KR100454984B1 - Process to remove rare earths from spent nuclear fuel - Google Patents

Abstract

The present invention relates to a method for removing rare earth elements (RE) from spent nuclear fuel. The spent fuel undergoes an oxidation step at a temperature of 200 to 800 ° C. and a heating step of 1000 to 1600 ° C. This process separates the spent fuel into a rare earth rich fluorite phase and a rare earth poor U ₃ O ₈ phase. The RE rich fluorite phase is separated from the RE poor U ₃ O ₈ phase by conventional separation techniques such as sieving, air fractionation, and precipitation.

Description

Process to remove rare earths from spent nuclear fuel

The present invention relates to a method for removing rare earth elements (eg, strong neutron absorbers neodymium and samarium) from spent nuclear fuel, in particular dry processing techniques therefor.

Rare earth elements (RE) (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, etc.) are produced by the breakdown of uranium fission and other fission products in nuclear fuel. Many rare earth elements have a large neutron cross-sectional area, making a significant contribution (approximately 50%) to neutron absorption of spent nuclear fuel. When neutron absorption becomes too high, the fuel must be removed from the reactor for processing. The effective removal of rare earth elements allows for the reuse of spent fuel, thus generating more energy per kilogram of starting material.

Several dry treatment techniques based on air oxidation and heat treatment of spent nuclear fuel are available but are not effective at removing significant amounts of rare earth elements.

Two known dry treatment technologies are the AIROX and OREOX processes. In the AIROX process the fuel may be declad by oxidation or conventional mechanical means. In decladding by oxidation, the fuel pins are heated to 400-600 ° C. in air after being punched out so that oxidation destroys the cladding by oxidizing UO ₂ to U ₃ O ₈ . The resulting U ₃ O ₈ powder can later be easily separated from the cladding. Thereafter, U ₃ O ₈ is reduced at 600 to 100 ° C. with hydrogen to regenerate UO ₂ . This oxidation / reduction step is carried out at a temperature high enough to cause the release of volatile fission products. By using oxidation / reduction cycles, the AIROX process can almost completely remove Xe, Kr, Cs and I.

The OREOX process is an improvement of the AIROX process. Oxidation is performed at a higher temperature (1200 ° C.) than the AIROX process, which allows more efficient removal of volatile fission products.

Wet reprocessing techniques (based on fuel dissolution and subsequent chemical separation) can be used to remove rare earth elements from spent fuel, but can be commercially viable due to the large amount of liquid waste generated and the need to maintain stringent plutonium useful protections. none. For example, Canadian Patent 589,122 discloses a method for reprocessing used nuclear reactor fuel. The patent discloses 99% removal of some rare earth elements, which includes contacting uranium with refractory oxides under non-oxidizing conditions in a molten state and separating decontaminated uranium from fission product containing oxides.

U.S. Patent 2,822,260 discloses a process for separating rare earth elements and other cleavage products from neutron bombarded uranium. This patent discloses melting uranium with metal oxides at a temperature of about 1150 to 1400 ° C. in an inert atmosphere to produce uranium dioxide scales that are highly enriched in fission products and highly concentrated.

The present invention relates to a dry treatment technique capable of removing significant amounts of rare earth elements from spent uranium dioxide fuel.

Summary of the Invention

The present invention relates to a method for removing rare earth elements from spent nuclear fuel. The method includes oxidizing spent fuel at a temperature of about 200-800 ° C. to oxidize UO ₂ to U ₃ O ₃ . The spent fuel is then heated to a temperature of 1000 to 1600 ° C to allow the U ₃ O ₈ to be separated into a rare earth (RE) rich fluorite phase and a rare earth (RE) poor U ₃ O ₈ phase. The rare earth (RE) rich fluorite phase is then separated from the rare earth (RE) poor U ₃ O ₈ phase.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the UO- (RE) O-O portion of a three-way U-RE-O phase diagram for a typical rare earth element in the temperature range of 1000-1500C.

