ASAXS and SANS Investigations of the Chemical Composition of Irradiation-induced Precipitates in Nuclear Pressure Vessel Steels

The deterioration of the mechanical properties of reactor pressure vessel (RPV) steels during their irradiation in a nuclear power plant is known as neutron embrittlement. The structural reason of the neutron embrittlement of low alloyed RPV steel is the radiation-induced formation of precipitates. Up to now the chemical composition of these precipitates in the VVER 440 type RPV steel 15Kh2MFA are not clarified.
A combination of small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) using methods of contrast variation were employed to determine the chemical composition of the irradiation-induced precipitates in the 15Kh2MFA steel.
The difference between the scattering intensities from irradiated and unirradiated specimens gives the small angle scattering effect of the radiation damages. Irradiation-induced precipitates were found with a mean diameter of = 1...2 nm. The defect volume fraction is depending on the neutron fluence and amounts 0.1 - 0.35 %.
Information about the chemical composition was obtained by using the magnetic scattering contrast in the SANS experiment and the anomalous scattering contrast in the SAXS measurements.
A scan of the X-ray energy in the range of the V-, Cr-, Mn-, and Fe-K-absorption edges proves that the precipitates are vanadium rich. The scattering density of the precipitates is lower than the scattering density of the matrix.
This facts and the results of the magnetic contrast variation in the SANS experiments are in agreement with the assumption that the irradiation-induced precipitates are vanadium carbides.