page_title: Uranium hydride bomb
section_title: Theory
hierarchical_section_title: Uranium hydride bomb / Theory
caption_attribution_description: English: Humorous depiction of the MK2, ELMER, the “unattractive and clumsy” uranium hydride bomb, and the “expensive” L.C. "ELSIE", the MK8 water penetrating fission bomb.
context_page_description: The uranium hydride bomb was a variant design of the atomic bomb first suggested by Robert Oppenheimer in 1939 and advocated and tested by Edward Teller. It used deuterium, an isotope of hydrogen, as a neutron moderator in a uranium-deuterium ceramic compact. Unlike all other fission-based weapon types, the concept relies on a chain reaction of slow nuclear fission. Bomb efficiency was adversely affected by the cooling of neutrons since the latter delays the reaction, as delineated by Rob Serber in his 1992 extension of the original Los Alamos Primer.
  The term hydride for this type of weapons has been subject to misunderstandings in the open literature. While the "hydride" might erroneously imply that the isotope used is hydrogen, only deuterium has been used for the bomb pits. The nomenclature is used in a manner similar to the term "hydrogen bomb", where the latter employs deuterium and occasionally tritium.
  Two uranium deuteride fueled bombs are known to have been tested, the Ruth and Ray test shots during Operation Upshot–Knothole. Both tests produced a yield comparable to 200 tons of TNT each, and were considered to be fizzles.
context_section_description: During early phases of Manhattan Project, in 1943, uranium deuteride was investigated as a promising bomb material; it was abandoned by early 1944 as it turned out such design would be inefficient. The "autocatalytic" design that emerged from this early research was "Elmer", the discontinued radial-implosion Mark 2 weapon. It made use of uranium deuteride particles coated with paraffin (to reduce the pyrophoricity of UD₃) and boron-10 carbide (B₄C) wax distributed uniformly throughout the solid core. A composite lead and B₄C tamper was envisioned, with about 10.5 kg of active material (i.e. UD₃) in one version, and a BeO tamper with 8.45 kg of active material in another.
  The heavy hydrogen (deuterium) in uranium deuteride (UD₃) or plutonium deuteride (PuD₃) moderates (slows down) the neutrons, thereby increasing the nuclear cross section for neutron absorption. The result should have been a lower required critical mass, concordantly reducing the amount of pure ²³⁵U or ²³⁹Pu needed for a weapon explosion. At the same time, due to the moderating effect of deuterium, the compression requirements are (at least in principle) relaxed somewhat, which would permit assembly of additional fissile material in the core, as well as a radial-implosion assembly, which was much simpler and compact than the one destined for the MK 3. In reality the result was that the slower neutrons delayed the reaction time too much by reducing the number of fission generations accomplished; especially as the core expanded to reach its snowplow region (where all nuclear reactions cease), more neutrons could escape from the turbulent surface of the core, and before enough energy (for military applications) could be produced. In all, neutron moderation sharply reduced the efficiency of the weapon before the inertial confinement failed. It was realised that the end result would be a fizzle instead of full-scale detonation of the device. The predicted energy yield was around 1 kilotonne of TNT (4.2 TJ), if the core operated as originally expected; the first rough estimate for the behaviour of the "hydride" bomb appeared in 1944, when James Conant forecasted that 1 kt of energy would be obtained from about 9 kg of UD₃.
  Post war, Los Alamos physicists continued research on the subject at low priority; while a Monte-Carlo simulation in December 1949 showed that the core could in principle work and result in a weapon considerably smaller than the MK 5, strong skepticism arose as the inherently low efficiency of the fuel would not improve even remotely as theoretically envisioned when a hollow core and boosting were incorporated, and a proposed test of such a core in an MK 4 high-explosive assembly was ultimately stricken from the preliminary shot schedule of operation Greenhouse.
caption: A cartoon by George Gamow showing the MK 2 "Elmer" and the MK 8 "Elsie" weapons, depicting the MK 2 (the "good fellow") as clumsy and unattractive.
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