Technical Fields

The invention relates to a method of non-destructively measuring the thickness of pipes that can only be accessed from the outside or just possibly from the inside. Particularly good results are obtained with thin-walled pipes of small diameter, such as nuclear fuel cladding or steam generator pipes. Measurement of the cladding thickness is important for downstream activities such as nuclear fuel inspections or thermomechanical calculations. The non-destructive form of measuring the thickness of thin- walled components such as fuel claddings opens up the possibility of expanding activities within the inspection of power production related equipment to other areas of industry, testing or structural diagnostics.

Background Arts

The measurement of the wall thickness of thin-walled parts in the machine industry is most often carried out by micrometer measurement. This measurement is carried out randomly on the production lines to check the accuracy of production taking into account the requirements of the end user. The measurement itself is carried out using a micrometer, the accuracy of which is around 10 microns and is mainly influenced by the temperature of the instrument, the object to be measured, but also by the personnel performing the measurement, the roughness of the surface and the degree of surface contamination. This method cannot be applied on closed pipes or in inaccessible locations, for example far from the pipe edges. An example of such a situation is a fuel rod containing nuclear fuel, the opening of which would result in contamination of the immediate environment.

A very precise destructive method is the preparation of metallographic cuttings for subsequent microscopic measurements. The great advantage of this method is its accuracy. This is in the order of several microns. The disadvantage is the low flexibility in the view of the location of thickness determination and the time and technologically consuming process of sample preparation for microscopy. In terms of industry, the use of this method is very limited and is reserved almost exclusively for laboratory environments. The technology can be performed on radioactive samples such as nuclear fuel claddings. However, the transport of fuel assemblies to the hot cells usually takes place several years after the fuel has been removed from the reactor core and the method is therefore in principle unsuitable.

The most popular method for the non-destructive determination of thickness, and not only of metal products, is the ultrasonic reflection method (ultrasonic pulse echo), based on measuring the time of flight of ultrasonic pulses. The principle of the method is that the signal sent by the ultrasonic probe through the material of the tested object, is reflected from its opposite wall and returned to the receiving gauge (the same ultrasonic probe). The thickness is then determined according to the physical properties of the tested material and the measured time of flight. This method is a frequently used non-destructive method in the engineering industry as well as in testing and diagnostics of structures, it is included in Czech and European standards (e.g. CSN EN ISO 16809, valid from 01.02.2020). The accuracy of the method depends on the sensitivity and resolution of the probes and the sensing device. For thin-walled components, measurement accuracy is in the tens of microns. In the case of measuring pipes with the really thin walls, such as nuclear fuel claddings, where the wall thickness is less than 600 microns, the accuracy of the method is in many cases insufficient.

Disclosure of Invention

These shortcomings are eliminated by the method of non-destructive thickness measurement of small-diameter thin-walled pipes, which works without the need to disassemble or damage the structure, in a relatively short time and at an arbitrarily chosen location in the pipe. It uses one or more resonant frequencies of the pipe, which are allowed to oscillate in the plane of the pipe cross-section.

It can be triggered by a short blow with a rigid object such as a hammer. This is the so- called impact-echo method.

It is also possible to excite by gradual sweeping, i.e. forced harmonic oscillation, where its wavelength gradually changes over time to cover a wider frequency spectrum, while the amplitude of the excited signal remains the same. This is a classical resonance method. The tube oscillates more intensely at fundamental frequencies, with greater amplitude. This is then scanned by an electro-acoustic probe, recorded and displayed by an oscilloscope. The frequencies with the highest amplitudes then correspond to the fundamental frequencies, i.e. the resonant oscillations of the pipe. The pipe's excited oscillations come in a variety of different shapes, but this method only deals with some of them. For thickness measurements of small diameter thin-walled pipes, it has proven useful to work with higher frequencies and to use the oscillation of the pipe in the plane of its cross section. These fundamental frequencies can be measured but also predicted by calculation. Important parameters that go into the fundamental frequency calculation are the material and geometrical properties of the pipe, and possibly also external influences that affect the pipe.

The method does not consider these external influences further for its calculations. However, they cannot be neglected. It is therefore advisable to avoid such influences during the measurement. These are mainly external boundary conditions such as the way the pipe is supported or suspended, the filling or wrapping of the pipe or its pressurisation. A specific case of such external conditions are coatings or oxidation layers. The use of these circumstances will be shown in the section of examples of the implementation of the invention.

Among the material properties of the pipe, the elastic modulus and the density of the pipe material are particularly important properties for predicting the fundamental frequency. Both properties can also be expressed in terms of the propagation speed of longitudinal elastic waves.

Among the geometrical properties, the diameter of the pipe and wall thickness are important for frequency prediction. From the computational relations (1) and (2) for predicting the selected natural frequencies, it is possible to express just the pipe wall thickness as a function of the above mentioned variables.

