RU2265902C2 - Method and device for drying nuclear fuel pellets after wet grinding - Google Patents

Abstract

FIELD: nuclear engineering; manufacture of nuclear fuel pellets for charging fuel elements of nuclear reactors.

SUBSTANCE: each trough is filled with pellets in bulk to 85% of its volume and drying is made in three zones under different temperature conditions. Troughs are displaced at linear speed of maximum 0.04 m/s. Gaseous nitrogen is heated and supplied to drying zones under pressure, drying is effected until hydrogen content of pellets becomes not over 6 x 10^-5%. Pellet drying device is provided in addition with heater and control system. Drying unit is produced from retort in the form of square-section horizontal pipe enclosed by heating wires and thermal converters. Charge-discharge units at retort ends have doors set in motion by means of pneumatic cylinders. Conveying facility is, essentially, conveyer with levers, heater is heated pipe feeding hot gaseous nitrogen to retort, and control system is built around programmable controller.

EFFECT: enhanced quality and operating reliability of pellets, enhanced capacity of pellet drying device.

3 cl, 3 dwg

Description

The invention relates to nuclear energy and may find application in enterprises manufacturing tablets of nuclear fuel for equipment of fuel elements for nuclear reactors.

It is known that in the process of manufacturing nuclear fuel tablets after sintering and wet grinding, the tablets are dried either with hot air at a temperature of about 120 ° C or in an atmosphere of nitrogen at a higher temperature (200-300 ° C) (see Development, production and operation of fuel elements power reactors. Book 1. Edited by F. G. Reshetnikov. - M.: Enogoatomizdat, 1995, p. 102). A mandatory characteristic of manufactured tablets is the total content of hydrogen in them, or less often, separately moisture and hydrogen (see ibid., P. 104).

It is known that fuel elements (fuel elements) have a shell made of an alloy of zirconium with niobium, and zirconium has a high chemical activity and a tendency to interact with hydrogen, with the formation of hydrides, embrittlement of the cladding of a fuel element, leading to depressurization.

The source of hydrogen inside the fuel rod is tablets containing adsorbed hydrogen and moisture (see ibid., Pp. 170-172).

In the known method (p.102), the essence of the method, in addition to drying in a nitrogen medium at 200-300 ° C, is not disclosed and the device is not disclosed.

The closest in technical specifications and the achieved effect is a method of drying tablets of nuclear fuel after wet grinding, including moving them in mesh boats and drying in a nitrogen medium at 200-300 ° C (see patent of the Russian Federation RU 2158030, IPC 7 G 21 C 3 / 62, 21/10 of November 18, 1998, published on October 20, 2000. A method of manufacturing pelletized fuel for fuel elements and a device for its implementation).

When unloading hot tablets of nuclear fuel, and the prototype method does not provide for their cooling, the tablets of nuclear fuel absorb moisture from the air, while the equilibrium content is reached relatively quickly - within 1-1.5 hours, regardless of the density of the tablets. With modern, well-developed technology for the production of fuel elements, the interaction of nuclear fuel tablets with a zirconium alloy shell is considered to be the main cause of premature depressurization of fuel elements, since the presence of hydrogen or moisture in the tablets leads to hydrogenation of the shell, its embrittlement and under the influence of stresses caused by temperature changes in the nuclear reactor , fuel elements can be destroyed (see ibid., pp. 86-87).

The closest in technical essence and the achieved effect is a device for drying tablets of nuclear fuel after wet grinding, containing a drying unit and a vehicle for transporting through it nuclear fuel tablets in mesh boats in a nitrogen environment (see patent of the Russian Federation RU 2158030, IPC 7 G 21 C 3/62, 21/10 of 11/18/1998, publ. 10/20/2000. A method of manufacturing pelletized fuel for fuel elements and a device for its implementation).

In the prototype patent, the device for drying tablets of nuclear fuel after wet grinding is not structurally disclosed.

As already mentioned, in the prototype patent, the device for drying nuclear fuel tablets after wet grinding contains a drying unit and a vehicle for transporting nuclear fuel tablets through it in mesh boats, but the device control system is not disclosed.

From practice it is known that the relay control system widely used in industry has such disadvantages as:

1) a small amount of time between failure for individual parts and components of the system, so the system is not reliable enough;

2) a frequent exit of the control system from a failure associated with the burning of relay contacts;

3) large dimensions (bulkiness), which requires the placement of the control system in several cabinets;

4) the control system of the drying unit requires frequent maintenance and repair due to the short time between failures of individual elements of the system;

5) there is no process visualization system;

6) high material costs for maintaining and upgrading equipment;

7) there is no possibility of rapid changes in the parameters of the technological process of drying tablets of nuclear fuel;

8) high power consumption;

9) low speed;

10) the ability to control temperature only according to the on-off law;

11) the inconvenience of recording and storing information on the progress of the technological process (see "The control system for the installation of drying tablets." Operation manual. Development OKPNO Novosibirsk plant of chemical concentrates 561.092.00.000 RE from 10/13/1998).

