ML20079J169
| ML20079J169 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 10/07/1991 |
| From: | Kovach T COMMONWEALTH EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9110150286 | |
| Download: ML20079J169 (8) | |
Text
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Commenwealth Edisen C
1400 Opus Place Downers Grove, Illinois 60515 October 7,1991 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555
Subject:
Byron Station Units 1 and 2 Response to Unresolved Itern inspection Report Nos. 50-454/91016; 50-455/91015 NBC_DochetNoA50-454_and.50-455
Reference:
(a) W. Shafer letter to Cordell Reed dated August 9,1991 Reference (a) provided the results of an inspection conducted by Mr. Kropp, et.
al., from June 19,1991 through August 1,1991 at Byron Station. The inspection identified an Unresolved item pertaining to offsite power sources. The attachment provides our response to the Unresolved item.
If there are any questions or comments regarding this response, please refer them to Denise Saccomando, Compliance Engineer at (708) 515 7285.
Respectfully, T
ach Nuclear Lice sing Manager Attachment cc:
A. Bert Davis, NRC Regional Administrator - Rlli W J. Kropp, NRC Senior Resident inspector - Byron 9110150286 911007 I
PDR ADOCK 05000454 i
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Attachment Commonwealth Edison Company Response tc Unresolved item inspection Report Nos. 454/91016,455/91015 Unresolved item 454/91016-02(DRP); 455/91015-02(DRP)
In a summary form, the Unresolved item stated:
A preliminary System Auxiliary Transformer (SAT) outage option evaluation was conducted at Byron Station. During this review the NRC was informed that two of the four 345 kv offsite power sources were on the same transmission tower. The two sources were lines LO621 and LO622. Two other offsite power sources were lost on 1
March 13,1991 due to Icing and " galloping" (lines L15501 and LO624) leaving only the LO621 & LO622 line operable. The disconnects to the ring bus for these two ines were open approximately 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> until the lines were restored. The NRC inspection report expressed the following concern.
The Station did not enter any limiting condition for operation action statement on W
t 13th when lines L15501 and LO624 were lost as required in Technical Specification 3.8.1.1 a. in addition the UFSAR states that the two preferred power circuits from the 345 kv transmission busses to each unit's Class IE distribution center enter through two physically separate right of ways with independent transmission line structures. The UFSAR further states that a single event will not simultaneously affect both circulis in such a way that neither can be restored to service within the time limit to exceed any design limits.
CECc Response The following discussion first identifies all Licensing Basis documents which were reviewed and are pertineni to this discussion. Subsequerrt to the identification of the licensing basis documents, a review of past operating practice as it relates to the UFSAR Licensing Basis is presented. Then an evaluation of the ident!fied requirements and related commitments is performed. Finally, the method of future operation with respect to Limiting Condition of Operation (LCO) 3.8.1.1(a) for offsite power sources is provided.
- 1. lDENIJFICATION OF LICENSING BASIS]QO11MENTS Chapter 8 of the Byron Station Safety Evaluation Report (SER) states that the " Electric Power Systems" for Byron Station Units 1 and 2 were reviewed in accordance with the July 1981 editisn c' the 'Siandard Review Plan for the Review of Safety Analysis Reports for Nuc%ar Power Plants"(SRP), NUREG 0800. Chapter 8.2 of the SRP, "OFFSITE POWER SYSTEM" s.ates that industry standards and regulatory guides refer to the offsite power system as the " preferred power system" (PPS). Throughout thic response these two terms mcy be considered synonymous.
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SRP Chapter 8.2 specifies Acceptance Criteria for an acceptable electrical PPS design. It begins by stating:
"In geners :h9 preferred power system is acceptable when it can be conclud. ' tnm two separate circuits from the transmission network to the onsite t
.5 power dictribution system are provided, adequate physical and elect...;al separation exists, and the system has the capacity and capability to supply power to all safety loads and other required equipment."
SRP Chapter 8.2 also identifies a number of General Design Criteria (GDC) as re)svant requirements but provides focus on three main GDC. GDC 5 (Sharing of Structures, Systems, and Components), GDC 17 (Electric Power Systems), and GDC 18 (Inspection and Testing of Electric Power Systems) were identified as the primary GDC's of concern. Neither GDC 5 nor GDC 18 are pertinent to this discussion. GDC 5 is primarily interested in the capacity and ca;aability of the PPS to provide power to the units under specified conditions and comp lance with GDC 18 is reviewed to assure the testability of the transfer of power between PPS and the standby power system. Lherefore. only GD_C_1ZIemainsas_theSIlmatylaguirement for the desigrLat the_ PES. The following excerpt from GDC 17 is pertinent to this discussion.
