Moderators and coolants

Why neutrons have to be slowed down

When a uranium-235 nucleus fissions it releases neutrons travelling at roughly 5% of the speed of light. A neutron that fast is unlikely to be captured by another U-235 nucleus. Slow it to the speed of the surrounding atoms, and its chance of causing a fission rises by a factor of several hundred.

That is the moderator's whole job. It sits between the fuel elements and takes energy out of neutrons through repeated collisions. A good moderator is light, so each collision removes a lot of energy, and it must not absorb the neutrons it is slowing down. Those two requirements pull in opposite directions, and the three practical answers each resolve the tension differently.

The three moderators in service

• Ordinary water. The lightest option, so it slows neutrons in the fewest collisions, which is why water-moderated cores are compact. It also absorbs a meaningful fraction of them, so the fuel has to be enriched to compensate. This is the reason almost every reactor on earth needs an enrichment plant upstream of it, and why 354 of 419 operating units depend on an industry that four organisations control.
• Heavy water. Deuterium absorbs so few neutrons that a reactor moderated with it can sustain a chain reaction on natural uranium. That removes the enrichment step from the fuel chain entirely. The price is a large inventory of heavy water, which is expensive to make, and a physically larger core. See PHWR.
• Graphite. Also a weak absorber, also compatible with natural or barely enriched fuel, and solid, so it can be built as a permanent structure with channels through it. It burns, it swells under irradiation, and dimensional change in the graphite is what eventually ends the life of a gas-cooled plant.

Fast reactors have no moderator at all

A fast reactor deliberately leaves the neutrons fast, which is why it uses liquid sodium or lead rather than water: those coolants are heavy enough not to moderate. Fission is less likely per neutron, so the fuel must be far more concentrated, typically 15% to 20% fissile or plutonium. It buys something in return: fast neutrons can fission isotopes that slow ones cannot, including much of the long-lived waste.

The coolant sets the ceiling

Water boils at 100 °C at atmospheric pressure, so a water-cooled reactor has to be pressurised to run hot, which is why a PWR vessel is a 20-centimetre-thick steel forging holding roughly 155 bar. Even then the steam leaves at around 285 °C and the plant converts about a third of its heat into electricity.

Helium and carbon dioxide have no such limit, which is how gas-cooled plants reach the best thermal efficiency of any reactor fleet, and how an HTGR can deliver process heat hot enough for industrial chemistry rather than just electricity. Sodium runs hot at atmospheric pressure but burns on contact with air and reacts violently with water.

Every operating family, by these two choices

Read the table left to right and the fuel chain follows from it: the enrichment column decides which of the world's four enrichers a plant can buy from, and which fabricator can build its assemblies.