The effect of phosphorus on precipitation in irradiated reactor pressure vessel (RPV) steels

Embrittlement of light water reactor pressure vessel (RPV) steels by fast neutron irradiation may limit extended nuclear plant life. Embrittlement, which is manifested as increases in various indexes of a ductile to brittle transition temperatures (AT), is primarily due to hardening by nanoscale precipitates containing Cu, Ni, Mn, and Si, which form under irradiation. In addition to these elements, P has also been found to play a role in embrittlement. While only slightly enriched in the precipitates, hardening and embrittlement increase with trace P concentrations in low-Cu steels. Here, we characterize the individual and synergistic irradiation precipitation and hardening mechanisms in a series of RPV steels containing no to low-Cu and with systematic variations in Ni and P. The steels were irradiated to a fluence of - 1.38x1020 n/cm2 at - 292 degrees C in the UCSB ATR-2 experiment. In nominally Cu-free medium and high-Ni RPV steels, atom probe tomography shows that P and Ni promote precipitation of P-Mn-Si-Ni and Mn-Si-Ni precipitates, respectively. The precipitate microstructure correlates with the observed irradiation hardening.

Automated summary

Embrittlement of light water reactor pressure vessel steels by fast neutron irradiation is primarily due to the formation of nanoscale precipitates containing Cu, Ni, Mn, and Si. However, phosphorus (P) also plays a role in embrittlement, with hardening and embrittlement increasing with trace P concentrations in low-Cu steels. Atom probe tomography analysis showed that P and Ni promote precipitation of specific precipitates, which correlates with the observed irradiation hardening.