ML20212A239

Safety Evaluation Supporting Amends 157 & 148 to Licenses NPF-10 & NPF-15,respectively

ML20212A239
Person / Time
Site:	San Onofre
Issue date:	09/07/1999
From:	NRC (Affiliation Not Assigned)
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Shared Package
ML20212A236	List: ML20212A232 ML20212A239
References
NUDOCS 9909160125
Download: ML20212A239 (5)

Text

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j NUCLEAR REGULATORY COMMISSION o

WASHINGTON, D.C. 3068 Hoot s...../

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.157 TO FACILITY OPERATING LICENSE NO. NPF-10 AND AMENDMENT NO.148 TO FACILITY OPERATING LICENSE NO. NPF-15 SOUTHERN CAllFORNIA EDISON COMPANY SAN DIEGO GAS AND ELECTRIC COMPANY THE CITY OF RIVERSIDE. CALIFORNIA THE CITY OF ANAHEIM. CALIFORNIA SAN ONOFRE NUCLEAR GENERATING STATION. UNITS 2 AND 3 DOCKET NOS. 50-361 AND 50-362 i

1.0 INTRODUCTION

By letter dated December 31,1998, as supplemented June 14,1999, Southern California Edison Company (SCE or the licensee) requested Technical Specification (TS) 3.3.5, changes i

for San Onofre Nuclear Generating Station (SONGS) Units 2 and 3. The proposed changes to l

TS 3.3.5, " Engineered Safety Features Actuation System (ESFAS) Instrumentation," would include restrictions on operation with a channel of the refueling water storage tank (RWST) level-low input to the recirculation actuation signal (RAS) and the steam generator (SG) pressure-low input or SG pressure difference-high input to the emergency feedwater actuation signal (EFAS) in the tripped condition. The current TS allows plant operation in this condition indefinitely. The licensee has determined that unacceptable consequences could result from a spurious trip of RAS or EFAS due to operation with a channelin trip condition. The proposed TS changes would improve the plant operational safety and, thereby, reduce plant risk.

The June 14,1999, letter provided clarifications and additional information that were within the scope of the original Federa/ Register (64 FR 40907) notice and did not change the staff's initial proposed no significant hazards consideration determination.

2.0 EVALUATION 2.1 Deterministic Evaluation The ESFAS initiation is based on a two-out-of-four logic. The ESFAS has a design feature that allows one of the four measurement channels to be placed in bypass indefinitely. When a channelis in bypass, the logic converts to a two-out of three configuration if two channels are inoperable, then one channel is placed in bypass and the second inoperable channel must 9909160125 990907 PDR ADOCK 05000361 P

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2-be placed in a tripped condition to satisfy the single failure criterion. With one channelin a tripped condition it takes only one additional channel trip, either due to channel failure or in response to actual plant conditions, to initiate an ESFAS signal.

. Previously, a spurious actuation signal due to channel failure had been considered failure in a safe condition. Now the licensee determined that there is a scenario in which a spurious RAS

. could lead to unacceptable results. The purpose of RAS is to change the mode of operation of the emergency core cooling system (ECCS) during a loss-of-coolant accident from the injection phase to the recirculation phase. It shifts the high-pressure safety injection pumps suction from the RWST to the containment emergency sump when the RWST is nearly depleted. The sump water supplies the inventory requirements for the ECCS pump suctions. However, a spurious RAS actuation (before the RWST level has reached the low level setpoint) will

' actuate the switchover action with the exception of closure of the ECCS miniflow valves because there is an interlock that prevents the miniflow valves closing unless there is sufficient water in the containment sump. If the miniflow lines remain open during the postulated scenario, it effectively breaches containment integrity, resulting in the unacceptable i

consequences for plant operation.

The licensee also determined that a spurious SG pressure-low or SG pressure difference-high signal could lead to undesirable consequences in the EFAS. A spurious SG pressure-low i

signal would block feedwater to the associated SG, as the system would treat that SG as ruptured. A spurious SG pressure difference-high could cause feeding of a ruptured SG, i

depleting condensate storage tank inventory and increasing the load on the containment cooling system and resulting in unacceptable consequences for plant operation.

