ML20204F538

Safety Evaluation of Util Preferred Ac Power Sys Conformance GDC 17.Proximity of Low Voltage Transformers Does Not Fully Meet GDC 17 Requirements for Physical Separation,But Deluge Sprinkler Sys Adequate

ML20204F538
Person / Time
Site:	Point Beach
Issue date:	04/25/1983
From:	NRC
To:
Shared Package
ML20204F529	List: ML20204F525 ML20204F538
References
TAC-12955, TAC-12956, TAC-46522, TAC-46523, NUDOCS 8305020137
Download: ML20204F538 (8)

Text

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Safety Evaluation of the Preferred Power Systems Conformance to General Design Criteria 17 Point Beach Nuclear Power Plant Docket Nos. 50-266 and 50-301 Introduction Following the issuance of a letter to all licensees, on August 8, 1979, concerning the adequacy of station electrical distribution system voltages, Region III management requested the resident inspectors to perform onsite followup inspections of the licensee's actions required by the letter.

During the course of these inspections at Point Beach Nuclear Plant (PBNP) the resident inspector identified two concerns relative to General Design Criteria (GDC) 17 of Appendix A to 10 CFR Part 50 (Reference 1).

These concerns were identified as:

1. A single fa tlure of the primary winding or a catastrophic failure of either secondary windings of the low voltage station auxiliary trans-formers 1-X04 or 2-X04 could render the entire transformer inoperable thus removing normal power availability from both trains of safeguards equipment.

2. The proximity of the 1-X04 and 2-X04 low voltage auxiliary transformers (approximately 16 feet apart) could conceivably render both inoperable if one were to have a catastrophic failure, thus eliminating all off-site power to the safeguards trains of both units.

This safety evaluation addresses only these two specific concerns of the resident inspector.

Evaluation General Design Criteria 17, which was first published on February 20, 1971, and became effective on May 21, 1971, states in part:

"An onsite electric power system and an offsite electrical power l system shall be provided to permit functioning of structures , systems ,

and components important to safety.---

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---Electric power from the transmission network to the onsite electric distribution system shall be supplied by two physically independent circuits (not necessarily on separate rights of way) designed and located so as to minimize to the extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions. A switchyard common to both circuits is acceptable.-- "

l 8305020137 B30425 PDR ADOCK 05000266 P PDR l

4 The PBNP Safety Analysis Report (SAR) states that the design bases for the auxiliary electrical systems is as follows:

"The function of the auxiliary electrical system is to provide reliable ,

power to those auxiliaries required during any normal or emergency mode '

? of plant operation.

The design of the system is such that sufficient independence or f

isolation between the various sources of electrical power is provided in order to guard against concurrent loss of all auxiliary power."

Auxiliary power required during unit startup, shutdown, and after a reactor

trip, and power for auxillaries associated with safeguards is supplied from the 345KV switchyard via the units high and low voltage station auxiliary transformers (see attached figure). The 345KV switchyard is served by four separate and independent lines. The 345KV system is the normal or preferred power supply for the auxiliary loads associated with plant engineered safe-guards. The two high voltage transformers (1-X03 and 2-X03) supply 13,800 volts to buses 1-HX04 for Unit No. I and 2-HX04 for Unit No. 2. A tie bus, H01, can be manually connected to intertie these buses and also the gas turbine generator to either low voltage transformer. Closing of the tie bus breakers into a common fault is prevented by trip and lockout, pre-venting automatic closure operations.

With the above described arrangement, either of the high voltage station auxiliary transformers may be removed from service and its associated low voltage station auxiliary transformer served via the tie bus from the opposite high voltage station auxiliary transformer and/or the 20 Mw gas turbine generator, thus permitting both units to operate.

With only one low voltage station auxiliary transformer operable, Section 4 15.3.7 of the Technical Specifications requires that only the reactor associated with the operable transformer shall be made critical or main-tained critical.

The 4160 volt system is divided into six buses per unit. Two buses for Unit No. 1, numbers 1-A03 and 1-A04 are connected to the 13,800 volt system via bus main breakers and the low voltage station auxiliary transformer Number 1-X04. These buses are used solely as switching buses. Buses 1-A03 and 1-A04 can be cross connected by manually switching to similar buses of Unit 2 (2-A03 and 2-A04). Thus, one low voltage transformer can feed the switching buses of the other unit as well as its own, supplying 4160 volt service to the safeguards buses and the 4160-480 volt safeguards transformers.

