Information Notice 2005-16, Outage Planning and Scheduling - Impacts on Risk
| ML050870005 | |
| Person / Time | |
|---|---|
| Issue date: | 06/20/2005 |
| From: | Hiland P NRC/NRR/DIPM/IROB |
| To: | |
| Laura R, IOLB/DIPM, 415-1031 | |
| References | |
| IN-05-016 | |
| Download: ML050870005 (5) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001 June 20, 2005 NRC INFORMATION NOTICE 2005-16: OUTAGE PLANNING AND SCHEDULING -
IMPACTS ON RISK
ADDRESSEES
All holders of operating licensees for nuclear power reactors, except those who have
permanently ceased operations and have certified that fuel has been permanently removed
from the reactor vessel.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform
addressees about recent experiences in which outage planning and scheduling and adverse
human performance for pressurized water reactors (PWRs) and boiling water reactors (BWRs)
have had a significant impact on shutdown risk. It is expected that recipients will review the
information for applicability to their facilities and consider actions, as appropriate, to avoid
similar problems. However, suggestions contained in this information notice are not NRC
requirements; therefore, no specific action or written response is required.
BACKGROUND
The NRC has published numerous reports on the generic issue of shutdown risk and specific
outage events, including NUREG-1269, Loss of Residual Heat Removal System, Diablo
Canyon Unit 2, April 10, 1987; NUREG-1410, Loss of Vital AC Power and the Residual Heat
Removal System During Mid-Loop Operations at Vogtle Unit 1", on March 20, 1990; and
NUREG-1449, Shutdown and Low-Power Operation at Commercial Nuclear Power Plants in
the United States, Final Report, dated September 1993. In response to the various studies
and events, the NRC has also issued generic communications to highlight the importance of
shutdown safety measures to the industry. The generic communications included
Bulletin 80-12, Decay Heat Removal System Operability," Generic Letter (GL) 87-12, "Loss of
RHR While the RCS is Partially Filled," and GL 88-17, Loss of Decay Heat Removal. This
listing of NRC documents regarding the control of shutdown activities and risk is not all inclusive
and other relevant studies and generic communications have been issued in this area. For
example, NUMARC 91-06, Guidelines for Industry Actions to Assess Shutdown Management, dated December 1991, provides guidance to licensees on managing shutdown safety issues.
DESCRIPTION OF CIRCUMSTANCES
Seabrook, a four loop Westinghouse PWR, has taken the initiative to develop an all-modes
probabilistic safety assessment model. Following issues identified during a recent refueling
outage, the risk associated with early midloop draindown and shutdown operations over a
seven day period was determined to be roughly equivalent to operating at full power for an
entire year. The instantaneous risk associated with draining the vessel to mid-loop exceeded
1.0x10-3 core damage frequency per year. This high instantaneous risk was discussed in other
NRC and industry studies, including the EPRI report, Low Power and Shutdown Risk
Assessment Benchmarking Study, dated December 2002. The Seabrook analysis provides a
relatively recent comparison of reactor risk.
During recent refueling outages, several work activities were conducted without appropriate
planning, resulting in challenges to operators and to the decay heat removal system. In each
case, operators responded appropriately and anomalous plant conditions were returned to
normal. However, continued attention is needed for work planning and execution during these
high-risk periods.
- At Point Beach Unit 1, the licensee authorized installation of the hot leg nozzle dams
prior to establishing an adequate reactor coolant system (RCS) vent path. The plant
was in midloop operations and the outage schedule had called for the pressurizer
manway to be removed to establish an RCS vent path before installation of the hot leg
nozzle dams. Due to unanticipated delays in removing the pressurizer manway, several
licensed and experienced personnel (including the shift outage manager, the outage
control center operations representative, the work control center supervisor, and the
shift manager on shift at the time) decided to begin installing the hot leg nozzle dams
before removing the manway. Fortuitously, problems delayed the installation of the hot
leg nozzle dams. The nozzle dams were not completely installed before the personnel
realized that installation of the hot leg nozzle dams without a RCS vent path would have
had a significant adverse impact on safety. Without an adequate vent path, the RCS
would become pressurized following a loss of shutdown cooling. If one of the cold leg
nozzle dams became dislodged, RCS inventory would quickly be discharged from the
vessel and the core could be uncovered within a very short time.
