JP7554728B2 - How to remove again - Google Patents

Description

The present invention relates to a method for removing metal containers that contain radioactive waste and are buried in backfill material from the backfill material.

Conventionally, radioactive waste has been disposed of by sealing it in a metal container, covering it with backfill material such as a compacted bentonite-based engineered barrier, and burying it deep underground. In light of the recent trend towards new regulatory standards for nuclear power, there is a demand to be able to quickly retrieve the radioactive waste again if migration of nuclides, for example, is confirmed in the radioactive waste after burial. To retrieve the radioactive waste again, it is necessary to remove the backfill material around the metal container. For example, as described in Patent Documents 1 and 2 below, a method is known in which salt water is sprayed on the backfill material to cause it to swell and collapse.

Patent No. 5553164 Patent No. 5747620

However, the methods described in Patent Documents 1 and 2 above have the problem that the salt water used in the treatment generates large amounts of bentonite mud and mud as new radioactive waste. In view of this problem, the present invention aims to provide a re-removal method that reduces the amount of new radioactive waste generated when removing metal containers from backfill material.

The re-removal method of the present invention is a method for re-removing from a backfill material a metal container that has stored radioactive waste in a radioactive waste disposal site and is buried in the backfill material, and includes a backfill material destruction step in which a magnetic attraction force is applied to the metal container from a magnet arranged to sandwich a part of the backfill material between the metal container and the magnet, and the backfill material is destroyed by a force acting from the metal container on the backfill material due to the magnetic attraction force.

In the backfill material destruction step, the metal container may be vibrated in the backfill material by magnetic attraction. The re-removal method of the present invention may include a container removal step in which the magnet is attached to a lifting device, and after the backfill material destruction step, the metal container is attracted to the magnet and removed from the backfill material by the lifting device.

The backfill material may be a bentonite-based engineered barrier material or a cement-based engineered barrier material that surrounds a metal container in a radioactive waste repository. The metal container may be an overpack or a PEM that has been buried in the radioactive waste repository.

The present invention provides a re-removal method that reduces the amount of new radioactive waste generated when removing metal containers from backfill material.

1A is a perspective view showing an underground tunnel in a radioactive waste disposal site to which the re-retrieval method of this embodiment is applied, and FIG. 1B is a cross-sectional view taken along line Ia-Ia thereof. 1A is a cross-sectional view of a tunnel including a lifting device and a magnet device, and FIG. 1B is a plan view of the magnet device. 1A is a cross-sectional view showing a cushioning material destruction step in the re-removal method of the present embodiment, and FIG. 1B is a cross-sectional view showing a container removal step. 1A is a plan view showing another example of a magnet device, FIG. 1B is a cross-sectional view showing yet another example of a magnet device, and FIG. 1C is a plan view showing yet another example of a magnet device. 13A is a plan view showing a state in which a magnet device according to still another example is placed on a cushioning material, and FIG. 13B is a cross-sectional view thereof. FIG. 2 is a perspective view illustrating an experiment conducted by the present inventors. 1A is a perspective view showing another example of an underground tunnel to which the re-retrieval method of this embodiment is applied, and FIG. 1B is a cross-sectional view showing the buffer destruction step of the re-retrieval method. 13 is a partially cutaway perspective view showing yet another example of an underground tunnel to which the re-taking method of this embodiment is applied. FIG.

Hereinafter, an embodiment of the re-retrieval method according to the present invention will be described in detail with reference to the drawings. The re-retrieval method of this embodiment is a method used to retrieve again a metal container that has contained radioactive waste and been buried in a radioactive waste disposal site. FIG. 1(a) is a perspective view showing an underground tunnel 1 in a radioactive waste disposal site to which this re-retrieval method is applied, and FIG. 1(b) is a cross-sectional view taken along line Ia-Ia. The underground tunnel 1 is provided, for example, in the ground at a depth of 300 m or more underground. At the bottom of the shaft 2 of the underground tunnel 1, a number of cylindrical holes 3 with a diameter of about 2.2 m are dug in a cylindrical shape approximately vertically downward and spaced apart in the longitudinal direction of the underground tunnel 1. A cylindrical metal container 5 is installed in the cylindrical hole 3 approximately concentrically with the cylindrical hole 3, and a buffer material 7 is filled in the cylindrical hole 3 as a backfill material so as to surround the metal container 5.

