What radioactive isotope is generally used to fuel a controlled nuclear chain reaction?

Answer 1

235U with a 5% enrichment of 238U is generally used to fuel a controlled nuclear chain reaction. In a navy nuclear application, such as a submarine, a higher enrichment of 238U is used, around 20%.

Answer 2

Uranium-235 is the main radioactive isotope used to fuel controlled nuclear chain reactions in nuclear reactors. It undergoes fission when bombarded with neutrons, releasing energy and more neutrons to sustain the chain reaction.