Only a select few materials are fissile, and then, only if they exist in sufficient purity. ^235 U and ^239 Pu are the most common fissile isotopes used in fission weapon cores. To detonate a fission core, usually the core is contained within an outer shell of conventional high explosive specifically shaped into explosive lenses (based on the positions of the detonators and speed of propagation of the detonation wavefront through the explosive). The multiple simultaneous explosions occurring within these explosive lenses create a symmetrical shockwave which acts to evenly implode the fissile material core, rapidly increasing its density and initiating the prompt-critical nuclear chain reaction in as many locations as possible, simultaneously. Neutrons released by every individual fission event at the atomic level may strike the nuclei of nearby fissile atoms, and there is a certain probability that this causes that nucleus to undergo fission itself, releasing more neutrons and thus propagating the chain reaction. While all this is happening, there are two things that are working to arrest the chain reaction: one is that the primary fissile core material is fissioning into child materials which have a much lesser (or even negligible) probability of further fission when struck by the reaction neutrons. Essentially, decay products are being created which are radioactive, but not fissile, so no further chain reaction is possible. The second thing is that the incredible energy release associated with the fissioning core is actively working to blow that core apart, both reducing it below the critical density first established by the conventional explosive trigger, and physically dispersing it and so reducing the probability of neutron collisions which result in fission. A goal of nuclear weapon design is to maintain supercritical density for as long as possible in order to maximize fission yield, but 100% fission of the core is not attainable. Some portion is always just blown apart to become a contribution to the radioactive fallout instead of actively contributing to the destructive yield.