The reactivity of any reactor is generally temperature dependent. Critical reactors have, for obvious safety reasons which we have discussed in chapter 3, a negative reactivity temperature coefficient. For example, PWRs have a coefficient between 5 x 10~5 and 10~4 per °C [48]. This means that a PWR reactor has a reactivity at zero power between 0.03 and

0. 015 higher than at nominal power. The temperature coefficient of fast reactors is, usually, smaller than that of thermal rectors. For sodium cooled fast reactors it is around 10~5 per °C [48]. A similar value has been calculated by Rubbia et al. [76] for their Fast Energy Amplifier.