05000443/LER-2006-003
| Docket Numbersequential Revmonth Day Year Year Month Day Year N/Anumber No. 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| 4432006003R00 - NRC Website | |
I. Description of Event
On February 5, 2006 at approximately 0400, personnel performing scheduled surveillance testing of the fire protection system [KP] inadvertently actuated one of five fire protection water deluge subsystems in the Seabrook Station cable-spreading room [NA]. The actuation of the deluge system, which was isolated in about two minutes, resulted in a discharge of approximately 1000 gallons of water to the cable spreading room. An operator responding to the event subsequently discovered water on the floor of the safety-related "A" train switchgear room located directly below the cable spreading room. The water was removed and the electrical distribution equipment in the switchgear room was inspected for damage and determined to be unaffected by the water.
The presence of water in the switchgear room was unexpected because the design of the cable spreading room floor is to function as a watertight hydrostatic barrier. The safety-related electrical equipment in the essential switchgear rooms does not have a comprehensive design tolerance for water. A subsequent investigation found a number of degraded cable spreading room floor seals [SEAL], two unsealed holes, and floor construction joints that were not designed to be watertight.
II. Cause of Event
An evaluation of this event identified the following root cause:
1. Design basis flood protection requirements contained in various design and licensing documents were not included in the penetration seal or concrete floor design specifications. As a result, the cable spreading room floor and associated seals were not originally specified, designed or installed as hydrostatic barriers.
2. The work control process did not ensure positive configuration control for cables that were removed, and work instructions did not clearly identify all breaches that would require repairs.
Ill. Analysis of Event Hydrostatic Seal Design and Licensing Basis Initial investigations following the event focused on large blockout seals containing multiple cable trays [TY] and associated cables [CBL]. These blockout assemblies extend into the 12" thick concrete floor and are built-up approximately 4 inches above the floor inside a kick plate enclosure. These assemblies are sealed with BISCO SF- 60 elastomeric material. The steel kick plate surrounding the penetration is sealed with caulking at the concrete floor interface. Several seals failed initial inspection criteria specified in plant procedures, with a total breach area of approximately 8 square inches. Each was repaired using approved methods. There were three SF-60 seals located in the actuated zone coverage area with sufficient defects to permit water leakage into the switchgear room. Their locations generally corresponded to wetted areas in the "A" switchgear room. A subsequent investigation of the cable spreading room and mechanical equipment room floor seals revealed five small penetrations that were sealed only with a foam material not meeting the hydrostatic design requirements. An inspection of the cable spreading room floor seals found two unsealed holes in the seals where cables had been removed and the seal was not subsequently repaired.
The specification for the wall and floor penetration seals in the cable spreading room and adjacent mechanical equipment rooms required the design, material and installation of the penetration seals to be performed in accordance with 10 CFR 50 Appendix B, Appendix R, and the Quality Assurance requirements in ANSI N45.2-1977.
Accordingly, the seal materials and installed penetration are classified as ANS safety-related items. Although the barrier requirements call for the seals to be hydrostatic barriers, no documented evidence exists that the seals were originally specified, designed, installed, or inspected as hydrostatic barriers.
A review of structural drawings and specifications revealed that the cable spreading room floor was not designed or constructed with features that would ensure its intended watertight integrity. A review of concrete placement drawings revealed a construction joint above a known leak location. Water could leak through this joint and follow the Q-decking to the wall inside the electrical switchgear room.
Seal Maintenance and Inspection Procedures Station procedures were originally developed to inspect fire rated penetration seals and barriers in accordance with Technical Requirement 11-4.7.9.5. The inspection attributes specified in these procedures addressed only fire rated qualifications of the barriers and seals. An evaluation performed in 1991 concluded that the procedures used for fire barrier and seal inspections would also demonstrate seal/barrier integrity for other safety-related applications, such as hydrostatic, air, and tornado barrier functions. However, the fire seal acceptance criteria allowed for certain defects to remain without repairs while, for hydrostatic seals, these allowable defects may not be sufficient to ensure a watertight seal. Although the procedures that provide administrative controls for the repair, removal, modification, and installation of the fire barrier penetration seals require a dedicated work order for penetration seal repairs and modifications, the guidance does not consider penetration seals in the cable removal steps. In addition, the post- maintenance and modification inspection attributes apply the fire rated seal inspection criteria and do not consider the hydrostatic barrier function.
Safety Consequences No consequences resulted from this event and, therefore, the event had no adverse impact on the plant or on the health and safety of the public. The degraded seals did not present a safety hazard and would not have prevented the fulfillment of any safety function. In the event of a fire, the seals will tend to expand as they are subjected to the heat of the fire, improving their effectiveness as a water barrier. The amount of leakage experienced during the deluge actuation was minimal and resulted in no adverse impact on equipment in the essential switchgear room. Further, although the equipment in the essential switchgear room is not designed to be waterproof; a certain amount of protection is provided by the inherent design of the metal enclosures. Most of the enclosures have solid tops with entry by conduit, and most vents, where provided, on the sides of the equipment have small hoods to prevent water intrusion. Vents on the top of inverters have drip shields covering the vents, and equipment mounted on the walls is offset by attachments to unistrut, preventing damage from water trickling down the walls. A fire in the cable spreading room is an extremely unlikely event due to the flame qualification of the cables, the absence of permanent combustibles in this area, and the rigid controls on transient combustible material in this area. From a PRA perspective, this area is one of the lowest risks of any fire areas with regard to fire frequencies. A conservative estimate of the frequency of loss of both buses from a fire deluge is 1E-7/yr.
Additionally, there have been hourly fire watches going into the cable spreading room since early 2005.
IV. Corrective Actions
The corrective actions taken to address this event include:
1. The deficient seals were repaired.
2. Design changes were developed to upgrade the cable spreading room seals and floor joints.
Planned corrective actions will provide a detailed barrier and seal basis document and revise the administrative controls for maintenance and configuration control of barriers and seals.
V. Similar Events required hydrostatic seals. The cause of the condition was attributed to a failure to transform the sealing requirements into field fabrication and installation drawings. Following this event, an inspection verified that all required hydrostatic seals were installed.