Please note! This essay has been submitted by a student.
The results of the internal event analysis (PRA level 1) for the Laguna Verde nuclear power plant (LVNPP), which is a typical BWR, shows that the SLC plays an important role in the estimation of the total CDF (Huerta and Nú˜nez, 1994). The study was conducted given special emphasis to the emergency core cooling, power conversion systems, and the SLC. The total CDF estimated for the LVNPP is 9.03e−5 per year for internal events.
The most dominant accident sequences found are the offsite power loss, station blackout accidents, and anticipated transient without scram (ATWS). Of course, the loss of coolant accident (LOCA) has a small contribution to the CDF (above 3%) due that the BWR is designed against LOCA. The interface between the system analysis results (PRA, Level 1) and the initial conditions require performing the accident progression analysis (PRA, Level 2). This interface is described by the plant damage state (PDS). The PDS is a grouping of the most dominant accident sequences of minimal cut sets that have similar characteristics.
The PDS that most contribute to the total CDF is:
In the first place, PDS associated with the station blackout. In this PDS the high-pressure core spray system (HPCS) is either failed or recoverable, later the support systems failures, which prompt the HPCS to fail, are restored. The reactor cooling insulated core(RCIC) is initially working and providing core cooling but subsequently, it fails. With no recovery of an AC power supply, the core damage begins approximately 4 h after RCIC failure with vulnerable containment. The main frequency of this PDS is 4.8e−5 and it contributes to 43% of the total CDF.
In the second place, the PDS that corresponds to the offsite power sequences loss with at least one emergency diesel generator working with either high pressure or low-pressure injection systems, once the vessel is depressurized. The residual heat removal system (RHR) fails and after the containment venting, the injection systems have to work under harsh environment until failure. This PDS has a frequency of 2.0e−5 and it contributes to 19% of the total CDF.
The last PDS corresponds to the ATWS, where the mechanical failure of the reactor protection systems avoids any possibility to shut down the reactor. The reactor operator tries to activate the SLC but it fails. With no reactor power control, the core damage is in short term. The main frequency of this PDS is 1.19e−5 and it contributes to the 11% of the total CDF. The level 1 PRA results for the LVNPP remarks the importance of the SLC in the contribution to the total CDF.