Compared to other reactor types, high-temperature gas-cooled reactors have a higher degree of safety. Sudden introductions of large amounts of reactivity are not possible in the foreseeable design. In the following paragraphs various types of accident will be analysed: the list includes all accidents foreseeable with the present designs, but any new design may carry with it new sources of accidents which will have to be analysed from case to case. In the evaluation of the consequence of the accidents it is very important to take into account the initial state of the system. The maximum temperatures reached during an accident vary considerably according to the initial power. Accidents at zero-power in a cold core seldom lead to dangerous temperatures if adequate safety equipment provides for a properly timed scram. In the analysis of the accidents occurring at power operation it is important to find the worst moment in which the accident can occur. This means considering the time when the fission product concentration leads to the worst temperature coefficient and when the configuration of the control rods already inserted in the core reduces to a minimum the reactivity effects of the insertion of the remaining rods. Very important in this respect is the Xe concentration which strongly influences the temperature coefficient and is dependent on the load diagram. In the following list we have in part followed the classification of accidents given by Stewart and Merrill,<25> limiting our analysis to the accidents which are of interest for the reactor physicist.