JP2015141197A - Method for recovering molten fuel in nuclear reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for safely recovering molten fuel.SOLUTION: A refrigerant such as a liquid nitrogen 9 is filled while causing water 8 to flow into a reactor containment vessel, the water leaking out from a leaking place is made frozen, a leakage of the radiation is stopped, and the water around a fuel debris 5 fallen inside a reactor pressure vessel 1 and the inner floor of a pedestal 3 is made frozen. Next, the fuel debris confined into ice is pulverized by injecting high temperature steam or high temperature air to the fuel debris from a tip end of a probe 19 such as an endoscope, and the fuel debris passing through a piping pump inside the probe by own expansive pressure is recovered into an accommodating container.

Description

The present invention relates to a recovery method and a recovery device for fuel melted in a nuclear reactor.

On March 11, 2011, the Fukushima Daiichi nuclear power plant from Unit 1 to Unit 3 melted due to the Great East Japan Earthquake.
There is an urgent need to recover the molten fuel as the first step in decommissioning measures. At present, no drastic construction method has been found.

Although the device for recovering fuel melted in the air or water is advanced at the research level, it is localized, and the process is fragmented and does not completely keep leaking, so it will contaminate immediately even after decontamination. Therefore, there is no method that captures the entire process from isolation to collection. An alternative method for removing fuel debris from the International Research Institute for Decommissioning of Technology is now open to the public.

  There is no method for safely recovering core melted fuel.

  The present invention is a reactor containment vessel in which the core is melted, while water is flowing inside and outside the pressure suppression chamber, water is frozen at the leaked portions filled with refrigerant such as ice particles, liquid nitrogen, dry ice, or cooling air, and the holes are closed. Stop leaking or flooding. After that, a certain amount of water is poured into the fuel debris melted in the reactor pressure vessel and pedestal floor, and ice particles containing copper are dropped until the water surface is covered. And ice are sealed in a frozen solid, bored from the work floor installed at the top of the reactor pressure vessel to the fuel debris, and a probe of a recovery device such as an endoscope is inserted into the bored tube. High-temperature high-pressure steam, high-temperature high-pressure air or liquid nitrogen is injected into the fuel debris from the tip, and the fuel debris is pulverized, and the fuel debris pulverized by the rapidly expanded gas is passed through the pressure feed tube in the probe and collected in the storage container.

  It is assumed that the containment vessel is cracked at an enormous location due to an explosion at the time of the accident, and it is necessary to stop all leaks.

  It is necessary to take drastic measures against the fact that the cooling water that cools fuel debris is activated and leaks from Kiretsu into the ground in large quantities and the pollution is spreading.

  It is necessary to take drastic measures to prevent the surrounding groundwater from invading into the concrete in the lower part of the reactor containment vessel and increasing radioactive contamination.

  Regardless of whether fuel debris is being stored or being recovered, the release of gas, liquid, and solid radioactivity released from fuel debris must be prevented and not released into the environment.

  Ensure safety for workers both physically and against radiation.

  Safe recovery of fuel debris without criticality

  The reactor containment vessel and the pressure suppression chamber that are cracked and leaked are filled with a coolant such as ice particles, liquid nitrogen, dry ice, or cooling air while flowing water.

  Water is poured into the reactor pressure vessel and pedestal where fuel debris is accumulated, the water level is raised to about half of the area, copper is mixed, and ice particles cooled with liquid nitrogen with a higher specific gravity than water are cooled on the water surface. Drop until covered.

  Inject liquid nitrogen to freeze water and copper. At this time, nitrogen vaporized from liquid nitrogen passes through a filter to remove radioactivity and release it to the atmosphere.

  High-temperature vapor, high-temperature air, and liquid nitrogen are injected into the fuel debris from the injection nozzle at the tip of the probe such as an endoscope for the fuel debris stored in the solid.

  Cooling water leaking from the reactor containment vessel and reactor pressure vessel and the concrete below the containment vessel freeze, and the outflow of gas, liquid and solid radioactivity released from the fuel debris stops. Prevent underground water from entering the concrete in the lower part of the containment vessel. The release of radioactivity released into the environment is almost eliminated.

