It seems there may be some confusion or misinformation in your question. Nuclear weapons, commonly known as nukes, do not possess the capability to self-replicate or continue splitting atoms indefinitely. The concept you mentioned, where a nuclear explosion could hypothetically trigger a chain reaction that would split every atom in the universe, leading to catastrophic consequences, is not based on scientific reality.
Nuclear weapons operate on the principles of nuclear fission or nuclear fusion. In nuclear fission, the nucleus of an atom is split into two or more smaller nuclei, releasing a tremendous amount of energy in the process. Nuclear fusion involves combining the nuclei of two lighter atoms to form a heavier nucleus, also releasing a substantial amount of energy.
In the design of nuclear weapons, the goal is to achieve a controlled and finite release of energy, rather than an uncontrolled and runaway reaction. This is accomplished through careful engineering and precise arrangements of fissile materials (such as uranium-235 or plutonium-239) or fusion fuel (such as deuterium and tritium).
Nuclear weapons are designed with specific mechanisms to prevent uncontrolled reactions and to ensure that the energy release remains within desired limits. These mechanisms include the use of carefully engineered explosives, neutron reflectors or tamper materials, and precise timing and geometry to control the chain reaction. The explosive lenses and other components in the weapon design are meant to focus the energy inward to achieve a powerful but finite explosion, rather than allowing the reaction to expand uncontrollably.
The fears you mentioned about a nuclear explosion causing a chain reaction to split every atom in the universe or create a black hole are not based on scientific understanding. Nuclear weapons, while capable of causing devastating damage, have a limited and finite destructive radius. The catastrophic scenarios you described belong more to the realm of science fiction rather than scientific reality.
It's important to note that nuclear weapons are highly complex and dangerous devices, and their proliferation and use are a significant concern for global security. International efforts and treaties, such as the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), aim to prevent the spread of nuclear weapons and promote disarmament, reducing the risks associated with these powerful weapons.