A Monte Carlo method has been developed for the computation of the eigenvalue, as a function of buckling, in an infinite lattice. This method has been used to test the accuracy of earlier, approximate, void-worth computations, computations that enter into the analysis of hypothetical accidents in which voids collapse. Test results indicate that reactivity effects due to the collapse of bubbles in a molten pool can be computed, with reasonable accuracy, by the Behrens method, used earlier by Goldsmith. On the other hand, the Webb estimates of eigenvalue changes, caused by the expansion of fuel pins into the voids of a previously voided lattice, appear to be somewhat too high.