AN EVALUATION OF LOW-TEMPERATURE RADIATION EMBRITTLEMENT MECHANISMS IN FERRITIC ALLOYS

Investigations underway at Oak Ridge National Laboratory (ORNL) into r easons for the accelerated embrittlement of surveillance specimens of ferritic steels irradiated at 50-degrees-C at the High Flux Isotope Re actor (HFIR) pressure vessel are described. Originally, the major susp ects for the precocious embrittlement were a highly thermalized neutro n spectrum, a low displacement rate. and the impurities boron and copp er. Each of these possibilities has been eliminated. A dosimetry exper iment made at one of the major surveillance sites shows that the spect rum at that site is not thermalized. A new model of matrix hardening d ue to point defect clusters indicates little effect of displacement ra te at low irradiation temperature. Boron levels are measured at 1 wppm or less, which is inadequate for embrittlement. Copper and nickel imp urities 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 ar gued that copper impurity is not responsible for the accelerated embri ttlement 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 h igh ratio of gamma rav flux to fast neutron flux at the pressure vesse l. Gamma rays can also induce atomic displacements, leading to the sug gestion that the accelerated embrittlement may be provoked by gamma ir radiation.