- During a Millstone Unit 2 refueling outage, shutdown cooling was temporarily lost when
the shutdown cooling heat exchanger outlet valve inadvertently closed and the heat
exchanger bypass valve opened. The valves changed position due to an instrument bus
power failure caused by an error in the procedure to synchronize the power supplies to
the instrument bus. Shutdown cooling was lost for 13 minutes and the RCS
temperature increased by approximately 14 degrees F. An Unusual Event was declared
for an uncontrolled heatup of the RCS greater than 10 degrees F. The risk significance
of this event was mitigated because operators had not completed preparations to drain
the reactor vessel to midloop operations. During previous outages this maintenance
activity had been performed with the power to shutdown cooling valves secured, and
later in the outage when decay heat was lower. * Calvert Cliffs Unit 1 had a partial loss of shutdown cooling during midloop operations.
Both component cooling water (CCW) heat exchangers were in service at the time of
the event. Salt water cooling flow to one CCW heat exchanger was lost when the heat
exchanger outlet valve failed closed. The valve closure was caused by the loss of
power to the valve controller when a control room maintenance activity inadvertently
grounded which resulted in the loss of power to an instrument bus and valve controller.
The maintenance activity that resulted in the grounded instrument bus should not have
been performed during midloop operations. Decay heat removal from one of the two
operating component cooling water heat exchangers, which were cooling two shutdown
cooling trains, was lost for 18 minutes resulting in an RCS heatup of 2 degrees F.
- During a Peach Bottom Unit 3 refueling outage, an unexpected decrease in reactor
vessel water level of approximately 42 inches (from +200 inches to +158 inches)
occurred over 4.5 minutes. Over 27 feet of water still remained above the top of active
fuel. This event occurred during a flush activity of the Unit 3 residual heat removal
(RHR) crosstie piping. The procedural controls for the flush activity did not contain
instructions to isolate the B train of RHR during the flush activity. This resulted in an
open flow path from the reactor vessel to the suppression pool. Additionally, shift
management did not conduct a pre-job brief with all personnel involved in the flush.
This event demonstrated the impact of adverse human performance on shutdown risk
controls.
DISCUSSION
Planning, scheduling, and execution of work activities during outages can have a significant
impact on overall plant risk. Refueling outages have become shorter, causing higher risk
evolutions, such as midloop operations at PWRs, to be entered sooner after reactor shutdown.
As a result there is reduced inventory in the reactor vessel at a time when the decay heat loads
are high and the time to boil and uncover the core is relatively low. During these high risk
evolutions, careful attention to work scheduling is necessary to ensure that decay heat removal
cooling systems remain functional.
It is also important that work activities be scheduled to minimize distractions to operators and
prevent unnecessary challenges to decay heat removal systems. Licensees need to continue
to properly implement commitments made to previous generic communications on shutdown
operations. Additionally, licensees need to continue to implement the controls specified by
NUMARC 91-06 to properly manage shutdown risk.
CONTACT
S
This information notice requires no specific action or written response. Please direct any
questions about this matter to the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
/RA/
Patrick L. Hiland, Chief
Reactor Operations Branch
Division of Inspection Program Management
Office of Nuclear Reactor Regulation
Technical Contacts: Glenn T. Dentel, Region I Stephen M. Schneider, Region I
(610) 337-1218 (610) 337-1211 E-mail: gtd@nrc.gov E-mail: smc2@nrc.gov
Mark A. Giles, Region I Paul G. Krohn, Region I
(610) 337-1202 (610) 337-5120
E-mail: mag@nrc.gov E-mail: pgk1@nrc.gov
NRR Project Manager: Richard Laura, NRR
(301) 415-1837 E-mail: ral1@nrc.gov
Note: NRC generic communications may be found on the NRC public website, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
ML050870005 DOCUMENT NAME: E:\Filenet\ML050870005.wpd
OFFICE OES:IROB:DIPM TECH EDITOR RI:DRP RI:DRP OES:IROB:DIPM
NAME RALaura (RALaura for) GTDentel (RALaura for) Bholian (RALaura for) IJung
DATE 05/17/2005 05/18/2005 05/19/2005 06/16/2005 05/19/2005 OFFICE OES:IROB:DIPM DSSA:SPSB NRR:IROB
NAME EBenner MPohida PLHiland
DATE 06/16/2005 05/19/2005 06/20/2005