RU2326052C2 - Method cleaning uranium hexafluoride from technetium-99 nuclide - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be used for cleaning hexafluoride of a raw uranium regenerate from a radio active technetium-99 nuclide. The method involves reaction of uranium hexafluoride in the gas phase with synthetic calcium fluoride, containing at least 98.0% of the mass of the basic substance at temperature between 20°C and 40°C and pressure of the gas phase from 1400 to 40000 Pa. Contact time is not less than 2.5-5.0 seconds. The process is carried out in an adsorber with a layer of pelleted calcium fluoride with linear speed of the gas phase in the 0.05 - 0.6 m³ /m²×s range.

EFFECT: simplification of the cleaning of uranium hexafluoride.

4 cl, 1 dwg, 2 tbl

Description

The invention relates to a technology for the recycling of nuclear energy materials and can be used to clean the raw uranium regenerate hexafluoride from the technetium-99 radionuclide.

Uranium isolated from spent nuclear fuel is a valuable source for reuse in light-water reactors, since it contains fissile uranium-235 isotope in an amount not less than natural uranium and allows saving the latter. For this purpose, the regenerated uranium hexafluoride (UF ₆₎ is to be enriched with fissionable isotope uranium-235.

Due to the presence of a residual amount of radiation hazardous impurities formed as a result of nuclear transformations, the nuclide composition of the raw uranium regenerate is different from natural uranium. Temporary exposure, repeated radiochemical cleaning [see: Patent RU No. 2184083 C2, IPC 7 C01G 43/00, 54/00. Priority from 08.25.2000. Publ. 06/27/2002. Bull. No. 18] and the conversion of regenerated uranium to hexafluoride form make it possible to reduce the content of some unwanted nuclides to an insignificant level. But even after this, the regenerated uranium hexafluoride remains to a significant degree contaminated with the beta-active nuclide technetium-99, which is in the form of volatile, presumably higher fluoride ( ⁹⁹ TcF ₆ ).

Since, during isotope enrichment, radiation-hazardous impurities are concentrated in commercial uranium products, they limit the content of undesirable nuclides in the initial hexafluoride of raw uranium regenerate. According to the international specification ASTM C 787-2003, the limit value for technetium-99 nuclide in UF6 directed to isotope separation is 0.001 μgTc / gU.

A known method of purification of uranium hexafluoride from undesirable volatile impurities on distillation columns, described in US Patent No. 2953431, class. 23-14.5. Publ. 09/20/1960; DE patent No. 1095264, cl. 12n 10. Publ. 12/22/1960 (analogues).

The use of analogue methods is hindered by the complexity of their implementation.

A method for the sorption purification of uranium hexafluoride from volatile impurities on metal fluorides, where granular sodium fluoride (NaF) is used as the sorbent, is proposed [US Patent No. 33328132, class. 23-355. Publ. 06/27/1967; GB patent No. 1185817, cl. CIA. Publ. 03/15/1970 (analogues)]. It was found that the use of NaF at a temperature of at least 400 ° C in the absence of associated sorption of UF ₆ reduces the content of fission products by 4.6 times. Purification on sodium fluoride at 100 ° С is more effective with simultaneous sorption of UF ₆ and undesirable volatile fluorides. Upon subsequent desorption with uranium hexafluoride, the coefficient of purification from radionuclides is provided at the level of 1000.

However, the need for desorption of UF _{6 at} an elevated temperature (from 350 to 450 ° C) causes the parallel thermal decomposition of uranium hexafluoride in the solid adduct UF ₆ · 2NaF:

at 200-400 ° C

UF ₆ · 2NaF → UF ₅ · 2NaF + 0.5F ₂ ;

above 400 ° C

UF ₅ · 2NaF → UF ₄ · 2NaF + 0,5F ₂ .

Even in small amounts, complex compounds of pentavalent and tetravalent uranium block the sorption of impurity fluorides in the pores of the sorbent and limit its sorption capacity.

In addition, above 400 ° C with sodium fluoride parallel to UF ₆ is the desorption of TcF ₆ .

