FI128657B

FI128657B - Apparatus and method for leak detection in a nuclear fuel assembly

Info

Publication number: FI128657B
Application number: FI20185241A
Authority: FI
Inventors: Mikhail Fedosovsky; Sergey Aleksanin; Vadim Dunaev
Original assignee: Joint stock company Diakont
Priority date: 2015-08-14
Filing date: 2016-08-15
Publication date: 2020-09-30
Also published as: CN108463857A; EA030889B1; EA201501008A3; CN108463857B; WO2017138835A1; EA201501008A2; HUP1800223A1; FI20185241A

Abstract

An apparatus and a method for leak detection in a nuclear fuel assembly are disclosed. The apparatus is arranged on an enclosure of a fuel handling machine for transferring the nuclear fuel and bubbling a coolant near the fuel assembly received in the enclosure. The feeding pipeline comprises atomizers facing the center of the enclosure, which enables to supply a pressurized gas directly under the bottom nozzle of the fuel assembly, increasing intensity of coolant bubbling near said fuel assembly, and thus increasing effectiveness of leak detection.

Description

APPARATUS AND METHOD FOR LEAK DETECTION IN A NUCLEAR FUEL ASSEMBLY

FIELD OF THE INVENTION The present invention relates to the field of nuclear power, and more particularly to an apparatus and a method for leak detection in a nuclear fuel assembly comprising a liquid coolant.

BACKGROUND OF THE INVENTION To ensure high efficiency and safe handling, nuclear fuel for nuclear reactors is received in special hermetic claddings referred to as fuel elements (FE). The FE are then assembled into fuel assemblies (FA). Leak detection in fuel elements while using the fuel is an important part of nuclear reactor operation safety measures. It is necessary to detect leak in nuclear fuel elements to prevent fuel fission products from entering the coolant, which may result in spreading radioactive elements beyond the reactor core. Standard bench control methods in FFDS (Failed Fuel Detection System) canisters are used for such checking, in which fuel assembly is transported into an enclosure filled with borated water, into which fission products are forced out of the leaky fuel element, then a water sample from said enclosure is analyzed. This method includes successive transfer of all fuel assemblies, without exception, into said enclosure, which results in sustained reactor downtime. Moreover, necessity to provide substantial amounts of borated water leads to increased cost of carrying out a standard bench test method. Desire to reduce nuclear reactor downtime and cost of carrying out a standard bench S control method led to solutions, in which it was proposed to conduct a predetection of leak in N fuel elements when they are transferred by a fuel handling machine (see RU2186429). S 25 RU2186429 teaches to combine a predetection of leak in a fuel assembly with operations of = its extraction and transfer in an enclosure of a fuel handling machine in order to replace or E rearrange the fuel assemblies inside the reactor. The aim of the predetection of leak was to IT reduce the number of fuel assemblies subjected to the standard bench test method, as fuel O assemblies, which in the result of predetection of leak were considered leaktight, were not 2 30 subjected to the standard test method. However, known solutions did not provide sufficiently high precision and reliability of radioactivity measurement of the gas sample extracted during the predetection of leak, increasing the likelihood of passing a leaky fuel assembly. 1

Hereinafter, measurement precision is a characteristic expressing the degree to which the measurement result corresponds to the actual value measured, and measurement reliability is a characteristic that defines a degree of confidence in the received measurement results. EA016571 (priority date 06 October 2010), which is incorporated herein in its entirety by reference, discloses an apparatus and a method for leak detection of a fuel assembly for a nuclear reactor, the fuel assembly being arranged within an enclosure of in a fuel handling machine of such reactor, that provides more precise and reliable gas sample radioactivity measurement, and as a consequence more efficient fuel assembly checking in comparison with other known methods and apparatuses. Said method and apparatus are the closest analogues of the claimed inventions. The known apparatus comprises a feeding pipeline for supplying gas under the enclosure, the feeding pipeline being disposed on the enclosure, a sampling pipeline for extracting a gas sample from the enclosure, the sampling pipeline being disposed on the enclosure, a pressurized gas supply unit connected to the feeding pipeline to supply a pressurized gas into it, a unit for extracting, preparing, and checking the activity of — the gas sample, the unit being connected to the sampling pipeline so as to extract the gas sample therethrough, a control and information processing unit communicatively connected to said pressurized gas supply unit and the unit for extracting, preparing, and checking the activity of the gas sample, and a remote control communicatively connected to the control and information processing unit. The apparatus is used to perform a corresponding method for leak detection.

