Simple - nuclear decay rate depends upon the physics of the nucleus, which is governed by the strong and weak nuclear forces. The energy scale of these forces is way above that of electromagnetism, that governs the atomic and chemical interactions. Thus, pressure, temperature, electricity, magnetism, etc cannot affect nuclear decay rates unless these reach the energy scale of the nuclear forces (which can occur in relativistic matter, such as in neutron stars and collapsars heading towards black holes.)
To help put this in perspective for the layman, think of the difference between dynamite and an atomic bomb. In fact, scientists already use TNT as a measuring stick for the explosive force of atomic bombs. A few kg's of uranium in an atomic bomb can yield the equivalent energy of several thousand tons of TNT which is a chemical reaction (and it is also worth mentioning that the yield of an atomic bomb is not 100%).
™ Version 4.2