When the waste is to be landfilled, the buffer material 7 is first placed in the cylindrical hole 3. A cylindrical installation hole with a bottom and approximately the same diameter as the metal container 5 is formed in the center of the buffer material 7, concentric with the cylindrical hole 3. The metal container 5 is then placed in the center of the cylindrical hole 3 by inserting the metal container 5 into the installation hole using heavy machinery or the like, and the buffer material 7 is then filled up to the top of the cylindrical hole 3 so as to cover the top of the metal container 5, so that the metal container 5 is buried in the buffer material 7 in the cylindrical hole 3. After one metal container 5 is buried in each cylindrical hole 3, the tunnel 2 of the underground tunnel 1 is also filled with a specified ground material. The upper surface of the metal container 5 is provided with a gripping portion 5a for gripping by the heavy machinery or the like.

The metal container 5 stores radioactive waste inside. This radioactive waste may be low-level radioactive waste or high-level radioactive waste. As a specific example, the metal container 5 of this embodiment is a cylindrical container with a diameter of about 80 cm, in which a vitrified body made by solidifying high-level radioactive waste (HLW) together with glass is sealed in a stainless steel cylindrical container called a canister, and this canister is further wrapped in steel. The aforementioned grip part 5a is provided on the cylindrical upper end surface of this container. Such a metal container 5 may be called an "overpack". Note that the metal container 5 is not limited to an overpack, and may be, for example, a PEM (Prefabricated Engineered Barrier System Module).

The buffer material 7 functions as an engineered barrier that blocks the influence of the surrounding ground on the metal container 5 by being interposed between the ground and the metal container 5. That is, the buffer material 7 is made of a water-resistant material and prevents the metal container 5 from being exposed to groundwater. The buffer material 7 also has the function of buffering the external force acting on the metal container 5 from the surrounding ground during an earthquake, etc. For example, a bentonite-based engineered barrier material, a cement-based engineered barrier material, etc. may be used as the buffer material 7. In this embodiment, the buffer material 7 is a bentonite-based engineered barrier material, and is made of a soil material formed by mixing bentonite and sand, for example.

The re-removal method for removing the metal container 5 from the buffer material 7 of the underground tunnel 1 as described above will now be described. In the re-removal method of this embodiment, first, the ground material filling the tunnel 2 of the underground tunnel 1 is excavated and removed, and a cylindrical hole 3 appears at the bottom of the tunnel 2. At this point, as shown in FIG. 1(b), the circular upper end surface of the buffer material 7 is exposed at the bottom of the tunnel 2 (i.e., the upper end of the cylindrical hole 3), and the metal container 5 is completely buried in the buffer material 7 in the cylindrical hole 3.

Next, as shown in FIG. 2(a), the magnet device 11 is introduced into the tunnel 2. In this embodiment, a lifting device 13 including a crane is constructed in the tunnel 2, and the magnet device 11 is suspended from the lifting device 13. The magnet device 11 can move horizontally in two dimensions in accordance with the two-dimensional horizontal movement of the lifting device 13, and can move up and down by pulling up/feeding out the suspension rope of the lifting device 13. As also shown in FIG. 2(b), the magnet device 11 has a housing 11a that is circular and has a larger diameter than the cushioning material 7 in a plan view, and an electromagnet section 11b built into the housing 11a. A known lifting magnet device may be used as the magnet device 11.

As shown in FIG. 3(a), the lifting device 13 moves the magnet device 11 by operating a crane, and positions the magnet device 11 above the upper surface of the buffer material 7 exposed at the bottom of the tunnel 2. At this time, the housing 11a of the magnet device 11 is positioned roughly concentrically with the metal container 5, and the suction surface 11c on the lower surface of the housing 11a is positioned along the upper surface of the buffer material 7. At this time, the suction surface 11c and the upper surface of the buffer material 7 may be in close contact with each other, or there may be a gap between the suction surface 11c and the upper surface of the buffer material 7. The electromagnet portion 11b has the function of attracting the metal container 5 in a direction perpendicular to the suction surface 11c (approximately upward).