Here fi and f _n are the ring mode and in-plane bending mode frequencies, n is the number of high harmonic, CL is the P-wave propagation speed, d is the diameter of the circular pipe, E is the elastic modulus, / is the wall thickness, and p the density of the pipe material. Generally the method works for all pipe sizes and shapes. However, its extraordinary accuracy is depended on the wall thickness, and also on the diameter of the pipe. For very small, thin-walled pipes, such as nuclear fuel claddings, the precision can achieve even in the range of microns.

Probes for measuring the local thickness of the cladding can be part of the standard equipment used for fuel inspection, for example in spent fuel pools. Being watertight and having high resistance to radiation are prerequisites.

The undeniable advantage of the non-destructive thickness measurement method is that the oscillation sensor can be an ultrasonic probe that is sufficiently resistant to water and radioactive gamma radiation. It is advantageous that the evaluation equipment, which is sensitive to radioactive radiation, can be concealed at a sufficient distance from the source of ionising radiation.

Brief Description of Drawings

The invention is further elucidated by means of the drawings, where Fig. 1 shows the oscillation shapes of a cross-section of a pipe having a circular cross-section. Fig. 2 shows an example of the frequency spectrum of the measured signal including the peaks of some interesting resonant frequencies of the pipe.

Made for Carrying out the Invention

One specific example of the use of this method is the measurement of the local thickness of nuclear fuel claddings during periodic inspections. The local geometry enters into further calculations and/or measurements. An example is the measurement of the pressure inside a fuel rod, where the wall thickness is determined by the design of the specific fuel, but long-term measurements show standard deviations of up to 20 microns from the designed value. However, such accuracy cannot be counted on in sensitive fuel rod calculations.

Another application of the method is to verify the thickness of the oxidation layer, or the thickness of the healthy pipe material. This is particularly advantageous when oxidation occurs inside the pipe and the accuracy of conventionally used methods is not sufficient. The method can be used even if the pipe is coated with a protective layer that cannot be removed for some reason. Even in this case, it is possible to determine the thickness of the pipe material accurately enough. However, slight variations due to the unknown stiffness of the coating must be taken into account.

There are many inherent oscillation shapes that could be used to predict pipe thickness. However, the best results are achieved by combining the fundamental frequency of oscillation in the plane of the pipe cross-section with in-plane bending mode 1 and ring mode 2. Incorporating both frequencies 5 and 6 into the appropriate relationship along with the pipe diameter will give a reasonably accurate pipe thickness result. One of the above-mentioned frequencies can then be replaced in the calculation of the pipe wall-thickness by the velocity of propagation of longitudinal elastic waves, or by the square root of the ratio of the elastic modulus and the density of the pipe material,

■■ ■ jl where CL is the propagation speed of longitudinal elastic waves, E is the elastic modulus and p is the density of the pipe material.

The fundamental frequencies can be found in Fig. 2 where amplitude, axis 3, is a function of frequency, axis 4. Fundamental frequencies appear as so-called frequency peaks 5, 6 or 7. However, without first estimating the two peaks of frequencies 5 and 6 as accurately as possible, it is impossible to find them in the complex frequency spectrum. The spectrum contains a number of peaks of high harmonic 7, noise and other obscured peaks. Also, the method, location and intensity of excitation must be adapted to the measurement of each frequency. East but not least, it is recommended to verify the gain of the correct values of the intended frequencies by fitting them to the pre-prepared relations (1) and (2) based on a good estimation of the pipe wall thickness and the longitudinal elastic wave propagation velocity. Industrial applicability

The method can be applied in all areas of industry where non-destructive determination of the wall thickness of a closed pipe is required. The method is particularly suitable for thin-walled closed pipes and tubes where the accuracy of standard methods is not sufficient and the measured value can have a significant influence on further work with the measurement results.

The method can be used, for example, to check the thickness of nuclear fuel cladding, which is also essentially a closed thin-walled tube of small diameter. The method is applicable for qualification of fuel assemblies for a deep repository where a repeat verification of the fuel condition will be required. The method can also be used to investigate fuel in hot cells.

Outside the nuclear industry, the method can be applied to the inspection of piping in laboratory apparatus where highly abrasive media are used - liquids or gases with solid particles or acid admixture. In this way, it is possible to check the degree of wear of the piping without dismantling the installation and thus interrupting the technological process for a significant length of time and potentially leaking liquids or vapours into the surroundings.

List of reference marks

1 - The shape of the pipe bend in the plane of the cross- section in in-plane bending mode

2 - The shape of the pipe bend in the cross-section plane in (in-plane) ring mode

3 - Signal amplitude axis

4 - Signal frequency axis

5 - Peak resonance frequency of the cross section (in in-plane bending mode)

6 - Peak of the resonant frequency of the cross section (in ring mode)

7 - Frequency peak of the high harmonic of the test body