The TRIGA fuel is a Uranium and Zirconium Hydride alloy. The intimate contact between the uranium and the hydrogen of the fuel results in a self-moderated reactor fuel. The prototype TRIGA Mark I reactor was designed to be slightly under-moderated through control of the hydrogen-to-uranium ratio. Thus, a decrease of the water density of the coolant surrounding the fuel element would produce a negative reactivity response. Because of the fundamentally different neutron thermalisation characteristics of hydrogen bound in a solid hydride as to hydrogen bound in water, the optimum (or minimum) critical mass and the amount of over- or under-moderation are complex functions of the hydride, water, and uranium ratios.

It should be noted that an over-moderated reactor would have an initial positive temperature coefficient. If the water temperature increases, then the water density decreases and the neutrons would be better moderated up to a point. If the temperature continued to increase past that point then the moderating properties would decrease. This would result in a negative temperature coefficient.