When the magnet device 11 is driven in this state and a magnetic attraction force is generated, the metal container 5 is attracted toward the attraction surface 11c, and the metal container 5 receives an upward force. This upward force on the metal container 5 acts to push up the upper part of the buffer material 7 covering the upper part of the metal container 5. When the electromagnet section 11b is repeatedly turned on and off, an upward magnetic attraction force is intermittently applied to the metal container 5, and an upward external force from the metal container 5 is intermittently applied to the upper part of the buffer material 7. Eventually, cracks occur in the upper part of the buffer material 7, and the upper part of the buffer material 7 is destroyed. Hereinafter, the process of applying a magnetic attraction force from the magnet device 11 to the metal container 5 and destroying the upper part of the buffer material 7 is referred to as the "buffer material destruction process."

Here, the metal container 5 may be vibrated up and down by periodically switching the electromagnet unit 11b ON/OFF. In this case, the ON/OFF switching period of the electromagnet unit 11b may correspond to the natural frequency of the system including the metal container 5 and the buffer material 7. With this configuration, the vibration of the metal container 5 is amplified and a large vibration is obtained, so that the destruction of the upper part of the buffer material 7 progresses efficiently. In addition, in order to find the natural frequency, which is generally unknown, in the buffer material destruction process, the vibration of the buffer material 7 may be monitored while gradually changing the ON/OFF switching period of the electromagnet unit 11b, and the switching period at which the vibration is the largest may be adopted. For this reason, the magnet device 11 may be equipped with a vibration detection device (not shown) for detecting the magnitude of the vibration of the buffer material 7.

Then, the upper part of the buffer material 7 is removed to expose the upper part of the metal container 5. Since the upper part of the buffer material 7 has already been destroyed in the buffer material destruction process described above, the removal of the upper part of the buffer material 7 can be performed relatively easily. This removal of the upper part of the buffer material 7 may be performed by other heavy machinery introduced into the tunnel 2. Then, as shown in FIG. 3(b), the lifting device 13 operates the crane to attract the exposed upper part of the metal container 5 to the attraction surface 11c of the magnet device 11 and lift it up, thereby removing the metal container 5 from the buffer material 7. This process of removing the metal container 5 from the buffer material 7 whose upper part has been destroyed in this way is referred to below as the "container removal process". This completes the re-removal of the metal container 5. The re-removed metal container 5 is appropriately transported out of the underground tunnel 1 through the tunnel 2.

As described above, when the metal container 5 is disposed of in a landfill, a cylindrical bottomed installation hole is formed in advance in the center of the cushioning material 7, and the metal container 5 is inserted into this installation hole. In order to facilitate the insertion of the metal container 5, the installation hole is formed to have a diameter slightly larger than that of the metal container 5, and therefore there is a slight radial gap between the metal container 5 and the cushioning material 7. In addition, the metal container 5 generates a certain amount of heat due to radiation emitted from the radioactive waste inside. This heat tends to dry out the cushioning material 7 around the metal container 5, and drying shrinkage of the cushioning material 7 may occur in the vicinity of the metal container 5. This drying shrinkage causes a radial gap between the metal container 5 and the cushioning material 7. Due to the presence of the radial gap as described above, after the upper part of the cushioning material 7 is removed, the metal container 5 can be relatively easily removed from the cushioning material 7 in the upward cylindrical axial direction. When removing the metal container 5 from the cushioning material 7 in the container removal process, the gripping portion 5a of the metal container 5 exposed at the top may be gripped by other heavy machinery to remove the metal container 5 from the cushioning material 7. The presence of the gap between the metal container 5 and the cushioning material 7 as described above also contributes to sufficiently vibrating the metal container 5 that is subjected to the magnetic attraction force inside the cushioning material 7.

A part or all of the series of operations performed by the lifting device 13 and the magnet device 11 as described above may be performed by remote control. By employing remote control, radiation exposure of workers can be reduced. In particular, the cushioning destruction process can be performed by relatively simple operations such as placing the magnet device 11 on the upper surface of the cushioning material 7 and driving the magnet device 11, and is therefore easy to perform by remote control. For this remote operation, the lifting device 13 and the magnet device 11 may be equipped with a wired or wireless communication device (not shown) for transmitting and receiving remote operation signals.