GDC 17 PEQUJREMfiRTS Electric power from the transmission network to the onsite electric distribution system shall be supplied by two physically independent circuits (not nect
'rily on separate rights-of-way) designed and located so as to minimize, tne extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions. A switchyard common to both circuits is acceptable.
IEEE Standard 308-1974,"lEEE Standard Criteria for Cisss 1E Electric Systems for Nuclear Power Generating Stations" and Regulatory Guide 1.32, " Criteria for Safety Related Electric Power Systems for Nuclear Power P! ants" are identified as guidance documents within SRP, Section 8.2 - Offsite Power System, but provide little or no real guidance for the design of the PPS as their titias would suggest. They provide design guidance for the onsite 1E Electrical Powe System. The scope 01 IEEE 308-1974 states "This Standard does not apply ta... switchyard, transmission lines, and the transmission network". A drawing within IEEE 308-1974 also suggests the PPS design is outside the scope of the document. The Standard contains a brief de'inition of the PPS and a discussion on the availability and capability of the PPS.
But neither of these references arovide any useful new information. The B/B UFSAR commits to the 1971 version of ' EEE 308 which is sim'ilar in content to the 1974 version. No changes, significant to this discussion, were made as a result of the revision.
The NRC Byron Safety Evaluation Report was reviewed to see if any additional plant specific requirements or guidance was identified as the NRC staff evaluation basis.
Section 8.1 listed a large number of references, Reg Guides and IEEE Standards, which applied to the offsite and/or the onsite power systems st,bsequently covered in SER Sections 8.2 and 8.3 respectively. As a result of the Byron SER review of Section 8.2.1 through 8.2.5 for the PPS, the previous findings of this section with regard to applicable GDC and Standards were confirmed.
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The following wording is part of Byron Technical Specification LCO 3.8.1.1(a) for Electrical Power Systems AC Sources and is obviously a requirement since it is part of the station operating license:
Tech Spec LCO 3.8.1.1(a)
"Each units System Auxiliary Transformer bank energized from an independent transmission circuit."
Interpretation of this statemerit in light of design requirements is the key issue of this discussion and will be discussed in the evaluation portion of this response, i
Finally, the last document from w's'ch pertinent references may be obtained is the B/B UFSAR. Sections 3 and 8 of the UFSAR contained applicable references. They are as follows:
UFSAR Section 3.1.2.2.8, Response to GDC 17 The two offsite electric power systems connections to the stations are designed to provide access to t diversity of reliable power sources and are physically and electrically isolated so that any single failure will affect only one supply and will not propagate to the alternate supply.
Each diverse power source, diesel generator and offsite, up to the point of connection 'o the engineered safety features system power buses, is physically and electrically independent.
UFSAR Section 8.1, Electric Power Introduction The 345 KV overhead lines exit the station via three separate rights-of-way and are connected into Commonwealth Edison's 345 KV system as shown on figure 8.2-2.
The station's 345 KV transmission terminal buses, which are continuously energized, serve as the preferred power source for the station's safety loads. Two physically independent circuits are provided from the switchyard for each unit, one via the unit's assigned system auxiliary transformers and the other from the system auxiliary transformers of the other unit.
UFSAR-Section 8.1.6, Use of IEEE 308-1971 Two physically independent circuits occupying separate rights-of way are available to each of the two reactor units so that each unit has immediate access to a second offsite power source. Offsite Power is available to each unit from the system auxiliary transformers of the opposite unit via non-redundant bus ties between 4 KV ESF buses.. Switchyard power is available to both units as long as one 345 KV transmission line is available at the switchyard (Section 8.2).
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UFSAR Section 8.2.1, Offsite Power System The station's 345 KV transmission terminal buses are continuously energized and serve as the preferred power source for the station's safety loads. The two preferred power circuits from the 345 KV transmission terrrinal buses to each unit's Class 1 E distribution center enter through two physically separate rights-of-way with independent transmission line structures. These lines enter the switchyard from the opposite sides to the lines leaving the switchyard and terminate at transformers located on the opposite sides of the reactor buildings. There are no other lines crossing these preferred power lines. A single event will not simultaneously affect both circuits in such a way that neither can be returned to service within the time limit to exceed any design limits.