However, an analysis of operation of said apparatus and applying the known method showed that some volume of gas flows past the fuel assembly in the enclosure during bubbling, whereby bubbling of such assembly lacks intensity. Said circumstance limits precision and reliability of gas sample radioactivity measurement and, as a consequence, N 25 — preliminary efficiency of leak detection. a

S MN SUMMARY OF THE INVENTION

T a = It is the aim of the present invention to overcome the disadvantages of the prior art and N to provide an apparatus and a method for nuclear reactor fuel assembly leak detection, = 30 reducing losses of gas passing through the fuel assembly, and increasing the intensity of N bubbling. 2

Said aim is achieved by an apparatus for leak detection in a fuel assembly for a nuclear reactor, the fuel assembly being arranged within an enclosure of a fuel handling machine of the nuclear reactor, the apparatus comprising: a feeding pipeline for supplying gas under the enclosure, the feeding pipeline being mounted on the enclosure; a sampling pipeline for extracting a gas sample from the enclosure, the sampling pipeline being mounted on the enclosure; a pressurized gas supply unit connected to the feeding pipeline so as to supply pressurized gas into it; a unit for extracting, preparing, and checking the activity of the gas sample, the unit being connected to the sampling pipeline so as to extract the gas sample therethrough; a control and information processing unit communicatively connectable to the pressurized gas supply unit and the unit for extracting, preparing, and checking the activity of the gas sample; a remote control communicatively connectable to the control and information processing; the feeding pipeline comprises at least two atomizers for supplying a pressurized gas, wherein the atomizers are mounted at an end of the fuel handling machine enclosure.

The apparatus is characterized in that the nozzles of the atomizers are directed in such a — way that the center axes of the nozzles intersect a central axis of the enclosure forming an intersection point located outside the enclosure.

Such arrangement of the atomizers for feeding pipeline the provides a gas supply directly under the central part of the enclosure, in which the fuel assembly is located during o predetection of leak, and which in this case is positioned vertically and in a transport O 25 position. Thus, the amount of gas flowing past the fuel assembly is reduced, and a sufficient N amount of gas passing through said assembly during bubbling (bubbling intensity is ~ increased) is provided, which leads to more intensive capture of radioactive elements from the T assembly in case it is leaky, and thus improved precision and reliability of gas sample = radioactivity measurement results, which reduces the likelihood that a leaky fuel assembly N 30 can remain undetected after the predetection of leak. 00 2 In one embodiment, the feeding pipeline comprises a removable portion mounted on the end of the enclosure along the periphery of the outer section of the enclosure. The at least two atomizers comprise three atomizers, each having a Laval nozzle. The three atomizers are 3mounted on said portion equally spaced from each other. The three atomizers ensure optimal gas volumetric flow rate over time during bubbling. The nozzles in the form of a Laval nozzle increase a throw distance of a gas jet in the coolant and reduce the sectional area of the jet, which further reduces gas losses during its supply to the fuel assembly.

In one particular embodiment, the feeding pipeline and the sampling pipeline each are made of tubes having a diameter of 7 to 10 mm and a wall thickness of 0.5 to 1 mm. Said dimensions, on the one hand, enable to integrate said lines into the fuel handling machine, and, on the other hand, they provide sufficiently low pneumatic resistance of the line and substantial gas pressure at the atomizers.

According to another embodiment, the feeding pipeline and the sampling pipeline each have quick-disconnect couplings for connection to the pressurized gas supply unit and the unit for extracting, preparing, and checking the activity of the gas sample, respectively. These connectors allow faster mounting of the proposed apparatus on the fuel handling machine and — its dismantling.

According to a particular embodiment, said quick-disconnect couplings have ends formed as conical sleeves with a taper angle of 70 to 78 degrees, and line tubes at the connection with the couplings have a taper angle of 60 to 70 degrees. These dimensions provide a good joint seal between line sections when mounting the apparatus onto the fuel handling machine.

