JP3456783B2 - How to replace core shroud - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for replacing a core shroud installed in a reactor pressure vessel such as a boiling water type light water cooling reactor.

[0002]

2. Description of the Related Art As shown in FIG. 3, a core shroud 2 is supported by a shroud support 3 in a reactor pressure vessel 1 of a boiling water reactor. This core shroud 2
Is used to hold a core support plate and an upper grid plate (not shown) and wrap the entire fuel assembly to form a core.

A jet pump 4 is installed on a jet pump support base 5 in the annulus space between the core shroud 2 and the reactor pressure vessel 1. Reference numeral 6 in the figure indicates a control rod drive mechanism housing.

Generally, the core shroud 2 of this type of nuclear reactor is constructed by welding stainless steel members. When the carbon content of the core shroud 2 is high, cracks may occur in the welded portion or its vicinity due to stress corrosion cracking or the like during long-term operation. When such an event occurs, it is necessary to replace the used core shroud with a new core shroud (hereinafter referred to as a new core shroud) in order to ensure the safety of the nuclear reactor.

However, the core shroud that has been used for a long period of time is brittle due to neutron irradiation, and when welded, finer cracks may occur around the weld metal, making repair by welding difficult. There is. A method of connecting the reinforcing members with bolts is also conceivable, but its adoption may be limited in nuclear reactors installed in high earthquake areas.

Therefore, the most preferable method is considered to be replacement with the new core shroud 2, but the core shroud 2 itself installed in the reactor pressure vessel is welded to the ring of the support 3 of the core shroud 2 at the lower end. ing.

In addition, there is interference with the in-core stabilizer, the core instrumentation guide tube, etc., and the radiation pressure inside the reactor pressure vessel 1 is high, and the upper lattice plate and the core installed inside the core shroud. Position adjustment with the support plate is difficult.

In addition, since there are various problems such as difficulty in handling the removed core shroud outside the reactor pressure vessel 1 and difficulty in processing a large amount of waste, the core shroud has been heretofore used. Was expected to be extremely difficult to replace.

By the way, in recent years when a long time has passed after the practical operation of a nuclear reactor, it is urgent to replace an old core shroud that has been used with a new core shroud. Various techniques as disclosed in Japanese Patent Laid-Open No. 63-36195 have been proposed.

However, in the prior arts up to now, all the operations for welding the new core shroud must be performed by remote control in order to increase the activation inside the reactor pressure vessel. On the other hand, in order for workers to enter the new core shroud and perform welding work, it is necessary to decontaminate the reactor pressure vessel and shield the high activation area. However, both of these operations require enormous effort and time.

[0011]

Therefore, in order to improve the drawbacks of the conventional techniques, for example, the following techniques have been proposed. That is, after removing the upper lid of the reactor pressure vessel and the steam dryer, the steam / water separator is removed and the fuel, the nuclear instrumentation detector of the reactor, the control rod, the fuel support fitting, the control rod guide tube, the control rod drive mechanism, etc. The internal structure of the shroud is removed, and the water supply sparger installed above the core shroud, core spray system piping, guide rods and other equipment installed above the shroud are removed.

Then, the upper lattice plate and the core support plate are removed, and the incore guide tubes, stabilizers and other lower structures in the lower part of the core are cut and removed to open the inside and outside of the core shroud, and in this state, decontamination of the core is performed. Conduct to reduce the radiation level in the reactor below the standard value at which personnel work is possible.

After that, a work platform at the height of the upper end of the shroud support cylinder is installed inside the core shroud, and the old core shroud is cut by a cutting device at least in the vicinity of the welded portion with the shroud support cylinder.
Remove as one or multiple pieces. Finally, the core shroud is replaced, which is characterized by fixing the new core shroud on the shroud support cylinder by shaping the upper surface of the shroud support cylinder, suspending and centering the new core shroud.

However, the above conventional techniques have the following problems. That is, as shown in FIG. 3, a worker 7 performing a welding operation inside the new core shroud 2
Is a highly activated jet pump 4 and reactor pressure vessel 1
Since it is in the vicinity of, it is constantly exposed to the radiation 8.