For the above reasons, for cleaning, preference is given to using those metal fluoride compounds, which, being effective sorbents for undesirable volatile impurities present in millionths of uranium hexafluoride, do not retain noticeable amounts of UF ₆ . Thus, a known method of purification of uranium hexafluoride from impurities of technetium [Patent WO No. 9852872, C01G 43/06. Publ. 11.26.1998 (analog)], where contaminated technetium UF ₆ in the liquid phase is brought into contact with metal fluoride, which is magnesium fluoride (MgF ₂ ), for a period of time sufficient to adsorb technetium fluoride. The implementation of the method is carried out at elevated temperature and pressure UF ₆ , significantly higher than atmospheric. The hardware design of the method requires autoclave performance.

The industrial use of the analogue method is limited by its potential hazard.

The closest in technical essence to the claimed is a method of purification of uranium hexafluoride from gaseous impurities [US Patent No. 3165376, class. 23-14.5. Publ. 01/12/1965], where the capture of higher fluorides of technetium, neptunium, molybdenum and vanadium is carried out in the gas phase on granular magnesium fluoride at temperatures from 93 to 121 ° C. It has been found that at the indicated temperature, MgF ₂ selectively delays said volatile undesirable fluorides. The method is proposed for use at concentrations of technetium-99 nuclide, commonly found in raw uranium regenerate hexafluoride - grams per ton UF ₆ . Sorption of impurities can be carried out by contacting the gaseous UF ₆ as with tableted MgF ₂ , and a fluidized bed of powder. Technetium can be removed from the sorbent by washing with water (up to 98% technetium is removed).

This method is selected as a prototype.

The disadvantage of the prototype method is the need for sorption purification at elevated temperatures, which in the conditions of large mass flows of external power of the isotope-separation cascades at which the raw uranium regenerate is enriched by the fissile uranium-235 isotope complicates its implementation. Operation of the adsorber at elevated temperatures may be accompanied by corrosive losses of uranium hexafluoride due to probable micro-leakage of atmospheric air moisture. The resulting nonvolatile uranium fluorides and oxyfluorides block the sorption of impurity fluorides in the pores of the sorbent and limit its sorption capacity. In addition, in the range of the claimed temperature range, where the highest efficiency of trapping of technetium fluoride, which is 95–99%, is achieved, magnesium fluoride still has a noticeable sorption capacity with respect to UF ₆ [Galkin N.P. and others. Capture and processing of fluorine-containing gases. M., Atomizdat, 1975, pp. 108-109], what causes additional losses of raw uranium regenerate on the sorbent. This does not allow to obtain a chemically pure concentrate of technetium for its subsequent use.

The present invention is directed to solving the problem of simplifying the purification of uranium hexafluoride from technetium-99 nuclide while reducing losses of UF ₆ .

The above problem is achieved by a technical solution, the essence of which is that in the method of purification of uranium hexafluoride from technetium-99 nuclide by contact of uranium hexafluoride in the gas phase with alkaline earth metal fluoride for a time sufficient to bind technetium fluoride, uranium hexafluoride in contact with synthetic calcium fluoride containing at least 98.0 wt.% the main substance.

In addition, the above task is also achieved by additional technical solutions, consisting in the fact that uranium hexafluoride is brought into contact with synthetic calcium fluoride for at least 2.5 ÷ 5.0 seconds; that uranium hexafluoride is brought into contact with synthetic calcium fluoride at a temperature of 20 ÷ 40 ° C and a gas phase pressure of from 1400 to 40,000 Pa; that uranium hexafluoride is brought into contact in the adsorber with a layer of tableted synthetic calcium fluoride at a linear flow rate of the gas phase from 0.05 to 0.60 m ³ / m ² × s.

