Loss of Residual Heat Removal (RHR) While the Reactor Coolant System (RCS) is Partially Filled (Generic Letter No. 87-12)
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20655
July 9, 1987
ALL LICENSEES OF OPERATING PWRS AND HOLDERS OF CONSTRUCTION PERMITS FOR PWRS
Gentlemen:
SUBJECT: LOSS OF RESIDUAL HEAT REMOVAL (RHR) WHILE THE REACTOR COOLANT
SYSTEM (RCS) IS PARTIALLY FILLED (GENERIC LETTER 87-12 )
Pursuant to 10 CFR 50.54(f), the NRC is requesting information to assess
safe operation of pressurized-water reactors (PWRs) when the reactor coolant
system (RCS) water level is below the top of the reactor vessel (RV). The
principal concerns are (1) whether the RHR system meets the licensing basis
of the plant, such as General Design Criterion 34 (10 CFR Part 50, Appendix
A) and Technical Specifications (TS), in this condition; (2) whether there
is a resultant unanalyzed event that may have an impact upon safety; and (3)
whether any threat to safety that warrants further NRC attention exists in
this condition.
Our concerns regarding this issue have increased over the past several
years, and lessons learned from the April 10, 1987 Diablo Canyon loss-of-RHR
event require an assessment of operations and planned operations at all PWR
facilities to ensure that these plants meet the licensing basis. Study of
the Diablo Canyon event has led to identification of unanalyzed conditions
that are of significance to safety. Although Diablo Canyon never came close
to core damage, and could have withstood the loss-of-RHR condition for more
than a day with no operator action, slightly different conditions could have
led to an accident involving core damage within several hours. One
unanalyzed condition involves boiling within the RCS in the presence of air,
leading to RCS pressurization with the potential for ejecting RCS water via
cold-leg openings, such as could exist during repair to a reactor coolant
pump (RCP) or to a loop isolation valve. The lost water would no longer be
available to cool the core, and if makeup water were unavailable, the core
could be damaged in a significantly decreased time. The pressurization could
also affect the capability to provide makeup water to the core. Other
unanalyzed situations are also possible, and occurred at Diablo Canyon
(e.g., boiling in the core). The seriousness of this situation is
exacerbated by the practice of conducting operations with the equipment
hatch removed, and by the lack of procedures that address prompt containment
isolation should the need arise.
Loss of RHR and related topics are not a new concern to the NRC staff. This
topic has been addressed in numerous communications with the licensee. Yet,
these events continue to occur at a rate of several per year. This condition
needs to, be fully considered in order to ensure compliance with the
licensing basis. Therefore, we request that you provide the NRC with a
description of the operation of your plant during the approach to a
partially filled RCS condition and during operation with a partially filled
RCS to ensure that you meet the licensing basis. Your description is to
include the following:
8707100112
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(1) A detailed description of the circumstances and conditions under which
your plant would be entered into and brought through a draindown
process and operated with the RCS partially filled, including any
interlocks that could cause a disturbance to the system. Examples of
the type of information required are the time between full-power
operation and reaching a partially filled condition (used to determine
decay heat loads); requirements for minimum steam generator (SG)
levels; changes in the status of equipment for maintenance and testing
and coordination of such operations while the RCS is partially filled;
restrictions regarding testing, operations, and maintenance that could
perturb the nuclear steam supply system (NSSS); ability of the RCS to
withstand pressurization if the reactor vessel head and steam generator
manway are in place; requirements pertaining to isolation of
containment; the time required to replace the equipment hatch should
replacement be necessary; and requirements pertinent to reestablishing
the integrity of the RCS pressure boundary.
(2) A detailed description of the instrumentation and alarms provided to
the operators for controlling thermal and hydraulic aspects of the NSSS
during operation with the RCS partially filled. You should describe
temporary connections, piping, and instrumentation used for this RCS
condition and the quality control process to ensure proper functioning
of such connections, piping, and instrumentation, including assurance
that they do not contribute to loss of RCS inventory or otherwise lead
to perturbation of the NSSS while the RCS is partially filled. You
should also provide a description of your ability to monitor RCS
pressure, temperature, and level after the RHR function may be lost.
