RU2297680C1 - Method and device for checking fuel element cans for tightness - Google Patents

Abstract

FIELD: nuclear engineering; water moderated water cooled reactors and cooling ponds for detecting leaky fuel elements in fuel assemblies.

SUBSTANCE: proposed device for checking tightness of fuel elements in fuel assemblies has rod, submersible pump, flow-type degasifier, and pipes for feeding pure distillate and compressed air. Rod mounts facility for connecting and packing fuel assembly. Flow-type degasifier is made in the form of vessel accommodating partially perforated and axially disposed tube. Degassed water drain pipes as well as air and water discharge pipes are provided in upper part of vessel and pipes for feeding air through bubbling manifold, in its lower part. Invention specification also includes relevant method for checking nuclear reactor fuel elements for tightness.

EFFECT: ability of on-line shooting of leaky fuel elements and evaluation of flaw size in faulty fuel element.

6 cl, 2 dwg

Description

The invention relates to the field of nuclear engineering and can be used in water-cooled nuclear reactors and storage tanks for nuclear fuel to detect leakage of fuel elements in fuel assemblies (FAs).

A known method for detecting leaks in fuel elements (RU patent 2094861, G21C 17/06, 1997). According to this method, fuel assemblies are placed in a separate sealed canister, repeatedly turning on the pump, they increase the external pressure, stand, and then release: This stimulates the release of readily soluble radionuclides I and Cs into the coolant. But the release of radioactive nuclides into the coolant is judged on the depressurization of the fuel elements.

The disadvantage of this solution is the ambiguity in determining the nature of the damage (size). The output of readily soluble I and Cs into the coolant depends on many factors: the influence of diffusion processes, the location of the defect along the height of the fuel rod, fuel burnup, and many other reasons, the actions of which are described by various laws.

With microcracks, water entering the inner space of the fuel element and exiting it is unlikely due to small pressure fluctuations, especially since a small space near the defect was already washed out significantly with water at relatively large pressure drops during reactor cooldown.

The fuel rods with a secondary defect in the region of the upper plug are completely filled with water and pressure fluctuations of the incompressible fluid inside the fuel rod do not lead to flushing. In this case, the ingress of iodine and cesium into the pencil case water is possible only by diffusion through the defect.

The highest yield of I and Cs can be obtained if the defect is in the lower part of the fuel element, and there is gas in the fuel collector of the fuel element. In this case, due to the compression of the gas space and the penetration of water into the internal space, I and Cs are flushed with pressure fluctuations from a significant surface inside the fuel rod. The flush is greater, the smaller the fuel burnup due to the greater gap between the tablets and the shell.

A known method and device for determining the leakage of fuel rods (RU, patent 2186429, G21C 17/06, 2003) for the release of gaseous radionuclides from the fuel rod into the space of the inner section of the working rod of the reloading machine, where the fuel assemblies are captured from the active zone to move to the holding pool. The gaseous radioactive isotopes Xe and Kr leave the leaky fuel rod in the water space of the inner section, dissolve, and then are removed from the water by sparging the gas supplied under the open lower part of the inner section. Sparged gas is collected in the surface volume of the internal section of the device of the refueling machine, from where it is supplied with a gas flow stimulator to a measuring device to determine the activity of Xe and Kr in the bubble gas.

The advantage of determining leaking fuel assemblies of fuel assemblies in the working rod of a reloading machine is the combination of the time of transport operations with the time of implementation of the determination method. However, with this method of monitoring the tightness of the cladding of the fuel rods, the reliability of the control is reduced.

The volume of the internal section of the working rod, where the fuel assembly to be checked, contains water taken directly above the core. In this water, there are enough dissolved gaseous fission products (GPA), which upon degassing of the water create a certain background, independent of the GPA output from the fuel assembly being tested. In this case, a fuel assembly having microcracks in the cladding of a fuel rod will be unnoticed. A false leakage signal can be generated by a fuel assembly extracted from a nearby cell having a known leaky fuel rod. From such a fuel assembly, the signal about the excess of the GPA output will be higher than the average background value of the activity of all water above the core.

