JPH0646233B2 - Ultrasonic processing method for nuclear reactor fuel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an ultrasonic processing method capable of efficiently performing a drilling or cutting process of a reactor fuel pellet in a dry manner.

The present invention is applicable to various types of sensors such as hollow machining of nuclear reactor fuel, thermocouples for fuel pins (comprising a small diameter cladding tube and a solid fuel contained therein), a SiC temperature monitor, and a wire monitor for measuring neutron dose. For deep hole drilling for mounting fuel, especially for burnup of the spent nuclear reactor fuel due to its high burnup, to collect, completely remove and recover the fuel of high hardness from the long cladding. It is a useful method for cutting, etc.

[Prior Art] A simple wet lathe drilling machine or a wet ultrasonic drilling machine using abrasives for drilling, cutting, removing, etc. of a solid reactor fuel before or after use. Is used.

[Problems to be Solved by the Invention] However, in the conventional wet processing method, it is necessary to perform processing for separating cooling oil and polishing powder as an abrasive from the processing fuel. However, there are problems such as the corrosion products of the cladding tube caused by the reaction product, the increase in the amount of radioactive waste including the processed fuel powder, and the low fuel recovery rate.

Since such a problem is caused by the wet processing method, it is possible to use the conventional apparatus for the dry processing as it is, but the following problems occur and it has not been put to practical use.

In the conventional technique, the amount of heat generated in the fuel processing portion at the tool tip is not controlled, so that the processing portion easily becomes hot, and softening deformation of the drill cutting edge and inability to cut occur.

In the case of the conventional ultrasonic vibration drilling apparatus, in addition to the above-mentioned problems, there is a problem that the wear loss of the drill cutting edge becomes more remarkable unless an abrasive is used.

In these processing devices, when the drill cutting edge is worn or deformed, the drill is likely to be broken due to uneven contact of the cutting edge at the processing portion.

Therefore, if the conventional technology is applied to dry processing as it is,
In any case, it is not practical because the drill cutting edge is frequently replaced.

By the way, in a spent reactor fuel pin, for the purpose of removing the solid fuel from the cladding tube, there is a technique of removing the fuel by applying a unidirectional pushing force only to the fuel. In the case of low burnup fuel, the fuel can be removed by such a simple extruding fuel removal method, but in the case of high burnup fuel, the fuel and the cladding pipe become stuck and the sticking force is strong. Therefore, it is difficult to take out fuel without damaging the cladding tube by a simple extrusion process at room temperature.

An object of the present invention is to solve the above technical problems. That is, deep hole drilling of pellets of arbitrary diameter and length of reactor fuel containing fuel pellets having a Vickers hardness of 800 or higher in a high hardness state, collection, removal, and recovery of fuel from a cladding tube with fuel The ultrasonic processing method of the reactor fuel that enables the cutting processing for the above to be performed dry and with the fuel recovery rate of 99% or more without using the abrasive while maintaining the soundness of the material to be processed. Is to provide.

[Means for Solving the Problems] The present invention capable of solving the above-mentioned technical problems includes a high-purity inert gas or nitrogen gas atmosphere having a water content and an oxygen concentration of 200 ppm by weight or less, or water content and oxygen. In a vacuum with a partial pressure of 20 Pa or less, a drill with a carbide tip at the tip of the ultrasonic longitudinal vibration horn tool,
Alternatively, by using an ultrasonic machining device equipped with a drill whose tip is cemented, a dry process of fuel is performed in a state where the wear of the drill tip is low, there are no drill defects, and the load at the tip of the drill edge does not become excessive. This is an ultrasonic processing method.

Here, the ultrasonic processing apparatus is installed in a cell (a section surrounded by a shielding wall that houses the test equipment body for handling a highly radioactive substance) or the like. The drill has a wear degree of 6 at the tip.
It is performed in a state of 0% or less, more preferably 40% or less.

The drill defect can be detected by detecting an abnormal decrease in ultrasonic output. Also, during processing, an excessive load prevention spring or the like is used to adjust and hold the drill cutting edge so that it has the optimum pressing load, so that an excessive load is not applied.

