Neutron spectrum (thermal and fast)

The range of speeds, and so energies, of the neutrons doing the fissioning in a reactor. A thermal reactor uses a moderator to slow its neutrons down to roughly the thermal energy of the surrounding material before they cause fission; a fast reactor uses no moderator and fissions with neutrons close to the energy they were born at. The spectrum decides a great deal downstream: it changes how enriched the fuel must be, which elements the reactor can fission or breed, and how much damage the neutrons do to structural materials, so a claim made about a thermal design cannot be assumed to hold for a fast one, or the reverse.

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concept:neutron-spectrum

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