Information Notice 2000-06, Offsite Power Voltage Inadequacies
| ML003695551 | |
| Person / Time | |
|---|---|
| Issue date: | 03/27/2000 |
| From: | Marsh L Operational Experience and Non-Power Reactors Branch |
| To: | |
| Petrone C | |
| References | |
| IN-00-006 | |
| Download: ML003695551 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001 March 27, 2000
NRC INFORMATION NOTICE 2000-06: OFFSITE POWER VOLTAGE INADEQUACIES
Addressees
All holders of operating licenses for nuclear power reactors, except those who have
permanently ceased operations and have certified that fuel has been permanently removed
from the reactor.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform
addressees of experience related to a possible concern regarding the voltage adequacy of
offsite power sources, that is, power from the transmission system grid to nuclear power plants.
It is expected that recipients will review the information for applicability to their facilities and
consider actions as appropriate to avoid similar problems. No specific action or written
response is required by this notice.
Description of Circumstances
On August 11, 1999, the Callaway plant experienced a rupture of a reheater drain tank line. As
a result, the plant operators initiated a manual reactor trip. Since the plant was shutdown, offsite power was required to supply the plant equipment loads. During this period, the grid
conditions were such that a substantial power flow was occurring from north to south through
the local Callaway grid. The licensee stated that the deregulated wholesale market contributed
to conditions in which higher grid power flows are likely to occur. The licensee stated that these
large flows were observed at this time. This power flow, coupled with a high local demand and
the loss of the Callaway generator, resulted in switchyard voltage at the site dropping below the
minimum requirements for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Although offsite power remained available during the
reactor trip transient, the post-trip analysis indicated that in the event that additional onsite
loads would have been in operation at the time of the event, 4-16 kV distribution voltage may
have decreased below the setpoint of the second-level undervoltage relays separating the loads
from offsite power. The NRC conducted a special inspection at Callaway from November 29 to
December 3, 1999, on the circumstances surrounding the event. The inspectors found that
similar conditions existed in 1995 that were undetected by the licensee (Licensee Event Report
(LER) 50-483/99-005 (Accession No. 9909200074); NRC Inspection Report 50-483/99-15 (Accession No. ML003684343), dated February 15, 2000).
The following events identify additional combinations of main generator unavailability, line
outages, transformer unavailability, high system demand, unavailability of other local voltage
support, and high plant load that could result in inadequate voltages. Common among all the
IN 2000-06 March 27, 2000 events is the inability to predict the inadequate voltages through direct readings of plant
switchyard or safety bus voltages, without also considering grid and plant conditions and their
associated analyses.
On July 11, 1989, safety systems at Virgil C. Summer Nuclear Station experienced a sustained
degraded voltage condition and, as a result, the safety buses were automatically transferred
from the offsite power system to onsite standby diesel generators. The degraded condition was
caused by a turbine trip and deficiencies in the offsite power system's transmission network
equipment. The transfer of power supplies was initiated by operation of degraded voltage
protective relays, as designed. Nonsafety system loads remained operable while being
powered for approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> from the degraded offsite power source
(LER 50-395/89-012 (Accession No. 8908140351)).
On November 5, 1991, the licensee for Arkansas Nuclear One, Units 1 and 2, reported that had
its 500-kV auto-transformer been lost during summer peak conditions, the 161-kV system might
not have been able to maintain adequate voltages to support the operation of the safety system
loads of both units (LER 50-313/91-010 (Accession No. 9111150021)).
On December 30, 1993, Northeast Nuclear Energy Company concluded that with the
switchyard at the worst case minimum voltage, Millstone Nuclear Power Station, Unit 1, loss-of- coolant accident (LOCA) mitigation loads could combine with normal loads that are not shed
upon receipt of an accident signal to produce a voltage drop that would actuate degraded
voltage relays resulting in separation from offsite power. The utility determined that this worst
case minimum switchyard voltage could occur after the loss of Millstone Unit 1 generation when
both Millstone Units 2 and 3 are off-line (LER 50-245/94-01 (Accession No. 950920001)).
On February 6, 1995, the licensee for Palo Verde Nuclear Generating Station, Units 1, 2, and 3, reported shortcomings in the plant site voltage regulation. Specifically, if a LOCA occurred with
the switchyard voltage in the lower two-thirds of its operating range, the engineered safety
feature (ESF) loads would begin sequencing onto the preferred offsite power source, and the
house loads would fast transfer to the startup transformer following the main generator or
turbine trip that would accompany the LOCA. The resulting voltage drops at the safety buses
would cause the bus degraded voltage relays to drop out during the ESF load sequencing and
subsequently resequence the loads onto the diesel generators. The licensee identified this
scenario as double sequencing (LER 50-528/93-011-01 (Accession No. 9502160195)).
