JP3823593B2 - Method for reprocessing spent nuclear fuel and method for reprocessing spent nuclear fuel - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for reprocessing spent nuclear fuel generated at a nuclear power plant and a method for reprocessing fuel from spent nuclear fuel, and in particular, uranium and plutonium are removed from spent fuel by utilizing the difference in volatility of fluorides. The present invention relates to a method suitable for reprocessing and recovering, and reprocessing the recovered uranium and plutonium as fuel.
[0002]
[Prior art]
As a method for reprocessing spent nuclear fuel, for example, M. Benedict et al., Kiyose Kyohei, Nuclear Chemical Engineering, Volume IV, Chemical Engineering for Fuel Reprocessing and Radioactive Waste Management (Nikkan Kogyo Shimbun), 18 The PUREX method, which is a wet method described below, is widely used. The Purex method is a method in which spent fuel is dissolved in nitric acid, and uranium (U) and plutonium (Pu) in the nitric acid solution are extracted and recovered with an organic solvent of tributyl phosphate.
[0003]
As a dry method that does not use an aqueous solution, two types of molten salt electrolysis methods have been developed, which are called the Russian method and the ANL method from the names of the developing countries and institutions. The Russian method is a method in which an oxide fuel is melted in a molten salt of chloride, U oxide is deposited on an electrode, and Pu oxide is precipitated and recovered. The ANL method is a technique for recovering U metal and Pu metal to separate electrodes (cathodes) by reducing the metal fuel as it is or by reducing the oxide fuel to a metal and melting it in a molten salt of chloride.
[0004]
Examples of the dry reprocessing method other than the molten salt electrolysis method include a fluoride volatilization method in which reprocessing is performed using the difference in volatility of fluoride described in Nuclear Industry Vol. Are known. The conventional fluoride volatilization method is a method for recovering U and Pu with high purity by utilizing techniques such as distillation, partial condensation, fractional distillation, and adsorption.
[0005]
[Problems to be solved by the invention]
The wet purex method as a reprocessing method for spent nuclear fuel is an aqueous solution system and uses an organic solvent, so that critical limits will become stricter in future high burnup fuel and fast reactor fuel with high Pu. It is said that degradation due to radiolysis of the solvent becomes a problem.
[0006]
On the other hand, the dry method that does not use an aqueous solution is said to easily avoid the problem of criticality and the problem of radiation degradation. Both dry methods are still under development, but the Russian method, which targets oxide fuels, has development issues such as improving the decontamination factor (DF) of products and treating salt waste. The ANL method, which mainly targets metal fuels, requires additional processes for oxide treatment, and has development issues such as improvement of product DF, salt waste treatment, and improvement of treatment speed.
[0007]
The conventional fluoride volatilization method, which is a dry reprocessing method other than the molten salt electrolysis method, recovers U and Pu with high purity, which complicates the process, and in the process of purifying Pu. There existed problems, such as the recovery rate of Pu falling by decomposition | disassembly.
[0008]
The object of the present invention is suitable for various fuels that can avoid the problems of criticality and radiation degradation, do not require salt waste treatment, can improve product DF, improve processing speed, simplify process, and improve Pu recovery rate. It is to provide a reprocessing method and a rational fuel reprocessing method following the reprocessing.
[0009]
[Means for Solving the Problems]
Among the objects of the present invention described above, criticality, radiation deterioration, avoidance of salt waste treatment, improvement of treatment speed, and improvement of applicability to various fuels can be achieved by adopting the fluoride volatilization method.
[0010]
Considering the use of the reprocessed fuel (U, Pu), the fuel that needs to be improved in the product DF is U. In the reprocessing method that is a feature of the present invention, the mixture of U, U and Pu, and fission generation By separating and recovering each product, stable DF improvement of fluoride can be easily achieved.
