[Federal Register Volume 62, Number 31 (Friday, February 14, 1997)]
[Notices]
[Pages 7075-7077]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-3737]
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NUCLEAR REGULATORY COMMISSION
Proposed Generic Communication; Loss of Reactor Coolant Inventory
and Associated Potential for Loss of Emergency Mitigation Functions
While in a Shutdown Condition (M92635)
AGENCY: Nuclear Regulatory Commission.
ACTION: Notice of opportunity for public comment.
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SUMMARY: The Nuclear Regulatory Commission (NRC) is proposing to issue
a generic letter that will request addressees to (1) assess the
susceptibility of their emergency core cooling system (ECCS) to common-
cause failure as a result of reactor coolant system (RCS) draindown
while in a shutdown condition, and (2) submit certain information,
pursuant to Sec. 50.54(f) of Title 10 of the Code of Federal
Regulations (10 CFR 50.54(f)), concerning their findings regarding
potential pathways for inadvertent RCS drain-down and the suitability
of configuration control and operating practices during reactor
shutdown cooling. This information will enable NRC staff to verify
whether addressees comply and conform with NRC regulatory and license
requirements; i.e., are adequately maintaining the residual heat
removal safety function to transfer fission product decay heat and
other residual heat from the reactor core (General Design Criterion
(GDC) 34 of Appendix A to 10 CFR 50), and the ECCS to provide abundant
emergency core cooling when required (GDC 35 of Appendix A to 10 CFR
part 50). The NRC is seeking comment from interested parties regarding
both the technical and regulatory aspects of the proposed generic
letter presented under the Supplementary Information heading.
The proposed generic letter has been endorsed by the Committee to
Review Generic Requirements (CRGR). The relevant information that was
sent to the CRGR will be placed in the NRC Public Document Room. The
NRC will consider comments received from interested parties in the
final evaluation of the proposed generic letter. The NRC's final
evaluation will include a review of the technical position and, as
appropriate, an analysis of the value/impact on licensees. Should this
generic letter be issued by the NRC, it will become available for
public inspection in the NRC Public Document Room.
DATES: Comment period expires March 17, 1997. Comments submitted after
this date will be considered if it is practical to do so, but assurance
of consideration cannot be given except for comments received on or
before this date.
ADDRESSES: Submit written comments to Chief, Rules Review and
Directives Branch, U.S. Nuclear Regulatory Commission, Mail Stop T-6D-
69, Washington, DC 20555-0001. Written comments may also be delivered
to 11545 Rockville Pike, Rockville, Maryland, from 7:30 am to 4:15 pm,
Federal workdays. Copies of written comments received may be examined
at the NRC Public Document Room, 2120 L Street, N.W. (Lower Level),
Washington, D.C.
FOR FURTHER INFORMATION CONTACT: Muhammad M. Razzaque (301) 415-2882.
SUPPLEMENTARY INFORMATION:
NRC Generic Letter 97-xx: Loss of Reactor Cooland Inventory and
Associated Potential for Loss of Emergency Mitigation Functions While
in a Shutdown Condition
Addressees
All holders of operating licenses for pressurized-water reactors
(PWRs), except those that have certified to the permanent cessation of
operations.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this
generic letter to request that addressees (1) assess the susceptibility
of their emergency core cooling systems (ECCSs) to common-cause failure
as a result of reactor coolant system (RCS) draindown while in a
shutdown condition, and (2) submit certain information, pursuant to
Sec. 50.54(f) of Title 10 of the Code of Federal Regulations (10 CFR
50.54(f)), concerning their findings regarding potential pathways for
inadvertent RCS drain-down and the suitability of configuration control
and operating practices during reactor shutdown cooling. This
information will enable NRC staff to verify whether addressees comply
and conform with NRC regulatory and license requirements; i.e., are
adequately maintaining the RHR safety function to transfer fission
product decay heat and other residual heat from the reactor core
(General Design Criterion (GDC) 34 of Appendix A to 10 CFR part 50),
and the ECCS to provide abundant emergency core cooling when required
(GDC 35 of Appendix A to 10 CFR part 50).
Background
The NRC issued Information Notice (IN) 95-03, ``Loss of Reactor
Coolant Inventory and Potential Loss of Emergency Mitigation Functions
While in a Shutdown Condition,'' on January 12, 1995, to alert
addressees to an incident at the Wolf Creek Plant involving the loss of
reactor coolant inventory while the reactor was in a shutdown
condition. In that event, operators were attempting to reborate
residual heat removal (RHR) train B, while at the same time maintenance
personnel were repacking an RHR train A-to-train B crossover isolation
valve. Train B is reborated by recirculating water through a loop that
contains the RHR system piping, the refueling water storage tank
(RWST), a containment spray pump, a manual RWST isolation
[[Page 7076]]
valve, and an RHR system crossover line. When the RWST isolation valve
was opened for the reboration process and the train A-to-train B
crossover isolation valve was opened for stroke testing, a drain-down
path was inadvertently created from the reactor coolant system (RCS) to
the RWST. This drain-down path included a suction header common to all
ECCS pumps.
