Automatic depressurization system (ADS)

The ADS is a part of the ECCS. The ADS depressurizes the RPV in the event of LOCA to allow the GDCS water injection into the vessel, preventing the core uncovery. Once the ADS actuates, it continuously operates to keep the reactor depressurized for GDCS injection after an accident initiation. The ADS in the ESBWR is composed of ten safety relief valves (SRVs) and eight depressurization valves (DPVs). The SRVs are mounted on the top of the main steam lines in the DW and discharge the steam through lines routed to quenchers in the SP. Four DPVs are horizontally mounted on horizontal stub tubes connected to the RPV at about the elevation of the main steam lines. The other four DPVs are horizontally mounted on horizontal lines branching from each main steam lines. Main function of the DPVs is to discharge the steam directly from the RPV to the DW in order to depressurize the RPV during the initial phase of LOCA.

The ADS automatically actuates on a reactor low level (Level 1) signal that persists for at least 10 seconds. A two-out-of-four Level 1 logic is used to activate the SRVs and DPVs. The 10-second persistence requirement for the Level 1 signal ensures that momentary system perturbations do not actuate the ADS when it is not required. The two-out-of-four logic ensures that a single failure does not cause spurious system actuation.

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