JPS6146406B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention makes it possible to increase the processing capacity for producing uranium trioxide and/or plutonium dioxide by pyrolytic denitrification of uranyl nitrate or/and plutonium nitrate using a flat plate fluidized bed having a spray nozzle. and plutonium nitrate denitrification equipment. At the uranium hexafluoride conversion plant, a purified uranyl nitrate solution is obtained using solvent extraction methods, and after concentrating it,
Denitration is performed by thermal decomposition to obtain uranium trioxide. Furthermore, in spent fuel reprocessing plants at nuclear power plants, uranium is separated from fission products and plutonium and is obtained as a uranyl nitrate solution, which is denitrified by thermal decomposition to obtain uranium trioxide. Furthermore, in the reprocessing of spent fuel, a method is being planned in which a mixed solution of uranyl nitrate and plutonium nitrate is simultaneously thermally decomposed to obtain a mixed oxide for reasons of nuclear non-proliferation. The present invention relates to the denitrification of uranyl nitrate and/or plutonium nitrate, but in order to simplify the description below, the case of uranyl nitrate will first be described. Methods for denitrating uranyl nitrate in spent fuel reprocessing include the pot method, stirred bed method, microwave heating method, and fluidized bed method, but the fluidized bed method is advantageous in terms of continuity and efficiency. It is being widely put into practical use. The fluidized bed currently used for the denitrification reaction of uranyl nitrate is cylindrical, but when dealing with slightly enriched uranium such as in the reprocessing of spent nuclear fuel, the diameter of the device is limited due to criticality control. Therefore, the processing capacity per unit radix is also limited. Therefore, when using a cylindrical fluidized bed for pyrolytic denitrification of uranyl nitrate, in order to increase the processing capacity, it is necessary to increase the number of equipment as well as the number of attached equipment. Recently, a flat plate type fluidized bed has been attracting attention as a solution to the problems of the cylindrical type fluidized bed. In general, in fluidized bed denitrification equipment for uranyl nitrate, the denitrification reaction is a remarkable endothermic reaction, so a supply of heat is required, and if this heat is supplied normally, the processing capacity is proportional to the cross-sectional area of the equipment. It can be assumed that the amount increases. When the flat plate type fluidized bed is adopted, the thickness of the device is limited by criticality control, but the width of the device can be increased in principle without limit in the longitudinal direction. Therefore, if a flat plate type fluidized bed is used and the cross-sectional area of the fluidized bed device is increased by increasing the length of the width, the processing capacity per unit number can be significantly increased if the above heat supply can be adapted. Therefore, in the case of plants with large processing capacity, the advantages of the flat plate fluidized bed are being emphasized. However, in a uranyl nitrate denitrification equipment using a flat plate fluidized bed, when increasing the processing capacity per unit base, even if the required amount of heat can be supplied, the following problems must still be taken into consideration. That is, (1) Generally, when denitrifying uranyl nitrate using a fluidized bed, a normal fluidization state is maintained;
In order to obtain the desired uranium trioxide product, the linear velocity of the fluidizing gas from the bottom of the fluidized bed should be set at 20 to 60 cm/cm.
