The maximum safe limits on the storage and shipment of slightly enriched fuels may be increased by the addition of fixed neutron poisons. A theoretical study, verified by experiment, is presented to show that an infinite amount of O.95 per cent U235 enriched fuel will be subcritical when the fuel elements are encased in tubing at least ⅛ in. thick. The fuel elements considered are solid rods of 1.34-in. diameter and tubular elements of 1.37-in. O.D. and O.48-in. I.D. Without the tubing it is possible to make as little as four tons of the fuel elements considered critical.