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The correct answer is Option B — 2 to 3 neutrons with an average of 2.5. When U-235 absorbs a slow neutron and undergoes fission it splits into two smaller nuclei (fission fragments) and releases approximately 2 to 3 fast neutrons along with enormous energy (about 200 MeV) and gamma radiation. The average is 2.5 neutrons per fission. These released neutrons when slowed by a moderator can go on to cause further fissions creating a chain reaction. Option A is incorrect: If only 1 neutron were released the chain reaction would barely sustain itself (k=1 at best). In practice due to neutron losses to moderators coolants and structural materials at least 2 neutrons must be released for practical reactors. Option C is incorrect: U-235 does not release 5 neutrons per fission event. The actual value is approximately 2 to 3. Five would be an overestimate that would make control much harder. Option D is incorrect: Ten neutrons per fission is far from the actual value of 2.5. Such a high value would make controlling the chain reaction extremely difficult requiring very large amounts of control rod material. The 2.5 average means enough neutrons for chain reaction while still being controllable.
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