<P> The CANDU reactor's moderator doubles as a safety feature . A large tank of low - temperature, low - pressure heavy water moderates the neutrons and also acts as a heat sink in extreme loss - of - coolant accident conditions . It is separated from the fuel rods that actually generate the heat . Heavy water is very effective at slowing down (moderating) neutrons, giving CANDU reactors their important and defining characteristic of high "neutron economy ." </P> <P> 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 . Other moderated designs were also considered by the Americans; proposals included using uranium deuteride 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 scientists 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 ." </P> <P> 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 designs was the exploration of deuterated polyethylene charge containing uranium as a candidate thermonuclear fuel, hoping that deuterium would fuse (becoming an active medium) if compressed appropriately . If successful, the devices could also lead to a compact primary containing minimal amount of fissile material, and powerful enough to ignite RAMROD a thermonuclear weapon designed by UCRL at the time . For a "hydride" primary, the degree of compression would not make deuterium to fuse, but the design could be subjected to boosting, raising the yield considerably . The cores consisted of a mix of uranium deuteride (UD), and deuterated polyethylene . The core tested in Ray used uranium low enriched in U, and in both shots deuterium acted as the neutron moderator . The predicted yield was 1.5 to 3 kt for Ruth (with a maximum potential yield of 20 kt) and 0.5 - 1 kt for Ray . The tests produced yields of 200 tons of TNT each; both tests were considered to be fizzles . </P> <P> The main benefit of using a moderator in a nuclear explosive is that the amount of fissile material needed to reach criticality may be greatly reduced . Slowing of fast neutrons will increase the cross section for neutron absorption, reducing the critical mass . A side effect is however that as the chain reaction progresses, the moderator will be heated, thus losing its ability to cool the neutrons . </P>

What characteristics of nuclear energy make it desirable