ML20096C207
| ML20096C207 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 01/03/1996 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20096C204 | List: |
| References | |
| NUDOCS 9601170167 | |
| Download: ML20096C207 (8) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION E
f WASHINGTON, D.C. 20066 4 001
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION 1
RELATED TO AMENDMENT NO. 109 TO FACILITY OPERATING LICENSE NO. DPR-80 AND AMENDMENT NO.
108 TO FACILITY OPERATING LICENSE NO. DPR-82 PACIFIC GAS AND ELECTRIC COMPANY
['.ABLO CANYON NUCLEAR POWER PLANT. UNITS 1 AND 2 DOCKET NOS, 50-275 AND 50-323
1.0 INTRODUCTION
By application dated September 29, 1995, Pacific Gas and Electric Company (or the licensee) requested changes to the Technical Specifications (Appendix A to Facility Operating License Nos. DPR-80 and DPR-82) for the Diablo Canyon Nuclear Power Plant, Units 1 and 2.
The proposed changes would add a one-time footnote to the Technical Specifications (TSs) related to the emergency diesel generator diesel fuel oil storage and transfer system to permit the existing storage tanks to be replaced with double walled tanks and piping that comply with new California regulations.
2.0 EVALUATION
2.1 Background
The diesel generator fuel oil (DFO) storage and transfer system is described in Section 9.5.4 of the updated Final Safety Analysis Report (FSAR). The diesel generator fuel oil system is provided to supply sufficient diesel oil to the six emergency diesel generators (EDGs) for Units 1 and 2 for seven days of onsite power generation. The supply of fuel beyond the seven-day period is ensured by the availability of offsite sources and a reliable delivery system.
Further, historical data on utility systems has shown that when there has been a loss of offsite power to a generating station, power has almost always been restored within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, so a 7-day supply of fuel for the onsite AC diesel generators is adequate.
There are two underground diesel fuel oil storage tanks, each with a storage capacity of 40,000 gallons. There are two diesel fuel oil transfer pumps located below ground level, each adjacent to a storage tank but in separate compartments. One pump is more than adequate to supply the six diesel generators.
The two diesel fuel oil supply headers to each unit are routed in separate trenches. The pumps and headers supply the 550-gallon capacity day tanks built into the base of each diesel generator. Any combination of one storage tank and one pump is capable of serving all six day tanks.
9601170167 960103 DR ADOCK 0500 5
. On May 5,1994, Title 23, " Waters," of the California Code of Regulations (CCRs) became effective.
Included in the CCRs are requirements for underground fuel oil storage tanks and piping. Specifically, the regulations require the following:
1.
All buried underground storage tanks containing engine fuel oil must be retrofitted with a secondary containment (i.e., double-walled);
2.
All buried underground piping must be double walled or installed within a structure designed to contain leakage; and 3.
A leakage detection system must be installed to identify leakage from piping.
To comply with the CCRs, the DF0 storage tanks must be modified by December 22, 1998. The required modifications will render one DF0 storage tank and its associated piping up to the transfer pump vault inoperable for up to 60 days. Both DF0 storage tanks and associated piping require modifications, and, therefore, a total of 120 days is required to complete i
these modifications.
j The specific work and modifications to the DF0 storage and transfer system include:
1.
Replacement of each 40,000 gallon storage tank with a new 50,000 gallon, double-walled tank; 2.
Replacement of the piping between each DF'O storage tank and the DF0 transfer pumps; 3.
Installation of a leakage detection system for the tank and piping; and 4.
Inspection, refilling and testing of the completed work to verify operability following the modifications.
PG&E estimates that the modifications will take approximately 60 days for each tank and its associated suction transfer piping (total of 120 days for both trains). This estimated time is from the tank and associated piping being removed from service to when it is returned to service, and is based on a schedule with work being performed on a 2-shifts per day, 7-days per week basis.
Each replacement DF0 storage tank has an increased nominal capacity over the existing storage tanks of 10,000 gallons (approximately 25 percent). This size tank was chosen because it provides the greatest capacity, while allowing use of the existing tank location (i.e., in close proximity to DF0 transfer pumps). The replacement tanks will be able to use the same concrete foundation as the existing tanks, are only slightly dimensionally larger, and -
therefore, are able to use the same location.