In addition, the linear velocity of the gas that leaves the fluidized bed, that is, the linear velocity of the gas near the upper surface of the fluidized bed, is controlled to 100 to 120 cm/sec from the viewpoint of controlling the particle size of the uranium trioxide produced. It is necessary to. The reason for controlling the linear velocity of the fluidizing gas to 20 to 60 cm/sec is that if the linear velocity of the fluidizing gas in the fluidized bed is too high, the fluidized bed will easily fall into a slugging state, making it impossible to maintain a stable fluidized state. It's for a reason. Desired uranium trioxide products for denitrification of uranyl nitrate include:
In consideration of reactivity in subsequent steps, the average particle size is preferably about 100 to 150 μm. In order to stably fluidize this level of product UO ₃ particles, it is necessary to control the linear velocity of the fluidizing gas to 20 to 60 cm/sec. In addition, the linear velocity of the gas that leaves the fluidized bed, that is, the linear velocity of the gas near the upper surface of the fluidized bed, is
The reason why it is necessary to control it to ~120 cm/sec or less is that if the linear velocity of the gas near the upper surface of the fluidized bed is too high, the fine particles of the UO ₃ particles contained in the gas that have come out of the fluidized bed will flow. This is to prevent the stability of the fluidized bed from being impaired due to the particles not returning to the bed, resulting in no or too small amounts of fine particles in the fluidized bed. Therefore, depending on the size of the device, that is, the cross-sectional area, the throughput of the device is limited in terms of linear velocity control of the gas. (2) The amount of gas leaving the fluidized bed reactor, that is, the total amount of gas leaving the fluidized bed mentioned above, the blowback gas amount of the solid-gas separation filter, and the purge gas amount of the overflow pipe is determined by The ventilation rate of the solid-gas separation filter (this ventilation rate is the value obtained by dividing the amount of gas vented through the solid-gas separation filter by the excess area of the filter, and the excess area of the filter is determined by the cross-sectional area and height of the device. The size of the filter For cylindrical filters, the diameter must be less than or equal to the thickness of the device, and a length that is too long, for example 2 m or more, is impractical from the viewpoint of strength, and even if the height of the device is high, There is a limit to the length). Normally, the ventilation speed of solid-gas separation filters is designed to be 2 cm/sec or less. That is, the processing capacity of the device is also limited by the ventilation rate of the solid-gas separation filter. In other words, if the processing capacity is increased unilaterally, the amount of gas produced will increase, and the ventilation speed of the filter will increase, resulting in a large pressure loss, increasing the pressure inside the device, and causing strength problems. . Furthermore, the power of the blower that sucks the off-gas increases, causing technical and economical problems. (3) When denitrating uranyl nitrate in a fluidized bed reactor, the denitrification reaction is a rapid endothermic reaction, and in order to continue the reaction stably, sufficient heat must be supplied from the outside system. Supply is needed. (4) Therefore, when increasing the processing capacity when thermally denitrifying uranyl nitrate in a fluidized bed reactor, it is necessary to take measures against the amount of cracked gas that increases with the increase in processing capacity, and to take measures accordingly. Consideration must be given to the increasing supply of required decomposition heat. That is, (1) above and
It is necessary to control the linear velocity of the gas in the fluidized bed so as to satisfy (2), and to supply a sufficient amount of decomposition heat necessary for the reaction. The processing capacity can only be increased in a state where the following conditions are simultaneously satisfied. In the above, the case where uranyl nitrate is the object of denitration has been described, but the same applies to the case where the object of denitration is plutonium nitrate or a mixture of uranyl nitrate and plutonium nitrate. Taking the above problems into consideration, the present inventors have developed a process for producing uranium trioxide and/or plutonium dioxide by thermally denitrifying uranyl nitrate and/or plutonium nitrate using a flat plate fluidized bed having a spray nozzle. As a result of our study to provide a denitrification device for uranyl nitrate and/or plutonium nitrate that can increase the capacity, we focused on the fact that a rapid increase in the amount of gas occurs in the fluidized bed section above the spray nozzle, and developed the present invention. Reached. That is, the gist of the present invention is to produce uranyl nitrate or/and plutonium dioxide