Next, the electromagnet section of the magnet device 11 will be further described. The magnet device 11 may be provided with an electromagnet section 11e shown in FIG. 4(a) instead of the electromagnet section 11b. The electromagnet section 11e has a circular ring shape concentric with the housing 11a in a plan view, and generates an electromagnet's north pole and south pole at positions facing each other in the radial direction across the center of the ring. The north pole and south pole can be moved in the circumferential direction or the north pole and south pole can be reversed by controlling the current. The magnet device 11 may also be provided with an electromagnet section 11f shown in FIG. 4(b). The electromagnet section 11f has a circular ring shape concentric with the housing 11a in a plan view. The electromagnet section 11f generates north poles and south poles in the vertical direction, and a portion capable of reversing the north pole and south pole from top to bottom is arranged in the circumferential direction of the ring.

The electromagnet portion 11b attracts the metal container 5 in a direction perpendicular to the attraction surface 11c, but the electromagnet portion 11h described below may be used instead of the electromagnet portion 11b. FIG. 5(a) is a plan view showing a state in which the magnet device 11 having the electromagnet portion 11h is placed on the cushioning material 7, and FIG. 5(b) is a side view thereof. As shown in FIGS. 5(a) and (b), the electromagnet portion 11h forms a ring shape concentric with the housing 11a in plan view. In plan view, the inner diameter of the electromagnet portion 11h is larger than the outer diameter of the metal container 5, and in the cushioning material destruction process, it is placed on the outer periphery so as to surround the metal container 5 in plan view.

The electromagnet section 11h has a function of switching the location where the magnetic attraction force is generated in the circumferential direction. Specifically, for example, the electromagnet section 11h has a large number of magnetic poles 12 arranged in the extension direction. In the example of FIG. 5, the 12 magnetic poles 12 of the electromagnet section 11h are arranged in the circumferential direction. The electromagnet section 11h can individually switch ON/OFF of the attraction force generation at each magnetic pole 12. When one of the magnetic poles 12 is turned ON, the metal container 5 is attracted diagonally upward toward the magnetic pole 12. According to such a configuration of the electromagnet section 11h, for example, by switching the magnetic poles 12 to be turned ON in sequence in the circumferential direction, the direction of attraction of the metal container 5 can be changed in the circumferential direction one after another, and a force that rotates the cylindrical axis of the metal container 5 in the circumferential direction can be applied to the metal container 5. In addition, by switching the ON/OFF of each magnetic pole 12 in a more complex order and combination, more complex forces can be applied to the metal container 5.

In the magnet device 11 having this electromagnet section 11h, by periodically switching the ON/OFF of each magnetic pole 12, complex vibrations including a vibration mode in which the direction of the cylindrical axis of the metal container 5 fluctuates can be applied to the metal container 5. As a result, the upper part of the buffer material 7 can be destroyed more efficiently. In addition, in the electromagnet section 11h, instead of switching the ON/OFF of each magnetic pole 12, the magnetism (N pole/S pole) of each magnetic pole 12 may be individually switched, or in addition to switching the ON/OFF of each magnetic pole 12, the magnetism (N pole/S pole) of each magnetic pole 12 may also be individually switched. In the electromagnet section 11h, as in the above-mentioned electromagnet section 11b, the period of switching the ON/OFF of each magnetic pole 12 or the switching of the magnetism may be made to correspond to the natural frequency of the system including the metal container 5 and the buffer material 7.

Note that it is not essential that an electromagnet is used in the magnet device 11, and the magnet device 11 may be provided with a permanent magnet portion 11j shown in FIG. 4(c) instead of the electromagnet portion 11b. The permanent magnet portion 11j extends longer than the diameter of the metal container 5 along the diameter of the housing 11a inside the housing 11a, and has a north pole and a south pole at both ends. The permanent magnet portion 11j has a rotation axis at the center position of the housing 11a, and rotates horizontally around the rotation axis by a predetermined rotation mechanism. Such a permanent magnet portion 11j can also impart vibration due to magnetic attraction to the metal container 5. As with the electromagnet portion 11b described above, the rotation period of the permanent magnet portion 11j may be set to correspond to the natural frequency of the system including the metal container 5 and the buffer material 7.

Superconducting electromagnets may be used for the electromagnet sections 11b, 11e, 11f, and 11h. By using superconducting electromagnets, a greater magnetic attraction force can be applied to the metal container 5, and the upper part of the buffer material 7 can be destroyed more efficiently. In addition, the magnet provided in the magnet device 11 may be configured by combining an electromagnet and a permanent magnet.

