The control of membrane pore size is critical for the separation efficiency of the isotopic separation of U-235/238 in the conventional uranium enrichment process by gaseous diffusion. Theoretical evaluations generally assume that the pores are cylindrical. however the actual pore morphology could be completely different. Here. we consider the case of pores with rectangular slit morphology ('groove-like' pores) of width l and length D (when D much greater than 1). We evaluate the separation factor a for membranes consisting of a mixture of 'cylindrical' and 'rectangular' pores. These two shapes may be considered as two extreme cases of pore morphology, and it is possible to represent any arbitrary actual morphology approximately as a mixture of such pores. In addition, the existence of defects on membranes, such cracks, crevices, grooves, etc. may severely affect membrane performance: and this will be more critical the smaller the pores. Here. we also use this 'rectangular' model to study the influence of such cracks on the separation efficiency of membranes.