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Uranium-236

Uranium-236 is an isotope of uranium, neither fissile with thermal neutrons, nor good fertile material, but is considered a nuisance and long-lived radioactive waste. It is found in the reprocessed uranium made from spent nuclear fuel; the fissile isotope uranium-235 fuels most nuclear reactors. When 235U absorbs a thermal neutron, one of two processes can occur. About 82% of the time, it will fission. Thus, the yield of 236U per 235U+n reaction is about 18%, the yield of fissile decay products is about 82%. In comparison, the yields of the most abundant individual fission products like caesium-137, strontium-90, technetium-99 are between 6% and 7%, the combined yield of medium-lived and long-lived fission products is about 32%, or a few percent less as some are destroyed by neutron capture; the second-most used fissile isotope plutonium-239 can fission or not fission on absorbing a thermal neutron. The product plutonium-240 makes up a large proportion of reactor-grade plutonium. 240Pu decays with a half-life of 6561 years into 236U.

In a closed nuclear fuel cycle, most 240Pu will be fissioned before it decays, but 240Pu discarded as nuclear waste will decay over thousands of years. While the largest part of uranium-236 has been produced by neutron capture in nuclear power reactors, it is for the most part stored in nuclear reactors and waste repositories; the most significant contribution to uranium-236 abundance in the environment is the 238U236U reaction by fast neutrons in thermonuclear weapons. The A-bomb testing of the 1940s, 1950s, 1960s has raised the environmental abundance levels above the expected natural levels. 236U, on absorption of a thermal neutron, does not undergo fission, but becomes 237U, which beta decays to 237Np. However, the neutron capture cross section of 236U is low, this process does not happen in a thermal reactor. Spent nuclear fuel contains about 0.4% 236U. With a much greater cross-section, 237Np may absorb another neutron, becoming 238Np, or fission. 236U and most other actinides are fissionable by fast neutrons in a nuclear bomb or a fast neutron reactor.

A small number of fast reactors have been in research use for decades, but widespread use for power production is still in the future. Uranium-236 alpha decays with a half-life of 23.420 million years to thorium-232. It is longer-lived than any other artificial actinides or fission products produced in the nuclear fuel cycle. Unlike plutonium, minor actinides, fission products, or activation products, chemical processes cannot separate 236U from 238U, 235U, 232U or other uranium isotopes, it is difficult to remove with isotopic separation, as low enrichment will concentrate not only the desirable 235U and 233U but the undesirable 236U, 234U and 232U. On the other hand, 236U in the environment cannot separate from 238U and concentrate separately, which limits its radiation hazard in any one place; the half-life of 238U is about 190 times as long as that of 236U. That is, in reprocessed uranium with 0.5% 236U, the 236U and 238U will produce about the same level of radioactivity. The ratio is less than 190.

The decay chain of uranium-238 to uranium-234 and lead-206 involves emission of eight alpha particles in a time short compared to the half-life of 238U, so that a sample of 238U in equilibrium with its decay products will have eight times the alpha activity of 238U alone. Purified natural uranium where the post-uranium decay products have been removed will contain an equilibrium quantity of 234U and therefore about twice the alpha activity of pure 238U. Enrichment to increase 235U content will increase 234U to an greater degree, half of this 234U will survive in the spent fuel. On the other hand, 236U decays to thorium-232 which has a half-life of 14 billion years, equivalent to a decay rate only 31.4% as great as that of 238U. Depleted uranium used in kinetic energy penetrators, etc. is supposed to be made from uranium enrichment tailings that have never been irradiated in a nuclear reactor, not reprocessed uranium. However, there have been claims that some depleted uranium has contained small amounts of 236U.

Depleted uranium Uranium market Nuclear reprocessing United States Enrichment Corporation Nuclear fuel cycle Nuclear power Uranium | Radiation Protection Program | US EPA NLM Hazardous Substances Databank - Uranium, Radioactive

John Randall (shipbuilder)

John Randall was an English shipbuilder. The son of John Randall, shipbuilder of Rotherhithe, he had a liberal education, on the death of his father, around 1776, continued the shipbuilding business under his own management, he worked on mathematics, naval construction. In addition to many ships which he built for the mercantile marine and for the East India Company, Randall built over 50 naval vessels, they included 74-gun ships and large frigates, among them being HMS Audacious, HMS Ramillies, HMS Culloden, noted in the French Revolutionary Wars. He took a prominent part in founding the Society of Naval Architects. On the Peace of Amiens, Randall lowered his rates of pay from the wartime level, his men went out on strike; the Admiralty permitted him to take on workmen from the Deptford dockyard, offered a military force to protect them, turned down. The Deptford men were prevented from working in his yard. At this point Randall died, at his house in Great Cumberland Street, Hyde Park, on 23 August 1802.

He left a family. Attribution This article incorporates text from a publication now in the public domain: Lee, Sidney, ed.. "Randall, John". Dictionary of National Biography. 47. London: Smith, Elder & Co