RU2098871C1 - Method for separating and replacing process channels in fuel assemblies of pressure-tube reactors - Google Patents

Abstract

FIELD: laser processing in nuclear power engineering. SUBSTANCE: method involves cutting spent process channel from upper plate, carrying it to separation zone wherein it is separated into low-reactivity and high-reactivity wastes. Then new process channel is conveyed to reactor core, laced in bottom plate shell and welded to top plate. All procedures are made inside channel by using laser beam. EFFECT: reduced number of tools used in procedure, eliminated manual labour and radioactive dust, reduced space requirement for waste disposal and reduced size and cost of waste storages. 5 cl, 5 dwg

Description

 The invention relates to the field of nuclear energy and can be used to replace the technological channels of fuel assemblies and similar structures (CPS channels, ionization chambers, etc.), channel-type reactors when they fail or are completely dismantled.

 It is known that during the operation of RBMK-type nuclear reactors, the radiation emitted by fuel elements (TVEL) affects the surrounding technological channel (TC). This leads to a gradual destruction of the crystal lattice of the material of the walls of the TC and, consequently, to a sharp decrease in ductility and viscosity, i.e. to embrittlement. As a result, the tightness of the fuel cell is violated, which can lead to leakage of the radioactive coolant. The embrittlement effect is further enhanced by the fact that upon irradiation, the microvolumes of the metal are heated, and then they experience self-hardening upon cooling.

To control the tightness of the technological channels, the TC integrity system (CCTC) is used, which controls the humidity and temperature in the area between the masonry and the TC. When an accident is detected, the reactor is stopped, the emergency fuel cell is cut out and replaced with a new one [1]
It is also known that the estimated life of the reactor is 30 years [2] After the completion of the life of the reactor, the reactor is dismantled and the spent equipment is replaced with new ones. Replacing equipment is a complex and expensive operation, further complicated by the fact that the equipment being replaced has high radioactivity. Therefore, to carry out the dismantling operation, the development of special, remotely controlled machines and mechanisms is required. In addition, for the preliminary and final storage of spent equipment, sufficiently large tanks (storages) are needed. One of the dismantling operations is the removal of fuel cells from the reactor core. TK is a vertically mounted pipe with a diameter of 90 mm and a wall thickness of 4 mm. The central part of the fuel cell located directly in the reactor core is made of heat-resistant zirconium alloy, two peripheral ones made of stainless steel.

 The operation of removing TC is as follows. According to the control signal of the operator, the unloading and loading machine (REM) moves in the reactor zone to the required fuel cell. The RZM spacesuit lowers and captures the assembly located in the TC. After that, using REM, the assembly is transferred to the storage of spent assemblies [3] Next, the REM is moved to the place of the freed TC. TK is manually cut off from the upper plate tract and removed from the reactor. REM moves the TC to the cutting zone. Cutting TC is as follows. To access the cutting site, the outer surface of the TC is previously cleaned of graphite remaining on its surface. This operation is laborious and, in addition, a lot of radioactive dust is generated during cleaning. After that, the steel parts of the fuel cell are mechanically cut off from the central zirconium and then the steel parts are transported as low-radioactive in the radioactive waste storage facility (CWO), and the central zirconium part having high radioactivity is transported to the holding pool, followed by transportation to the CRO.

 In the case of dismantling the entire reactor, this operation is performed sequentially for all the fuel cells located in it.

 When changing the emergency fuel cell in a running reactor, the reactor is stopped, fuel assemblies are removed, cut off from the top plate of the fuel cell reactor and transferred to the cutting zone and then to the holding pool and XPO. After that, from the weighing unit, the new fuel cell is transferred to the reactor, installed on the base plate of the cell on the bottom plate of the reactor and manually welded to the upper plate from the outer surface [4] Next, the rare-earth metal component installs the next fuel assembly with fuel rods in the new fuel cell.

The disadvantages of the known method of replacing TC are:
1. The need to use various equipment for TC replacement operations:
for cutting TC,
for welding a new TC,
for cleaning the surface of spent fuel from radioactive graphite,
for cutting TC into the central and peripheral zones.

2. The manual method of welding and cutting TC. The consequence of this is:
a) low productivity of the process;
b) the need for workers to be located directly in the reactor zone;
c) low quality of the seam (manual or arc welding methods), leading to lower reliability of the TC;
d) the need to supply electricity with high parameters directly to the reactor zone
3. Removal of residual graphite for subsequent cutting from the surface of the spent fuel, leading to the occurrence of radioactive dust.