TS 3.3.5 currently allows operation with one channel in bypass and one channel in trip for all ESFAS functions. Following identification of the RAS and EFAS issues discussed above, the licensee placed administrative limits on channelinoperable for RAS and EFAS signals. These j

administrative limits, which allow operation for only 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> with a channel in trip before requiring a plant shutdown, do not provide plant operational flexibility.

The licensee performed a risk-informed assessment to determine acceptable limits of allowable completion time (CT) for operation with a channel of RAS and EFAS in trip. Based on that assessment, the licensee proposed to include restrictions on operation of the RWST level-low for the RAS function and also restrictions on SG pressure-low or SG pressure difference-high for the EFAS function. When two automatic ESFAS trip channels inoperable for RWST level-low for the RAS function or SG pressure-low or SG pressure difference-high for the EFAS function, the proposed TS requires the restrictions on CT to restore the tripped channels of RAS and EFAS functions to operable status within 7 days which is more conservative than the current TS and will reduce plant risk.

2.2 Probabilistic Risk Assessment (PRA) Evaluation SCE performed a quantitative risk-informed assessment to determine acceptable limits for operation with a channel of RAS in a tripped condition and a qualitative risk-informed assessment to determine acceptable limits for operation with a channel of EFAS in a tripped condition.

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. The staff used a three-tiered approach to gain risk insights and to evaluate the risk associated with the proposed risk-informed amendment. The first tier evaluated the PRA model and the impact of the change on plant operational risk. The second tier addressed the need to preclude potentially high risk configurations if additional equipment will be taken out of service simultaneously or other risk-significant operational factors such as the potential for concurrent system or equipment testing. The third tier evaluated the licensee's configuration risk management program (CRMP) to ensure that equipment removed from service prior to or during the proposed CT will be appropriately assessed from a risk perspective. Each tier and associated findings are discussed below.

2.2.1 Tier 1: PRA Evaluation of Proposed CT The Tier 1 staff review of the licensee's PRA involved two aspects: (1) evaluation of the PRA model and its application to determine acceptable limits for operation with a channel of RAS or EFAS in a tripped condition, and (2) evaluation of PRA results and insights stemming from the application. The review did not warrant an assessment of any unconventional PRA practices or unique features that could impact the reasonableness of PRA findings and conclusions.

(1) Evaluation of PRA Model and Application to the CT The staff's review consisted of an initial screening process that examined the attributes of the licensee's PRA including the plant protection system (PPS) model. However, this did not involve an in-depth review of the licensee's PRA to the extent necessary to validate the licensee's overal! quantitative estimates, j

The staff determined that the licensee actively uses PRA on a daily basis during at-power operation when making safety decisions of various plant activities. The licensee's PRA staff has used their PRA model extensively for this and other previous risk-informed applications.

The licensee used its '1iving" PRA model as the basis for the calculations to support the proposed changes. The licensee's current PRA model has been updated since the development of the individual plant examination. The current PRA model has undergone intemal and external peer reviews, and the licensee utilizes a proceduralized change process to control modification of the PRA to reflect the as-built, as-operated, plant condition.

The licensee's PRA model contains a detailed PPS model based on generic Combustion Engineering (CE) fault-tree logic. The CE PPS model was adapted for SONGS Units 2 and 3.

The staff believes that the licensee's at-power PRA has adequate scope and detail to conclude that proposed restrictions will reduce plant risk and for evaluating acceptable limits for operation with a channel of RAS or EFAS in a tripped condition.

(2) Evaluation of PRA Results and insights Associated with the Proposed Change The proposed TS change limits the time during which an ESFAS trip channel for RWST level-low for the RAS function or SG pressure-low or SG pressure difference-high for the EFAS

- function is placed in a tripped condition. Since a spurious trip of the RAS or the EFAS may result in undesirable consequences, it can be concluded that the proposed TS amendment, which limits the allowable CT from an indefinite time period to 7 days, will reduce plant risk.