The present invention also provides a method for leak detection. The method is performed by using the claimed apparatus. The method comprises: arranging the fuel o assembly within an enclosure of a fuel handling machine; supplying gas under the enclosure O using atomizers arranged on the enclosure; extracting a gas sample from above the fuel N 25 assembly; and analyzing the gas sample to predetect leak in the fuel assembly.

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I = BRIEF DESCRIPTION OF THE DRAWINGS N The following is a description of a preferred embodiment of the apparatus of the present = invention, which by way of illustration is accompanied by drawings, in which:

N FIG. 1 shows a diagram of an apparatus for leak detection in accordance with the present invention, 4

FIG. 2 illustrates a simplified pneumatics diagram of a pressurized gas supply unit and a unit for extracting, preparing, and checking the activity of a gas sample, of the apparatus of the present invention; FIG. 3 shows a partial view illustrating the arrangement of atomizers in the feeding pipeline at the end of the outer section of the enclosure; FIG. 4 illustrates connection of the pipelines using a quick-disconnect coupling; FIG. 5 shows an enlarged view of the atomizer mounted according to the present invention at the end of the enclosure in the fuel handling machine.

DETAILED DESCRIPTION OF THE INVENTION As can be seen in FIG. 1, an apparatus 1 is generally arranged on a fuel handling machine (FHM) for a nuclear reactor and moves along with it as the reactor fuel is being transferred. An enclosure of the fuel handling machine is cylindrical in shape. The enclosure comprises an outer section A which bears elements of the apparatus 1 mounted directly on it.

— Further, the enclosure comprises an inner section B to accommodate a fuel assembly during transfer and to predetect leak. The fuel assembly is held and transferred with a gripper provided in the inner section. The fuel assembly is arranged exactly in the center of the enclosure inner section using said grip, which is a part of the fuel handling machine.

The apparatus 1 comprises a feeding pipeline 2 for supplying gas under the enclosure — through atomizers. The feeding pipeline 2 is made of a tube having a diameter of 7 mm and a wall thickness of 0.5 mm. The apparatus 1 comprises a sampling pipeline 3 for extracting a o gas sample from the space in the enclosure inner section above the level of a coolant (for O example, water) of a nuclear reactor. The sampling pipeline is made of a tube having a N diameter of 7 mm and a wall thickness of 1 mm. Said dimensions enable to minimize N 25 pneumatic resistance of the pipeline and maximize gas pressure at the atomizers, and to I integrate the pipelines 2, 3 into the fuel handling machine.

a 3 The sampling pipeline 3 is brought into the enclosure in two points. Sampling points are 0a located near the surface of the coolant. Extracting samples in several points minimize the 2 influence of arbitrary factors on the result of radioactivity check of a target gas sample extracted from above the liquid coolant surface. The apparatus 1 also comprises a pressurized gas supply unit 7 connected with the feeding pipeline 2 to supply a pressurized gas through it.

5

Preferably, the gas is air. The feed and sampling pipelines 2, 3 are connected with other elements of the apparatus 1 through quick-disconnect couplings, as described in further detail below. Thus, the parts of the apparatus 1 can be quickly and securely connected when it is mounted on the enclosure.

Furthermore, the apparatus 1 comprises a unit 5 for extracting, preparing, and checking the activity of a gas sample for extracting gas through the sampling pipeline 3. Further, the apparatus 1 comprises a control and information processing unit 6 connected with the unit

5. The control and information processing unit 6 comprises a transceiver and a programmable logic controller configured to receive and process signals and to output electrical control pulses. The control and information processing unit 6 is also connected to a pressurized gas supply unit 7. The control and information processing unit 6 controls the operation of the unit 7 and the unit 5 by transmitting corresponding control signals. The control signals are transmitted to the control and information processing unit 6 through a remote control 9 connected thereto, which is also a part of the apparatus 1. The remote control 9 is provided — outside the reactor core, i.e. inside the reactor hall, and is connected to the unit 6 via the data communication channel (wired or wireless) configured according to the RS-422 standard. The remote control 9 allows the personnel outside of the reactor hall (outside of the containment area) to predetect leak in the fuel assembly. The remote control 9 comprises display means, controls, computing means, and storage media. For the convenience of the operator, the remote control 9 is provided with display means and controls, such as a graphical interface, a keyboard and a mouse. FIG. 2 illustrates details of the pressurized gas supply unit 7 and the unit 5 for extracting, preparing, and checking the activity of a gas sample extracted from the apparatus