Thus, the proposed method is based on the ability of synthetic CaF ₂ , containing at least 98.0 wt.% Of the basic substance, to actively bind volatile technetium fluorides from the gas phase of raw uranium regenerate hexafluoride. We note as a particular example that, at the initial content of technetium-99 nuclide 3 ÷ 4 μgTc / gU, purification with synthetic CaF ₂ is possible up to 0.0008 μgTc / gU. The studies also obtained the results of purification of UF ₆ to a technetium-99 nuclide content below the sensitivity limit of the used analytical methods (0.0003 μgTf / gU). These highly effective properties of synthetic calcium fluoride were not expected in the patent of the prototype, the description of which says about the inefficiency for the capture of volatile impurities from uranium hexafluoride other alkali earth metal fluorides, in particular CaF ₂ .

Selective sorption technetium fluoride on synthetic jet CaF ₂ was the basis for the implementation of a simple and highly efficient process of cleaning gas phase raw uranium hexafluoride regenerate impurities from volatile fluoride technetium nutritional isotopically separating stages.

Is established (see. Table 1) that the synthetic calcium fluoride is volatile compound effectively binds technetium at room or close to the temperature of the gas phase in the absence of uranium hexafluoride UF ₆ passing sorption [Galkin NP and others. Capture and processing of fluorine-containing gases. M., Atomizdat, 1975, p. 109].

The authors' studies did not concern the nature of the process — chemical or physical sorption accompanies the capture of volatile technetium fluorides. It is assumed that technetium in uranium hexafluoride is in the form of higher TcF ₆ fluoride, although other forms of volatile fluorides are known for technetium, primarily pertechnetium fluoride (TcO ₃ F). Less volatile forms of technetium include technetium oxytetrafluoride (TCOF ₄ ), TcF _4, or TcF ₅ [see: Basic Properties of Inorganic Fluorides: Reference. Ed. N.P. Galkina. - M., Atomizdat, 1975, p.114, 367].

Table 1 Capture efficiency ⁹⁹ TcF ₆ on synthetic CaF ₂ Temperature ° C twenty 26 26 thirty thirty thirty thirty 40 40 40 Contact time, s 2,5 0.5 5,0 2,5 4.0 5,0 10.0 1,0 5,0 10.0 The degree of extraction at a height of the sorbent layer of 0.38 m, wt.% 98.5 83,4 one hundred 99.7 one hundred one hundred one hundred 96.6 one hundred one hundred The degree of extraction at a height of the sorbent layer of 1.20 m, wt.% one hundred 91.5 one hundred one hundred one hundred one hundred one hundred 98.9 one hundred one hundred

The capture of volatile technetium fluorides on synthetic calcium fluoride can be carried out both in a tablet (granular) sorbent layer and in a fluidized powder layer. Moreover, the term "sorption" in this description refers to both the adsorption of TcF ₆ on the surface of CaF ₂ particles and inside the pores of a granular (pelletized) material.

In the claimed invention, UF _{6 is} used in the gas phase at a temperature of 20 ÷ 40 ° C to achieve the results described in the examples of its implementation below. The partial pressure of the gas phase of uranium hexafluoride in the indicated temperature range is from 10,000 to 40,000 Pa. These values are taken as the maximum pressure of using the method. The lower value of the gas phase pressure UF _{6 is} limited by the acceptable performance of the process for cleaning the external feed stream of the isotope separation cascades and cannot be lower than 1400 ÷ 2000 Pa.

As can be seen from Table 1, the purification of UF ₆ from the technetium-99 nuclide was excellent at a linear gas flow rate of more than 0.05 m ³ / m ² × s. Without an obvious decrease in efficiency, purification was successfully demonstrated up to a gas velocity of 0.6 m ³ / m ² × s. Velocity values are obtained by dividing the height of the sorbent layer by the contact time of the gas phase with the sorbent at a 100% or close degree of sorption extraction of TcF ₆ and a temperature of the sorbent of 40 ° C as the worst temperature conditions of sorption. The established interval of linear velocities of blowing the gas phase of uranium hexafluoride through an adsorber, on the one hand, ensures the industrial applicability of columns filled with granular (pelletized) sorbent in the form of synthetic calcium fluoride, for the indicated purposes with their actual overall design (the smallest value of the velocity interval). On the other hand, it guarantees the conditions for the most complete use of the external surface and internal pores of the sorption material for the extraction of technetium fluoride from the gas phase stream (the highest value of speed).

