Created by: paulromano

For accurate beginning-of-cycle power distribution calculations in an LWR, it's important to account for the rapid increase in Xenon concentrations. Normally this would require depleting the model until Xenon reaches equilibrium conditions, but an alternative method is to solve a reduced system of equations that only accounts for I135 and Xe135 alone, assuming that equilibrium has already been reached (setting derivatives to zero). The first implementation of such a method was by Griesheimer in the MC21 code. Griesheimer demonstrated that this can actually be done on-the-fly during the solution by tallying I135 and Xe135 production rates and the Xe135 absorption rate, solving the reduced system of equations, and adjusting the number densities (all as part of the inactive batches).