by denitrifying uranyl nitrate or/and plutonium nitrate by thermal decomposition using a flat plate fluidized bed having a spray nozzle. / and plutonium nitrate denitrification equipment, in which the side wall of the equipment above the spray nozzle attachment part is directly connected to an inclined part having an angle of repose greater than or equal to the angle of repose of the uranium trioxide and/or plutonium dioxide particles to be produced, and the upper end of the inclined part. An appropriate number of inserts lower than the height of the spray nozzles are vertically connected to the vertical part and are lower than the spray nozzles at the bottom of the fluidized bed, and are vertically connected to or in close contact with the inner walls of the device facing each other in the width direction, and a heat source is provided to the inserts. Uranyl nitrate or/
and plutonium nitrate denitrification equipment. Next, the present invention will be explained with reference to the drawings. FIG. 1 is a front sectional view of a basic embodiment of the present invention, FIG. 2 is a view taken along the line A-A in FIG. 1 of the present invention, and FIG. 3 is a front sectional view of a basic embodiment of the present invention. FIG. 2 is a perspective view (conceptual diagram) showing the concept of an insert portion. In the figure, a uranyl nitrate solution 12 is blown into a fluidized bed 3 together with a spraying gas 13 from a spray nozzle 4 attached to the side wall of the apparatus. The uranyl nitrate solution 12 is thermally denitrified in the fluidized bed 3 under the fluidizing gas 11 supplied from the fluidizing gas supply section 1 to produce uranium trioxide particles 15, NOx (gas),
Generates H ₂ O (gas). The generated uranium trioxide particles 15 are discharged to the outside of the fluidized bed 3 through an overflow pipe 6. At this time, heated air is blown into the overflow pipe 6 as a purge gas toward the fluidized bed 3. on the other hand,
The gas leaving the fluidized bed 3 is sent to the off-gas treatment system 14 via the solid-gas separation filter 5, and blowback gas is injected from the solid-gas separation filter 5 in the direction of the fluidized bed 3. In this way, in the fluidized bed 3 portion above the spray nozzle 4 attachment part, uranyl nitrate 12
The amount of gas increases rapidly due to the atomizing gas 13 for atomizing and decomposed gas generated by thermal decomposition of the uranyl nitrate solution 12. In order to solve the above problem, this embodiment has a side wall of the device above the attachment part of the spray nozzle 4, which is directly connected to the upper end of the inclined part 9 and the inclined part 9, which has an angle greater than the angle of repose of the uranium trioxide particles 15 to be generated. Vertical section 10
It becomes more. The volume of the fluidized bed 3 above the spray nozzle 4 attachment part is expanded to correspond to the above-mentioned rapid increase in the amount of gas, and the linear velocity of the gas flowing out of the above-mentioned fluidized bed 3 is increased.
In addition to removing the restriction on the processing capacity based on controlling it to 100 to 120 cm/sec or less, the excess area of the solid-gas separation filter 5 can be increased by increasing the number of filters 5, and the air flow rate of the solid-gas separation filter 5 can be increased. This also removes the limitations on processing power that would otherwise occur, thereby making it possible to increase processing power. The angle of the inclined portion 9 is such that the uranium trioxide particles 15 that are generated do not remain on this inclined portion 9.
The angle of repose should be at least 45℃, preferably at least 45℃ for safety reasons. As a result of the above structure, the volume of the fluidized bed 3 above the spray nozzle mounting part is expanded, which may cause problems in the strength of the device and in supplying the required amount of heat. Nozzle 4
The side wall of the device above the mounting part is constituted by the above-mentioned inclined part 9 and vertical part 10, and the fluidized bed 3
An appropriate number of (three in the figure) spray nozzles 4 are installed in the lower part of the insert body 16 in a vertical direction by being joined or closely connected (in the figure) to the inner wall surfaces of the device that are opposite to each other in the width direction of the insert body 16, which is lower than the height of the spray nozzles 4. It has a structure in which a heat source 17 (an electric heater or a heat medium) is built in. Although the structure of the insert 16 is not particularly limited, it is generally a flat plate having an appropriate thickness and has a function inside thereof to heat the insert 16 itself with a heat source. By increasing the temperature of the insert 16 itself with this configuration,
The heat necessary for the denitrification reaction is supplied to the fluidized bed by heat transfer from the surface of the insert 16 in contact with the fluidized bed.
