Method of reprocessing nuclear fuel based on uranium metal

 

(57) Abstract:

The invention relates to the field of metallurgy, namely, processing of nuclear fuel based on uranium metal containing silicon and/or aluminum. Nuclear fuel pre-melted in a vacuum melting furnace and maintain the melt in the presence of oxygen at a residual pressure in the furnace below the equilibrium pressure of volatile oxides lower degrees of valences of silicon and/or aluminum, after which the ingot is dissolved in acid and extract the uranium extraction process. The method allows to completely remove the spent fuel silicon and partially aluminum, as well as to reduce the volume of vitrified waste. 5 C.p. f-crystals.

The invention relates to the field of metallurgy, namely pyrometallurgical reprocessing of nuclear fuel based on uranium metal, mainly metallic uranium-aluminum fuel, including aluminum-silicon layer, and uranium-silicide fuel.

There is a method of reprocessing spent uranium metal-aluminum fuel nuclear reactors method of dissolution in the alkali in the presence of sodium nitrate [H. Peterson, R. Wye is eskay scheme, large amounts of equipment, occupying valuable floor space, large volumes of produced water solutions containing radionuclides that must be disposed of.

The closest in technical essence and the achieved result is a method of reprocessing spent uranium metal-aluminum fuel, including the dissolution in nitric acid in the presence of nitrate of mercury, which is a catalyst, followed by separation of the uranium from the solution by the method of solvent extraction [H. Peterson, P. Weimer. Chemistry in nuclear technology. - M.: Atomizdat, 1967, S. 212-213]. Remaining after the extraction of uranium solution containing aluminum and radionuclides, evaporated and sent to vitrification. The disadvantages of this method are:

the presence of silicon in the solution fed to the extraction, resulting in the reduction of uranium extraction in this operation;

- the formation of large amounts of glass, the main share of which take radionuclides, and aluminum.

Technology pre-separation of the aluminum and silicon from uranium and radionuclides in solutions up to the present time does not exist.

A method for processing aderno varenie in acid and solvent extraction separation of uranium from radioactive fission products and stable impurities, according to which nuclear fuel is pre-melted in a vacuum melting furnace and incubated in the presence of oxygen at a residual pressure in the melting chamber below the equilibrium pressure of volatile oxides lower degrees of valences of silicon and/or aluminum.

The process is conducted to remove the silicon from uranium.

The process is carried out to a residual aluminum content in the uranium 0.5 to 35 wt.%.

The process is conducted in a vacuum induction furnace metal split-cooled crucible, transparent to electromagnetic fields, electromagnetic stirring of the melt.

As a nuclear fuel based on uranium metal containing silicon and/or aluminum, use processed metallic uranium-aluminum fuel with aluminium-silicon layer and uranium-silicide fuel.

Frequency induction currents is 50-200000 Hz.

It was established experimentally that the surface of the melt, consisting of uranium metal, silicon and/or aluminum, in the presence of a certain amount of oxygen to form a film composed of oxides of silicon - SiO2and/or aluminum - Al2O3. When distillate components of the oxide film with Si and/or Al, contained in the melt:

SiO2+2Si=2SiO (gas) (1)

Al2O3+4Al2=3Al2O (gas) (2)

with the formation of gaseous oxides of lower degrees of valences of silicon and/or aluminum, mainly SiO and Al2Oh, which have very high vapor compared with SiO2, Al2ABOUT3, SiO, Al2O, Si and Al. In addition, thermodynamic calculations has been revealed and experimentally confirmed that in the temperature range 900-1800oWith needed to melt the uranium-aluminum, uranium-silicon and uranium-aluminum-silicon nuclear fuel compositions, the vapor pressure of SiO is almost always higher than the vapor pressure of Al2O. This allows, feeding oxygen to the zone of the melt within a certain period of time and while maintaining the vacuum in the melting chamber is lower than the equilibrium pressure of volatile oxides lower degrees of valences of silicon and aluminum, to remove the silicon as SiO from melt completely, and aluminum in the form of Al2About only partially. Uranium and the estimated amount of aluminum will remain in a cold crucible in the form of a melt.

SiO and Al2O captured on a special water cooled condenser is installed above the crucible. If cond is.)+Si(TV.) (3)

Al2O(gas.)=Al2O3(TV.)+Al(TV.) (4)

The implementation process in a vacuum induction furnace with crucible, transparent to electromagnetic fields, mainly metal split water-cooled crucible (IPHT), provides electromagnetic stirring of the melt, which intensify the mass transfer of silicon and/or aluminum from the depth of the molten metal on the surface where he(and) enter into reactions according to equations (1-2).

The use of induction currents of relatively low frequency range (50-200000 Hz) allows the use of simple and reliable sources of power supply, which greatly simplifies the design IPT, and provides a release of melt from the crucible walls. When this occurs, the increased surface melt compared to the flat, and therefore increases the amount of volatile oxides of silicon and aluminum, are removed from the crucible in a unit of time.

