Method for reprocessing spent nitride fuel in molten salt

Abstract

A method for post-processing nitride spent fuel in molten salt is proposed, which includes chlorinating the fuel in a melt of a mixture of alkali metal and / or alkaline earth metal chlorides containing cadmium dichloride. The chlorination reaction is carried out in an apparatus for post-processing nitride spent fuel using an inert gas atmosphere. The apparatus has a heating zone and a cooling zone, the heating zone containing a reactor with molten chloride and nitride spent fuel immersed therein, and the cooling zone is arranged below the reactor. During the chlorination process, the area of ​​the apparatus containing the reactor is heated to a temperature exceeding 700°C, the nitride spent fuel is maintained in the melt until fully chlorinated, and the cooling zone of the apparatus is used to crystallize the metallic cadmium formed during the chlorination process. The technical effects of the present invention include increasing the conversion rate of nitride spent fuel in molten chloride to 100%, which reduces or completely eliminates the necessity for additional stages of repeated filtration and post-processing of the spent fuel.

Description

Technical Field

[0001] The present invention relates to nuclear power engineering, in particular to a method for reprocessing spent fuel (SNF), wherein the spent fuel is extracted from a heating element, and the method can be used in the spent fuel reprocessing technology in the operation of a closed nuclear fuel cycle. Background Art

[0002] The most important task of modern nuclear power engineering is the creation of a closed nuclear fuel cycle (CNFC) for fast neutron reactors. CNFC allows to significantly increase the energy efficiency of operating technologies for nuclear energy production, reduce the consumption of natural resources, minimize the amount of radioactive waste (RW), and thus solve problems related to the transportation, storage or reprocessing of this waste.

[0003] At present, nuclear energy is preferably produced using oxide or metal fuels (Olander D., Theoretical foundations of the heat-dissipating elements of nuclear reactors, M., 1982), the reprocessing of which by existing methods implies an initial extended storage (3-7 years) of the heat-dissipating elements (fuel elements). Further reprocessing is carried out by hydrochemical methods. The disadvantage of this technology is the generation of large amounts of liquid radioactive waste (radioactive water).

[0004] The greatest prospects for the creation of CNFCs are related to the reprocessing of SNFs stored for a shorter period of time in molten salts. In particular, this concerns a nitride fuel that can be sent to the reprocessing stage practically immediately after its recovery from the reactor, thanks in particular to its higher degree of burnout in the reactor (RU 2524681 C2 published on August 10, 2014). Existing hydrochemical methods for the treatment of SNFs cannot be used to treat SNFs stored for a shorter period of time, to which nitride SNFs belong. This is associated with the release of a large amount of energy, which would lead to the evaporation of radioactive water.

[0005] A method for post-treatment of nitride SNF by chlorination is known, which method comprises anodic dissolution of its conductive components in the electrolysis of a chloride melt (RU 2079909 published on May 20, 1997). The known method is carried out at a temperature range of 450 to 700° C., an anodic current density of 0.3 A / cm and a cathodic current density of up to 0.4 A / cm, wherein the nitride SNF is used as anode and a low-melting alloy or metal such as cadmium is used as cathode, which are immersed in a chloride melt in an apparatus representing an electrolysis cell for chlorinating the nitride SNF. During the anodic dissolution of the components of the nitride fuel (for example, nitrides UN and PuN), nitrogen is released and actinides are dissolved in the chloride melt (for example in the form of chloride UCl). 3 and PuCl 3 The chloride melt is the source for the electrodeposition of uranium and plutonium at the cathode.

[0006] The advantages of this method are the stability of the chloride melt (more precisely, the stability of its physicochemical properties under the influence of radiation irradiation) and the possibility of selecting the electrolysis parameters to achieve a high separation factor of the nitride SNF component returned to the fuel cycle.

[0007] The disadvantage of this method is that the nitride SNF forms the compound UOCl when it is anodic polarized in a chloride melt. y and UNCl (Journal of Nuclear Materials 344 (2005), 128-135). The formation of these compounds prevents complete anodic dissolution of nitride SNF in chloride melts and results in additional filtration steps and aqueous chemical dissolution of unreacted SNF, thus limiting the integrity of fuel conversion.

[0008] The closest to the claimed method is the method for reprocessing nitride spent fuel in a salt melt (RU2603844 published on December 10, 2016). The method consists of chlorination of the fuel, which consists of continuous anodic dissolution of the fuel and then electrochemical reduction on a liquid metal cathode in a chloride melt of LiCl-KCl-CdC at a temperature not exceeding 500°C. The method is carried out in an inert gas atmosphere in an apparatus for chlorination of nitride SNF, which represents an electrolyzer. In this case, up to 5% by mass of CdCl is present in the melt. 2 , which triggers the chemical chlorination of the nitride SNF, and the anodic dissolution also increases the dissolution rate of UN in the LiCl-KCl melt.