Buses 1-A05 and 1-A06 are connected to buses 1-A03 and 1-A04 using manually closed tie breakers.

i Buses 1-A05 and 1-A06 each serve one of the two 4160-480 volt station service I transformers for the unit's 480 volt safeguards equipment and one of the two safety injection pumps. No transfer is required for the safeguards equipment in the event of a turbine generator trip. .In addition to being served by buses 1-A03 and 1-A04, buses 1-A05 and 1-A06 are directly served by emergency diesel generators G01 and G02, respectively. Each emergency diesel generator 2

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will be automatically started and placed on the line upon undervoltage on the 4160 volt buses to which it is associated. One diesel will handle both the loads of one reactor in an accident mode and the other in hot shutdown mode.

The six 4160 volt buses for Unit No. 2 have the same arrangement as described for Unit No. 1.

The preferred power systems at PBNP are shared in that the two independent systems can be used for either unit upon failure of one component. Although present standards and guides do not address the sharing of offsite preferred power systems, the sharing of onsite standby safety-related systems is per-mitted, with certain restrictiors, for facilities constructed prior to 1973.

Standard Review Plan (SRP) NUREG-0800, Section 8.2, Offsite Power Systems, covers the review of the preferred power system (normal offsite external commercial power). The acceptance criteria states in part:

"In general, the preferred power system is acceptable when it can be concluded that two separate circuits from the transmission network to the onsite Class IE power distribution system are provided adequate physical and electrical separation, and the system has the capacity to supply power to all safety loads and other required equipment."

The Power Systems Branch's acceptance criteria for the preferred power system are based on meeting'the relevant requirements and guidelines of GDC-17 as it relates to the preferred power system's (i) capacity and capability to permit functioning of structures, systems, and components important to safety; (ii) provisions to minimize the probability.of losing electric power from any of the remaining supplies as a result of, or coin-cident with, the loss of power generated by the nuclear power unit or loss of power from the onsite electric power supplies; (iii) physical independence; (iv) availability; and the guidelines of Regulatory Guide 1.32 (see also IEEE 308-1974) as related to the availability and number of immediate access circuits from the transmission network.

Regulatory Guide 1.32 gives guidance only on the availability of offsite power within a few seconds following a loss-of-coolant accident. The Standard concerns only standby onsite safety-related power systems. Other electrical RG's and standards referred to in the SRP are silent on what is acceptable for a preferred power system's physical independence.

We have reviewed the two concerns using these acceptance criteria and guidance.

The first concern was that one failure in one of the low voltage auxiliary transformers (1-X04 or 2-X04) could remove preferred power from both trains of safeguards equipment; therefore, the preferred power supply was not independent or redundant as required by GDC-17.

The electrical independence of the two preferred systems as described above gives the flexibility to switch A.C. power around a failed low voltage 3

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transformer via the associated unit's transformer. This can be accomplished manually in a few seconds meeting the guidance of R.G. 1.32.

In the above case, Technical Specifications (TSs) require that the reactor associated with the failed transformer be in or placed in the hot shutdown condition. If preferred power is not restored immediately, the diesel generators automatically start and one can carry the vital loads for about 13 days without replentishing onsite fuel.

Calculations by the licensee (Reference 2) indicate that in the instance of one unit in hot shutdown, the other in an accident situation, and only one low voltage transformer in service, the voltage on the 4160 volt safe-guards buses would be 3714 volts, below the present degraded voltage trip setpoint of 3762 volts + 2% and the proposed setpoint of 3875 volts + 2%

(Reference 3). In this case, both preferred power systems would be auto-matica11y disconnected from the buses and unavailable until loads were reduced, one to two hours later. During this period, no offsite preferred power would be available contrary to the requirements of GDC-17. The short duration power disconnection was found to be acceptable in the Technical Evaluation Report and NRR Safety Evaluation (Reference 7).

The second concern of the resident inspector was that the proximity of the 1-X04 and 2-X04 transformers did not meet the requirements of GDC-17, e.g.

"two physically independant circuits be designed and located so as to minimize to the extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions."

The two low voltage station auxiliary transformers are Westinghouse 13.8 KV to 4.16 KV 3 phase, 60 cycle, Class OA/FA and each use 4406 gallons of oil (140 C Flash Point) as the cooling medium. The transformers are located outside and about 12 feet away from the Gas Turbine Building (sheetmetal construction). The four output breaker cabinets are located between the transformers which are separated by about 16 feet. The breaker cabinets are standard weather proof metal outdoor cabinets. The inlet and outlet leads to the transformers and breakers are underground and with the excep-tion of the Gas Turbine Building wall there are no overhead towers or cables that could fall and damage the transformers. An automatic deluge sprinkler system with pneumatic rate-of-rise detection is provided for oil fire suppression. The transformers are mounted on concrete bases about 2 to 4 inches above grade. The ground has a slight slope away from the Gas Turbine Building and the transformers.