(3) Identification of all pumps that can be used to control NSSS inventory.
Include: (a) pumps you require be operable or capable of operation
(include information about such pumps that may be temporarily removed
from service for testing or maintenance); (b) other pumps not included
in item a (above); and (c) an evaluation of items a and b (above) with
respect to applicable TS requirements.
(4) A description of the containment closure condition you require for the
conduct of operations while the RCS is partially filled. Examples of
areas of consideration are the equipment hatch, personnel hatches,
containment purge valves, SG secondary-side condition upstream of the
isolation valves (including the valves), piping penetrations, and
electrical penetrations.
(5) Reference to and a summary description of procedures in the control
room of your plant which describe operation while the RCS is partially
filled. Your response should include the analytic basis you used for
procedures development. We are particularly interested in your
treatment of draindown to the condition where the RCS is partially
filled, treatment of minor variations from expected behavior such as
caused by air entrainment and de-entrainment, treatment of boiling in
the core with and without RCS pressure boundary integrity, calculations
of approximate time
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from loss of RHR to core damage, level differences in the RCS and the
effect upon instrumentation indications, treatment of air in the
RCS/RHR system, including the impact of air upon NSSS and
instrumentation response, and treatment of vortexing at the connection
of the RHR suction line(s) to the RCS.
Explain how your analytic basis supports the following as pertaining to
your facility: (a) procedural guidance pertinent to timing of
operations, required instrumentation, cautions, and critical
parameters; (b) operations control and communications requirements
regarding operations that may perturb the NSSS, including restrictions
upon testing, maintenance, and coordination of operations that could
upset the condition of the NSSS; and (c) response to loss of RHR,
including regaining control of RCS heat removal, operations involving
the NSSS if RHR cannot be restored, control of effluent from the
containment if containment was not in an isolated condition at the time
of loss of RHR, and operations to provide containment isolation if
containment was not isolated at the time of loss of RHR (guidance
pertinent to timing of operations, cautions and warnings, critical
parameters, and notifications is to be clearly described).
(6) A brief description of training provided to operators and other
affected personnel that is specific to the issue of operation while the
RCS is partially filled. We are particularly interested in such areas
as maintenance personnel training regarding avoidance of perturbing the
NSSS and response to loss of decay heat removal while the RCS is
partially filled.
(7) Identification of additional resources provided to the operators while
the RCS is partially filled, such as assignment of additional personnel
with specialized knowledge involving the phenomena and instrumentation.
(8) Comparison of the requirements implemented while the RCS is partially
filled and requirements used in other Mode 5 operations. Some
requirements and procedures followed while the RCS is partially filled
may not appear in the other modes. An example of such differences is
operation with a reduced RHR flow rate to minimize the likelihood of
vortexing and air ingestion.
(9) As a result of your consideration of these issues, you may have made
changes to your current program related to these issues. If such
changes have strengthened your ability to operate safely during a
partially filled situation, describe those changes and tell when they
were made or are scheduled to be made.
Enclosure 1 contains insight which experience indicates should be well
understood before commencing operation with a partially filled RCS. Your
response to this 50.54(f) letter request should encompass the topics
contained in Enclosure 1. Additional information is contained in the NRC
Augmented Inspection Team report, NUREG-1269, "Loss of Residual Heat Removal
System, Diablo Canyon Unit 2, April 10, 1987." A copy of NUREG-1269 is
enclosed.
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Your response addressing items 1 through 9 (above) is to be signed under
oath or affirmation, as specified in 10 CFR 50.54(f), and will be used to
determine whether your license should be modified, suspended, or revoked. We
request your response within 60 days of receipt of this letter. This
information is required pursuant to 10 CFR 50.54(f) to assess conformance of
PWRs with their licensing basis and to determine whether additional NRC
action is necessary. Our review of information you submit is not subject to
fees under the provision of 10 CFR 170. If you choose to provide a portion
of your response in association with your owners group, such action is
acceptable.
This request for information was approved by the Office of Management and
Budget under clearance number 3150-0011 which expires December 31, 1989.
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.
