After the Fukushima accident, beyond design basis accidents (BDBA), even though their frequency is predicted by probabilistic safety assessment (PSA) to be low, have moved to the center of attention. Since then an increased number of analyses on the long term station blackout accidents have been performed. The present paper highlights selected aspects of another accident which, even if its frequency to occur is very low, may be severe in its consequences - a main steam line break with failure of all main steam isolation valves in the affected steam line in a Boiling-Water Reactor (BWR). Following the accident the nuclear power plant (NPP) quickly loses coolant inventory directly to the environment (containment bypass). Without proper operator actions, core heat up and core damage may occur at an early stage in the accident. A plant input model which has been developed at the University of Pisa has been used to run three cases, using the best estimate thermal hydraulic system code Relap5 mod 3.3. The first case assumes no operator actions and availability of passive systems only. It is used as baseline to be compared to the two other cases. The second case assumes availability of active and passive systems, and plausible operator behavior following symptom based emergency operating procedures (EOP), considering plant states. The third case presents an optimized strategy, assuming that the operator was able to clearly identify the root cause of the accident. The analysis confirms that, without proper operator actions, the accident can quickly progress to a severe accident (the situation is aggravated by the containment bypass). It shows further that procedures based on plant states, if this special case has not been considered, may not lead to desirable results. Finally it can be seen that, adopting a suitable strategy, the grace period can be extended up to a point where additional sources of coolant can be made available.
Main Steam Line Break Analysis with Failure of all Main Steam Isolation Valves in a BWR (Peach Bottom)
D'AURIA, FRANCESCO SAVERIO;
2012-01-01
Abstract
After the Fukushima accident, beyond design basis accidents (BDBA), even though their frequency is predicted by probabilistic safety assessment (PSA) to be low, have moved to the center of attention. Since then an increased number of analyses on the long term station blackout accidents have been performed. The present paper highlights selected aspects of another accident which, even if its frequency to occur is very low, may be severe in its consequences - a main steam line break with failure of all main steam isolation valves in the affected steam line in a Boiling-Water Reactor (BWR). Following the accident the nuclear power plant (NPP) quickly loses coolant inventory directly to the environment (containment bypass). Without proper operator actions, core heat up and core damage may occur at an early stage in the accident. A plant input model which has been developed at the University of Pisa has been used to run three cases, using the best estimate thermal hydraulic system code Relap5 mod 3.3. The first case assumes no operator actions and availability of passive systems only. It is used as baseline to be compared to the two other cases. The second case assumes availability of active and passive systems, and plausible operator behavior following symptom based emergency operating procedures (EOP), considering plant states. The third case presents an optimized strategy, assuming that the operator was able to clearly identify the root cause of the accident. The analysis confirms that, without proper operator actions, the accident can quickly progress to a severe accident (the situation is aggravated by the containment bypass). It shows further that procedures based on plant states, if this special case has not been considered, may not lead to desirable results. Finally it can be seen that, adopting a suitable strategy, the grace period can be extended up to a point where additional sources of coolant can be made available.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