2 shows the UO- (RE) OO portion of a three-way U-RE-O phase diagram showing the behavior of a UO ₂ sample doped with 1.7 mole percent of rare earth elements.

3 is a DTA result for the oxidation of UO ₂ in air.

4 is a DTA result for the oxidation of 2% Nd doped UO ₂ in air.

FIG. 5 is an X-ray diffraction pattern for the powder generated by subjecting neodymium doped UO ₂ powder to two stage air oxidation (580 ° C., 2 hours) and heat treatment (1400 ° C., 1 hour).

FIG. 6 shows the ratio (F) of fluorite separated from U ₃ O ₈ to Nd% of neodymium doped UO ₂ oxidized to U ₃ O ₈ and annealed in air at 1400 ° C. for 8 hours.

FIG. 7 shows the ratio (F) of fluorite separated from U ₃ O ₃ to Ce% of cerium doped UO ₂ oxidized to U ₃ O ₈ and annealed in air at 1400 ° C. for 8 hours.

FIG. 8 shows the ratio (F) of fluorite separated from U ₃ O ₃ to Yb% of ytterbium doped UO ₂ oxidized to U ₃ O ₈ and annealed in air at 1400 ° C. for 8 hours.

10 is an X-ray diffraction pattern of a powder generated by oxidizing neodymium doped sintered pellets by the method of the present invention.

FIG. 11 is an XRD pattern of powder generated by treating a SIMFUEL sample (4 atomic% fuel consumed mock sample) by the method of the present invention.

12 (a) and (b) show the used H.B. obtained by air oxidation (440 ° C., 4.5 hours) and heat treatment (1400 ° C., 4 hours). Arrows in XRD and SEM patterns for Robinson LWR fuel samples indicate the location of XRD peaks relative to the fluorspar phase.

FIG. 13 shows typical wavelength dispersion X-ray emission (WDX) of U ₃ O ₈ grains and rare earth-rich nodules in samples obtained by two-stage air oxidation (440 ° C., 4.5 hours) and heat treatment (4 hours at 1400 ° C.) of spent LWR fuel. ) Shows the spectrum.

FIG. 14 is an SEM image of powder produced by oxidizing Nd doped sintered UO ₂ at 400 ° C. for 16 hours and heating in air at 1400 ° C. for 8 hours.

FIG. 15 is an SEM image of a powder produced by oxidizing Nd doped sintered UO ₂ at 400 ° C. for 16 hours.

16 shows the number of particles as a function of diameter in neodymium doped UO ₂ after treatment with the method of the present invention.

Rare earth (or RE) elements in the spent fuel make up about 30% of the fission product and half the neutron loading. Thus, modifications of dry treatment techniques for spent fuel, including the removal of significant amounts of rare earth elements, have a significant impact on the commercial viability of these processes.

The present invention relates to a process using a high temperature treatment under non-reducing conditions and the RE rich fluorite phase (U, RE) O _{2 + X} from the U ₃ O ₈ phase (for simplicity the fluorite phase is indicated throughout the present application. is used herein to mean the ₂ and O _{2 + X} samples (U, RE) belongs to the same crystal structure) with separates the RE element effectively. Experimental results performed by the Applicant show that not only does RE separation occur, but also the way in which smaller particles become rich in rare earth elements. Therefore, with the difference in density between U ₃ O ₈ and (U, RE) O _{2 + X} , a simple mechanical separation (ie sedimentation, air classification, sieving) can be used to remove a significant amount of RE from U ₃ O ₈ . have. The sample does not heat up to a sufficiently high temperature where a significant degree of RE separation occurs compared to conventional AIROX and OREOX processes. In addition, reduction in AIROX and OREOX introduces a small amount of RE that can be separated onto a single fluorite so that no actual RE separation can be achieved.

Various aspects of the three-way U-RE-O phase diagram were intensively studied (Fujino, Miyake 1991, Handbook on the Physics and Chemistry of the Actinides, Elsevier Science Publishers, New York, Vol. 6, p. 155). Although there are subtle differences in the phase diagrams for the various REs, the general characteristics are the same for most rare earth elements. It is shown in FIG. 1 at 1000-1500C of the UO- (RE) O-O portion of a typical U-RE-O phase diagram.

