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s" UNITED STATES i j NUCLEAR REGULATORY CdMMISSION
[ WASHINGTON, D.C. 2066H001 SAFETY EVALUATION FOR THE OFFICE OF NUCLEAR REACTOR REGULATION TIA 94-012 - EMERGENCY POWER SUPPLY j TECHNICAL SPECIFICATION REQUIREMENTS OCONEE NUCLEAR STATION l INTRODUCTION In a letter dated March 24, 1994, Region II staff asked NRR staff to determine l if a specific modification to the Keowee emergency power supply logic must be reviewed by the staff prior to implementation of the modification. Also the .
l NRR staff was requested to determine if the licensee's intent to I simultaneously use both Keowee hydro units in parallel to the offsite grid for commercial purposes is acceptable when considering the requirements for independence between the two onsite emergency sources to avoid common mode failures.
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The NRR staff is concerned about the connon-mode vulnerability presented by paralleling both Keowee hydro units to the offsite grid. Also the staff feels that the modification (with further revision as discussed in the licensee's letter dated August 25,1994), which adds more interlocks and complex ~
circuitry to an already complex breaker control system might not be an improvement. Based on these concerns NRR staff is considering possible future action pertaining to the licensee's use of the Keowee hydro units simultaneously in parallel with the offsite power system for commercial purposes.
DISCUSSION The emergency electrical power sources for the Oconee Nuclear Station are the two Keowee hydro units which supply power via one overhead path and one underground path. Each hydro unit can be aligned'to either path with interlocks which allow only one unit to be selected for alignment to the underground path at a time. On October 12, 1992, it was determined that a single electrical fault could cause the loss of both emergency power paths due to the isolating action of protective relaying (see LER 269/92-16). Immediate corrective action involved aligning one Keowee hydro unit to the overhead path and dedicating the other hydro unit to the underground path by opening its overhead breaker's disconnect switches.
As a long-term corrective act' ion, the licensee will modify (per NSM #0N-52966) the control circuitry for each hydro unit's underground breaker by providing additional interlocks which automatically close the breaker which was initially not selected for the underground path. The concurrent conditions which generate the closure signal are:
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- 1. Tripping of the lock-out relay on the Keowee output transformer to the overhead path.
- 2. Tripping of the lock-out relay on the opposite Keowee hydro unit.
- 3. Existence of an emergency start signal from the Keowee Emergency Start Logic.
The licensee has proposed to add an additional surveillance to the Technical Specifications to demonstrate the operability of the automatic closure circuitry on an annual basis.
NRC Inspection Report No. 269, 270, 287/93-31 identified a violation to the 10 CFR 50.59 process for failure to identify the need for a change to the plants' Technical Specifications prior to implementation of the modification discussed above. Specifically, Region II concluded that Technical Specification 3.7.1.(b) requires that two independent onsite emergency power paths be operable at all times with one Keowee hydro unit aligned to the underground power path and the other Keowee hydro unit aligned to the overhead path. Also i Region II concluded that the installation of the new interlocks which !
automatically realign the Keowee overhead unit to the underground path j represented a significant and complex change to the design of the emergency !
power paths and the plants' licensing basis. The new interlock logic did not appear to be bounded by existing technical specification requirements for dedicated and independent power paths. Also the inspectors found that the l licensee did not intend to add surveillance requirements to the Technical Specifications for the new logic even though it could be considered to be a primary success path for postulated events.
In a February 24, 1994, submittal, the licensee stated that after the l modification, two separate and independent power paths would still exist because existing interlocks would prevent simultaneous alignment of both Keowee hydro units to the same power p th. The licensee .further stated that ,
the new automatic realignment would only occur if both the preasigned underground unit and overhead path are lost during an emergency. Also the licensee's 10 CFR 50.59 process indicated that it.was not necessary to include the new interlocks and their surveillance requirements in the Technical Specifications. This conclusion was based upon a determination that the added circuitry was not significant and that only limited guidance was available for the 10 CFR 50.59 process.
