JPH0316639B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an apparatus for dissolving nuclear fuel in heated nitric acid and separating it from an insoluble cladding material in a process of reprocessing spent ceramic nuclear fuel. ,
The present invention relates to an apparatus suitable for continuously dissolving nuclear fuel with a high concentration of fissile material at high efficiency.

[Background of the invention]

Ceramic nuclear fuel is usually produced by sintering nuclear fuel material into cylindrical pellets, sealing it in a metal cladding tube to form a fuel body, and then assembling it into a bundle to form a so-called nuclear fuel assembly.

Nuclear fuel materials include fissile materials that fission in the presence of neutrons and nuclear source materials that generate fissile materials in the presence of neutrons.

Uranium with mass number 233 or 235 or mass number
Plutonium, which has a mass number of 239 or 241, is a fissile material, uranium, which has a mass number of 238, is a nuclear source material that produces plutonium, and thorium, which has a mass number of 232, is a nuclear source material that produces uranium, which has a mass number of 233. .

The concentration of fissile material varies depending on the type and purpose of the reactor in which the fuel is used. Reactors that sustain fission using neutrons slowed down to low energy usually have a low concentration of fissile material, whereas reactors that sustain fission using high-energy neutrons usually have a low concentration of fissile material. The concentration of is high.

The nuclear fuel assembly is inserted into the core of a nuclear reactor, and the atoms of the fissile material are fissioned in the presence of neutrons into 2
Fission products of atomsWhen they change into atoms, they generate heat energy and radiation. After some atoms of the nuclear source material absorb neutrons, they undergo a series of nuclear reactions to become atoms of fissile material, and some of them undergo nuclear fission in the nuclear reactor.

A fuel assembly whose fissile material has been consumed and whose calorific value has decreased and is no longer suitable for use in a nuclear reactor is removed from the reactor and is called spent fuel.

Reprocessing is the process of separating spent fuel fissile materials and nuclear source materials from fission products for reuse.

The reprocessing process generally consists of a variety of unit steps, but in a commonly known commercial method called the Piurex process, the first step is to shear the nuclear fuel assembly into small pieces, and then the nuclear fuel material is exposed to nitric acid. dissolved in The cladding material of a nuclear fuel body is generally a zirconium alloy or stainless steel and is not dissolved in nitric acid, so it is separated after the nuclear fuel material is dissolved.

The first technical issue to be considered in the nuclear fuel melting process is criticality safety measures, that is, measures to prevent chain fission reactions.

Criticality safety measures depend on the concentration of fissile material in the nuclear fuel, but are generally treated very conservatively and care is taken to ensure criticality safety is achieved in any case.

In general, the concentration of fissile material is high before it is consumed in the reactor, and there is no neutron absorption of fission products or residual flammable neutron poisons. Assume that conditions are such that it is easy for this to occur.

In the design of nuclear fuel melting devices, it is common to limit the geometry of the constituent parts in accordance with the amount of fissile material that may be loaded.

In other words, for a cylindrical container, its diameter,
For flat containers, the thickness is controlled.
If the container has a jacket on the outside to allow water or steam to pass through, the dimensions are limited by including the jacket. An annular container with a limited thickness is sometimes used as a compromise between a cylinder and a flat plate.

Particularly in the case of an annular container, a thin cadmium plate that effectively absorbs thermal neutrons is pasted on the outside of the inner cylinder, and a material that is effective for decelerating neutrons and converting them into thermal neutrons is placed inside the inner cylinder. It is known to increase safety or relax thickness restrictions by arranging

When using a cage for loading nuclear fuel, the dimensions of the cage must be limited separately from the container.

The second technical issue to be considered in the nuclear fuel melting process is the control of the reactions involved in melting.

When uranium dioxide, a typical substance in ceramic nuclear fuel, is dissolved in nitric acid, the dissolution rate is affected by the surface area of uranium dioxide, the concentration of nitric acid, and temperature. In actual cases, it is further influenced by the rate at which nitric acid is replaced by an aqueous solution of uranyl nitrate, which is a dissolved product, on the surface of uranium dioxide.

