05000315/LER-2001-001

LER-2001-001,

Docket Number
Event date:
Report date:
3152001001R00 - NRC Website

Conditions Prior to Event Unit 1 was in Mode 1, Power Operation, at 100 percent Rated Thermal Power

Description of Event

On February 15, 2001, at 1417 hours0.0164 days <br />0.394 hours <br />0.00234 weeks <br />5.391685e-4 months <br />, while at approximately 100 percent power, a low vacuum trip on the Unit 1 east main feedwater pump turbine occurred. The operator manually tripped the reactor in accordance with plant procedures. The post trip investigation indicated that all safety related systems performed as designed.

Prior to the event, the main feedpump water box differential pressure and backpressure was being monitored on an increased frequency due to a rising trend on both parameters. The previous shift had attempted to improve this condition by flushing the circulating water side of the main condenser waterboxes. A downpower had been previously scheduled to allow cleaning of the main feedwater condenser. However, the fouling occurred more rapidly than anticipated.

In accordance with 10 CFR 50.72 (b)(3)(iv)(B)(1), a four hour ENS notification was made to the NRC at 1545 hours0.0179 days <br />0.429 hours <br />0.00255 weeks <br />5.878725e-4 months <br /> on February 15, 2001 as an event or condition that results in actuation of the reactor protection system when the reactor is critical. LER 315/2001-001 is being submitted in accordance with the requirements of 10 CFR 50.73 (a)(2)(iv)(A) for a condition or event that resulted in a manual or automatic actuation of the RPS system.

Cause of Event

The cause of the feedwater pump trip was loss of condenser vacuum as the result of corrosion debris obstructing flow at the condenser's inlet-side tubesheet. This condition lead to an elevated condenser backpressure and a subsequent low vacuum trip of the east main feedwater pump. The corrosion debris was a product of general corrosion that occurred in the vertical portion of the 24-inch carbon steel circulating water lines several feet above and below the standing water level.

The corrosion was limited to the portion of the lines that were repeatedly wetted by fluctuating water level during the extended unit 1 outage.

Analysis of Event

As detailed in the UFSAR, the main feedwater system has one accident mitigation function; namely, feedwater isolation.

This event had no impact on the ability of the main feedwater system to perform its safety function. The function of providing feedwater flow to support power operation is a not a safety related or accident mitigating function. Therefore, with respect to the main feedwater system, this condition has no safety significance.

The circulating water system provides a heat sink for waste heat from the plant thermal cycle, including the main feedwater pump condenser. As detailed in the UFSAR, the circulating water system has no safety or accident mitigation functions.

The once-through circulating water system supplies cooling water to the condensers and turbine auxiliary coolers.

Therefore, with respect to the Circulating Water System, this condition has no safety significance.

The variable speed turbine-driven main feedwater pumps are designed to provide the required feedwater flow to the steam generators. The main feedwater pump turbines are non-safety related, non-safety interface, balance of plant equipment.

Chapter 14 of the UFSAR includes an analysis that addresses loss of normal feedwater flow. Results of the analysis show that a loss of normal feedwater does not adversely affect the core, the RCS, or the steam system.

Corrective Actions

Following the reactor trip, both the east and west main feed pump condensers were opened and cleaned. A significant amount of debris was removed from the east main feed pump condenser, which consisted mostly of corrosion scale. A sample of the corrosion scale removed from the Unit 1 east main feedwater pump condenser was sent to a laboratory for chemical analysis. Laboratory results indicated that the corrosion scale was indicative of corrosion material derived from low-alloy carbon steel.

A visual examination was conducted to locate the source of the corrosion scale. Divers completed inspections of the following areas: (1) The forebay from the travelling water screens up to and including the circulating water pumps, (2) the circulating water discharge pipe from the circulating water pumps to the common discharge area, (3) the common discharge area downstream of the circulating water pumps, and (4) the 24-inch carbon steel lines leading to the east and west feed pump condensers. The inspections determined that the source of corrosion scale was the 24-inch carbon steel lines. No signs of corrosion scale were found in any of the other inspected areas. As stated earlier, the corrosion scale originated from the vertical portion of the 24-inch carbon steel circulating water lines several feet above and below standing water level. Upon discovery of the corrosion, the vertical circulating water piping leading to both the east and the west feed pump condensers were cleaned with high-pressure sprays and the resulting debris was removed by vacuum. Based on the thickness of corrosion scale removed during cleaning, wall thinning was considered negligible.

Between March 30 and April 1, 2001, Units land 2 respectively were reduced to approximately 60 percent power to clean and inspect the feed pump condensers. No new scale formation was observed.

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