ML20209G305
| ML20209G305 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom, Brunswick, 05000000 |
| Issue date: | 08/09/1985 |
| From: | Leeds E NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML20209G223 | List: |
| References | |
| TASK-AE, TASK-E511 AEOD-E511, NUDOCS 8509190213 | |
| Download: ML20209G305 (15) | |
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AEOD ENGINEERING EVALUATION REPORT
- UNITS: Brunswick Units 1 & 2 EE REPORT NO: AE0D/E511 l Peach Bottom Unit 3 DATE: August 9, 1985
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EVALUATOR / CONTACT: E. Leeds
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50-325/50-324 ;
DOCKETN0j.:- 50-278 ,
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LICENSEE: Carolina Power and Light Co.
Philadelphia Electric Co.
I NSSS/AE United Engineers and Constructors / !
General Electric i Bechtel/ General Electric 4 SUPNARY On June 1,1984, an engineering review at Brunswick deterr.ined that the control logic for the core spray system minimum flow valves did not permit the valves to perform their required containment isolation function. Based on this finding, the normally open minimum flow valves for both trains of the Brunswick 1 and 2 core spray systems were closed, and deactivated. The operating staff did not declare the core spray systems to be inoperabic, hv.;&ver, even though closing and deactivating the minimum flow valves rerdered the minimum flow bypass line inoperable. Only later did the operating staff recognize that the minimum flow line provided an essential pump protection feature. The Brunswick ,
event, along with similar events involving closed minimum flow bypass valves at Peach Bcttom Unit 3, were investigated to evaluate the underlying cause(s),
the potential safety significance and the generic applicability of operating events involving closed Emergency Core Cooling System (ECCS) minimum flow valves.
The study found that the minimum flow bypass lines provide an essential pump protection feature and that pump operability is generally depender on minimum flow valve operability. This finding leads to the conclusion thac the affected safety system trains at both the Brunswick and Peach Bottom units should have been oeclared inoperable when the minimum flow valves for these systems were closed and deactivated. Additionally, an evaluation of a data search for similar events at other plants coupled with the reported Brunswick and Peach Bottom events clearly indicates that not all licensees may recognize the impor- !
tance of minimum flow valves for ECCS pump operability. In view of these ;
conclusions, the study suggests that the Office of Inspection and Enforcement !
consider issuing an Information Notice to remind licensees of the importance ,
of the minimum flow bypass line pump protection feature and the dependency of .
pump operability on minimum flow valve operability.
8509190213 DR 850815 "
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- This document supports cngoing AE00 and NRC activities and does not represent the position or requirements of the responsible NRC program office.
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T. E. Hicks, Region II _
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- T. Johnson, Region I '
M. Grotenhuis, NRR G. Gears, NRR ,, !
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1 i C. E. Rossi', IE l
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r The control logic of the Brunswick core spray system minimum flow valves was ;
also reviewed to assess the generic safety implications. The evaluation con-cludes that the design was inadequate to assure that the valves could perform their containment isolation function in all required situations. Additionally, the Brunswick units have operated for over eight years with the logic error undetected. It would appear possible, therefore, that the.same mipimum flow -
valve control logic problem may exist and remain undetected at one or more other light water reactors. Based on these findings, the study also suggests that the IE Information Notice emphasize that licensees review the control i logic of ECCS minimum flow valves to ensure that it is adequate to satisfy containment isolation requirements. '
INTRODUCTION i
A study was initiated to evaluate an event which occurred at the Brunswick i plant in May,'1984. The event involved the discovery that the core spray j system minimum flow bypass valves could not fulfill their containment isolation i function for all required conditions because of an inadequacy in the control !
logic for the valves. The minimum flow valves provide a protection function .
for the core spray system pumps. Upon discovering the control logic problem, j the licensee closed, deactivated and declared the valves inoperable at both !
units. The licensee took this action to comply with the plant's technical j specifications for containm?nt isolation. However, the licensee did not recognize nor fully evaluate the protection provided the core spray pumps by the minimum flow valves. As a result, the licensee failed to declare either of the two core spray system trains inoperable while the valves were closed i and deactivated. f This study reviews the closed minimum flow valve event at the Brunswick units and similar events at Peach Bottom Unit 3. The study examines: (1) the importance of minimum flow valves to ECCS pump operability, (2) minimum flow valve functional requirements, and (3) minimum flow valve control logic. This study also evaluates the events at each plant with respect to the underlying cause(s), the potential safety significance and the potential applicability to other nuclear plants.
