Once a reactor is critical and at a constant power level, the net reactivity at any time is zero. The major contributions to changes of reactivity in the V.P.I. Argonaut reactor are: 1) Fuel and coolant temperature changes 2) Graphite temperature changes 3) Xenon poisoning The control rods have to be continuously positioned so that the total reactivity change of these parameters is nullified. The reactivity effects of the fuel and coolant have been combined into one temperature coefficient because for this investigation they are assumed to have simultaneous temperature changes. An experiment was conducted to measure the temperature coefficient of fuel and coolant which was found to be negative. The temperature coefficient for graphite was measured by installing heaters in two graphite blocks which were placed in the core. This experiment showed the reactivity effect of the graphite to be positive. The reactivity effect due to xenon poisoning was calculated using the xenon-iodine equations. The results of these experiments and calculations were used to predict the position of the control rods during long power operation. The predicted position was in good agreement with the actual position. / Master of Science