A major concern for ferritic/martensitic steels in light-water reactors, fast reactors, and future fusion reactors is the effect of irradiation on fracture [1,2], as exhibited in a Charpy V-notch (CVN) test as an increase in the ductile-brittle transition temperature (DBTT) and a decrease in the upper-shelf energy (USE) (see Fig. 14.1) [3]. (Note that values for the Charpy energy in Fig. 14.1 and other figures in this chapter appear low relative to values normally reported for such steels; this is due to the use of the subsize Charpy specimens discussed in Chapter 8.) Such a change in properties is referred to as irradiation embrittlement [1,2] or low-temperature irradiation embrittlement to distinguish it from elevated-temperature helium embrittlement discussed in Chapter 13.