[Operation] In the present invention, the ultrasonic processing device is installed in a cell or the like, and the moisture and oxygen concentrations are respectively in a high-purity inert gas or nitrogen gas atmosphere having a concentration of 200 ppm by weight or less, or less than or equal to that. Since dry processing is performed in a vacuum that does not include, it is possible to prevent deterioration of the workpiece during processing.
Since the dry processing method is used in the present invention, various problems associated with the wet method can be solved without inclusion of impurities.

The wear rate (%) of the drill tip can be detected by a cutting speed meter. If the drill tip is greatly worn and the processing fuel approaches an abnormal heat generation state, the cutting speed will drop extremely,
The start of abnormal heat generation is predicted by monitoring the cutting speed, and the drill replacement time is detected. Further, when a drill defect occurs, the ultrasonic output sharply decreases. Therefore, the abnormal operation of the output is detected by the ultrasonic output indicator to stop the machining operation when the defect occurs.

Further, if an excessive load is applied to the drill blade, damage and wear of the blade will progress, abnormal heat generation of the sample, sample damage and the like will occur.
In the present invention, these problems are solved by maintaining the pressing load of the drill blade in an appropriate state and preventing overload.

[Embodiment] FIG. 1 is a block diagram showing an example of an apparatus used in the method of the present invention. As shown in the figure, the present invention includes an ultrasonic vibration drilling machine body 11, a vibrator cooling device 12, and a processed fuel dust collector 13, which are installed in a cell 14. The ultrasonic vibration drilling machine main body 11, the vibrator cooling device 12, and the processed fuel dust collecting device 13 are mechanically connected by a coolant pipe 16 and a dust collecting pipe 17, respectively. Further, each device operates by being supplied with power from outside the cell. Control device 18 that controls the operation of each device and performs measurement
Is installed outside the cell 14.

The ultrasonic vibration drilling apparatus main body 11 is shown in FIG. This is equipped with an ultrasonic transducer processing section 21 provided on one side of the bed section 20 as a base, and a fuel pin holding / driving section 22 provided on the opposite side thereof, and remote control is possible.

The ultrasonic vibrator processing unit 21 connects a longitudinal vibration horn tool 24, a drill 25 with a cemented carbide tip that is detachably attached to the tip of the horn tool by a manipulator for remote operation, and a coolant pipe to the vibrator cooling device. It has a connection part for. The fuel pin holding portion is composed of a collet chuck 28 for fixing the processing fuel 26, a collet chuck tightening lever, a collet chuck replacement handle, etc., and a sample driving section is a sample rotating motor 30, a step motor for sending a processing fuel pin, and various types. It is equipped with a limit switch and a striker.

Next, an example of the ultrasonic transducer is shown in FIGS. This ultrasonic transducer has a structure in which a drill 25 is attached to the tip of a longitudinal vibration horn tool 24, and by exchanging the drill 25 at the tip according to the application, a sample of arbitrary diameter and hardness can be processed. It is like this. With such a tool, it is possible to process up to a reactor fuel having a Vickers hardness H _V = 1000. However, the higher the hardness of the processed sample, the greater the wear of the tip of the drill and the more time it takes to process. When precision processing is required, it is divided into two steps, rough processing and finishing, and when sampling a sample, a hollow circular drill is used. Cost reduction can be achieved by dividing the drill and the horn tool, but in order to further improve remote operability and more effectively transmit vibration energy, the drill and horn tool can be integrated. Good.

As shown in FIG. 4, a processed fuel receiving tank 36 for collecting the fuel powder, debris, and the like that fall during processing and an acrylic resin cover for covering the processed fuel receiving tank 36 for preventing scattering of the processed fuel are provided at approximately the center of the bed portion 20. A dust collection box 37 is provided, and a dust collection pipe 17 connected to the processed fuel dust collection device is attached to an upper portion of the dust collection box 37. Dust collection box 37
An inlet / outlet port 38 of the processed fuel pin portion and an inlet / outlet port 39 of the drill portion are opened on both side walls of the.

In the ultrasonic vibration drilling machine body 11 described above, in order to facilitate the processing of a long sample, once the processing from one side reaches a fuel pin central portion for a fuel pin of 80 to 100 mm, it is processed once. The fuel pin is automatically stopped and the fuel pin at the collet chuck is turned over to allow reworking.

