The expansion effect due to a temperature rise in structures is concerned with thermal utilization factor in the six-factor formula. Since structures are usually solids, their temperature coefficient is small. In the evaluation of the fuel or moderator temperature coefficient, it was assumed that the atomic density of fuel or moderator decreases with each temperature rise assuming a constant fuel or moderator volume fraction. This assumption was made because the variation in volume of structures due to a change in temperature is small and therefore almost a constant volume fraction of fuel or coolant is maintained. Hence, the structure temperature coefficient was disregarded in the discussion above.

However, the structure temperature coefficient should be necessarily regarded in the practical evaluation. Generally, a volume expansion due to a temperature rise of structures leads to a decrease in atomic density and an increase in the thermal utilization factor, and therefore has a positive reactivity effect. The structure expansion essentially presses liquid or gas coolant out of the system. It is somewhat complicated to formulate the structure temperature coefficient.