An evaluation of low temperature radiation embrittlement mechanisms in ferritic alloys

Investigations underway at Oak Ridge National Laboratory (ORNL) into reasons for the accelerated embrittlement of surveillance specimens of ferritic steels irradiated at 50°C at the High Flux Isotope Reactor (HFIR) pressure vessel are described. Originally, the major suspects for the precocious embrittlement were a highly thermalized neutron spectrum, a low displacement rate, and the impurities boron and copper. Each of these possibilities has been eliminated. A dosimetry experiment made at one of the major surveillance sites shows that the spectrum at that site is not thermalized. A new model of matrix hardening due to point defect clusters indicates little effect of displacement rate at low irradiation temperature. Boron levels are measured at 1 wppm or less, which is inadequate for embrittlement. Copper and nickel impurities are shown to promote radiation strengthening at high doses but not at the low doses pertinent to the surveillance data. It is shown that a copper embrittlement scenario has other drawbacks, and it is argued that copper impurity is not responsible for the accelerated embrittlement of the HFIR surveillance specimens. The dosimetry experiment revealed unexpectedly high levels of reaction products in some of the fast flux monitors, which are found to be caused by an exceptionally high ratio of gamma ray flux to fast neutron flux at the pressure vessel. Gamma rays can also induce atomic displacements, leading to the suggestion that the accelerated embrittlement may be provoked by gamma irradiation.