Next, the effect of the re-retrieval method of this embodiment will be described. In this re-retrieval method, the magnet device 11 is placed along the upper surface of the buffer material 7, and the magnet device 11 applies a magnetic attraction force to the metal container 5. Due to this magnetic attraction force, a force from the metal container 5 acts on the upper part of the buffer material 7 sandwiched between the magnet device 11 and the metal container 5, and this force destroys the buffer material 7. Therefore, in this re-retrieval method, treatment such as spraying water on the buffer material 7 as in the conventional method is not performed, and new radioactive waste from the sprayed water is not generated. In addition, cutting dust is not generated compared to cutting treatment of the buffer material 7, and almost no dust is generated before cracks occur in the buffer material 7. Therefore, according to the re-retrieval method of this embodiment, it is possible to reduce new radioactive waste generated when removing the metal container 5 from the buffer material 7.

In addition, in the buffer destruction process of this re-removal method, the upper part of the buffer material 7 is destroyed by the force of the metal container 5 pushing up on the upper part of the buffer material 7 from below. The force pushing up on the upper part of the buffer material 7 from below acts as a force that mainly generates tensile stress in the buffer material 7. Here, the buffer material 7 is, for example, a bentonite-based artificial barrier material, and therefore has a smaller tensile fracture strength than its compressive fracture strength. Therefore, by using the magnetic attraction force to the metal container 5 to generate tensile stress in the buffer material 7, the buffer material 7 can be destroyed with a relatively weak force.

Furthermore, in the cushioning destruction process, when the electromagnet section 11b is periodically switched ON/OFF to apply a magnetic attraction force intermittently to the metal container 5 to vibrate the metal container 5, the above-mentioned tensile stress acts repeatedly and intermittently on the upper part of the cushioning material 7, and the destruction of the cushioning material 7 progresses more efficiently. At this time, if the period of switching ON/OFF of the electromagnet section 11b is made to correspond to the natural frequency of the system including the metal container 5 and the cushioning material 7, the vibration of the metal container 5 is amplified, and the destruction of the cushioning material 7 progresses more efficiently.

The magnet device 11 is also attached to the lifting device 13, and in the container removal process following the buffer destruction process, the metal container 5 is attracted to the magnet device 11 and removed from the buffer 7. This makes it possible to carry out the buffer destruction process, the container removal process, and the movement of the metal container 5 within the tunnel 2 using the lifting device 13 and the magnet device 11.

Next, the experiment conducted by the present inventors will be described. In the experiment, a specimen 90 shown in FIG. 6 was prepared. The specimen 90 was formed by embedding three metal balls 93 inside a buffer material 97, compacting it, and forming it into a cylindrical shape with a diameter of about 40 mm and a height of about 40 mm. The buffer material 97 was made of the same material (bentonite-based artificial barrier material) as the buffer material 7 in the above-mentioned underground tunnel 1, and was compacted to a hardness equivalent to that expected for the buffer material 7. The dry density of the buffer material 97 was also set to be approximately the same as that expected for the buffer material 7 in the underground tunnel 1. For the metal balls 93, pachinko balls with a diameter of about 11 mm were used. The three metal balls 93 were arranged approximately along the cylindrical axis inside the buffer material 97.

This specimen 90 was placed on a magnetic stirrer 92, and a magnet 94 with an attractive force of about 20 kg was placed on the top surface of the specimen 90, and the internal magnet of the magnetic stirrer 92 was rotated at 1500 RPM. This caused vibrations in the specimen 90, and after about three days to one week, cracks appeared in the buffer material 97, and it eventually broke into four pieces. In the broken pieces, gaps had appeared between the buffer material 97 and the metal ball 93 around it. The above experiments confirmed the effectiveness of a method of destroying buffer material with metal objects buried in it by using the magnetic attractive force of the metal objects from the outside.

The present invention can be implemented in various forms, including the above-described embodiment, with various modifications and improvements based on the knowledge of those skilled in the art. In addition, the following modified examples can be constructed by utilizing the technical matters described in the above-described embodiment. The configurations of each embodiment, etc. may be used in appropriate combination.