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. The DF0 storage tanks and the piping between the tanks and the DF0 transfer pumps are buried and, therefore, require modifications to comply with the CCRs. Each DF0 transfer pump is located in a concrete vault and is connected to the buried piping via a flanged connection upstream of the pump suction cross-tie line. The CCRs do require modifications to the DF0 transfer pump vaults. However, these modifications will not require the DF0 transfer pumps to be taken out of service. No modification to the DF0 transfer pumps and suction cross-tie piping is required during the tank and piping replacement.
2.2 Proposed TS Changes
TS 3.8.1.lb.2, A.C. Sources - Operating, limiting condition for operation (LCO) in Modes 1 - 4 requires:
Two supply trains of the Diesel Fuel Oil Storage and Transfer System containing a minimum combined storage of 33,000 gallons of fuel for one unit operation
- and 65,000 gallons of fuel for two unit operation.
TS 3.8.1.2b.2, A.C. Sources - Shutdown, LCO in Modes 5 and 6 requires:
One supply train of the Diesel Fuel Oil Storage and Transfer System containing a minimum storage of 26,000 gallons
- of fuel in addition to the fuel required for the other unit.
The licensee proposes to add U e following footnote to TS 3.8.1.lb.2:
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The performance of modifications to the diesel fuel oil storage and transfer system requires one fuel oil storage tank at a time to be drained and replaced with a new storage tank. During this period, the diesel generator fuel oil storage requirement for two unit operation in 4
Modes 1 - 4, or for one unit operation in Modes 1 - 4 and one unit in Mode 5 or 6 is 35,000 gallons. A total of up to 120 days may be required to complete the replacement of both tanks.
For the duration of the tank replacement, temporary onsite storage of 30,000 gallons will be maintained.
Prior to removal of a tank from service, the offsite circuits required by Technical Specification 3.8.1.la. will be verified to be OPERABLE.
The licensee proposes to add the following footnote to TS 3.8.1.2b.2:
The performance of modifications to the diesel fuel oil storage and transfer system requires one fuel oil storage tank at a time to be drained and replaced with a new storage tank. During this period, the diesel generator fuel oil storage requirement for one or two unit operation in Modes 5 and 6 is 35,000 gallons. A total of up to 120 days may be required to complete the replacement of both tanks.
For the duration of the tank replacement, temporary onsite storage of 30,000 gallons will be maintained. Prior to removal of a tank from service, the offsite circuits required by Technical Specification 3.8.1.2a. will be verified to be OPERABLE.
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. 2.3 Assessment The licensee has stated that the fuel oil inventory that will be maintained in the operable permanent DF0 storage tank will be at least 35,000 gallons, which is sufficient for the EDGs to power the minimum engineered safety features (ESF) systems for approximately four days. This is more than sufficient time to cool down both units from full power operation, if necessary, under postulated transient or accident conditions.
As an additional compensatory measure, the licensee is planning to provide a temporary onsite DF0 storage of 30,000 gallons (useable) so that there would be a total of 65,000 gallons of diesel fuel available onsite. This is as much fuel oil as is required by the present TSs (3.8.1.lb.2) for two unit operation. To assure that this additional fuel oil is available, if needed, the licensee has also proposed the following actions:
a.
The tank providing the temporary storage will be seismically supported to assure the tank maintains its function following a seismic event. The temporary storage complies with the requirements of FSAR Appendix 9.58, Section F.
FSAR Appendix 9.58 requires that all above-ground DF0 storage tanks be located at least 50 feet from any building containing safety-related equipment.
b.
To assure that the DF0 in the temporary, onsite storage tank is of acceptable quality, TS 4.8.1.1.3c. and TS 4.8.1.1.3d. will be applied to the temporary, onsite storage tank.
c.
A DF0 tanker truck will also be located onsite inside the plant-protected area to transport fuel oil from the temporary storage tank to the in service permanent DF0 storage tank, if required to make up DF0 inventory in the permanent tank, d.
