05000413/LER-2004-003

LER-2004-003, Unanalyzed Condition Due To Inadequate Evaluation of Fire Interactions

Docket Number
Event date:
Report date:
4132004003R01 - NRC Website

BACKGROUND AND EVENT DESCRIPTION

This event is being reported in accordance with the 30-day reporting requirement of the Catawba Facility Operating License Section 2F and 50.73(a)(2)(ii)(B).

NUREG-0800 Standard Review Plan and subsequent Safety Evaluation Reports (NUREG-0954) state that one train of systems necessary to achieve and maintain Hot Standby conditions from the control room is required to be free of fire damage. The separation criteria for components to be free from fire damage are:

1.Separation of cables and equipment and associated circuits of redundant trains by a fire barrier having a 3-hour rating; or, 2.Separation of cables and equipment and associated circuits of redundant trains by a horizontal distance of more than 20 feet with no intervening combustible or fire hazards. In addition, fire detectors and an automatic fire suppression system should be installed in the fire area; or, 3.Enclosure of cable and equipment and associated circuits of one redundant train in a fire barrier having a 1-hour rating. In addition, fire detectors and an automatic fire suppression system should be installed in the fire area.

If the separation criteria cannot be met, then an alternative, dedicated shutdown capability should be provided.

During the Catawba initial licensing in the early 1980s, Catawba elected to meet the existing fire protection regulations using the following three fire response strategies:

1.Plant shutdown from the control room using A-train equipment.

2.Plant shutdown from the control room using B-train equipment.

3.Plant shutdown from a dedicated, safe shutdown facility independent from the control room.

On June 23, 2004, electrical components were identified within the same fire area that did not meet the cable separation criteria in that a postulated single fire may damage both charging pumps and damage the standby makeup pump cable. The standby makeup pump is used for alternate shutdown to provide reactor coolant pump seal injection flow.

The components identified within the fire area for 2ETA include:

• Unit 2 A-train charging pump breaker and cables

• Unit 2 valve 2NV188A breaker and cables (volume control tank outlet valve)

• Unit 2 standby makeup pump cable

• Reactor coolant pump thermal barrier heat exchanger valve breakers and cables [EIIS:CC] If the Unit 2 B-train charging pump is initially in service and a fire in 2ETA causes valve 2NV188A to hot short to spuriously close, then the 2B charging pump is assumed to be damaged due to a loss of suction.

The fire in 2ETA is also assumed to damage the 2A charging pump cable and standby makeup pump cable. The thermal barrier cooling water system is initially in service and should provide cooling to the reactor coolant pump seals unless another hot short causes a second spurious actuation. The Catawba Licensing Basis is to postulate "one worst case spurious actuation". The scenario that includes spurious actuation of both the volume control tank outlet valve and thermal barrier isolation valve is beyond the design basis but is being evaluated for safety significance.

A similar vulnerability exits for the fire area for the Unit 1 B-train 4160V switchgear room (1ETB). Components within lETB include:

• Unit 1 B-train charging pump breaker and cables

• Unit 1 valve 1NV189B breaker and cables (volume control tank outlet valve)

• Unit 1 standby makeup pump cable

• Reactor coolant pump thermal barrier heat exchanger valve breakers and cables The plant Safe Shutdown Analysis did not consider the postulated common failure mechanism for both charging pumps due to a spurious closure of a volume control tank outlet valve. The analysis assumed that one train of charging pumps would be available following a fire in a 4160V switchgear room.

The 4160V switchgear rooms are equipped with fire detection capability but are not protected by an automatic fire suppression system. The electrical components within each of the 4160V switchgear rooms, lETB or 2ETA, are not separated by a 3-hour fire barrier.

Because the cable separation criteria for a 3-hour fire barrier could not be satisfied, operations personnel initiated hourly fire watches in accordance with the Selected Licensee Commitment for Fire Rated Assemblies. Fire watches were started on June 23, 2004 for all four switchgear rooms (ZETA, lETB, 2ETA, and 2ETB).

The phone notification to the NRC Operations Center was completed June 23, 2004 for Unit 2 and June 24, 2004 for Unit 1.

Subsequent review of the cable routing for the volume control tank outlet valves identified a fire in the following fire areas may also result in damage to both charging pumps:

Fire Area 6, 1B Penetration Room Fire Area 13, lA Penetration Room Fire Area 15, 1A Switchgear Room Fire Area 32, Unit 1 Auxiliary Shutdown Panel A-train Fire Area 34, Unit 1 Auxiliary Shutdown Panel B-train Fire Area 5, 2B Penetration Room Fire Area 7, 2B Switchgear Room Fire Area 12, 2A Penetration Room Fire Area 31, Unit 2 Auxiliary Shutdown Panel A-train Fire Area 33, Unit 1 Auxiliary Shutdown Panel B-train Fire Area 46, Unit 2 Corridor The Safe Shutdown Analysis for these fire areas was based on one charging pump being available.

A noteworthy design feature of Catawba is the extensive use of grounded, shielded armor cables. The NUREG-0954 Safety Evaluation Report dated February 1983 description of cables states:

"The power, control, and instrumentation cables used in Catawba are of an interlocked armor design in a galvanized steel jacket. All cables pass the IEEE standard 383-1974 flame test. In addition, the applicant has submitted samples of the cable for testing at Underwriters Laboratories in their "corner test" configuration. When subjected to a 400,000 BTU/hr heat flux, the cable exhibited no tendency to propagate fire. In addition, the applicant has conducted tests that demonstrate that no fire propagation from cable to cable or tray to tray occurs as a result of an electrically initiated fire. The staff finds this acceptable.

