Physics · Nuclear and quantum physics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
In fission a heavy nucleus splits into two smaller nuclei and a few neutrons. It can happen on its own (spontaneous fission) or after the nucleus absorbs a neutron (induced fission). The products have a higher binding energy per nucleon, so mass is lost and energy is released, mostly as kinetic energy of the fission fragments.
The neutrons released can cause more fissions. This is a chain reaction. It keeps going if, on average, at least one neutron from each fission causes another. The fuel must exceed a critical mass, or too many neutrons escape.
A power station controls the chain reaction and uses the thermal energy to make steam, which drives turbines.
Students swap the moderator and the control rods. The moderator slows neutrons down and does not stop the reaction. The control rods absorb neutrons and so control the rate.
Answer: 2 neutrons
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