The Fire Protection Handbook states that, "Although transformer failures

, appear to be common, percentage wise they are few. A study of all trans-former failures over a 5-year period showed a fire developing in 64 of 430 losses caused by lightening, electrical breakdown and fire"; and

" Transformer containing appreciable quantities of flammable, inhibited mineral insulating oils with flash point ranging from 130C to 135C are placed at least 25 ft away from windows or other important structures or placed in a vault of the type specified in the National Electric Code (NEC)".

NEC, Article 450-27 (1981), Oil-insulated Transformers Installed Outdoors, states in part " Space separations, fire-resistant barriers, automatic water 4

spray systems, and enclosurcs that confine the oil of a ruptured transformer tank are recognized safeguards. One or more of these safeguards shall be applied according to the degree of hazard involved in cases where the transformer installation presents a fire hazard."

The effect of one transformer failure on another adjacent transformer cannot be reliably predicated. Projectiles from a transformer rupture failure are not likely to damage the adjacent transformer; however, fire from expelled hot oil could cause damage if the burning oil is not contained or extinguished.

The NRR fire protection safety evaluation of the PBNP (Reference 4) found that building walls adjacent to the transformers were adequately protected by automatic deluge sprinkler systems and the existing yard fire protection systems were adequate to protect equipment required for safe shutdown.

Other causes of both transformers being damaged simultaneously can be postulated; such as lightning, windstorm, tornado, earthquake, falling aircraft, etc. Further separation distance between the transformers could reduce, to a certain extent, the probability of both transformers being incapacitated by a single event; but, it would not entirely eliminate the possiblity.

The probability of the above events occurring and deactivating both trans-formers simultaneously is small, but real and the proximity of the trans-formers and outlet breakers do not meet the intent of GDC-17 for physical separation.

The licensee correctly points out (References 5 and 6) that the PBNP was designed and constructed before GDC-17 was promulgated and that the preferred and emergency standby systems are as described in the Final Safety Analysis Report (FSAR). He also states that they have adequately considered the potential consequence of the loss of both transformers; however, aside from having ample diesel fuel available, no formal plans or procedures are in place to restore offsite A.C. power to the facilities in such an emergency as simultaneous failure of both low voltage transformers.

Since they are unique and require some months to construct, the licensee is considering the procurement of a spare low voltage transformer which could be installed in a few days. The licensee is also studying the possibility of backfeeding offsite power through the main power transformer, which if possible, could take a few hours to accomplish. If these pre-cautions are taken, we feel the safety of the PBNP is not reduced to an unacceptable level.

Conclusion We have determined that the PBNP preferred offsite A.C. power systems do not fully meet the requirements of GDC-17, since with the failure of one low voltage transformer in the worst case situation, voltage on the 4160 volt safeguards buses could drop below the degraded voltage set point, disconnecting the offsite power from the buses for a short period of time.

The NRR safety evaluation considered this departure from GDC-17 requirements to be acceptable (Reference 7).

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We have also determined that the proximity of the low voltage transformers do not fully meet the intent of GDC-17 requirements for' physical separation.

However, the NRR Fire Prevention Safety Evaluation found the deluge sprinkler system adequate to protect the transformers and the Gas Turbine Building from fire (Reference 4). Other' simultaneous failures by interactive causes or natural events would not be substantially reduced by further separation of the transformers, i

The following NRC personnel have contributed to this Safety Evaluation.

K. R. Ridgway P. A. Barrett i

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REFERENCES

1. Memorandum from W. G. Guldemond, Senior Resident Inspector, PBNP, to T. Colburn, Licensing Project Manager, PBNP, Office of Nuclear Reactor Regulation, NRC, June 11, 1981.

2. Lettar from C. W. Fay, Assistant Vice President, Wisconsin Electric Power Company (WEPC) to H. R. Denton, Director, NRR, September 10, 1982.

3. Letter from C. W. Fay, Assistant Vice President, WEPC, to H. R. Denton, Director, NRR, June 1, 1982.

4. -Letter from A. Schwencer, Chief of Operating Reactor Branch No. 1, NRR, to S. Burstein, Executive Vice President, WEPC, August 2, 1979 (Amendment Nos. 39 and 44 to Facility Operating License Nos. DPR-24 and 27).

5. Letter from C. W. Fay, Assistant Vice President, WEPC, to H. R. Denton, Director, NRR, October 29, 1982.

6. Letter from C. W. Fay, Assistant Vice President, WEPC, to H. R. Denton, Director, NRR, March 14, 1983.

7. Memorandum from L. S. Rubenstein, Assistant Director for Core and Plant Systems, NRR, to G. C. Lainas, Assistant Director for Operating Reactors, March 2, 1983.

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