[0011]
The mixture of U and Pu recovered as a product in the same manner as U does not need to improve the DF in consideration of its use as a mixed oxide fuel, and therefore the purification process can be eliminated, which simplifies the process. And improvement of Pu recovery rate. In addition, the mixture of U and Pu, which has a high value as a reprocessed fuel, is obtained by directly oxidizing the mixture of U and Pu fluoride in the fuel reprocessing method, which is a feature of the present invention, and again using a vibration filling method or the like. By processing as a mixed oxide fuel, it is possible to simplify the series of processes from reprocessing to fuel reprocessing and to improve the Pu recovery rate in the series of processes.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
[Example 1]
A method for reprocessing spent nuclear fuel rods generated in a light water reactor, which is a first embodiment of the present invention, will be described with reference to FIG.
[0013]
By loading spent fuel rods into the fluorination unit 3 after de-coating or as-is and supplying the fluorinating agent 1 to this unit, U, which is most likely to become a fluorinated product, is first converted into uranium hexafluoride (UF ₆ ). Volatilizes. By selecting a fluoride such that only U is fluorinated as the fluorinating agent 1, a relatively high purity U can be obtained. In this case, either know the production of UF ₆ for previously supplied amount of fluorinating agent 1, by measuring the amount of UF ₆ to generate, the amount of U to be mixed with Pu into the fluoride device Leave it. It volatilizes and the UF ₆ is stored as it is or after purification as needed and stored in a cylinder. When re-concentrate the U is still stored the more efficient of UF _6, if not reconcentrated better converted into oxides can be reduced storage space.
Next, a fluorinating agent 2 stronger than the fluorinating agent 1 is supplied to the fluorination device, and Pu and the remaining U are fluorinated and volatilized. Since the fluorinating agent 2 is used to volatilize the entire amount of Pu (approximately 100%), a sufficiently powerful reagent is applied. When the fluorinating agent 2 has an oxidizing action, the volatilization of Pu proceeds relatively smoothly. At this time, nuclides other than U and Pu, such as neptunium (Np), americium (Am), iodine (I), and technesium (Tc), are partially or fully fluorinated, but the mixture of U and Pu is a remote technology Because it is processed into fuel, no problems arise. In addition, by adjusting the amount of Pu, sufficient combustion occurs in the nuclear reactor. Rather, it is more convenient in terms of nuclear non-diffusion when the impurity nuclides are mixed, making handling as nuclear material difficult. The recovered mixture of U and Pu is converted from fluoride to oxide and reused as a mixed oxide fuel (MOX). Upon conversion, the liberated fluorine is recycled as a fluorinating agent.
Eventually, non-volatile fission products remain in the fluorination apparatus 3 as high level waste. Since these remain as oxides or metals, they can be stably stored and disposed of by compression molding, vitrification, ceramic solidification, and artificial rock solidification.
[0014]
As described above, according to the present embodiment, it is possible to separate and collect U, U and Pu mixture and fission products from spent nuclear fuel rods by using only the fluorination device 3, and thus simplify the reprocessing system. In addition, the reprocessing plant can be constructed and the operating cost can be reduced. The reprocessing system is simplified, the generation site of waste such as FP is limited, and fluorine is recycled, so that the amount of radioactive waste can be reduced. Since the recovered U is relatively high in purity, it is easy to store and re-concentrate, and since the mixture of U and Pu contains a relatively large amount of impurity nuclides, it has the effect of preventing nuclear proliferation (diversion of nuclear weapons). By selecting an appropriate fluorinating agent, long half-life nuclides such as Np, Am, I, and Tc can be recovered, which has the effect of reducing the burden of managing high-level waste.
[0015]
[Example 2]
A method of reprocessing spent nuclear fuel rods generated in a fast reactor and reprocessing the mixture of U and Pu to be recovered as nuclear fuel material, which is a second embodiment of the present invention, will be described with reference to FIG.
[0016]
After the spent fuel rod is sheared as necessary, it is loaded into the pyrolysis furnace 4 and a hydrogenating agent such as hydrogen gas is supplied to temporarily convert the nuclear fuel material and the cladding tube into a hydride. After the hydride conversion, an oxidant such as oxygen gas is supplied to convert the hydride into an oxide. As the crystal form and lattice constant change during each conversion process, the nuclear fuel material and the cladding tube are pulverized. Further, volatile fission products (FP) such as iodine are volatilized and removed. To increase the degree of pulverization and volatilization, it is effective to increase the number of times of heating and hydride-oxide conversion. Volatilized iodine is removed by a silver-based adsorbent or the like, and the powdered nuclear fuel material and the cladding tube are transferred to the fluorination tower 5.
