In the above, the principal effects of the moderator temperature coefficient have been looked at based on the six-factor formula. For a liquid moderator, two prominent effects were seen. (i) Due to the large expansion coefficient, the temperature coefficients of p and f have a negative or positive reactivity effect, respectively, to the extent of 10_4Ak/k/K. (ii) When 239Pu builds up with fuel burnup, the resonance of 239Pu leads to a negative or positive reactivity effect, respectively, on the temperature coefficients of n and f to the same extent, 10_4Ak/k/K.

As first mentioned, the temperature coefficients must be essentially negative for stable operation of the reactors. As a practical result of the effects, (i) and (ii), necessary conditions for a negative reactivity effect are discussed below.