[0009] The disadvantage of this method is that anodic dissolution and lower temperatures increase the formation of insoluble compounds UNCl on the surface of the chlorinated nitride SNF. This leads to passivation phenomena, low fuel conversion, filtration of unreacted SNF and additional operations of water chemical dissolution, which significantly reduces the energy efficiency of the process.

[0010] The above-mentioned disadvantages of the known methods do not fully solve the problems associated with developing energy-saving and low-waste methods of performing technological operations within the framework of a closed nuclear fuel cycle. Summary of the invention

[0011] An object of the present invention is to provide an energy-efficient method for reprocessing nitride SNFs which is suitable for incorporation into schemes for a closed nuclear fuel cycle.

[0012] For this purpose, a method is proposed, which as a prior art comprises chlorinating nitride spent fuel / spent nuclear fuel in a melt of a mixture of chlorides of alkali metals and / or alkaline earth metals containing cadmium dichloride, the chlorination reaction being carried out in a device for post-processing nitride spent fuel using an inert gas atmosphere. The method of the invention differs from the prototype in that the chlorination reaction is carried out in an apparatus having a heating zone and a cooling zone, the reactor having the chloride melt and the nitride spent fuel immersed therein being located in the heating zone, the cooling zone being placed below the reactor, and in that during the chlorination process, the area of ​​the apparatus with the reactor is heated to a temperature above 700° C., the nitride spent fuel is kept in the melt until it is completely chlorinated, wherein the cooling zone of the apparatus is used to crystallize the metallic cadmium formed in the chlorination reaction. The method is also characterized in that chlorides of lithium, potassium, sodium, cesium, calcium, magnesium, strontium, barium are used as chlorides of alkali metals and / or alkaline earth metals.

[0013] The essence of the claimed method is as follows: When the nitride SNL (for example the nitride UN) comes into contact with the chlorinating agent cadmium dichloride in a LiCl-KCl melt, an exchange reaction will occur, for example:

[0014] UN+1.5 CdCl 2 =UCl 3 +1.5 Cd+N2(g) (1)

[0015] UN+0.5 CdCl 2 =0.5 UNCl+0.5 Cd (2)

[0016] LiCl-KCl melt is also used as the starting CdCl 2 And the obtained UCl 3 (and other actinides) solvents.

[0017] The data for thermodynamic modeling, i.e., Figure 1 The negative value of the standard Gibbs energy (D(7°)) shown indicates that reactions (1) and (2) proceed in parallel at all temperatures, so that the escape of a portion of the uranium into the insoluble phase of UNCl seems inevitable. However, calculations also show (see Figure 1 ), at temperatures of 700°C and above, UNCl dissolves in excess CdCl according to reaction (3) 2 middle:

[0018] UNCl+CdCl 2 =UCl 3 +Cd+0.5 N 2 (r) (3)

[0019] This produces a product containing UCl 3The melt does not contain impurities in the form of UNCl and other insoluble actinides.

[0020] The basic chlorination product is therefore a LiCl-KCl melt containing actinide chlorides, which is used for further electrolytic deposition of actinides on a solid or liquid cathode.

[0021] The method is carried out in an apparatus with an inert atmosphere, wherein the nitride SNF is immersed in a reactor with a chloride melt located in one area of ​​the apparatus, which is heated to a chlorination temperature exceeding 700°C. It is recommended that the chlorination temperature not exceed 900°C, which is consistent with the CdCl 2 The vapor pressure of CdCl 2 The boiling point of CdCl is 960°C. Increasing the temperature above 950°C means that 2 The evaporation of CdCl 2 , the process efficiency is reduced, and the CdCl 2 The vapors are captured in the chlorination plant.

[0022] A distinctive feature of the claimed method is the formation of metallic cadmium according to reactions (1) and (2), which completely evaporates from the melt at the process temperature. In order to capture it, the equipment for post-treatment of the nitride SNFs has a cooling zone in which the formed cadmium crystallizes. Unlike the prior art, the claimed method does not require substantial anodic dissolution of the nitride SNFs, which increases the passivation phenomena on the surface of the post-treated samples, nor does it require the presence of a certain content of UCl in the initial melt. 3 .

[0023] The above description of the invention is provided to carry out the method in a mixture of chlorides LiCl-KCl, which is the most common melt for the thermochemical post-treatment of SNFs. However, the technical result will also be achieved by using mixtures of chlorides of alkali metals and / or alkaline earth metals (e.g. chlorides of lithium, potassium, sodium, cesium, calcium, magnesium, strontium, barium) that do not interact with nitride SNFs. The mixtures of the listed chlorides can be used as chloride CdCl in the claimed method. 2 and UC1 3 The solvent used is a mixture of chlorides LiCl-KCl. The influence of the composition of the mixture on the outcome of the chlorination reaction of nitride SNF has not yet been determined.

