Intrinsic barrier controlled reaction series

If in the event of such a transient not all the control rods drop into the core, the transient will continue, based solely on the intrinsic reactions in the reactor core which continue to evolve freely (ATWS). Studies performed since the early stages of the 900 MW series have demonstrated that the loss of feedwater without falling of the control assemblies is the transient with the most restricting effect on category 2 operating conditions, in relation to the first two barriers (the clad and the primary system). 

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