Moderated nuclear explosion

A moderated nuclear explosion results from a nuclear chain reaction mediated by moderated neutrons. Unlike most nuclear weapons, that rely on fast neutrons to sustain the chain reaction, a moderated nuclear explosion is sustained by moderated or slow neutrons. A moderated explosion may be the result of severe criticality accident or reactor accident, the most notable example being the Chernobyl disaster. Moderated nuclear explosions have also been tested in nuclear weapons.
With moderated neutrons fissile cross-sections are much higher than with fast neutrons, hence also the critical mass is much smaller. In the presence of a neutron moderator less fissile material is required to cause an uncontrolled chain reaction than that needed for a traditional nuclear explosive. Less than 1 kg of plutonium or enriched uranium dispersed in a water moderator may form a critical mass. Under the right circumstances such a critical assembly may explode in a moderated nuclear explosion.
It has been speculated by low-dose radiation researcher and anti-nuclear activist Christopher Busby that one of the explosions at the Fukushima I nuclear reactors was a "nuclear" one, rather than a hydrogen explosion as reported. However, he did not use the term "moderated", and attributed that explosion to a reaction in the spent fuel cooling ponds. In the same Russia Today broadcast, he referred to calculations made with his colleagues estimating that Chernobyl had killed 1,400,000 people, and that Fukushima's death toll would be in the same range, if not worse.
Nuclear weapon design
Early speculation about nuclear weapons assumed that an "atom bomb" would be a large amount of fissile material, moderated by a neutron moderator, similar in structure to a nuclear reactor or "pile". Only the Manhattan project embraced the idea of a chain reaction of fast neutrons in pure metallic uranium or plutonium. Moderated designs were also considered by the Americans; proposals included using uranium hydride as the fissile material. In 1943 Robert Oppenheimer and Niels Bohr considered the possibility of using a "pile" as a weapon. The motivation was that with a graphite moderator it would be possible to achieve the chain reaction without the use of any isotope separation. In August 1945, when information of the atomic bombing of Hiroshima was relayed to the scientist of the German nuclear program, interned at Farm Hall in England, chief scientist Werner Heisenberg hypothesized that the device must have been "something like a nuclear reactor, with the neutrons slowed by many collisions with a moderator."
After the success of the Manhattan project, all major nuclear weapons programs have relied on fast neutrons in their weapons designs. The notable exception is the Ruth and Ray test explosions of Operation Upshot-Knothole. The aim of the University of California Radiation Laboratory design was to produce an explosion powerful enough to ignite a thermonuclear weapon, with the minimal amount of fissile material. The core consisted of uranium hydride, with hydrogen, or in the case of Ray, deuterium acting as the neutron moderator. The predicted yield was 1.5 to 3 kt for Ruth and 0.5-1 kt for Ray. The tests produced yield a 200 tons of TNT, both tests were considered to be fizzles.
Again quoting Heisenberg:
::One can never make an explosive with slow neutrons, not even with the heavy water machine, as then the neutrons only go with thermal speed, with the result that the reaction is so slow that the thing explodes sooner, before the reaction is complete.
While a nuclear bomb working on thermal neutrons may be impractical, modern weapons designs may still benefit from some level of moderation. A beryllium tamper used as a neutron reflector will also act as a moderator.
Nuclear reactors
Nuclear reactors are controlled by keeping the criticality of the reactor in the narrow range between delayed criticality and prompt criticality. About 1% of neutrons in a reactor are delayed neutrons, that are emitted seconds after fission. The central principle of nuclear safety of nuclear reactors is that the reactor should never reach prompt criticality. Usually this is achieved by passive safety features, such as a negative void coefficient.
Prompt criticality of a nuclear reactor will result in a uncontrolled power excursion. At best it will result in steam explosion, where the coolant or moderator is ejected from the reactor; at worst it result in a meltdown and destruction of the reactor core and ejection of fissile material.
In the Chernobyl disaster the RBMK-1000 type reactor of the Chernobyl Nuclear Power Plant reached prompt supercriticality. This was partly due to the "xenon poisoning" of the reactor. Shutdown of the reactor and the low power level before the accident caused the buildup of xenon 135, a decay product of iodine 135 and a strong nuclear poison. The poisoning of the reactor forced the operators to completely pull out the control rods to start the reactor. A power surge burnt up the remaining xenon 135, driving the reactor to a high state of supercriticality. The result was an explosive disassembly ["explosion"?] of the reactor, and the spread of radioactive contamination throughout Europe.

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