METHOD FOR RECOVERING METALLIC NUCLEAR FUEL MATERIALS FROM SPENT NUCLEAR FUEL AND METHOD FOR REPROCESSING SPENT NUCLEAR FUEL

Abstract

A spent oxide form nuclear fuel in a spent nuclear fuel assembly which has been taken out from a light water reactor is reacted with fluorine in fluorination treatment process and then separated into gaseous UF6 and solid converted fluoride. The UF6 is purified in UF6 treatment Process. In electrolysis using fused fluoride process, the converted fluoride is dissolved into a fused fluoride salt (a mixture of LiF and BeF2) filled into an electrolysis cell of an apparatus for electrolysis. A first electrode, which is an anode, and a second electrode, which is a cathode, are submerged into the fused fluoride. A mixture of the oxides Li2O and BeO are added to the fused fluoride. A metallic plutonium and a metallic uranium contained in the fused fluoride is deposited onto the second electrode by energizing of the first and second electrodes.

Description

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent application serial no. 2008-298918, filed on Nov. 25, 2008, the content of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a method for recovering metallic nuclear fuel material from spent nuclear fuel and a method for reprocessing spent nuclear fuel, and more particularly, to a method for recovering metallic nuclear fuel material from spent nuclear fuel and a reprocessing method for reprocessing spent nuclear fuel, suitable for obtaining metallic nuclear fuel.

Plutonium (mainly plutonium 239) is generated by neutron absorption of uranium 238 in a core of a light water reactor in which uranium is used as the nuclear fuel material. Spent fuel assemblies that are loaded into the light water reactor are taken out from the light water reactor and then reprocessed. A fast breeder reactor exists as a nuclear reactor which generates electricity by using as nuclear fuel material the plutonium recovered by the reprocessing of the spent fuel assemblies and which generates at the same time more plutonium than the loaded plutonium. Among this fast breeder reactor is type of a reactor which uses metallic nuclear fuel containing uranium and plutonium as nuclear fuel materials.

The spent fuel assemblies taken out from the fast breeder reactor loaded fuel assemblies including the metallic nuclear fuel are reprocessed by a fused-salt electrolysis method, thereby providing reuse of recovered Pu formed into metallic nuclear fuel (see, for example, Masashi Koyama et al., “Dry Reprocessing Technology”, Denchuken Review, No. 37, pp. 26-37 (2000)). Nevertheless, a great majority of the nuclear reactors in current operation are light water reactors which use an oxide form nuclear fuel as the nuclear fuel material. Spent fuel assemblies taken out from the light water reactors include oxide form nuclear fuels containing about 1% Pu. Accordingly, an example of producing from oxide form nuclear fuels a metallic nuclear fuel which is used in a fuel assembly which is originally loaded into a fast breeder reactor using the metallic nuclear fuel is described by Tsuyoshi Usami et al. in “Adoption of Dry Reprocessing Technology for Oxide Form Nuclear Fuel”, Denchuken Review, No. 37, pp. 40-46 (2000). In this manufacturing method of the metallic nuclear fuel, uranium oxide in the spent fuel assemblies used in light water reactors is reduced by lithium; thereby, metallic uranium produced is recovered.

As a method for reprocessing spent nuclear fuel, using as fluorides uranium and plutonium contained in spent nuclear fuel materials, separating uranium, and uranium and plutonium mixtures by utilizing the difference in their volatilities, and recovering them has been proposed (Japanese Patent Laid-open No. 2000-284089). This reprocessing method volatizes a large portion of the uranium contained in a spent nuclear fuel material by using a fluorination treatment that uses a first fluorination agent and then separates the large portion of the uranium. Later, the remaining uranium and plutonium are volatized by using a second fluorination agent and then recovered, followed by oxidation of the recovered uranium and plutonium. In this way, mixtures of uranium and plutonium are generated.

Japanese Patent Laid-open No. 2004-233066 also discloses a reprocessing method for spent nuclear fuel by way of a fluorination treatment. This reprocessing method reacts spent nuclear fuel materials with fluorine gas and volatizes them in the form of UF₆ and PuF₆. Gas mixed with these is supplied to a Pu recovery trap loaded with pelletized UO₂F₂, PuF₆ is adsorbed in the form of PuF₄ onto UO₂ and then separated. UF₆ is transformed into UO₂, and UO₂F₂ and PuF₄ are transformed into a mixed oxide of UO₂ and PuO₂.

Japanese Patent Laid-open No. 2002-257980 discloses a reprocessing method for spent nuclear fuel, which uses a PUREX process to obtain a mixed oxide nuclear fuel containing uranium and plutonium obtained from a spent nuclear fuel.

Another reprocessing method, which uses a fused salt electrolysis method, for the spent nuclear fuel is explained in Japanese Patent Laid-open No. 2003-43187. An oxide form nuclear fuel that is a spent nuclear fuel material is charged into fused salt filled in a vessel and chlorine gas is blown into the fused salt. UO₂ and PuO₂ contained in the oxide form nuclear fuel are dissolved as chlorides into the fused salt. An anode and a cathode are soaked into the fused salt. When a current is passed between the anode and the cathode, UO₂ and PuO₂ are deposited onto the cathode. A reprocessing method for spent nuclear fuel, which uses a fused salt electrolysis method is also disclosed in Japanese Patent Laid-open No. 2000-284089.

SUMMARY OF THE INVENTION

In order to obtain metallic nuclear fuel for use in fuel assemblies to be loaded into fast breeder reactors, the inventors studied reprocessing method for oxide form nuclear fuels included in spent nuclear fuel assemblies taken out from light water reactors. As a result, in order to obtain metallic nuclear fuel from spent oxide form nuclear fuel, the inventors arrived at the conclusion that the use of a fused-salt electrolysis method would be good.

However, amount of uranium contained in metallic nuclear fuel present in fresh fuel assemblies that are loaded in the core of the fast breeder reactor is 3 to 4 times amount of plutonium in the metallic nuclear fuel, contrasting to amount of uranium contained in oxide form nuclear fuel in spent fuel assemblies taken out from light water reactor, which is about 100 times amount of plutonium contained in the oxide form nuclear fuel. Therefore, prior to process of fused salt electrolysis, a large portion of the excess uranium must be removed from spent oxide form nuclear fuel.

In order to remove the excess uranium prior to the fused salt electrolysis process used to obtain the metallic nuclear fuel, the inventors gave thought to subjecting spent oxide form nuclear fuel are previously subjected to a fluorination process, as is disclosed in Japanese Patent Laid-open No. 2000-284089, Japanese Patent Laid-open No. 2004-233066, and Japanese Patent Laid-open No. 2002-257980. By conducting previously the fluorination process on spent oxide form nuclear fuel, a large portion of the uranium is converted into a fluoride, volatized, and removed. Additionally, as disclosed in Japanese Patent Laid-open No. 2000-284089 and Japanese Patent Laid-open No. 2004-233066, each fluoride of the remaining uranium and plutonium are converted into their respective oxides. The obtained uranium and plutonium oxides are supplied into the fused salt in the vessel and these oxides are subjected to the fused salt electrolysis. As a result, the metallic nuclear fuel containing metallic uranium and metallic plutonium can be generated. Since excess uranium can be removed from spent oxide form nuclear fuel, an equipment for conducting the fused salt electrolysis on uranium and plutonium can be made compact. Furthermore, it is possible to shorten the time required to obtain the metallic nuclear fuel by the fused salt electrolysis.

The inventors discovered a new issue of the need to further simplify a process for recovering from the spent oxide form nuclear fuel the metallic nuclear fuel materials acting as the raw material for metallic nuclear fuel.

An object of the present invention are to provide a method for recovering metallic nuclear fuel material from spent nuclear fuel and a method for reprocessing spent nuclear fuel, which can simplify a process for recovering metallic nuclear fuel from spent oxide form nuclear fuel.

To achieve the above-mentioned object, the present invention is characterized by

generating nuclear fuel fluorides by reacting fluorine with spent oxide form nuclear fuel taken out from a nuclear reactor,

removing one part of fluorinated uranium from among the nuclear fuel fluorides,

dissolving the remaining nuclear fuel fluorides and oxides into fused fluoride, and

energizing a first electrode which is an anode and a second electrode which is a cathode, both of which were immersed into the fused fluoride, and depositing onto the second electrode a metallic nuclear fuel material dissolved in the fused fluoride.

