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Physics · Glossary

What is The reactor, tamed?

Also known as: moderator · control rod

Definition 33.14 High School Physics · Chapter 33 — Nuclear Energy: Fission, Fusion, E=mc2

A power reactor holds the chain at exactly one neutron per fission. The fuel: uranium pellets modestly enriched in uranium-235. The moderator (ordinary water, usually) slows the fast fission neutrons by collisions — slow neutrons are far better at triggering the next fission; the control rods (boron, cadmium) devour neutrons — pushed in, the chain dies; drawn out, it quickens. The fragments’ agitation becomes heat, then steam, then electricity (Chapter 9).

Examples

Example 33.15 (A peppercorn against a truck)

One gram of uranium-235 holds N=1.0×10−3/(235×1.660 54×10−27)≈2.56×1021N = 1.0 \times 10^{-3} / (235 \times 1.660\,54 \times 10^{-27}) \approx 2.56 \times 10^{21} nuclei; at 200 MeV=3.2×10−11 J200\,\mathrm{MeV} = 3.2 \times 10^{-11}\,\mathrm{J} each, fissioning them all yields about 8.2×1010 J8.2 \times 10^{10}\,\mathrm{J} — the chemical energy of 8.2×1010/3.0×107≈2.7×103 kg8.2 \times 10^{10} / 3.0 \times 10^{7} \approx 2.7 \times 10^{3}\,\mathrm{kg} of coal: a peppercorn against three tonnes.

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