i DISCUSSION j Operating Experience j
- 1. Brunswick Units 1 and 2 On or about May 23, 1984, while performing local leak rate testing on l, soveral primary containment penetrations, operations personnel at Brunswick l Unit 2 observed that the minimum flow valve for the 2A Core Spray System l (CSS) pump would not stay in the closed position (Reference 1). When the A control switch for the minimum flow valve was turned to the "close" "
position, the valve closed, but then immediately reopened. Engineering ;
personnel determined that the control logic for the minimum flow valves for both units was such that the valves would reopen after closure whenever a low ficw condition was sensed in the core spray line. This could occur
either when the pump was secured or when an actual low flow condition existed. On June 1, an engineering review determined that the minimum '
flow valve control logic did not appear to meet General Design Criterion (GDC) 57 in Appendix A of 10 CFR 50. Based on this determination, the-normally open minimum flow valves for both CSS trains for Brunswick Units ,
1 and 2 wers. declared inoperable, .olosed and deactivated (see-Figure 1). 5 Operations p~ersonnel took these actions to comply with the plant technical specification action statement for inoperable primary containment isolation valves (PCIVs) that requires either the inoperable PCIV be restored tc. an operable status within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> after it is discovered inoperable, or the affected line be isolated.
At the time the minimum flow valves were closed, Unit 2 was defueled and Unit 1 was operating at power. The Unit I valves were also placed under shift foreman clearance to ensure effective operator action to minimize the potential for pump damage in the event of a pump start. Therefore, although the Unit 1 CSS minimum flow valves were closed, Carolina Power and Light (the licensee) believed that any potential for pump damage could be easily avoided by timely operator action.
From subsequent discussions with the pump vendor, the plant operating staff learned that damage to the CSS pump could occur within as little as one minute after initiation if the pumps were run at shutoff head without the minimum required flow. The plant staff re-evaluated the situation and concluded that the risk of possible pump damage with the valves closed was unacceptable.
Accordingly, after a plant shutdown on June 12, the minimum flow valves were reopened and actuation power restored. During the subsequent Unit 1 startup on June 13, administrative controls and special procedures were effected to ensure closure of the valves when required for containment isolation.
In review, Unit 1 operated at power from June 1, 1984 to June 12, 1984 with the CSS minimum flow valves closed and deactivated. During this period, the plant staff did not consider either train of the CSS to be inoperable. Prior to taking this action, the licensee's technical staff did not recognize the need to perform a 10 CFR 50.59 review to evaluate the potential adverse effects of closing the minimum flow valves on the operability of the CSS pumps. The licensee's planned permanent corrective action is to develop and implement appropriate modifications which will provide a remote isolation capability for the valves so that the minimum flow and the containment isolation functions are both assured.
- 2. Peach Bottom Unit 3 During an NRC inspection at Peach Bottom Unit 3, from July 16 to August 31, 1984, the site resident inspectors performed a review of the plant procedures and practices for assuring minimum flow protection for the ECCS pumps (References 2 and 3). From a review of plant logs, maintenance records and inspection
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j notes, inspectors determined that the minimum flow valves, for separate trains [
i of the Residual Heat Removal (RHR) system, had been closed and deactivated on '
two different occasions while the reactor was operating at power. Records j showed that the first event.had occurred on April 27, 1982, when the 'D' RHR
- pump minimum flow valve was. closed and deacti,vated for several hours (see !