On August 8, 1995, Pacific Gas & Electric Company (PG&E) reported that during peak system
loading, all transmission lines and a local fossil power plant (Morro Bay) needed to be in service
to meet Diablo Canyon Nuclear Power Plant voltage requirements. A review of the available
data by PG&E on the offsite power supplies identified 47 instances in which the system
configuration could have resulted in a degraded voltage condition between 1990 and 1995.
PG&E identified a potential double sequencing scenario at Diablo Canyon if a LOCA occurred
during these degraded voltage conditions (LER 50-275/95-007-01 (Accession No.
9608140037)).
IN 2000-06 March 27, 2000 On July 22, 1997, the licensee for Clinton Power Station sought an exemption from offsite
power regulatory requirements because of its analysis that offsite power would become
inadequate under certain summer peak conditions following the loss of the nuclear unit. The
exemption request was eventually withdrawn by the licensee.
NRC inspection findings and licensee event reports have indicated instances in which grid
stability analyses had not been updated by the licensees to reflect changes in the grid power
system. An Office of Nuclear Regulatory Research report, The Effects of Deregulation of the
Electric Power Industry on the Nuclear Plant Offsite Power System: An Evaluation, dated
June 30, 1999 (Accession No. 9907120008), recommended that the staff take certain followup
actions to ensure that licensees will continue to maintain their licensing bases in this area.
Discussion
NRC Information Notice (IN) 98-07 discussed the possibility that the changes occurring as a
result of deregulation of the electric utility industry could affect the reliability of the offsite power
systems in nuclear power plants. Offsite power problems highlighted in licensee event reports
were identified as potential sources of concern if not properly managed following the
restructuring that occurs as a result of deregulation. NRC IN 95-37 alerted licensees to
circumstances that could result in inadequate offsite power system voltages during design basis
events.
The most recent problem, which was reported by the licensee for Callaway Unit 1, potentially
tied the inadequate offsite system voltage problem to industry deregulation. The licensee
stated in LER 50-483/99-005 (Accession No. 9909200074) that the magnitude of the power
being transported across the grid during the period had not been previously observed and was
far in excess of typical levels. LER 50-483/99-005 (Accession No. 9909200074) also stated
that the deregulated wholesale power market contributes to conditions in which higher grid
power flows are likely to occur, and these large flows were observed at this time.
Because the Callaway generator was supporting the grid voltage in the vicinity of the plant, the
low grid voltage had not been observed until the Callaway generator voltage support was no
longer available. However, if a design basis event had occurred during the period of high
system demand, the consequential loss of the Callaway generator, combined with the plant
electrical requirements associated with the event, could have actuated the plants degraded
voltage protection and separated safety loads from offsite power, which is the preferred power
supply under these circumstances.
The reports referenced in this notice also identify additional combinations of circumstances than
those seen at Callaway that could result in inadequate offsite voltages. These circumstances
include main generator unavailability, line outages, transformer unavailability, high system
demand, unavailability of other local voltage support, and high plant load. The common
characteristic of these problems is that the true capability of the offsite source cannot
necessarily be verified through direct readings of plant switchyard or safety bus voltages.
IN 2000-06 March 27, 2000 Instead, analyses of grid and plant conditions must be relied upon to determine this capability, considering the postulated occurrence of an event. If these analyses are not accurate and up
to date, licensees could inadvertently operate their plants in regions of inadequate voltages for
some periods of time.
As demonstrated by the Callaway event, industry deregulation can heighten the need to update
the analyses on a more frequent basis. Some utilities have utilized on-line contingency analysis
techniques in their grid control centers and implemented arrangements to be notified when the
offsite system to their plant is in jeopardy of not providing its required capability. When the on- line capability is not available, other utilities have provided for updating of the analyses on a
more frequent basis and have implemented procedures to identify when the plant and grid
conditions are outside the bounds of the assumptions of the analyses, thereby providing the
information to take compensatory actions as necessary.
Maintaining plant operation in a region of adequate offsite voltage is especially important for
licensees that may not have evaluated their plant safety systems for the double-sequencing
scenario identified in the Palo Verde and Diablo Canyon LERs. The safety consequences that
would result if an event occurred during a period of inadequate voltage can, therefore, be
difficult to assess.
