Cruise missles and stand-off weapons (like Hounddog for B52) are another delivery method for nuclear weapons. These nuclear tipped cruise missles are destabilizing because like ICBMs and SLBMs are nearly invulnerable to interception.
China includes decoys in its MIRVs further hindering defense.
Russia/US now spending Trillion $ to upgrade nuclear weapons (current issue). Tritium half life is 12 yrs. Polonium (Polonium-Beryllium neutron sources for Uranium 235 bombs) half life is <one year. Plutonium over time becomes contaminated with decay products and less reliable as a nuclear weapon. Result is after a decade, current nuclear weapons require replacement or at least refurbishing. Separating contaminating Plutonium isotopes is difficult.
U235 has a very high cross-section (probability) for absorbing low energy neutrons. Result is U236m (or U236 at a higher energy level) which usually fissions (breaks apart) and gives off 2.4 fast neutrons (to continue reaction) but 18% of reactions (fusion of U235 + neutron) only emits a gamma ray. Hence, with no moderator, nuclear bomb reaction is propagated by high energy (high MeV) neutrons, which split U235 & U238 at near the same lower probability rate. The higher loss of high MeV neutrons and gamma rays results in unreacted fuel hence lower than ideal yields.
Re largest bomb, each fissile material has its own critical mass (& volume) such that enough neutrons are retained to propagate a chain reaction. Minimum size bomb just larger than critical mass formed from one or more subcritical masses. Limit for larger bomb is number of subcritical pieces kept apart. (Imploding a hollow sphere of Pu239 with explosives, also.)
Re “Tamper”: Could use Thorium? Thorium (and Bismuth et al) also fission from impact of high MeV neutrons and gamma rays. Shjacks45 (talk) 22:43, 15 November 2019 (UTC)
After the use of Nuclear Weapons in World War II, it showed the world why these types of bombs should not be used in war again. If these bombs were used today, there very well may be a nuclear fallout. — Preceding unsigned comment added by LyonsJ7 (talk • contribs) 18:47, 16 February 2020 (UTC)