1. The pressurized gas supply unit 7 comprises a compressor 10, a receiver 11, a filtering S 25 — pressureregulator 12, distribution valves 13 and 14 engaging each other to supply the N pressurized gas into the feeding pipeline 2. The compressor 10 pumps gas, preferably air, into S the receiver 11 in response to a control signal. Then the air flows to the unit 7 outlet through = the filtering pressure regulator 12 which dries the air and concurrently stabilizes its pressure at z the unit 7 outlet. The unit 7 is connected to the feeding pipeline 2 via the distribution valve 13 3 30 and a fitting pipe. For a bubbling procedure, the distribution valve 13 is opened and o pressurized air is fed under the fuel assembly through the feeding pipeline 2 comprising N atomizers. The duration of the bubbling procedure is set by software settings. After the end of a fuel assembly control cycle, before a control cycle of the next, optionally the overwater volume of the enclosure is purged to remove gaseous fission products. To purge the overwater 6volume, the air is fed from the unit 7 into the sampling pipeline 3 through the distribution valve 14 switched to the purge mode, and the fitting pipe. The distribution valves 13 and 14 are controlled by an electric signal from the unit 6. The unit 5 comprises a vacuum water separator 20, a vacuum pump 21 for delivering the sample to the unit input, a cooler 22, a microfilter 23, a submicrofilter 24, an air dehumidifier 25, a pressure regulator 26, a throttle 27, a pressure sensor 28, a temperature and humidity sensor 29, a radioactivity analyzer 30, an air flow-rate sensor 31 which controls passage of the proper volume of the sample through the analyzer 30 chamber, a pump 32 for forcing the extracted sample through the regulator 26, the sensor 28 and the sensor 29. Said — details of the unit 5 engage each other so as to take a gas sample and prepare it for a corresponding radioactivity analysis. To take a gas sample from the overwater volume, the distribution valve 14 is switched to a sampling mode to connect the unit 5 with the sampling pipeline 3. Meanwhile, the vacuum pump 21 is used to pump the gas sample from the overwater volume of the enclosure, previously passing it through the vacuum water separator

20. Then, using the pump 32, the gas sample is pumped through the cooler 22, the microfilter 23, the submicrofilter 24 and the air dehumidifier 25 which form means of sample preparation. Meanwhile, the microfilter 23 and the submicrofilter 24 are designed for two-step purification of the sample to prevent contamination the dehumidifier 25, positioned downstream along the path of the sample from it. Then, the sample passes through the pressure regulator 26 and the throttle 27, which adjust required flow rate and pressure of the gas supplied to the inlet of the analyzer 30. It is recommended to use a beta radiometer as the analyzer, since it provides the most accurate measurement of the radioactivity level in the given conditions. Thus, the sample is prepared, i.e. it is brought into a state, in which its temperature, pressure and humidity conform to requirements of the measuring equipment, in o 25 — this case a beta-radiometer. Meanwhile, the pressure sensor 28, the temperature and humidity O sensor 29 control the state of the gas sample at the inlet of the analyzer 30, and the air flow- S rate sensor 31 monitors the passage of the sample through the analyzer 30 chamber. The gas ~ sample exits into the environment through the "OUTLET" branch pipe. In response to an =E electric signal from the control and information processing unit 6 (not shown in FIG. 2) = 30 pumps 21 and 32 are activated in order to take a sample. Signals from all the sensor of the N unit 5, as well as readings of the analyzer 30, are input in the unit 6, where signals are = transformed and subseguently processed by a computer of said unit in order to determine the N state of the fuel assembly under control. A graphical interface and indicators of the unit 6 and the remote control 9 (not shown in FIG. 2) display information about the present state of the 7apparatus.