 4. The existing method of cutting fuel into three parts with subsequent storage in the chemical waste disposal is irrational in terms of waste volumes, because a pipe with a small wall thickness S = 4 mm and, therefore, a small volume of the metal itself occupies a large total volume. This leads to the well-known problem of the need to build large-volume HWP, which accordingly leads to large investments.

 The tasks to be solved by the invention are the exclusion of manual labor from the removal and installation of a new fuel cell, which leads to an increase in the productivity of the process, an increase in the quality of the weld and, therefore, an increase in the reliability of the fuel cell and the entire reactor as a whole, and an increase in work safety by eliminating the presence of personnel in the reactor core, as well as electricity with high parameters; elimination of the occurrence of radioactive dust during the process of cutting spent fuel, a more compact placement of radioactive waste in the radioactive waste, a significant reduction in equipment used for the replacement of fuel.

 The above objectives of the invention are solved due to the fact that a segment of the spent TC and welding of a new lead from the inside with a laser beam passing through the radiation transportation system and further inside the TC; cutting TC lead the same beam using rotational and translational (vertical) movement; the division of the TC into the central and peripheral parts is conducted by the same beam.

 The laser can be located on the fixed parts of the rare-earth metals or closer to the place of cutting on the moving parts of the rare-earth metals. The beam can be supplied to the cutting zone using swivel mirrors or a light guide.

 The cutting and welding of the technological channel to the upper plate with a laser beam eliminates the presence of personnel directly in the reactor zone, the manual labor used in this operation, as well as the supply of electricity with high parameters, due to the fact that the laser beam can be transported from the laser for considerable distances.

 Welding the TC to the top plate with a laser beam will provide a higher quality of the weld, which will increase the reliability of the TC and the reactor as a whole.

 The cutting of the spent fuel cell into the central and 2 peripheral parts is carried out from the inside of the fuel cell, and not from the outside. Knowing the thickness of the TC, its material, it is not difficult to choose cutting conditions (radiation power, focal length, cutting speed) so that the metal is cut strictly to the thickness of the TC, and graphite on its surface is not cut. This eliminates the operation of removing graphite from the surface of the TC and, consequently, the formation of radioactive dust.

 After cutting the TC in a new way, separation into the central and peripheral parts will not be difficult, because even if the parts cut into metal are connected due to the graphite remaining on their surface, their separation from each other does not present serious difficulties due to the low strength and high fragility of graphite. Radioactive dust does not occur.

 The pipe can be cut using a laser beam passing through it, which can vertically move and rotate in a horizontal plane, can be made into segments having any arbitrarily small size and any shape and which can be nested into each other. This ensures a compact storage of radioactive waste compared to solid pipes and, consequently, a significant reduction in XP.

 The use of a laser beam as a tool for all the above-mentioned TC replacement operations will significantly reduce the amount of equipment used, reduce auxiliary time, eliminate the work of personnel of various professions (welders, carvers, etc.), irradiate it, and reduce the number of possible equipment failures.

 The radiation inside the TC can be supplied using a fiber, which will greatly facilitate its supply to hard-to-reach places.

 To facilitate the exit of gaseous products of laser cutting of TC, it is possible to supply radiation coaxially.

 It is possible to supply radiation inside the suit. This eliminates additional devices for replacing and cutting TC.

 An example of a specific application of the proposed method.

 Nuclear reactor RBMK-100 (RBMK-1500). The REM of the reactor is a bridge structure and a spacesuit (see, for example, [1] p. 221). REM workspace reactor core, cutting sites for fuel assemblies, fuel elements, etc.

 The laser beam from the laser, which can either be located outside the reactor zone on the stationary parts of the rare-earth metals, or on the moving parts of the rare-earth metals, for example, directly next to the spacesuit, is fed through the radiation transportation system to its focusing system, and then to the processing zone. According to the design, the nodes for delivering radiation to the processing zone, as well as the mechanisms of vertical movement and rotation of the focusing device, can be different.

 An example of a design for implementing the method (Fig. 1): 1 - REM (schematically), 2 radiation transportation system, 3 rotary mirrors, 4 laser, 5 mechanism for vertical movement of the focusing device, 6 mechanism of rotation of the focusing device around a horizontal axis, 7 focusing device , 8 TC capture mechanism.

 An example of a specific application of the proposed method.

