Abstract Average BWR assembly cross-sections for nominal conditions, namely for zero bypass void, can be utilized in the analysis of transient conditions with boiling in the bypass. A model is developed to yield an effective channel void for such conditions. The use of this void in conjunction with the “nominal conditions” cross-section library approximately preserves the assembly K ∞ corresponding to the true channel and bypass voids. The effective void is an augmentation of the actual channel void. The augmentation is proportional to the bypass-to-channel volume ratio, to the bypass void and to a weight W which is introduced to quantify the fact that a water molecule in the bypass has a different assembly criticality worth from 1 in the channel. The formula developed is superior to the practice of choosing W = 1, namely a simple, non-weighted, transfer of water from the channel to the bypass. The use of this approximate effective channel void reproduces actual K ∞-values of assemblies to better than 5 mk.