The most important features of FIG. 1 in the dry oxidation of the fuel investigated are as follows:

1.Single fluorite phase (F) consisting of a solid solution of UO _{2 + X} and (RE) O _1.5

2. A triangular zone bounded by two phase regions, A and U ₄ O ₉ and U ₃ O ₈ , in which both the fluorite and U ₃ O ₈ solid solutions are present. The solubility of (RE) O _1.5 in U ₃ O ₈ is reported to be below the detection limit of the X-ray diffraction (XRD) technique (ie less than 0.2 to 0.5 mol%) (Keller, 1975, Ternare und polynare oxide des urans Gmelins Handbuch der Anorganischen Chemie, 8 Erganzungswerk, Band 55, Teil C3, Springer-Verlag, Berlin, p. 97).

3. The “M” zone of FIG. 1 where (RE) O _1.5 coexists with other RE rich compounds (eg, UO _{3 ·} 6PrO _1.5 ). This zone is not detailed because it is independent of the present invention.

Oxidation of UO ₂ and Consumed Fuel

Oxidation of UO ₂ not consumed

The main features of the UO binary phase diagram are well established. When the pure UO ₂ sample is oxidized its composition shifts from the UO ₂ point in FIG. 1 towards the oxygen vertex of the phase diagram. Oxygen anions are inserted in the interstitial lattice gap between the fluorite grid and between UO ₂ and U ₄ O ₉ ; At the same time, the change in the average uranium ion valence maintains the fluorite lattice. Only one phase (fluorspar) is found in the sample in the UO ₂ to U ₄ O ₉ zone in the temperature range of 1000 to 1500 ° C.

If a pure UO ₂ sample is oxidized beyond the U ₄ O ₉ point and the total O / U ratio is between 2.25 and 2.67, two phases U ₄ O ₉ (fluorite) and U ₃ O ₈ (orthorhombic) are present in the sample. do. The intermediate phase is only stable (or metastable) at low temperatures (U ₃ O ₇ ) or at high pressures (U ₂ O ₅ ). In these two-phase zones, the relative amounts of orthorhombic and fluorite phases can be calculated by lever rules.

If more oxygen is added to the system so that the overall composition is at 0 in U ₃ O ₈ , U ₃ O ₈ is present along with the gaseous oxygen. Above 1100 ° C U ₃ O ₈ loses a small amount of oxygen and above 1500 ° C. U ₃ O ₈ decomposes in air to form UO _{2 + X} (x is ˜0.25).

Oxidation of spent fuel

The oxidation aspect of the spent fuel is more complicated than the unused UO ₂ due to the differences in the fuel microstructure and the numerous cleavage products found in the spent fuel. When estimating the chemistry of RE in spent fuel, the spent fuel can be regarded as a solid solution of one RE in UO ₂ . The concentration of a single RE is ˜1.7 atomic percent, which is the amount of total rare earth elements in a PWR fuel after typical 35 MW · d / kg U fuel consumption. The RE content of typical spent PWR fuel is given in Table 1.

In this application, the terms atomic% and molecular% refer to the ratio of the total metal content on the basis of oxygen free. For example 1.7 atomic% RE is a mixture with a molar ratio RE / [RE + U] of 0.017.

TABLE 1a

Rare Earth Element Content of Typical PWR Fuel Consumed After 35MW · d / kg U Fuel Consumption