Figure ?). The, inspectors also found that tile m.inimum flow valve for the 'A'
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L, q l ~RHR pump had been closed and deactivated for approximately three days between , p
]' June 22 and June 25, 1984. The inspectors' investigation could not determine s why the minimum flow valves had been closed and deactivated. However, the inspectors believed that the actions were taken to perform maintenance on the l valves or the valve motor operators. The inspectors also found that in both i i events, Philadelphia Electric Co. (the licensee). did not consider (or declare) i i the associated Low Pressure Coolant Injection (LPCI) system train to be inoper-
, able while the minimum flow valve was closed. The circumstances surrounding i
- the two events were brought to the licensee's attention by the resident l i inspectors. The licensee acknowledged that if a loss of coolant accident l
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(LOCA) were to occur in which reactor pressure remained above the LPCI pump shutoff head for a considerable length of time, pump damage could occur if
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minimum flow protection was unavailable.
t To prevent a recurrence, operations personnel were instructed not to deactivate t
any of the ECCS minimum flow valves when system operability is required. For
- the long term, the licensee is investigating an alternative to declaring an RHR y
- 1000 inoperable when a minimum flow valve is inoperable. To provide adequate i justification for operating with the minimum flow valves de-energized in the j open position, an LPCI pump flow test was performed with the valve fully open.
i The test showed that the pump can supply the minimum required injection flow i
even with the minimum flow valve fully open. If operating with the minimum flow valve always open is found to be acceptable, a written safety evaluation
! will be prepared. The licensee also plans to perform a similar test for the other ECCS pumps in order to verify that they are also capable of satisfying i the technical specification flow requirements with their associated minimum l
J flow valve open.
- ANALYSIS AND EVALUATION
{ Minimum Flow Bypass Valves and ECCS Pump Operability i ,
} Each ECCS pump in a Light Water Reactor (LWR) is provided with a minimum flow
- bypass line. The minimum flow bypass line provides an alternative discharge [6
, flowpath for the pump when the pump is operating with its associated injection I j valve closed. In this way, the minimum flow bypass line prevents overheating or possible damage to the pump when it is running in an otherwise deadheaded h condition. Minimum flow bypass lines are usually equipped with an automatic [H i
valve which opens or closes depending on the measured pump discharge flow rate.
- If pump discharge flow is less than that required for adequate pump cooling, the associated minimum flow valve automatically opens to allow sufficient flow ((
[ to pass through the pump. When the measured discharge flow is adequate to [
prevent pump overheating, the minimum flow valve automatically closes to i ensure that all pump flow is directed to the reactor vessel.
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The protection provided by the minimum flow lines is especially important to the low pressure ECCS pumps during a postulated small break LOCA. A small break would cause an ECCS iaitiation signal which would' automatically start all of the high and low prissure ECCS pumps. In the initial phase of the accident, it should be expected that reactor pressure will re, main relatively high (i.e. ,
wpli. above the sh'Jtoff , head of the low pressure ECCS pumps). Dur'ing thts early .-
period, the low pressure ECCS injection valves would be closed with the pumps ,
running. Spray / injection flows would not commence until reactor coolant system (RCS) pressure decreased below the opening pressure for these valves. Without minimum flow during the first several minutes of a small break LOCA, the low pressure ECCS pumps would operate deadheaded until the valves opened and injec-tion. flow was established. Pump operation without minimum flow cculd cause significant pump damage or failure in a short period of time. Minimum flow bypass capability is, therefore, considered an essential pump protection feature.
The Final Safety Analysis Reports (FSARs) for LWRs typically describe the ;
purpose and design of the ECCS pump minimum flow bypass feature. From these FSAR descriptions, it can be concluded that an operable minimum flow line is '
necessary to prevent pump damage and to ensure pump operability. Plant-speci fic ;
technical specifications, however, usually do not explicitly address the need '
to ensure an operable minimum flow valve in the minimum flow line as a require-ment for ECCS pump operability. However, boiling water reactor (BWR) and .
pressurized water reactor (PWR) Standard Technical Specifications include a statement in the definition for equipment operability that says, in effect, that all necessary auxilliary equipment (e.g. , cooling water systems) must be capable of performing their related support functions if the ECCS train is to be considered operable. From this perspective, it would appear clear that ECCS pump operability is dependent on minimum flow valve operability.
For Brunswick, the importance of the CSS minimum flow valves is specifically cited in the Brunswick CSS operability test procedure and the Brunswick FSAR.
