The way pyrochemical regeneration nuclear fuel

 

(57) Abstract:

The invention relates to the field of processing of irradiated and defective nuclear fuel, in particular mononitrides uranium-plutonium fuel. The inventive container load nitride fuel and use the electrolyte, the composition of which in % by weight is not less than 15% of trichloride uranium, heat the electrolyte to a temperature of at least 600oC and create the container, the current density is not more than 0.3 A/cm2and on the electrode is not more than 0.4 A/cm2in addition , fusible electrode to create a current density of not more than 0.1 A/cm2and the melt is heated to a temperature not exceeding 700oC, and the container with nitride fuel is placed fusible metal or alloy, more electrophoretically than uranium nitride, and, in addition, is dissolved in a container of this metal or alloy and precipitate the uranium and plutonium on fusible electrode is the cathode. 4 C.p. f-crystals, 2 tab.

The invention relates to the field of processing obolgannogo and defective nuclear fuel, in particular mononitrides uranium-plutonium fuel.

Improvement of operational characteristics of fast reactors, increasing their betapace monocarbide. Fundamental properties mononitride fuel when used in the active zone a long way to ensure inherent safety, to reduce the sodium void reactivity effect (reactors, sodium cooled), to increase the reactivity margin on the fuel burnup, etc.

The known method hydrometallurgical regeneration mononitride fuel (PUREX process), which consists in dissolving in nitric acid, the extract solution, extraction of actinides 30% solution of TBP in n-dodecane [1] the final product of which are oxides of uranium and plutonium.

The disadvantages of this method are the large water volumes, which contain 15 to 20 percent fissile elements (U and Pu); the need for preliminary lengthy excerpts extracted from the reactor fuel to reduce activity to avoid decomposition of the extractants; a significant number of high-level waste solutions 150 700 l/tonne fuel and more (3 to 5 m3) solutions average activity; obtaining the final product in the form of oxides of uranium and plutonium, of which the synthesis and manufacture of cores (tablets) mononitride is a more complex technology, requiring for redlagaemoe method is a method of pyrochemical regeneration of nuclear fuel, namely, that in the molten eutectic mixture of salts of potassium chloride and lithium enter the uranium chloride and immersed container with nuclear fuel and two electrodes, one of which is made of fusible metal, then connect the container with the positive and negative electrodes with the poles of the current source and conduct electrolysis, after which subjected deposited on the electrodes (cathodes) products dividing the heat-emitting pure metals [2]

According to this method, the irradiated metal doped fuel is subjected to anodic dissolution in the electrolyte and the cathode to the recovery of metals on the cathode.

The advantage of this method according to the authors is its compact size, small number of waste, reducing the cost of capital construction, the possibility of NPP allocation.

The disadvantage of this method is its applicability only for the regeneration of the metal fuel, because other fuels, such as nitride, compounds of uranium, plutonium and other elements of the fission products are chemically stable compounds. In particular, the values of free energy of formation of mononitride uranium G298= -63,4 kcal/mol vikluchaetsya in the obtaining of metallic uranium and plutonium from irradiated and/or defective nuclear fuel, in particular, mononitrides uranium-plutonium fuel, for the production of nuclear fuel, suitable for the needs of the nuclear industry.

The result of solving the aforementioned problem provides the decomposition mononitride fuel and the transfer of uranium, plutonium and actinides in the form of chlorides in the electrolyte, the recovery and allocation of metallic uranium and plutonium removal of fission products (PD) of nuclear fuel.

According to the invention in the method of pyrochemical regeneration of nuclear fuel, which consists in the fact that in the molten eutectic mixture of salts of potassium chloride and lithium enter the uranium chloride, immerse the container with nuclear fuel and two electrodes, one of which is made of fusible metal, then connect the container with the positive and negative electrodes with the poles of the current source and conduct electrolysis, after which subjected deposited on the electrodes (cathodes) products dividing the heat-emitting pure metals, container load nitride fuel and use the electrolyte, which in the mass is not less than 15% trichloride uranium, heat the electrolyte to a temperature of at least 600oC and create on the container or road which case the electrode to create a current density of not more than 0.1 A/cm2and the melt is heated to a temperature not exceeding 700oC, and the container with nitride fuel is placed fusible metal or alloy, more electrophoretically than uranium nitride, and, in addition, after running out of fuel is dissolved placed in the container more electrophoretically than uranium nitride, a low-melting metal or alloy and precipitate the uranium and plutonium on fusible electrode the cathode.

Irradiated or defective mononitride fuel is subjected to electrochemical decomposition (at the anode). During anodic dissolution occurs the separation of PD in the form of gases (nitrogen, xenon, krypton, partially iodine) and in the form of insoluble sludge residue (zirconium nitride, molybdenum, technetium, and noble metals). In the electrolyte together with the uranium, plutonium and actinides are transferred and accumulated chlorides of alkali, alkaline earth and rare earth metals.

The restoration of the main mass of the uranium is held on the electrode located at the operating temperatures in the solid state, with the formation of metallic crystals dendritic structure, and plutonium, actinides and the remnants of the uranium restore on fusible electrode (cadmium) with the formation of the alloy.

Received on lit.

Separation of plutonium and other actinides from the fusible alloy is carried out by distillation of the metal cathode.

To exclude the presence in the electrolyte of the oxygen containing compounds and obtain the given content trichloride uranium latter is introduced into the electrolyte by anodic dissolution of metallic uranium.

Example 1.

The following examples of implementation was conducted in an electrolyzer coupled with a glove sealed camera service with dry inert atmosphere (argon).