TABLE 1b

The equilibrium aspect of a UO ₂ sample doped with 1.7 atomic% RE is shown in FIG. 2. The stoichiometric solid solution of UO ₂ and (RE) O _1.5 lies along the line connecting the points representing these compounds in FIG. 2. The composition corresponding to 1.7 atomic% is point (B). Oxidation of this sample moves its composition along the line BC toward the oxygen vertex of the upper diagram. Along with UO ₂ , a single fluorite phase exists from this region to point (C). However, if the sample is further oxidized above point C, a large difference between UO ₂ and RE doped material is observed. For RE doped materials, the sample composition along line CD consists of two phases RE depleted U ₃ O ₈ phases and an RE rich fluorspar phase. Therefore, as the oxidation proceeds above the point C, as the U ₃ O ₈ phase separates, the composition of the fluorspar phase moves along the line segment CA. When oxidation proceeds so that the total sample composition is at point D, the total mixture consists of the fluorite phase of composition A and U ₃ O ₈ . The relative quantities of U ₃ O ₈ and fluorspar can be calculated according to the lever rules. The composition of the fluorspar phase can be calculated by extrapolating the line OA up to the UO- (RE) O axis and determining the relative ratio of RE and U from point E.

In fact, RE doped UO ₂ treated by the process of the present invention does not exactly follow this equilibrium path because the oxidation rate is much faster than the RE separation rate. The initial oxidation product is therefore a metastable RE doped U ₃ O ₈ phase. Therefore, oxidation and separation occur in two steps in the process described below. The final set of phases, however, is as described for point D in the preceding passage.

Two parameters have been recognized on U-RE-O that are important for possible applications in the method of the present invention. First, it was assumed that U ₃ O ₈ separated in the U ₄ O ₉ -AU ₃ O ₈ region had negligible solubility for RE. Through experiments, the solubility of RE in U ₃ O ₈ is as low as 0.2 to 0.5 atomic%, which is the upper limit of typical solubility. The position of point A in the second phase diagram is important (FIGS. 1 and 2). The method of the present invention aims at the production of RE rich fluorite phases. This scenario corresponds to the case where A is located as close as possible to the (RE) O vertex of the top diagram. The exact location of point A changes with temperature and with the element type of RE.

37 g of 1 kg of spent PWR fuel containing 1.7 atomic% RE when the spent fuel was separated into fluorspar with essentially U ₃ O ₈ and composition ((RE) _0.35 , U _0.65 ) O ₂ . RE rich fluorspar waste can be obtained. Less than 3% uranium is lost in RE rich waste in one kilogram of the first fuel. An important difference between UO ₂ and spent fuel is the fact that there is a significant amount of plutonium in the spent fuel. The similarity in oxidation of UO ₂ and hybrid oxide (U, Pu) O ₂ fuels indicates that plutonium does not significantly affect the RE separation step of the process of the present invention. However, the presence of plutonium is important because no dry treatment technique should be used to separate plutonium from spent fuel and there must be a practical way to do it for safety reasons. The maintenance of most plutonium in the treated fuel is also economically desirable.

Two different forms of neodymium doped UO ₂ are prepared to assess the possibility of separating rare earths from uranium oxide. Neodymium was chosen as a typical rare earth element because it is the most abundant RE cleavage product (Table 1). The first doped UO ₂ sample consisted of a powder prepared by coprecipitation of neodymium and uranium with ammonium diuranate and then reduction to (U, Nd) O ₂ . The second sample consists of a sintered UO ₂ pellets portion is doped with 2% neodymium is a part is SIMFUEL, i.e. the UO ₂ doped with a mixture of a cleavage product, designed to mimic the spent fuel released by Lucuta (1991, Microstructure Characteristics of SIMFUEL, Journal of Nuclear Materials 178, 48-60). Most tests are performed on Nd-doped powders and pellets. The degree of separation from U ₃ O ₈ on Nd rich fluorspar is measured by X-ray diffraction (XRD). A crude particle size classification test (by sedimentation or filtration) was performed on the powder oxidized by the method of the present invention and the Nd content of the various fragments was analyzed. Thus, it can be determined whether a simple mechanical separation can remove a significant amount of rare earth elements from the doped UO ₂ .

Experiment on RE doped UO ₂ powder

Sample manufacturing

UO ₂ samples doped with various amounts (0, 0.1, 0.2, 0.5, 1.0, 2.0 atomic%) of RE (Nd, La, Yb or Ce) were reported by Clayton and Aronson (1961, Some preparative methods and physical characteristics of Uranium Dioxide Powders, Journal of Chemical and Engineering Data 6, 43-51). Most experiments on RE doped powders were performed on samples with RE = Nd. Ammonium hydroxide (25%) was used to co-deposit neodymium and uranium from their nitrate solutions. The resulting Nd-doped ammonium diuranate (ADU) is collected by centrifugation and washed several times with dilute ammonium hydroxide before drying with 105 ° C. air. The ADU powder is then heated in air and converted to U ₃ O ₈ . Analysis of the resulting sample by XRD showed only U ₃ O ₈ peaks (primarily α-U ₃ O ₈ and ˜6% β-U ₃ O _8, as determined by the intensity of the XRD peak). There is no indication of the isolated Nd rich phase and there is no correlation between the relative amounts of neodymium and α- and β-U ₃ O ₈ . Just before the powder is air oxidized, the doped U ₃ O ₈ is reduced from Ar / 3% H ₂ to UO ₂ at 1150 ° C. for 3 hours. XRD analysis of the reduced powder shows only UO ₂ and in some cases shows a small amount of U ₃ O ₇ but no U ₃ O ₈ or other impurities.

Air Oxidation Experiments on Nd-doped UO ₂ Powders

Doped UO ₂ samples prepared via the ADU synthesis route are oxidized in air flowing in a simultaneous differential thermal analyzer (DTA) / thermal weight analyzer (TGA). The sample is heated from room temperature to 400 ° C. at a rate of 10 ° C./min and held at that temperature for 16 hours. Although a temperature of 400 ° C. may be used, the temperature may range from 200 to 600 ° C. The sample is then heated to 1400 ° C. at a rate of 10 ° C./min and held at this temperature for 8 hours. Temperatures between 1000 and 1600 ° C. can be used during this step. Peaks associated with the well-known two-stage oxidation of UO ₂ are observed but no peaks associated with the separation of the RE rich fluorspar phase (FIGS. 3 and 4). However, since the separation process is controlled by diffusion, this result is not surprising, and a fairly low diffusion rate provides a difficult to discern reaction exotherm.

X-ray diffraction analysis of the powder produced by DTA gives clear evidence that the neodymium rich fluorite phase has been separated from U ₃ O ₈ during oxidation and heat treatment. 5 shows the XRD peaks associated with the Nd rich fluorspar phase. The quantitative data described below show the correlation between the neodymium content of the original neodymium doped UO ₂ and the peak intensity of the fluorspar in the oxidized powder. The average cell parameters of the fluorspar phase at 1% and 2% Nd-doped materials are calculated to be 0.543 ₄ nm, which is consistent with the reported value of 0.543 ₅ nm for Nd-doped UO ₂ sintered at 1200 to 1400 ° C (Keller and Boroujerdi, 1972, Journal of Inorganic and Nuclear Chemistry 34, 1187-1193).

Solubility of rare earths in U ₃ O ₈

The low solubility of rare earths in U ₃ O ₈ as described above is one of the key factors of the process of the invention. Thus, the oxidation product of the doped UO ₂ powder is quantitated by XRD to determine the solubility of neodymium in U ₃ O ₈ at 1400 ° C. This is in agreement quantitation double integral area at ~26.0 ° is used for the intensity (I _U308) Measurement of U ₃ O ₈ signal area of the peak at ~28.4 ° is the intensity (I _{calcium fluoride)} type of Gargoyle signal. The amount of fluorite present in the sample is considered to be proportional to F = I _fluorite / (I _fluorite + I _U308 ). Although this approximation affects the slope of the F graph as a function of neodymium content at low neodymium concentrations, assuming the same absorption coefficients for U ₃ O ₈ and the fluorspar phase (FIG. 6), the measured neodymium solubility, ie x -Does not affect the intercept significantly. The ratio F is calculated for each sample and the results are shown in Table 2. The data in Table 2 is graphed in FIG. 6 from which it was estimated that the solubility of neodymium in U ₃ O ₈ at 1400 ° C. is less than 0.3 atomic percent.

