- Preventing Disassembly and Increasing Efficiency
By the time a significant percentage of the atoms have fissioned, their thermal kinetic energy is so high that the pit will expand enough to shut down the reaction in only a few of shakes. This severely limits the efficiency of fission weapons (percentage of material fissioned). The practical efficiency limit of a typical pure fission bomb is about 25%, and could be much less. The Fat Man implosion bomb was 17% efficient (counting only the energy produced by the fissile core, the natural uranium tamper contributed another 4% through fast fission). Little Boy had an efficiency of only 1.4%. Very large pure fission bombs can achieve efficiencies approaching 50% but have been supplanted by thermonuclear weapon technology. Anything that will increase the confinement time of the fissionable core or decrease the generation time, even slightly, can cause a significant increase in bomb yield.
U235 is about as dangerous as DU. Pu239 is more dangerous than either U235 or DU.
I believe that many fission bombs use one or the other of beryllium
and tungsten as neutron reflectors. The Fission Weapon Physics
webpage I linked to above, however, cites uranium as one of the metals typically composing tampers. Presumably, tampers always double as reflectors.
- A layer of dense material called a "tamper" (typically made of natural or depleted uranium or tungsten) surrounds the critical mass....
The tamper has an additional benefit, it can also scatter or "reflect" neutrons back into the critical mass after they escape from its surface. This means that a smaller amount of fissionable material is necessary to make the critical mass. The importance of this effect is often overstated in the nuclear weapons literature however. Only a portion of the neutrons are scattered back, and since it takes on average several shakes for the neutrons that do return to reenter the critical mass, their significance is further reduced through "time absorption" (see section 2.1.3). This is offset somewhat by the fact that some neutron multiplication occurs in natural uranium tampers through fast fission of U-238.
- A Hiroshima gun-type weapon would be much easier to make, especially with weapons-grade uranium instead of the lower-enriched material available in August 1945, and a non-uranium reflector such as beryllium or tungsten carbide to minimize the possibility of pre-detonation.