Evaluating the variations of point kinetics parameters in pressurized heavy water reactor analyses

Abstract

Since the pressurized heavy water reactor design utilizes natural uranium bundles as its fuel, there is a need to keep replenishing the reactor core with unirradiated fuel bundles in order to keep the reactor critical. This practice causes the fuel inventory in the core to be very dynamic since the changes are not solely due to the depletion of the fuels due to irradiation but also due to the presence of new fuel bundles in the core. An average behaviour of the core after operating in an equilibrium stage for a long time can be captured through the time-average approach. This approach has been extensively used in the pressurized heavy water reactor fuel management, especially during the design phase where the actual data from the operating reactor are not readily available. The time-average core is utilized for many purposes such as to determine the average fueling rate, to determine the average worth of various reactivity control devices, as well as to determine the point kinetics parameters for transient analyses. The objective of this paper is to quantify potential variations in the point kinetics parameters (namely the decay constants and the delayed neutron fractions) as a function of core inventories with respect to the time-average-based values. In particular, 100 core configurations representing various snapshots from an equilibrium operation of the core have been utilized to quantify these potential variations. It should also be noted that the point kinetics equation (PKE) is utilized in this study, instead of full-core space–time model, since the objective of this paper is to bring about the awareness of potential variations and the PKE should be sufficient. The results of this study indicate that, for a point kinetics equations system involving six precursor groups, the variations in precursor decay constants and delayed neutron fractions could be as high as ±0.45% and ±4.85%, respectively. Using the point kinetics parameters in a simple transient involving an insertion of positive reactivity, it is demonstrated that the variation in the trip time for the transient could vary up to approximately ±3.55%, depending on the amount of positive reactivity insertion. These potential variations should be acknowledged since it is an important part of safety analysis. However, it is also important to note that, based on the amount of positive reactivity insertion evaluated in this study, the higher variation corresponds to a lower level of positive reactivity insertion which means that the transient will be slowly developing, could be observed more readily, and, therefore, could be easily terminated.