Solubility and Air Oxidation of Rare Earths in U ₃ O ₈ when RE = Ce, La, or Yb

The air oxidation experiment and the solubility test described above for neodymium doped samples were performed in the same manner even when RE = Ce, La or Yb. The results are shown in Tables 3, 4 and 5 and the data of Tables 3, 4 and 5 are graphed in FIGS. 7, 8 and 9 respectively. From these figures, the solubility in U ₃ O ₈ of cerium, lanthanum and ytterbium at 1300 ° C. is estimated to be less than 0.3 atomic%.

TABLE 2

F of product obtained by two-step air oxidation and heat treatment of Nd-doped UO ₂ powder

TABLE 3

F of product obtained by two-step air oxidation and heat treatment of Ce-doped UO ₂ powder

TABLE 4

F of product obtained by two-step air oxidation and heat treatment of La doped UO ₂ powder

TABLE 5

F of product obtained by two-step air oxidation and heat treatment of Yb-doped UO ₂ powder

Experiment on Sintered Pellets

Sample manufacturing

Oxidation experiments are performed on two atomic percent neodymium doped UO ₂ sintered pellets obtained from J. Sullivan (Fuel Materials Branch, Chalk River Laboratories). Sintered pellets are obtained by mixing the finely divided oxide powder in an appropriate amount and subsequently sintering in hydrogen atmosphere. Nd-doped UO ₂ pellet slices were polished to 0.05 μm and inspected by scanning electron microscopy / energy dispersive X-ray spectrometer. Since these tests show no evidence of neodymium separation, it can be concluded that the sintering process was successful in forming a solid solution between neodymium and uranium at (U, Nd) O _{2 + X.}

Tests were also performed on simulated high energy consumption fuel (ie SIMFUEL) samples. A quaternary% simulated fuel consumption material was prepared by mixing and sintering the appropriate powder material in Chalk River Laboratories (Lucuta, 1991, supra, Journal of Nuclear Materials 178, 48-60).

Oxidation Test of Nd-doped UO ₂ Pellets

Four Nd-doped UO ₂ specimens cut from the sintered pellet were oxidized to U ₃ O ₈ at 400 ° C. for 16 hours and heated in air at 750, 1000, 1250 and 1380 ° C. for ˜ 16 hours. XRD analysis of the resulting powder shows only U ₃ O ₈ in the samples subjected to 750 and 1000 ° C. heat treatment. However, the fluorite phase was separated from U ₃ O ₈ during the heat treatment at 1250 and 1380 ° C. (FIG. 10). The intensity of the XRD peak associated with the Nd rich fluorescence phase for the U ₃ O ₈ peak is approximately equal to that produced by oxidizing the 2 atomic% Nd doped UO ₂ pellets (compare FIGS. 5 and 10). The lattice parameters on the fluorspar were 0.5437 ₀ nm for samples oxidized at 1250 ° C. and 0.5433 ₂ nm for samples oxidized at 1380 ° C.

Oxidation experiment of SIMFUEL

The test is done on 4% SIMFUEL samples. The disk pieces from which the sintered pellets were cut are powdered by oxidizing in air at 400 ° C. for 4 hours and heated to 1200 ° C. in air for an additional 16 hours. The XRD pattern shown in FIG. 11 clearly shows the presence of a significant amount of fluorite phase, indicating that separation of RE has occurred. This result is important because it implies that many of the nonvolatile fission products present in spent fuel samples do not significantly affect the U-RE-O phase relationship. The lattice parameter calculated for the fluorspar phase separated from the SIMFUEL sample is 0.5428 ₃ , which is consistent with the known results for doped UO ₂ (Keller and Boroujerdi, 1972, Journal of Inorganic and Nuclear Chemistry 34, 1187-1193). .

Experiment on spent PWR fuel

Experiment

The method of the invention was applied to spent PWR fuel samples in a series of tests performed at Chalk River Laboratories and subjected to product analysis in Whiteshell Laboratories. The spent PWR fuel sample was H.B. Obtained from Robinson Unit 2. Fuel consumption is 672 MWh / kg U.

