While important in other reactor types, these accidents either cannot occur or have negligible effects in high-tempeature gas-cooled reactors. In pebble-bed reactors the voidage between the fuel elements can change locally, but the effects on reactivity are always very small.

(d) Loss of coolant and blower failure

These accidents do not change directly the neutron multiplication properties of the reactor, but affect the heat-removal mechanism. A failure of this mechanism can either be the result of a depressurization or of a blower failure.

The first of these accidents can be treated considering that the coolant pressure decreases with time according to a given law, which depends on the type of leak. In case of blower failure the mass flow will decrease following a curve of exponential type. It is usually possible to guarantee an emergency cooling with a mass flow reduced to 6-10% of the original value (e. g. one of the blowers continues to operate at reduced power). An emergency cooling for the removal of the decay heat after a scram has occurred is in any case necessary since extreme temperatures have to be avoided.