24.2 Chain Reaction, Criticality, and Neutron Economy

Each induced fission can release multiple neutrons. Some neutrons trigger new fissions, some leak out of the fuel region, and some are absorbed by non-fuel materials. Chain-reaction behavior depends on this balance. Reactor control is fundamentally neutron accounting over repeated generations.

If fuel mass is too small, neutron leakage dominates and the chain dies out. Increasing fuel quantity and selecting favorable geometry lowers leakage fraction, making sustained chain reactions possible. This threshold condition is described as critical mass, but in practice composition, moderation, and reflector choices matter as much as total kilograms.

Students often think critical means dangerous. In reactor operation, critical is the normal steady-power condition: each generation replaces itself on average. Safety concerns rise when k-effective drifts persistently above unity while cooling and control systems fail to compensate.

Two loss channels dominate simple chain models: leakage beyond the active core and absorption by materials that do not fission under the current neutron spectrum. Real reactors tune moderation and core layout so enough neutrons survive these losses to keep k-effective near 1 under changing temperature and fuel composition.