In normal operation the temperatures of the various materials in the core vary in a regular way and it is useful to sum up their combined effects. There are various ways of doing this, one of which is to determ­ine the “isothermal temperature coefficient”. This is the rate of change of reactivity with temperature if the temperatures throughout the core change by the same amount. It includes the effects of axial and radial expansion of the structure, expansion of the coolant, and the Doppler effect. Because of the variation of the Doppler coefficient it depends slightly on the fuel temperatures. For a typical sodium-cooled breeder it might be -2.5 x 10-5 K-1 when the fuel is cold (i. e. the reactor is operating at very low power), and -1.5 x 10-5 K-1 at normal oper­ating temperature. It can be thought of as measuring the response of the reactor to a change in coolant inlet temperature when power and coolant flow-rate are held constant.

Another useful parameter is the power coefficient. This is the rate of change of reactivity with power, assuming the coolant flow-rate changes proportionately to the power and the inlet temperature is constant. The coolant and structure temperatures everywhere are thus constant so that the power coefficient depends only on the Doppler effect. A typical value of the power coefficient for a 2500 MW (heat) sodium-cooled breeder is -3 x l0-6 (MW)-1.