Therefore, it is necessary to install the shield 9 between the jet pump 4, the reactor pressure vessel 1 and the worker 7. However, the shield thickness of the iron plate required to reduce the radiation level in the reactor to a value below the standard value at which personnel can work is about 10 times the thickness of the core shroud 2, and the procedure for installing and removing the shield 9 is complicated. In addition, there is a problem that it takes a lot of time and effort to remove it after use.

Further, the shield 9 needs to be provided with an opening 11 for installing the welding machine 10. Considering the influence of the radiation 8 leaking into the shield 9 from this portion, the worker 7 Needs to work away from the welder 10 and the weld 12 to an extent that is sufficient to reduce the radiation level to below a standard value (1 mSv / h) at which human work is possible.

Therefore, the operation of the welding machine 10 and the welded portion 12
All work, including inspections, must be performed remotely. This poses a problem that the merits of work efficiency and quality improvement due to the worker 7 entering the reactor pressure vessel 1 are significantly impaired.

In FIG. 3, reference numeral 13 is a furnace upper floor, 14 is an elevator, 14a is a rail, 15 is a capsule, 16 is a work platform, and a worker 7 rides on the capsule 15 and a rail 14a of the elevator 14.
Work platform 16 in the reactor pressure vessel 1 from the reactor upper floor 13
To get to work.

In addition, a large amount of gas is generated during welding, which becomes bubbles and rises to the surface of the water in the state of taking in the radioactive material, so that a special recovery device is required, which requires installation work and cost. is there.

The present invention has been made to solve the above problems, and when it becomes necessary to replace the core shroud in the reactor pressure vessel, the thickness of the iron plate required for shielding is reduced so that the inside of the new core shroud is reduced. Reduction of the shield installed in the
Core shroud replacement method that simplifies the procedure of installation and removal work, reduces the removal work after use, and efficiently replaces a new core shroud with a structure similar to that of a used core shroud in a short time. The purpose is to provide.

[0021]

In order to achieve the above object, in the core shroud replacement method according to the present invention, after removing the reactor pressure vessel upper lid and the steam dryer, the steam-water separator is removed and the fuel, In-core nuclear instrumentation detectors, control rods, fuel support fittings, control rod guide tubes, control rod drive mechanisms, and other structures inside the shroud are removed, and water spargers installed above the core shroud, core spray system piping, and guides The rod and other equipment installed above the shroud are removed, and the upper lattice plate and core support plate are removed, and the incore guide tube, stabilizer and other lower structures of the core lower part are cut off to open the inside and outside of the core shroud, Before cutting the core shroud, place the work platform at the height of the upper end of the shroud support cylinder inside the core shroud. Installed, and cut with the weld vicinity of at least shroud support cylinder by cutting device core shroud, and removal as an integral or divided bodies, performing furnace decontamination reduce furnace radiation levels,
In the shroud replacement method in which the new core shroud is welded onto the shroud support cylinder by shaping the upper surface of the shroud support cylinder, suspending and centering the new core shroud, the backing ring previously attached to the lower end of the new core shroud. After sealing welding between the shroud support cylinder and the shroud and installing a cylindrical shield inside the new core shroud, the inside of the shield is drained and put into the air, and the reactor pressure vessel, shroud support cylinder, backing ring,
It is characterized in that the water is enclosed by the new core shroud and the shield to form a water shield near the jet pump, which is a high radiation source, and a worker enters the shield container to perform welding work.

Further, the present invention is characterized in that a backing ring is welded in advance over the entire circumference of the lower end of the new core shroud, and seal welding is performed from the outside between the backing ring and the shroud support cylinder.

[0023]

[Function] By enclosing the vicinity of the jet pump, which is a high radiation source, with a shield and by water shielding, the thickness of the iron plate required for shielding is reduced, and the weight of the shield installed in the core shroud is reduced. Simplified installation / removal work procedure,
Also, the amount of disposal work after use can be reduced.

Further, due to the effect of water shielding, the amount of radiation leaking from the opening of the shield during welding work can be reduced, so that the worker can approach the welding portion, the finish and quality of the welding portion are improved, and work error and It is possible to prevent measurement errors, work for a short time, and reduce costs.

Further, clevis-like pseudo defects which may occur on the outer side of the weld when the conventional welding is performed only from the inner side can be prevented by previously performing the seal welding from the outer side.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the core shroud replacement method according to the present invention will be described with reference to FIGS. Figure 1
2 is for explaining an embodiment of a core shroud replacement method according to the present invention. FIG. 1 is a partial sectional view showing a state in which a backing ring and a shroud support cylinder are seal-welded, and FIG. It is a longitudinal cross-sectional view for explaining a welding operation state of a new core shroud and a shroud support in a reactor pressure vessel.