Events of this nature are considered particularly significant
because they can result in loss of emergency core cooling capability
and involve the potential for containment bypass. On March 25, 1996,
the staff issued a supplement to IN 95-03 that further analyzed the
event. The NRC has also issued a number of other communications
describing events at reactor facilities involving inadvertent loss of
reactor coolant inventory while the reactor was in a shutdown
condition. The Office for Analysis and Evaluation of Operational Data
(AEOD) published AEOD/E704, ``Discharge of Primary Coolant Outside of
Containment at PWRs While on RHR Cooling,'' in March 1987, which
documented six events involving RCS backflow into the RWST. In Generic
Letter 88-17, ``Loss of Decay Heat Removal (DHR) 10 CFR 50.54(f),''
dated October 17, 1988, the NRC requested several actions to address
loss-of-DHR events that occurred while reactors were in a shutdown
condition. In IN 91-42, ``Plant Outage Events Involving Poor
Coordination Between Operations and Maintenance Personnel During Valve
Testing and Manipulations,'' dated June 27, 1991, the NRC discussed
inadvertent loss-of-inventory events.
Discussion
At Wolf Creek, all ECCS pump suction lines are tied into a common
suction header. When the draindown event occurred at Wolf Creek, hot
RCS water was introduced into this common suction header between the
RWST and the ECCS pumps. This hot water flashed to steam, resulting in
a steam/water mixture in the header. In the event of an ECCS actuation,
this mixture would have been introduced into the suction of the ECCS
pumps. If operators had not been able to terminate the event, the hot
water in the RWST suction piping might have led to steam binding, which
could have affected all pumps in both ECCS trains. In addition, water
flashing to steam in the header and the RWST could have caused serious
mechanical damage to the RHR piping and the RWST as a result of water
hammer. Finally, steaming through the RWST establishes a containment
bypass path.
The licensee estimated (using actual plant conditions) that for an
unmitigated event, the reactor vessel water level could have drained to
the bottom of the hot leg within 5 minutes and, as a consequence, RHR
pump A would have lost suction, cavitated, and failed. Shortly
thereafter, the common ECCS suction header could have reached a 90-
percent steam/water ratio. The licensee also estimated that continued
boil-off could have caused the pressure vessel water level to drop to
the point of core uncovery in less than 1 hour.
The AEOD report ``Reactor Coolant System Blowdown at Wolf Creek on
September 17, 1994,'' (AEOD/S95-01), dated March 1995, noted 19 events
in which RCS water was transferred to the RWST. On the basis of this
history and the potential for containment bypass, the staff has
concluded that additional information is required to confirm the
adequacy of existing ECCS configuration control and operating practices
regarding residual heat removal.
Requested Actions
Addressees are requested to determine whether their ECCSs are
susceptible to common-cause failure, e.g., as a result of events
similar to the Wolf Creek RCS drain-down event of September 17, 1994.
If ECCSs are found to be susceptible to common-cause failure,
addressees are expected to take corrective action, as appropriate, in
accordance with the requirements stated in Section XVI of Appendix B to
10 CFR Part 50, to ensure compliance with NRC regulatory and license
requirements.
Requested Information
Within 120 days of the date of this generic letter, addressees are
requested to submit a written summary report that includes a
description of the evaluation conducted and the conclusions reached
concerning the susceptibility of the RCS to drain-down events with a
potential for consequential common-cause ECCS failure, and the
corrective actions that were taken, or that are planned to be taken, if
any, in response to the above requested actions. If the RCS is found to
be susceptible to drain-down events, describe each potential drain-down
flow path (include piping sizes, identify flow path valves and their
normal positions, and identify valve interlocks and provisions for
valve position indication in the control room), describe potential
valve testing manipulations or uses, and describe any administrative
controls that are intended to be used to control valve manipulations to
preclude RCS drain-down events.
Required Response
Within 30 days of the date of this generic letter, addressees are
required to submit a written response indicating (1) whether or not the
requested actions will be taken, (2) whether or not the requested
information will be submitted, and (3) whether or not the requested
information will be submitted within the requested time period.
Addressees who choose not to complete the requested actions, or choose
not to submit the requested information, or are unable to satisfy the
requested completion date must describe in their response any
alternative course of action that is proposed to be taken, including
the basis for establishing the acceptability of the proposed
alternative course of action and the basis for continued operability of
affected systems and components, as applicable.
Address the required written responses to the U.S. Nuclear
Regulatory Commission, ATTN: Document Control Desk, Washington, D.C.
20555-0001, under oath or affirmation under the provisions of Section
182a of the Atomic Energy Act of 1954, as amended, and 10 CFR 50.54(f).
In addition, submit a copy to the appropriate regional administrator.
Backfit Discussion
The actions requested in this generic letter, if required, would be
backfits in accordance with NRC procedures and are necessary to ensure
that addressees are in compliance with existing NRC rules and
regulations. Specifically, 10 CFR 50.46 requires that the ECCS be
designed to provide adequate flow capability to maintain the core
temperature at an acceptably low value and to remove decay heat for the
extended period of time required by the long-lived radioactivity
remaining in the core. The Wolf Creek event has demonstrated that the
adequacy of ECCS configuration control and operating practices
regarding residual heat removal can adversely impact ECCS performance
and could prevent the ECCS from performing its safety function
following events at reactor facilities involving inadvertent loss of
reactor coolant inventory while the reactor is shut down. Therefore,
this generic letter is being issued as if the requested actions were
compliance backfits under the terms of 10 CFR 50.109(a)(4)(i). A full
backfit analysis was not performed. However, in accordance with NRC
procedures, an evaluation was prepared stating the objectives of and
the reasons for the requested actions and the basis for invoking the
compliance exception if
[[Page 7077]]
the requested actions were to be required. A copy of this evaluation
will be made available in the NRC Public Document Room.
Dated at Rockville, Maryland, this 10th day of February 1997.
For the Nuclear Regulatory Commission.
Thomas T. Martin,
Director, Division of Reactor Program Management, Office of Nuclear
Reactor Regulation.
[FR Doc. 97-3737 Filed 2-13-97; 8:45 am]
BILLING CODE 7590-01-P
