JPH02176599A - Ultrasonic working method for nuclear reactor fuel - Google Patents

Abstract

PURPOSE:To prevent a work from deteriorating when worked by performing dry working of fuel in an atmosphere of high-purity inert gas of <=200wt.ppm moisture and oxygen concentration or gaseous nitrogen. CONSTITUTION:An ultrasonic working device is equipped with an ultrasonic vibration drill working device main body 11, a vibrator cooling device 12, and a worked fuel dust collecting device 13. This working device is installed in a cell 14 wherein the atmosphere of high-purity inert gas of <=200wt.ppm. water and oxygen concentration or gaseous nitrogen is produced. The fuel is dry- worked by the working device which has a drill 25 with a carbide tip mounted atop of an ultrasonic longitudinal vibration horn tool 24 with a low drill tip wear extent without any drill defective so that a drill blade edge pressing load does not become excessive.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an ultrasonic machining method that allows drilling, cutting, etc. of nuclear reactor fuel pellets to be carried out dryly and efficiently.

The present invention includes, for example, ■ hollow processing of nuclear reactor fuel, ■ thermocouples, 5iC 4 degree monitors, wire monitors for neutron irradiation measurement, etc. on fuel pins (composition of small-diameter cladding tubes and built-in solid fuel). Processing of deep holes in the fuel to install various sensors; Collection and complete removal of spent nuclear fuel from long cladding tubes, which have been burned and hardened on the inside of the cladding tubes due to their extremely high burn-up; This is a useful method for processing and cutting for recovery.

[Prior art] For drilling, cutting, removing, etc. of solid reactor fuel before use or spent, a simple wet-type lathe drilling device or a wet-type ultrasonic vibration drilling device that uses an abrasive material is used. is used.

[Problems to be solved by the invention] However, in the conventional wet processing method, processing is necessary to separate cooling oil and abrasive grains from the processing fuel, and fission products and coolant moisture are separated. Corrosion of the cladding occurs due to reaction products of radioactive waste including processed fuel powder! There were problems such as an increase in the amount of IL and a low fuel recovery rate.

Since such problems are caused by the wet processing method, it is conceivable to use conventional equipment as is for dry processing, but the following problems have occurred and this has not been put to practical use.

■With conventional technology, the amount of heat generated in the fuel machining section at the tip of the tool is not controlled, so the machining section tends to reach high temperatures, causing softening and deformation of the drill cutting edge and the inability to cut.

In the case of conventional ultrasonic WX dynamic drilling equipment, in addition to the above-mentioned problems, there is also the problem that if an abrasive is not used, the wear area of the drill cutting edge becomes more significant.

- In these processing devices, if the drill cutting edge becomes worn or deformed, the cutting edge tends to hit unevenly in the processing area, causing drill chipping.

Therefore, if the conventional technology was applied directly to dry processing,
In any case, the drill tip will have to be replaced more frequently, making it impractical.

By the way, for the purpose of removing solid fuel from the cladding tube in a spent nuclear reactor fuel bin, there is a technique of applying a unidirectional pushing force only to the fuel to remove the fuel. In the case of low-burnup fuel, fuel can be removed by such a simple extrusion fuel removal method, but in the case of high-burnup fuel, the fuel and cladding stick together, and the sticking force is strong. Therefore, it is difficult to extract the fuel without damaging the cladding tube using simple extrusion processing at room temperature.

An object of the present invention is to solve the above technical problems. In other words, it is possible to drill deep holes into pellets of arbitrary diameter and length for reactor fuel containing fuel pellets with a high hardness of 800 or higher on the Pinkers hardness, and to extract, remove, and recover fuel from fuel-containing cladding tubes. We have developed an ultrasonic processing method for nuclear reactor fuel that allows cutting, etc., to be carried out dryly and without the use of abrasives, with a fuel recovery rate of 99% or more, while maintaining the integrity of the processed material. It is to provide.

[Means for Solving the Problems] The present invention, which can solve the above-mentioned technical problems, can be carried out in a high-purity inert gas or nitrogen gas atmosphere with moisture and oxygen concentrations of 200 wt ppm or less, or in an atmosphere of moisture and oxygen. In a vacuum with an oxygen partial pressure of 20 Pa or less, an ultrasonic machining device equipped with a drill with a carbide tip or a drill with a carbide-treated tip on the tip of an ultrasonic longitudinal vibration horn tool can reduce wear on the drill tip. This is an ultrasonic processing method for nuclear reactor fuel in which the fuel is dry-processed in a state where there is no drill missing and the load applied to the drill tip does not become excessive.