  Ice grains float on the water surface because they are lighter than water, but contain copper powder and bullet-shaped hollow copper material at the time of manufacture, make the specific gravity larger than water, sink to the bottom of the water, and make direct contact with fuel debris.

Liquid nitrogen freezes copper and ice and indirectly cools fuel debris. This cooling method has a three-layer structure, the first layer is a layer of copper powder in contact with fuel debris and soaked in cooling water, the second layer is arranged with ice particles containing copper powder on the top, The third layer has a structure for flowing liquid nitrogen. The fuel debris is confined in the solid formed by copper and ice, and the liquid nitrogen is cooled from the outside through these substances, so the liquid nitrogen will not be heated suddenly and the gas will expand rapidly even if it vaporizes. Fuel debris can be stored safely without any radioactive material spreading.
In these layers, the specific gravity of copper, water, ice, and liquid nitrogen placed from below gradually decreases to 9, 1, 0.9, and 0.8 in order. Cooling can be performed stably.
The state of each layer is expected.
In the first layer, the boundary with the fuel debris is in the state of film boiling due to the decay heat from the fuel debris, and in the center of the layer, the copper powder is concentrated and the water gas and liquid are in the space between the copper powders. It is in a mixed state. The boundary surface of the fuel debris is expected to be about 100 ° C. at maximum because water evaporates.
At this time, the copper that is in contact also becomes about 100 ° C., and due to the material properties of copper, the copper is ductile at this time and comes into close contact with the fuel debris, further increasing the thermal efficiency.
In the second layer, vapor condensation and ice liquefaction are balanced at the boundary with the first layer, and the copper powder and water are frozen in the center of the layer. Since the temperature distribution is ice, the lower part is 0 ° C., the upper part in contact with liquid nitrogen is about −200 ° C., and the temperature between them is expected to be proportional.
In the third layer, liquid nitrogen is vaporized and film boiled at the boundary with the second layer, and liquid nitrogen is flowing in the center of the layer. The temperature is about -200 ° C.

  At present, all reactors melted at the core in Fukushima generate about 100 KW of decay heat from the fuel debris that has fallen onto the pedestal floor with a diameter of about 4.8 m. When copper with high thermal conductivity is put into ice powder, the thermal conductivity of the ice layer is increased, and the thickness of the frozen layer of ice and copper is estimated to be about 70 cm, forming a layer that can be called a pressure shell, which occurs when fuel debris is recovered The structure is stable even in a high temperature and high pressure environment.

At present, all reactors melted at the core in Fukushima use 4 t / h or more of cooling water to cool the fuel debris, and a significant part of it is discharged into the building.
In this construction method, fuel debris is indirectly cooled through copper and ice fixed with liquid nitrogen, so that no cooling water is required. Since liquid nitrogen is also indirectly cooled, there is little pollution contained in the vaporized nitrogen, and even if nitrogen is passed through the filter, the radioactivity is removed and released to the atmosphere. The groundwater that had entered the concrete under the containment was also frozen, preventing the spread of radioactive contamination. For these reasons, almost no radioactivity is released into the environment.

High-temperature high-pressure water, high-temperature high-pressure air, or liquid nitrogen is injected and pulverized into fuel debris stored in a solid. The following eight types of pulverization processes are assumed.
First, fuel debris is directly hit by high temperature and high pressure injection forces.
Secondly, fuel debris is composed of various materials such as nuclear fuel, zircaide, concrete, and iron materials, and these materials have greatly different temperature expansion coefficients. Applying high-temperature steam or high-temperature air to cooled fuel debris A temperature difference of 300 ° C to 500 ° C is created, and the thermal stress that exceeds the strength of most materials reaches a maximum of 1000 N / mm ² in the fuel debris due to the difference in thermal expansion of each substance Occurs and separates in units of substances.
Thirdly, it is known that fuel debris has many voids due to hydrogen and gaseous FP generated during melting. When sufficiently cooled, water penetrates into this gap. Some are ice. When high-temperature high-pressure steam or high-temperature high-pressure air is received, water or ice in the gap evaporates all at once, the pressure rapidly increases in the gap, destroys the boundary of the gap, and the fuel debris is destroyed from the inside.
Fourthly, water vapor and ice pieces generated by melting surrounding ice are repeatedly subdivided into fuel debris that has risen in the air.
Fifth, the soaring fuel debris becomes finer by repeatedly colliding with the ice wall.
Sixth, when the copper powder contained in the ice powder is exposed to high-temperature high-pressure water or high-temperature high-pressure air, it jumps into the space and becomes blast and collides with fuel debris.
Seventh, when the bullet-type hollow copper material included in the ice particles is exposed to high-temperature and high-pressure water or high-temperature and high-pressure air, the surrounding ice melts, the bullet-type hollow iron material jumps into the space, and the spindle surface faces downward due to the weight. When the ice inside turns, it gains propulsion and collides with the fuel debris just like a bullet.
Eighth, if the space in the ice becomes larger and the cooling effect or fuel debris recovery effect diminishes, low-temperature high-pressure water is injected from the probe injection nozzle, water is infiltrated into the fuel debris, and then liquid nitrogen is injected from the probe injection. The fuel debris freezes and the fuel debris rapidly contracts at this time, so that a temperature stress is generated, and cracks are easily generated and easily collapse. Then repeat steps 1-7.