Region II, in the March 24, 1994, letter, expressed disagreement with the licensee's responses and stated that the purpose of the modification was to permit both Keowee hydro units to simultaneously supply electric power to the offsite system. Region II cbncluded that this practice had never been formally approved or condoned by the staff and that the Technical Specifications specifically require both hydro units to remain independent at all times (not just at the onset of an emergency when the hydro units receive a emergency start signal and realign to the dedicated paths). In the March 24, 1994, letter, Region II requested NRR staff to determine if this modification needs to be reviewed prior to its implementation and if it is acceptable to use both hydro units in parallel for commercial purposes.
EVALUATION Region II specifically requested the NRR staff to consider three issues. l Those issues and the NRR staff's responses are as follows: ]
- 1. . Does existing Technical Specification 3.7.1.b allow the modification to be implemented without prior NRC approval?
Technical Specification 3.7.1.b states (with specific exceptions / limiting conditions) that an Oconee reactor shall not be above 200*F unless:
- Two independent on-site emergency power paths shall be operable and shall consist of:
- 1. One Keowee hydro unit; through the underground feeder path; through transformer CT4; through the Keowee standby bus feeder breakers (SKl and SK2) to two standby buses; and capable of supplying emergency power through the standby bus to main feeder bus breakers (51 and S2).
- 2. The other (redundant) Keowee hydro unit; through the Keowee main step-up transformer and breaker PCB-9; the 230 kV switchyard yellow bus and safety related PCB-18, 27, or 30; through the respective 1 operating unit's startup transformer (CT-1, 2, or 3) or aligned and I connected alternate startup transformer; and capable of supplying
, emergency power through the startup transformer to main feeder bus breakers (El and E2). One startup transformer may not be aligned to supply power to more than one unit.
Technical Specification 3.7.1.b reflects the design basis for the plant as discussed in the FSAR. LER 269/92-16 identified a design deficiency that could result in a total loss of emergency power due to a single failure.
This design deficiency and the modification to correct it are not addressed in the plaats' F. CAR and, consequently, are not evaluated in any staff . safety evaluation report. The FSAR discusses the electrical independence and l
automatic operation of the Keowee hydro units under various conditions including a LOCA/ LOOP scenario. The proposed modification involves interlocking parts of the Keowee hydro units' breaker control circuitry to preclude the loss of all emergency power under the postulated fault conditions. This modification could potentially impact system independence and, thus, could be construed to constitute an unreviewed safety question. If this were the case, implementation without prior staff approval would place the plants outside their licensing basis (see 10 CFR 50.59(a)(2)(ii). The l staff was unable to reach a consensus as to whether the modification
! constitutes an unreviewed safety question.
Notwithstanding the fact that Technical Specification 3.7.1.b is subject to interpretation and the guidance for the 10 CFR 50.59 process may indeed be limited, the staff believes that surveillance requirements should have been proposed for the plants' Technical Specifications and an explicit discussion 1 of the automatic path-altering capability of this modification should have i been proposed for the plants' FSAR and Technical Specifications Bases for the l
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staff's review prior to implementation of the modification. Thus, a violation of 10 CFR 50.59 occurred due to this deficiency alone.
l It should be noted that the modification could be looked upon as a safety improvement to the plants' onsite emergency power system since it does provide additional automatic operational flexibility, albeit for a specific failure mode. Since this apparent improvement from the modification is obtained by adding complexity to an already complex system, the staff is concerned about the implementation of this modification and is reviewing its acceptability along with the proposed surveillance requirements as a separate effort which is scheduled for completion in January,1995.
- 2. Does the modification require that a surveillance requirement be !
incorporated into the existing Technical Specifications and therefore require prior NRC approval?
As stated above in the discussion, the modification to the plants' emergency onsite electrical sources is intended to eliminate a single failure vulnerability which could cause the loss of both emergency power paths. The two emergency power paths and their breakers, which this modification impacts, I are part of the primary success path for a LOCA/ LOOP event.