Another disadvantage is the inefficiency of degassing water by air sparging with significant dilution with air supplied from the bottom in a local way. As a rule, air will pass where there is less resistance, i.e. along the walls of the inner space where the fuel assembly is located, and may not reach the space near the defective fuel rod, where the resistance is high and where the largest amount of dissolved gases is. Therefore, ineffective degassing, insufficient time (only 3 min) until the GPA completely dissolves in the entire volume, and significant dilution (40 L) leads to a decrease in the activity signal. With the proposed method, the task of accelerated leakage monitoring runs in contradiction with the completeness and reliability of measurements for deciding on a fuel rod leakage.

The very method of determining leakage by exceeding the signal by comparing the average background indicator is not accurate enough and non-operational. Indications of the background value of GPA activity in the reactor water can vary for various reasons: changes in the temperature of the water in the reactor over time during overload, the amount of residual heat and location of the fuel assembly, etc.

As a result, the efficiency of determining an unsealed fuel assembly (immediately when checking one fuel assembly) is lost. A verified fuel assembly with a defect in the output of the engine must be sent to the overload pool, and the decision for further use can be made only after checking all the fuel assemblies (for comparison with the average value of the background indicators).

Therefore, the tightness control of the claddings of fuel assemblies of fuel assemblies at a stopped reactor and in storage facilities used for fuel assemblies is carried out in two stages: search for leaky fuel assemblies by the GPA activity of ^{133, 131m} Xe and ⁸⁵ Kg in the working rod of the reloading machine, and determining the nature of the defect for deciding on further use FA - according to the yield of ¹³¹ I and ^{134, 137} Cs readily soluble in water.

The above devices and methods are the closest analogues of the invention.

The technical problem solved by the present invention is to carry out an operational search for an untight fuel rod and determine the size of a defect in an untight fuel rod, to obtain the most accurate criteria for deciding on the further operation of spent fuel assemblies in the reactor.

The specified technical result is achieved in a device for monitoring the tightness of the shells of the fuel elements, comprising a rod, on which there is a device for connecting and sealing with a fuel assembly, a submersible pump, a flow degasser, pipes for supplying clean distillate, compressed air, and the flow degasser is made in the form a vessel, inside of which a pipe is coaxially located, part of which is perforated, in the upper part of the vessel there is a pipe for draining degassed water, pipes for drainage ode of air and water, and in the lower part of the pipe for supplying air through the bubbler manifold.

The perforated part of the degasser is made with holes ⌀ 1.5 ÷ 2.0 mm, placed around a circle through 90 ° with a distance between the rows of at least 100 mm with a displacement of the holes relative to the other row w ∠45 °. A pipe for draining water is located at least 200 mm above the supply of compressed air.

The specified technical result is achieved using the method of tightness control, which consists in connecting the tightness control device for the cladding of the fuel rods of a nuclear reactor with a fuel assembly (FA), supplying pure distillate to the interior of the fuel assembly to displace the reactor water, keeping the distillate in the fuel assembly, supplying air to the degasser, reactor water is supplied to the degasser by turning on the pump, water is degassed by the flowing air of small particles of water, formed due to a sharp drop in pressure and the dynamic effect of water jets on the inner wall of the degasser, measure the activity of gaseous fission products of xenon and krypton in air samples, which are used to judge the presence of a defect, take samples of water displaced from the internal cavity of the fuel assembly due to the supply of distillate, measure activity of radionuclides I and Cs in water, which is used to judge the size of a defect in a fuel assembly.

Pure distillate is fed into the inter-fuel space of the fuel assembly with a temperature of at least 30 ° C lower than the temperature of the water in the reactor.

After the device is installed on the fuel assembly, pure distillate is supplied in an amount of at least 2.5 internal volume of the fuel assembly plus the internal space of the pump and degasser. This operation displaces the reactor water from the internal cavity of the fuel assembly and flushes the pressure pipe of the degasser together with the pump. An exposure is made for 20 minutes, as a result of which the cold distillate is heated to the water temperature in the reactor with the simultaneous dissolution of the Xe and Kr radioactive gases released from the untight fuel rod in water. Turn on the pump to supply air to the degasser. Then they turn on the pump for pumping water from the fuel assembly to the flow degasser, where due to a sharp drop in pressure and decomposition into small particles inside the degasser and ventilation of the internal space with a flow rate of 40 l / h, and supplying it to the measuring device, the GPA activity is determined. At the same time, a sample of water displaced from the internal cavity of the fuel assembly is taken. The activity of ¹³¹ I and ^{134, 137} Cs is measured with a semiconductor detector. The device’s operating time in degassing mode is 5 minutes.

