Where formula_19 is reactivity and "T" is temperature. The relationship shows that formula_20 is the value of the partial differential of reactivity with respect to temperature and is referred to as the "temperature coefficient of reactivity". As a result, the temperature feedback provided by formula_20 has an intuitive application to passive nuclear safety. A negative formula_20 is broadly cited as important for reactor safety, but wide temperature variations across real reactors (as opposed to a theoretical homogeneous reactor) limit the usability of a single metric as a marker of reactor safety.