In order to increase the efficiency of a melting device with a limited shape, there are already methods such as raising the temperature of nitric acid to the boiling point, circulating the nitric acid in a container using a thermosiphon, or making it flow by blowing gas. Proposed.

On the other hand, the reaction of uranium dioxide dissolving in nitric acid generates a large amount of nitrogen oxide gas, so if the dissolution rate is too high, bubbles will erupt from the container, causing the highly radioactive solution to spread unnecessarily. There is a fear. This becomes a particularly serious problem when fine powder generated by shearing nuclear fuel dissolves.

As a nuclear fuel melting device, the necessary amount of nitric acid is added to the container loaded with nuclear fuel to complete the melting.
There is a so-called batch method in which a solution is taken out, a so-called continuous method in which a container is loaded with nuclear fuel and nitric acid while the solution is taken out, and a semi-continuous method that is a compromise between the two.

Continuous methods are generally highly efficient, but there is a problem in removing insoluble materials from the coating material. A device for processing this has been proposed. However, this device is disadvantageous in terms of earthquake resistance due to its vertical structure, and there is also concern that the hull may clog the channel due to its complex structure.

[Purpose of the invention]

The purpose of the present invention is to maintain a large processing capacity while maintaining criticality safety when dissolving nuclear fuel with a high concentration of fissile material in a dissolution treatment solution, to complete the progress of the dissolution process, and to load and coat sheared fragments. To provide a continuous melting and processing device for nuclear fuel that does not cause clogging in a path when taking out materials, maintains mechanical durability in a highly corrosive atmosphere, and is easy to maintain even in the event of a failure.

[Summary of the invention]

The melting device of the present invention suspends a nuclear fuel loading basket from an annular container and an annular drive frame supported above the liquid surface of the container, and moves horizontally in one direction in the annular container while dipping the lower part of the basket in a nitric acid solution or the like. It is characterized by having a structure that allows it to be moved.

According to the features of the present invention, since an annular container is used, the internal volume can be increased despite the small thickness of the container, and by adopting a nuclear fuel loading basket with a fan-shaped cross section, nuclear fuel can be Loading capacity can also be increased.

Another feature of the invention is that the annular vessel is provided with a partition, which allows the nuclear fuel to come into contact with countercurrent nitric acid as it travels.

According to yet another feature of the present invention, the opening area of the loading cage is large in relation to the volume of the nuclear fuel to be melted, and a communicating space is also present in the upper part. It can avoid the effects of foaming.

[Embodiments of the invention]

Examples of the present invention will be described below.

FIG. 1 is a perspective view showing the basic configuration of a melting device according to an embodiment.

The device basically consists of an annular container consisting of an outer wall 1, an inner wall 2, and a bottom plate 3, and a plurality of fuel loading baskets 4 having the same shape. The bottom plate 3 has a continuous and smooth slope, and small insoluble pieces that fall through the holes of the fuel loading basket 4 gradually move to the deepest part. The lower half of the fuel loading basket 4 is immersed below the surface of the dissolving treatment liquid 5.

The basic configuration of the device is shown in Figure 1. When nuclear fuel is loaded into a loading basket at a fixed location, it is sequentially moved in one direction, and in another fixed location, the insoluble residue is discharged, thereby allowing continuous operation. It is something that can be driven.

FIG. 2 is a diagram showing the lower structure of the embodiment. What has been added to FIG. 2 is an annular frame 6. The annular frame 6 is supported on the upper part of the main body portion of the annular container and rotates. The annular frame 6 is integrally constituted by an inner ring 7, an outer ring 8, and a ring 9, forming a fan-shaped frame 10 from which the fuel loading basket 4 is suspended.

FIG. 3 is a schematic diagram illustrating a further lower structure of the embodiment. FIG. 3 shows details of the fuel loading basket 4.