Here, the ultrasonic processing device is installed in a cell (a compartment surrounded by a shielding wall that houses a test equipment body for handling highly radioactive substances) or the like. For machining, the wear rate of the drill tip is 60
% or less, more preferably 40% or less.

Drill damage can be detected by detecting an abnormal drop in ultrasonic output. Also, during machining, an overload prevention spring or the like is used to adjust and hold the drill tip to the optimum pressing load to prevent excessive load from being applied.

[Function] In the present invention, an ultrasonic processing device is installed in a cell, etc., and the ultrasonic processing device is installed in a cell or the like in a high purity inert gas or nitrogen gas atmosphere with a moisture and oxygen concentration of 200 ppm by weight or less, or only a moisture and oxygen concentration of the same level or less. Since dry processing is performed in a vacuum containing no substances, deterioration of the workpiece can be prevented during processing. Since the present invention uses a dry processing method, there is no contamination of impurities and all the various problems associated with wet processing can be solved.

The wear degree (%) of the drill tip can be detected by a cutting speed meter. The tip of the drill is thick (if it wears out and the machining fuel approaches an abnormal heat generation state, the cutting speed will drop dramatically).
By monitoring cutting speed, it is possible to predict the start of abnormal heat generation and detect when it is time to replace the drill. Furthermore, since the ultrasonic output sharply decreases when a drill chipping occurs, the machining operation is stopped when a chipping occurs by detecting an abnormal drop in the output using the ultrasonic output display needle.

Furthermore, if an excessive load is applied to the drill cutting edge, damage to the cutting edge, accelerated wear, abnormal heating of the sample, damage to the sample, etc. will occur.

The present invention solves these problems by maintaining the pressing load of the drill cutting edge in an appropriate state and preventing overload.

[Example] 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 device main body 11, a vibrator cooling device 12, and a processing fuel dust collector 13, which are installed in a cell 14. The ultrasonic vibration drilling apparatus main body 11, the vibrator cooling device 12, and the processing fuel dust collector 13 are mechanically coupled through a coolant pipe 16 and a dust collecting pipe 17, respectively. Further, each device operates by being supplied with power from outside the cell. A control device 18 that controls the operation of each device and performs measurements is installed outside the cell 14.

The main body 11 of the ultrasonic vibration drilling device is shown in FIG. This includes an ultrasonic transducer processing section 21 provided on one side of a bed section 20 serving as a base, and a fuel bottle holding/driving section 22 provided on the opposite side, and can be remotely controlled.

The ultrasonic transducer processing section 21 has a longitudinally vibrating horn tool 24, a drill 25 with a carbide tip that can be attached and detached by a remote control manipulator at the tip of the horn tool, and a coolant pipe connected to the transducer cooling device. It is equipped with a connection section for The fuel pin holding section consists of a collet chuck 28 that fixes the machining fuel 26, a lever for tightening the collet chuck, a handle for replacing the collet chuck, etc., and the sample drive section includes a sample rotation motor 30, a step motor that sends the machining fuel pin, Various limit switches and strikers
etc.

Next, an example of an ultrasonic transducer is shown in FIGS. 3A to 3C. This ultrasonic transducer has a structure in which a drill 25 is attached to the tip of a longitudinally vibrating horn tool 24, and by replacing the drill 25 at the tip according to the purpose, it is possible to process samples of any diameter and any hardness. It looks like this. With such a tool, it is possible to process up to nuclear reactor fuel with a Vickers hardness of about 1000 Hv. However, the harder the sample to be processed, the greater the wear on the tip of the drill, and the longer it takes to process it. When precision machining is required, it is divided into two stages: rough machining and finishing machining, and when sampling samples, a hollow circular drill is used. Although cost reduction can be achieved by separating the drill and horn tool, it is also possible to integrate the drill and horn tool into one in order to further improve remote operability and transmit vibration energy more effectively. good.

As shown in FIG. 4, approximately in the center of the head section 20, there is a processing fuel receiver 36 for collecting fuel powder and debris that falls during processing, and an acrylic resin container covering it to prevent processing fuel from scattering. A dust collection box 37 is provided, and a dust collection pipe 17 connected to the processing fuel dust collection device is attached to the upper part of the dust collection box 37. Dust collection box 37
An entrance/exit 38 for the processing fuel bottle section and an entrance/exit 39 for the drill section are opened in both side walls of the machine.