As mentioned above, the thermal decomposition (denitrification) reaction of uranyl nitrate is a rapid endothermic reaction, and in order to increase the processing capacity, it is essential to supply sufficient heat from an external system. In the example of the present invention, by incorporating a heat source in the insert 16, heat is supplied from the insert 16 in addition to the heat supplied from the outer wall portion, and the upper part of the fluidized bed is expanded (thereby, the heat source in the fluidized bed is The linear velocity of the gas can be controlled within a practical range), making it possible to increase the amount of heat supplied corresponding to the increase in processing capacity. Although an electric heater can be used as the built-in heat source 17, liquid inorganic salt heat carriers (for example, certain nitric acid and nitrite mixtures can be used up to a maximum temperature of about 500°C, which is the optimum temperature range for the denitrification reaction). is 270 to 330°C, so it can be used as a practical heating source.Organic heating media are difficult to use due to their operating temperature range.)
Indirect heating by circulating is preferred; in the figure the heat source 17 represents a protective tube or a flow path for the heat medium. When an indirect heating method using a heat medium is used as a heat source, the heat source 17 consists of a pipe through which the heat medium passes and a heat medium heated to a high temperature, and the heat medium enters from one end of the pipe and passes through the other end. come out from the edge. The number of inserts 16 can be determined appropriately depending on the desired amount of heat, but considering manufacturing technology, it is appropriate to set the distance between each insert to be about 10 cm or more. For a fluidized bed device with a width of 60 cm, there will be about 5 pieces. The height of the insert 16 is approximately determined by the height of the lower part (vertical part) of the fluidized bed that is directly connected to the lower end of the spray nozzle 4 attachment part, but it is necessary to set it at a level lower than the height of the spray nozzle 4. The difference between the height of the spray nozzle 4 and the level is
The length is preferably about 10 cm or more. Further, in the method of the present invention, the following effects are also produced by providing the insert body 16 in contact with or in close contact with the inner wall surfaces of the device facing each other in the width direction. That is, this configuration makes it possible to increase the processing capacity, reinforce the device by the rib-like action of the joined insert 16, and ensure the necessary amount of heat is supplied by the built-in heat source 17. It is something. When the insert 16 is provided closely to the inner wall surface of the device,
Although the device exhibits a reinforcing effect against external pressure, it does not provide reinforcement against internal pressure, so it exhibits the same effect as when joined together with a separately provided reinforcing means for internal pressure. In the above, the present invention has been mainly described in the case where only uranyl nitrate is the object of denitrification, but it goes without saying that the present invention can also be applied to the case where only plutonium nitrate or a mixture of uranyl nitrate and plutonium nitrate is the object of denitrification. . As described above, by adopting the above configuration, the present invention uses a flat plate fluidized bed having a spray nozzle to thermally denitrify uranyl nitrate and/or plutonium nitrate, thereby converting uranyl trioxide or/and plutonium dioxide into uranium trioxide or/and plutonium dioxide. The present invention provides a denitrification device for uranyl nitrate and/or plutonium nitrate that enables an increase in processing capacity for producing uranyl nitrate and/or plutonium nitrate, and is extremely useful in nuclear fuel processing. Next, the effects of the device of the present invention will be explained using specific numerical values. In FIG. 1, ₁ ; inclined part 9 of the fluidized bed apparatus;
Height ₂ of the lower vertical part 18 of the fluidized bed directly connected to the lower end of the fluidized bed; Height a from the upper end level of ₁ to the upper end of the overflow pipe 6; Lower vertical part 18 of the fluidized bed directly connected to the lower end of the inclined part 9 of the fluidized bed apparatus Width b: Length of the width of the upper vertical part 10 of the fluidized bed that is directly connected to the upper end of the inclined part 9 of the fluidized bed apparatus Device dimensions Thickness 8 cm Angle of inclined part 45° ₁ 30 cm ₂ 60 cm Linear velocity of fluidizing gas 40cm/sec Linear velocity of gas leaving the fluidized bed 100cm/sec Aeration speed of solid-liquid separation filter 2cm/sec Aeration temperature of solid-liquid separation filter 200℃ Solid-gas separation filter specification diameter 6cmφ Length 150cm Number of solid-liquid separation filters Installation 1 tube per 10 cm of fluidized bed width Reaction temperature 285℃ Amount of gas such as purge gas, blowback gas, etc.