The Sandia National Laboratories, Circuit Failure Mode and Likelihood Analysis, draft revision 2 December 20, 1999 page 15 states:

"For an armored (metal jacketed) cable, cable-to-cable shorting without a short to ground would be considered highly unlikely, if not impossible. Armoring might also influence the likelihood and duration of non-grounded conductor-to-conductor shorts within the cable. In effect, the armor represents a readily accessible ground plane. The ready availability of a strong ground plane may increase the likelihood of ground shorts, especially considering that the heating during a fire will occur from the outside in.

Hence, conductors (or insulation) nearest the cable surface will likely fail first.

Some experimental evidence regarding armored cables is available, in particular, from testing by EdF (EF.30.15.R/96.442). In this program several samples of various armored cables were tested.

Most showed evidence of the initial failures involving one conductor and the armor, and relatively few showed conductor-to­ conductor shorts independent of the shield. Hence, the experimental evidence indicates that in comparison to non-armored multi-conductor cables, the likelihood of conductor-to-conductor hot shorts is substantially reduced.

Additional correspondence on armored cables is contained in the February 19, 2003 NRC Risk-Informing Post-Fire Shutdown Circuit Analysis Inspection meeting transcript and September 15, 2003 Duke letter to the NRC entitled, "Comments on Proposed Generic Communication Risk-Informed Inspection Guidance for Post-Fire Safe Shutdown Inspection".

At the time of this event, Unit 1 and 2 were operating in Mode 1 at 100 percent power. No structures, systems, or components were removed from service that had any effect on the event or conflicted with Technical Specifications.

CAUSAL FACTORS

This condition is historical and dates to the original development of the assumptions used to support the Safe Shutdown Analysis.

Consequently, a root cause evaluation was not performed. The apparent cause is attributed to an inadequate original Safe Shutdown Analysis of certain spurious hot short valve operations.

CORRECTIVE ACTIONS

Immediate:

1. Hourly fire watches established for switchgear rooms lETA, lETB, 2ETA, and 2ETB.

Subsequent:

1.Cable routing locations for Unit 1 and Unit 2 charging pump suction valves within ETA and ETB were identified. Valves included the two volume control tank outlet valves (NV188A and NV189B) and the two refueling water storage tank valves (NV252A and NV253B).

2.Following the review and evaluation of the cable routes for NV188A, NV189B, NV252A, and NV253B, guidance was provided to the operators such that control room actions would be taken to maintain at least one charging pump available for post-fire recovery following a fire in the 13 identified fire areas.

3.Following the implementation of the operator guidance, the hourly fire watches were terminated.

4. Cable routing evaluations were completed for the charging pumps, standby makeup pump, suction and discharge valves for the standby makeup pump, thermal barrier isolation valves, offsite power cables for the 4160V switchgear, and diesel generator power cables for the 4160V switchgear.

Planned:

1. Update the Safe Shutdown Analysis based on the results of the cable routing evaluation.

The planned corrective actions are being addressed within the Catawba Corrective Action Program. There are no NRC commitments contained in this LER.

SAFETY ANALYSIS

There were no fire events that challenged the operability of the charging pumps, standby makeup pump, reactor coolant pump seal cooling by seal injection, or thermal barrier heat exchanger operation. The conditions addressed by this report are related to postulated accidents and potential failures and had no direct effect on the health and safety of the public.

The 4160V switchgear rooms have minimal in-situ combustible materials and are maintained free of significant transient combustible materials by administrative controls. Ignition sources are limited to the electrical cabinets and switchgears. The switchgear rooms are equipped with fire detectors to alarm the operators of fire events. The fire brigade members are trained to immediately respond to any plant fire.

Test data from the December 2002 EPRI report, Characterization of Fire- Induced Circuit Faults, supports the conclusion that spurious hot shorts will not occur for approximately 30 minutes following fire initiation.

Within the 4160V switchgear rooms, the Unit 2 volume control tank outlet valve cable and the standby makeup pump cable are separated by a horizontal distance of greater than 20 feet with no intervening combustibles. The Unit 1 volume control tank outlet valve cable and the standby makeup pump cable are separated by a horizontal distance of approximately 9 feet with no intervening combustibles.

The risk of core damage was analyzed for a potential spurious closure of each unit's volume control tank outlet valve. The analysis was performed using the NRC Fire Significance Determination Process (SDP), Phase II procedure, including use of the SDP Fire Ignition Frequencies, Zone of Influence Charts, and manual suppression curves. The analysis for each area was performed conservatively, with some cables not traced, but assumed failed by any fire. Additionally, numerous conservatisms were applied throughout.

The Phase II analysis showed an overall Core Damage Frequency of less than 1E-06/year for each unit. A number of fire areas were qualitatively screened as low risk, due to a number of factors such as not affecting the SSF and other important components in the PRA scenarios analyzed. A more detailed Fire SDP Phase III calculation would likely result in core damage estimates much lower than the above Phase II results.

The analysis used extensive cable tracing, circuit analysis and walk downs provided by site engineering as a good-faith approximation of cable locations and cable interactions. As Catawba continues with the transition to the NFPA 805, Performance-Based Standard for Fire Protection for Light Water Reactor Generating Plants, a more formal verification of cable locations will be documented. Therefore, the results of the PRA analysis cannot be considered finalized until the completion of the formal cable location verification.

ADDITIONAL INFORMATION

Within the last three years, no other LERs occurred at Catawba involving fire events or the safe shutdown analysis. Therefore, this event was determined to be non-recurring.

Energy Industry Identification System (EIIS) codes are identified in the text as [EIIS: XX].

.� , This event did not meet the criteria for a Safety System Functional Failure.

There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.