In particular, if readings of the sensors 28 and 29 do not correspond to the set conditions, then the graphical interface outputs a message about the measurement being carried out on a sample, which does not match the measurement conditions.

Moreover, said graphical interface and indicators show a number of a fuel assembly subjected to control, the operator identification data, a control mode and access level, current sensor and beta radiometer readings, and also a result of predetection of leak in a fuel assembly after the end of this fuel assembly control cycle.

A sample is considered representative if it has been taken in the prescribed location, it is not mixed with ambient air, conforms to the temperature, humidity and pressure — conditions.

Checking according to the first two criteria is performed before the start of transferring and checking operations by verifying the condition of the equipment.

Compliance check is performed via corresponding sensors while extracting and analyzing the sample.

However, if the sample does not conform to any one of the conditions, a display means informs that the measurement was performed on a sample, which does not match the measuring equipment operating conditions.

As can be seen in FIG. 3, the feeding pipeline has a removable annular portion 40 arranged along the end face 41 of the outer section of the enclosure and attached thereto by screws 42 or other suitable fastening means.

The removable annular portion 40 is connected with the feeding pipeline 2 via a quick-disconnect coupling 43. Any other method of connection can be used, providing for optional detachment of the annular portion 40 from the feeding pipeline 2 without dismantling the entire enclosure.

Three atomizers 44 are positioned on the annular portion 40 for supplying pressurized gas.

The atomizers 44 are equally spaced from each other. o FIG. 4 illustrates a guick-disconnect coupling 43 connecting the feeding pipeline 2 with O 25 — the branch pipe 47 of the removable annular portion 40. It should be understood that such a N coupling may also be used for connecting other portions of lines of the apparatus according to ~ the invention.

As can be seen in FIG. 4, the line 2 and the branch pipe 47 have conical flanged T portions at the junction with the coupling 43. The values of the taper angle are within the = range of 60 to 70 degrees, and in the illustrated embodiment the taper angle is 60 degrees.

The N 30 coupling 43 has two nipples 48, mounted on the respective conical portions.

The coupling 43 = also comprises a feedthrough element 50 which has an axial cylindrical channel, which, N during installation of the coupling 43 in the operating position, is mounted coaxially with therespective lines to be connected, and coupled with them, forming a portion of the line.

To

8facilitate installation of the element 50 its ends are formed as conical sleeves with the taper angle of 70 to 78 degrees, in this example 70 degrees. The element 50 has two threaded portions with an external thread. The coupling 43 also comprises two nuts 51 which are mounted on respective ends of the lines by screwing them over the nipples 48 onto the threaded portions of the element 50. Corresponding end of the line and the element 50 are brought together by screwing the nuts 51 onto the threaded portions of the element 50, providing a hermetic connection between the lines. Optionally, the nuts 51 may be sealed with a seal 52 which is installed through special holes formed in the base of the nuts 51. FIG. 5 illustrates an atomizer mounted according to the present invention at the end 41 of the fuel handling machine enclosure. By way of illustration, FIG. 5 shows a sectional view of the annular portion 40 in the mounting location of the atomizer 44. As can be seen in FIG. 5, the atomizer 44 is mounted directly within the annular portion 40, wherein the atomizers 44 are equally spaced from each other, and their nozzles are directed in such a way that the central axes of these nozzles intersect the enclosure central axis to form an intersection point outside the enclosure. According to the illustrated embodiment, the axes of the nozzles of the atomizers 44 are arranged at an angle of 15 degrees to the horizontal plane. This arrangement of the atomizers 44 reduces the losses of pressurized gas passing through the fuel assembly by increasing the intensity of bubbling. This ensures supply of maximum amount of gas under the fuel assembly, and removal of the maximum amount of gaseous fission products from the liquid coolant in the volume above its surface, if there are non-hermetic fuel elements. This increases precision of gas sample radioactivity level measurement and effectiveness of leak detection in a fuel assembly. The atomizers 44 each have a Laval nozzle, which allows for maximum pressurized gas jet throwing distance and further improves precision of gas sample radioactivity level measurement. Location of the S 25 atomizers 44 on the end 41 of the outer section allows free movement of the fuel assembly N within the enclosure.