This information notice requires no specific action or written response. If you have any
questions about the information in this notice, please contact one of the technical contacts
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
/RA/
Ledyard B. Marsh, Chief
Events Assessment, Generic Communications
and Non-Power Reactors Branch
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Technical contacts: Ronaldo V. Jenkins, NRR James J. Lazevnick, NRR
301- 415-2985 301- 415-2782 E-mail: rvj@nrc.gov E-mail: jjl@nrc.gov
Thomas Koshy, NRR Jeffrey L. Shackelford, RIV
301-415-1176 817-860-8144 E-Mail: txk@nrc.gov E-mail: jls2@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
IN 2000-06 March 27, 2000 Instead, analyses of grid and plant conditions must be relied upon to determine this capability, considering the postulated occurrence of an event. If these analyses are not accurate and up
to date, licensees could inadvertently operate their plants in regions of inadequate voltages for
some periods of time.
As demonstrated by the Callaway event, industry deregulation can heighten the need to update
the analyses on a more frequent basis. Some utilities have utilized on-line contingency analysis
techniques in their grid control centers and implemented arrangements to be notified when the
offsite system to their plant is in jeopardy of not providing its required capability. When the on- line capability is not available, other utilities have provided for updating of the analyses on a
more frequent basis and have implemented procedures to identify when the plant and grid
conditions are outside the bounds of the assumptions of the analyses, thereby providing the
information to take compensatory actions as necessary.
Maintaining plant operation in a region of adequate offsite voltage is especially important for
licensees that may not have evaluated their plant safety systems for the double-sequencing
scenario identified in the Palo Verde and Diablo Canyon LERs. The safety consequences that
would result if an event occurred during a period of inadequate voltage can, therefore, be
difficult to assess.
This information notice requires no specific action or written response. If you have any
questions about the information in this notice, please contact one of the technical contacts
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
/RA by/
Ledyard B. Marsh, Chief
Events Assessment, Generic Communications
and Non-Power Reactors Branch
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Technical contacts: Ronaldo V. Jenkins, NRR James J. Lazevnick, NRR
301- 415-2985 301- 415-2782 E-mail: rvj@nrc.gov E-mail: jjl@nrc.gov
Thomas Koshy, NRR Jeffrey L. Shackelford, RIV
301-415-1176 817-860-8144 E-Mail: txk@nrc.gov E-mail: jls2@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
ACCESSION #: ML003695551 TEMPLATE #: NRR-052
- See previous concurrence
To receive a copy of this document, indicate in the box C=Copy w/o attachment/enclosure E=Copy with attachment/enclosure N = No copy
OFFICE EEIB EEIB REXB Tech Ed. SC:EEIB C:EEIB
NAME RJenkins* JLazevnick* CPetrone* BCalure* DThatcher* JCalvo*
DATE 02/17/00 02/17 /00 3/15/00 2/17/-3/17/00 02/18/00 3/09/00
OFFICE DD:DE D:DE REXB:DRIP C:REXB:DRIP
NAME RWessman* JStrosnider* TKoshy* LMarsh
DATE 3/ 07/00 3/08/00 3/22/00 3/ 27/00
OFFICIAL RECORD COPY
Attachment
IN 2000-06 March 27, 2000 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
________________________________________________________________________________
2000-05 Recent Medical 3/06/2000 All medical licensees
Misadministrations Resulting
from Inattention to Detail
2000-04 1999 Enforcement Sanctions 2/25/2000 All NRC licensees
for Deliberate Violations of
NRC Employee Protection
Requirements
2000-03 High-Efficiency Particulate Air 2/22/2000 All NRC licensed fuel-cycled
Filter Exceeds Mass Limit conversion, enrichment, and
Before Reaching Expected fabrication facilities
Differential Pressure
2000-02 Failure of Criticality Safety 2/22/2000 All NRC licensed fuel-cycled
Control to Prevent Uranium conversion, enrichment, and
Dioxide (UO2) Powder fabrication facilities
Accumulation
2000-01 Operational Issues Identified in 2/11/2000 All holders of licenses for nuclear
Boiling Water Reactor Trip and power reactors
99-34 Potential Fire Hazard in the 12/28/99 All holders of licenses for nuclear
use of Polyalphaolefin in reactors and fuel cycle facilities
Testing of Air Filters
99-33 Management of Wastes 12/28/99 All medical licensees
Contaminated With
Radioactive Materials
99-32 The Effect of the Year 2000 12/17/99 All NRC medical licensees
Issues on Medical Licensees
99-31 Operational Controls to Guard 11/17/99 All NRC licensed fuel cycle
Against Inadvertent Nuclear conversion, enrichment and
Criticality fabrication facilities
____________________________________________________________________________________
OL = Operating License
CP = Construction Permit