In the above-mentioned ultrasonic vibration drilling apparatus main body 11, in order to facilitate the processing of long samples, once the fuel pins ranging from 80 to 100a++n are machined from one side, once they reach near the center of the fuel bottle. Machining is automatically stopped and the fuel pin is reversed at the collet chunk part, allowing re-machining.

The processing fuel dust collector 13 has a slightly larger particle size (0.3 μm particle size).
A pre-filter that traps particulate matter (about 500 yen or more) and a high-performance air filter that collects particulate matter less than that are arranged in two stages in series, and the dust collection status of the intake blower and filter is also monitored. Equipped with a differential pressure gauge to indicate when to replace the filter. For example, a paper filter with a mesh size of about 0.3 μm is used as the pre-filter.

The control device 18 that measures and controls each of the above devices from outside the cell includes a transducer cooling device drive system, a sample rotation feed control system, and an ultrasonic oscillation system. It consists of 11 series etc.

Since this device is used under high radiation dose rate, among the ultrasonic vibration drilling device main body 11, vibrator cooling device 12, and processing fuel dust collector 13, the collet chuck operating section and horn tool are frequently used and replaced. All parts are of a simple lever type or rotary type using a torque wrench, and the dust collection filters are also divided and assembled so that they can be disassembled and replaced using a manipulator.

In particular, the workpiece in the present invention is a nuclear reactor fuel, and is required to be less affected by the reaction between the fission products in the fuel and the atmosphere during processing. Figure 5 shows an austenitic stainless steel cladding tube containing spent reactor fuel, which was dry-processed in a nitrogen atmosphere with oxygen and moisture concentrations as impurities as parameters, and then processed in a nitrogen atmosphere containing the same impurities. In addition, the tensile elongation at break was measured at high temperatures.

As is clear from the figure, the elongation at break decreases in an atmosphere containing 1000 ppm or more of moisture and/or oxygen. This is thought to be because fission products seeping from inside the fuel to the surface and adhering to the inner surface of the cladding react with moisture, causing the cladding to deteriorate under high temperatures. Therefore, in order to avoid the influence of the processing atmosphere on the reactor fuel, for example, a high purity inert gas or a highly purified nitrogen gas with low reactivity is used as the processing atmosphere. The concentrations of moisture and oxygen, which are impurities in the atmospheric gas, are each set to 200 ppm by weight or less so that their influence is reduced.

Of course, in addition to the above-mentioned conditions, the processing atmosphere may also be a vacuum containing less than the same amount of moisture and oxygen.

Now, in the present invention, the following measures are further taken to perform dry processing. These are monitoring the wear condition of the drill, detecting drill breakage, maintaining and adjusting the optimal pressing load of the drill cutting edge during machining, and preventing overload.

Dry cutting of highly hard nuclear reactor fuel may generate an increased amount of heat during processing, and the increased temperature will soften the drill bit, making it impossible to cut. Moreover, as the temperature rises, it has a negative effect on the sample.

As a result of trying various studies to prevent these problems, it became clear that the increase in heat generation at the tip of the drill and drill wear are closely related, and it is important to use this to properly understand the condition of the drill. Dry processing becomes possible.

First, it was found that when the tip of the drill wears out by 40 to 50% or more, the cutting speed decreases rapidly, and if the tip wears out by 60% or more, the heat generation at the sample and the tip of the drill increases significantly. This relationship is the sixth
As shown in the figure. The figure shows cutting speed (%) versus drill tip wear (%) using an alumina sintered body with hardness Hv = 830 as a sample.
ffiIII/min). The region where the degree of wear of the drill tip exceeds 60% becomes the sample heating region.

Therefore, in order to solve this problem, the present invention uses a cutting speed meter and a cutting speed recorder to detect the degree of wear on the drill, and takes advantage of the fact that when the tip of the drill is heavily worn and the fuel approaches a heat-generating state, the cutting speed drops dramatically. The system predicts the start of abnormal heat generation and detects when it is time to replace the drill.

Next, as shown in FIG. 7, it was found that when a drill breakage occurs during dry machining, the ultrasonic output decreases to about 1/2. Therefore, this fact can be used to prevent damage to the fuel cladding tube and fuel zero 4 of the sample by stopping the machining operation when an abnormal decrease in output is detected by the ultrasonic output indicator.

If an excessive load is applied to the drill cutting edge through which ultrasonic vibrations are transmitted, damage to the cutting edge, progression of wear, abnormal heat generation of fuel, fuel damage, etc. will occur. In order to prevent these problems, it is necessary to keep the pressing load of the drill cutting edge at an optimum value at all times during machining.

