Higher accuracy measurement of coating and wall thickness in tubes and claddings

Exact determination of wall thickness can be a very challenging task for thin walled tubes. Example of such a tube is nuclear fuel cladding. Usually its diameter is bellow 10 mm and the wall thickness less than 1 mm. There are many conventional methods of wall thickness determination, among others ultrasonic measurement, eddy current, metallographic-semi-destructive measurement and others. In particular case, there is need of extremely high accuracy because the nuclear fuel can lose its integrity and the breakage of the 1st safety barrier of the nuclear fuel. In such a situation, the costs of spent fuel assembly exchange, its consequent storage, contaminated medium filtering and prolongation of the outage of nuclear power plant, could be beyond the planned (regime). Expected accuracy of nuclear fuel cladding's wall thickness measurement is less than 1 micrometer. The uncertainty effects consequent measurements, calculations and predictions, model errors and others. All together, such uncertainty, e.g. 700 nm, can lead to a false judgment of the current state of inspected fuel in the view of e.g. fretting or grid-to-rod fretting. Recent commonly used NDT methods provide accuracy of the measurement in range of several micrometers. In case of metallographic cut and following measurement, it can be even bellow one micron. Nevertheless, the obstructions connected to sample preparation are enormous. Newly developed method combining small number of resonance measurements can lead in specific cases to measurement errors about 300 nm. Thus the wall thickness can be determined for example as 0.5850 ± 0.0003 mm. This thickness measurement represents fuel cladding type E110 targeted to VVER light water reactors. Accurate measurement can be applied elsewhere, in tubes unreachable from the end, unknown coating thickness or any other tasks requiring high precision measurement. The presented work has been realized within the Institutional Support by Ministry of Industry and Trade of the Czech Republic, No. 11/2023.