In calculating a closely packed lattice, it is well known that one-velocity integral transport theory is the most useful method. Results are briefly presented for calculation of the ratio of moderator to fuel flux in a lattice, , by first and second approximations which have been developed by Corngold. In order to compare these approximations with various other calculating methods, some approximate formulations of a penetrability factor are discussed. After comparing the numerical results from the first and second approximations with the ones of Wachspress, Amouyal, Bengston, and the blackness method, the second approximation is considered to be the best. Consequently the values of the flux ratio in a lattice of fuel cylinders are calculated by using a unit cell method, and the second approximation in the case of a slab lattice which has a mean chord length equivalent to that of the actual lattice, and the results are compared with experiment.