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In a pressurized water reactor, what role does water play in controlling the fission of uranium-235?
AIt speeds up neutrons to make fission harder
BIt absorbs all neutrons, stopping fission completely
CIt has no meaningful effect on the neutrons
DWater slows the neutrons to induce more fission
Answer & Solution
Correct answer: D. Water slows the neutrons to induce more fission
1. Fast neutrons released by fission are less effective at inducing further fission in uranium-235.
2. Water slows the neutrons down, since they collide with protons in water molecules and lose energy.
3. If the reactor overheats and the water boils away, this slowing effect is lost and the chain reaction stops, which is a built-in safety feature.
4. Water does not speed neutrons up, and it does not permanently absorb every neutron.
5. So water slowing down neutrons is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 22 "The Atom", section 22.4 Nuclear Fission and Fusion_
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