Spatial effects in specifying reactivity and application of point model in power reactors

The aim of this research is to specify a method by which large reactors can be described by point model, and point kinetic equations can be applied. By this method the in-core reactivity can be measured and the reactor size and dimensions do not have any effect on the kinetic equations and the measured reactivity. The effective multiplication factor of a reactor (k) is a value that is a function of time (not a function of location in the core). By solving the reactor modeling equations through which the dimensions of the reactor are considered, different values are obtained for k. If these values differ very much with each other, the spatial effects should be considered in measuring reactivity. These effects affect the needed time for showing the correct reactivity by reactimeter and eliminating these effects reduces aferesaid time. In this study it is shown that in large reactors if at any time we consider the in core neutron flux equal to the maximum flux at that time, the reactor can be considered as a point. In this research the other schemes which specify the reactor reactivity are also studied, compared with the maximum flux scheme, and it is concluded that the maximum flux scheme is the most precise and applicable method that can be used to approximate a reactor as a point.