Since the nuclear fuel fluorides and oxides are dissolved in the fused fluoride, and the metal nuclear fuel material dissolved in the fused fluoride is deposited onto the second electrode by energizing the first electrode which is the anode and the second electrode which is the cathode, both of which were immersed into the fused fluoride, a process, which is a pretreatment for electrolysis using fused fluoride, for converting the nuclear fuel fluorides into oxides becomes unnecessary. Accordingly, the process of the method for recovering the metallic nuclear fuel materials from the spent oxide form nuclear fuels can be simplified.

According to the present invention, the process for recovering the metallic nuclear fuel materials from the spent oxide form nuclear fuel can be simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing showing a process of a method for recovering metallic nuclear fuel from the spent nuclear fuel according to Embodiment 1 which is one suitable embodiment of the present invention.

FIG. 2 is an explanatory drawing showing material balance of nuclear fission products in the embodiment shown in FIG. 1.

FIG. 3 is a characteristic drawing showing a relationship between number of treatments of a process of electrolysis using fused fluoride and amount of nuclear fission products transferred to a treatment process of waste salt from an electrolysis cell.

FIG. 4 is an explanatory drawing showing a process of a method for recovering spent nuclear fuel according to Embodiment 2, which is another embodiment of the present invention.

FIG. 5 is an explanatory drawing showing a process of electrolysis using fused fluoride in a method for recovering spent nuclear fuel according to Embodiment 3, which is another embodiment of the present invention.

FIG. 6 is an explanatory drawing showing a purification process in a method for recovering spent nuclear fuel according to Embodiment 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors investigated of a discovered new process for reprocessing spent oxide form nuclear fuels, that is, a treatment flow of a fluorination treatment for spent oxide form nuclear fuel, converting respective fluorides of uranium and plutonium obtained thereby into respective oxides of uranium and plutonium, and fused salt electrolysis of the respective oxides of uranium and plutonium. As a result, the inventors found knowledge that it is possible to further simplify the process of reprocessing used oxide form nuclear fuels by fused salt electrolysis of the respective fluorides of uranium and plutonium obtained by a fluorination treatment of oxide form nuclear fuels. In order to realize this knowledge, (a) use of a fused fluoride acting as fused salt used in the fused salt electrolysis, and (b) the need to add an oxide to the fused fluoride were newly discerned.

As the fused fluorides, for example, a mixture of LiF and BeF₂ is used and, as the added oxides, a mixture of Li₂O and BeO is used. By doing so, fused salt electrolysis of the respective fluorides of uranium and plutonium obtained by the fluorination treatment of the oxide form nuclear fuel is made possible. Accordingly, conversion treatment of the respective fluorides of uranium and plutonium into oxides, which is pretreatment for fused salt electrolysis, becomes unnecessary, and in the method for recovering metallic nuclear fuel materials from used oxide form nuclear fuels, the process of recovering the metallic nuclear fuel materials from spent oxide form nuclear fuels can be simplified.

Reflecting the abovementioned investigation results, embodiments of the present invention are explained below.

Embodiment 1

A method for recovering metallic nuclear fuel materials from spent nuclear materials according to embodiment 1 which is one suitable embodiment of the present invention, is explained below by referring to FIG. 1.

Spent fuel assemblies being loaded into a core of a light water reactor are taken out from a reactor pressure vessel of the light water reactor and stored for a specified period in a fuel storage pool. This spent fuel assemblies are then taken out from the fuel storage pool and transferred to a nuclear fuel reprocessing facility from the nuclear power generation facility where the light water reactor is located. Nuclear fuel materials, that is oxide form nuclear fuels, contained in the spent fuel assemblies are reprocessed in the nuclear fuel reprocessing facility.

	TABLE 1
	Spent oxide form nuclear fuel	Converted fluoride
	Component		Amount of		Amount of
Line		Atomic		Mass	Radioactivity	substance		Mass	Radioactivity	substance
number	Element	weight	Form	(kg)	(Ci)	(kmol)	Form	(kg)	(Ci)	(kmol)
1	U	238.029	UO2	9.54E+02	4.05E+00	4.01E+00	UO2F2	1.91E+01	8.10E−02	8.02E−02
2	Pu	244	PuO2	9.03E+00	1.08E+05	3.70E−02	PuF4	9.03E+00	1.08E+05	3.70E−02
3	Np	237.0482	NpO2	7.49E−01	1.81E+01	3.16E−03	NpO2F2	7.49E−01	1.81E+01	3.16E−03
4	Am	243	Am2O3	1.40E−01	1.88E+02	5.76E−04	AmF3	1.40E−01	1.88E+02	5.76E−04
5	Cm	247	Cm2O3	4.70E−02	1.89E+04	1.90E−04	CmF3	4.70E−02	1.89E+04	1.90E−04
6	H	1.0079	H2O	7.17E−05	6.90E+02	7.11E−05	—	0.00E+00	0.00E+00	0.00E+00
7	Se	78.96	SeO2	4.87E−02	3.96E−01	6.17E−04	SeF4	4.87E−02	3.96E−01	6.17E−04
8	Br	79.904	Br2	1.38E−02	0.00E+00	1.73E−04	—	0.00E+00	0.00E+00	0.00E+00
9	Kr	83.8	Kr	3.60E−01	1.10E+04	4.30E−03	—	0.00E+00	0.00E+00	0.00E+00
10	Rb	85.4678	Rb2O	3.23E−01	1.90E+02	3.78E−03	RbF	3.23E−01	1.90E+02	3.78E−03
11	Sr	87.62	SrO	8.68E−01	1.74E+05	9.91E−03	SrF2	8.68E−01	1.74E+05	9.91E−03
12	Y	88.9059	Y2O3	4.53E−01	2.38E+05	5.10E−03	YF3	4.53E−01	2.38E+05	5.10E−03
13	Zr	91.22	ZrO2	3.42E+00	2.77E+05	3.75E−02	ZrF4	3.42E+00	2.77E+05	3.75E−02
14	Nb	92.9064	Nb2O5	1.16E−02	5.21E+05	1.25E−04	NbF5	1.16E−03	5.21E+04	1.25E−05
15	Mo	95.94	MoO3	3.09E+00	0.00E+00	3.22E−02	MoF6	3.09E−01	0.00E+00	3.22E−03
16	Tc	97	TcO2	7.52E−01	1.43E+01	7.75E−03	TcF6	7.52E−02	1.43E+00	7.75E−04
17	Ru	101.07	RuO2	1.90E+00	4.99E+05	1.88E−02	RuF6	1.90E−01	4.99E+04	1.88E−03
18	Rh	102.9055	Rh2O3	3.19E−01	4.99E+05	3.10E−03	RhF3	3.19E−01	4.99E+05	3.10E−03
19	Pd	106.4	Pb2O3	8.49E−01	0.00E+00	7.98E−03	PdF3	8.49E−01	0.00E+00	7.98E−03
20	Ag	107.868	Ag2O	4.21E−02	2.75E+03	3.90E−04	AgF	4.21E−02	2.75E+03	3.90E−04
21	Cd	112.4	CdO	4.75E−02	5.95E+01	4.23E−04	CdF2	4.75E−02	5.95E+01	4.23E−04
22	In	114.82	In2O3	1.09E−03	3.57E−01	9.49E−06	InF3	1.09E−03	3.57E−01	9.49E−06
23	Sn	121.75	SnO	3.28E−02	3.85E+04	2.69E−04	SnF4	3.28E−02	3.85E+04	2.69E−04
24	Sb	121.75	Sb2O5	1.36E−02	7.96E+03	1.12E−04	SbF5	1.36E−03	7.96E+02	1.12E−05
25	Te	127.6	TeO3	4.85E−01	1.34E+04	3.80E−03	TeF6	4.85E−02	1.34E+03	3.80E−04
26	I	126.9045	I2	2.12E−01	2.22E+00	0.00E+00	—	0.00E+00	0.00E+00	0.00E+00
27	Xe	131.3	Xe	4.87E+00	3.12E+00	0.00E+00	—	0.00E+00	0.00E+00	0.00E+00
28	Cs	132.9054	Cs2O	2.40E+00	3.21E+05	1.81E−02	CsF	2.40E+00	3.21E+05	1.81E−02
29	Ba	137.34	BaO	1.20E+00	1.00E+05	8.74E−03	BaF2	1.20E+00	1.00E+05	8.74E−03
30	La	138.9055	La2O3	1.14E+00	4.92E+02	8.21E−03	LaF3	1.14E+00	4.92E+02	8.21E−03
31	Ce	140.12	Ce2O3	2.47E+00	8.27E+05	1.76E−02	CeF3	2.47E+00	8.27E+05	1.76E−02
32	Pr	140.9077	Pr2O3	1.09E−01	7.71E+05	7.74E−04	PrF3	1.09E−01	7.71E+05	7.74E−04
33	Nd	144.24	Nd2O3	3.51E+00	9.47E+01	2.43E−02	NdF3	3.51E+00	9.47E+01	2.43E−02
34	Pm	145	Pm2O3	1.00E−01	1.00E+05	6.90E−04	PmF3	1.00E−01	1.00E+05	6.90E−04
35	Sm	150.4	Sm2O3	6.96E−01	1.25E+03	4.63E−03	SmF3	6.96E−01	1.25E+03	4.63E−03
36	Eu	151.96	Eu2O3	1.26E−01	1.35E+04	8.29E−04	EuF3	1.26E−01	1.35E+04	8.29E−04
37	Gd	157.25	Gd2O3	6.29E−02	2.32E+01	4.00E−04	GdF3	6.29E−02	2.32E+01	4.00E−04
38	Tb	158.9254	Tb2O3	1.25E−03	3.02E+02	7.87E−06	TbF3	1.25E−03	3.02E+02	7.87E−06
39	Dy	162.5	Dy2O3	6.28E−04	0.00E+00	3.86E−06	DyF3	6.28E−04	0.00E+00	3.86E−06
40	Total amount	9.94E+02	4.54E+06	4.27E+00		4.79E+01	3.60E+06	2.81E−01
41	MA + FP	3.09E+01	4.44E+06	2.25E−01		1.98E+01	3.49E+06	1.64E−01
42	FP	2.99E+01	4.42E+08	2.21E−01		1.88E+01	3.47E+06	1.60E−01