The fuel loading basket 4 has a fan-shaped cross section and a fan-shaped bottom plate 11;
It consists of a side plate 12 and an edge frame 13. The liquid contact portions of the bottom plate 11 and the side plates 12 have a large number of holes (not shown). The shape of the fuel loading basket 4 is set so as to maintain equal distances from the surfaces of the outer wall 1 and inner wall 2 of the annular container. The edge frame 13 cooperates with the sector frame 10 of the annular frame 6 and operates to suspend the fuel loading basket 4.

FIG. 4 is a schematic diagram illustrating the configuration of the apparatus in the embodiment. In FIG. 4, the annular container is partitioned by partition walls 14. In FIG. In this case, the nuclear fuel loading cage 4 sequentially moves counterclockwise. When the nuclear fuel loading basket 4 is positioned directly below the fuel loading port 15, nuclear fuel sheared pieces from a fuel body shearer (not shown) pass therethrough, and the nuclear fuel is loaded into the nuclear fuel loading basket 4. Exhaust port 16
is connected to exhaust equipment (not shown) in order to exhaust gases generated within the melting device and maintain a negative pressure inside. The transport section 17 is installed at the upper part of the annular container so as to straddle the partition wall 14. First, the nuclear fuel loading basket 4 that has reached the fixed position is lifted up to the lifting position 18, and the door 19 is opened and the delivery device is opened through the opening 20. (not shown) and returned to the original lifting position 18 after the undissolved contents have been evacuated. The nuclear fuel loading basket 4 is transported to the hanging position 21 by the transport section 17 and immersed in the dissolution treatment liquid 5.

FIG. 5 is a schematic diagram showing the structure of one cross section of the device in the embodiment. The annular frame 6 is a lower roller 21
It is supported horizontally and without eccentricity by the side rollers 22 and the annular guide 23. A rack tooth 24 is also attached to the annular frame 6, and the pinion gear 2
5 and a drive device connected thereto (not shown)
It is rotated by The top of the annular container forms an annular flange 26 wider than the wetted part, and a lid 27.
is tightened with a remotely controllable screw 29 via a gasket 28 to maintain airtightness. Here, a case is shown in which the exhaust port 16 is integrated with the lid 27.

As is clear from FIG. 5, once the lid 27 is removed, the annular frame 6 can be easily removed by pulling it straight upwards, and the lower roller 21,
Inspection and maintenance of the side rollers 22 and the rack teeth 24 can be easily performed.

The jacket 30 is used for heating by introducing steam or for cooling by introducing cold water. The jacket 30 is surrounded by an extension of the outer wall 1 and inner wall 2 of the annular container, the bottom plate 3 and the jacket bottom 31. This configuration is designed to ensure that even if the solution should enter the jacket 30, there will be no problem in terms of criticality safety.

The thermal neutron absorption plate 32 is firmly attached to the gap between the water tank 33 and the inner wall 2, and absorbs the neutrons generated by the nuclear fuel after they decelerate in the water tank 33 and become thermal neutrons. Prevent the multiplication factor from increasing.

FIG. 6 is a sectional view showing a part of the device in the embodiment. One or more air lifters 34 pump the solution from the deepest part of the annular container and return it to the nuclear fuel loading basket 4 through a discharge port 35 and a notch 36 provided in the outer ring 8 of the annular frame 6. The solution is stirred at the same time as the insoluble pieces remaining at the bottom of the annular container are returned to the nuclear fuel loading basket 4. An air inlet 37 is also installed at the deepest part of the annular container to stir and homogenize the solution. The solution outlet 38 drains the overflow from the solution surface, and the solution drain pipe 39 is used in conjunction with a steam ejector (not shown) to drain all the liquid in the annular container. The fuel loading port 15 is connected to the lid 3
1 and is tightened to the annular flange 26 via a gasket 28 with a screw 29 that can be remotely operated.

