Nondestructive characterization of neutron induced embrittlement in nuclear pressure vessel steel microstructure by using electromagnetic testing

By using nuclear power for energy generation the pressure vessel wall is exposed to neutron fluences of different levels depending on the distance to the core. Hence materials undergo a change in their microstructure in terms of embrittlement, to be measured as toughness reduction and shift of the Ductile-to-Brittle Transition Temperature (DBTT) to higher temperatures. Normally plant safety concerning this change in microstructure is assured by destructively testing surveillance samples. These are standard tensile and ISO V-specimen which consist of exactly the same material as the pressure vessel and its weld metal, being exposed to accelerated irradiation rates within special irradiation channels allowing a pronounced ageing. It is demonstrated that electromagnetic parameters allow to characterize the changes in the microstructure generated through neutron irradiation. After a defined calibration process a quantitative characterization of the embrittlement especially in terms of the shift of the DBTT is possible. This has been demonstrated for reactor pressure vessel steels used in nuclear power plants of eastern and western designs. As testing methods 3MA (Micromagnetic, Multiparameter, Microstructure and stress Analysis) [1] and the dynamic magnetostriction using EMAT (Electromagnetic Acoustic Transducers) have been applied in a nondestructive combination measurement system. Further experiments show the possibility to measure 3MA and dynamic magnetostriction quantities through an 8 mm thick austenitic stainless steel cladding.