Microstrucutural Characterization of RPV Steels Irradiated to High Fluences

Abstract

Neutron radiation embrittlement of reactor pressure vessel (RPV) steels is one of critical issues for the structural integrity assessment of the RPVs. Especially, the embrittlement at high fluences is of great interest for the long term operation of light water reactors because information on the mechanical property changes as well as embrittlement mechanisms is limited at high fluences. In this study, microstructural analyses were conducted on the RPV steels irradiated to high fluences in order to confirm the applicability of the trend curve at high fluence region. Steels investigated are five base metals and a weld metal with their copper content ranging from 0.02 to 0.25 wt. %. These steels were irradiated in the material test reactors to fluence up to 1.3 × 1020 n/cm2, E > 1MeV, at temperature of about 290 °C. After irradiation, transmission electron microscope (TEM) observations were performed to characterize the nano-meter scale microstructural changes due to irradiation. Formation of dislocation loops was observed. Number density and diameter of dislocation loop was investigated. Effects of chemical composition of steel and fluence on dislocation loop formation are discussed.

Effects of dose rate change under irradiation on hardening and microstructural evolution in A533B steel

“…The data followed the general trend line shown by the dashed line. These results showed that the formation of the solute atom clusters was the main microstructural evolution occurring by the heavy ion irradiations; this has been reported in the same steels irradiated by neutrons [3,14,15]. It was concluded that the microstructural changes similar to those of the neutron irradiation were caused during the heavy ion irradiation.…”

Current understanding of radiation-induced degradation in light water reactor structural materials

“…However, recent microstructural analyses have provided new knowledge mainly from TEM observations and PA measurements. TEM observations with improved specimen preparation techniques using focused ion beam techniques confirmed that dislocation loops are commonly formed in irradiated commercial steels [58,59,77,[98][99][100][101]. Figure 10 shows example TEM images indicating interstitial dislocation loops in A533B steels irradiated with heavy ions [98].…”

“…However, since the minimum size of the dislocation loops is almost comparable to the resolution limit of TEM (∼1 nm), the reported size and density should be recognized as the values for 'visible' dislocation loops. As the dose increases, the number density shows a clear increase, while the size shows a very slow increase [100].…”

“…Interstitial dislocation loops are thought to be one of the microstructural features of SMD. It was pointed out in several papers that the contribution of visible dislocation loops to hardening estimated using an Orowan-type hardening model was smaller than that of solute clusters [77,100,104]. However, since the estimated contribution of dislocation loops to hardening or transition temperature shift exceeded 10% of the total value, dislocation loops might have non-negligible contribution to radiation embrittlement.…”