Reactor is the use of a neutron moderator with a lower neutron absorption cross section than protium. Options include heavy water as used in CANDU type reactors or graphite as used in Magnox or RBMK reactors. Obtaining heavy water however also requires isotope separation, in this case of Hydrogen isotopes, which is easier due to the bigger variation in atomic weight. Both Magnox and RBMK reactors had undesirable properties when run with natural uranium, which ultimately led to the replacement of this fuel with low enriched uranium, negating the advantage of foregoing enrichment. Pressurized Heavy Water Reactors such as the CANDU are still in active use and India which has limited domestic uranium resources and been under a partial nuclear embargo ever since it became an atom bomb state in particular relies on heavy water moderated reactors for its nuclear power. A big downside of heavy water reactors is the enormous upfront cost of the heavy water. Practical methods of separation Diffusion Often done with gases, but also with liquids, the diffusion method relies on the fact that in thermal equilibrium, two isotopes with the same energy will have different average velocities. The lighter atoms (or the molecules containing them) will travel more quickly through a membrane, whose pore diameters are not largr than the mean free path length (Knudsen flow). The speed ratio is equal to the inverse square root of the mass ratio, so the amount of separation is small. For example for 235UF6 versus 238UF6 it is 1.0043. Hence many cascaded stages are needed to obtain high purity. This method is expensive due to the work needed to push gas through a membrane and the many stages necessary. The first large-scale separation of uranium isotopes was achieved by the United States in large gaseous diffusion separation plants at Oak Ridge Laboratories, which were established as part of the Manhattan Project. These used uranium hexafluoride gas as the process fluid. Nickel powder and electro-deposited nickel mesh diffusion barriers were pioneered by Edward Adler and Edward Norris. See gaseous diffusion. Centrifugal Centrifugal schemes rapidly rotate the material allowing the heavier isotopes to go closer to an outer radial wall. This too is often done in gaseous form using a Zippe-type centrifuge. Centrifuging plasma can separate isotopes as well as separating ranges of elements for radioactive waste reduction, nuclear reprocessing, and other purposes. The process is called "plasma mass separation";