Security measures will be established to protect the temporary onsite storage tank.
It is highly unlikely that Diablo Canyon would have a loss of offsite power (LOOP) where the units would have to rely on the six onsite diesel generators for more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. To reduce the already low probability of a LOOP, the licensee will minimize planned maintenance on the 230 kV and 500 kV offsite power transmission system when a DF0 storage tank is removed from service.
If there is anticipated severe weather or potential grid disturbances (e.g., a wildland fire), the DF0 storage tank replacement will be postponed.
If restoration of offsite power were to be delayed following a postulated LOOP event, DF0 can be transported to the site from the fuel oil supplier within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
Surveillance Requirement 4.8.1.1 lb requires that the required independent circuits between the offsite transmission network and the onsite Class IE distribution system be determined to be OPERABLE at least once per seven days by verifying correct breaker alignments and indicated power availability.
- During the time that a DF0 storage tank is removed from service, the licensee i
is proposing to perform this surveillance at a frequency of once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> instead of once per 7 days.
Increasing the surveillance frequency will provide increased assurance that offsite power will be available to the Class IE distribution system, thus reducing the potential importance of the diesel generators, although the proposed surveillance frequency seems excessive.
The licensee has also proposed several other compensatory measures when the DF0 storage tanks are being replaced:
a.
Two supply trains of the DF0 storage and transfer system (downstream of the suction pipe cross connection), required by TS 3.8.1.1.b.2, will be verified to be OPERABLE for DF0 transfer from the tank remaining in service within seven days of removing the other tank from service, b.
An independent means of fuel oil transfer is also provided by the portable DF0 transfer pump. This is a non-safety related diesel powered pump which can bypass the tank foot valve, suction line and transfer pump and supply DF0 from the tank to the transfer pump discharge. This pump can take suction directly from the DF0 storage tank and discharge into the DF0 piping distribution system. To assure that the pump is operable, it is now tested every 3 months.
s To assure the portable DF0 transfer pump will be available following a seismic event, the portable DF0 transfer pump is located in a seismically sound, accessible location in the turbine building. The portable DF0 transfer pump will be tested within seven days of removing the DF0 tank from service.
c.
The EDG day tanks are recuired by TS 3.8.1.lb.l. to contain a minimum volume of 250 gallons. This amount of DF0 is adequate to start and operate the EDGs for approximately 60 minutes at rated load. To provide additional DF0 in each EDG day tank, following any run of an EDG, the EDG day tank will be filled to at least 500 gallons.
As discussed above, the licensee will have about 65,000 gallons of diesel fuel onsite during the tank replacement project, which is sufficient for about 1
seven days operation for the six diesel generators and as much fuel as is required by the present TSs.
The seven day supply would permit operation of the diesels for considerably longer than it took to restore offsite power to Turkey Point 3 and 4 after the devastation of Hurricane Andrew. The four day supply in the operating tank is more than adequate for any likely loss of offsite power event. Considering the other compensatory actions proposed by the licensee, the one-time changes to the TSs are acceptable.
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- 3.0 PROBABILISTIC RISK ASSESSMENT As discussed above, the licensee performed a comprehensive safety assessment of the proposed changes to the TSs using the traditional approach. Using the same deterministic approach, the NRC staff has determined that the proposed changes are acceptable.
Pacific Gas and Electric Company (PG&E) also supported the proposed TS changes with a probabilistic risk assessment (PRA). The PRA analysis was performed to quantify the increase in risk associated with the proposed one-time allowed outage time (A0T) extension for the DF0 storage and transfer system. The PRA considered the effect on risk of removing each of the DF0 storage tanks for 60 days (in sequence) and effectively having only one tank supplying DF0 to both 3
t trains of the DF0 transfer system for a total of 120 days.
