The crack-arrest fracture toughness (K_a) of unirradiated and irradiated specimens machined from a low upper-shelf, high-copper weld was determined. The specimens were machined from the beltline weld of the Midland Unit 1 nuclear reactor pressure vessel (RPV). This weld was fabricated from one heat of weld wire and one lot of welding flux and bears the designation WF-70. It is of particular interest because it has been used in several RPVs that may still be in operation. The effect of irradiation has been measured in terms of the shift of the mean and lower-bound crack-arrest fracture toughness versus temperature curves. These shifts are compared to (1) 41-J transition temperature shift from Charpy V-notch (CVN) tests, ΔTT_41-J; (2) shift in the Master Curve transition temperature, T₀, at the 100 MPa√m level, ΔT₀; and (3) shift of the “arrest force” F_a temperature determined from instrumented CVN testing, ΔTF_a. Preliminary results indicate that the shift of K_a of this low upper-shelf energy weld is approximately the same as the shift ΔTT_41-J, larger than ΔT₀, and significantly larger than ΔTF_a.