RU2253158C1 - Method for dismounting spent defective fuel assemblies - Google Patents

Abstract

FIELD: nuclear power engineering; handling radioactive wastes and spent nuclear fuel assemblies disposed in floating or coastal stores.

SUBSTANCE: proposed method used in particular for dismounting spent defective fuel assemblies disposed in store of floating technical depot includes removal of shielding plug from charging port of swivel slab combined with one of concentrically arranged rows of subchannels in tube plate holding fuel assemblies and detection of all defective fuel assemblies in this row. Upon removal of shielding plug swivel stab is rotated, charging port is aligned with head of box holding defective fuel assembly, and annular drilling is made in tube plate body through this port through depth exceeding weld penetration in tube plate, inner diameter of this drilling being slightly greater than outer diameter of bushing. After that box holding defective fuel assembly is separated from tube plate by blasting through its thickness value followed by its lifting from subchannel together with bushing over height of about 0.8 m from bottom edge of box bushing to top plane of swivel slab. Then auxiliary snap ring is fitted onto cylindrical surface of box, outer diameter of this ring being slightly greater than that of bushing, whereupon box is moved back downward and placed in same subchannel of tube plate. After that charging port of swivel slab is sequentially placed above each box holding defective fuel assembly in this row, and all above-described operations are resumed. Upon completion of all these operations all disrupted boxes are extracted one after the other together with bushings and auxiliary snap rings through charging port. Such method takes minimal time for extracting all defective fuel assemblies from floating technical deport store and ensures essential (by several times) reduction in gamma-ray dose absorbed by personnel and reduced escape of radionuclides into environment due to reduced time taken for auxiliary operations associated with specific process equipment installation on, alignment, and removal from working position.

EFFECT: reduced process time, enhanced safety for personnel.

2 cl, 8 dwg

Description

The invention relates to the field of nuclear energy, in particular to methods for the management of radioactive waste and spent nuclear fuel, and can be used to dismantle defective spent fuel assemblies (SFAs) located in floating or onshore temporary storage facilities.

At present, in the temporary storage facilities for spent nuclear fuel (SNF) located in floating technical bases (PTB), there are a large number of SFAs that must be dismantled for further processing or disposal.

For various technological reasons, some SFAs located in special cases and covers may be partially damaged (deformed), the fuel cell shells are leaky, their attachment points are corroded, separate mechanical destruction of SFA elements may occur, or the positions of some SFAs relative to the standard one may change. Therefore, the standard method of handling SNF, including gripping and retracting the assembly from the canister into a transfer container for transportation to reprocessing, cannot be used.

In addition, unloading from SFA temporary storage facilities with the above defects can lead to their destruction and, consequently, to uncontrolled release of radioactive substances into the environment (SNF spills, etc.), which increases the risk of radiation exposure for maintenance personnel, as well as deterioration of the production and environmental environment.

There is a method of dismantling defective SFA located in individual canisters in the storage facilities of the PTB. This method is described in the article “Lepse Floating Storage” (journal “Nuclear Technology Abroad”, 1997, No. 3, pp. 20-21). According to this method, the protective plug of the loading hole of the rotary plate is removed by rotating the rotary plate, this hole is centered relative to the case with the SFA and the condition of the upper parts of the SFA is evaluated.

In the future, according to one of the options, to ensure access to the SFA, the rotary plate is removed, after which the cases with defective assemblies are removed entirely using remotely controlled dismantling means. This method involves the removal of pencil cases after removing the turntable, which has large dimensions and weight, therefore, its removal requires a lot of labor. In addition, after removing the turntable, access to all SFAs, including those with various defects, is opened, as a result of which the release of radioactive products and the flux of γ radiation into the environment is possible. For the purposes of radiation safety of personnel, in this case, the most rapid removal of all canisters from the SFA using remote dismantling means (robotic systems) is required, which is associated with large economic costs.

There is also known a method of dismantling defective SFAs, set forth in patent No. 2154864 on “Method for dismantling defective spent fuel assemblies” and adopted as a prototype. According to this method, the protective plug of the loading hole located in the rotary plate above one of the concentrically arranged rows of cells with canisters with SFAs is first removed and, by rotating the rotary plate, defective SFAs are determined through this hole. Then, the loading hole is centered relative to the case with the defective SFA and through this hole an annular groove is cut out in the body of the tube plate, the inner diameter of which is slightly larger than the outer diameter of the sleeve, to a depth exceeding the penetration in the tube plate. After that, the pencil case is seized by the technological grooves available in the upper part of the pencil case, it is torn from the tube plate and removed through the aforementioned hole together with the defective SFA and the sleeve.