After the oxide form nuclear fuel are removed from the nuclear fuel rods by removing claddings and then pulverized, a fluorination treatment is applied to the spent oxide form nuclear fuels (fluorination treatment process 1). When 1 ton of the oxide form nuclear fuel is loaded into the core of the light water nuclear reactor, at the point in time when one operation cycle is finished, the amount of the oxide form nuclear fuel has reduced to 0.994 ton due to nuclear fission occurring during operation in the light water reactor, as shown in Table 1. The spent oxide form nuclear fuel included in the spent fuel assemblies taken out from the light water reactor have compositions shown in a section of the spent oxide form nuclear fuel shown in Table 1, and contained, as the major components, uranium; transuranic elements such as plutonium, neptunium, americium, and curium, generated from the uranium; and fission products (hereinafter called “FP”). In a section of the spent oxide form nuclear fuel shown in Table 1, the weight, masses (number of moles), and radioactivity of each element contained in about 1 ton of the spent oxide form are shown.

In fluorination treatment process 1, the oxide form nuclear fuel are reacted with fluorine and then divided into gaseous uranium hexafluoride (UF₆) and a solid converted fluoride. The UF₆ generated in fluorine treatment process 1 is transferred to UF₆ treatment process 2 and then refined and converted into UO₂. This UO₂ is filled into fuel rods for flesh nuclear fuel assemblies for the light water reactors.

A flame reactor (or fluidized bed) is used in fluorination treatment process 1. After the oxide form nuclear fuel are supplied to the flame reactor, fluorine is supplied to the flame reactor and fluorination treatment is conducted on the oxide form nuclear fuel within the flame reactor. A percentage of uranium being volatized in the fluorination treatment process 1 can be adjusted between from 80% to 98% by controlling the amount of fluorine (F₂) supplied to the flame reactor.

The amount of uranium remaining in the solid converted fluoride can be adjusted by adjusting the percentage of uranium volatilization. The compositions of the converted fluorides obtained by applying the fluorination treatment to the spent oxide form nuclear fuel having the constitutions disclosed in the section of the spent oxide form nuclear fuel of Table 1 and by making volatilization of 98% of the uranium, are shown in a section of the converted fluoride of Table 1. Each compound contained in the spent oxide form nuclear fuel prior to the fluorination treatment is converted after the fluorination treatment into each of the compounds shown in a section of the form in the section of the converted fluoride of Table 1. Due to being removed as UF₆ during fluorination treatment, a large portion of the uranium is reduced to 2% of the amount of uranium contained in the spent oxide form nuclear fuel. Furthermore, H, Br, Kr, I and Xe contained in the spent oxide form nuclear fuel, since they are gasses, are removed in the fluorination treatment process 1, and are no longer contained in the converted fluorides which were generated in the fluorination treatment process 1. Since the fluorides of Se, Nb, Tc, Mo, Ru, and Sb are volatile, only about 10% of them remain in the converted fluorides.

The fluorination treatment process which uses a fluidized bed occurs batchwise, so at the point in time when just the essential amount of uranium is volatized, supply of fluorine to the fluidized bed discontinues, and the remaining substance is removed from the fluidized bed. By conducting such an operation, the percentage of uranium volatized can be adjusted even when a fluidized bed is used.

After the abovementioned fluorination treatment process 1 was finished, a process 3 of electrolysis using fused fluoride is carried out. Converted fluoride generated in the fluorination treatment process 1 is supplied into an electrolysis cell used in the process 3 of electrolysis using fused fluoride. An apparatus for electrolysis using fused fluoride, used in the process 3 of electrolysis using fused fluoride in the electrolysis cell filled with the fused fluoride, and the electrolysis cell is provided with a first electrode, which is an anode, and a second electrode, which is a cathode. The first and second electrodes are submerged into the fused fluoride (see FIG. 5, described below). The converted fluorides are supplied into the fused fluoride. A mixture of LiF and BeF₂ is used, for example, as the fused fluoride. In place of the mixture of LiF and BeF₂, a mixture including LiF, NaF, and KF may be used as the fused fluoride.

By using the fused fluoride, the uranium present in the form of UO₂F₂ containing the converted fluoride and the neptunium present in the form of NpO₂F₂ containing the converted fluoride is dissolved in the fused fluoride by the reactions shown below.

UO₂F₂(solid)→UO₂²⁺(in fused salt)+2F⁻(in fused salt)  (1)

NPO₂F₂(solid)→NpO₂²⁺(in fused salt)+2F⁻(in fused salt)  (2)

Other converted fluorides generated in fluorination treatment process 1 are in the form of AnFm, so, for example, they are dissolved into the fused fluoride by the following reaction below.

PuF₄(solid)→Pu⁴⁺(in fused salt)+4F⁻(in fused salt)  (3)

LaF₃(solid)→La³⁺(in fused salt)+3F⁻(in fused salt)  (4)

SrF₂(solid)→Sr²⁺(in fused salt)+2F⁻(in fused salt)  (5)

CsF(solid)→Cs⁺(in fused salt)+F⁻(in fused salt)  (6)

The first electrode and the second electrode are energized and when the fused fluoride in the electrolysis cell is electrolyzed, the reactions of each of expressions (7) through (10) shown below, occur in the fused fluoride, and metallic uranium is deposited on the second electrode that is a cathode. That is, while electricity passes between the electrodes, the fused fluoride generates the reaction in expression (8) due to the high degree of electrolysis of O²⁻ in contrast to fused chloride, and the uranium forms into metallic uranium by passing from U⁴⁺ to U³⁺.

UO₂²⁺(in fused salt)+2e⁻(cathode)→UO₂(near the anode)  (7)

UO₂(near the anode)→U⁴⁺(in fused salt)+2O²⁻(in fused salt)  (8)

U⁴⁺(in fused salt)+e⁻(cathode)→U³⁺(in fused salt)  (9)

U³⁺(in fused salt)+3e⁻(cathode)→U(deposition on the anode)  (10)

By summarizing expressions (7) to (10), the reaction arising at the second electrode results in expression (11).

