[en] In an LWRWIMS calculation of a fuel assembly, the user normally designates several fuel pins as being of the same type. If these pins are physically identical, and similarly situated with respect to poisons and water gaps this seems a reasonable thing to do. However, unless the pins so designated are symmetrically placed within the assembly they will in practice burn up at different rates so that at any given irradiation they will have different isotopic compositions and macroscopic cross sections. The simple option in LWRWIMS assumes that all pins designated to be of a certain type deplete at an appropriate average rate and therefore have equal cross sections at all irradiations. The original differential option which attempted to compensate for this approximation has been shown to have serious defects and a new method has been developed to replace it. The new differential option in LWRWIMS, in which each pin is depleted independently at a rate determined by the local flux and spectrum, is a significant improvement on the old method. It is conceptually simpler and therefore less prone to failure, it is particularly suited to calculating isotopic spatial distributions and it costs no more in computing time. (U.K.)