The use of pure distillate eliminates the influence of the activity of the GPA contained in the reactor water.

Pure distillate is fed into the inter-fuel space of a fuel assembly with a temperature of at least 30 ° C lower than the temperature of the water in the reactor to cool the claddings of the fuel rods and the space between the cladding and fuel and to facilitate the penetration of water through the cladding defect.

The degassing of water with a GPA is carried out by running air of small particles of water formed due to a sharp drop in pressure from 0.3 MPa to atmospheric and dynamic effects of water jets on the inner wall of the degasser.

The activity indications are obtained not from the average sample from the total volume of pumped air during degassing by bubbling (40 L), but from that part of the flowing air (3 L) that comes into contact with water having the maximum amount of dissolved GPA, resulting in an excess of activity (signal) much higher.

The method allows you to quickly identify gas leaks with microdefects immediately after registering a signal about the presence of GPA activity.

The activity of the GPA is measured in dynamics, the increase in activity is recorded in the form of a peak and corresponds to that part of the water that entered the degasser from the space located near the defect of the fuel rod cladding.

By the activity of ^{133, 131m} Xe it is possible to determine the leakage of fuel assemblies within 1.5 months after the reactor shutdown, and by the activity of ⁸⁵ Kr - during 10 or more years of storage of fuel assemblies in the storage pools.

The device (Fig. 1) for achieving the indicated technical result consists of a rod (1), on which there is a device for connecting and sealing with fuel assemblies (2), an underwater pump (3), a flow degasser (4), and a pure distillate supply system (5, 6), a drain pipe (7), an air supply system (8), a radionuclide control system in air and water (9, 10) with shutoff valves.

The rod is a pipe dy ÷ 50 with a sealing device in the form of an expansion plug made of rubber, the compression of which is carried out remotely by rotating the inner pipe.

The degasser (figure 2) is a vessel from a pipe dy ÷ 80, inside which a pipe (11) is placed for water supply and spraying (12) through holes Ш 1,5 ÷ 2,0 mm, placed around a circle through ∠90 ° Distance between the rows of nozzles 100 mm, the offset of the nozzles relative to the other row by ∠45 °. The total number of nozzles is determined by the capacity of the pump for pumping water in the internal space of a fuel assembly. To exclude the transport of water with air to the sample, the height of the internal space of the degasser above the nozzles should be at least 500 mm. Air supply for degassing (8) is made through a bubbler collector (13) with holes отверстия 2.0 mm. The drain pipe for degassed water (7) is located 200 mm above the air supply to create a layer of water for degassing by bubbling. Pipes for air exhaust (14), water (15) are located on the upper flange of the degasser, the diameter of the pipes is not less than 10 mm.

The washing system includes a measuring tank (6) with a volume of 2.5 volumes of water in the fuel assembly plus the internal space of the pump and degasser, and pipelines for the supply of distillate (5) with fittings. The measuring tank is located above the upper flange of the degasser by 500 mm.

The air supply and control system for the content of radionuclides consists of a cylinder of compressed air, a flow regulator, a shut-off valve and a flow meter, pipelines for supplying air to the degasser and outlet to the measuring device. To determine the contents of I and Cs, water is supplied from the central pipe of the degasser through the upper flange of the rod. To drain the water from the degasser there is a water trap with a height of 1500 mm.

The control unit carries out operations on and off the pump and shutoff valves according to a specific algorithm. The measuring devices have a continuous detector, a unit for processing and displaying information about the activity of the GPA in ventilated air. The activity of ¹³¹ I and ^{134, 137} Cs in water samples is measured by a semiconductor detector.