Sincerely,
Frank J. Miraglia
Associate Director for Projects
Office of Nuclear Reactor Regulation
U.S. Nuclear Regulatory Commission
Enclosures:
As stated
ENCLOSURE 1
INFORMATION PERTINENT TO LOSS OF RESIDUAL HEAT REMOVAL SYSTEMS
WHILE THE RCS IS PARTIALLY FILLED
Many maintenance and test activities conducted during an outage require
lowering the water level in the reactor coolant system (RCS) to below the
top of the reactor vessel (RV) or (as is done many times) to the centerline
elevation of the RV nozzles. This operating regime is sometimes known as
"mid-loop" operation. It places unusual demands on plant equipment and
operators because of narrow control margins and limitations associated with
equipment, instrumentation, procedures, training, and the ability to isolate
containment. Difficulty in controlling the plant while in this condition
often leads to loss of the residual heat removal (RHR) system (Table 1).
Although this issue has been the topic of many communications and
investigations, such events continue to occur at a rate of several per year.
Recent knowledge has provided additional insight into these events. Although
the full implications of this knowledge remain to be realized, our
preliminary assessments have clearly established real and potential
inadequacies associated with operation while the RCS is partially filled.
These include: not understanding the nuclear steam supply system (NSSS)
response to loss of RHR, inadequate instrumentation, lack of analyses
addressing the issue, lack of applicable procedures and training, and
failure to adequately address the safety impact of loss of decay heat
removal capability.
The following items are applicable to these conclusions:
(1) Plants enter an unanalyzed condition if boiling occurs following loss
of RHR. For example:
(a) Unexpected RCS pressurization can occur.
No pressurization would occur with a water/steam-filled RCS with
water on the steam generator (SG) secondary side, because RCS
steam
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would condense in the SG tubes and the condensate would return to
the RV. Air in the RCS can block the flow of steam through
passages, such as the entrance portion of SG tubes, so that steam
cannot reach cool surfaces. Failure to condense the steam causes
pressurization in the RCS until the air compresses enough for
steam to reach cooled tube surfaces. This pressurization occurred
during the April 10, 1987 event at Diablo Canyon since the RCS
contained air. Pressure reached 7 to 10 psig, and would have
continued to increase if RHR had not been restored. The operators
began to terminate the event by allowing water to flow from the
refueling water storage tank (RWST) into the RCS. Increasing
pressure would have eliminated this option, and would have
jeopardized options involving pumps with suction lines aligned (in
part) to the RCS.
(b) Water that ordinarily would be available to cool the core might be
forced out of the RV, thereby reducing the time between loss of
RHR and initiation of core damage.
This is a potential concern whenever there is an opening in the
cold leg, such as may exist for repair of reactor coolant pumps
(RCPs) or loop isolation valves. Upper vessel/hot-leg
pressurization could force the RV water level down with the
displaced water lost through the cold-leg opening. A corresponding
decrease in level would occur in the SG side of the crossover
pipes between the SGs and the RCPs.
This occurrence could be particularly serious if the cold-leg
opening were large or if makeup water flow to the RCS were small,
as from a charging pump. Cold-leg injection with elevated pressure
in the upper vessel may not provide water to the core.
(2) RCS water level instrumentation may provide inaccurate information.
There are many facets to this issue. Instrumentation may be indicating
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a level that differs from level at the,RHR suction line, a temporary
instrument may be in use that has no indication or alarms in the
control room, and design and installation deficiencies may exist. We
have observed the following:
(a) Connections to the RCS actually provide a water level indication
up-stream of the RCP location. This water level is higher than the
water level at the RHR suction connection because of flow from the
injection to the suction locations and because of entering water
momentum, which increases level on the RCP side of the cold-leg
injection location.
Ingestion of air at the RHR suction connection will result in
transporting air into the cold legs; this can potentially increase
pressure in the air space in the cold legs relative to the hot
legs. Level instrumentation may respond to such a pressure change
as though RCS level were changing. In addition, such a
pressurization would move cold-leg water into the hot legs and
upper RV (or the reverse if a depressurization occurs).