FIG. 7 is a diagram showing another partial cross section of the device in the embodiment. In this cross section, the nuclear fuel loading cage 4 is located at the lifting position 18. The inner door 40 is always closed to maintain negative pressure in the annular container, but is opened before the lifting device 41 is lowered. The lifting device 41 is attached to the transport section 17 and lifts the nuclear fuel loading basket 4 from inside the fan-shaped frame 10 of the annular frame 6. The spray 42 injects water or steam to wash off the dissolution treatment liquid adhering to the nuclear fuel loading cage 4. The radiation detector 43 selectively measures radiation generated by nuclear fuel material remaining dissolved in the nuclear fuel loading basket 4. If it is determined that there is no melting remaining, door 1
9 is opened, and the nuclear fuel loading basket 4 is taken out to the outside by the delivery device 44 through the opening 20. Delivery device 44
has a rotation mechanism, and after transferring the contents to a container 46 via a receiving tray 45, delivers the nuclear fuel loading basket 4 to the lifting device 41. After closing the door 19, the transport section 17
transports the nuclear fuel loading basket 4 to the hanging position 20. Open the inner door 40 directly below the hanging position 20 and suspend the nuclear fuel loading basket 4 into the fan-shaped frame 10.
After the lifting device 41 is retracted, the inner door 40 is closed.
The lower part of the empty nuclear fuel loading basket 4 is immersed in the dissolution treatment liquid 5, and then loading is performed when the empty nuclear fuel loading basket 4 is moved directly below the fuel loading port 15.

A plurality of dissolution treatment liquid supply ports 47 are installed, and at least one of them directly supplies the dissolution treatment liquid to the nuclear fuel loading basket 4 through the notch 36. In the next step, a new dissolution treatment liquid having sufficient dissolution ability is poured into the fuel loading basket 4 to be lifted, and the soluble nuclear fuel material adhering to the insoluble covering material is effectively removed.

FIG. 8 is a side development view for explaining the operating conditions of the embodiment device. The maximum number of nuclear fuel loading baskets 4 that is two fewer than the number of fan-shaped frames 10 arranged in the annular frame 6 is arranged in the device.

The movement of the nuclear fuel loading basket 4 does not necessarily have to be continuous at all times; rather, it is preferable that at least the loading of nuclear fuel and the lifting and lowering of the nuclear fuel loading basket 4 be carried out with the annular frame 6 stopped at a fixed position. Although the method of loading nuclear fuel into the nuclear fuel loading basket 4 is outside the scope of the present invention, the loading speed is limited in order to prevent a large amount of nitrogen oxide from foaming due to the dissolution reaction when operating this device.

Next, an embodiment will be described in which the present invention is applied to a fast breeder reactor with a high concentration of fissile material and melting of core fuel.

Fast breeder reactor, core fuel has an outer diameter of 6.5 mm and a wall thickness of 0.47 mm.
The fuel rod consists of a stainless steel cladding tube with a diameter of 1.5 mm, filled with 220 g of uranium-plutonium mixed oxide pellets containing 20% fissile plutonium, and 150 g of depleted uranium oxide pellets, with the end plugs sealed. The weight of the fuel rod is 520g.

After this fuel has been used up to its rating in the reactor,
The amount of fissile plutonium in the fuel rods has been reduced by about half, but when explaining the melting equipment for reprocessing, it is necessary to prepare for cases where combustion has not progressed, and the content of fissile plutonium is high. It must also be accommodated that only parts are loaded into the melting device.

Criticality safety is ensured when melting the above-mentioned high fissile material concentration fuel in an annular melting vessel made of nitric acid-resistant metal material with a thin cadmium plate attached to the inner wall and a water tank placed in the center. The container thickness limit required for this purpose is 75 mm based on the internal method.
The thickness limit of the cage for loading the sheared pieces of fuel rods was 50 mm based on the internal method.

Prior to melting, the fuel rods are sheared to a length of about 30 mm and loaded into a nuclear fuel loading basket at 4.8 kg per volume. Of this, 3.4 kg is nuclear fuel oxide.

The dissolution processing solution capacity of the annular container with an inner diameter of 2 m, a container wall thickness of 10 mm, an inner thickness of 75 mm, a deepest liquid depth of 500 mm, and a shallowest liquid depth of 300 mm is 197.

On the other hand, the nuclear fuel loading basket will have arc lengths of 532 mm and 505 mm, a thickness of 50 mm, and a depth of 450 mm, and 10 will be loaded into the melting device.

