³ , the weight of scrap is 11.4 kg, the density of backfill of scrap is 0.285 kg / dm ³ , height layer of nozzle scrap in each column is 0.50 m. The contact time of HFCs with the sorbent is ~ 10 seconds.

During the experiment, samples of uranium hexafluoride “before” and “after” adsorption columns were taken to determine the content of radionuclides, including ruthenium-106. The content of γ-emitting nuclides in the samples was measured by a γ-spectrometer. According to the results of the analysis of the samples, the efficiency of purification of uranium hexafluoride from Ru-106 nuclide (the amount of Ru-106 nuclide retained by the sorbent relative to the initial content of ruthenium nuclide in the medium to be purified, expressed as a percentage) was 24.9%.

Example 2

The purification of uranium hexafluoride from the nuclide Ru-106 was carried out according to the conditions of example 1, only the contact time of HFCs with the crowbar of nozzles was increased to 25 seconds. The purification efficiency of uranium hexafluoride from Ru-106 was 47.6%.

Example 3

Uranium hexafluoride with a content of ruthenium fluorides in an amount of 0.65 × 10 ² BqRu-106 / gU at a pressure of 40.0 × 10 ³ Pa and a temperature of 24.0 ° C was passed through two adsorption columns filled in series with nozzle crowbars from diffusion enrichment machines uranium, cut into fragments with a length of (5-8) × 10 ^-3 m. The total scrap volume is 40 dm ³ , the mass of scrap is 24.8 kg, the density of the backfill of scrap is 0.62 kg / dm ³ , the height of the scrap layer in each adsorber ~ 0.50 m. The contact time of HFCs with the sorbent in the experiment is ~ 30 seconds. According to the results of the analysis of samples of uranium hexafluoride “before” and “after” the adsorption unit, the efficiency of purification of HFCs from Ru-106 nuclide was 75.2%.

Example 4

Uranium hexafluoride, which is in a solid state in the initial process vessel with an equilibrium vapor pressure above a solid surface of (10.7-12.0) × 10 ³ Pa, was purified from Ru-106 nuclide by condensation into an empty (receiving) vessel through an adsorber, filled with scrap filtering nozzles from uranium diffusion enrichment machines in the form of tapes cut into fragments with a length of (20-40) × 10 ^-3 m. The receiving container was frozen to the temperature of liquid nitrogen. The total scrap volume is 11.5 dm ³ , the mass of scrap is 7.9 kg, the density of the backfill of scrap is 0.69 kg / dm ³ , the height of the scrap layer in the adsorber is ~ 0.30 m.

The presence of ruthenium nuclides in uranium hexafluoride was determined by measuring the technological capacities with a portable gamma spectrometer. Measurement of gamma radiation was carried out in the energy range from 50 to 1500 keV.

Recondensation conditions: pressure (10.7-12.0) × 10 ³ Pa; temperature - 22.0 ° С, contact time of HFCs with sorbent ~ 130 seconds.

The coefficient of purification of uranium hexafluoride from ruthenium fluorides was determined as the ratio of two quantities: the first value is the ratio of the intensity of γ-radiation Ru-106 (peak energy 621.7 keV) to the intensity of γ-radiation U-235 (peak energy 185.6 keV) from the initial capacity, and the second value is the ratio of the intensity of γ-radiation Ru-106 to the intensity of γ-radiation U-235 from the receiving technological capacity. The results are presented in table 1.

Table 1 Nuclide Γ-ray intensity, imp./s Technological capacity Source The reception Ruthenium-106 12.3 (bottom)
13.9 (mid)
average 13.1 5.14 (bottom)
6.69 (mid)
average 5.92 Uranium-235 71.0 (bottom)
141.0 (mid)
average 106.0 88.7 (bottom)
125.0 (mid)
average 106.9 The ratio of the intensity of γ-rays Ru-106 / U-235, average 13.1: 106.0 = 0.124 5.92: 106.9 = 0.055 Cleaning ratio 0.124: 0.055 = 2.25 Cleaning efficiency (0.124-0.055): 0.124 × 100% = 55.6%

After recondensation, a scrap sample was taken from the adsorber. The specific activity of the initially “pure” sorbent after contact with HFCs was 5.67 × 10 ³ BqRu-106 / kgNi (or 2.09 × 10 ^-6 μgRu-106 / kgNi).

Example 5

The conditions of the experiment corresponded to example 4. The contact time of HFCs with the sorbent is ~ 150 seconds; HFC recondensation temperature - 25.0 ° С; the volume of nozzle scrap in the adsorber is 25.4 dm ³ ; nozzle scrap weight - 17.5 kg; the density of the filling of scrap nozzles - 0.7 kg / DM ³ ; the height of the scrap layer in the adsorber is ~ 0.70 m.

13.6 kg of HFCs were recondensed. The efficiency of purification of uranium hexafluoride from ruthenium fluorides was determined analogously to example 4. The results are presented in table 2.

table 2 Nuclide Γ-ray intensity, imp./s Technological capacity Source The reception Ruthenium-106 12.3 (bottom)
13.9 (mid)
average 13.1 2.23 (bottom)
2.78 (mid)
average 2.51 Uranium-235 71.0 (bottom)
141.0 (mid)
average 106.0 91.2 (bottom)
125.4 (mid)
average 108.3 The ratio of the intensity of γ-rays Ru-106 / U-235, average 0.124 0,023 Cleaning ratio 0.124: 0.023 = 5.39 Cleaning efficiency (0.124-0.023): 0.124 × 100% = 81.5%

Thus, the proposed sorbent in the form of porous metallic nickel (tapes from scrap filtering porous nozzles from uranium diffusion enrichment machines) makes it possible to clean uranium hexafluoride from trace elements of ruthenium-106 nuclide in the form of its fluorides. The sorbent does not have dusty properties, and also lends itself to liquid regeneration for reuse.

It is economically important that the filter nozzles from uranium diffusion enrichment machines are decommissioned, stored, and do not require large expenditures for use in the technology for purifying uranium hexafluoride from radioactive impurities.

The method is simple and safe technologically, does not require complex hardware design.

Claims (5)

1. The method of purification of uranium hexafluoride from ruthenium fluorides, comprising the interaction of gaseous uranium hexafluoride with a sorbent, characterized in that uranium hexafluoride is passed through porous granules of sintered metal powder of nickel or nickel containing up to 10 wt.% Copper. 2. The method according to claim 1, characterized in that they use granules of sintered metal powder, which are plates or pieces of tape with a thickness of (0.05-0.15) · 10 ^-3 m, the pore size of which is (0, 5-5.0) ^10-6 m. 3. The method according to claim 2, characterized in that the granules use scrap of porous filtering nozzles from uranium diffusion enrichment machines. 4. The method according to claim 2 or 3, characterized in that the interaction of gaseous uranium hexafluoride with granules is carried out at a temperature of 22-25 ° C and a pressure of (3.9-40.0) · 10 ³ Pa for 10-150 s at the density of the filling of the granules of the sorbent (0.28-0.70) kg / DM ³ . 5. The method according to claim 1, characterized in that the granules after loading into the adsorber are treated with fluorine or inorganic fluoroxidants.