(I apologize in advance if this is in the wrong section of the forum) I found an article online describing the safety concerns of the W88 thermonuclear warhead. Apparently, water can turn the uranium-235 coating of the W88's secondary into a "runaway boiling hot water reactor" (not sure what that means...), causing the secondary to go critical. Since my knowledge of nuclear physics is practically nonexistent, I can't really understand what the article is talking about. Can someone explain to me why/if this process would occur? From http://www.fas.org/sgp/eprint/morland.html This use of uranium-235 in the outer shell, or pusher, of the W-88 secondary raised a curious environmental problem that was first noted with the Hiroshima bomb, Little Boy, in 1945. If uranium-235 is used as reactor fuel, the moderator can be ordinary salt or fresh water (not heavy water, or graphite as required with uranium-238), and the reactor can be quite small. Most research reactors use highly enriched fuel for this reason, including the one on the campus of the Massachusetts Institute of Technology in the heart of greater Boston, but that's another story. Uranium-235 plus water equals a nuclear reactor, especially if the geometry of the metal allows the water to get between metal surfaces where neutrons can be moderated, or slowed down, as they travel from one surface to another. In theory, the hollow uranium-235 projectile of Little Boy could become a runaway boiling water reactor if it fell into the ocean. (Until John Coster-Mullen's book Atomic Bombs was self-published in 2002, all descriptions and graphic depictions of Little Boy had the large hollow uranium piece as the stationary target and a smaller cylindrical piece as the projectile. Coster-Mullen's contrary account comes from years of interviewing men of the 509th Composite Group who assembled and dropped the bomb.) The problem with Little Boy was ignored. However, by the time of the first Polaris Submarine, the standard primary design employed a hollow plutonium shell which could theoretically fill with water and become a reactor. Because the Polaris W-47 warhead was deployed at sea, a test was done on May 11, 1960, code-named the Stardust Program. A hollow plutonium pit was filled with water in an underground tunnel at the Nevada Test Site. The pit did not go critical. But the secondary is much more massive than the primary. When the decision was made with the W-88 to replace the uranium-238 pusher shell in the secondary with uranium-235, the boiling water reactor problem was undeniable. If the submarine sank, or a warhead was damaged and dropped into the ocean, the secondary would go critical as soon as water got inside it. Considering the damage a nuclear war is designed to cause, such safety considerations are of course trivial, but the problem was studied, anyway.