Small, high-temperature, fast-spectrum reactors, of the type proposed for auxiliary space-power applications, cannot be conveniently controlled by fuel or control-rod motion in the core. Consequently, the reflector (which may be a moderator) must provide the needed reactivity control. Critical experiment measurements employing conventional as well as pulsed-neutron techniques on various reflector control methods show that at least 5% Δk control worth is easily achieved, and full shutdown of over 12% Δk can be achieved by complete reflector removal. Though pulsed-neutron techniques are convenient for evaluating large changes in system reactivity, the interpretation of such measurements is far from straightforward, particularly on fast reactors with moderating reflectors.