0.50m ³ /min (when b = 120cm) 0.35m ³ /min (when b = 60cm) Spraying gas / uranyl nitrate 500 (room temperature, volume ratio) Temperature when supplying the same gas 200℃ Concentration of uranyl nitrate solution The maximum processing capacity calculated from the linear velocity of the gas leaving the fluidized bed and the aeration velocity of the solid-gas separation filter under conditions of 1200 g-U/ or more is shown in the table below. a and b in the table refer to the above-mentioned a: length of the width of the lower vertical part of the fluidized bed 18 directly connected to the lower end of the inclined part 9 of the fluidized bed apparatus b: upper vertical part of the fluidized bed directly connected to the upper end of the inclined part of the fluidized bed apparatus The length of the width of the portion 10 is shown.

[Table] The calculated values of maximum processing capacity in the table were calculated according to the following calculation example. (Calculation example 1) Value calculated from the linear velocity of gas leaving the fluidized bed (case of the present invention) The amount of gas at the surface of the fluidized bed is 100cm/sec x 8cm x 120cm = 96000cm ³ /sec... (a) Fluidization The amount of gas is 40cm/sec x 8cm x 60cm = 19200cm ³ /sec...(b) The amount of gas such as purge gas and blowback gas (converted to 285℃) is 0.50cm ³ /min x 1000000 ÷ 60 x (273 +285)/273+200)=9830cm ³ /sec...(c) (Note) If b=60cm/s, calculate at ^0.35cm3 /min. The total amount Q of the gas generated by the thermal decomposition reaction and the gas for spraying must be equal to (a) - (b) - (c), so Q = 96000 - (19200 + 9831) = 66969 cm ³ / second... ( d) On the other hand, the thermal decomposition reaction of uranyl nitrate occurs according to the following equation. UO ₂ (NO ₃ ) ₂・6H ₂ O→UO ₃ +NO ₂ +NO+O ₂ +6H ₂ O Therefore, since 9 moles of gas are generated from 1 mole of U, the amount of gas per 1 Kg-U (285℃ conversion ) is (1000/238) x 22.4 x 9 x (273 + 285) / (273) = 1731.35...(e) Similarly, the amount of atomizing gas per 1 Kg-U (converted to 285℃) is (1000/1200 ) × 500 × (273 + 285) / (273 + 20) = 793.52…(f) Therefore, when 1 kg of U is processed, (e) + (f) = 2524.87 = 2524870 cm ³ (at 285°C)… (g) of gas Occur. Maximum processing capacity calculated from the above conditions [=
Q/(g)] is 66969/2524870 = 0.02652 Kg - U/sec = 95.49 Kg - U/hr. Comparative examples can also be calculated using the same method. (Calculation example 2) Value calculated from the aeration rate of the solid-gas separation filter (case of the present invention) In the case of the present invention, it is possible to install 11 solid-gas separation filters (in the case of b = 60 cm in the comparative example, 5 Book)
Therefore, the amount of gas that can pass through the solid-gas separation filter is 11 pipes x 6 cm x π x 150 cm x 2 cm/sec = 62172 cm ³ /sec...(a) The amount of fluidizing gas (converted to 200℃) is 40 cm/sec x 8 cm x 60 cm x (273 + 200) / (273 + 285) = 16275 cm ³ / sec... (b) The amount of gas such as purge gas and blowback gas (200℃ conversion) is 0.50 cm ³ / min x 1000000 ÷ 60 = 8333 cm ³ /sec...(c) The total amount Q of the gas generated by the thermal decomposition reaction and the gas for spraying must be equal to (a) - (b) - (c), so Q = 62172 - (16275 + 8333) =37564cm ³ /sec...(d) On the other hand, the amount of gas per 1Kg-U (200℃ conversion)