UO₂²⁺(in fused salt)+6e⁻→U(deposition on cathode)+2O²⁻(in fused salt)  (11)

Similar to UO₂F₂, NpO₂F₂ is deposited on the cathode as Np. Fluorides existing in the form of AnFm are, by the following reactions, metals deposited on the cathode.

Pu⁴⁺(in fused salt)+4e⁻(cathode)→Pu(deposited on the cathode)  (12)

La³⁺(in fused salt)+3e⁻(cathode)→La(deposited on the cathode)  (13)

Sr²⁺(in fused salt)+2e⁻(cathode)→Sr(deposited on the cathode)  (14)

Cs⁺(in fused salt)+e⁻(cathode)→Cs(deposited on the cathode)  (15)

The fused fluoride was stated above as being electrolyzed, but as it is, when electrolysis is conducted, A reaction in expression (16) occurs at the first electrode 1 that is a anode with respect to the reaction in expression (11) and fluorine gas is generated near the first electrode. Therefore, the first electrode readily corrodes.

6F⁻(in fused salt)→3F₂+6e⁻(anode)  (16)

In the present embodiment, in order to prevent the generation of fluorine near the first electrode, oxide is dissolved in advance into the fused fluoride in the electrolysis cell. As the oxide to be added to the fused fluoride, use of an oxide having a cation in common with the fused fluoride is preferable. That is, by using the oxide common to the fused fluoride, for example, as shown below in expression (19), reacting excess fluorine in the fused fluoride the generated fluoride has the same composition as the fused fluoride. Therefore, the composition of the fused fluoride can be maintained. In the present embodiment, since a mixture of LiF and BeF₂ is used as the fused fluoride, a mixture of Li₂O and BeO is used as the oxide to be added. By using a mixture of Li₂O and BeO, LiF and BeF₂, which are the composition of the fused fluoride used in the present embodiment, are generated based on the reaction in expression (19). In the present embodiment, use of a carbon anode as the first electrode promotes the reaction of expression (11). Thus, the anode reactions occurring at the first electrode become like those in expressions (17) and (18).

Li₂O+BeO→2Li⁺(fused salt)+Be²⁺(fused salt)+2O²⁻  (17)

3O²⁻(fused salt)+3/2C(anode material)→3/2CO₂+6e⁻(anode)  (18)

In the reaction of expression (18), carbon dioxide is generated from the first electrode. Nevertheless, depending on the temperature of the fused fluoride, the generated carbon dioxide may be decomposed and carbon monoxide may be generated.

In reaction of expression (11) to recover metallic uranium from UO₂F₂ by the electrolysis using fused fluoride, two O²⁻ are generated, so the reaction of expression (18) arises on the first electrode. Therefore, Li₂O as well as BeO may be added to the mixture of LiF and BeF₂ as the fused fluoride, such that just one mole of O²⁻ are generated per mole of UO₂F₂. The entire reaction is shown in the following expression.

1/2(Li₂O+BeO)+UO₂F₂+3/2C→Li⁺+1/2Be²⁺+2F⁻+U+3/2CO₂  (19)

Expression (11) shows what materials in the recovered metallic uranium accompany. Among the reactions shown in expressions (7) to (10), the reaction which must place the greatest negative value on the second electrode is the reaction of expression (10). The electric potential of the deposition reaction by the fused fluorides, due to a paucity of data, will be explained below, using the electric potential occurring with fused chlorides.

The electric potentials of the deposition reactions with fused fluoride and fused chloride differ, but the order of the electric potentials to deposit each ion (deposition electric potentials) does not change for either. That is, although electric potential E1 to make U³⁺ into metallic uranium and electric potential E2 to make La³⁺ into metallic La differ depending on the type of fused salt, even if the fused salts differ, electric potential E2 being a more negative value than the electric potential E1 does not change.

	TABLE 2
	Converted fluoride	Electrode-
	Component		Amount of	position
Line		Atomic		Mass	Radioactivity	substance	potential
number	Element	weight	Form	(kg)	(Ci)	(kmol)	(V)	Remarks
1	Te	127.6	TeF6	4.85E−02	1.34E+03	3.80E−04	−0.1
2	Ru	101.07	RuF6	1.90E−01	4.99E+04	1.88E−03	−0.142
3	Pd	160.4	PdF3	8.49E−01	0.00E+00	7.98E−03	−0.214
4	Rh	102.9055	RhF3	3.19E−01	4.99E+05	3.10E−03	−0.231
5	Ag	107.868	AgF	4.21E−02	2.75E+03	3.90E−04	−0.637
6	Mo	95.94	MoF6	3.09E−01	0.00E+00	3.22E−03	−0.638
7	Sb	121.75	SbF5	1.36E−03	7.96E+02	1.12E−05	−0.67
8	Sn	121.75	SnF4	3.28E−02	3.85E+04	2.69E−04	−1.082
9	In	114.82	InF3	1.09E−03	3.57E−01	9.49E−06	−1.104
10	Nb	92.9064	NbF5	1.16E−03	5.21E+04	1.25E−05	−1.19
11	Cd	112.4	CdF2	4.75E−02	5.95E+01	4.23E−04	−1.316
12	Tb	158.9254	TbF3	1.25E−03	3.02E+02	7.87E−06	−1.465	Approximate with Yb
13	Dy	162.5	DyF3	6.28E−04	0.00E+00	3.86E−06	−1.465	Approximate with Yb
14	Cm	247	CmF3	4.70E−02	1.89E+04	1.90E−04	−1.505
15	Am	243	AmF3	1.40E−01	1.88E+02	5.76E−04	−1.623
16	Pu	244	PuF4	9.03E+00	1.08E+05	3.70E−02	−1.733
17	Sm	150.4	SmF3	6.96E−01	1.25E+03	4.63E−03	−1.819
18	Zr	91.22	ZrF4	3.42E+00	2.77E+05	3.75E−02	−1.86
19	Np	237.0482	NpO2F2	7.49E−01	1.81E+01	3.16E−03	−2.068
20	U	238.029	UO2F2	1.91E+01	8.10E−02	8.02E−02	−2.253
21	Sr	87.62	SrF2	8.68E−01	1.74E+05	9.91E−03	−2.58	Approximate with Mg
22	Ba	137.34	BaF2	1.20E+00	1.00E+05	8.74E−03	−2.58	Approximate with Mg
23	Gd	157.25	GdF3	6.29E−02	2.32E+01	4.00E−04	−2.823
24	Nd	144.24	NdF3	3.51E+00	9.47E+01	2.43E−02	−2.854
25	Y	88.9059	YF3	4.53E−01	2.38E+05	5.10E−03	−2.866
26	La	138.9055	LaF3	1.14E+00	4.92E+02	8.21E−03	−2.883
27	Pr	140.9077	PrF3	1.09E−01	7.71E+05	7.74E−04	−2.883	Approximate with La
28	Pm	145	PmF3	1.00E−01	1.00E+05	6.90E−04	−2.883	Approximate with La
29	Eu	151.96	EuF3	1.26E−01	1.35E+04	8.29E−04	−2.883	Approximate with La
30	Ce	140.12	CeF3	2.47E+00	8.27E+05	1.76E−02	−2.94
31	Rb	85.4678	RbF	3.23E−01	1.90E+02	3.78E−03	−3.14	Approximate with Na
32	Cs	132.9054	CsF	2.40E+00	3.21E+05	1.81E−02	−3.14	Approximate with Na
33	Se	78.96	SeF4	4.87E−02	3.96E−01	6.17E−04
34	Tc	97	TcF6	7.52E−02	1.43E+00	7.75E−04
35	Total amount	4.79E+01	3.60E+06	2.81E−01
36	MA + FP	1.98E+01	3.49E+06	1.64E−01
37	FP	1.88E+01	3.47E+06	1.60E−01
38	FP mixed in	2.44E+01	1.14E+00	6.12E−02

Table 2 presents a summary, in order from small to large, of the electric potentials to deposit each compound dissolved in the fused chloride. Each of the elements in line numbers 1 to 19 has an electrodeposition potential which is more positive than that of uranium, and deposits more readily than uranium. Therefore, when uranium is deposited onto the second electrode, each of the elements in line numbers 1 to 19 is also deposited onto the second electrode. Since the elements in line numbers 21 and below have electrodeposition potentials which are more negative than uranium, they deposit less readily than uranium and when uranium is deposited onto the second electrode, they are included in the fused salt in the electrolysis cell. Although there is no data of the electrodeposition potentials for Se and Tc, conservatively, when uranium is deposited onto the second electrode, Se and Tc are also deposited onto the second electrode. Naturally, plutonium is also deposited onto the second electrode.