As shown in FIG. 1, a backing ring 27 is attached to the lower end of the new core shroud 2 in advance by welding over the entire circumference of the new core shroud 2. The lower end of the backing ring 27 is in contact with the shroud support cylinder 3. A remote automatic welding machine 28 is installed below the new core shroud 2, and the remote automatic welding machine 28 hangs down from a guide 29 mounted below the new core shroud 2.

In such a state, the remote automatic welding machine 28 carries out seal welding between the backing ring 27 and the shroud support cylinder 3 while moving the lower end of the new core shroud 2 along the guide 29 in a circumferential shape. .

Next, the welding operation state is shown in FIG. A new core shroud 2 is installed in the reactor pressure vessel 1, and the new core shroud 2 is fixed to an upper part of a shroud support cylinder 3. A work platform 16 is installed below the reactor pressure vessel 1, and inside the new core shroud 2,
A shield 18 and a center shield 19 are installed.

The center shield 19 is an intermediate platform
Suspended from 20, the intermediate platform 20 is supported by a shield support 21, and the shield support 21 is suspended from a flange portion 17 of the reactor pressure vessel. Jet pump 4
A seal 21a is installed at the upper end of the reactor pressure vessel 1,
The space enclosed by the shroud support cylinder 3, the backing ring, the new core shroud 2, and the shield 18 and the center shield 19 is filled with water 22.
To block water.

In this state, the worker 23 carries a welding machine and an inspection device (not shown), and approaches the welded portion 25 through the ladder 24 installed inside the shield 18. welded part
After completing the preset inspection for 25, start the welding operation. During the welding work, the radiation 8 from the jet pump 4, which is a high radiation source, is shielded by the water 22 and the radiation level in the working atmosphere is reduced to a value below the reference value at which personnel work is possible. After the welding work is completed, a preset inspection is performed, and the worker 23 carries a welding machine and an inspection device (not shown), passes through the inside of the center shield 19 and separates from the welded portion.

During work, the work condition and work safety are constantly monitored by the preliminary worker 26 from the intermediate platform 20 above the shield 18. After the welding work is completed, the inside of the center shield 19 is returned to a full state again, and the center shield 19, the shield 18, and the work platform 16 are lifted and removed in this order.

Using the backing ring 27, the backing ring 27 is preliminarily provided over the entire circumference of the lower end of the new core shroud 2.
When the backing ring 27 and the shroud support cylinder 3 are seal-welded from the outside, the seal welding is performed from the outside, and then the overlay welding is performed from the inside over the entire thickness of the new core shroud 2 (full-wall welding). Say). In this case, it can be expected that the welding is complete over the entire thickness of the new core shroud 2, so that there is an effect of increasing the welding strength.

On the other hand, when the backing ring 27 is not used, the lower end of the new core shroud 2 is grooved and directly welded to the shroud support cylinder 3. In this case, the tip of the groove is set at a position 3 to 4 mm apart from the upper end of the shroud support cylinder 3, and overlay welding is sequentially performed from the outside to the inside. In this case as well, full-wall welding is performed.

Since it is assumed that the welding of the outer portion is incomplete, strength cannot be expected over the entire wall thickness. However, the advantage of not using the backing ring 27 is that it does not require welding from the outside.

Therefore, in the above-mentioned embodiment, even when the backing ring is not attached to the lower end of the shroud and the groove with the lip is formed, the procedure similar to that of the above-mentioned embodiment can be carried out and the same effect as that of the above-mentioned embodiment can be obtained. To be

[0037]

According to the present invention, by shielding the vicinity of the jet pump, which is a high radiation source, with water, the thickness of the iron plate required for shielding is reduced, and the weight of the shield installed in the core shroud is reduced. It has the effects of simplifying the installation and removal work procedures and reducing the amount of removal work after use.

Further, the amount of radiation leaking from the opening of the shield during welding work can be reduced due to the effect of the water shield.
Workers can approach the welded portion, which improves the finish and quality of the welded portion, prevents work errors and measurement errors, shortens work time, and reduces costs.

