Decommissioning methods for nuclear reactors that have experienced core meltdown

Abstract

This invention provides a method for decommissioning a nuclear reactor that has experienced a core meltdown. [Solution] In the decommissioning of a reactor that has experienced a core meltdown, the melted debris is covered with gravel and cooled by immersing it in water to suppress heat generation and thus shield and seal off radiation. In the removal of the debris after filling and sealing, it is possible to excavate the gravel and debris together using a rotary excavation method (rock auger method). The auger is used to lift the contents of the casing, and the recovered debris can be transported in a sealed state by using a flexible hose and piping to transport it from the outlet at the top to the recovered debris storage facility. Next, in order to remove any radioactive contaminated material remaining inside the reactor, the material is dissolved and removed using aqua regia circulation.

Description

Technical Field

[0001] With the power outage of the Fukushima Daiichi Nuclear Power Plant caused by the tsunami of the Great East Japan Earthquake, the reactor core melted down (melted), hydrogen explosions occurred, and radioactive contamination damage was caused to a vast area. Although 13 years have passed since then, the current situation is that we are struggling to find a solution. The top priority is to remove the debris that has melted and fallen inside and outside the reactor core. However, debris emitting high levels of radiation is not easy to handle and requires cooling with cooling water during the operation. Moreover, radioactive emissions to the outside are strictly prohibited. As a countermeasure, the debris is covered with gravel that can block radiation to shield and seal the radiation, and after creating an environment where the removal work can be carried out, the removal of the debris is started. The removal method considered is the rotary excavation method (rock auger method) which has a lot of experience in civil engineering and building demolition work. The casing and the auger are operated simultaneously to excavate the gravel and debris while lifting the inside of the casing and raising it. The excavated debris is connected from the discharge box provided at the upper part to a flexible hose and a pipe, and sent to the recovered debris storage to prevent radioactive emissions. After completing the recovery of the debris, then, using the technology of dissolving and recovering the built-in objects remaining inside the reactor by circulating aqua regia and the furnace washing technology, the reactor removal plan is drawn up.

Background Art

[0002] The removal of debris inside the reactor is considered by the underground obstacle removal work (rotary excavation method), which has many construction achievements in Japan and is considered very suitable for removing the built-in objects of a vertical tank with a height of more than 20 m like a reactor. And the radioactive contaminated built-in objects inside the reactor after the debris removal are dissolved and recovered by circulating aqua regia based on my Patent No. 6579469, and further, the furnace is washed using a cleaning agent. After creating a safe working environment, the reactor is disassembled and removed, and the debris buried in the concrete foundation part at the bottom of the removal site is also discharged by the same rotary excavation method as the background art.

Patent Literature

[0003] The method for decommissioning a nuclear reactor, for which I filed a patent application and received patent approval, was published in the patent gazette on September 25, 2019, as Patent No. 6579469. This forms the foundation of the decommissioning technology of this invention, and the method for removing debris was based on applied technologies such as rotary excavation methods used in civil engineering and building demolition work. [Non-patent literature] In my separate application filed on April 15, 2021, with publication number JP 2021-60379, the title of the invention is "Method for Processing Dissolved Substances Discharged from a Nuclear Reactor," and references techniques such as dilution evaporation, separation of liquid from mixed substances using a thin-film evaporator, and further separation of powder using a drying mixer. [Overview of the project]

[0004] This invention was devised as a solution to the extremely dangerous task of removing and recovering molten debris from a damaged nuclear reactor. First and foremost, shielding and sealing off the radiation emitted by the debris is crucial. By using gravel, which is effective in blocking radiation, as the material to fill the seal, the sealing objective can be achieved. By filling the reactor containment vessel surrounding the reactor with gravel, the debris that is the source of radiation emitted from the entire reactor facility is shielded and sealed off, creating a highly safe working environment. Furthermore, we believe that the debris can be removed using the rotary excavation method described in technical section

[0001] . Then, the radioactive contaminated material remaining inside the reactor is dissolved using aqua regia, which has superior dissolving power, and circulated. The dissolved material, recovered over a long period of time, is placed in a dissolution and separation tank, where the mixed dissolved material, which is denser than the aqua regia, is allowed to settle in the lower tank. The tank is then steam-heated to vaporize only the aqua regia for separation. After evaporation, the remaining residue is discharged and transferred to a dissolved material storage container using a gear pump. The processing method for this residue will be described separately. Reason for adopting gravel as packing material

[0005] The reason gravel was chosen for radiation shielding and sealing is that it can block radiation, which is its primary purpose, and it is expected to have a shielding effect comparable to that of concrete. Furthermore, it can be filled into narrow spaces, is easy to handle, and easy to clean up after use. It also has the advantage of suppressing debris heat generation by utilizing the permeability of gravel and spraying cooling water from above. Additionally, a rotary excavation method is possible for debris removal, and the excavated hole can be sealed again by operating the auger up and down, thus re-shielding and sealing off radiation. Radioactive contaminants are dissolved using aqua regia circulation.