In the process 3 of electrolysis using fused fluoride of the present embodiment, the first electrode and the second electrode are energized such that the electric potential difference between the first electrode and the second electrode, which were provided to the electrolysis cell, becomes equal to the electrodeposition potential of the element (uranium), among the elements required for metallic nuclear fuels to be manufactured, with the lowest electrodeposition potential within the fused fluoride. By doing so, elements having an electrodeposition potential equal to or greater than the element with the lowest electrodeposition potential are deposited onto the second electrode that is a cathode.

Since 98% of the uranium contained in the spent oxide form nuclear fuels in the fluorine treatment process 1 has been removed, the proportion (enrichment) of metallic plutonium to the metallic uranium and metallic plutonium deposited on the second electrode becomes 0.32, according to Table 2. That is, according to the present embodiment, after removal of excess uranium from the spent oxide form nuclear fuel, the remaining uranium and plutonium can be converted into metallic nuclear fuel having a plutonium enrichment of at least 0.25. The second electrode, to which the recovered metallic nuclear fuel materials (each of the metallic elements, such as metallic uranium and metallic plutonium, of line numbers 1 to 20 shown in Table 2) adhere, is transported to another plant for implementing a purification process 4 set up at a different location, this plant differing from the plant for implementing the recovery method for the metallic nuclear fuel material of the present embodiment.

	TABLE 3
	Type of metallic	Amount recovered
	nuclear fuel material	(kg/month)
	Component	U	14.66
		Pu	6.936
		MA	0.719
		RE	0.536
		AM	0.000
		AEM	0.000
		NM	1.075
		Zr	2.627

Table 3 shows a summary of how much of each element in line numbers 1 to 20 of Table 2 is jointly recovered when Pu is attempted to be recovered at a rate of 6.93 kg/month. In Table 3, MA refers to minor actinides, RE refers to rare earths, AM refers to alkaline metals, AEM refers to alkaline earths, and NM refers to noble metals. In Table 2 the MA are Am, Cm, and Np; RE is Tb, Dy, and Sm; NM are each of the elements from line numbers 1 to 11. AEM and NM are the elements in line numbers 21 and below of Table 21 and they remain in the fused fluoride without being deposited onto the second electrode.

Since Li₂O and BeO are added to the fused fluoride for the purpose of conducting the reaction of expression (14), the amount of fused fluoride in the electrolysis cell increases. Accordingly, after the electrolysis using fluoride, the increased portion of fused fluoride (excess fused fluoride) is removed from the electrolysis cell together with the FP and then transferred to a treatment process 6 of waste salt.

	TABLE 4
			Mole
		Component	percentage of
	Category	material	element
	Base materials of acidic glass	SiO2	39.49%
		B2O3	20.77%
		Al2O3	4.99%
	Base materials of basic glass	Li2O	10.22%
		CaO	2.72%
		ZnO	1.88%
		Na2O	16.43%
	FP	Rb2O	0.06%
		Cs2O	0.26%
		SrO	0.15%
		BaO	0.16%
		ZrO2	0.60%
		MoO3	0.51%
		MnO2	0.22%
		RuO2	0.28%
		Rh2O3	0.06%
		PdO	0.15%
		Ag2O	0.01%
		CdO	0.01%
		SnO2	0.01%
		SeO2	0.01%
		TeO2	0.06%
		Y2O3	0.08%
		Sm2O3	0.08%
		Eu2O3	0.01%
		Gd2O3	0.01%
		LaO3	0.13%
		CeO2	0.10%
		Pr6O11	0.13%
		Nd2O3	0.42%
	Total	100.00%
	Total base materials of acidic glass	65.25%
	Total base materials of basic glass	31.25%
	Total FP	3.50%

Table 4 shows the composition, converted into mole percentages, of P0798 simulated glass which is a model glass of high level waste. Base materials of glass include base materials of acid oxide which build network structures, such as SiO₂, and base materials of basic oxide which sever networks and lower melting points. The base materials of basic oxide includes boron, which decomposes fluorides.

The excess fused fluorides (LiF and BeF₂) including FP, Li₂O, and BeO, removed from the electrolysis cell, are supplied to a high level radiation waste treatment apparatus. In the high level waste treatment apparatus, The FP and excess fluoride compounds are poured into a solidification vessel, the base materials of acid oxide and base materials of basic oxide are additionally poured into the solidification vessel, and the mixture is further agitated. Later, the solidification vessel is sealed and the solidification vessel is stored in a sealed state in a storage room.

	TABLE 5
	Converted fluoride	Electrode-
	Component		Amount of	position		Amount	Amount of
Line		Atomic		Mass	Radioactivity	substance	potential		Electric	of O2−	increase
number	Element	weight	Form	(kg)	(Ci)	(kmol)	(V)	Remarks	charge	consumed	in salt
1	Te	127.6	TeF6	4.85E−02	1.34E+03	3.80E−04	−0.1		6	1.14E−03
2	Ru	101.07	RuF6	1.90E−01	4.99E+04	1.88E−03	−0.142		6	5.64E−03
3	Pd	160.4	PdF3	8.49E−01	0.00E+00	7.98E−03	−0.214		3	1.20E−02
4	Rh	102.9055	RhF3	3.19E−01	4.99E+05	3.10E−03	−0.231		3	4.65E−03
5	Ag	107.868	AgF	4.21E−02	2.75E+03	3.90E−04	−0.637		1	1.95E−04
6	Mo	95.94	MoF6	3.09E−01	0.00E+00	3.22E−03	−0.638		6	9.66E−03
7	Sb	121.75	SbF5	1.36E−03	7.96E+02	1.12E−05	−0.67		5	2.79E−05
8	Sn	121.75	SnF4	3.28E−02	3.85E+04	2.69E−04	−1.082		4	5.39E−04
9	In	114.82	InF3	1.09E−03	3.57E−01	9.49E−06	−1.104		3	1.42E−05
10	Nb	92.9064	NbF5	1.16E−03	5.21E+04	1.25E−05	−1.19		5	3.12E−05
11	Cd	112.4	CdF2	4.75E−02	5.95E+01	4.23E−04	−1.316		2	4.23E−04
12	Tb	158.9254	TbF3	1.25E−03	3.02E+02	7.87E−06	−1.465	Approximate	3	1.18E−05
								with Yb
13	Dy	162.5	DyF3	6.28E−04	0.00E+00	3.86E−06	−1.465	Approximate	3	5.80E−06
								with Yb
14	Cm	247	CmF3	4.70E−02	1.89E+04	1.90E−04	−1.505		3	2.85E−04
15	Am	243	AmF3	1.40E−01	1.88E+02	5.76E−04	−1.623		3	8.64E−04
16	Pu	244	PuF4	9.03E+00	1.08E+05	3.70E−02	−1.733		4	7.40E−02
17	Sm	150.4	SmF3	6.96E−01	1.25E+03	4.63E−03	−1.819		3	6.94E−03
18	Zr	91.22	ZrF4	3.42E+00	2.77E+05	3.75E−02	−1.86		4	7.50E−02
19	Np	237.0482	NpO2F2	7.49E−01	1.81E+01	3.16E−03	−2.068		6	3.16E−03
20	U	238.029	UO2F2	1.91E+01	8.10E−02	8.02E−03	−2.253		6	8.02E−02
21	Sr	87.62	SrF2	8.68E−01	1.74E+05	9.91E−03	−2.58	Approximate	2	0	9.91E−03
								with Mg
22	Ba	137.34	BaF2	1.20E+00	1.00E+05	8.74E−03	−2.58	Approximate	2	0	8.74E−03
								with Mg
23	Gd	157.25	GdF3	6.29E−02	2.32E+01	4.00E−04	−2.823		3	0	4.00E−04
24	Nd	144.24	NdF3	3.51E+00	9.47E+01	2.43E−02	−2.584		3	0	2.43E−02
25	Y	88.9059	YF3	4.53E−01	2.38E+05	5.10E−03	−2.866		3	0	5.10E−03
26	La	138.9055	LaF3	1.14E+00	4.92E+02	8.21E−03	−2.883		3	0	8.21E−03
27	Pr	140.9077	PrF3	1.09E−01	7.71E+05	7.74E−04	−2.883	Approximate	3	0	7.74E−04
								with La
28	Pm	145	PmF3	1.00E−01	1.00E+05	6.90E−04	−2.883	Approximate	3	0	6.90E−04
								with La
29	Eu	151.96	EuF3	1.26E−01	1.35E+04	8.29E−04	−2.883	Approximate	3	0	8.29E−04
								with La
30	Ce	140.12	CeF3	2.47E+00	8.27E+05	1.76E−02	−2.94		3	0	1.76E−02
31	Rb	85.4678	RbF	3.23E−01	1.90E+02	3.78E−03	−3.14	Approximate	1	0	3.78E−03
								with Na
32	Cs	132.9054	CsF	2.40E+00	3.21E+05	1.81E−02	−3.14	Approximate	1	0	1.18E−02
								with Na
33	Se	78.96	SeF4	4.87E−02	3.96E−01	6.17E−04			4	1.23E−03
34	Tc	97	TcF6	7.52E−02	1.43E+00	7.75E−04			6	2.33E−03
35	Total amount	4.79E+01	3.60E+06	2.81E−01		2.78E−01	9.84E−02
36	MA + FP	1.98E+01	3.49E+06	1.64E−01
37	FP	1.88E+01	3.47E+06	1.60E−01
38	FP mixed in	2.44E+01	1.14E+00	6.12E−02

