Defining material volumes
Getting the right volumes for burnable materials is crucial for normalizing the reaction rates in burnup calculation. If the volume used in the calculation is incorrect, the reaction rates are over- or under-estimated, which leads to accumulating errors in material compositions as they are burnt. This can be pretty confusing for the user, since the results are correct for the initial composition, and if the volumes are not off by several factors, it may take several burnup steps before the errors become apparent. Serpent tries to calculate the volumes automatically, and can do that pretty well for 2D assembly geometries constructed using the "pin" geometry type. If burnable regions are more complex, the code prints out an error message saying that the volumes must be entered manually, which is usually not a major problem either.
The real danger lies in geometries in which the code thinks that the volumes are appropriately calculated, and has no way of telling the user that something may be wrong. The most typical case where this happens is that the burnable regions are defined using the pin-structure, but some of the pins are cut by the boundaries of a universe at another geometry level. Imagine, for example, a 1/4 model of a PWR fuel assembly: