June 16 NRC event report: Ft. Calhoun nuke plant: “Potential flooding issue in the Intake Structure” — “There is one penetration of concern” that could impact water pumps

[Emphasis Added]

Operations identified a potential flooding issue in the Intake Structure 1007 ft. 6 in. level. The area of concern is a the hole in the floor at the 1007 ft. 6 in. level where the relief valve from FP-1A discharge pipe goes through the raw pump bay and discharges into the intake cell. There is one penetration of concern. Flooding through this penetration could have impacted the ability of the station’s Raw Water (RW) pumps to perform their design accident mitigation functions.

Licensee Event Report 2011-003, Revision 1, for the Fort Calhoun Station, NRC, May 16, 2011:

[Emphasis Added]

As a result of a Nuclear Regulatory Commission (NRC) inspection conducted from January 1 to June 21, 2010, the NRC determined that Fort Calhoun Station (FCS) did not have adequate procedures to protect the intake structure and auxiliary building against external flooding events. [...]

During identification and evaluation of flood barriers (condition report (CR) 2010-2387), in response to NRC findings previously noted, unsealed through wall penetrations in the intake structure were identified that are below the licensing basis flood elevation. These penetrations were installed during the installation of upgrades to the plant fire protection system. As a result of the penetrations not being sealed, the intake structure was vulnerable to water inflow during an extreme flooding event. This inflow had the potential to affect the operability of both trains of safety related raw water pumps (ultimate heat sink).

On February 4, 2011, an eight (8) hour report was made under 10 CFR 50.72 (b)(3)(v)(D) to the NRC Headquarters Operation Office (HOO) at 1717 CST (Event Number (EN) 46594). The report should have been made on September 9, 2009. [...]

SAFETY SIGNIFICANCE

The Fort Calhoun Station is required to be protected from flooding within the station’s licensing basis. The safety related equipment required to mitigate the consequences of an accident were affected by these findings. The openings could have jeopardized the ability of the safety related equipment to perform their design basis function during an accident. [...]

Therefore, this external flooding concern has substantial importance to safety as indicated by the Yellow Finding issued for this event.

NRC: The ultimate heat sink (UHS) is the source of cooling water provided to dissipate reactor decay heat and essential cooling system heat loads after a normal reactor shutdown or a shutdown following an accident, including a loss-of-coolant accident (LOCA).