[en] The perturbation is quantified for the surface heat flux and surface temperature in blowdown heat transfer (BDHT) bundle 1 fuel pin simulators due to the presence of the thermocouple and groove, gaps, heater eccentricity, and power zone breaks. Steady-state and transient-case studies are presented to illustrate the azimuthal and axial variations in the surface conditions. The conceptual designs of BDHT bundle 3 fuel pin simulators are also evaluated. Recommendations are made for improvements in the heater designs and for the development of computer codes to analyze the responses of the heater thermocouples