TRIGA reactors may be licensed to operate with:

1. Standard Al-clad TRIGA fuel (20% enriched)
2. FLIP TRIGA fuel (70% enriched)
3. Standard SST-clad TRIGA fuel (20% enriched)
(Note: The 20% enriched fuel may have a fuel loading of 8.5, 11, 20 or 30 wt-%)

The basic parameter which allows TRIGA reactors to operate safely during either steady-state or transient conditions is the prompt negative temperature coefficient associated with the TRIGA fuel and core design. This temperature coefficient allows great freedom in steady-state and transient operations.

A major factor in the prompt negative temperature coefficient for the TRIGA cores is the core spectrum hardening that occurs as the fuel temperature increases. The rise in temperature of the hydride increases the probability that a thermal neutron in the fuel element will gain energy from an excited state of an oscillating hydrogen atom in the lattice. As the neutrons gain energy from the ZrH, the thermal neutron spectrum in the fuel element shifts to a higher average energy (i.e., the spectrum is hardened). This spectrum hardening is used differently to produce the negative temperature coefficient in standard and FLIP fuel.