(b) Use of long lengths of small-diameter tubing which can lengthen
instrument response time and cause perturbations such as RCS
pressure changes to appear as level changes, installation with
tubing elevation changes which can trap air bubbles or water
droplets, and installation which makes it possible for tubing to
be kinked or constricted.
(c) Some installations provide no indication in the control room, yet
level is important to safety. Some provide one indication. Others
provide diversity via different instrumentation, but do not
provide independence because they share common connections.
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(d) Tygon tube installations faintly marked at 1-foot intervals that
have no provision for holding the tube in place.
(e) Instrumentation in which critical inspections were not performed
after the installation.
(f) Instrumentation in which no provisions were made to ensure a
single phase in connection tubing or that tubing was not plugged.
(g) Use of instrumentation without performing an evaluation of
indicated RCS level behavior and instrument response.
(3) Vortexing and air ingestion from the RCS into the RHR suction line are
not always understood, nor is NSSS response understood for this
condition.
(a) On April 10, 1987, Diablo Canyon operators reduced indicated RCS
level to plant elevation 106' 6" immediately after steam generator
tubes drained, and indications of erratic RHR pump current were
observed. Restoring the RCS level to 106' 10" was reported to have
eliminated the problem. RHR operation was terminated a few hours
later at an indicated level of 107' 4" because the operators
observed erratic RHR pump current indications. The licensee later
reported that vortexing initiated under those conditions at 107'
5-1/2" ,and was fully developed at 107' 3-1/2". Procedures in
place at the time of the event indicated the minimum allowable
level to be 107' 3" (the hot- and cold-leg centerline elevation)
or 107' 3".
(b) Additional phenomena appear to occur under air ingestion
conditions. These include:
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o RHR pumps at Diablo Canyon were reported to handle several
percent air with no discernible flow or pump current change
from that of single-phase operation.
o A postulate is that air in the RHR/reactor coolant system can
migrate or redistribute, and thus cause level changes which
are at variance with those one would expect. This is a
possible explanation for observed behavior in which lowering
the RCS water level is followed by a level increase. Water in
the RHR appears to be replaced by air. Similarly, an increase
in RCS water level that is followed by a decreasing level may
be due to voids in the RHR system being replaced by RCS
water. Failure to understand such behavior leads operators
to mistrust level instrumentation and to perform operational
errors.
(c) Operators typically will start another RHR pump if the operating
pump is lost. Experience and an understanding of the phenomena
clearly show that loss of the second pump should be expected. The
cause of loss of the first pump should be well understood and
normally should be corrected before attempting to run another RHR
pump.
(d) Typical operation while the RCS is partially filled provides a
high RHR flow rate, which may be required by TS, but which may be
unnecessary under the unique conditions associated with the
partially filled RCS. Air ingestion problems are less at low flow
rates.
(4) Only limited instrumentation may be available to the operator while the
RCS is partially filled.
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(a) Level indication is many times available only in containment via
a Tygon tube. Some plants provide one or more level indications in
the control room, and additionally provide level alarms.
(b) Typically, RHR system temperature indication is the only
temperature provided to the operators. Loss of RHR leaves the
operator with no RCS temperature indication. This can result in a
TS violation, as occurred at Diablo Canyon on April 10 when the
plant entered Mode 4, unknown to the operators, with the
containment equipment hatch removed. It also resulted in failure
to recognize the seriousness of the heatup rate, or that boiling
had initiated.
(c) RHR pump motor current and flow rate may not be alarmed and scales
may not be suitable for operation with a partially filled RCS.
(d) RHR suction and discharge pressures may not be alarmed and scales
may not be suitable for operation with a partially filled RCS.
(5) Licensees typically conduct operations while the RCS is partially
filled, the containment equipment hatch has been removed, and
operations are in progress which impact the ability to isolate
containment. Planning, procedures, and training do not address
containment closure in response to loss of RHR or core damage events.
This is inconsistent with the sensitivity associated with partially
filled RCS operation and the history of loss of RHR under this
operating condition.
(6) Licensees typically conduct test and maintenance operations that can
perturb the RCS and RHR system while in a partially filled RCS
condition. The sensitivity of the operation and the historical record
indicate this is not prudent.
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