[0017]
The nuclear fuel material and the cladding tube transferred to the fluorination tower 5 are fluorinated in two stages as in the first embodiment. That is, by first supplying a fluorinating agent 1 such as BrF ₅ , most of U is volatilized in the form of UF ₆ . In this case, either know the production of UF ₆ for previously supplied amount of fluorinating agent 1, by measuring the amount of UF ₆ to generate, the amount of U to be mixed with Pu into fluoride in the tower Leave it. Next, a fluorinating agent ₂ such as F ₂ is supplied to the fluorination tower 5, and the entire amount of Pu and the remaining U are fluorinated and volatilized. At this time, nuclides other than U and Pu, such as neptunium (Np), americium (Am), iodine (I), and technesium (Tc), are partially or fully fluorinated, but the mixture of U and Pu is a remote technology. Because it is processed into fuel, no problems arise. In addition, by adjusting the amount of Pu, sufficient combustion occurs in the nuclear reactor. Rather, it is more convenient in terms of nuclear non-diffusion when the impurity nuclides are mixed, making handling as nuclear material difficult.
[0018]
UF ₆ volatilized in the first stage of fluorination is purified by removing impurity nuclides coexisting in the adsorption tower and / or condensing tower such as a NaF trap, and stored in a cylinder, or reconcentrated or fueled. Be transported. If purification is not required, leave it as is. U after being refined or sent to the fuel production process without purification, and converted to uranium dioxide (UO ₂ ) in the fuel production process, U is a nucleus (seed) for converting the mixed fluoride of U and Pu into oxides described below. ) Can be used. The FP removed in the purification process is managed as a high-level waste together with the FP removed in other processes.
[0019]
The mixed fluoride of U and Pu volatilized in the second stage of fluorination is supplied together with water vapor and hydrogen gas to the oxide conversion tower 6 in which uranium dioxide particles (powder) as seeds are loaded in advance. It is converted into mixed oxide particles of Pu. The mixed fluoride becomes an oxide on the surface of the seed particles by the action of water vapor and hydrogen gas, and is laminated to form large particle diameter particles. At this time, the size of the mixed oxide particles can be controlled by adjusting the size of the seed uranium dioxide particles, the flow rate and composition of the supply gas, and the reaction time. The mixed oxide particles after the reaction are not spheres but distorted spheres. These mixed oxide fuel particles of U and Pu are sent to the fuel processing step and processed again as fuel by vibration filling. A gas containing fluorine generated upon conversion to a mixed oxide is recycled as a fluorinating agent.
[0020]
Eventually, non-volatile FP remains as high-level waste in the fluorination tower 5. Since these remain as oxides or metals, they can be mixed with FP generated in other processes and compression-molded as they are, or formed as waste by vitrification, ceramic solidification, artificial rock solidification, etc. It can be stored and disposed of stably.
[0021]
As described above, according to the present embodiment, a simple system including only a pyrolysis furnace, a fluorination tower, an adsorption tower, an oxide conversion tower, and a vibration packing device is used to remove U, U and Pu mixture and FP from spent nuclear fuel. Therefore, it is possible to construct a reprocessing plant and reduce operating costs. Since the reprocessing system is simplified and the fluorine is recycled, the amount of radioactive waste can be reduced. The recovered U becomes highly pure after the purification process, so it is easy to store, re-concentrate, and produce fuel (pellets). The mixture of U and Pu contains a relatively large amount of impurity nuclides, so it can be used for nuclear proliferation (diversion of nuclear weapons). There is an effect that can be prevented. By selecting an appropriate fluorinating agent, long half-life nuclides such as Np, Am, I, and Tc can be recovered, which has the effect of reducing the burden of managing high-level waste. In addition, since the fuel is refined by the pyrolysis furnace, the recovery of the nuclear fuel material can be improved by promoting the fluorination of U or a mixture of U and Pu. Since the mixed oxide of U and Pu generated in the oxide conversion tower is used as the fuel for vibration filling as it is, the recovery rate of nuclear fuel material, particularly Pu can be improved. At this time, there is no need to bother producing the vibration filling fuel, so that there is an effect of reducing the fuel production processing cost. The mixed oxide of U and Pu contains impurity nuclides and has a relatively high dose rate. However, since it is reprocessed as fuel by vibration filling, the exposure dose can be reduced.