Residual aluminum content in the melt of 0.5-35 wt.% is determined by the composition of the glass used for immobilization of radionuclides allocated for long-term storage.

The proposed method can be applied for processing of otrabotavshego, and for the reprocessing of irradiated uranium metal alloys with silicon and/or aluminum. With this purpose, the source material is loaded into the vacuum melting furnace and melted. The melt was kept for a certain period of time, feeding him oxygen and maintaining residual pressure in the melting chamber below the equilibrium pressure of volatile oxides lower degrees of valences of silicon and/or aluminum. The exposure time is determined by the composition of the source material and the amount of aluminum you want to remove from the melt. After complete removal of the molten silicon and partially of aluminum it is crystallized, then removed from the crucible, dissolved in acid and the resulting solution extracted uranium method of extraction with organic extractants. The remaining solution containing radioactive fission products and other impurities, evaporated, Steklova and sent for storage.

Example 1. Alloy mass of 4 kg, containing, wt.%: Al - 70, U - 26 and Si - 4, were loaded in a vacuum induction furnace with copper split water-cooled crucible, transparent to electromagnetic fields, and melted by pointing directly at the alloy induction currents of a frequency of 2400 Hz. The melt was kept within the residual pressure in the melting chamber was maintained at a level of 510-2mm RT. Art. Values of the equilibrium pressures of Al2Oh and SiO in this temperature range are within 0,062-0,48 and 35.7-330 mm RT. Art. respectively. To the melt was continuously applied to the oxygen with a speed of 1.5 g/S. Evaporates from the surface of the melt SiO and Al2O was caught on a copper water-cooled condenser is installed above the melting crucible. Extracted from the crucible ingot uranium contained, wt.%: Si - 0,23, Al - 5,7.

Example 2. Alloy mass of 4 kg, containing, wt.%: Al - 75 and U - 25 was melted in the same furnace. The melt was kept for 18 min, maintaining the temperature of the melt within 1250-1650oC. the Residual pressure in the melting chamber was maintained at a level of 510-2mm RT. Art. Is the equilibrium pressure of Al2In this temperature range is within 0,062-0.48 mm RT. Art. In the melt was continuously applied to the oxygen with a rate of 0.9 g/s is Extracted from the crucible ingot uranium contained of 33.4 wt.% Al.

Example 3. Alloy weight 7.5 kg, containing, wt.%: Si - 30 and U - 70 was melted in the same furnace. The melt was kept for 30 min, maintaining the temperature of the melt within 1250-1750oC. the Residual pressure in the melting chamber was maintained at a level of 20 mm RT. Art. Is roadvale oxygen at a speed of 0.75 g/sec. Extracted from the crucible ingot uranium contained 0.08 wt.% Si.

As follows from the obtained results show the effectiveness of the proposed method. The implementation of this method will allow you to:

1) almost completely removed from the spent fuel silicon as drastically reducing the uranium extraction in the extraction process;

2) because the aluminum is included with the glasses for immobilization of HLW and does not affect the extraction of uranium for subsequent operations, then delete it (aluminum) from spent fuel is only partially, leaving in the melt such amount, which will provide content in the glass at the required level. As a result, in proportion to the quantity of the removed aluminum reduces the volume of vitrified waste generated during reprocessing of spent nuclear fuel.

1. Method of reprocessing nuclear fuel based on uranium metal containing silicon and/or aluminum, including the dissolution in acid and solvent extraction separation of uranium from radioactive fission products and stable impurities, characterized in that the nuclear fuel pre-melted in a vacuum melting furnace and maintain the melt in the presence of CI is ti silicon and/or aluminum.

2. The method according to p. 1, wherein the process is conducted to remove the silicon from the melt.

3. The method according to PP. 1 and 2, characterized in that the process is carried out to a residual aluminum content in the melt of 0.5-35 wt. %.

4. The method according to p. 1, characterized in that as a vacuum melting furnace using a vacuum induction furnace with a metal split-cooled crucible, transparent to electromagnetic fields in electromagnetic stirring of the melt.

5. The method according to p. 4, characterized in that the frequency of induction of currents is 50-200000 Hz.

6. The method according to p. 1, characterized in that as a nuclear fuel based on uranium metal containing silicon and/or aluminum, use of spent metallic uranium-aluminum fuel with aluminium-silicon layer and uranium-silicide fuel.

 

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SUBSTANCE: claimed method includes junk charge into premelted flux at ratio of 1:(5-10); heating up to melt temperature; smelting under flux layer, and separation of metal from flux. Equimolar mixture of sodium chloride and potassium chloride with addition of 2.9-52.6 % (in respect to total flux weight) magnesium fluoride is used as flux, and in melting process flux layer with thickness of 4.5-20 cm is maintained. Method affords the ability to conserve original composition and eliminate additional burdening with magnesium.

EFFECT: decreased burn-off loss, especially for magnesium, metal of improved quality.

4 cl, 3 tbl, 5 ex

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