These conditions provide guaranteed sorption purification of uranium hexafluoride from technetium-99 nuclide in accordance with the requirements of the customer of enriched uranium products at acceptable cost parameters.

Synthetic calcium fluoride is a selective sorbent for volatile fluorides of technetium-99 nuclide. Since the technetium-99 nuclide is a valuable beta-emitting isotope, which is used as a corrosion inhibitor in aqueous media, for research purposes in the chemistry of transuranium elements, etc., the simultaneous production of its concentrates is, in principle, desirable. In this case, adsorbers with calcium fluoride can be preceded by adsorbers with fluorides of other elements, which are ineffective with respect to volatile technetium fluorides, but well retain other volatile radiation hazardous impurities. An example is the combination of AlF ₃ and CaF ₂ sorbents.

The accompanying description of the drawing shows a pilot plant for the implementation of sorption purification of uranium hexafluoride from impurities of volatile fluorides of technetium-99 nuclide on a sorbent of calcium fluoride. In the drawing, positions: 1 - cylinder with uranium hexafluoride, equipped with a heater 2; 3 and 4 - adsorption columns filled with layers 5 of a tabletted sorbent CaF ₂ ; 6 - critical apertures to create a normalized value of the gas phase flow UF ₆ ; 7 - a cylinder for receiving purified uranium hexafluoride, placed in a dewar 8 with liquid nitrogen; 9, 10 - sensors for measuring the surface temperature of the cylinder 1, as well as columns 3 and 4, respectively; 11 - vacuum gauges for measuring pressure of uranium hexafluoride in front of the critical diaphragms; 12 - sampling points of the gas phase UF ₆ .

It is clear that the specific design of an industrial installation may differ slightly from that shown in the drawing. Other instrumentation can be used to determine the linear flow velocity and temperature of the gas phase UF ₆ , as well as to calculate the contact time of the gas phase UF ₆ with the CaF ₂ sorbent.

The following are examples of specific performance of the proposed method.

Example 1. The study of the sorption extraction of technetium fluoride on tableted calcium fluoride was carried out using the experimental setup shown in the drawing. Column 3 was loaded with 100 grams of specially prepared tabletted sorbent from natural fluorspar (CaF ₂ ), column 4 - 90 grams of tabletted sorbent from synthetic CaF ₂ with a basic substance content of at least 98.0 wt.%. Sorbents in columns 3 and 4 were covered with layers 5, respectively A1, ..., A5 and B1, ..., B5, which were separated by porous partitions. The total height of the sorbent in layers A1-A5 and B1-B5 during various experiments ranged from 0.38 to 1.20 m.

During the experiments, samples from an industrial batch of fluorspar according to GOST 29219-91, grade 95 (A), containing at least 95.0 wt.% Of the main substance, were used as feedstock for producing CaF ₂ tablets with a diameter of 9 mm and a height of 5 mm and reagent "calcium fluoride" qualification "h" according to GOST 7167-77 with a basic substance content of at least 98.0 wt.%. To prepare a homogeneous charge, a sample with the starting material was placed in a Teflon glass and 40 wt% hydrofluoric acid was added if the starting material was fluorspar or distilled water if it was synthetic CaF ₂ . The use of hydrofluoric acid was explained by the need to remove silica and calcium carbonate impurities from natural fluorspar. The pasty mixture was mixed. Sorbent tablets were formed from the obtained homogeneous mass. The crude tablets were dried and then calcined.

The calcined tablets were placed in adsorption columns, vacuum dried at a temperature of 120 ÷ 150 ° C, and then treated with elemental fluorine at a temperature of 250 ° C to finally remove traces of moisture. And the sorbent already prepared in this way was contacted with UF ₆ . Columns included in parallel.

Uranium hexafluoride containing 3 grams of technetium per tonne of UF _{6 was} prepared by mixing specially synthesized ⁹⁹ TcF ₆ with natural (natural) uranium hexafluoride. The resulting mixture was homogenized in the liquid phase by heating the container 1 above 64 ° C.