The processed fuel dust collector 13 is slightly larger (particle size 0.3 μm).
A pre-filter for trapping fine particles (about m or more) and a high-performance air filter for collecting fine particles of less than 2 are arranged in series in combination, and the dust collection status of the intake blower and filter is also monitored. It is equipped with a differential pressure gauge that indicates when to replace the filter for clogging. As the pre-filter, for example, a paper filter of about 0.3 μm mesh is used.

The control device 18 for measuring and controlling each of the above devices from the outside of the cell includes a vibrator cooling device drive system, a sample rotary feed control system, an ultrasonic oscillation control system, and the like.

Since this device is used under a high radiation dose rate, a collet chuck operating part, a horn tool, which is frequently used or replaced among the ultrasonic vibration drilling device main body 11, the oscillator cooling device 12, and the processed fuel dust collecting device 13. The parts and the like are all simple lever type or rotary type using a torque wrench, and the dust collecting filters are also divided and assembled so that they can be disassembled and replaced by a manipulator.

In particular, the material to be processed in the present invention is a reactor fuel, and it is required that the fission products in the fuel are less affected by the reaction with the atmosphere during processing. FIG. 5 shows an austenitic stainless steel cladding tube containing a spent nuclear reactor fuel, which was dry-processed in a nitrogen atmosphere having oxygen and water concentrations as impurities as parameters, and then in a nitrogen atmosphere containing the same impurities. In addition, the tensile elongation at break at high temperature is measured. As is clear from the figure, the elongation at break is lowered in the atmosphere containing 1000 ppm by weight or more of water and / or oxygen. It is considered that this is because the fission products that exude from the inside of the fuel to the surface and adhere to the inner surface of the cladding tube react with moisture to deteriorate the cladding tube at high temperature. Therefore, in order to avoid the influence of the processing atmosphere on the reactor fuel, for example, a high-purity inert gas or a low-reactivity high-purity nitrogen gas is used as the processing atmosphere. The moisture and oxygen concentrations of impurities in the atmosphere gas are
Each of them is set to 200 ppm by weight or less so that their effects are reduced.

Of course, the processing atmosphere may be, in addition to the above-described conditions, a vacuum containing water and oxygen in the same amount or less.

In the present invention, the following measures are further taken to perform dry processing. They are to monitor the wear condition of the drill, detect the loss of the drill, hold and adjust the optimum pressing load of the drill blade edge during machining, and prevent overload.

When dry cutting a high hardness reactor fuel, the amount of heat generated may increase during the processing, and the cutting becomes uncut due to the softening of the drill blade due to the temperature rise. Moreover, the sample is adversely affected as the temperature rises. As a result of various studies to prevent these problems, it became clear that the increase in the amount of heat generated at the drill tip and drill wear are closely related. This makes dry processing possible.

First, it was found that when the drill tip wears 40 to 50% or more, the cutting speed sharply decreases, and when the drill tip is 60% or more, heat generation in the sample and the drill tip increases remarkably. This relationship is the sixth
Shown in the figure. This figure shows the relationship between the cutting speed (mm / min) and the wear rate (%) of the drill tip using an alumina sintered body having a hardness H _V = 830 as a sample. Drill tip wear is 6
The area over 0% is the heat generation area of the sample. Therefore, in the present invention, in order to solve the problem, the degree of wear of the drill is detected by a cutting speed meter and a cutting speed recorder, and it is utilized that the cutting speed is extremely reduced when the tip of the drill is greatly worn and the fuel is in a heat generation state. Then, it predicts the start of abnormal heat generation and detects the drill replacement time.

Next, as shown in FIG. 7, it was found that the ultrasonic output was reduced to about 1/2 when a drill defect occurred during dry machining. Therefore, by utilizing this fact, the processing operation is stopped when an abnormal decrease in output is detected by the ultrasonic output indicator to prevent damage to the fuel cladding tube and fuel of the sample.

When an excessive load is applied to the drill blade to which ultrasonic vibrations are transmitted, the blade edge is damaged or worn, abnormal fuel heat is generated, and fuel is damaged. In order to prevent these, it is necessary to always keep the pressing load of the drill blade edge at the optimum value during machining.
FIG. 8 shows the relationship between the cutting load of the drill blade tip (kg) and the cutting speed obtained by using an alumina sintered body having H _V = 800 as a simulated fuel. As is clear from the figure,
It shows the best cutting efficiency in the range of 0.8 to 1.2 kg of the pressure applied to the drill edge, the cutting speed is low at 0.7 kg or less, and conversely the wear rate of the drill increases and the cutting speed also rapidly decreases at 1.4 kg or more. . In the present invention, based on such a relationship, the pressing load of the drill cutting edge against the processing fuel is optimized. An example thereof is shown in FIG.

