Control of the exit temperature of the coolant from each fuel channel of the Hallam Nuclear Power Facility reactor is obtained by adjusting the coolant flow rate by a remotely operated variable orifice. Two variable orifices were designed and the hydraulic characteristics determined. Both orifice designs utilized a tapered plug moving in and out of a restricted flow passage at the upper end of the fuel channel. Data were obtained on pressure drop as a function of flow rate at different orifice plug positions; all measurements were made using water, and data were converted to equivalent values for sodium. Either type of orifice was capable of adjusting flow rate to match the power output of a fuel element at any location in the reactor core. The temperature sensitivity (change in exit temperature per unit change in orifice plug position) of the first type of orifice was low (lO deg F/in.) when used in combination with a central fuel element, and high (7OO deg F/in.) when used with a peripheral element. The temperature sensitivity of the second type was more uniform (varying from 90 to 250 deg F/ in.). Consequently, the second type of orifice was selected for the HNPF. (auth)