For example, in the above embodiment, an example was described in which the magnet device 11 was suspended from the lifting device 13 including a crane, but the magnet device 11 may also be attached as an attachment to the end of the arm of heavy machinery capable of self-propelling within the tunnel 2. In this case, in the container removal process, the metal container 5 may be attracted to the magnet device 11 and removed from the buffer material 7 by operating the arm of the heavy machinery. In addition, the buffer material 7 is not limited to bentonite-based artificial barrier material, and may be cement-based artificial barrier material. Furthermore, the present invention can also be applied to the re-removal of the metal container 5 buried in various other backfill materials.

The re-retrieval method of the present invention can be applied to an underground tunnel 51 of the type shown in FIG. 7(a) instead of the underground tunnel 1 (FIG. 1). In the underground tunnel 51, a metal container 5 is placed at the center of the tunnel 52. A plurality of metal containers 5 are arranged at a predetermined interval along the extension direction of the tunnel 52, with their cylindrical axes facing the extension direction of the tunnel 52. The tunnel 52 is filled with a buffer material 7, which is a bentonite-based engineered barrier material. In the re-retrieval of the metal container 5 in such an underground tunnel 51, as shown in FIG. 7(b), for example, a magnet device 11 is attached as an attachment to the tip of the arm of a heavy machine 14. By operating the arm of this heavy machine 14, the magnet device 11 can be placed along the almost vertical end surface of the buffer material 7. Then, as in the above-mentioned embodiment, the magnetic attraction force from the magnet device 11 is applied to the metal container 5, and the buffer material 7 is destroyed.

The re-extraction method of the present invention can also be applied to an underground tunnel 61 of the type shown in FIG. 8 instead of the underground tunnel 1 (FIG. 1). A plurality of rectangular parallelepiped-shaped metal containers 65 are arranged in the tunnel 62 of the underground tunnel 61 in the width, height and length directions of the tunnel. Filling material 67 (backfilling material) is provided in the gaps between the metal containers 65. In addition, a concrete pit 69 made of reinforced concrete, a low diffusion layer 71 made of precast concrete, and a buffer layer 73 are present so as to surround the filling material 67 in multiple layers, and the space between the buffer layer 73 and the wall surface of the underground tunnel 61 is backfilled with ground material 75. The above-mentioned filling material 67 is a cement-based artificial barrier material, and is formed by filling and hardening the space between the metal containers 65 with cement milk. When re-extracting the metal container 65 from such an underground tunnel 61, the ground material 75, the buffer layer 73, the low diffusion layer 71, and the concrete pit 69 are first removed. Thereafter, when the metal container 65 buried in the filler 67 is to be removed again, for example, heavy machinery 14 (see FIG. 7(b)) is introduced into the tunnel 62, and as in the previous embodiment, a magnetic attraction force is applied to the metal container 65 from the magnet device 11 arranged outside the filler 67, destroying the filler 67.

1, 51, 61...underground tunnel (radioactive waste disposal site), 5, 65...metal container, 7...buffer material (backfill material), 11...magnetic device, 13...lifting device, 11b, 11e, 11f, 11h...electromagnet section (magnet), 11j...permanent magnet section (magnet), 14...heavy machinery (lifting device), 67...filling material (backfill material, cement-based engineered barrier material).

Claims (5)

A method for re-removing a metal container that stores radioactive waste and is buried in a backfill material at a radioactive waste disposal site from the backfill material, comprising the steps of:
The re-removal method includes a backfilling material destruction process in which a magnetic attraction force is applied to the metal container from a magnet arranged with a portion of the backfilling material sandwiched between the magnet and the metal container, and the backfilling material is destroyed by a force acting from the metal container on the backfilling material due to the magnetic attraction force. The method of claim 1, wherein the backfill material destruction step vibrates the metal container within the backfill material by the magnetic attraction force. The magnet is attached to a lifting device;
3. The method according to claim 1, further comprising a container removal step of, after the backfilling material destruction step, attracting the metal container to the magnet and removing the metal container from within the backfilling material by the lifting device. The re-retrieval method according to any one of claims 1 to 3, wherein the backfill material is a bentonite-based artificial barrier material or a cement-based artificial barrier material that surrounds the metal container in the radioactive waste disposal site. The re-retrieval method according to any one of claims 1 to 4, wherein the metal container is an overpack or a PEM that has been buried in the radioactive waste disposal site.

Images