An example of a mass balance of FP, Li, Be, etc. in the present embodiment is explained below. Table 5 summarizes the number of electrons relating to the reactions of the converted fluorides shown in Table 2, the amount of O²⁻ consumed on the first electrode (anode) during the electrolysis using fused fluoride, and, for each treatment of about 1 t of the spent oxide form nuclear fuel, amount of substance accumulating in the fused fluoride for each treatment of about 1 t of the spent oxide form nuclear fuel.

A summary of the mass balance of FP, Li, and Be is shown in FIG. 2. From about 1 t of the spent oxide form nuclear fuel generated by a light water reactor, 1.6×10⁻¹ kmol of the FP is introduced to the fused fluoride in the process 3 of electrolysis using the fused fluoride process 3, and among this FP, 6.12×10⁻² kmol is transferred to another plant where is conducted the purification process 4 of embodiment 2 (or embodiment 3) described below, together with the metallic nuclear fuel materials (metallic uranium, metallic plutonium, etc.) recovered by the process of the electrolysis using fused fluoride. The remaining 9.84×10⁻² kmol of the FP is discharged from the process 3 of electrolysis using fused fluoride to the treatment process 6 of waste salt.

In the process 3 of electrolysis using fused fluoride, 2.78×10⁻¹ kmol of O²⁻ is consumed, due to the recovery of the metallic nuclear fuel materials (metallic uranium, metallic plutonium, etc.). To supplement these O²⁻ ions, each of Li₂O and BeO are added to the fused fluoride only 39×10⁻¹ kmol respectively. As a result of the electrolysis using fused fluoride, these oxides are respectively changed into LiF and BeF₂, which are the main components of the fused fluoride. The amount of the fused fluoride increases to a total of 4.17×10⁻¹ kmol, with 2.78×10⁻¹ kmol of LiF and 1.39×10⁻¹ kmol of BeF₂. This increased portion of the fused fluoride (4.17×10⁻¹ kmol) is transferred to the treatment process 6 of waste salt.

In order to vitrify radioactive waste generating due to the treatment process 6 of waste salt, according to Table 4, with respect to 3.5% FP, 65% base materials of acidic glass and 30% base materials of basic glass are required. With respect to 9.84×10⁻² kmol of the FP, 1.83 kmol of the base materials of acidic glass and 8.79×10⁻¹ kmol of the base materials of basic glass are required. After the process of the electrolysis using fused fluoride completed, since Li and Be discharged from the electrolysis cell are employed as the base materials of basic glass, 4.62×10⁻¹ kmol of the base materials of basic glass must be supplied to the electrolysis cell in the process 3 on the electrolysis using fused fluoride.

When converted substances by fluorination generated in the fluorination treatment process 1 are converted into a metallic nuclear fuel materials by using new fused fluoride without FP, since concentration of the FP in the fused fluoride is low, the amount of the FP transferred to the treatment process 6 of waste salt is lower than 9.84×10⁻² kmol. Nevertheless, when the concentration of the FP is increased by repeating the process 3 of electrolysis using fused fluoride, 9.84×10⁻² kmol of the FP is steadily discharged from the electrolysis cell and transferred to the treatment process 6 of waste salt. A relationship between the number of treatments of the process 3 of electrolysis using fused fluoride and the amount of the FP transferred to the treatment process 6 of waste salt is shown in FIG. 3. The fused fluorides contain 2 kmol of LiF and 1 kmol of BeF₂, as initial components. Therefore, in the present embodiment, F⁻ becomes 4 kmol. In an electrolysis method using fused chloride, the concentrations of uranium and plutonium are about 1 to 5%. When 1 t of the spent oxide form nuclear fuel is processed in the process 3 of electrolysis using fused fluoride, amount of substances of fluorinated uranium and fluorinated plutonium dissolved in the fused fluoride contained in the converted substances by fluorination generated from the oxide form nuclear fuel, is total 1.17×10⁻¹ kmol, from a section “amount of substance (kmol)” of the converted fluorides of Table 1; thus, the necessary amount of the fused fluoride is about 0.2 to 10 kmol. Accordingly, the aforementioned 4 kmol is the appropriate amount for the present embodiment.

According to the present embodiment, since the converted fluorides and oxides generated by removing a large portion of the uranium in the fluorination treatment process tare are supplied into the fused fluoride in the electrolysis cell and the electrolysis using fused fluoride is carried out, prior to this electrolysis using fused fluoride, there is no need for treatment to convert into the converted fluorides such as the fluorinated uranium, fluorinated plutonium, etc. which were generated in the fluorination treatment process 1. Therefore, the process of the method for recovering metallic nuclear fuel material from spent nuclear fuel can be simplified.

In the present embodiment, since the electrolysis using fused fluoride is conducted, the spent oxide form nuclear fuel generated in light water reactors can be dissolved into the fused fluoride and the metallic uranium and metallic plutonium contained in this oxide form nuclear fuel can be readily recovered.

Since the present embodiment has the fluorination treatment process 1 as the pretreatment of the process of the electrolysis using fused fluoride, a large portion of the uranium contained in the spent oxide form nuclear fuel can be preliminarily removed. Therefore, the apparatus for the electrolysis using fused fluoride, which is used in the process 3 of electrolysis using fused fluoride, can be made compact and the amount of the fused fluoride used can be reduced.

In the present embodiment, since oxides are added into the fused fluoride, generation of fluorine gas near the first electrode that is an anode can be prevented and carbon gas near the first electrode can be generated. Therefore, corrosion of the first electrode can be prevented and the life of the first electrode can be extended. In particular, in the present embodiment, since oxides having cations common to the fused fluoride are used, for example, as shown in expression (19), when reacted with excess fluorine in the fused fluoride, the generated fluorides will have the same composition as the fused fluoride. Therefore, the composition of the fused fluoride can be maintained. When oxides not having cations common to the fused fluoride are used, the compositions of the fused fluoride change.

Furthermore, in the present embodiment, since a carbon electrode is used as the first electrode that is the anode, carbon dioxide (or carbon monoxide) is generated from the anode first electrode that is the anode. Therefore, emission of fluorine gas from the first electrode can be prevented.

