JPS61172100A - Method of removing deposit of spent nuclear fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention mainly relates to an optimal method for removing limescale adhering to the surface of a spent nuclear fuel assembly loaded in a light water-cooled nuclear reactor.

[Background of the invention]

Conventional equipment, as described in Japanese Unexamined Patent Publication No. 59-90200, seals the nuclear fuel assembly in a drying chamber, removes reactor water, rapidly dries it with high-temperature, high-pressure gas, and peels it off, while forcing the reactor water. It was supposed to clean the fuel rods by supplying them with water. However, the calorific value of a nuclear fuel assembly, for example, one week after the reactor is shut down due to the decay of fission products, is approximately 10 kca Q.
/see and the surface of the fuel rods would become hot.

[Purpose of the invention]

The purpose of the present invention is to seal water scales attached to spent nuclear fuel assemblies in a container installed in a fuel storage pool, remove water from the container, dry the scales while cooling, and inject high-pressure air to dry the water scales. In addition to speeding up the process, the fuel rods are subjected to contact impact with high-pressure air, and cleaning water mixed with air is injected to remove limescale from the fuel rods with the force of extinguishing air bubbles, and the peeled scale is transported to a receiving tank. The goal is to provide a method to do so.

[Summary of the invention]

Traditionally, as a method for removing limescale adhering to nuclear fuel assemblies,
The use of chemicals, underwater high-pressure water injection method, ultrasonic method, and dry removal method were considered, but there were concerns that using chemicals would not only affect the nuclear fuel assembly itself, but also that Processing of chemicals is difficult. In addition, in the underwater high-pressure water injection method, water scale is not removed because the fuel rods arranged inside the nuclear fuel assembly are in the shadow of the outer fuel rods because the high-pressure water is attenuated in the water. Mainly, only a portion of the outer fuel rods are affected. On the other hand, even when ultrasonic waves are used, water scale can only be removed from the outer surface of the outer peripheral fuel rod, which is in contact with the ultrasonic waves, which is very inefficient.In addition, the water scale attached to the fuel rods has a spinel structure in the part near the surface of the fuel rod. Since it has a double structure with a strongly adhered limescale layer, it is difficult to remove the strong limescale layer only by injecting high-pressure water as described above. Based on the above, the best way to remove limescale is to expose the nuclear fuel assembly with limescale to the atmosphere and dry the scale, which removes the scale based on the difference in volume between the swollen scale and the dry scale. There is a law, but nuclear fuel assemblies are
There is an effect of decay heat due to the decay of nuclear fission products, and for example, the decay heat after one week of reactor shutdown is approximately 10 kca Q /
reach see. In order to dry the limescale adhering to the fuel rods in this state, the surface of the fuel rods becomes hot and requires cooling, which requires the use of cooling gas or a larger scale cooling system. However, spent nuclear fuel assemblies are stored in spent fuel storage pools for six months to one year before being transported for reprocessing, and during this time the surface temperature of the fuel rods is 150 to 150°C.
Focusing on this temperature drop, the present invention aims to provide a device for effectively removing limescale adhering to a cooled spent nuclear fuel assembly. The spent nuclear fuel assembly is sealed inside the container, and while the inside of the container is cooled with simple cooling equipment, the water inside the container is removed, air cooling is used to dry the limescale attached to the spent nuclear fuel assembly, and prevent temperature rise. do. Next, high-pressure air is injected to speed up drying, and the contact impact of the high-pressure air on the fuel rods causes limescale to be peeled off from the fuel rods. Furthermore, cleaning water mixed with air is injected into the fuel rods, and scale is again removed from the fuel rods by utilizing the extinguishing power of air bubbles. That is, the present invention is a method for removing scale by drying scale from a spent nuclear fuel assembly cooled in a spent fuel storage pool while cooling the pool in air, and utilizing contact impact and the vanishing power of bubbles.

[Embodiments of the invention]

FIG. 2 shows the basic structure of the nuclear fuel assembly 1. This nuclear fuel assembly 1 consists of a plurality of fuel rods 2 and 7 arranged in a square grid.
The fuel rod 2 is comprised of spacers 3, upper and lower tie plates 4, 5 for holding the fuel rods 2, a channel box 6 surrounding the fuel rod 2, and channel fasteners 7 for fixing the channel box 7 to the upper tie plate 4.

Cooling water flows through the channel box 6 from the nose piece of the lower tie plate 5, cools the fuel rods 2, and then flows toward the upper tie plate 4. If the fuel rod 2 is used for a long period of time, water scale 8 will adhere to the surface of the fuel rod 2.

