Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 86-74
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
August 20, 1986
Information Notice No. 86 74: REDUCTION OF REACTOR COOLANT INVENTORY
BECAUSE OF MISALIGNMENT OF RHR VALVES
Addressees:
All boiling water reactor facilities holding an operating license or a
construction permit.
Purpose:
This notice is provided to inform recipients of NRC's continuing concern
with the potential for draining the reactor coolant system because of
residual heat removal system (RHR) valve misalignment. This is a recurring
situation previously addressed in Information Notice (IN) 84-81 and General
Electric Company's (GE's) Service Information Letter (SIL) 388. This notice
is considered an interim action pending further review of this problem.
Suggestions contained in this notice do not constitute NRC requirements
therefore, no specific action or written response is required. It is
expected, however, that recipients will carefully review the information for
applicability to their facilities and consider actions to preclude similar
problems from occurring at their facilities.
Description of Circumstances:
A number of events involving reduction of reactor coolant inventory because
of RHR valve misalignment have occurred since the issuance of IN 84-81. Six
events that occurred in 1985 have been selected for inclusion in this
notice. A seventh case was included since RHR was intentionally used to
drain water from the reactor cavity, (Refer to Attachment 1 for diagram and
definition of pathways.)
SUSQUEHANNA 2, 04/27/85
While attempting to align B-loop RHR in the shutdown cooling mode, a
portion of the RHR, piping was inadvertently drained to the main
condenser during the warmup process. When the operator opened the heat
exchanger bypass valve F048B, water drained rapidly from the vessel,
refilling the RHR piping, and waterhammer occurred. The reactor vessel
level decreased 35 inches. Reactor scram and primary containment
isolation system (PCIS) actuation occurred on the reactor
low-level signal. (Pathway 5)
Typical scram and PCIS actuation set point is +12.5 inches or 162
inches above active fuel.
8608180091
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IN 86-74
August 20, 1986
Page 2 of 4
WNP-2, 05/07/85
While shifting from shutdown cooling to low pressure coolant injection
(LPCI) lineup, suppression pool suction valve F004 was opened before
shutdown cooling suction valve F006 was shut (valves F008 & F009 were open).
The operator did not allow for the stroke times of the valves. Water level
rapidly fell to the scram and PCIS actuation set point. The final water
level was +5 inches. (Pathway 1)
SUSQUEHANNA 1, 05/16/85
The minimum flow bypass valve F064A failed open while starting C-pump for
shutdown cooling with valves F008A and F009A open. (Pathway 3)
SUSQUEHANNA 1, 05/20/85
The minimum flow bypass valve F064A failed open while starting A-pump for
shutdown cooling with valves F008A and F009A open. (Pathway 3)
SHOREHAM, 07/26/85
While shifting from shutdown cooling to LPCI lineup, the F004 valve was
opened before the F006 valve shut (valves F008 & F009 were open).
Approximately 7500 gallons of reactor inventory was drained to the
suppression pool. The incident was terminated by PCIS actuation on the low
reactor water level signal. The final water level was -10 inches as opposed
to the normal level of about +39 inches. (Pathway 1)
PEACH BOTTOM 2, 09/24/85
The 2A RHR pump was being realigned from shutdown cooling to the full flow
test mode. With valves F008A, F009A and F006C open, the discharge valve to
the suppression pool F024A was opened in the 2A line, allowing reactor water
to flow to the torus. On reaching the reactor water low-level set point, the
reactor scrammed and PCIS actuated to terminate the event. The final water
level was -10 inches. (Pathway 2)
VERMONT YANKEE, 05/22/86
During refueling operations, the equipment pool, cavity and reactor, and
spent fuel pool were communicating. With no one on the refueling floor to
observe level and less control room attention than necessary to prevent the
event, water level increased to the point that water was flowing into the
ventilation ducts. To lower water level, operators aligned shutdown cooling
to the torus. Although effective in this case to stop the overflow, use of
the RHR pump and valves to drain the cavity is not recommended in GE SIL
388.
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IN 86-74
August 20, 1986
Page 3 of 4
Discussion:
The RHR system has several modes of operation including the shutdown cooling
mode. Shutdown cooling makes use of the same piping, valves, pumps, and heat
exchangers that the LPCI function uses. The misalignment of shutdown cooling
valves with its potential for RHR system damage and draining of the reactor
coolant system is of regulatory concern.
Generic Letter 83-28, dated July 8, 1983, Required Action 2.2: "Equipment
Qualification and Vendor Interface," paragraph 2, states that "licensees and
applicants shall establish, implement and maintain a continuing program to
ensure that vendor information for safety-related components is complete,
current and controlled throughout the life of their plants, and
appropriately referenced or incorporated in plant instructions and
procedures." A relevant vendor communication is GE SIL 388 which states that
shutdown cooling "is entirely controlled by manual operator actions and is,
therefore, subject to operator error, which could result in hydraulic and
thermal conditions not specifically considered in the design process." It
recommends review and upgrading of operating procedures and operator
training programs to minimize operator error during operation of the
shutdown cooling mode of RHR.
Possible operator errors identified in SIL 388 include the opening of the
F004 valve before the F006 valve is shut, intentional use of shutdown
cooling for vessel level reduction, opening of the minimum flow bypass line,
use of the test return line, and opening of the upper containment pool
return line (BWR/6). This latter possibility is designated pathway 4 in
Attachment 1. Also, GE cautions against, the use of RHR shutdown cooling to
drain the cavity because of possible thermal and hydraulic conditions not
explicitly considered in the design process and possible equipment damage.
Since the RHR pumps each have a capacity of 10,000 gpm, two operating pumps
can transfer 20,000 gpm (that is, more than 1,000,000 gallons per hour) from
the vessel to the suppression pool. Also, the paths involving gravity flow
are through large diameter piping systems with a considerable differential
height between the vessel and the suppression pool. The reactor and cavity,
the equipment pool, and the spent fuel pool above the bottom of the fuel
transfer slot contain around 800,000 gallons of water. Quantities of 5000 to
50,000 gallons have been rapidly drained in previous incidents, usually
terminated by the operation of PCIS. PCIS has mitigated several of the
events, although technical specifications generally do not require PCIS to
be operable in modes where shutdown cooling is required.
Because the problems of draining the vessel through misalignment of the RHR
valves continue to occur in spite of GE and NRC communications alerting
licensees to them, the NRC is considering what further action should be
taken.
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IN 86-74
August 20, 1986
Page 4 of 4
No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or this office
Edward L. Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Mary S. Wegner, IE
(301) 492-4511
Attachments:
1. Reactor Coolant Inventory Pathways Created by RHR Valve Misalignment
2. List of Recently Issued IE Information Notices