In response to Generic Letter (GL) 88-20, PG&E submitted an Individual Plant Examination (IPE) for severe-accident vulnerabilities on April 14, 1992. The basis for the IPE was a full-scope level 1 PRA originally performed for Unit 1 in 1988 (and reviewed by the NRC), which was updated in 1991 to reflect plant design as of June 1990 and included a level 2 containment performance analysis. The licensee estimated that the annual core damage frequency (CDF) at Diablo Canyon due to internal initiating events (including flood events) to be 8.8 E-5 per _ reactor year, which, at the time, was in the upper range for Westinghouse four-loop PWRs. The licensee's estimate of the contributions of the major initiating event groups to the total CDF were 41 percent for loss of offsite power, 26 percent for general transients, 9.3 percent for all loss of coolant accidents, 8.2 percent for loss of any one of three vital 125 V DC buses, and 6.2 percent for loss of the auxiliary salt water system (ASW) or the component cooling water (CCW) system. The licensee has maintained a "living" PRA, periodically updating it to reflect various plant modifications, improved procedures, revisions to the training provided to plant staff and use of more plant-specific data. For example, at the time PG&E submitted the IPE, Diablo Canyon 1 and 2 had five diesel generators, with one of these being a swing diesel generator that could supply power to either units safety bus.
Failure of this swing diesel contributed to sequences which accounted for 34 percent of the estimated CDF.
PG&E subsequently installed a sixth diesel generator. A sensitivity-type analysis at the time indicated that with each vital bus having a dedicated diesel generator, the core damage frequency was reduced by 14 percent to 18 percent, since the possible impact of a LOOP was reduced. The current average annual CDF is 5.5E-5 per reactor year.
PG&E management has been and is committed to using a probabilistic risk assessment program. The PRA personnel interact with engineering and i
operations personnel to assess the potential impacts of significant design and/or operational changes on the PRA result.
In the September 29, 1995 submittal, the licensee discussed the possible effect of removing each of ne DF0 storage tanks for 60 days.
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The PRA considered one permanent DF0 storage tank in service, the transfer system alignments, and availability of the portable DF0 transfer pump. The proposed operating sequence, described above, was also considered in calculating the new core damage frequency resulting from internal and seismic initiators. Additional core damage probability (CDP) due to internal fires, was initially screened out due to the low probability of fire initiating events coincident with an independent loss of offsite power in which the fuel oil system would be demanded. Further, additional CDP due to seismic
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initiators is a small fraction of that due to internal initiators.
l The PRA analysis was performed for the system alignment with the standby l
transfer pump in "AUT0".
The PRA analysis calculated an additional increase in CDP of 3.1 x 10*, which reflects a 6 percent increase in annualized core damage frequency.
j The PRA analysis was also performed for the system alignment with the standby transfer pump in "On".
This alignment would enable continuous operation of the DF0 transfer pump, thereby reducing the potential for a pump start demand failure. Utilizing this system alignmept, the PRA analysis calculated an additional increase in CDP of 1.8 x 10 *, which reflects a 3.3 percent increase in annualized core damage frequency.
1 As discussed previously, planned maintenance that would affect the availability of the 230 kV and 500 kV offsite power sources will be minimized during the time that a DF0 storage tank is removed from service. The effects on the PRA analysis of 230 kV line outages of up to 9 hou,rs were considered and determined to not be risk significant (i.e., <1 x 10' ).
The compensatory measures, as described above, that are not credited in the PRA analysis, will give added assurance that the overall plant risk is being managed acceptably during the DF0 storage and transfer system replacement period.
The probabilistic safety assessment of the proposed one-time changes to the TSs to permit replacement of the DF0 storage tanks fully supports and complements the deterministic assessment discussed in Section 2.0 of this safety evaluation. The PRA analysis confirms that having one DF0 storage tank out of service for 60 days results in no significant reduction in the margin of safety and that the proposed changes to the TSs are 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.
- The computed CDP is based upon PRA modeling which conservatively does not include the availability of fuel oil from both the temporary, onsite storage tank and the onsite DF0 tanker truck.
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5.0 ENVIRONMENTAL CONSIDERATION
These 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 and change surveillance requirements. The NRC staff has determined that the amendments involve no significant increase in the amounts, l
and no significant change in the types, of any effluents that may be released i
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 (60 FR 58403). 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 Contributor:
R. Clark Date:
January 3, 1996 i
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