It should be apparent that a definition for the PPS derived from each of these 3revious UFSAR references is not easily arrived at as will be discussed later in the eva uation.
- 2. REVIEW OF PAST OPERATING PRACIICEMD UFSAR LLCENSING BASIS The UCSAR statements identified in the inspection report Unresolved item were taken from the following paragraph in Section 8.2.1 entitled "Offsite Power System," of the Byron /Braidwood UFSAR:
1 "The station's 345 KV transmission terminal buses are v 'tInuously energized and sente as the Isreferred power source foi tree station's safety loads. The two pre.-, red power circuits from the 345 KV.
transmission terminal buses to each unit's Class 1E distribution center enter through two physically separate rights-of-way with independent transmission line structures. These lines enter the switchyard from the opposite sides to the lines leaving the switchyard and terminate at transformers located on the opposite sides of the reactor buildings.- There are no other lines crossing these preferred power lines. A single event will Do1 simultaneously affect both.
circuits in such a way that neither can be returned to service within the time limit to exceed any design limits."
In the first sentence of this paragraph the words "The station's 345 KV transmission terminal buses. " are roferring to the switchyard 345 KV feed buses to each unit's SAT. This definition is supported by the words in the following two sentences.
The first sentence states that the station's 345 KV transmission terminal buses serve as the pretened p_owsl sogcst for the station's safety loads. The second sentence goes on to state the two prefened poyzer ciLcuits from the 345 KV transmission terminal buses to each unit's Class 1E Distribution Center enter through two physically separate rights-of-way with independent transmission line structures. This sentence is attempting to show compliance with the physical separation requirement of GDC 17, for the preferred power circuits. The third sentence further supports the limited definition of the preferred power circuits, by indicating they are on the opposite side of the switchyard to the transmission lines. leaving the switchyard to interconnect with the rest of the grid.
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Similar wording to this paragraph of UFSAR Section 8.2.1 can be found on page 8.1-3 of UFSAR Section 8.1. A summary description of the Offs te Power Systems includes the following paragraph:
"The station's 345 KV transmissico terminal buses, which are continuously energized, serve as ao preferred power source for the station's safety loads. Tvu abysically independent circuits are provided from the switelyard for each unit, one via the unit's assigned system auxiliary transformers and the other from the system auxiliary transformers of the other unit."
GDC 17 refers to "two physically independent circuits" providing electric aower "from the transmission network to the onsite electric distribution system". Simi ar wording is used in IEEE 308 where limited reference is made to the Preferred Power Supp!y.
The question which arises from this wording is where does the transmission network end and the " Preferred Power Circuit" begin. Station switchyards can be viewed as simply transmission substations through which power may be switched to various transmission lines. This viewpoint would include the station switchyard as part of the transmission network, hence the Preferred Power Circuit would begin on the station feed side of the switch yard relative to the rest of the grid. This interpretation is further supported by the statement in UFSAR Section 8.1.6 regarding use of IEEE-3081971 which states, " Switchyard power is available to both units as long as one 345 KV transmission line is available at the switchyard".
In light of the two UFSAR references, the past operating practice of identifying two electrically independent transmission lines to satisfy the Technical Specification Surveillance requirements for Offsite Power would appear conservative considering these " Preferred Power Circuit" descriptions
- 3. EVALUAIJOtLOEBEQUEEMENTSLCOMMLTAIENIS O
in summary, the design regt iements which apply to this discussloo, as determined in section 1, arc found in GDC 17, No IEEE Standard or Regulatory Guide referenced by the SRP or the B/B UFSAR as a commitment provides true additional guidance or definition to the PPS scope. The scope of IEEE Standard 3081974 is restricted to the Onsite 1E Electrical Power System. Because of the interface between the onsite power system and the offsite power system reference to the PPS was unavoidable.
The capability and availability of the PPS is briefly addressed in the standard but have no impact on this discussion.
The Technical Spec:ficn%n operational requirement, known as Limiting Condition for Operation (LCO) 3.8.1.ha), states in part, Each unit's System Auxiliary Transformer bank energized from an independent transmission circuit. The Tech Spec bases section 3/4.8.1 for AC sources offers little definition other than a cross reference to GDC 17. Terms like " transmission network"in GDC 17 and " independent transmission circuit" in the LCO, left undefined as they were, cause interpretation problems. The PPS beginning and end can be very subjective as a result.