This mechanism is provided between an upper slide board 43 on which the fuel pin holding / driving section 22 is mounted and a lower slide board 42 on which the fuel pin holding / driving section 22 is mounted and which is driven at a constant speed. That is, an overload prevention spring 44 is provided between the slide plates 42 and 43.
Further, the span adjusting screw 45 and the limit switch 46 for stopping the machining operation when the overload prevention spring 44 is applied with a reaction force of the machining portion which is equal to or more than a prescribed value are provided.

Lower cutting board 4 by step motor during cutting
2 is driven at a constant speed in the direction of the outlined arrow a. A driving force is applied to the upper slide plate 43 via the overload prevention spring 44, and a processing portion reaction force is applied in the opposite direction (white arrow b). The span of the overload prevention spring 44 is set to the optimum cutting pressing load by the adjusting screw 45 and the cutting process is performed. In this way, the overload prevention spring 44 also functions as an optimum pressing load element for the drill tip. When the pressing load of the drill blade exceeds the specified value, the overload prevention spring 44 contracts according to the reaction force,
The limit switch 46 operates. As a result, a step motor for feeding the processed fuel pin and a motor 3 for rotating the sample are provided.
0 stops, and the machining operation stops.

Generally, in order to increase the processing speed by ultrasonic waves, it is sufficient to increase the amplitude of ultrasonic vibration of the tip processing tool,
For this purpose, it is preferable to increase the electric input to the vibrator or lower the resonance frequency of the vibrator as much as possible. However, if the oscillator frequency becomes low, it may become audible, so
Practically, frequencies in the range of 16 to 30 kHz are selected. Considering that this device is installed in an airtight cell,
A resonance frequency of around 16 kHz is selected, and an ultrasonic vibration control system that maximizes electric input under this condition is adopted.
In practice, the control device 18 is provided with an ultrasonic vibration adjustment changeover switch so that the maximum output can be adjusted and maintained.

In the cutting process of nuclear reactor fuel pellets, it is strongly required to collect fuel substances such as fuel debris and fuel powder produced by the process at a high recovery rate without mixing substances other than fuel. For example, in the fuel removal processing of mixed oxide fuels of uranium and plutonium, recovery of 0.1 g order is desired. In addition, reducing the level of pollution due to the scattering of fuel powder during nuclear fuel processing and reducing radioactive waste during decontamination are also important issues. In the present invention, the processing fuel pin is moved and processed in the processing fuel receiving tank provided in the central portion of the ultrasonic vibration drill processing device main body,
Fuel powder and debris that fall during processing are first collected, and the finely divided fuel powder is designed to have a high recovery rate by a processed fuel dust collector. The fine fuel powder sucked from the dust collecting box 37 of the main body is trapped by the prefilter. The fine powder captured by the pre-filter can be collected by taking out it together with the pre-filter. The high-performance air filter traps the fuel powder that has passed through the pre-filter and prevents the pollution of the fuel fine powder in the cell, thus reducing the radioactive waste when removing the pollution.

Next, Table 1 shows an example of recovery when alumina sintered pellets as a simulated fuel were processed by the method of the present invention. As shown in the table, the recovery rate is very good, and the average value is 99% or more.

The simulated fuel pellet used in the test was a homogeneous cemented carbide alumina sintered body that was shrink-fitted into a stainless steel cladding tube, and was a dense one without any fine cracks. However, a typical object to be actually processed is one in which oxide combustion pellets are contained in a cladding tube, and in such a spent reactor fuel pellet, many fine holes and cracks are generated.
For this reason, it can be judged that it is extremely easy to be fragmented and pulverized, and can be processed more easily than in the case of the simulated fuel pellet in a densely packed state. Therefore, according to the method of the present invention, it is possible to perform the deep hole drilling and the cutting of the ultra-hard reactor fuel having the Pickers hardness of 800 or more by the dry manipulator without using the abrasive, and the fuel processing powder. Can be recovered with a high efficiency of 99.9% or more.

