All components within the reactor system have natural temperature time constants which are included in the model equations (1.26)—(1.28). They are:

Fuel

rf=WfCf/Af (1.33)

For a graphite moderated thermal reactor with a Magnox fuel element, this time constant could be 5-10 sec; for a light water thermal system fueled with plates or small pins, the time constant is reduced to 1-5 sec; while for a fast reactor fuel pin, the time constant is 1 or 2 sec at the most. The fast reactor fuel therefore reacts very rapidly to power changes and the time which it takes to reach a higher temperature is reduced compared to the thermal systems.

Structural components

rs = macjha (1.34)

For a graphite moderated system, the graphite itself might have time con­stants in the range of 10 to 15 min; however, in the fast reactor almost all structure reacts in times of less than a few seconds.

Coolant

rc = L/vc (1-35)

The effective time constant here is the channel transit time which might be 0.5 sec for a light water thermal reactor but is about 0.1 sec for a sodium — cooled fast breeder.

Thus it can be seen that the fast reactor reacts to disturbances very much more rapidly than thermal systems in almost all respects. It is therefore less liable to some of the instabilities exhibited in the other systems, but it requires faster operating control and protective systems to ensure an accep­table response to reactivity and flow disturbances.