FIG. 8 shows the relationship between the drill tip pressing load (kg) and the cutting speed, which was determined using an alumina sintered body of HV=800 as a simulated fuel. As is clear from the figure, the best cutting efficiency is shown when the drill tip pressing load is in the range of 0.8 to 1.2 kg, the cutting speed is low when it is less than 0.7 kg, and conversely when it is more than 1.4 kg. The wear rate of the drill increases and the cutting speed decreases rapidly. In the present invention, based on such a relationship, the pressing load of the drill cutting edge against the machining fuel is optimized. An example is shown in FIG.

This mechanism is provided between an upper slide board 43 on which the fuel bottle holding/driving section 22 is mounted and a lower slide board 42 on which these are mounted and driven at a constant speed. That is, an overload prevention spring 44 is provided between both slide plates 4243,
Its span adjustment screw 45 and overload prevention spring 44
A limit switch 46 is provided which is activated to stop the machining operation when a reaction force exceeding a specified value is applied to the machining part.

During cutting, the lower slide plate 4 is moved by a step motor.
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 reaction force of the processed portion 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 using the adjusting screw 45, and cutting is performed.

In this way, the overload prevention spring 44 also serves as a load element that optimally presses against the tip of the drill. When the pressing load of the drill cutting edge exceeds a specified value, the overload prevention spring 44 contracts in response to the reaction force, and the limit switch 46 is activated. As a result, the step motor for feeding the machining fuel pin and the sample rotation motor 30 are stopped, and the machining operation is stopped.

Now, generally speaking, in order to increase the machining speed using ultrasonic waves, it is sufficient to increase the amplitude of the ultrasonic vibrations of the tip machining tool.
For this purpose, it is best to increase the electrical input to the vibrator or to lower the resonant frequency of the vibrator as much as possible. However, if the transducer frequency becomes low, there is a risk that the sound will become audible.
Practically speaking, a frequency in the range of 16 to 30 kllz is chosen. Considering that this device will be installed in an airtight cell, a resonance frequency of around 16 kllz is selected, and an ultrasonic vibration control system is used to maximize electric power under this condition. In reality, the control device 18 is provided with an ultrasonic vibration adjustment switch, thereby making it possible to adjust and maintain the maximum output.

In the cutting process of nuclear reactor fuel pellets, no substances other than fuel are mixed in, and in the process of removing fuel materials such as fuel fragments and fuel powder generated by the process, such as mixed oxide fuel of uranium and plutonium, 0.1g
It is hoped that orders will be collected. In addition, reducing the level of contamination caused by flying fuel powder during reactor fuel processing and reducing radioactive waste during decontamination are also important issues. In the present invention, the machining fuel pin is moved and machined in a machining fuel receiver provided in the center of the ultrasonic vibration drilling machine body, and fuel powder and debris that fall during machining are first collected and pulverized. The recovered fuel powder is collected using a processed fuel dust collector to increase the recovery rate. Main body dust collection box 3
The fine fuel particles sucked from 7 are trapped by the pre-filter. The fine powder trapped in the prefilter can be collected by taking out the entire prefilter. For high-performance air filters, use pre-filters! The system traps the contaminated fuel particles to prevent contamination caused by scattering of the fuel particles inside the cell, and to reduce the amount of radioactive waste produced during decontamination.

There is a strong need to recover the waste with a high recovery rate.

Next, Table 1 shows examples of recovery when alumina sintered pellets, which are simulated fuel, were processed by the method of the present invention. As shown in the table, the recovery rate was very good, with an average value of 99% or more.

Table 1 The simulated fuel pellet 7) used in the test was a homogeneous cemented carbide alumina sintered body shrink-fitted into a stainless steel cladding tube, and was dense with no fine cranks. However, a typical workpiece in reality is a cladding tube containing oxide fuel pellets, and such spent reactor fuel pellets have many minute holes and cranks.

For this reason, it is extremely easy to fragment and pulverize (it can be judged that it can be processed more easily than in the case of densely packed simulated solidified fuel pellets. Therefore, by the method of the present invention, ultra-hard atoms with a Vickers hardness of 800 or more can be processed. Deep hole machining and cutting of furnace fuel can be performed dryly using a remote control manipulator without using abrasive materials, and fuel processing powder can be recovered with a high efficiency of over 99.9%. .