In the crucible of steel HIT load the mixture of potassium chloride and lithium medictions composition (for example, 55 and 45% by weight, respectively), melt it and saturate the melt trichloride uranium (for example, by anodic dissolution of uranium in the electrolyte). Then the melt was placed a container in the form of, for example, perforated molybdenum basket loaded with pills from mixed (uranium-plutonium) nitride fuel with a diameter of 6.9 mm, a length of 11 mm, a density of 83 95% of TP and two electrodes, one of which is made of fusible metal. Connect the container with a positive and "solid" electrode with the negative poles of the power source. By passing direct current produced by the kind of the cathode in the form of dendritic powder. Because in the process of electrolysis of spent fuel also stand out DD is cleaned metallic uranium and plutonium from harmful impurities.

For the extraction of metals from the cathode sludge spend dividing melting at a temperature of 1250oC, after which the released electrolyte is returned to the electrolysis, and an ingot of an alloy of uranium with plutonium, purified from the AP, sent on receipt of the fuel alloy.

The way pyrochemical regeneration nuclear fuel spent in different modes and the electrolyte composition.

Obtained empirically the results are shown in tables 1 and 2.

Based on these data, it was determined that to obtain the claimed technical result is needed in the eutectic mixture of salts of potassium chloride and lithium chloride to input at least 15% by weight of uranium chloride, heated to a temperature of at least 600oC while maintaining the current density at the anode is not more than 0.3 A/cm2and on "hard" the cathode is not more than 0.4 A/cm2because when the temperature is less than 600oC and the content in the melt of trichloride uranium in an amount not less than 15% by weight yield of uranium reaches industrial values and uneconomical, and at current densities on Antioch processes: dissolution of the material of the container and/or salt decomposition of the electrolyte, the formation of fine crystals of uranium and plutonium.

When these values were processed 600 g will smeet of mononitrides uranium and plutonium for 15 hours of continuous operation. The dissolution rate was 0.4 0.45 g/cm2o'clock In the resulting cathode product contained 564 g of uranium and plutonium, and 245 g (30%) salt electrolyte. The extraction of uranium and plutonium during electrolysis was 95 97%

From the obtained uranium and plutonium method of hydrogenation and nitriding was again obtained nitride to produce fuel pellets. Electrochemical processing smelting and separation did not practically affect the quality of remanufactured cores (for example, the increase in oxygen occurred from 0.1% to 0.15 wt%).

Example 2.

When carrying out the regeneration process of example 1 PU stands for "solid" electrode is the cathode in the form of fine powder, which leads to significant losses of plutonium. For more complete extraction of uranium and plutonium second electrode, made of a fusible metal, such as cadmium, are also connected to the negative pole of the current source and create it a current density of 0.1 A/cm2. At the anode the decomposition of mononitride is and fusible plutonium with the formation of an alloy with the metal electrode.

It was found that increasing the current density on the fusible electrode

the cathode of more than 0.1 A/cm2is to restore it along with plutonium, uranium, resulting in solidification of the alloy and the selection of the metal uranium and plutonium in the form of fine powder that prevents a more complete extraction of uranium and plutonium.

Example 3.

In the melt mixture of salts (see example 1), heated over 700oC, electrolysis was performed when indicated in example 1 modes and determined that the output of uranium and plutonium current has not changed. But increased consumption and corrosion of the equipment and require special cooling system sealed assemblies of equipment.

Example 4.

In order to improve the contact of the fuel with the container, the retention of fission products and localization slurry the residue in the container has napravila layer of fusible metal (cadmium, tin or lead) or alloy, more electropolishing than uranium nitride.

The pyrochemical process of regeneration was carried out at current density at the anode is not more than 0.3 A/cm2on hard the cathode is not more than 0.4 A/cm2and fusible (cadmium) the cathode is not more than 0.1 is, for example by transferring them to a container of fuel pellets, resulting in improved electrical contact of the tablet container and stopped the removal of the talus with tablets in the electrolyte, which contributed to more complete fuel processing and removal of PD.

Direct extraction of uranium and plutonium amounted to 93.7% of the Rest of the nitrides in the container, and the electrolyte is not found.

Example 5.

To increase the degree of regeneration of the fuel after being in the container-anode tablets nonconforming mixed nitride fuel dissolved in the electrolyte under the same conditions described in example 4, to produce a partial dissolution at the anode weld on him fusible metal, saturate it with salt electrolyte and thereby provide further separation of the uranium and plutonium on fusible electrode is the cathode.

1. The pyrochemical method of regeneration of nuclear fuel, which consists in the fact that in the molten eutectic mixture of salts of potassium chloride and lithium enter the uranium chloride, immerse the container with nuclear fuel and two electrodes, one of which is made of fusible metal, then connect the container with the positive and negative electrodes with the poles of the current source and spend Elke emitting pure metals, characterized in that the container load nitride fuel and use the electrolyte, which includes at least 15 wt. trichloride uranium, heat the electrolyte to a temperature of at least 600oWith and build on the container-anode current density of not more than 0.3 A/cm2and on the electrode-the cathode is not more than 0.4 A/cm2.

2. The method according to p. 1, characterized in that the fusible electrode-cathode create current density is 0.1 A/cm2.

3. The method according to PP. 1 and 2, characterized in that the melt is heated to a temperature not exceeding 700oC.

4. The method according to PP. 1 to 3, characterized in that in the container with nitride fuel is placed fusible metal or alloy, more electrophoretically than uranium nitride.

5. The method according to PP. 1 to 4, characterized in that after running out of fuel in the container dissolve placed in it more electrophoretically than uranium nitride, a low-melting metal or alloy and then precipitated onto fusible electrode-cathode uranium and plutonium.

 

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