However, the removal of canisters with defective SFAs in this way is also quite laborious, since the total technological time spent on the determination of defective assemblies, cutting an annular groove, undermining and removing a blown canister consists not only of the machine time intended for these operations, but also preparatory time for installation, alignment and removal after the end of each operation of special technological equipment (STO), designed to support these operations. Moreover, the machine time of the described method is significantly (several times) less than the preparatory time.

The objective of the present invention is to minimize the technological time spent on dismantling all defective SFAs from the PTB storage, developing an effective and radiation-safe method of dismantling as much as possible.

The technical result in the implementation of the invention will be a significant (several-fold) reduction in the absorbed dose of gamma radiation by maintenance personnel and the release of radionuclides into the environment when dismantling defective SFAs from the storage facility by reducing the time of auxiliary operations of installing, centering and removing special technological equipment.

The specified technical result is achieved due to the fact that in the proposed method for dismantling defective SFAs from the storage located on the PTB, remove the protective plug of the loading hole of the rotary plate, combined with one of the rows of cells with assemblies concentrically located in the tube board and determine all defective assemblies located in this row. After that, rotating the rotary plate, center this loading hole relative to the head of the pencil case with a defective assembly and cut an annular groove in the body of the tube plate, the inner diameter of which is slightly larger than the outer diameter of the sleeve, to a depth exceeding the penetration in the tube plate. After this, the canister is blown up with a defective assembly from the tube plate by the value of its thickness and its subsequent rise from the cell along with the sleeve to a height of about 0.8 m from the lower edge of the pencil sleeve to the upper plane of the turntable. Then, a technological detachable spring ring is put on the cylindrical surface of the canister, the outer diameter of which is slightly larger than the outer diameter of the sleeve, and the canister is lowered back into the same cell of the tube plate. Next, the loading hole of the turntable is sequentially installed over each pencil case with a defective assembly in this row and all the above operations are repeated. At the end of these operations, through the loading hole, all the blown-up pencil cases are removed one by one, together with the bushings and technological split spring rings.

The operation of blasting a pencil case with a defective assembly from the cell of the tube plate by the value of its thickness is necessary to overcome the effect of “cold welding” (corrosion diffusion) between the outer surface of the sleeve of the pencil case and the inner surface of the cell.

The operation of lifting the canister from the cell tube plate to a height of 0.8 m from the lower edge of the sleeve to the upper plane of the turntable is required for the installation of a technological detachable spring ring covering the canister. The lift of 0.8 m was determined due to the optimal combination of the following factors: the position of the assembly when lifting relative to the turntable when its fuel part (95% of radioactivity) is under the rotary protective plate, as well as the ergonomic capabilities of the staff.

The installation of a technological detachable spring ring ensures that the sleeve of the pencil case does not jam when the pencil case is lowered back into the cell of the tube plate.

Thanks to the support of the surface of the ring on the surface of the annular groove, the exact alignment of the pencil case will be guaranteed, preventing it from tilting, which could occur if the butt end of the blasted sleeve had tears, burrs, etc. To enhance this effect in the particular case of the proposed solution, a spring ring with a horizontal supporting surface is used. Thus, in the final analysis, the subsequent reliable operation of gripping the reloading container when removing the canister will be ensured without additional work on centering the canister in the cell of the tube plate.

The operation of lowering the pencil case back into the same cell of the tube plate against the stop on the technological ring allows, without removing the service station to cut the ring-shaped groove, to move the loading hole of the turntable radially and center relative to the pencil case head following this radial row of cells with pencil cases, and, in conclusion, through the loading holes of the turntable, remove all the blown-up pencil cases with defective assemblies, in turn, together with the bushings and technological split spring rings.

The invention is illustrated by the following drawings:

figure 1 - General view of the storage;

figure 2 is a section of the storage on aa;

figure 3 is a diagram of a notch annular grooves;

figure 4 - diagram of the operation of undermining the canister with SFA;

5 is a diagram of the lifting case with a SFA and the installation of the process ring;

6 is a diagram of lowering the pencil case into the cell until the technological ring abuts against an annular groove in the tube plate;

Fig.7 - node support blown up pencil case on the technological ring on an enlarged scale;

Fig. 8 is a diagram of unloading a blown up pencil case and installing it in a cover of a transport container.

The storage consists of the following components: tank 1, tube plate 2, rotary plate 3, protective plug 4, loading hole (in the rotary plate) 5, pencil case 6, SFA 7 and sleeve 8.

The flange of the sleeve 8 is welded to the tube plate 2 and to the canister 6 containing the SFA. To install the rotary plate 2 in a certain position, there is a latch 9.

