Analysis and results of pulsed-neutron experiments performed on the Fort St. Vrain high-temperature gas-cooled reactor

Pulsed-neutron experiments were performed on the Fort St. Vrain reactor to determine the reactivity of the core for various control rod configurations while the reactor was still subcritical. For all configurations the reactivity was inferred from the in-hour equation using the measured decay constant and a calculated generation time. For the configurations near critical, both the reactivity and generation time were determined using the extrapolated area-ratio method. The predicted reactivities agreed poorly with those inferred from the experiments. However, by adding 5 ppM of boron to the reflector calculational model, the calculated generation time was significantly reduced. This brought the inferred reactivity into good agreement with that calculated for all control rod configurations. This emphasizes the dependence of the interpretation of pulsed-neutron experiments on calculations and the importance of the reflector in a large HTGR