In a breeder reactor:

CONCEPT

A reactor is classified by its conversion (breeding) ratio CR: the number of new fissile nuclei created per fissile nucleus destroyed. When CR is greater than one, the core is described as breeding.

Every neutron absorbed in the fuel destroys one fissile nucleus and, on average, releases η neutrons (η is the neutron yield per neutron absorbed in fuel). Exactly one of those η neutrons must be absorbed in the next fissile nucleus to hold the chain critical, and a fraction L is lost to leakage and to parasitic capture in coolant, cladding and structure. Whatever is left is free to be captured in fertile nuclei, so CR = η − 1 − L. Breeding therefore requires η > 1 + L.

Fertile nuclei are the ones that absorb those surplus neutrons and transmute into fissile nuclei: U-238 and Th-232 are fertile, while U-235, Pu-239 and U-233 are fissile.

APPLICATION

1. Take Pu-239 in the fast neutron spectrum of a breeder core. Its yield is η ≈ 2.4 neutrons released per neutron absorbed in the fuel (against η ≈ 2.1 at thermal energies).

2. Reserve one of those neutrons to be absorbed in the next fissile nucleus and keep the chain critical. Surplus = η − 1 ≈ 1.4 neutrons per fissile nucleus destroyed.

3. Subtract the loss term L — leakage out of the core plus parasitic capture in coolant, cladding and structure. In a compact, sodium-cooled fast core these come to roughly L ≈ 0.2 to 0.3 neutrons.

4. Let the remainder, about 1.1 to 1.2 neutrons, be captured in the fertile U-238 of the blanket and core: U-238 + n → U-239 → (β⁻) Np-239 → (β⁻) Pu-239. Each such capture manufactures one new fissile nucleus.

5. Evaluate the ratio: CR = η − 1 − L ≈ 2.4 − 1 − 0.25 ≈ 1.15, which is greater than one. About 1.15 fissile nuclei are manufactured for every one destroyed, so the fissile inventory grows with operating time. Operating fast breeders report breeding ratios of roughly 1.1 to 1.3, which matches this budget.

So a breeder reactor produces more fissile material than it consumes — that surplus is what the word "breeder" names.

CROSS-CHECK AND CONTRAST

• A conversion ratio below one — fissile material destroyed faster than it is created — is the ordinary burner/converter regime of a conventional thermal power reactor, which must be refuelled with externally enriched fuel. That is the opposite of breeding, not a description of it.

• Moderation is not part of the definition, and the dominant breeder design deliberately avoids it. Slowing the neutrons down drops η for Pu-239 from about 2.4 to about 2.1, which cuts CR = η − 1 − L to roughly 0.85 and wipes out the surplus that breeding depends on. A fast breeder reactor is therefore built with no moderator and uses liquid sodium, not water, as coolant. (A few thermal-spectrum thorium breeders do use a moderator, because U-233 keeps a usable η at thermal energies, but they are the exception and moderation is still not what makes a reactor a breeder.)

• The fissile/fertile labels on U-238 and Pu-239 are the reverse of the physics: U-238 is the fertile nucleus that captures the neutron and Pu-239 is the fissile nucleus produced from it, so the chain runs from mass 238 to mass 239, not from 239 back to 238.

Result: in a breeder reactor, more fissile material is produced than is consumed.