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The Technical Specifications in a license define certain features, characteristics, and conditions governing operation of a facility that cannot be changed without prior NRC staff approval. In a more specific sensa the Technical Specifications in a license specify minimum equipment availability requirements. These requirements are specified to assure design features of a nuclear power plant considered necessary for safe operation, or needed to mitigate the consequs,1ces of an accident are maintained functional.
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It is therefore appropriate to use the original design basis requirement, GDC 17, to better define the current license oaerational requirement in Technical Specifications.
Especially since it was referencec in the Bases Section for LCO 3.8.1.1(a). A document used to interpret the GDC 17 requirements with regards to PPS design and scope was SRP Section 8.2. The Byron SER was also used. Though these documents did not present the scope of the PPS in a definitive summary form, design features of the PPS that were individually evaluated were integrated to define the PPS scope as presented later in Section 4. With regards to PPS design the PPS must be designed to prevent any single event from causing a !oss of all offsite 13ower (simultaneous loss of both transmission circuits). Common switchyarcls, where physical separation standards cannot be maintained, are specifically allowed in GDC 17. Please note that the words " single failure" have been avoided because the term implies a necessary assumption lD_ addition.to an event for other accident analyses.
When less than the original design minimum of two electrically independent and physinallyleRarate PPS circuits are available, it appears that it would be appropriate i
to enter action statement "a" for LCO 3.8.1.1. This would be a departure from the previous practice of considering the worcs " independent transmission circuit" of LCO 3.8.1.1(a) as only requiring electricalindependence of the two c ualifying circuits.
Physical separation of the qualifying transmission circuits woulc also be a consideration. One or more lines carried on the same transmission tower may never be considered more than one qualifying transmission line. As a separate point, qualifying transmission lines do not necessarily have to be in separate rights-of-way to be determined physically separate. If a single event, such as a failure on a tower will not result in failure of the second tower the two lines in the same right-of-way on separate towers are acceptable.
There are statements in the previous section, excerpted from B/B UFSAR, which are ambiguous and poorly define the PPS. Many of the entries attempt to show compliance of a portion of the PPS, such as transmission lines or SAT feeds to the 4 KV ESF buses, to the General Des'gn Criteria. In doing so, wording such as, "The station's 345 KV transmission terminal buses, which are continuously energized, serve as the preferred power source for the station's safety loads. Two physically independent circuits are provided from the switchyard for each unit, one via the unit's assigned system auxiliary transformers and the other from the system auxiliary transformers of the other unit."
similar to wording found in two separate UFSAR entries can redefine the PPS for the reader. The entire PPS and all applicable requirements can be interpreted as the 345 KV switchyard buses feeding the SATs, the SATs themselves, and lines between the 345 KV buses, SATs and onsite power system. The term " transmission network" of GDC 17 could also be incorrectly interpreted as inclusive of the station switchyard thereby excluding transmission lines from physical separation considerations. From this discussion it should be apparent why there is difficulty interpreting LCO 3.8.1.1 requirements for the PPS.
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- 4. METHOD _OF FUIUBE_OfEBATION in conclusion, the Preferred Power System should be considered as having 3 major sections, each of which must provide two
- physically separate and electrically independent circuit paths between the onsite power system and the transmission network (the transmission network excludes the station switchyard). The three sections are:
1.
The transmission lines entering the station switchyard from the transmission network 2.
The station switchyard 3.
The overhead transmission lines, SATs, and buses between the switchyard and the onsite power system The words " independent transmission circuit" in LCO used 3.8.1.1(a) should be interpreted as meaning both electrically independent and physically separate through all 3 sections of the PPS. When thuse definitions are not met and only a single
" independent transmission circuit"is available, Action A of LCO 3.8.1.1 shouId be entered. No single event such as a breaker failing open, a bus fault in the switchyard, or a failure on a transmission tower should cause simp.taneous loss of both offsite power sources when assuming two independent trarumission circuits are available.
Changes to the B/B UFSAR Sections 3 and 8 v;iil be initiated to better define the PPS and mora clearly word entries specific to ir.Jividual portions of the PPS, U 1/2 BOS 8.1.1.1. a-1 " Normal & Alternate Offsite AC Power Availability Weekly Surveillance" and U 1/2 BOS B.1.2-1 "Offsite AC Power Availability Weekly Surveillance" will be revised by 11/1/91 to incorporate the revised PPS definition.
- A common switchyard is allowed by GDC 17
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