Research On Surface Defect Detection Of Ceramic Coating For Nuclear Fuel Rod

The zirconium alloy casing is the most important part of nuclear fuel rods and its internal structure is related to the operation of the nuclear reactor.Usually,a layer of COF will be added to its shell to improve the wear resistance and enhance its mechanical strength.COF is a kind of new material,which has been widely used in nuclear reactor,microelectronics,optics,sensors,machinery,chemical industry and other fields due to the rapid development of science and technology.The defects caused by mass production or improper use.It is difficult to check these defects by human inspection,detect error will cause serious consequences.In order to detect the defects of COF effectively and achieve the purpose of quality monitoring,this paper has carried out research from two aspects.First of all,a DC voltage will be applied to two arbitrary fixed points on the surface and a Nano-amps current will be measured at the electrode by detection circuit due to the zirconia has high resistance characteristics.This design use STM32F103C8T6 as microprocessor,meanwhile using Buck converter topological circuit to reduce the 110 V DC source,and then use the linear regulator circuit to further stabilize the output voltage at the sets,the voltage accuracy is not less than 0.5% and ripple less than 5m V.The Nano-amps current detection module is used ADA4625 to construct trans-impedance amplifier,it can control by microprocessor to achieve multi-step amplification and automatic measurement.Multiple measurement method is used during the measurement process to reduce the error caused by the noise of the DC source.Polynomial fitting is carried out on the obtained data and the defects of the specimen is judged by the polynomial coefficients obtained from the fitting.Secondly,in order to get further judge on the defects,an adjustable frequency sinusoidal signal is used to stimulate eddy current coil which place on the surface of COF and using the phase shift and the voltage amplitude ratio between excitation signal and feedback signal.The sinusoidal signal generator uses AD9859 and control by microprocessor,it can output the frequency in the range of 10 k ～ 80 MHz.AD8336 is used to design a gain-controlled amplifier,which are composed of fixed gain Amp and Post-Amp.For purpose of further reduce the impact of accidental factors,the amplitude and the phase shift measurement curves of nondestructive zirconia ceramic samples and defective samples in different frequency bands were obtained by using the scanning frequency working mode,and the defect condition was determined by comparing the similarity of the curves.Finally,the program of DC potentiality detection system and eddy current detection system has been designed in detail,and the functional verification prototype is made to carry out simulation experiments.Sample resistances which value can up to 1GΩ is used to simulation in DC potentiality detection system,and it can distinguish the error of 1%resistance.In the eddy current testing method,it is found that the excitation frequency of COF defect detection has to greater than 40 MHz,or the detection effect is not obvious,and the defect depth can reach the micrometer scale.The experimental results show that two methods proposed in this paper are feasible for the defect detection of ceramic coating of nuclear fuel rods,which can provide a detection method and diagnostic basis for the next engineering design.