In the above embodiment, the ultrasonic vibration drilling machine body is a horizontal type, but the ultrasonic vibrator machining section is installed vertically and the fuel pin holding / driving section is vertically driven. Good.

The present invention can be used for the following applications.

Efficiently recovering only the fuel from the cladding tube with spent reactor fuel while keeping the cladding tube sound, and collecting the cladding tube sample to investigate the irradiation characteristics of the material. Used for drilling holes for embedding and wire monitoring for neutron dose measurement Reactor fuel machining for providing embedding holes, drilling for hollow fuel pellets, and deep fuel drilling for other purposes To do.

EFFECTS OF THE INVENTION As described above, the present invention has a water content and an oxygen content of 2
Since dry processing is performed in an atmosphere of high-purity inert gas or nitrogen gas of less than 00 ppm by weight, or in a vacuum containing water and oxygen at the same level or less, deterioration of the work piece due to the processing atmosphere can be prevented. it can. Deep drilling and cutting of ultra-hard reactor fuel with a Vickers hardness of 800 or more is performed because the fuel is processed in a state in which the wear of the drill tip is low, there are no drill defects, and the load at the tip of the drill edge does not become excessive. And the like can be carried out in a dry manner without using an abrasive, and the fuel processing powder can be recovered at a high recovery rate of 99% or more.

By utilizing the present invention as described above, it is possible to promote the development of a highly reliable high-performance nuclear reactor fuel, which is a key technology among nuclear reactor technologies, and has a long life.

Further, the fuel can be processed with an extremely high recovery rate, and since it is a dry type, the reactor fuel can be recovered in a state where impurities are not mixed, so that the reactor fuel can be effectively reused. In addition, it is possible to prevent radioactive contamination due to scattering of reactor fuel powder in the work environment, reduce the amount of radioactive waste including reactor fuel substances, and keep radioactive substances released to the environment around the post-irradiation test facility extremely low. It has a number of excellent effects.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of an ultrasonic machining apparatus to which the method of the present invention is applied, FIG. 2 is a perspective view of an ultrasonic vibration drill machining apparatus body, and FIGS. 3A, 3B and 3C are ultrasonic transducers, respectively. FIG. 4 is an explanatory view of the dust collection box, and FIG. Further, FIG. 5 is an explanatory diagram showing material characteristics with respect to the impurity concentration in the atmosphere during machining, FIG. 6 is an explanatory diagram showing the relationship between the drill tip wear rate and cutting speed, and FIG. 7 is the output when a drill defect occurs. FIG. 8 is an explanatory diagram showing changes, FIG. 8 is an explanatory diagram showing a relationship of cutting speed with respect to a drill blade tip pressing load, and FIG. 9 is a mechanism for holding / adjusting an optimum pressing load of the drill blade tip during processing and preventing overload. FIG. 11 ... Ultrasonic vibration drill processing device main body, 12 ... Oscillator cooling device, 13 ... Processing fuel dust collector, 14 ... Cell, 21 ... Ultrasonic vibrator processing part, 22 ... Fuel pin holding / Drive part, 24 ... Horn tool, 25 ... Drill with carbide tip, 26 ... Processing fuel.

Claims (3)

[Claims] 1. A water content and an oxygen content are each 200 weight pp.
Ultrasonic machining equipment with a carbide tip drill attached to the tip of an ultrasonic vertical vibration horn tool in an atmosphere of high-purity inert gas or nitrogen gas of m or less, with low wear of the drill tip and no drill defects And a method for ultrasonically processing a fuel for a nuclear reactor, characterized in that the fuel is dry-processed in a state where the load applied to the drill blade tip does not become excessive. 2. A drill having a low wear at the drill tip by an ultrasonic machining apparatus in which a drill with a cemented carbide tip is attached to the tip of an ultrasonic longitudinal vibration horn tool in a vacuum with a water and oxygen partial pressure of 20 Pa or less, respectively. A method for ultrasonically processing a fuel for a nuclear reactor, characterized in that the fuel is dry-processed in a state where there is no loss and the load applied by pressing the drill blade is not excessive. 3. The ultrasonic machining method according to claim 1, wherein the machining is performed when the wear of the tip of the drill is 60% or less.