All pencil cases in the tube plate are located on concentric circles (figure 1). One concentric opening 5 is provided for each concentric row, closed by a protective plug 4.

The proposed method can be implemented as follows.

From the loading hole 5, the protective plug 4 is removed and, by rotating the rotary plate 3, visually determine through the cameras (not shown) all defective assemblies in this row. Then center the loading hole 5 relative to the pencil case 6 with a defective assembly. Checking this alignment can be done using a caliber. In the installed position, the rotary plate is fixed with a latch 9.

Next, an annular groove 12 coaxial with this pencil case is cut out in the body of the tube plate. The cutting operation can be performed, for example, by means of an annular drill 10 fixed in the spindle 11 of the device for cutting pencil cases with spent defective fuel assemblies (decision to grant a patent dated 05.06.03 for an invention according to application No. 2002103837/06 of 02.11.2002) .

Cutting the annular groove 12 in the body of the tube plate 2 and the flange of the sleeve 8 is made so that the inner diameter of the groove is slightly greater than the outer diameter of the sleeve 8 to prevent failure of the canister 6 in the storage tank 1 (see Fig. 2). In this case, the depth of the groove must exceed the depth of penetration in the tube plate 2 (see Fig. 3). The penetration depth is determined by a method known in the art.

Undermining and removing the pencil case 6 is as follows. A blasting device 13 with a hand grip is installed in the loading hole 5 of the rotary plate 3. The hand grip 14 hooks the pencil case 6 into the technological grooves and the pencil case 6 is power blasted from the tube plate 2 by the value A of the thickness of the tube plate 2 (see FIG. 4).

The standard lifting and transport device 15, the blasting device 13, together with the canister with the SFA, is raised to a height B of about 0.8 m from the lower edge of the sleeve 8 to the upper edge of the turntable 3 (see Fig. 5). Then put on the surface of the canister 6 technological split ring 16, while if it has a flat surface 17, then it is placed below. After that, a standard lifting and transporting device 15 lowers the blasting device 13 to the stop of the process ring 16 on the horizontal surface of the annular groove 12 of the same cell of the tube plate 2 (see Fig.6).

The wrist grip 14 of the blasting device 13 is unclenched, maintaining the alignment of the pencil case due to horizontal contact - the rings 16 and the annular groove 12 abut against each other (see Fig. 7). After that, the hoisting device 15 raises the blasting device from the cavity of the loading hole 5 in the turntable. In this case, the case 6 with the SFA together with the process ring 16 remains in the annular groove 12 of the same cell of the tube plate 2.

Then the loading hole 5 of the rotary plate 3 is moved radially and centered relative to the head of the pencil case 6, following this radial row of pencil cases with a defective assembly. In this position, all the above operations are also carried out, and so all cells of this radial row pass sequentially.

At the end of work in this row, through the loading hole 5 of the rotary plate 3, one by one, using the guiding funnel 18, the gripper 19 and the reloading container 20, remove all the blown up canisters 6 with defective SFAs together with the bushings 8 and process rings 16 (see Fig. 8 ) and load them into the corresponding nests of the cover 21 with the emphasis of the ring 16 on the horizontal surface of the spacer lattice 22 of the cover, which provides the specified positions of the cases in the cover located in the transport container 23 or in a regular coastal or floating storage looking for SFA.

Claims (2)

1. A method of dismantling defective spent fuel assemblies from the storage of a floating technical base, in which the protective plug of the loading hole of the rotary plate is removed, which is combined with one of the rows of cells concentrically located in the tube board with assemblies in the canisters and by visually identifying the defective assembly in this hole through this hole In a row, an annular groove with an inner diameter slightly exceeding the outer diameter of the bushing through which the pencil case is cut installed in the tube plate, then undermine the pencil case from the tube plate by the value of its thickness and remove it from the storage together with the bushing, characterized in that after undermining the said case, it is lifted together with the sleeve from the tube plate cell to a height of at least 0.8 m from the lower edge of the sleeve to the upper plane of the turntable, a technological detachable spring ring is put on the pencil case, the outer diameter of which is slightly larger than the outer diameter of the sleeve, and the pencil case is lowered back into the cell of the tube plate, after which the open loading hole of the turntable is sequentially installed over each defective assembly in this row and all the above operations are repeated, and then all the blown-up canisters are successively removed through the loading hole together with the bushings and process rings. 2. The method of dismantling defective spent fuel assemblies from the storage of the floating technical base according to claim 1, characterized in that the cross section of the said spring ring is a circle with a segment cut parallel to the plane of the ring, the height of which is less than the radius of the circle, and the ring with the cut surface is installed in the groove in the pipe board.

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