The tightness control is performed as follows. A bar attached to a reloading machine or hoisting mechanisms is installed on the fuel assembly under test. From a pre-filled distillate tank, water is supplied to displace the reactor water from the degasser and fuel assembly. Water is kept in the fuel assembly for 20 minutes and then the pump is turned on to supply water from the fuel assembly to the degasser, to the central pipe with nozzles Ш 15 ÷ 20 mm, where due to a sharp drop in pressure and spraying on small particles, the water is degassed by air ventilation at a speed of no more 1 m / s

Water is discharged into the lower part of the degasser and through a water trap into the reactor, and flowing air with a flow rate of 40 l / h enters the measuring device. At the moment water arrives from the place of the defect in the fuel assembly, the concentration of GPA increases. The signal has a peak character, and is larger in magnitude than during degassing by bubbling, since the volume of air for dilution is 3.3 liters instead of 40 liters with bubbled degassing. The size of the defect is determined by the activity of ¹³¹ I and ^{134, 137} Cs in the selected water sample.

The advantages of the proposed method for determining the leakage of fuel assemblies and the size of the defect in the shell is as follows.

The effect of dissolved GPA in the aqueous medium inside the fuel assembly due to its filling with pure distillate is excluded.

GPA degassing takes place in a more efficient way: a sharp drop in pressure of the medium with GPA, spraying onto small particles in a degasser and ventilation with running air, with significantly (an order of magnitude) less dilution.

In the bubble degassing method, the recorded GPA activity (signal + background) has an integral character, since in the upper cavity of the volume of the inner section of the working rod of the reloading machine, all GPA extracted from both the reactor water and the non-leaking fuel rod are mixed. And in this method, the activity of the GPA increases at the time of delivery of that portion of water to the degasser that was near the defect, therefore, the received signal is peak in nature and carries operational information about the presence of the defect, which does not require data to verify a certain batch of verified fuel assemblies by the comparison principle.

The tightness control of the fuel assembly shells is carried out without moving the fuel assembly from the reactor, which reduces the radiation hazardous operations during overload.

Claims (6)

1. A device for monitoring the tightness of the shells of the fuel elements, containing a rod on which there is a device for connecting and sealing with a fuel assembly, a submersible pump, a flow degasser, nozzles for supplying clean distillate, compressed air, characterized in that the flow degasser is made in the form of a vessel , inside of which a pipe is located coaxially, part of which is perforated, in the upper part of the vessel there is a pipe for draining degassed water, pipes for venting air and water, and in the lower part A nozzle for supplying air through the bubbler manifold. 2. The device according to claim 1, characterized in that the perforated part of the degasser is made with holes ⌀ 1.5 ÷ 2.0 mm, placed around the circumference in an amount of 4 pcs. through an angle of 90 ° with a distance between rows of holes of at least 100 mm and an offset of 45 ° relative to another row. 3. The device according to claim 1, characterized in that the outlet pipe for degassing water is located at least 200 mm above the nozzle for supplying compressed air. 4. A method for controlling the tightness of the cladding of the nuclear fuel rods of a nuclear reactor, which consists in connecting the tightness control device for the cladding of the fuel rods of a nuclear reactor with a fuel assembly (FA), supplying clean distillate to the interior of the fuel assembly to displace the reactor water, keeping the distillate in the FA, supplying air to the degasser, reactor water is supplied to the degasser by turning on the pump, water is degassed by the flowing air of small particles of water formed due to a sharp drop in pressure and dynamic the effect of water jets on the inner wall of the degasser, measure the activity of gaseous fission products of xenon and krypton in air samples, which are used to determine the defect, take samples of water displaced from the internal cavity of the fuel assembly by feeding distillate, measure the activity of radionuclides I and Cs in water , by which they judge the size of the defect in the fuel assembly. 5. The method according to claim 4, characterized in that the pure distillate is fed into the inter-fuel space of the fuel assembly with a temperature of at least 30 ° C below the temperature of the water in the reactor. 6. The method according to claim 4, characterized in that pure distillate is supplied in an amount of not less than 2.5 internal volumes of fuel assemblies plus the internal space of the pump and degasser.

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