In the above embodiment, the main body of the ultrasonic vibration drilling machine is installed horizontally, but it is also possible to use a vertical structure in which the ultrasonic vibrator processing section is installed vertically and the fuel pin holding/driving section is driven vertically. good.

The present invention can be used for the following purposes.

■ Efficient recovery of only the fuel from spent reactor fuel-containing cladding while the cladding remains healthy, and collection of cladding samples to examine the irradiation characteristics of the material; ■ To measure fuel temperature. Reactor fuel processing to create holes for embedding thermocouples and wire monitors for neutron irradiation measurement, hole drilling for making hollow fuel pellets, and deep hole drilling for fuel for other purposes. to be used for.

[Effect of the invention 1 As described above, the present invention has a water content and an oxygen concentration of 2.
Dry processing is performed in a high purity inert gas or nitrogen gas atmosphere of 00 ppm by weight or less, or in a vacuum containing moisture and oxygen to the same extent or less, so deterioration of the workpiece due to the processing atmosphere can be prevented. can. In addition, in order to process fuel with low drill tip wear, no drill breakage, and no excessive drill tip pressing load, it is possible to perform deep hole drilling and cutting of ultrahard nuclear reactor fuel with a Pinkers hardness of 800 or higher. This process can be carried out in a dry manner without the use of abrasives, and it has become possible to recover processed fuel powder with a high recovery rate of over 99%.

By utilizing the present invention as described above, it is possible to promote the extension of the life of nuclear fuel assemblies, which are key technologies in nuclear reactor technology, and the development of highly reliable, high-performance nuclear reactor fuels.

Furthermore, it is possible to process fuel with an extremely high recovery rate, and since it is a dry process, reactor fuel can be recovered without contamination with impurities, making it possible to effectively reuse the reactor fuel. It also prevents radioactive contamination due to the scattering of reactor fuel powder in the working environment, reduces the amount of radioactive waste including reactor fuel materials, and keeps the amount of radioactive materials released into the environment around the test facility extremely low after irradiation. It has many excellent effects, such as the ability to

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram showing an example of an ultrasonic machining device to which the method of the present invention is applied, Fig. 2 is a perspective view of the main body of the ultrasonic vibration drilling device, and Fig. 3 A, B, and C are each an ultrasonic vibrator. A front view of the part, an enlarged view of the drill cutting edge, and a side view thereof, and FIG. 4 are explanatory views of the dust collection box. Furthermore, Fig. 5 is an explanatory diagram showing material characteristics with respect to impurity concentration in the atmosphere during machining, Fig. 6 is an explanatory diagram showing the relationship between drill tip wear and cutting speed, and Fig. 7 is an explanatory diagram showing the relationship between drill tip wear and cutting speed. Fig. 8 is an explanatory diagram showing the relationship between the cutting speed and the pressing load on the drill cutting edge, and Fig. 9 is a mechanism for maintaining and adjusting the optimum pressing load on the drill cutting edge during machining and preventing overload. FIG. DESCRIPTION OF SYMBOLS 11... Ultrasonic vibration drill processing device main body, 12... Vibrator cooling device, 13... Processing fuel dust collector, 14...
・Cell, 21... Ultrasonic vibrator processing section, 22... Fuel bottle holding/driving section, 24... Horn tool, 25.
... Drill with carbide tip, 26... Processing fuel. Patent applicant: Power Reactor and Nuclear Fuel Development Corporation Agent Shigeru Mi Seed box diagram Figure 5 Oxygen purity IX (I amount ppm) Moisture j1 degree (I precious ppm + n)

Claims (1)

[Claims] 1. In a high purity inert gas or nitrogen gas atmosphere with moisture and oxygen concentrations of 200 ppm or less by weight, an ultrasonic vibration horn tool with a drill with a carbide tip attached to the tip of the ultrasonic longitudinal vibration horn tool A method for ultrasonic machining of nuclear reactor fuel, characterized in that fuel is dry-processed using a sonic machining device with low drill tip wear, no drill breakage, and no excessive drill tip pressing load. 2. In a vacuum with moisture and oxygen partial pressures of 20 Pa or less, an ultrasonic machining device equipped with a carbide-tipped drill at the tip of an ultrasonic longitudinal vibration horn tool has low drill tip wear and no drill breakage. , and a method for ultrasonic processing of nuclear reactor fuel, characterized in that the fuel is dry-processed under a condition where the pressing load of the drill cutting edge does not become excessive. 3. The ultrasonic machining method according to claim 1 or 2, wherein the machining is performed with a wear degree of the tip of the drill being 60% or less.