The modification involves an issue that was unknown at the time the plants were licensed. Consequently, the FSAR does not discuss this common mode 4 failure or the modification needed to prevent it. The modification involves changes to the Keowee hydro units' breaker control circuitry that ensures proper operation under the fault conditions. Upon staff approval the modification would become part of the licensing basis for Oconee Nuclear ;
Station. Since Technical Specifications reflect the licensing basis, it is i appropriate that a surveillance to verify proper functioning of that portion of the Keowee hydro units' breaker control circuitry which precludes the common mode failure be included in those Technical Specifications. Therefore, a technical specification encompassing surveillance of the new hardware associated with this modification should have been proposed by the licensee to support the operability and to maintain the quality of the plants' emergency power sources (see 10 CFR 50.36(c)(3)). The licensee has proposed surveillance requirements for the hardware associaited with the modification.
However, the specific amendment request was submitted for the staff review and approval in a February 24, 1994, letter after the 10 CFR 50.59 violation was issued, and as a Cost Beneficial Licensing Action item.
- 3. Is alignment of both Keowee units to the electrical grid simultaneously in compliance with Technical Specification 3.7.1.b as presently written or related requirements as they apply to Oconee?
On Page 3-20 of the plants' FSAR, it is stated that NRC General Design Criterion 39 requires that:
l Alternate power systems shall be provided and designed with adequate independence, redundancy, capacity, and testability to permit the functioning required of the engineered safety features. As a minimum, the on-site power system and the off-site power system shall each
independently, provide this capacity assuming a failure of a single active component in each power system.
In a discussion of how the Oconee electrical systems meet the intent of this criterion, the FSAR states that:
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Three alternate emergency electric power supplies are provided...These are the 230 KV switching station with multiple off-site interconnections and two on-site independent 87,500 KVA hydroelectric generating units.
Also on Page 8-15 of the Oconee FSAR it states:
The emergency power sources are independent of each other... Paralleling of emergency power sources is prevented by redundancy in transfer logic equipment and interlocking.
In a May 25, 1994, letter, the licensee stated that the Keowee hydro units were designed and built for two purposes; (1) to serve as the emergency onsite power sources for Oconee and (2) to generate commercial peaking power to the offsite system. Further, the licensee stated that the specific words from the FSAR refer to the Keowee hydro units in their role as emergency power sources after they are separated from the offsite system by emergency signals and switching logic. While the licensee has applied a rather limited interpretation of the required independence of the offsite power system and the two onsite power paths as scenario dependent, it must be recognized that the licensee has used both Keowee hydro units simultaneously for peaking in the past with no apparent formal staff approval (though possible tacit staff approval). The licensee's interpretation is supported by the following statements in the plants' FSAR:
This underground feeder is connected at all times to one hydroelectric generator on a predetermined basis and is energized along with Transformer CT4 whenever that generator is in service in either emergency or normal mode. (Page 8-9)
If the units are already operating when ett.her of the above conditions occur, they are separated from the network and continue to run on standby until needed. (Page 8-9)
Also further support, as quoted below, is found in the Bases of the plants' Technical Specifications ,
, i l The Keowee Hydro units, in addition to serving as the emergency power
! sources for the Oconee Nuclear Station, are power generating i sources...As power generating units, they are operated frequently, i normally on a daily basis at loads...The frequent starting and loading of these units to meet Duke system power requirements assures the !
continuous availability...
Based on this we must conclude that the simultaneous alignment of both Keowee units to the offsite system in the past could not have clearly been considered as a violation to the plants' Technical Specifications. Prohibition of this ,
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i 6-configuration would have been difficult to justify because of the statements contained in the FSAR which were available for the staff's review and form the basis for a tacit (at least) approval claim. Prevention of future operation in this mode would most likely constitute a plant-specific backfit.
Principal Contributor: F. Burrows Date: December 29, 1994 i
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