7.19 understand that a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei

During nuclear fission, a slow-moving neutron gets absorbed by the nucleus of a U-235 atom. When this occurs, the atom splits into 2 daughter nuclei, whilst also releasing a small number of nuclei. If these nuclei his other uranion-235 atoms, these atoms will split and release more nuclei. The process will repeat. this is known as a chain reaction.