The immersion volume of one nuclear fuel loading basket is determined by the immersion depth.
In the case of 300mm, it is 7.77, and if the filling height of nuclear fuel is 200mm, it becomes 5.18, the fuel rod loading amount is 24.86Kg, and the amount of nuclear fuel oxide is 17.6Kg. This amount is equivalent to approximately 48 fuel rods.

The loading cycle for one nuclear fuel loading basket is 1 hour, and the melting time from the start of loading to unloading is 9 hours. The maximum weight of nuclear fuel rods that this device can melt in one day is 596 kg, and the amount of nuclear fuel oxide contained in this rod is 422 kg.

The amount of nitric acid consumed to dissolve the oxide fuel was 630 kg per day, and 1250 nitric acid of 8 regulations was supplied per day. The average concentration of the solution was 384gU+Pu/
The nitric acid concentration was according to 3 regulations.

During dissolution, the liquid temperature in the dissolution reaction section was maintained at 90° C. by appropriately supplying heating steam to jackets installed separately at the bottom of the annular container.

Convection of the lysis solution, air blowing agitation, and circulation by an air lifter kept the concentration of the lysis solution virtually uniform.

On average, 7.3 kg of stainless steel cladding material remained in the nuclear fuel loading basket held in the melter for 9 hours, and was removed from the reactor once every hour and transferred to a storage container.

Irrespective of the description of the embodiments, the effects of the present invention can also be exhibited in other modifications.

For example, the dimensions of the annular vessel do not necessarily limit the effectiveness of the present invention, but are dictated by the needs of the reprocessing process and criticality safety constraints.

However, it is not preferable to make the depth of the container extremely deep in order to maintain a constant concentration of the solution that is the product of the dissolution device.

The various configurations, parts, and structures used in the embodiments of the apparatus according to the present invention are not limited in type as long as they can achieve the purpose.

For example, the method for driving the annular frame may be selected from a constant speed rotation mechanism using various general gears, an inconstant speed rotation mechanism using a ratchet, etc., regardless of the description of the embodiments. The required condition for this mechanism is not rotational movement itself, but accuracy and certainty of stopping at a predetermined position, which is one of the sub-concepts of the present invention, which is to achieve the attachment and detachment of the nuclear fuel loading basket to the annular frame. It is necessary for For this purpose, a system in which the number of reciprocating movements corresponds to the rotation angle is preferable to a system in which rotational motion is transmitted to gears.

The present invention has a favorable effect under the special conditions peculiar to nuclear fuel materials such as ensuring criticality safety, and for this purpose, the shape of the device is accompanied by an increase in the thickness of the container and the thickness of the cage. Deformation must be strictly prevented. In this respect, the annular structure has more structural and mechanical stability than a simple plate structure, but the addition of further reinforcement does not impair the effectiveness of the present invention.

In applying the present invention, the target nuclear fuel material in the examples was a mixed oxide of plutonium and uranium, but the present invention dissolves the material with nuclear fuel material containing fissile material to the extent that it is necessary to consider criticality safety. Effective in combination with dissolving treatment liquid. The material constituting the device only needs to be corrosion resistant to the heated dissolution treatment solution and radiation resistant. The sliding parts of moving parts are easily damaged by corrosion, so they can be made of a different material from other parts.

〔Effect of the invention〕

According to the present invention, melting is performed continuously while moving the nuclear fuel loading basket in one direction in the annular container, so that the following effects can be achieved.

(1) Even if the thickness of the container is limited to a certain value or less, the volume of the container is large and the installation area is small.

(2) Annular containers have more structural and mechanical stability than flat containers.

(3) The movement mechanism rotates an annular frame while supporting it horizontally at a distance from the solution surface, so position control is reliable and there are fewer corrosion problems in the drive part.

(4) Nuclear fuel loaded into the equipment remains in the same cage throughout the process, so clogging in the route is minimized.

(5) The state of the nuclear fuel loading basket, including residual materials, can be monitored for each process cycle, making it easy to manage the process and to take measures against the occurrence of unsteady conditions such as poor melting.

