NRC Bulletin 86-01
text
Bulletin 86-01: Minimum Flow Logic Problems That Could Disable RHR Pumps
SSINS No.: 6820
OMB No.: 3150-0012
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
May 23, 1986
IE COMPLIANCE BULLETIN NO. 86-01: MINIMUM FLOW LOGIC PROBLEMS THAT COULD
DISABLE RHR PUMPS
Addressees:
All GE boiling water reactor (BWR) facilities holding an operating license
(OL) or a construction permit (CP).
Purpose:
The purposes of this bulletin are: (1) to inform BWR licensees and
applicants of a recently identified problem with the minimum flow logic for
which a single failure could disable all RHR pumps, (2) to request that
licensees affected by the problem immediately provide appropriate
instructions and training to plant operators on how to recognize the problem
if it occurs and take appropriate mitigating actions, (3) to request that
licensees notify the NRC of the existence of the problem at their facility
within 7 days of receipt of this bulletin, and (4) to request that licensees
inform the NRC of measures taken to correct design or installation problems
that are identified as a result of this bulletin.
Description of Circumstances:
During a recent review of IE Information Notice 85-94, "Potential For Loss
Of Minimum Flow Paths Leading To ECCS Pump Damage During A LOCA," the
Pilgrim Nuclear Power Plant discovered that a single failure under certain
accident sequences could result in all RHR minimum flow bypass valves being
signaled to close while all other pump discharge valves are also closed.
This condition could result in no flow through the RHR pumps and could lead
to the pumps running dead headed with potential for pump damage in a few
minutes. If this single failure occurred in conjunction with an automatic
start of the RHR system, RHR pump damage may occur if unrecognized by the
operator. This event could disable RHR functions including Low Pressure
Coolant Injection (LPCI), head spray, drywell spray, shutdown cooling, torus
spray and suppression pool cooling. As a result of the loss of suppression
pool cooling over a long period of time, core spray pumps could ultimately
lose net positive suction head and also be unavailable.
The NRC staff has judged that the overall probability of a serious core
damage accident due to this problem is low. Nevertheless, such a single
failure vulnerability is not in compliance with the regulatory requirement
for independence in emergency core cooling systems and could compromise
several important systems.
8605230308
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IEB 86-O1
May 23, 1986
Page 2 of 3
GE's initial review identified the following plants with some potential for
having the single failure problem: Pilgrim; Browns Ferry 1, 2 and 3; Peach
Bottom 2 and 3; Duane Arnold; Millstone 1; Monticello; Quad Cities 1 and 2;
Dresden 2 and 3; Cooper; and Fermi 2. The staff understands that GE has
notified these potentially affected facilities and informal communications
from GE indicate that the list was narrowed to Pilgrim, Quad Cities and
possibly Dresden during preparation of the bulletin. The above listed plants
are being provided the bulletin by telecopy. Since there is not complete
certainty that other BWR's do not have the problem, they are also addressed
by this bulletin. Their copies are being sent by mail.
At Pilgrim, to prevent the pump from running dead headed, each pair of RHR
pumps is provided with a minimum flow bypass capability. The minimum flow
bypass consists of an orificed flow bypass which allows a flow of
approximately 10 percent of rated flow. The minimum flow bypass lines for
each pair of RHR pumps are connected to a single line and controlled by a
single minimum flow bypass valve. The minimum flow bypass valve is normally
open. The valve will close upon sensing flow in either of the RHR loops.
The current logic configuration for Pilgrim minimum flow bypass valves is
that a high flow signal from either the A or B RHR loops will close both A
and B bypass valves. Thus, a postulated single failure of a flow sensing
instrument may result in all RHR pumps running without bypass flow. If this
failure occurs during an event with a high drywell signal or low water level
signal, such as during small or intermediate size loss of cooling accidents
(LOCA) or spurious actuation, the RHR pumps may start and run dead headed.
One of the potential fixes being proposed by GE is to remove the automatic
closing signal from the RHR minimum flow bypass valves. This fix will result
in some of the LPCI flow being diverted through the minimum flow line. For
other RHR modes of operation, the values may be manually closed.
Although safety analyses may justify this interim fix, there are a number of
problems that need to be considered. For example, on many plants the minimum
flow bypass valves must be closed during shutdown cooling in order to
prevent draining the reactor vessel inventory to the torus. The minimum flow
bypass valves are considered containment isolation valves on some plants.
REQUIRED ACTION FOR GE BWR FACILITIES
1. Promptly determine whether or not your facility has this single failure
vulnerability.
2. If the problem exists, immediately instruct all operating shifts of the
problem and measures to recognize and mitigate the problem.
3. Within 7 days of receipt of this bulletin, provide (a) a written report
to the NRC which identifies whether or not this problem exists at your
facility, (b) if the problem exists, identify the short-term
modifications to plant operating procedures or hardware that have been
or are being implemented to assure safe plant operations.
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May 23, 1986
Page 3 of 3
4. If the problem exists, provide a written report within 30 days of
receipt of this bulletin informing the NRC of the schedule for
long-term resolution of problems that are identified as a result of
this bulletin.
Should a licensee determine that any action requested by this bulletin
jeopardizes overall plant safety, the NRC should be notified of that fact
and provided with appropriate justification for not implementing the
requested action. Such notification shall be made within 7 days of receipt
of this bulletin.
The written reports shall be submitted to the appropriate Regional
Administrator under oath or affirmation under provisions of Section 182a,
Atomic Energy Act of 1954, as amended. Also, the original copy of the cover
letters and a copy of the reports shall be transmitted to the U.S. Nuclear
Regulatory Commission, Document Control Desk, Washington, D.C. 20555 for
reproduction and distribution.
This request for information was approved by the Office of Management and
Budget under a blanket clearance number 3150-0011. Comments on burden and
duplication may be directed to the Office of Management and Budget, Reports
Management, Room 3208, New Executive Office Building, Washington, D. C.
20503.
If you have questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office or one of the technical
contacts listed below.
James M. Taylor, Director
Office of Inspection and Enforcement
Technical Contacts: Eric Weiss, IE
(301) 492-9005
M. Wayne Hodges, NRR
(301) 492-7483
Attachment: List of Recently Issued IE Bulletins