Embodiment 2

A method for reprocessing spent nuclear fuel, to which the method for recovering metallic nuclear fuel materials from spent nuclear materials of the embodiment 1 is applied, according to embodiment 2 which is another embodiment of the present invention, is explained by referring to FIG. 4. The method for reprocessing spent nuclear fuel of the present embodiment is a method to which a purification process 4 and a metallic nuclear fuel manufacturing process 5 have been added to all processes of the method for reprocessing metallic nuclear fuel from the spent nuclear fuel of the embodiment 1, that is, added to the fluorination treatment process 1, the UF₆ treatment process 2, the process of the electrolysis using fused fluoride, and the treatment process 6 of waste salt. Since the fluorination treatment process 1, the UF₆ treatment process 2, the process 3 of electrolysis using fused fluoride, and the treatment process 6 of waste salt in the present embodiment are conducted using treatments which are the same as these processes of the embodiment 1, explanations of these processes are omitted. The purification process 4 and metallic nuclear fuel manufacturing method 5 are explained.

In the purification process 4, the recovered metallic nuclear fuel materials are purified by a dry reprocessing treatment applying the electrolysis using fused chloride disclosed in, for example, Masashi Koyama et al., “Dry Reprocessing Technology”, Denchuken Review, No. 37, pp. 26-37 (2000). By conducting the dry reprocessing treatment applying this electrolysis using fused chloride, a metallic nuclear material with a composition in which all high level radiation NM were removed from the metallic nuclear fuel materials with the composition shown in Table 3 can be obtained. The obtained metallic nuclear fuel has the composition shown in Table 6.

	TABLE 6
		Amount required for
		manufacturing fresh
	Type of metallic	fuel assemblies
	nuclear fuel	(kg/month)
	Component	U	26.44
		Pu	6.92
		MA	0.18
		RE	0.089
		AM	0
		AEM	0
		NM	0
		Zr	3.737

Table 6 shows a metallic nuclear fuel used in the manufacture of fresh fuel assemblies. This metallic nuclear fuel shown in Table 6 is shown in FIGS. 4-5-1 on page 37 of Masashi Koyama et al., “Dry Reprocessing Technology”, Denchuken Review, No. 37, pp. 26-37 (2000).

	TABLE 7
	Type of metallic	Fresh fuel
	nuclear fuel	assembly
	Component	U	3.82081
		Pu	1
		MA	0.02601
		RE	0.01286
		AM	0
		AEM	0
		NM	0
		Zr	0.54003

Table 7 has the metallic nuclear fuel components of Table 6, but rewritten based on a Pu amount of 1. The fresh fuel assembly having metallic nuclear fuel with the composition shown in Table 6 includes a quantity of U which is 3.82 times that of Pu. In the fluorination process 1, U is removed such that the amount thereof will be 3.82 or less that of the amount of Pu contained in the converted fluoride supplied to the process 3 of electrolysis using fused fluoride.

The metallic nuclear fuel (in Table 3, metallic nuclear fuel having a composition with 0 for NM) obtained in the purification process 4 becomes nuclear fuel substance to be loaded into each fuel rod included in the fresh fuel assemblies to be used in a fast breeder reactor. When the composition of the metallic nuclear fuel obtained in the purification process 4 is compared with the composition (Table 6) of the metallic nuclear fuel required for the fresh fuel assemblies, the metallic nuclear fuel obtained in the purification process 4 becomes less than that of U and Zr shown in Table 6. In the metallic nuclear fuel obtained in the purification process 4, Pu, MA, and RE fulfill the specifications shown in Table 6.

In the metallic nuclear fuel manufacturing process 5, the fuel rods are manufactured by using a metallic nuclear fuel manufacturing apparatus. That is, using the metallic nuclear fuel obtained in the purification process 4, as well as metallic uranium and metallic zirconium added such that the amount of each of U and Zr will be that shown in Table 6, a plurality of fuel pellets are manufactured. These fuel pellets are loaded into a fuel cladding and then the fuel rods are manufactured by sealing the fuel cladding. As the metallic uranium to be added, for example, one part of the UF₆ generated in the fluorination treatment process 1 is used, as metallic uranium. Metallic uranium manufactured based on depleted uranium generated by uranium enrichment may be used as the metallic uranium to be added.

A plurality of fuel rods bundled together by a plurality of fuel spacers are attached to a lower tie plate, the fuel rod bundle is passed through a wrapper tube, and a lower end portion of the wrapper tube is attached to the lower tie plate. In this way, the fresh fuel assemblies to be loaded into the core of the fast breeder reactor are manufactured.

The present embodiment can also achieve each effect obtained in the embodiment 1. According to the present embodiment to which the method for recovering metallic nuclear fuel materials from the spent nuclear fuel materials of the embodiment 1 is applied, the process for the method for reprocessing spent nuclear fuel can be also simplified.

Embodiment 3

A method for reprocessing spent nuclear fuel, to which the method for recovering metallic nuclear fuel materials from spent nuclear materials of the embodiment 1 is applied, according to embodiment 3 which is another embodiment of the present invention, is explained by referring to FIGS. 5 and 6. In the method for reprocessing spent nuclear fuel of the present embodiment, each process shown in FIG. 4 is carried out. The process 3 of electrolysis using fused fluoride of the present embodiment is explained in detail based on FIG. 5 and the purification process 4 is explained in detail based on FIG. 6.

In the process 3 of electrolysis using fused fluoride of the present embodiment, a fused fluoride electrolysis apparatus 10, shown in FIG. 5, is used. The fused fluoride electrolysis apparatus 10 is provided with an electrolysis cell 11, a first electrode (electrode for electrolysis) 13, which is an anode, and a second electrode (first recovery electrode) 14, which is a cathode. For example, a circular dam 12 is installed on a, bottom surface of the electrolysis cell 11. The second electrode 14 is disposed at the inner side of the dam 12 in the electrolysis cell 11. The first electrode 13 facing the second electrode 14, is arranged above the dam 12 in the electrolysis cell 11. The first electrode 13 and the second electrode 14 do not contact the dam 12. As the fist electrode 13, for example, a carbon electrode is used and for the second electrode 14, for example, an iron electrode is used. Fused fluoride 15, which is a mixture of LiF and BeF₂, is filled into the electrolysis cell 11, and a liquid surface of the fused fluoride 15 reaches above the upper end of the dam 12. The first electrode 13 and the second electrode 14 are immersed in the fused fluoride 15. Converted fluoride 17 obtained in the fluorination treatment process 1 is poured into an area outside of the dam 12 in the electrolysis cell 11. The converted fluoride 17 is dissolved into the fused fluoride 15.

A mixture of Li₂O and BeO that are oxide is added to the fused fluoride 15. Similar to Embodiment 1, a voltage is applied between the first electrode 13 and the second electrode 14 such that the potential of the second electrode 14 provided in the electrolysis cell will be equal to the electrodeposition potential of the element (uranium), among the elements required for the metallic nuclear fuels being manufactured, for which the electrodeposition potential of the fused fluoride is lowest. The elements having at least the electrodeposition potential of the element with the lowest electrodeposition potential will deposit onto the second electrode 14. The composition of the metallic nuclear fuel material deposited on the second electrode 14 is as shown in Table 3. Carbon oxide gas is generated from the first electrode 13.

After the process 3 of electrolysis using fused fluoride was finished, in order to remove metallic impurities (shown in Table 2, from Te in line number 1 to Cd in line number 11) deposited onto the second electrode 14, the purification process 4 is carried out. Prior to carry out this purification process 4, the first electrode 13, which was used in the process 3 of electrolysis using fused fluoride, is removed and a fresh first electrode (second recovery electrode) 16 is attached to the electrolysis cell 11 (see FIG. 5). The first electrode 16 is also immersed in fused fluoride 15.