Examination by SEM revealed that RE rich phase separation occurs when the spent fuel is treated according to the present invention. Detailed analysis was performed on HB Robinson fuel samples that were oxidized at 440 ° C. for 4.5 hours and subsequently heated at 1400 ° C. for 4 hours. Examination by SEM revealed the presence of RE-rich nodules and the XRD pattern shows significant peaks associated with the phase of the fluorspar structure (FIG. 12). Detailed investigation of the RE-rich nodules by wavelength-dispersive X-ray emission (WDX) shows that significant amounts of major RE cleavage products (Nd, Ce, La, Pr and Sm) are present in spent PWR fuel. A similar investigation of U ₃ O ₈ grains shows that rare earths do not appear (FIG. 13).

Particle Size and Rare Earth Separation of Products

Characteristics of Particle Size Distribution

The powder produced by oxidizing the Nd-doped sintered pellet at 400 ° C. and heating to 1250 ° C. showed the presence of U ₃ O ₈ crystals with many large (˜10 μm) planes by electron scanning microscopy (SEM). 14). In addition, there are a number of small (˜1 μm) particles that adhere to the faces of the larger U ₃ O ₈ grains. These microstructures are compared to materials heated to 750 ° C. or 1000 ° C. in a two step process of the present invention. Samples treated at these lower temperatures exhibit an irregular "popcorn" shape that is characteristic of U ₃ O ₈ formed by low temperature oxidation of UO ₂ . This shows that solid-state recrystallization of U ₃ O ₈ occurs at 1250 ° C but not below 1000 ° C, and secondary crystallization of RE-rich fluorspar occurs simultaneously with this recrystallization. The investigation of U ₃ O ₈ grains and smaller RE-rich fluorescence phases by energy dispersive X-ray emission (EDX) shows that the latter show significant amounts of neodymium, but not the former.

Samples treated with the method of the present invention were slurried a small amount of product in 100 ml water and analyzed for particle size distribution using a Climet Particle Counter. The measured particle size distribution (FIG. 16) shows that groups of 10-20 μm in diameter and less than 5 μm in diameter were observed in these powders. Therefore, simple tests are used to examine the possibility of separating some REs by particle size based processes.

Rare earth separation test

A small amount (˜0.25 g) 2 atomic% Nd doped U ₃ O ₈ treated at 1250 ° C. by the method of the present invention is slurried in 75 cm ₃ of distilled water and ultrasonically dispersed for 2 minutes to weakly adhere to larger U ₃ O ₈ grains. Small Nd-rich particles. Very small amounts of ultrafine particles are observed to float on the water surface; Pour this material and filter through a 0.8 μm nylon filter. The remaining mixture is stirred vigorously and the crude particles are allowed to settle for 40 seconds. The finer particles still suspended are then filtered through a 0.8 μm nylon filter. Finally, the remaining "crude" debris is removed from the water by filtration. All three sizes of debris (crude, fine, ultrafine) are washed with isopropanol and air dried. To determine the Nd / U ratio, fragments of various sizes were analyzed by inductively coupled plasma (ICP) spectral analysis, and the results (Table 6) show that there are significant concentrations of Nd in finer particle fragments. However, it should be noted that only a small amount of starting material is present in the fine and ultrafine fragments.

TABLE 6

Nd / U in granulated, fine and ultrafine debris separated by sedimentation of powder

In the second test, discs made of Nd-doped sintered pellets were treated by heating the method at 400 ° C. for 16 hours at 1250 ° C. for 16 hours by the method of the present invention. A small amount (0.26) of material is slurried in 100 ml water and the mixture is filtered through an 8 μm filter. The filtered particles are washed several times with distilled water and the solid sample is stored. The filtration process is repeated to filter the remaining solution sequentially through 5 μm, 1.2 μm and 0.22 μm filters. The resulting powder is analyzed by ICP spectra to determine the neodymium and uranium content. The results are given in Table 7. Although the total amount of material in the finer fragments is very low, a significant amount of neodymium enrichment can be seen in the 1.2 μm fragments.

