Following irradiation in the FFTF-AC01 test at 360°C to 5.5 x 10²² n/cm², two HT9 samples tested at 30°C were measured to have fracture toughness levels of 28.2 and 31.9 MPa m^1/2, respectively, whereas a third identical specimen tested at 205°C gave 126 MPa m^1/2. Based on testing of notched tensile specimens from the same irradiation test, the low toughness was a result of brittle fracture. A similar low level of toughness has also been demonstrated in HT9 following irradiation at 250°C, and therefore such behavior is reproducible. Using ASTM Standard E 1921-02, which characterizes the fracture toughness of ferritic steels that experience onset of cleavage cracking at instabilities, it is shown that these data can be analyzed by a Master Curve approach, and that the trend of the fracture toughness over a wider range of temperatures can be estimated. Master Curve analysis demonstrates that toughness will remain low over a wide range of temperatures near 30°C, but will degrade only slightly when temperatures drop to -10°C.