[0022]
【The invention's effect】
According to the present invention, when the spent nuclear fuel is reprocessed, the effect can be exhibited in various points as compared with the conventional method. That is, since uranium is recovered with high purity, it is easy to reuse uranium, for example, re-concentration, and it is very easy to manage uranium during storage. Moreover, since uranium and plutonium can be recovered as a mixture and directly as a fuel reworking raw material, the fuel reworking cost can be reduced, and high purity plutonium can not be handled alone, so that the nuclear non-proliferation can be enhanced. Compared to the conventional method, the equipment and processing facilities can be simplified, so the economy can be improved.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a spent fuel reprocessing apparatus used in a spent nuclear fuel reprocessing method according to a preferred embodiment of the present invention.
FIG. 2 is a configuration diagram of a reprocessed fuel reprocessing apparatus used in a fuel reprocessing method from spent nuclear fuel according to another embodiment of the present invention.
[Explanation of symbols]
3 ... fluorination device, 4 ... pyrolysis furnace, 5 ... fluorination tower, 6 ... oxide conversion tower.

Claims (6)

In the reprocessing method that recovers uranium and plutonium by fluorinating spent fuel and recovering uranium and plutonium, uranium, which is easily fluorinated, is volatilized and recovered by using weak fluorinating agent or reducing the concentration of fluorinating agent. Then, by using a strong fluorinating agent or increasing the concentration of the fluorinating agent, plutonium, which is difficult to be fluorinated, is volatilized and recovered as a fluoride, leaving fission products as the residue, uranium, plutonium, fission products Separated into each
A method of reprocessing fuel from spent nuclear fuel, characterized in that a mixture of fluoride of uranium and plutonium that volatilizes in fluorination is directly oxidized, recovered as a mixed oxide of uranium and plutonium, and processed again as fuel.   The spent nuclear fuel according to claim 1, wherein the spent nuclear fuel is an oxide, a metal, or a nitride, and the spent nuclear fuel is refined by oxidation, reduction treatment or the like before being fluorinated as necessary. Fuel reprocessing method.   The fuel from spent nuclear fuel according to claim 1, wherein fission products such as iodine and technetium having a long half-life and minor actinides such as neptunium and americium are volatilized by fluorination and recovered together with uranium or plutonium. Rework method.   2. The method of reprocessing fuel from spent nuclear fuel according to claim 1, wherein the recovered mixed oxide of uranium and plutonium is used as fuel for a light water reactor, a high conversion reactor, and a fast reactor.   2. The fuel reprocessing method from spent nuclear fuel according to claim 1, wherein the mixture of uranium and plutonium recovered as oxide is powdered or granulated, and these are vibrated and filled into a fuel cladding tube. In the reprocessing method that separates and collects a mixture of plutonium and uranium and high-purity uranium by fluorinating spent fuel and volatilizing components, it is fluorinated by using a weak fluorinating agent or by reducing the concentration of the fluorinating agent. Easy uranium is volatilized and recovered as fluoride, and then a strong fluorinating agent is used or the concentration of the fluorinating agent is increased, so that plutonium that is difficult to fluorinate and the remaining uranium is volatilized and recovered as fluoride, and fission products are recovered. A method for reprocessing spent fuel that separates and recovers uranium, a mixture of plutonium and uranium, and fission products by leaving them as residues,
When recovering volatile uranium, which is easily fluorinated by using a weak fluorinating agent or reducing the concentration of the fluorinating agent, as a fluoride, monitor the amount of uranium fluoride recovered to detect a weak fluorinating agent or a low concentration of fluorination. A method for reprocessing spent fuel, comprising adjusting a supply amount of the agent and adjusting a ratio of uranium and plutonium in a mixture of plutonium and uranium.

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