One has to decide which are the relevant parameters for controlling the evolution (fixed ks=eff, fixed power, …). For example, in the study by Brandan et al. [77], the average reactor power density (per fuel volume unit) was fixed at 330W/cm3, which permits local values of the order of 600W/cm3 at the reactor centre, an acceptable value from a thermo­dynamical point of view. The proton beam intensity (in this study a true spallation target was generated by FLUKA) is adjusted during the reactor evolution to compensate the reactivity fluctuations from one cycle to the next. This restriction imposes a constant number of fissions per unit time, so that it is possible to associate a fixed burn-up to all cycles (defined at fixed operation periods). In turn, this allows the direct comparison among inventories after different cycles for all types of reactor fuel.