(6) The basic structure of the device is such that all parts can be removed by pointing upwards, making it easy to disassemble and maintain by remote control.

[Brief explanation of the drawing]

1 to 4 are perspective views showing the basic configuration of a continuous melting device for spent nuclear fuel according to an embodiment of the present invention, FIG. 5 is a cross-sectional view of a portion of the melting device of the embodiment, and FIG. 7 are views showing other partial cross sections of the melting device of the example, and FIG. 8 is a developed view from the side showing the operating status of the melting device of the example. 1... Outer wall, 2... Inner wall, 3... Bottom plate, 4...
Nuclear fuel loading basket, 5...Dissolution processing liquid, 6...Annular frame, 7...Inner ring, 8...Outer ring, 9...Mr., 1
0... Fan-shaped frame, 11... Fan-shaped bottom plate, 12... Side plate, 13... Edge frame, 14... Partition wall, 15... Fuel loading port, 16... Exhaust port, 17... Conveying section, 1
8... Lifting position, 19... Door, 20... Opening,
21... Hanging position, 22... Roller, 23... Annular guide, 24... Rack tooth, 25... Pinion gear, 26... Annular flange, 27... Lid, 28
... Gasket, 29 ... Screw, 30 ... Jacket, 31 ... Jacket bottom, 32 ... Thermal neutron absorption plate, 33 ... Water tank, 34 ... Air pump, 35 ... Discharge port, 36 ... ... Notch, 37 ... Air inlet, 38 ... Solution discharge port, 39 ... Solution discharge pipe, 40 ... Inner door, 41 ... Lifting device, 42 ...
...Spray, 43...Radiation detector, 44...Delivery device, 45...Saucer, 46...Container, 47...
...Dissolution processing liquid supply port.

Claims (1)

[Claims] 1. A plurality of corrosion-resistant nuclear fuel loading baskets are arranged in a corrosion-resistant annular container that holds a dissolution treatment solution, and the lower part of the nuclear fuel basket is immersed in the dissolution treatment solution, and the nuclear fuel baskets are sequentially loaded in one direction. A continuous melting device for spent nuclear fuel, characterized in that it is moved. 2 A nuclear fuel loading basket is suspended from a ring-shaped frame,
the annular frame is supported at the top of the annular container;
2. The continuous melting device for spent nuclear fuel according to claim 1, wherein the spent nuclear fuel continuous melting device is configured to be able to rotate horizontally together with the cage. 3. The spent nuclear fuel continuous melting device according to claim 1 or 2, wherein the nuclear fuel loading basket has a fan-shaped horizontal cross section when installed in the annular container. 4. The annular container is partitioned by a partition at at least one location, and the container liquid depth at one partitioned end is deeper than the other end, and the bottom is smooth and continuous between the partitions. A continuous melting device for spent nuclear fuel according to claim 3. 5. When the nuclear fuel loading basket sequentially moves in one direction within the annular container, it is taken out of the container in front of the partition wall, moved horizontally, and relocated to the rear side of the partition wall. An apparatus for continuously melting spent nuclear fuel according to claim 3. 6. The device according to claim 1, characterized in that the liquid is continuously pumped up from the deepest part of the annular container and transferred into one or more of the nuclear fuel loading cages. 7. In the annular container, the nuclear fuel loading basket is moved from an end with a deep liquid depth to an end with a shallow liquid depth,
5. The continuous melting apparatus for spent nuclear fuel according to claim 4, wherein the dissolving process is carried out in opposition to the liquid flow generated by supplying the dissolving treatment liquid and discharging the dissolving solution. 8. The integrated annular frame for suspending the nuclear fuel loading basket has a structure that can be lifted upward and easily removed by removing the upper structure of the annular container. A continuous melting device for spent nuclear fuel according to scope 2. 9 In the annular frame for suspending the nuclear fuel loading basket and moving it sequentially in one direction, a partial notch is provided for introducing liquid continuously pumped from the deepest part of the annular container into the nuclear fuel loading basket. 7. A continuous melting device for spent nuclear fuel according to claim 6, characterized in that the device has a continuous melting device for spent nuclear fuel.