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Event Inquiry into the Nuclear Regulatory Commission’s Oversight of the Auxiliary Feedwater System at Diablo Canyon Nuclear Power Plant

Report Description: 

In the last few years, the OIG received multiple allegations regarding the NRC’s oversight at DCNPP. Several of those involved the NRC’s oversight of safety-related structures, systems, and components (SSCs). One such SSC is the auxiliary feedwater (AFW) system, which is important to a commercial nuclear power plant because it is a backup water supply that can be used to cool the reactor if normal feedwater is out of service.

After a July 2020 AFW system failure that required one of DCNPP’s nuclear reactors to shut down for 8 days, we received specific allegations that the NRC had inadequately inspected the AFW system prior to the event. Our investigation revealed that the NRC failed to identify piping insulation on the AFW system that had long been in a degraded condition, and that led to a leak. Our investigation further revealed that the NRC had not inspected the area where the leak occurred, even though its inspection report indicated that the inspectors had conducted a complete walkdown of the AFW system in April 2020, 3 months prior to the leak and shutdown. A complete walkdown is a physical inspection that verifies that the selected system is correctly aligned and able to perform its intended safety function. Our investigation also demonstrated that the number of hours NRC staff spent directly inspecting both reactors’ AFW systems for the complete walkdown in April 2020 was fewer than recommended in the applicable NRC inspection procedures.

Since the event, described more fully in our Event Inquiry Report, the licensee has remedied the AFW system failure and made improvements to the system, and DCNPP continues to operate safely. Additionally, the NRC has since verified that the AFW system complies with regulatory requirements.

Submitting OIG-Specific Report Number: 

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