Nuclear reactors and safety

Taming the chain

A thermal reactor holds the chain reaction from the last lesson at a steady tick. On average exactly one neutron from each fission goes on to cause the next. Three components share the work, and each is best learnt as a verb.

The moderator, filling the core around the fuel rods, slows the fast fission neutrons to thermal speeds so uranium-235 can capture them. The control rods absorb neutrons. Lower them further in and they thin the chain, so the power falls; withdraw them and it quickens. The coolant carries the thermal energy of the slowed fragments out of the core to boilers, where it raises steam for the turbines. No specific reactor design is examined, and this generic picture is all that is asked for.

Fuel for the common light-water designs is enriched to only a few per cent uranium-235, and some designs run on natural uranium with a graphite or heavy-water moderator; either way the fuel is far too dilute for the runaway growth a weapon needs, and the geometry and control systems hold the chain at one. A reactor can overheat catastrophically if mismanaged, which is serious enough; a nuclear detonation it cannot do.

Why the moderator is light

The moderator's physics is a collision problem borrowed from mechanics. In a head-on elastic collision between a neutron of mass m and a stationary nucleus of mass M, the neutron keeps a fraction of its kinetic energy,

Equal masses swap completely. Strike a proton head-on and the neutron stops dead, like one snooker ball on another. Against carbon-12 it keeps 72%, and against uranium-238 over 98%. An effective moderator is therefore made of light nuclei, and it must also absorb few neutrons and stay cheap and stable inside a reactor core. Water and graphite are the standard examples. Control rods want the opposite virtue, strong neutron absorption, so boron and cadmium take that job. Coolants must carry heat well without absorbing neutrons or corroding, and water again, or carbon dioxide, does it.

The safety case

Fresh fuel is only mildly radioactive; almost everything else in the safety case follows from what fission makes. The neutron-rich fragments are fierce β⁻ and γ emitters, so spent fuel is handled remotely, by machinery behind barriers, from the moment it leaves the core. The core itself sits inside metres of concrete and steel shielding that attenuate the neutrons and gamma rays of a working reactor down to controlled levels outside.

For emergency shut-down, the control rods drop fully into the core, absorbing enough neutrons to stop the chain reaction outright. Around all of it sits a sealed containment structure, the last barrier, whose job is to hold radioactive material in if the inner ones are breached.

Waste is half-life arithmetic. Spent rods typically spend some years in cooling ponds on site, the schedule depending on the design and the country's rules, where water shields the radiation and carries away the heat while the short-lived fragments burn themselves out. What remains is dominated by long-lived isotopes, so it is sealed, often in glass, and destined for secure deep storage lasting many thousands of years. Storage time is set by the half-lives of the longest-lived components, exactly the logic of the decay lesson.

Risk, benefit and the decision

The course ends by asking for judgement. On one side, a reactor delivers vast, reliable, low-carbon energy out of very little fuel. On the other, rare accidents with severe consequences, and waste that must be stewarded for generations. Physics does not make the decision. It quantifies both sides, in doses, half-lives and joules, and that is what lets a society choose with open eyes instead of by slogan. Expect a question asking you to weigh one named benefit against one named risk, and answer it in exactly that shape.