The flow of UF ₆ through columns 3 and 4 was created by maintaining the temperature of cylinder 1 in the range of 20–40 ° C and cooling the cylinder 7 with liquid nitrogen in the dewar 8. The gas flow rate was determined by selecting the flow area of the critical orifices 6 and heating the cylinder 1 with heater 2; controlled according to the gauges 11. During the experiment, we measured the temperature of cylinder 1, columns 3 and 4, and also conducted sampling of the gas phase of uranium hexafluoride at the inlet and outlet of each column in sections 12. The pressure of the gas phase UF6 in the columns was varied in the range of 10,000 ÷ 40,000 Pa The temperature of the sorbent was 20–40 ° С. Under these conditions, the estimated time of gas contact with the layers of each sorbent was in the range from 0.5 to 10.0 seconds.

At the end of the experiments, cylinders 1 and 7 with UF ₆ were removed from technological places and weighed. In columns 3 and 4, a sample was taken from each layer of the CaF ₂ sorbent to determine the content of technetium-99 nuclide.

The experimental results are shown in table 2.

table 2 The content of technetium-99 nuclide in the sorbent layers, mgTf / gCaF ₂ Sorbent brand Sorbent bed number (along the gas) one 2 3 four 5 CaF ₂ (natural) 0.0015 0.0035 0.0033 0.0021 0,0006 CaF ₂ (synthetic) 0.104 0.000055 0.000007 0.000004 0.000005

As can be seen from Table 2, on synthetic CaF ₂ all technetium is retained in the frontal layer B1, whereas on the sorbent prepared from natural fluorspar, technetium is distributed over all layers A1-A5 of the sorption material. In the latter case, a slip of volatile technetium fluorides appeared very quickly at the output of column 4. The material balance of the technetium-99 nuclide content in UF ₆ in parts of the installation made it possible to establish the efficiency of TcF ₆ capture on synthetic CaF ₂ , which amounted to 99.97%.

The desorption of technetium fluoride from the sorbent was carried out by washing the sorption material with water. At the same time, it was possible to remove about 98.5 wt.% Technetium-99 nuclide.

Example 2. Experiments were conducted on the purification of UF ₆ obtained in large-scale fluoride and containing about 0.01 g of technetium-99 per 1 ton of UF ₆ . It was shown that at such an initial concentration of ⁹⁹ TcF ₆ after passing 1.5 tons of UF ₆ per kilogram of sorbent in the form of synthetic CaF ₂ , the cleaning efficiency was not less than 97%. At the same time, the degree of extraction on the sorbent, for example, of the nuclide ¹⁰⁶ Ru did not exceed 4–6% of the initial content in UF ₆ .

Thus, the claimed method for the purification of uranium hexafluoride from the beta-active nuclide technetium-99 has the following advantages compared with the known technical solutions:

- allows with a high degree of selectivity to retain unwanted technetium fluoride on synthetic calcium fluoride;

- use a widely available reagent for cleaning UF ₆ , the tabletted sorbent from which has good mechanical properties.

To implement the method in an industrial environment does not require the development of special technological equipment.

Claims (4)

1. The method of purification of uranium hexafluoride from technetium-99 nuclide upon contact of uranium hexafluoride in the gas phase with alkaline earth metal fluoride for a time sufficient to bind technetium fluoride, characterized in that the uranium hexafluoride is brought into contact with synthetic calcium fluoride containing not less than 98 , 0 wt.% The main substance. 2. The method according to claim 1, characterized in that the uranium hexafluoride is brought into contact with calcium fluoride for at least 2.5 ÷ 5.0 s. 3. The method according to claim 1, characterized in that the uranium hexafluoride is brought into contact with calcium fluoride at 20 ÷ 40 ° C and a gas phase pressure of from 1400 to 40,000 Pa. 4. The method according to claim 1, characterized in that the uranium hexafluoride is brought into contact in the adsorber with a layer of tableted calcium fluoride at a linear flow rate of the gas phase from 0.05 to 0.6 m ³ / m ² · s.