I

. The licensee also used quantitative risk-informed assessment methods to determine acceptable limits for operation with a channel of RAS in a tripped condition. This risk-informed assessment followed the approach outlined in Regulatory Guide (RG) 1.174 and RG 1.177. The staff review of the licensee's risk analysis focused on the reasonableness of the overall approach and PRA technique. To analyze the risk of a spurious RAS trip signal, the licensee applied the PPS fault-tree logic using component failure rates from the SONGS Units 2 and 3 living PRA and the rNelear computerized library for assessing reactor reliability generic component failure rate database for light-water and liquid sodium reactor PRAs prepared by EG&G Idaho. The staff finds that there are no significant weaknesses or deficiencies associated with this approach, and PRA methodology used to determine acceptable limits for operation with a channel of RAS in a tripped condition is reasonable for this application.

Although instituting the proposed time limits during which an ESFAS trip channel for SG pressure-low or SG pressure difference-high for the EFAS function is placed in a tripped condition results in an overall increase in operational safety, the licensee used a qualitative assessment to determine acceptable limits for operation with a channel of EFAS in a tripped condition. The licensee concluded that the risk stemming from a spurious EFAS actuation during the proposed limits for operation is low. The staff finds that the licensee's conclusion is reasonable for this application.

The staff finds that the overall proposed limits will reduce risk and has, therefore, met the intent of the Tier 1 guidance in RG 1.174 and RG 1.177.

2.2.2 Tier 2: Avoidance of Risk-Significant Plant Configurations The staff finds that SONG's risk-informed CRMP provides reasonable assurance that risk-significant plant equipment outage configurations will not occur when a channel of RAS or EFAS is in a tripped condition consistent with the proposed TS change. The CRMP has provisions for assessing the need for additional actions if additional equipment-out-of service 1

conditions exist while the plant is in the risk-ir; formed completion time.

The staff, therefore, believes that the licensee's CRMP satisfies the intent of Tier 2 to avoid risk-significant plant configurations.

1 2.2.3 Tier 3: Risk-Informed Plant Configuration Management The staff believes that the licensee's risk-informed CRMP will allow an evaluation of the risk associated with both scheduled and unscheduled plant activities with a channel of RAS or EFAS in a tripped condition. The licensee actively uses PRA to control risk using its online i

safety monitor. The licensee has already incorporated the CRMP descriptions into TS 5.5.2.14 j

" Configuration Risk Management Program," and the staff finds it acceptable. The licensee I

stated in its submittal that TS 5.5.2.14 is applicable to the proposed TS change since the j

proposed change is a risk-informed completion time. The staff concludes that the licensee has met the intent of the Tier 3 guidance.

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i 5-3.0

SUMMARY

The proposed TS changes on allowable CT for tripped channels of the RAS and EFAS functions in TS 3.3.5 do not involve either a physical alteration of the plant or affect the design 4

basis of the ESFAS and the engineered safety feature systems. When two automatic ESFAS trip channels are inoperable for RWST level-low for RAS function or SG pressure-low or SG pressure difference-high for the EFAS function, the additional restrictions on allowable CT to restore the tripped channels of RAS and EFAS functions to operable status within 7 days are more conservative than the current TS. It will improve the plant operational safety and reduce plant risk. Also, the staff believes that the licensee's CRMP provides a proceduralized risk-informed method to manage the risk associated with this risk-informed TS changes. Therefore, the staff concludes that PRA analysis supports the proposed limitations on ESFAS trip channels i

for RWST level-low for the RAS function and SG pressure-low or SG pressure difference-high for the EFAS function. The staff finds this proposal acceptable.

4.0 STATE CONSULTATION

in accordance with the Commission's regulations, the California State official was notified of the

- proposed issuance of the amendments. The State official had no comments.

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5.0 ENVIRONMENTAL CONSIDERATION

l The amendments change a requirement with respect to the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendments involve no significant increase in the amounts and no

. significant change in the types of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on such finding (64 FR 40907). Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.

6.0 CONCLUSION

The Commission has concluded, based on the considerations discussed above, that (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.

Principal Contributors: K.,Desai S. Malik H.1.1 Date: September 7,1999 L.