TABLE 7

Uranium and Neodymium content of each fragment after treatment by this method.

^* The neodymium content of the 0.22 μm sample could not be detected due to the very small sample volume.

conclusion

The ternary U-RE-O phase diagram shows that air oxidation of RE doped UO ₂ (or spent fuel) and heat treatment at 1000 to 1600 ° C. form a RE-rich fluorite phase and U ₃ O ₈ . Although there is a significant difference between the various rare earth elements, the RE content in the fluorspar phase is approximately 25 to 40 mole percent, while the RE content in the U ₃ O ₈ phase should be quite low (about 0.3 atomic percent or less at the temperature used in this process).

Experimental results with neodymium (typical RE) doped UO ₂ show that this separation actually occurs. A typical experiment is carried out in two steps:

1. low temperature oxidation (400-600 ° C.) to convert UO ₂ to U ₃ O ₈ powder;

2. High temperature heat treatment (1250-1400 ° C.) to allow rare earths to separate into fluorspar.

The use of the method of the present invention shows good separation of the Nd rich fluorspar phase for the UO ₂ powder (coprecipitation) method prepared by ammonium diuranate and the sintered pellets. Further testing with SIMFUEL also shows RE isolation. Application of the present method to spent PWR fuel confirms that this treatment results in phase formation of the fluorspar structure and that a significant amount of main RE cleavage product is found in this phase.

Precipitation and filtration experiments showed that the RE rich fluorspar phase had a much smaller particle size distribution than U ₃ O ₈ . Thus, significant amounts of RE rich material can be removed by adsorbing inert phases such as Al ₂ O ₃ , ZrO _2, or SiO ₂ that can absorb mechanical separation, volatilization, or rare earths such as sieving, air fractionation or precipitation. . Removal of RE can be enhanced by post-separation sample processing by sample reduction (converting U ₃ O ₈ to UO ₂ ) or other methods of reducing the RE-rich nodules that attach to larger U ₃ O ₈ grains.

Various modifications are possible without departing from the spirit of the invention and in particular the method can be used in a variety of hard water reactors, heavy water reactors and rapid reactors.

Claims (10)

Oxidizing the spent fuel to 200-800 ° C. to oxidize UO ₂ to U ₃ O ₈ ; From the spent nuclear fuel, which is then heated to 1000 to 1600 ° C. to separate U ₃ O ₈ into a RE (rare earth) high fluorite phase and a RE (rare earth) low U ₃ O ₈ phase. How to get rid of rare earths. The method of claim 1, wherein the rare earth is selected from the group consisting of neodymium, samarium, cerium, lanthanum, praseodymium and ytterbium. 3. The method of claim 2 wherein the rare earth is selected from the group consisting of neodymium, cerium, lanthanum and ytterbium. The method of claim 1 wherein the oxidation of spent nuclear fuel is carried out in the presence of an oxidant selected from the group consisting of air, oxygen, N ₂ O, NO and NO ₂ . The method of claim 1 wherein the spent fuel is oxidized at 400 to 600 ° C. The method of claim 1, wherein the temperature of the heating step is 1250 to 1500 ℃. The method of claim 1, wherein the RE (rare earth) high fluorite phase is separated by sieving, air fractionation, electrical or magnetic separation, precipitation, absorption or absorption into an inert matrix. The method of claim 1, wherein the remaining RE (rare earth) with low content U ₃ O ₈ phase method of RE (rare earth) was reduced to the low content UO ₂ phase comprising the step of sintering the RE (Rare Earth) low content UO ₂ phase to the fuel pellets. 9. The process according to claim 8, wherein the RE (rare earth) low content U ₃ O ₈ phase is reduced by a gas phase reducing agent. 10. The method of claim 9, wherein the gaseous reducing agent is selected from the group consisting of H ₂ and CO.