Application of the Seebeck effect for monitoring of neutron embrittlement and low-cycle fatigue in nuclear reactor steel

The monitoring of neutron embrittlement and low-cycle fatigue in nuclear reactor steel is an important topic in lifetime extension of nuclear power plants. Among several material parameters that may change due to material degradation are the thermoelectric properties. Therefore, we investigated the application of the Seebeck effect for determining material degradation of common reactor pressure vessel (RPV) steel. The Seebeck coefficients (SC) of several irradiated Charpy specimens made from Japanese reference steel JRQ were measured. The specimens suffered fluences from 0 up to 4.5 E19 neutrons/cm2, with energies higher than 1 MeV. Measured changes of the SC within this range were about 500 nV/°C, increasing continuously in the range under investigation. Some indications of saturation appeared at fluencies larger than 4.5 E19 neutrons/cm2. We obtained a linear dependency between the SC and the temperature shift ΔT41 of the Charpy energy versus temperature curve, which is widely used to characterize material embrittlement.Similar measurements were performed on fatigue specimens made from the austenitic stainless steel X6CrNiTi18-10 (according to DIN 1.4541) that were fatigued by applying cyclic strain amplitudes of 0.28%. A clear correlation between the change of the SC and the accumulated plastic strain, i.e. number of cycles was obtained.Further investigations were made to quantify the size of the gauge volume in which the thermoelectric power (TEP), also called thermoelectric voltage, is generated. It appeared that the information gathered from a thermoelectric power measurement is very local. This fact can be used to develop a novel TEP-method acting as a thermoelectric scanning microscope (TSM).Finally, we conclude that the change of the SC has a potential for monitoring of material degradation due to neutron irradiation and thermal fatigue, but it has to be taken into account that several influencing parameters could contribute to the TEP in either an additional or extinguishing manner. A disadvantage of the method is the requirement of a clean surface without any oxide layer. This disadvantage can partially be avoided by using the proposed new TSM.