  The pulverized fuel debris is stored in the storage container by the expanded gas through the pressure feeding tube in the probe.

  When exposed to high-temperature high-pressure water or high-temperature high-pressure air, the copper and ice that covered the fuel debris will become hot, the ice will evaporate to form a space, and the copper powder will remain sponge-like, but it will withstand the high temperature and high pressure and maintain its shape. To do.

  At present, the reactor core melted in Fukushima is expected to generate about 100 kW of fuel debris. The thickness of ice formed by cooling from liquid nitrogen is estimated to be about 70 cm. If the average specific gravity of this layer is 6, the radiation shielding effect of this layer is equivalent to 4m of water, greatly reducing the radiation emitted from the fuel debris, and the amount of exposure to workers on the operation floor above the reactor vessel Can be reduced.

  The fuel debris is pulverized and pumped in small capillaries little by little and is not critical because it does not reach the critical mass.

  FIG. 1 is an overall image diagram of a construction method for preventing leakage and flooding and collecting fuel debris.   FIG. 2 shows a method for cooling fuel debris.   FIG. 3 shows the tip of the recovery device.

FIG. 1 is an overall image diagram showing a state in which the fuel debris 5 is being collected. Cooling leaking from the reactor containment vessel 2 and the pressure suppression chamber 4 through the pedestal cooling pipe 11 and the pressure suppression chamber cooling pipe 13 in which water 8 and liquid nitrogen 9 are installed in the reactor containment vessel 2 and the pressure suppression chamber 4 by this method. Freeze water and concrete in the lower part of the reactor containment vessel 2 to stop the outflow of gas, liquid, and solid radioactivity released from the fuel debris 5 and enter the lower concrete in the reactor containment vessel 2 Stop the underground water. The nitrogen 10 vaporized from the liquid nitrogen 9 is discharged from the nitrogen exhaust pipe 12, passed through a filter, removed from radioactivity, and released to the atmosphere. There is almost no pollutant release into the environment.
The fuel debris 5 is intermittently cooled from liquid nitrogen 5 through 30 copper 31 ice. Details are described in FIG.
When the fuel debris 5 is isolated and stored in the solid, the recovery operation is started. The reactor containment vessel lid 7 and the reactor pressure vessel lid 6 are released, and water is filled and frozen to form ice 14 for shielding the reactor pressure vessel upper lid, and a work floor iron plate 15 is provided on the upper portion thereof to maintain airtightness. Is installed. A plurality of recovery devices and excavation drill nozzles 16 are installed on the iron plate to stably protect against radiation even during recovery and excavation.
When the working floor iron plate is laid, the excavation drill 17 is used to excavate the fuel debris 5.
When excavating, a borehole 18 is installed. The probe of the collection device 19 is inserted through the borehole. The recovery device 19 will be described with reference to FIG. The fuel debris 5 is recovered by the recovery device 19,
The fuel debris 5 is stored in a fuel debris storage container 21, and the gas sent under pressure removes radioactive substances through a filter 22 and is released to the atmosphere.

