Please pay attention to your topic, which is uranium 235. This is uranium that can carry out nuclear reactions. It is declining, with a half-life of 700 million years. However, the half-life of uranium 238 is 4.6 billion years, and the reaction is too slow to cause a chain reaction. However, it is usually not pure uranium 235, and it needs "uranium blocks with sufficient volume". If pure uranium 235 reaches its critical volume, three conditions can be met. At this time, the concentration of uranium 235 is 100%, and the number of slow neutrons is enough to reach the critical volume.

Different uranium concentrations correspond to different critical volumes. The uranium used in ordinary atomic bombs is high-purity uranium with purity exceeding 95% of uranium-235. The critical volume is small, and only a few small pieces of uranium can react together. The concentration of uranium 235 in nuclear fuel of nuclear power plant is 3.2%, which can control the chain reaction. The abundance of uranium in nature is about 0.7 1%, and there will be no chain reaction.

Source of critical volume: the radius of atom is 10- negative 10, and the radius of nucleus is 15- negative 10. Neutrons quickly pass through uranium and the probability of hitting the uranium core is extremely small. In order to hit enough uranium, you need enough uranium-235 and use slow neutrons. In fact, there is a neutron reflection layer around the uranium block of the atomic bomb, so it can increase the probability of neutron collision with the nucleus and speed up the reaction.