An instrument for nondestructively mapping core to cladding heat transfer quality in nuclear fuel elements has been developed. Heat transfer defects down to in. diameter in Zircaloy-2 clad and down to in. diameter in X-8001 aluminum alloy clad uranium fuel elements have been detected. An infrared radiometer was used to detect fuel element surface temperature variations during induction or plasma arc jet heating. The unusual application of plasma arc heating in this instrument made heat transfer testing of nonuniform fuel elements feasible.