WO2019069122A1

WO2019069122A1 - Method of measuring the deformation of the fuel system using ultrasound

Info

Publication number: WO2019069122A1
Application number: PCT/IB2017/056754
Authority: WO
Inventors: Pavel Nerud
Original assignee: Centrum Vyzkumu Rez sro
Current assignee: Centrum Vyzkumu Rez sro
Priority date: 2017-10-05
Filing date: 2017-10-31
Publication date: 2019-04-11
Anticipated expiration: 2020-04-05
Also published as: CZ2017617A3; CZ307569B6

Abstract

The method of contactless ultrasonic measurement and evaluation of the distance between the ultrasonic probe and the reflecting surface of the nuclear fuel system and the energy reflected from the reflecting surface for calculating the geometry and deformation of the nuclear fuel system is that the reading of the correct distance value is carried out after determining the ratio of energy values (1) and the cylindrical surface of the fuel rods (2), the measurement being carried out in parallel movement of the ultrasonic probe (3) and the fuel system in the direction of its longitudinal axis. The measured values are processed in the measuring device (4) for processing the signal from the measurement and output display. Multiple probes are used for the measurement, and for each or some of them, the previous measurement method is used. When multiple probes are used (3), some probes (3) are used in transmission mode and others in Ultrasonic wave receiving mode.

Description

Method of measuring the deformation of the fuel system using ultrasound

Technical fields

The invention relates to the method of contactless measurement of the geometry of a nuclear fuel assembly consisting of a head, a socket, a supporting structure (including spacing grids), and a bundle of fuel rods, by means of ultrasound. Deformation measurement of irradiated nuclear fuel always takes place deep below the coolant level, to remove residual heat, and simultaneously to shield radioactive radiation. Measurements are conducted using remote-controlled devices that are equipped with contact or non-contact sensors or optical systems.

Background Arts

The beginnings of the measurement of the deformation of the fuel systems (elongation, deflection, and torsion) date back to the 1970s. The reason for this approach was to maintain a high safety standard for fuel assemblies after irradiation, to determine their deformation properties, and to predict behaviour in subsequent irradiation cycles. Several different systems and methods, based on different forms of scanning, have been used to measure deformation. Gradually, a form of partial automation of the measurement process was introduced to reduce the human factor's negative impact on accuracy, precision, and measurement speed.

Initially, contact sensors or contact sensor fields were used for measurement, touching the sides of the spacing grids. However, the use of contact sensors (e.g. LVDT) has a significant disadvantage. This disadvantage is the safety risk in the event of a fault condition of the sensor when stuck in the area of the fuel rods, where an undesired interaction of the sensor with the fuel rods may occur. Another dangerous condition can occur even if the sensor does not come into contact with any part of the fuel system, and its position relative to the fuel system is unknown. The fuel rods can be damaged by the sensor when removing the fault condition, so that a risk to nuclear and radioactive safety can occur. The advantage of measurement by contact sensors is the high accuracy of the measured values and the low degree of influence on the refrigerant flowing around the fuel system. Another approach used in the past was the use of optical systems, in particular, using cameras. These systems are based on the uniformity and simplicity of the geometry measurement system and the simultaneous use of this visual inspection system. The progress of visual systems compared to measurements by contact sensors is in the use of contactless measurement, and thus the elimination of the risk condition of a sensor failure (mostly a camera) or its control. The disadvantage of these visual systems is the high dependence of measurement accuracy on the optical conditions occurring in the environment between the camera and the fuel system (waves in the refrigerant due to temperature fluctuation, bending of the light, cooling of the refrigerant), and the need for special radiation-resistant optical systems (ordinary camera lenses in the radiation environment lose transparency, and used semiconductors degrade under the influence of radiation). Because of the difficulty of evaluating image output, the use of optical systems is significantly influenced by the human factor. The use of an automated image processing system is also relatively slow, due to the high computational complexity, and the complexity of decision algorithms.

Another innovative step is to use a contactless ultrasonic sensor array, positioned in fixed positions so that an ultrasonic beam contacts the fuel system via the side plate of the spacing grids. This method of measurement, like the use of contact sensors, requires a precise stationary state of the fuel system, which can be considered a disadvantage.

Another disadvantage is the need to use a large number of sensors and their interaction. On the contrary, a significant advantage is the elimination of the risk condition of damage to the fuel system when the ultrasonic sensor or its control fails.