FIG. 1 shows an embodiment of the present invention. Reference numeral 10 is a rectangular and elongated container equipped with a clamp-type opening/closing lid that is installed underwater, and a spent nuclear fuel assembly 9 with limescale attached therein is stored and sealed. do. The spent nuclear fuel assembly 9 is obtained by removing the channel box 6 from the nuclear fuel assembly 1 shown in FIG. 2, and FIG. 1 schematically shows this. container 1
Inside the 0, there are an adapter 11 for holding the spent nuclear fuel assembly 9, an intermediate support 12, an air sparger pipe 13, a cleaning water injection header pipe 14, and a cooling coil 15.
is provided. The container 10 also has a cleaning water supply system 21.
．． (22, 23, 24) and air supply system 18 (19, 2
0), a water discharge treatment system 25 (26 to 3o), and a thermocouple 17. The cooling coil 15 covers the entire span of the spent nuclear fuel assembly, and a cooling gas such as argon gas is sealed inside the cooling coil 15, and a motor 16 compresses the cooling gas to cool the entire container. On the other hand, the air is injected from the air sparge pipe through the air compressor 19, and the cleaning water is injected through the 6-valve 22. Further, the washing water is injected from the washing water injection header 14 via a valve 23 having a high pressure pump 24. A discharge treatment system 25 for water in the container 10 includes a drainage pump 28 for discharging water from an outlet 26 of the container 10 via a valve 27, a filter 29 for discharged water, and a treated water tank 30. Next, a method for removing limescale using this device will be explained.

(1) The channel box is removed from the spent nuclear fuel assembly 9 that has been cooled in the spent fuel storage pool for six months to one year, and the channel box is sealed and stored in the container 10 installed underwater.

(2) Operate the motor 16 to cool the inside of the container 1o.

(3) The temperature inside the container 10 is measured by the thermocouple 17, the valve 27 is opened, and the water inside the container 10 is discharged from the drain port 26 using the drain pump 28. The discharged water passes through a filter 29 and enters a treated water tank 30.

(4) Open the air valve 20 to adjust the pressure via the compressor 19, and then inject high-pressure air into the spent nuclear fuel assembly 9 from the air sparger pipe 13 to speed up the drying of scale and to bring the high-pressure air into contact with the fuel rods. The impact separates scale from the fuel rods.

(5) Stop the high-pressure air injection and lower the injection pressure set value to some extent. Next, open the valve 22 to send the pure water mixed with air to the air sparger pipe 13, and open the air valve 20 again to inject the air. Inject the pure water mixed with the spent nuclear fuel into the spent nuclear fuel assembly. As a result, water scale is peeled off from the surface of the fuel rod due to the impact of the pressurized pure water and the extinguishing force of the bubbles.

In this case, since the pure water is injected under pressure in the air, there is no pressure attenuation, and the water reaches the inside of the nuclear fuel assembly through the spacing between the fuel rods, and water scale can be efficiently removed by the impact force and the extinguishing force of air bubbles. .

(6) The drain pump 28 is operated to collect limescale remaining at the bottom of the container 10 from the drain port 26 through the valve 27 and into the filter while stopping it at the air valve 20 and the pure water valve 22.

(7) The high-pressure pump 24 is operated to inject wash water from the wash water injection header 14 into the spent nuclear fuel assembly via the valve 23. In this case, the purpose of cleaning spent nuclear fuel assemblies is to reverse the flow of cooling water from the lower side of the nuclear fuel assembly during operation and to flow cleaning water from the upper side to further remove limescale. It's been a long time.

〔Effect of the invention〕

According to the present invention, spent nuclear fuel assemblies can be cooled with very simple cooling equipment, scale can be effectively removed, and no chemicals are used.

It can be carried out inside the spent fuel storage pool in the Satoshi reactor building.

Additionally, since limescale is removed in a sealed container, worker exposure to radiation can be reduced.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing an embodiment of the apparatus for removing limescale from a spent nuclear fuel assembly according to the present invention, and Fig. 2 is a partial sectional view of the nuclear fuel assembly and a diagram showing the form of limescale adhering to the surface of the fuel rods. . DESCRIPTION OF SYMBOLS 1...Nuclear fuel assembly, 2...Fuel rod, 3...Spacer, 4...Upper tie plate, 5...Lower tie plate, 9...Spent nuclear fuel assembly, 10... - Sealed container, 11... Lower adapter, 12... Intermediate support, 14... Cleaning water injection header, 15... Cooling coil, 16... Cooling operation motor, 27... Valve, 2
8... Drain pump, 29... Filter, 30... Treated water tank.

Claims (1)

[Claims] 1. Spent nuclear fuel assemblies with limescale attached are sealed in a storage container with a clamp-type opening/closing lid inside a spent fuel storage pool, and the pool water inside the container is removed from the drain pipe connected to the container. The temperature inside the container is monitored by a thermocouple, and the inside of the container is cooled by a cooling coil built into the container, and water scale is dried while cooling the container.Then, high-pressure air is injected from a spargeer pipe installed in the container. In addition to speeding up the drying of limescale, the contact impact between high-pressure air and fuel rods causes limescale to be peeled off from the fuel rods, and cleaning water mixed with air is injected into the fuel rods, making use of the vanishing power of air bubbles. A method for removing limescale from spent nuclear fuel assemblies, characterized by peeling the limescale from the fuel rods again.