[0006] After the removal of molten debris from inside the reactor is completed, it is necessary to remove any remaining radioactive contaminated materials such as fuel cladding tubes and control rods from the reactor. This is achieved by using aqua regia circulation to dissolve and recover the internal components. While this process takes some time, it is the most reliable method for handling radioactive contaminants, and its automated operation ensures worker safety. (Expected volume of debris to be processed)

[0007] The actual amount of molten debris to be processed is reported to be approximately 880 tons in total for units 1-3. Even with a generous estimate, this amounts to 300 tons per unit. However, converting the uranium density of 18.11 to a volume of 17 m³, and adding the molten internal components, the volume is estimated to be approximately double that, around 35 m³. The problem the invention aims to solve...

[0008] In debris removal operations, the greatest concern is the leakage of radioactivity. While it is generally considered best to enclose the area in reinforced concrete to block and seal off radiation, this concept is based on the idea that gravel is a suitable alternative. The idea behind using gravel is that covering the debris with gravel blocks radiation, and the permeability of the gravel allows for the spraying of cooling water from above to suppress the heat generated by the debris. Furthermore, the rotary drilling auger used during debris removal can be used without hindrance, and the drilled holes can be sealed with gravel by manipulating the auger's vertical movement, thus sealing off radiation. This is a means to solve the problem.

[0009] Recovering the debris from inside a nuclear reactor is extremely difficult, but dismantling the reactor itself is even more challenging. One method involves circulating aqua regia, which has exceptionally high dissolving power, inside the reactor to dissolve the internal materials and discharge them to the outside. This method is described in detail in my patent No. 6579469, as mentioned in [Patent Document], so please refer to it. [Brief explanation of the drawing]

[0010] [Figure 1] Overall Structure Diagram [Explanation of symbols]

[0011] Explanation of symbols in Figure 1 1) Nuclear reactor, 2) Nuclear reactor lid, 3) Nuclear reactor containment vessel, 4) Dissolved debris, 5) Fallen dissolved debris, 6) Gravel for filling the containment vessel 7) Gravel for filling the reactor 8) Rotary drilling auger (rock auger method) 9) Storage facility for recovered debris 10) Mixed recovered debris 11) Dissolution and separation tank 12) Multi-tube heat exchanger 13. Aqua regia reduction tank 14) Dissolved material storage container (residue storage container is made of glass) 15), Reactor installation building 16) Radiation shielding dome 17) Requirements for implementation of the dissolution treatment facility building (installation of aqua regia circulation system for dissolution)

[0012] I believe there is no worse way to decommission a nuclear reactor than one that has already caused an accident. The radiation emitted by the melted and fallen debris is enormous, enough to instantly kill a human. Therefore, the method of removing the debris is the top priority, and as a countermeasure, it is necessary to cover and seal the radiation-emitting debris and create a safe working environment. Mortar filling is one option, but if the mortar hardens and solidifies by the time of debris removal, it will hinder the removal work, and post-disposal will also be difficult. We believe that using gravel will solve these problems. Gravel is fluid, can be pumped into narrow spaces with water, and can also serve as a filler. Furthermore, by utilizing the permeability of gravel and spraying cooling water from above, the heat generated by the debris can be suppressed. Next, for debris removal, a rotary excavation method (rock auger method) is used, where the casing and auger are operated simultaneously to excavate the gravel and debris. The debris is then raised while being hoisted up inside the casing and sent to the recovered debris storage facility via a flexible hose and piping from a discharge box installed at the top of the screw auger, thereby preventing the release of radiation and allowing for the safe recovery of the debris. While debris recovery is largely complete, there are areas where excavation methods are insufficient, and small amounts of debris remain. This will be resolved by dissolving these debris, along with any remaining radioactive materials inside the reactor, through aqua regia circulation. This method, even if it takes many years, will enable unmanned, automated operation and is considered the safest recovery and processing method.

Claims

[Claim 1] This invention addresses the fact that the most important priority for decommissioning a reactor that has experienced a core meltdown is the recovery of the debris that has melted and fallen inside and outside the reactor. As a countermeasure, gravel has been selected as a packing material that can shield and seal off radiation. Gravel can block radiation, and its effectiveness is expected to increase when used in combination with water. In particular, it can block highly penetrating neutron radiation. Furthermore, gravel is easy to handle, can be packed into narrow spaces, and is easy to excavate. Moreover, by utilizing the permeability of gravel and spraying cooling water from above, the heat generation of the debris can be suppressed. The method for recovering the debris involves adopting a rotary excavation method, which has a proven track record in civil engineering and building demolition work. The debris excavated by the rotation of the auger is lifted up within the casing, and a non-rotating discharge box is installed at the top of the auger. The discharge port is connected to the box with a flexible hose and piping to prevent the release of radiation, and the debris is then sent to a storage facility. After the debris has been removed, the method for dissolving and recovering any radioactive contaminants remaining inside the reactor without melting is to dissolve them using aqua regia circulation. The return molten liquid from the reactor is steam-heated in a dissolution and separation tank to vaporize only the aqua regia and separate it. The residue remaining after the aqua regia evaporates is transferred to a molten material storage container as needed, and the vaporized aqua regia is condensed in a multi-tube heat exchanger and stored in an aqua regia reduction tank for recirculation. This is the method for decommissioning a reactor that has experienced a core meltdown.

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