In the last ten years, other methods of measuring fuel deformation have begun to emerge. These are methods using, for example, an oblique view of a fuel system and image analysis. Using this, the positions of characteristic elements of the fuel system are found, and a geometric network is created that is compared with a similar network of a non-deformed fuel system. This method of measuring deformation is very fast. It is also very safe, as relates to failures and unpredictable states of the system, as there is no interaction (or close connection) between the fuel system and parts of the measuring device. However, one deficiency is the low accuracy of measurement.

An important element that occurs in the measurement of the deformation of fuel systems is a certain degree of automation of the measurement and evaluation process, especially the processing of signals and images, thereby limiting the inappropriate influence of the human factor on the measurement and evaluation.

Disclosure of Invention

These deficiencies eliminate the method of measuring the deformation of the fuel system by means of ultrasound, where the measurement is carried out with the parallel movement of the ultrasonic probe and the fuel system in the direction of its longitudinal axis. The basic principle of the present invention is the simultaneous measurement of two variables, while detection of the second value is used to derive the correct value of one quantity needed to calculate the deformation. Changes in the values of the first magnitude (measured distance of the grid or fuel rods from the ultrasonic probe) are in the order of tenths of a millimetre, and it is difficult to tell which value is the reflection of ultrasound waves from the spacing grid or the fuel rods, respectively. Therefore, the second measured quantity (energy reflected from the surface of the spacing grid or fuel rods), which is significantly more sensitive to changes in the shape and gradient of the reflecting surface, is used to detect the distance grids. When reflecting ultrasound waves from fuel rods, due to the high curvature of the surface, most of the probe transmitted energy is reflected off the ultrasonic probe. The ratio of reflected energy returning to the probe to energy transmitted by the probe (this value is considered as the base - 100%) reaches low values. When reflecting ultrasound waves from the lateral surface of the spacing grid (the surface approaching the plane, and perpendicular to the ultrasound beam), most of the energy emitted by the surface incident probe is reflected back into the ultrasound probe. In this case, the ratio of the reflected energy returning to the probe to the energy transmitted by the probe is high. The difference in the reflected energy between the high and low values is tens of percent, and therefore, when moving the ultrasonic probe in the direction of the longitudinal axis of the fuel system, it clearly identifies the parts with significantly higher values corresponding to the spacing grids, and the lower values corresponding to the fuel rods. The correct value of the distance of the distance grid from the ultrasonic probe is determined by subtracting the value of the measured distance at the defined point of the section, denoted by the reflected energy as the spacing grid.

This innovative measurement and evaluation method makes it possible to measure the geometry of the entire fuel system (meaning deflection and torsion) in a single motion, during the extraction or lowering of the fuel system to or from the reactor or storage grid, storage container, or other location. By taking the measurements in the course of movements that would have been carried out without this measurement (it is not necessary to bring the fuel system into a stationary state), it significantly saves time in performing the measurements, and obtains information about all of the fuel systems with which it is being handled. It is not necessary to set the spacing grids in the appropriate position relative to the ultrasonic measuring probes; their detection is performed automatically from a measured distance and energy signals. This can also be considered to be one of the advantages of this measurement method. Another advantage is the ability to measure fuel systems with different numbers of spacing grids, without changing the location of the ultrasonic probes. An important advantage is also the simple automation of the process, and thus the limitation of the influence of the human factor on the measurement and evaluation of the results. The method detects an ambiguous evaluation of the position of the distance grid when the lateral surface of the grid is significantly deformed, or when the perimeter of the ultrasound beam is not secured to the side surface. The method allows measurement even in cases where the angle of the beam is not kept on the lateral surface of the distance grid, but the reflected beam energy is high enough to detect the resolution of the reflection from the distance grid and from the fuel rods.

Brief Description of Drawings

The invention is illustrated in more detail by means of drawings, in which: Figure 1 shows a basic measurement scheme and the principle of the measured signals. At the bottom right, the appearance of the signals of the measured quantities and their mutual relation to each other is schematically shown. Fig. 2. shows the difference between the reflection of the ultrasound waves from the level plane of the distance grid (upper part) and from the fuel rods (the lower part).

Made for Carrying out the Invention

The method of measurement and evaluation was verified in an experimental plant at the laboratory of the Rez Research Center, where simulations and measurements of different changes in the geometry of the fuel system can be made by changing the positions and positions of the spacing grid I, between the fuel rods 2, similarly to with the actual fuel system. These activities take place in the experimental device only on a fuel system imitator.