 Nuclear reactor RBMK-1000 (1500). The REM of reactor 1 is a bridge structure (see, for example, [1] p. 221), which has vertical spacesuit manipulators for unloading and loading fuel 8 and a device for working with fuel cells. At the signal of the operator, the REM brings the spacesuit to the required fuel cell and the fuel assemblies and fuel elements are unloaded. After this, the REM brings the device for working with the TC to the vacant TC and the vertical movement mechanism 5 sets the focusing device 7 inside the TC so that its nozzle would be at the height of the cut. The laser 4 and the laser beam are turned on by the radiation transport system 2, deviating on three rotary mirrors and focusing in the focusing device 7, is directed to the inner wall of the TC. By rotating the focusing device using the rotation mechanism around the horizontal axis 6, the TC is cut out from the upper plate. The laser turns off. After that, the TC 8 capture mechanism captures the cut-out TC, removes it from the reactor, and the rare-earth metals transfers the TC to the cutting zone. Cutting TC can be carried out as follows. The vertical movement mechanism 5 lowers the focusing device 7 inside the TC to the very bottom. With a joint reverse vertical and rotational movement of the focusing device, as shown in Fig.2 (trajectories A and B), diamond-shaped "scales" are formed. The shape of the flakes is shown in FIG. "Scales" fall under their own weight down into the container intended for them. Moreover, having low strength and high brittleness, graphite breaks. The cutting is carried out from inside the TC, and the processing modes (radiation power, focal length, cutting speed) are selected so that only the TC metal is cut through and, therefore, no radioactive dust is generated from the TC located on the outer surface. The shape of the flakes allows them to be placed much denser than a solid TC having the shape of a pipe.

 By successively rising, the focusing device performs the cutting of the lower low-radioactive part of the TC.

 After this, REM moves the remaining part of the fuel cell to the waste treatment area with high radioactivity. The cutting cycle of the middle part of the TC with high radioactivity is similar to the above.

 After cutting the middle part, REM moves the remaining upper part to the cutting zone of low-level radioactive waste and the cycle repeats again. This completes the TC cutting. In the case of dismantling the reactor after cutting the fuel cell, the rare-earth metal component again moves the spacesuit and the device for working with the fuel cell to the active zone and the entire cutting and cutting cycle is repeated for all fuel cells.

 The trajectory of cutting TC may be different. Figure 4 shows the vertical cuts (cuts B) along the radius of the TC. After cutting with ring cuts at different heights (cuts B), rectangular waste is generated (Fig. 5), which also provides a high storage density.

 In the case of replacing the TC after cutting the REM from the weighing unit, it captures a new TC and transfers it to the place of the cut out. The lower end of the TC is installed in the support cup of the cell of the lower reactor plate. The focusing system moves vertically to the welding site. At the same time, the laser and the rotation drive of the focusing system are turned on and the upper part of the TC is welded to the upper plate of the reactor. After that, in the usual way, fuel assemblies with fuel elements are loaded.

 This completes the replacement cycle.

 Drives and actuators for vertical movement and rotation around the horizontal axis of the focusing device can be performed in various ways: pinion gear, rolling screw-nut, linear actuator, etc.

Literature
1. Margulova T.Kh. Nuclear power plants. -M. High School, 1984, p. 112.

 2. Margulova T.Kh. Nuclear power plants. -M. High School, 1984, p. 24.

 3. Monakhov A.S. Nuclear power plants and their technological equipment. -M. Energoatomizdat, 1986, p. 159-160.

 4. Girnis V.V. Fedulov V.A. Filatkin G.F. and other installation of equipment of nuclear power plants. -M. High School, 1980, p. 93-97.

Claims (5)

 1. The method of cutting technological channels of fuel assemblies of channel type nuclear reactors, in which the replaced technological channel is cut from the reactor vessel, transferred to the cutting site, cut, transferred to storage, a new channel is mounted in the reactor vessel, characterized in that the channel is cut into vertical position from the inside with laser radiation to a depth equal to the thickness of the channel wall, while the channel walls are divided into segments that are put into each other during storage, and the cut segments sequentially from bottom to top along the length of the channel.  2. The method according to claim 1, characterized in that when dividing the walls of the channel into segments, longitudinal cuts are made along the length of the channel and subsequent horizontal circular cuts.  3. The method according to claim 1, characterized in that when dividing the walls of the channel into segments, intersecting sawtooth cuts are performed.  4. The method according to PP.1 to 3, characterized in that the radiation in the processing zone is carried out using a fiber.  5. A method of replacing the technological channels of fuel assemblies, in which the channel is cut from the reactor vessel and the new channel is welded into the reactor vessel, characterized in that the channel is cut and the new channel is welded by laser radiation from the inside of the channel.

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