[0024] The technical effects achieved by the claimed method include increasing the conversion of nitride SNF in a chloride melt to 100%, thereby reducing or completely eliminating the necessity for additional stages of repeated filtration and post-treatment of the SNF. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The invention is illustrated in the accompanying drawings, in which:

[0026] Figure 1 is a schematic diagram of an apparatus for chlorinating nitrogen compounds SNF according to the claimed method. Figure 2 is a diagram for which the process is carried out at temperatures exceeding 700° C. for thermodynamic reasons. Figure 3 It shows that when the nitride SNF (UN nitride) is immersed in a LiCl-KCl melt, the UCl in the melt 3 and CdCl 2 The table shows the parameters and results using the prototype and the claimed method. DETAILED DESCRIPTION

[0027] The experimental confirmation of the chlorination of the nitride SNF according to the present method is carried out in a closed device with an inert atmosphere, for which anhydrous argon is used. The device includes a housing 1, a heater 2, a reactor 3, a chloride melt 4, a nitride spent fuel 5, a metal collector 6, and a cadmium 7. The device has a heating zone I and a cooling zone II.

[0028] In the heating zone of the device, a reactor 3 in the form of a ceramic crucible made of oxide MgO is placed, which has previously prepared chlorides KCl, LiCl, CdCl 2 The eutectic mixture of LiCl-KCl was prepared by zone recrystallization to maximize the removal of oxygen impurities and mixed with pure CdCl in a "dry box". 2 The equipment area with the reactor was heated to a temperature of 750°C, and then the UN nitride sample, which is the main component of the nitride SNF, was immersed in LiCl-KCl-CdCl 2 The reaction is then carried out in a melt and kept there until, according to chemical analysis, the reaction is complete.

[0029] During the experiment, melt samples were collected by a special locking device to analyze CdCl 2 and the resulting UCl 3 Based on the data obtained, the conversion of nitride UN to UCl in the chlorination reaction was calculated. 3 The table gives the main parameters and experimental results, and Figure 3 shows CdCl in LiCl-KCl melt 2 and UCl 3 Typical curve of the interdependence of the content. Figure 3 It can be seen that during the chlorination process, CdCl 2 The concentration of UCl 3This demonstrates the direct progress of reactions (1), (2) and (3). At an experimental temperature of 500 °C, the metallic cadmium formed according to reactions (1) and (2) was concentrated at the bottom of the reactor, while at 750 °C, the cadmium was collected in a special metal collector (see Figure 2 ). It can be seen from the table that at temperatures of 700°C and above, the conversion rate of nitride SNF (UN nitride) is 100%, while at lower temperatures, the conversion rate is only 20% to 60%.

[0030] For industrial use, the plant must be designed so that the cooling zone is located below the reactor where the chlorination reaction is carried out. In a particular case, a metal collector in the form of a crucible is placed in this zone to crystallize the metallic cadmium formed during the chlorination process and to condense the cadmium vapor. As a result, the reactor of the plant produces a LiCl-KCl melt containing actinide chlorides and separately collected cadmium.

[0031] After the chlorination reaction is completed, the LiCl-KCl melt containing actinide chloride and cadmium is recovered from the device and enters the electrolytic deposition step of actinides. In a specific embodiment, the electrolytic deposition of actinides can be carried out directly in the reactor carrying out the chlorination reaction.

[0032] Mixtures of alkali metal and / or alkaline earth metal chlorides (chlorides of lithium, potassium, sodium, cesium, calcium, magnesium, strontium, barium) which do not interact with the nitride SNF can be used as chloride melt containing cadmium chloride in order to optimize the process parameters of the method.

[0033] The claimed method for reprocessing nitride spent fuel thus makes it possible to increase the conversion of nitride SNF in a chloride melt up to 100%, which reduces or completely eliminates the necessity for additional stages of repeated filtration and reprocessing of the SNF.

Claims

1. A method for post-processing nitride spent fuel, comprising chlorinating the nitride spent fuel in a melt of LiCl-KCl and CdCl2, wherein the chlorination is performed in an apparatus for post-processing nitride spent fuel using an inert gas atmosphere, characterized in that: A LiCl-KCl melt is prepared by zone recrystallization to remove oxygen impurities and mixed with CdCl2 to obtain a LiCl-KCl-CdCl2 chloride melt (4); The device for post-processing nitride spent fuel comprises a heating zone I and a cooling zone II, wherein a reactor (3) having a LiCl-KCl-CdCl2 chloride melt (4) and nitride spent fuel (5) immersed therein is located in the heating zone I, and the cooling zone II is placed below the reactor (3); During the chlorination process, a heating zone I of a device for post-processing nitride spent fuel, which has a reactor (3), is heated to a temperature above 700° C., and the nitride spent fuel (5) is kept immersed in a LiCl-KCl-CdCl2 chloride melt (4) until it is completely chlorinated; In the cooling zone II, the evaporated metallic cadmium formed in the chlorination reaction during the chlorination of LiCl-KCl-CdCl2 with complete evaporation of CdCl2 in the heating zone I of the apparatus for reprocessing nitride spent fuel is crystallized.

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