Further, by installing the guide below the new core shroud, the remote automatic welding machine can be easily aligned, the seal welding work efficiency and the quality of the seal welded part are improved, and the new core shroud is improved. By monitoring the welding work situation by a preliminary worker from the top of the shield during the welding work of the shroud support cylinder and
A quick response in an emergency is possible, and the effect of improving the safety of welding work is achieved. Further, when the backing ring is used, the welding can be completed completely over the entire wall thickness of the new shroud, which has the effect of increasing the welding strength.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view showing a welded state of a core shroud for explaining an embodiment of a core shroud replacement method according to the present invention.

FIG. 2 is a vertical cross-sectional view for explaining a working state in a reactor pressure vessel according to an embodiment of the present invention.

FIG. 3 is a vertical sectional view for explaining a conventional core shroud replacement method.

[Explanation of symbols]

1 ... Reactor pressure vessel, 2 ... New core shroud, 3 ... Shroud support cylinder, 4 ... Jet pump, 5 ... Jet pump support base, 6 ... Control rod drive mechanism housing, 7 ... Worker, 8 ... Radiation, 9 ... Large tubular shield, 10
… Welder, 11… Opening, 12… Weld, 13… Furnace floor, 14…
Elevator, 14a ... Rail, 15 ... Capsule, 16 ... Work platform,
17 ... Flange part, 18 ... Shield, 19 ... Center shield,
20 ... intermediate platform, 21 ... shield support, 22
… Water, 23… Worker, 24… Ladder, 25… Welding part, 26… Preliminary worker, 27… Backing ring, 28… Remote automatic welding machine,
29 ... Guide.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Moto Kato, 2-4 Suehiro-cho, Tsurumi-ku, Yokohama-shi, Kanagawa Kanagawa, Ltd. (72) Inventor Sumi Ishikawa 2-Suehiro-cho, Tsurumi-ku, Yokohama No. 4 within Toshiba Keihin Office (72) Inventor Yuji Yamada 2-4 Suehiro-cho, Tsurumi-ku, Yokohama-shi, Kanagawa Within Toshiba Keihin Office (56) Reference JP-A-8-68889 (JP, A) ) JP-A-63-36195 (JP, A) JP-A-7-270577 (JP, A) JP-A-8-211185 (JP, A) JP-A-5-80187 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G21C 19/02 G21C 13/00 G21C 13/02

Claims (4)

(57) [Claims] 1. A reactor pressure vessel top lid is removed, a steam dryer inside said reactor pressure vessel is removed, and then a steam separator is removed and fuel, in-core nuclear instrumentation detector, control rod, fuel support Remove the metal fittings, control rod guide tubes, control rod drive mechanism and other internal structures of the shroud, and remove the water sparger installed above the core shroud, core spray system piping, guide rods and other equipment installed above the shroud. In addition, remove the upper lattice plate and core support plate, cut and remove the incore guide tube, stabilizer and other lower structures below the core to open the inside and outside of the core shroud, and before cutting the core shroud Install the work platform at the height of the upper end inside the core shroud, and at least shroud the core shroud with a cutting device. Cut in the vicinity of the weld with the loudspeaker support cylinder and remove it as one or more divided bodies, decontaminate the inside of the furnace to reduce the radiation level inside the furnace, shape the upper surface of the shroud support cylinder, and remove the new core shroud. In the method of replacing the core shroud by hanging and centering and welding the new core shroud onto the shroud support cylinder, seal welding is performed from the outside between the lower end groove portion of the new core shroud and the shroud support cylinder. After the cylindrical shield is installed in the new core shroud, the inside of the shield is drained to an air state, and is enclosed by the reactor pressure vessel, the shroud support cylinder, the new core shroud and the shield. The water near the jet pump installed in the reactor pressure vessel While subjected to shielding, replacing the core shroud, characterized in that to perform the welding operation within the shield. 2. The method of exchanging a core shroud according to claim 1, wherein in the seal welding step, the remote automatic welding machine is operated along a guide rail previously attached below the new core shroud. . 3. The core shroud replacement method according to claim 1, wherein a scaffold is installed on the upper part of the shield. 4. The backing ring is welded in advance over the entire circumference of the lower end of the new core shroud, and seal welding is performed between the backing ring and the shroud support cylinder from the outside. How to replace the core shroud.