Local temperature increases can be due to wrong gagging, channel blocking, loading errors, spatial instability, incorrect spatial distribution of the inserted control rods. The main problem posed by these local temperature increases is the difficulty of their detection. Monitoring of local temperature is not always possible or reliable in HTRs and one must often rely on proper planning of the operational procedure in order to avoid these temperature disturbances.

In many cases an accident of this type will only be detected by the increase of the primary circuit activity due to the fuel damage.

(e) Start-up accidents

Any one of the above-mentioned accidents can happen during start-up, but the typical start-up accident is a continuous insertion of reactivity resulting in too short a reactor period. Provided a scram occurs if the period gets smaller than a given value, no start-up accidents can lead to dangerous temperatures. If the reactor is cold the high thermal capacity of graphite gives plenty of time for the control system to intervene.

In the treatment of start-up accidents it is important to consider that only the prompt heat contributes to the core temperatures because delayed heat is not yet present.