The technical task is to improve the quality of nuclear fuel pellets, the reliability of the operation of fuel rods in a nuclear reactor and increase the productivity of the device for drying nuclear fuel pellets by increasing the reliability of its operation.

This technical problem is solved by the fact that in the method of drying the tablets of nuclear fuel after wet grinding, including moving them in mesh boats and drying in a nitrogen environment, according to the invention, tablets are poured in bulk into each boat, not more than 85% by volume of the boat, drying of tablets of nuclear fuel carried out in at least three zones, where a temperature of 300 ± 10 ° C is maintained in the first zone, a temperature of 200 ± 20 ° C is maintained in the second zone, and in the third zone, a temperature of 120 ± 20 ° C is maintained, moving mesh boats with tablets through s Drying is carried out with a linear speed of not more than 0.04 m / s; gaseous nitrogen is preheated to a temperature of not less than 120 ° C and fed into the drying zone under pressure, MPa (kg / cm ³ ) 0.15-0.16 (1, 5-1.6), and the nuclear fuel pellets are dried to a residual hydrogen content of not more than 6 × 10 ^-5 %.

This technical problem is solved in that in the device for drying nuclear fuel tablets after wet grinding, comprising a drying unit and a vehicle for transporting nuclear fuel tablets through them in net boats in a nitrogen medium, according to the invention, the device for drying nuclear fuel tablets after wet grinding is additionally equipped with a heater and a control system for a device for drying a tablet of nuclear fuel, where the unit for drying a tablet of nuclear fuel is made of a retort in the form of a horizontal square pipe, covered by at least three wire heaters with thermal converters, at the ends of the retort there are loading and unloading units with doors driven by pneumatic cylinders, the vehicle for conducting net boats through the tablet drying unit is made in the form of a conveyor with levers spacing the boats on the conveyor, air heater made in the form of a heated pipe for supplying hot gaseous nitrogen to the retort, and the control system for the device for drying nuclear fuel tablets is made on the basis of the program Ammountable controller.

Another difference is that the SIMATIC S-300 programmable controller is used as a programmable controller, which is electrically connected to the electrical equipment of the tablet drying unit, the conveyor drive, the electric heater, sound and light signaling devices. This embodiment of the method and device for drying nuclear fuel tablets after wet grinding will improve the quality of nuclear fuel tablets and the reliability of the fuel rods equipped with these tablets by achieving a total hydrogen content in the tablets of 2 × 10 ^-5 % instead of 6 × 10 ^-5 %, acceptable TU.

The new control system based on the industrial controller SIMATIC S7-300 drying device for nuclear fuel tablets after wet grinding allows you to:

1) to significantly increase the reliability of the device for drying a tablet of nuclear fuel by eliminating mechanical switching contacts when regulating temperature conditions, which significantly reduces the cost of maintaining the operability of the equipment and increases its productivity;

2) implement various laws of process control, which ultimately significantly affects the improvement of product quality;

3) to work out the process flow clearly and informatively;

4) be able to reprogram the controller of the operational change of the process parameters;

5) through the unification of the automation system to be able to upgrade equipment;

6) have a small size control system, low power consumption and high speed, the convenience of recording and saving information and transmitting data on the process for processing on an electronic computer.

The drawings show a device for drying tablets of nuclear fuel after wet grinding, where:

- figure 1 is a General view of the device for drying tablets

- figure 2 is a device for drying tablets (side view);

- figure 3 is a download node of the device for drying tablets.

The device for drying tablets of nuclear fuel after wet grinding contains a drying unit 1, a vehicle 2 for passing through it nuclear fuel tablets in mesh or perforated boats 3 in a nitrogen atmosphere.

The device for drying tablets of nuclear fuel after wet grinding is additionally equipped with a heater 4 and a control system for drying a tablet of nuclear fuel tablets.

The unit 1 for drying the tablets of nuclear fuel is made of retort 5 in the form of a horizontal square pipe covered by at least three wire heaters 6, 7, 8 with thermal converters 9, 10, 11. At the ends of the retort 5 there are loading units 12 and unloading 13 s doors 14, 15, driven by pneumatic cylinders 16, 17.

Vehicle 2 wiring mesh or perforated boats 3 through the tablet drying unit 1 is made in the form of a conveyor with levers 18 spacing boats 3 on the conveyor.

The heater 4 is made in the form of a heated pipe 19 for supplying hot gaseous nitrogen to the retort 5.

The control system for the drying device for drying nuclear fuel pellets is based on programmable controller 20. As programmable controller 20, the programmable controller SIMATIC S7-300 is used, which is electrically connected with the electrical equipment of the tablet drying unit 1, the conveyor drive 2, the electric equipment of the air heater 4, and sound and light signaling devices.