O ~ The apparatus 1 operates according to the method disclosed in the present T invention. When a bubbling operation is needed a control signal is supplied to the pressurized = gas supply unit 7 via a control channel. The unit 7 supplies a part of the pressurized gas N 30 through the atomizers 44 directly under the center of the lower end of the enclosure = containing the fuel assembly. Said gas intensively bubbles the coolant near the fuel assembly s and exits into the overwater volume, from where it is taken through the sampling pipeline 3 by the unit 5 for extracting, preparing, and checking the activity of the sample, where it is 9successively prepared for a radiation analysis and subjected to it. Said preparation includes water separation in the water separator 20, cooling in the cooler 22, successive purification in the microfilter 23 and the submicrofilter 24, drying in the air dehumidifier 25, and reaching the required pressure using the pressure regulator 26, the throttle 27 and the pressure sensor 28 Then, the prepared gas sample enters the analyzer 30 where radiation level data of said sample is determined, wherein the pump 32 pumps the gas sample through the analyzer 30 chamber, and the air flow-rate sensor 31 enables to control passage of the sample in the proper amount through the analyzer 30 chamber. Said data is transmitted to the information processing and control unit 6 and then output on the display means of the remote control 9 — (for checking by an operator), which automatically compares the received data with a predetermined threshold value and determines whether to send the fuel assembly for standard bench test method for leak detection. The apparatus and the method of the present invention enable to effectively predetect leak after the end of a checking cycle of a single fuel assembly and provide the results of such — detection on the graphical interface of the remote control 9.

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Claims

1. An apparatus (1) for leak detection in a fuel assembly for a nuclear reactor, the fuel assembly being arranged within an enclosure of a fuel handling machine of the nuclear — reactor, the apparatus (1) comprising: a feeding pipeline (2) for supplying gas under the enclosure, the feeding pipeline (2) being mounted on the enclosure; a sampling pipeline (3) for extracting a gas sample from the enclosure, the sampling pipeline (3) being mounted on the enclosure; a pressurized gas supply unit (7) connected to the feeding pipeline (2) to supply a pressurized gas into it; a unit for extracting, preparing, and checking the activity of the gas sample, the unit being connected to the sampling pipeline (3) to extract the gas sample therethrough; a control and information processing unit (6) communicatively connectable to the — pressurized gas supply unit (7) and the unit for extracting, preparing, and checking the activity of the gas sample; a remote control (9) communicatively connectable to the control and information processing unit (6); wherein the feeding pipeline (2) comprises at least two atomizers (44) mounted at an end (41) of the enclosure, characterized in that the nozzles of the atomizers (44) are directed in such a way that the center axes of the nozzles intersect the central axis of the enclosure forming an intersection point located outside the enclosure.

Q 2. The apparatus (1) according to claim 1, wherein the feeding pipeline (2) comprises a N removable portion mounted at the end (41) of the enclosure along the periphery of an outer > section of the enclosure, S wherein the at least two atomizers (44) comprise three atomizers (44), each having a E 30 Laval nozzle, the three atomizers (44) being installed on said portion with an equal spacing = from each other.

2 S

3. The apparatus (1) according to claim 1, wherein the feeding pipeline (2) and the sampling pipeline (3) each are made of a tube having a diameter of 7 to 10 mm inclusive and a wall thickness of 0.5 to I mm inclusive. 11

4. The apparatus (1) according to claim 1, wherein the feeding pipeline (2) and the sampling pipeline (3) each have quick-disconnect couplings for connection to the pressurized gas supply unit (7) and the unit for extracting, preparing, and checking the activity of the gas sample, respectively.

5. The apparatus (1) according to claim 4, wherein the quick-disconnect couplings are formed as conical sleeves with a taper angle of 70 to 78 degrees, and pipeline tubes at the connection with the couplings have a taper angle of 62 to 70 degrees.

6. A method for leak detection of a fuel assembly by the apparatus (1) according to any of the claims 1-5, the method comprising: arranging the fuel assembly within an enclosure of a fuel handling machine; supplying gas under the enclosure using atomizers (44) arranged on the enclosure; extracting a gas sample from above the fuel assembly; and analyzing the gas sample. o

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