lower power density. A two-fluid design might operate with a bigger blanket and keep the high power density core (which has no thorium and therefore no protactinium). However, a group of nuclear engineers argues in Nature (2012) that the protactinium pathway is feasible and that thorium is thus "not as benign as has been suggested . . ." Proliferation of neptunium-237 – In designs utilizing a fluorinator, Np-237 appears with uranium as gaseous hexafluoride and can be easily separated using solid fluoride pellet absorption beds. No one has produced such a bomb, but Np-237's considerable fast fission cross section and low critical mass imply the possibility. When the Np-237 is kept in the reactor, it transmutes to short lived Pu-238. All reactors produce considerable neptunium, which is always present in high (mono)isotopic quality, and is easily extracted chemically. Neutron poisoning and tritium production from lithium-6 – Lithium-6 is a strong neutron poison; using LiF with natural lithium, with its 7.5% lithium-6 content, prevents reactors from starting. The high neutron density in the core rapidly transmutes lithium-6 to tritium, losing neutrons that are required to sustain break-even breeding. Tritium is a radioactive isotope of hydrogen, which is nearly identical, chemically, to ordinary hydrogen. In the MSR the tritium is quite mobile because, in its elemental form, it rapidly diffuses through metals at high temperature. If the lithium is isotopically enriched in lithium-7, and the isotopic separation level is high enough (99.995% lithium-7), the amount of tritium produced is only a few hundred grams per year for a 1 GWe reactor. This much smaller amount of tritium comes mostly from the lithium-7 – tritium reaction and from beryllium, which can produce tritium indirectly by first transmuting to tritium-producing lithium-6. LFTR designs that use a lithium salt, choose the lithium-7 isotope. In the MSRE, lithium-6 was successfully removed from the fuel salt via isotopic enrichment. Since lithium-7 is at least 16% heavier than lithium-6, and is the most common isotope, lithium-6 is comparatively easy and inexpensive to extract. Vacuum distillation of lithium achieves efficiencies of up to 8% per stage and requires only heating in a vacuum chamber. However, about one fission in 90,000 produces helium-6, which quickly decays to lithium-6 and one fission in 12,500 produces an atom of tritium directly (in all reactor types). Practical MSRs operate under a blanket of dry inert gas, usually helium. LFTRs offer a good chance