05000275/LER-2001-003

LER-2001-003,

Event date:
Report date:
2752001003R00 - NRC Website

I. Plant Conditions

Unit, has operated at 100 percent reactor thermal power (RTP) with the described condition.

A. Background

Technical Specification (TS) 3.7.6, "Condensate Storage Tank (CST) and Fire Water Storage Tank (FWST)," requires that "the FWST level shall be > 22.2% for one unit operation and > 41.7 % for two unit operation.

TS 3.7.6, Bases, Limiting Condition for Operation (LCO) requires that "Tc.) satisfy the Hosgri analysis assumptions, the CST and FWST must contain sufficient cooling water to remove decay heat following a reactor trip from 102% RTP, and then to cool down the (reactor coolant system; RCS to (residual heat removal) RHR entry conditions, assuming a coincident loss of offsite power and the most adverse single failure.

Supplemental Safety Evaluation Report (SSER) '6, dated November 15, 1978, "... require(s) that redundant flow paths be provided to bypass any assumed single valve failure (valve jammed shut)," between the FWST and auxiliary feedwater (AFW) Pumps suction as a result of a seismic event.

On June 20, 2001, indication of contamination of the condensate system (SD) was identified in Unit 1 due to increased sodium levels. During the investigation, PG&E determined that in-leakage from an undefined water source had increased the sodium level in the CST (BA). Valve 1-FCV-437 between the Unit 1 AFW Pumps suction and the alternate long term cooling water sources was determined to be leaking-by, allowing high sodium content water to contaminate the CST.

Valve 1-FCV-437 is a normally sealed-shut valve and valve MU-1-297 is a normally sealed-open valve. Valve MU-0-1557 is a normally sealed-open valve. Refer to the diagram on page 6.

B. Event Description

On June 22, 2001, during trouble-shooting of sodium contamination of the Unit 1 CST, valve MU-1-297 was closed to stop water from the raw water reservoir (RWR) leaking-by valve 1-FCV-437 and into the CST.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) OF 2001 3 5 0 5 0 Diablo Canyon Unit 1 2 0 0 During preparation of a 10 CFR 50.59 evaluation fo7 permanent closure of valve MU-1-297, PG&E E igineering personnel identified that SSER 8 specifically evaluated the fiow path from the FWST to the AFW Pumps suction. Closure of MU-1-297 isolated the AFW Pumps with ; : tingle valve, which must be assumed to fail jammed shut following a seismic event, making the FWST inventory unavailable to the AFW Pumps for alternate decay heat removal. This is similar to the TS 3.7.6 prohibited condition A, "CST or FWST level not within limit" for which the required Action A.2 is to "Restore the CST or FWST level to within limit" within 7 days. The flow path was isolated for approximately 80 days.

On September 12, 2001, Valve MU-1-297 was reopened restoring the flow path between the FWST and the AFW Pumps suction. Valve MU-0-1557 was closed isolating the raw water contamination through 1-FCV-437.

On September 13, 2001, PG&E determined that the alternate cooling water flow path isolation constituted a violation of TS 3.7.6 on June 29, 2001, following the first 7 days of isolation.

C. Inoperable Structures, Components, or Systems that Contributed to the Event FCV-437 was operable but degraded due io seating-leakage.

D. Other Systems or Secondary Functions Affected

None.

E. Method of Discovery

PG&E Engineering personnel discovered the condition while performing a 10 CFR 50.59 evaluation for permanent closure of valve MU-1-297.

F. Operator Actions

None.

Safety System Responses

None.

(3) PAGE LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6)

OF

- 0 10 2001 Diablo Canyon Unit 1 5 2 0 0 0 7 0 5 III. � Cause of the Problem

A. Immediate Cause

Valve MU-1-297 was closed; thereby, isolating the non-redundant flow path from the FWST to the AFW Pumps suction.

B. Root Cause

PG&E believes the cause of the flow path being isolated in violation of TS 3.7.6 is personnel error, "knowledge exceeded," due to licensing-basis documentation not readily available to the plant staff.

C. Contributory Cause PG&E's licensing history spanned many years resulting in 34 separate SSERs. As a result the SSER 8 requirement to maintain redundant flow paths between the FWST and the AFW Pumps suction had not been clearly identified as a licensing requirement.

IV. � Analysis of the Event The CST supplemented by the FWST provides a safety grade source of water to the steam generators (SG) for removing decay and sensible heat from the RCS.

The CST and FWST provide a passive flow of water, by gravity, to the AFW System. The AFW System provides water to the SGs to accomplish the heat removal function. The steam produced is released to the atmosphere by the main steam safety valves or atmospheric dump valves if the main steam iso!ation valves are closed.

The CST and FWST provide cooling water to rarrov.-a decay heat and to cool down the unit following all events in the accident analyses as discussed in the FSAR Update, Chapters 6 and 15. The limiting event for AFW inventory, i.e., CST and FWST minimum volumes, is based on a loss of offsite power, which assumes a reduced RCS cooldown rate on natural circulation and requires seismically qualified water sources. The lower RCS cooldown rate increases the cooldown period until the RHR system can be used to remove further decay heat. The extended cooldown time thus requires more AFW supply than can be provided by the seismically qualified portion of the CST.

Because the CST and FWST are the principal components for removing residual heat from the RCS, they are designed to withstand earthquakes and other natural phenomena, including missiles that might be generated by natural LER NUMBER 6 LOCENSEE EVENT REPORT (LER) TEXT CCNTINJATION FACILITY NAME (1) DOCKET NUMBER (2) 2001 0 � 0 0 phenomena. The CST and FWS-r-ar, _ :signed Seismic Category I to ensure availability of the AFW inventory. The operability of the AFW cooling water supply is assured by verifying the CST is properly aligned and by verifying the FWST is capable of being aligned to the AFW Pumps suction.

Since there was insufficient volume in the CST alone for Icng-term cooling needs, the NRC required in SSER 8 that the FWST have a seismically-qualified flowpath to the AFW Pumps suction capable of withstanding an assumed seismic failure of any single valve (including a valve jammed shut).

Should valve MU-1-297 have failed in the closed position, water from additional sources could have been transferred to the CST to supply the AFW Pumps suction, as described in the FSAR Update, Chapter 6, "Auxiliary Feedwater System," subsection 6.5.2.1.1, "Water Sources.

Therefore, the event:

• Is of very low risk significance;

• did not adversely affect the health and safety of the public; and

• was not a Safety System Functional Failure.

V. � Corrective Actions A. Immediate Con-ective.Actions Valve MU-1-297 was opened and sealed in the open position. Valve MU-0-1557 was closed to prevent leak-by of raw water into the CST or loss of FWST inventory through this valve due to a stuck-open valve.

B. Corrective Actions to Prevent Recurrence

TS 3.7.6 Bases will be revised to include the critical valve line-up between the FWST and the AFW Pumps suction.

VI. � Additional Information

A. Failed Components

None.

B. Previous Similar Events

None.

LER NUMBER (6) FACILITY NAME (1) DOCKET NUMBER (2) PAGE (3) OF Diablo Canyon Unit 1 0 5 0 0 0 2 7 5 2001 - 0 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 100

ABOVE

MU.

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FEADER

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PFWPp 143. 2 N3. 3 FCV-437 Auxiliary Feed Water Pumps Suction from Alternate Water Sources

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UNIT2 FCV-437 PFVVPp Nb. 1 PFWPp No. 2 No. 3

RESEVOIR

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TURBINE

UNIT1 LC.

FP-0-307 1-FCV-437 leak-by allowed raw water to enter and contaminate the CST. (N0002129)