For a symmetric fault, the reactivity increase is uni­form over the core, becoming modified as tempera­tures increase as a result of the effect of the fuel and moderator temperature coefficients of reactivity. The temperatures increase most rapidly in the chan­nels with the highest powers. The protection may involve fuel element thermocouples, channel gas outlet thermocouples or neutron flux measuring instrumen­tation. Which of these lines of protection is effec­tive depends on the initial starting point of the fault in terms of absolute reactor power, the channel-by­channel power distribution and the rate of the reac­tivity release.