This device consists of a tank filled with water, in which a fuel system imitator is located, and a system allowing movement of the ultrasonic probe 3, pointing to the side of the imitator. The ultrasonic probe 3 is positioned at a predetermined distance, verified by another method (ruler, calibration), and the perpendicularity of the beam 3 of the probe to the imitator surface, where the focusing on the spacer is assumed, is verified on the reflected energy curve, and should reach high values of the defined distances. When the ultrasound probe 3 is moved in a direction other than in the direction of the longitudinal axis of the fuel system imitator, the reflected energy value should decrease.

At the same time, starting the motion of the ultrasonic probe 3 in a direction parallel to the axis of the fuel assembly imitator, recording of the axis values 7 is also started along longitudinal co-ordinates from the beginning of the measurement, the progression of the measured distance graph K⁾, and the progress of the reflected energy graph 1J_, from the measuring device 4, which controls the ultrasound probe 3. After the ultrasonic probe 3 moves towards the fuel system imitator, the measured data is stored. After reading the signals of all three variables, the signal of the progress of the reflected energy graph Π_. is analyzed by the computer. In sections where its value reaches low values in relation to the energy transmitted by the probe, the imitator surface is curved or non-perpendicular to the beam of the ultrasonic probe 3, and is therefore assigned to the range of fuel rods bundle 2. Sections that contain high energy from the energy-reflected proportions of the energy transmitted by the probe, are assigned to distance grid L In the next processing step, the longitudinal coordinates of each section are determined in the axis signal by the longitudinal co-ordinates 7 from the beginning, which is referred to as the distance grid J_, where the positioning centre of the grid 1 is determined by the reflected energy. All of these longitudinal co-ordinates are then assigned the distance values of the distance grid I from the ultrasonic probe 3 from the signal in the measured distance graph 10.

When plotting the values determined by the distances for the individual spacing grids I in the measured distance graph 10 in relation to the longitudinal co-ordinate 7 and the connection curve, the deflecting curve of the imitator of the fuel system is interpreted in the direction of the beam of the ultrasonic probe 3. To determine the spatial deformation curve of the imitator, multiple probes are required, each of which uses the above-mentioned method, along with another mathematical procedure.

A special case for determining the distance of elements of the fuel system from the plane of ultrasonic probe 3 is the case of replacing one ultrasonic probe 3 in the transmitter- receiver mode with two probes, each of which having a separate function (one being a transmitter, the other being a receiver). These two probes must be placed in a pair, so that the distance between the probes is known, and the plane passing through these probes is almost parallel to the side plane of the fuel system imitator. In this case, the measured distance is determined from the ultrasound wave flight time along the transmitter-reflecting surface- receiver track. Subsequent signal processing is the same as using one ultrasonic probe 3.

This method of measuring the deformation of the fuel system using ultrasound was also verified at the Temelin nuclear power plant, under real stress conditions.

Industrial Application of the Invention

The method of measurement of ultrasound geometry and the measurement of the measured signal can be applied in devices handling fuel systems, devices for inspection, and measurement of fuel systems or their storage.

These facilities are mainly found in nuclear power plants, but also in industrial business nuclear power plants, research institutes, and even wet storage sites for spent nuclear fuel.

This method can also be used to measure the geometry of other objects in which there is a significant difference in the shape of the reflecting surface (flatness-curvature).

Claims

PATENT CLAIMS

1. The method for contactless ultrasonic measurement and evaluation of the distance between the ultrasonic probe and the reflective surface of the nuclear fuel system and the energy reflected from the reflecting surface for calculating the geometry and deformation of the nuclear fuel system, characterized in that the correct distance value is deduced after determining the ratio of the values (3) and the fuel assembly in the direction of its longitudinal axis, the measured values being processed in the measuring device (1) and the cylindrical surface of the fuel rods (2), the measurements being carried out in a parallel movement of the ultrasonic probe (4) for signal processing from measurement and output display.

2. The method according to claim 1, characterized in that more probes (3) are used for the measurement, and for each of them or for some of them, the method of measurement is according to claim 1.

3. The method according to claims 1 and 2, characterized in that, if multiple probes (3) are used, some probes (3) are used in the transmission mode and the others in the ultrasound wave receiving mode.