As in calculation example 1, (1000/238) x 22.4 x 9 x (273 + 200) / (273) = 1476.61...(e) Similarly, the amount of atomizing gas per 1 Kg-U (200℃ (conversion) is (1000/1200) x 500 x (273 + 200) / (273 + 20) = 672.64... (f) Therefore, when 1 kg of U is processed, (e) + (f) = 2149.25 = 2149250 cm ³ (at 200℃)... ...(g) gas is generated. Maximum processing capacity calculated from the above conditions [=
Q/(g)] is 37564/2149250 = 0.01748 Kg - U/sec = 62.92 Kg - U/hr. Comparative examples can also be calculated using the same method. Furthermore, the effect of the insert 16 as a heat source of the present invention can be explained as follows. As an example, a=60cm, b=120cm, ( ₁ =30cm, ₂ =
60 cm), the effective heat transfer areas are 1.91 m ² and 1.79 m ² with and without the inserts 16 (the number of inserts is 5 and the height is 30 cm), respectively. The amount of heat required for the thermal decomposition reaction of uranyl nitrate is approximately
550kcal/Kg-U, and the practical design values are temperature difference (△t) = 150℃, heat transfer coefficient value (U ₀ ) =
Taking 120kcal/ ^m2・℃・time, the maximum processing speed calculated from the amount of heat transfer is 150×120×1.97÷550=64.5Kg-U/hour Comparative example (a=60cm, b=120cm , without insert), the limit is 150 x 120 x 1.79 ÷ 550 = 58.6 Kg-U/hour. Therefore, in the device shown in this example, the maximum processing capacity is determined by the aeration rate of the solid-gas separation filter, and the device of the present invention has a processing capacity of about twice as much per unit number as that of the comparative example. is obtained.

[Brief explanation of the drawing]

Fig. 1 is a front sectional view of a basic embodiment of the present invention, Fig. 2 is a view taken along the line A-A in Fig. 1, and Fig. 3 is a portion of an insert body of a basic embodiment of the present invention. It is a perspective view (conceptual diagram) showing the concept of. In the figure, 1... Fluidization gas supply section (wind box), 2... Rectifier, 3... Fluidized bed, 4
... Spray nozzle, 5 ... Solid-gas separation filter, 6
... Overflow pipe, 7 ... Extraction pipe, 8 ... Uranium trioxide seed hopper, 9 ... Inclined part, 10 ... Upper vertical part, 11 ... Fluidization gas, 12 ... Uranyl nitrate solution, 13... Gas for spraying, 14... Off-gas treatment system, 15... Uranium trioxide particles, 16... Insert, 17... Heat source (electric heater or heat medium), 18... Lower vertical part, ₁ ... Lower vertical part 1 of the fluidized bed directly connected to the lower end of the inclined part 9 of the fluidized bed apparatus
8 height, ₂ ... Height from the upper end level of ₁ to the upper end of the overflow pipe 6, a... Length of the width of the lower vertical part 18 of the fluidized bed directly connected to the lower end of the inclined part 9 of the fluidized bed apparatus, b... Inclined part 9 of fluidized bed apparatus
The length of the width of the upper vertical section 10 of the fluidized bed that is directly connected to the upper end of the fluidized bed.

Claims (1)

[Claims] 1 In a uranyl nitrate and/or plutonium nitrate denitrification equipment that produces uranium trioxide or/and plutonium dioxide by denitrating uranyl nitrate or/and plutonium nitrate by thermal decomposition using a flat plate fluidized bed having a spray nozzle, The side wall of the device above the nozzle mounting part consists of an inclined part having an angle greater than the angle of repose of the uranium trioxide and/or plutonium dioxide to be produced, and a vertical part directly connected to the upper end of the inclined part, and at the bottom of the fluidized bed. Uranyl nitrate or / and plutonium nitrate denitrification equipment.