Reactor thermal hydraulics

The coupled analysis of heat generation, heat transfer and coolant fluid mechanics in a nuclear reactor. A change in fuel geometry can affect neutron moderation, hydraulic resistance and thermal margins together, so holding one quantity fixed does not generally hold the others fixed.

An accident involving loss of reactor coolant through a breach of its pressure boundary. Depressurization, loss of inventory and changes in heat removal require a transient assessment; stopping the fission chain reaction does not remove decay heat.

A system intended to supply emergency cooling to limit fuel damage and maintain heat removal following a coolant-loss event. Its performance depends on the accident conditions and flow paths as well as nominal injection capability.

Thermal power generated per unit axial fuel-rod length, measured in $\mathrm{W}{\mathrm{m}}^{-1}$. At fixed rod radius and material state, it controls the conductive fuel-to-surface temperature rise; coolant conditions determine the surface temperature from which that rise begins.

Thermal power per active-core volume under a stated volume convention. Coolant channels can be included in that core volume even when excluded from fuel volume, so core and fuel power densities need not change together.