United States Nuclear Regulatory Commission - Protecting People and the Environment

Loss of Residual Heat Removal (RHR) While the Reactor Coolant System (RCS) is Partially Filled (Generic Letter No. 87-12)

                                UNITED STATES
                           WASHINGTON, D.C. 20655

                                July 9, 1987




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: 


                                  - 2 -

(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 

                                  - 3 -

     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 

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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 

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. 


                              Frank J. Miraglia 
                              Associate Director for Projects 
                              Office of Nuclear Reactor Regulation 
                              U.S. Nuclear Regulatory Commission 

As stated

                                 ENCLOSURE 1

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 

                                    - 2 -

          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 

                                    - 3 - 

     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. 

                                    - 4 - 

     (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 

     (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: 

                                    - 5 - 

          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 

     (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 

     (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 

(4)  Only limited instrumentation may be available to the operator while the
     RCS is partially filled. 

                                    - 6 -

     (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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