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The correct answer is Option B. For a nuclear reactor to sustain a chain reaction two conditions are essential. First each fission of U-235 must release more than one neutron on average (U-235 releases approximately 2.5 neutrons per fission). This ensures that even if some neutrons are lost or absorbed there are enough remaining to trigger further fissions. Second the fast neutrons produced in fission must be slowed down to thermal energies by a moderator (heavy water, graphite, or ordinary water). Slow thermal neutrons have a much higher probability of being captured by U-235 nuclei and causing fission compared to fast neutrons. Option A is incorrect: High temperature and high pressure are requirements for nuclear fusion (in the Sun and hydrogen bombs) not for fission chain reactions in reactors. Fission reactors operate at moderate temperatures. Option C is incorrect: Heavy water is a moderator and lead is a radiation shield. While these may be present they are not the two defining conditions for self-sustained chain reaction. Option D is incorrect: 90% enrichment is for nuclear weapons not reactors. Reactors use only 3-5% enriched uranium.
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