FIG. 2 shows a method for cooling the fuel debris 5. Water is poured into the pedestal 3 from the cooling pipe 11 in the pedestal, the water level is raised to about half of the area, copper is mixed, and the ice particles containing copper that has a higher specific gravity than water and cooled with liquid nitrogen are covered on the water surface. To drop. Copper-filled ice particles reach fuel debris. Thereafter, liquid nitrogen is injected to cool and freeze the copper and water.
Three layers of a mixed layer 23 of copper, water vapor, and water, a mixed layer 24 of copper and ice, and a layer 25 of liquid nitrogen are formed, and the fuel debris 5 is stored in a cooled state.

  FIG. 3 shows the tip of the recovery device. Fuel debris stored in solids is injected from the probe tip like an endoscope into the fuel debris by high temperature steam, high temperature air, and liquid nitrogen, and the fuel debris is crushed. The debris is collected in a storage container arranged at the top of the reactor pressure vessel. High-temperature high-pressure water, high-temperature high-pressure air, and liquid nitrogen are discharged from the injection nozzle 26. A TV camera 27 is arranged for remote monitoring. The pulverized fuel debris equipped with the dosimeter 28 is collected through the pressure feeding tube 29 by the expanded gas.

  The present invention can be used to remove high-temperature hazardous materials and improve the efficiency of heat exchange.

1. 1. Reactor pressure vessel 2. Reactor containment vessel Pedestal4. 4. Pressure suppression chamber Fuel debris 6. 6. Reactor pressure vessel lid Reactor containment lid8. Water 9 Liquid nitrogen10. Nitrogen 11. Pedestal cooling tube 12. Nitrogen exhaust pipe 13. Pressure suppression chamber cooling pipe 14. 15. Reactor pressure vessel top shielding ice Floor iron plate for work 16. 18. nozzle for recovery device and drill for drilling Drill for drilling 18. Boring hole 19. Collection device 20. Recovery device tip 21. Fuel debris storage container 22. Filter 23. Mixed layer of copper, water vapor and water24. Mixed layer of copper and ice 25. Liquid nitrogen layer 26. Injection nozzle 27. TV camera 28. Dosimeter 29. Pumping tube 30. Copper 31. ice

Claims (7)

  While flowing water into and out of the reactor pressure vessel, reactor containment vessel, and pressure suppression chamber that have melted the core, they are filled with a refrigerant such as liquid nitrogen, dry ice, or cooling air, and the leaked cracks and gaps are filled with water. The construction method is characterized in that it freezes at the reactor and prevents leakage from the reactor containment vessel and the reactor pressure vessel.   Fill the pressure suppression chamber with ice particles, liquid nitrogen, dry ice, or cooling air, freeze the concrete in the lower part of the containment vessel, and stop the groundwater that has entered the concrete in the lower part of the containment vessel A construction method characterized by that.   A three-layer cooling structure with water containing copper on the fuel debris and ice particles containing copper on the top and liquid nitrogen on top of the fuel debris is safe without vaporizing liquid nitrogen and causing rapid expansion. This method is characterized by cooling the fuel debris and completely confining the fuel debris.   In heat exchange for cooling fuel debris, copper powder, which has high heat conductivity on both the low temperature side and high temperature side, is mixed with water and ice to improve the heat transfer and transfer area and increase the amount of heat exchanged. .   The trapped fuel debris is injected from the injection nozzle at the tip of the probe, such as an endoscope, with high-temperature steam, high-temperature air, or liquid nitrogen injected into the fuel debris, and the fuel debris is crushed and passed through the pressure feed tube in the probe, and the fuel debris powder A method characterized by collecting the waste in a storage container.   Inclusion of copper powder or bullet-shaped copper material in the ice particles increases the specific gravity, allowing the ice particles to sink to the bottom of the cooling water and cover the fuel debris, and is exposed to high-temperature high-pressure water or high-temperature air. Then, the copper powder jumps out of the ice into the space and collides with the fuel debris like blast, and the bullet-type material gets propulsive force by evaporation of the internal ice and collides with the fuel debris like a bullet and fuels. A method characterized by subdividing debris.   When exposed to high-temperature high-pressure water or high-temperature air, the copper and ice that covered the fuel debris also become hot and the ice evaporates to form a sponge, but the remaining copper forms a shell that can withstand high-temperature and high-pressure. Construction method to do.

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