Retort 5 of unit 1 is mounted on frame 21 with adjustable legs.

Thermocouples 9, 10, 11 are designed to automatically maintain temperatures in each of the three heating zones by wire heaters 6, 7, 8. From the side of the loading unit 12, the retort 5 has a pipe 22.

On the sides of the retort 5 to control the presence or absence of the boat at the beginning or end of the retort, light sources 23, 24 and photosensors 25, 26 are installed.

The method of drying tablets of nuclear fuel after wet grinding is carried out in the device as follows.

After wet grinding, the nuclear fuel tablets are loaded into boat 3 in bulk no more than 85% by volume of boat 3 and transported to the unit.

The loading unit 12 on the retort 5 of the unit 1 is opened by lifting the door 14 with the pneumatic cylinder 16, and a mesh or perforated boat 3 is loaded with the pellets of nuclear fuel loaded into the open loading unit 12. After closing the door 14 with the pneumatic cylinder 16, the boat 3 is transported by vehicle 2 in the form of a conveyor with levers 18 with a speed of not more than 0.04 m / s through an area where, with the help of heaters 6 and a temperature converter 9, a temperature of 300 ± 10 ° С is maintained with a simultaneous supply of retort 5 from a heated tube 19 of a nitrogen gas air heater 4 with a temperature of at least 120 ° C with a gas outlet through the pipe 22 of the loading unit 12 together with moisture vapor from the tablets.

Then, boat 3 with tablets, at the same speed of 0.04 m / s, moves through the second zone, where, with the help of heater 7 and thermal converter 10, the temperature is maintained at 200 ± 20 ° С, and the next mesh comes into the zone with a temperature of 300 ± 10 ° С or perforated boat 3 with nuclear fuel tablets.

After moving through the third zone with a temperature of 120 ± 20 ° C, supported by a heater 8 and a thermal converter 11, a mesh boat with nuclear fuel pellets having a residual hydrogen content of 2 × 10 ^-5 % instead of 6 × 10 ^-15 %; according to the specification, through the unloading unit 13, when the door 15 is opened with the help of a pneumatic cylinder 17, it is unloaded from the retort 5, into which hot gaseous nitrogen is constantly supplied under pressure, MPa (kg / cm ³ -0.15-0.16 (1.5-1, 6). After unloading the boat 3, the pneumatic cylinder 17 closes the door 15 and the drying cycle continues, and the unloaded dry tablets in the nitrogen environment in the box are cooled.

The control system is based on a programmable controller 20 SIMATIC S7-300, which is electrically connected to the electrical equipment of the unit 1 drying tablets, the conveyor drive 2, the electrical equipment of the air heater 4, sound and light signaling devices, with light sources 23, 24 and photo sensors 25, 26 of availability control or lack of boats at the beginning or end of retort 5.

Claims (3)

1. A method of drying tablets of nuclear fuel after wet grinding, including moving them in mesh boats and drying in a nitrogen medium, characterized in that tablets are poured in bulk into each boat in bulk not more than 85% by volume of the boat, at least drying of the tablets of nuclear fuel is carried out , in three zones, where a temperature of 300 ± 10 ° С is maintained in the first zone, a temperature of 200 ± 20 ° С is maintained in the second zone, and in the third zone, a temperature of 120 ± 20 ° С is maintained; mesh boats with tablets are moved through the zones with a linear speed not b more than 0.04 m / s, nitrogen gas is preheated to a temperature of at least 120 ° C and fed into the drying zone under pressure MPa (kg / cm ² ) - 0.15-0.16 (1.5-1.6) and drying the nuclear fuel pellets is carried out to a residual hydrogen content of not more than 6 · 10 ^-5 %. 2. A device for drying nuclear fuel tablets after wet grinding, comprising a drying unit and a vehicle for transporting nuclear fuel tablets through them in net boats in a nitrogen medium, characterized in that the drying device for nuclear fuel tablets after wet grinding is additionally equipped with a heater and a drying device control system nuclear fuel tablets, where the drying unit for nuclear fuel tablets is made of a retort in the form of a horizontal square tube covered by at least three wires heaters with thermoconverters, at the ends of the retort are loading and unloading units with doors driven by pneumatic cylinders, the vehicle for conducting net boats through the tablet drying unit is made in the form of a conveyor with levers spacing the boats on the conveyor, the air heater is made in the form of a heated hot feed pipe nitrogen gas in the retort, and the control system for the device for drying the tablets of nuclear fuel is based on a programmable controller. 3. The device according to claim 2, characterized in that the programmable controller used is a programmable controller SIMATIC S7-300, electrically connected with the electrical equipment of the tablet drying unit, the conveyor drive, the electrical equipment of the air heater, sound and light signaling devices.

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