In the process 3 of electrolysis using fused fluoride, the amount of uranium in the metallic nuclear fuel material deposited on the second electrode 14 is the greatest. In the purification process 4, without applying a voltage to the first electrode 16 and the second electrode 16, the electric potential of the second electrode 14 approaches the electrodeposition potential of uranium. Then, when a slight electric potential difference is applied between the first electrode 16 and the second electrode 14 such that the first electrode 16 becomes negative, the electric potential of the second electrode 14 becomes slightly more positive than the electrodeposition potential of uranium. Therefore, the uranium deposited on the second electrode 14 begins to dissolve into the fused fluoride 15. Since the first electrode 16 is more negative than the second electrode 14, the uranium dissolved in the fused fluoride 15 begins to deposit onto the first electrode 16. When the electric potential difference applied to the first electrode 16 and the second electrode 14 increases, the electric potential of the second electrode 14 then progressively becomes positive and rises above the electrodeposition potential of Np. At this time, the Np deposited on the second electrode 14 dissolves into the fused fluoride 15 and the dissolved Np deposits onto the first electrode 16. In this way, by adjusting the electric potential difference applied between the first electrode 16 and the second electrode 14 such that the value of the electric potential of the second electrode 14 will become more positive than that of the electrodeposition potential of Tb, and the electrodeposition potential will become a more negative value than that of the electrodeposition potential of Cd, each element deposited onto the second electrode 14 and having an electrodeposition potential of that from Tb to U dissolves into the fused fluoride 15 and then deposits onto the first electrode 16. At this time, each element having an electrodeposition potential of that from Te to Cd deposits onto the second electrode and then remain. In the purification process 4 of the present embodiment, the first electrode 16 becomes the cathode and the second electrode 14 becomes the anode. Due to the abovementioned, the purification process 4 finishes, and the metallic nuclear fuel having the same composition as when the purification process 4 of the embodiment 1 finishes can be obtained. In the purification process 4, each element remaining on the second electrode 14 may be recovered, the second electrode 14 being takenout from the electrolysis cell 11 and cleansed, as required.

After the purification process 4 finished, the first electrode 16 is taken out from the electrolysis cell 11, and this first electrode 16 is transported to the metallic nuclear fuel manufacturing apparatus of the metallic nuclear fuel manufacturing process 5. Using the metallic nuclear fuel deposited onto the first electrode 16, similar to the embodiment 1, the fresh fuel assembly is manufactured by the metallic nuclear fuel manufacturing apparatus.

The present embodiment can obtain each of the effects which generate in the embodiment 2. The present embodiment can conduct the purification process 4 by using the fused fluoride electrolysis apparatus 10, exchanging the first electrode 16 for the first electrode 13 with the fused fluoride 15 loaded as it is in the electrolysis cell 11 of the fused fluoride electrolysis apparatus 10 used in the process 3 of fused fluoride electrolysis, and making the first electrode 16 the cathode and the second electrode 14 the anode and applying an electric potential. That is, since the present embodiment can carry out the process 3 of electrolysis using fused fluoride and the purification process 4 by using the fused fluoride electrolysis apparatus 10, the process of the method for reprocessing spent nuclear fuel can be made even more simple than that of the embodiment 1 which conducts the process 3 of electrolysis using fused fluoride with using a fused fluoride electrolysis apparatus, and the purification process 4 with using a fused chloride electrolysis apparatus. The present embodiment does not require the use of a fused fluoride electrolysis apparatus different from one like that in the embodiment 1.

The fused fluoride electrolysis apparatus 10 used in the process 3 of electrolysis using fused fluoride and purification process 4 is one example, and as this apparatus, an apparatus, which applies anode electrolysis to the metal deposited onto the second cathode 14 by the electrolysis using fused fluoride, and deposits dissolved metal onto a different electrode, may be used. For example, in the fused fluoride electrolysis apparatus 10, although the first electrode 13 was exchanged with the first electrode 16, a fused fluoride electrolysis apparatus provided with an electrolysis cell 11 with the first electrodes 13 and 16 and second electrode 14 may be used. In this case, in the process 3 of electrolysis using fused fluoride, as previously stated, the electric potential difference between the first electrode 13 and the second electrode 14 may be adjusted, and in the purification process 4, as stated above, the electric potential difference between the first electrode 16 and the second electrode 14 may be adjusted.

The principles of the embodiment 3, when consolidated, are as follows. The fused fluoride in which the spent oxide form nuclear fuel is dissolved includes a plurality of recovered elements (called “group A elements”), a plurality of elements which more readily dissolve into the fused fluoride than each element of group A (called “group B elements”), and a plurality of elements which less readily dissolve into the fused fluoride than each element of group A (called “group C elements”). When the first electrode is made an anode and the second electrode is made a cathode and a voltage is applied between the electrodes, and the elements of the group A deposit onto the cathode second electrode from the fused fluoride, the elements of the group C also deposit onto the second electrode and the elements of the group B do not deposit onto the second electrode. When a voltage is applied between the electrodes such that the second electrode becomes an anode and the first electrode becomes a cathode, each element of the group A adhering to the second electrode deposits onto another first electrode after dissolving into the fused fluoride. Nevertheless, each element of the group C remain adhered to the second electrode, not dissolving into the fused fluoride. That is, each element of the element group for which recovery is desired and contained in the spent oxide form nuclear fuel can be separated in a metallic state by electrolyzing metals for which recovery is desired and by transferring them another electrode among the elements that once deposit onto a certain electrode.

Claims

1. A method for recovering metallic nuclear fuel materials from spent nuclear fuel, comprising steps of:

generating nuclear fuel fluorides by reacting fluorine with spent oxide form nuclear fuel taken out from a nuclear reactor,

removing one part of fluorinated uranium from among said nuclear fuel fluorides,

dissolving the remaining nuclear fuel fluorides and oxides into fused fluoride, and

energizing a first electrode which is an anode and a second electrode which is a cathode, both of which were immersed into the fused fluoride, and depositing onto the second electrode a metallic nuclear fuel material dissolved in said fused fluoride.

2. The method for recovering metallic nuclear fuel materials from spent nuclear fuel according to claim 1, wherein the oxide is an oxide having a cation contained in the fused fluoride.

3. The method for recovering metallic nuclear fuel materials from spent nuclear fuel according to claim 1, wherein an electrode containing carbon is used as the first electrode.

4. A method for reprocessing spent nuclear fuel, comprising steps of:

generating nuclear fuel fluorides by reacting fluorine with used oxide form nuclear fuel taken out from a nuclear reactor,

removing one part of fluorinated uranium from among said nuclear fuel fluorides,

dissolving the remaining nuclear fuel fluorides and oxide into fused fluoride, and

energizing a first electrode which is an anode and a second electrode which is a cathode, both of which have been immersed into the fused fluoride, and depositing a metallic nuclear fuel material dissolved in the fused fluoride onto the second electrode, and purifying the metallic nuclear fuel material deposited onto the second electrode.

5. The method for reprocessing spent nuclear fuel according to claim 4, wherein the oxide is an oxide having a cation contained in the fused fluoride.

6. The method for reprocessing spent nuclear fuel according to claim 4, wherein the purification of the metallic nuclear fuel material is carried out by using a fused chloride electrolysis apparatus having an electrolysis cell filled with a fused chloride.

7. The method for reprocessing spent nuclear fuel according to claim 4, wherein the purification of the metallic nuclear fuel material is carried out by,

energizing the second electrode and a third electrode such that the second electrode becomes a cathode and the third electrode becomes an anode when the metallic nuclear fuel material deposited onto the second electrode is immersed into the fused fluoride, and the third electrode is immersed into the fused fluoride; and

depositing a plurality of materials used in the manufacture of metallic nuclear fuel, among materials contained in said metallic nuclear fuel materials, onto the third electrode.

8. The method for reprocessing spent nuclear fuel according to claim 4,

Wherein when the metallic nuclear fuel material deposited onto the second electrode includes a first metallic material used in the manufacture of the metallic nuclear fuel and a second metallic material which less readily dissolves into the fused fluoride than the first metallic material and which is unnecessary for the manufacture of the metallic nuclear fuel, and the fused fluoride includes a third metallic material which more easily dissolves into the fused fluoride than the first metallic material, a third electrode is immersed into the fused fluoride with the metallic nuclear fuel material deposited onto the second electrode being immersed in the fused fluoride; and

the purification of the metallic nuclear, fuel material is carried out by generating between the second electrode and the third electrode an electric potential in which the first metallic material deposited onto the second electrode is dissolved into said fused fluoride, in which the second metallic nuclear material deposited onto the second electrode is not dissolved into said fused fluoride, in which the first metallic material dissolved in the fused fluoride deposits onto the third electrode, and in which the third metallic material does not deposit onto the third electrode the third metallic material.

9. The method for reprocessing spent nuclear fuel according to claims 4, wherein an electrode containing carbon is used as the first electrode.