The Brunswick CSS operability test is a comprehensive test which is performed (
at least once every 92 days. In regard to minimum flow valves, the test i verifies that each valve opens when the associated CSS pump discharge flow rate decreases below the specified rate (required to provide sufficient pump cooling) and closes when the specified pump discharge flow rate (that l provides adequate pump cooling) is met. During the test, each CSS pump is '
required to operate without any discharge flow paths available other than the minimum flow line. At this point, the Brunswick procedure states, " CAUTION: y If a core spray pump is operated on minimum flow for extended periods of time, pump damage may occur." From this caution statement, it may be concluded that t if a CSS pump was to operate deadheaded (without minimum flow available), pump ,
damage may occur in a short period of time. Additionally, the Brunswick FSAR j specifically discusses the importance of the minimum flow line for CSS pump protection. The Brunswick FSAR states in part that, "A low flow bypass line l runs from the pump discharge...to the suppression pool. The bypass flow is required to prevent the pump from overheating when pumping occurs against a closed discharge valve." However, the Brunswick technical specifications do i not explicitly address the need to have an operable (or open) minimum flow I valve in order for the associated CSS pump to be considered operable. l
After the minimum flow valves were closed in June 1984, a detailed analysis 5 performed by the vendor for the Brunswick CSS pumps confirmed that pump seal datage could begin within three minutes when operating the pumps deadheaded.
A7 analysis supplied to the licensee by General Electric Co. (the NSSS vendor) at that time also showed that the CSS pumps would be expected to operate dead-headed for more than 3 mir4tes (before injection flow would occur) for a -
small break LOCA of less than 0.2 ft2. Therefore, the recent vendor analyses are consistent with the Brunswick FSAR statement that ".. bypass flow is required to prevent the pump from overheating...." It would appear clear that opera-bility of the Brunswick CSS pumps is dependent on the operability of the minimum flow valves even though the plant technical specifications do not explicitly state this dependence. y Frc, the preceding discussion, it can be concluded that both trains of the Ercaswick CSS should have been declared inoperable from June 1,1984, when the f
rrinimum flow valves were closed and deactivated, until June 12,'when both f val.es were reopened. The fact that the valves were closed without declaring the system to be inoperable can be traced to the operating staff's failure to f i f.dly recognize the importance of the CSS system minimum flow valves and the licensee's failure to perform a safety evaluation in accordance with 10 CFR g 53.59. .
The two Peach Bottom events discussed in the previous section are, in a j ru-ber of respects, very similar to the Brunswick event. The importance of ;
the minimum flow valves for RHR system operability is also specifically cited ?
in the Peach Bottom FSAR and RHR system operability test procedure. The RHR system operability test procedure requires a periodic, comprehensive test of the LPCI mode of the system. The minimum flow valves are tested to ensure that they open and close at the specified flow rates. The FSAR states, "A bypass lire to the suppression pool is provided so that the (RHR) pumps are not caraged by operating with the discharge valves shut." However, as is the case for the Brunswick plant, the Peach Bottom technical specifications do not explicitly address the need to have an operable (or open) minimum flow valve to consider the associated RHR pump operable. It is left to the plant cperating staff to recognize that the minimum flow protection feature must be available for the associated ECCS train to be considered operable.
For the Peach Bottom event, closure of the RHR system minimum flow valves also }
can be traced to a f ailure by the plant operating staff to fully recognize the r importance of the valves and the licensee's failure to perform a safety j evaluation in accordance with 10 CFR 50.59. The licensee has since '
acknowledged that tFa affected RHR system trains should have been declared I in perable when the minimum flow valves were closed and deactivated. l To generically assess the extent to which LWR licensees recognize the im:ortant relationship between minimum flow valve operability and ECCS pump j cperability, a search was conducted for all Licensee Event Reports (LERs) ;
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J submitted for the three year period between January 1,1981 and December 31, 1983. The search was performed to determine if license _es typically reported failures of ECCS minimum flow valves. The search revealed that problems with -
ECCS minimum flow valves were reported by only 9 of the 39 licensees.
Additionally, only 13 plants (three PWRs and ten BWRs) reported minimum flow valv4; problems. Seven of .the nine licensees submitted more tha_n one LER j-during the time period covered by the search. Most licensees did not submit a single LER relating the ECCS minimum flow valves. To ensure that the LER search was thorough, a second search, utilizing a different search strategy, was performed yielding similar results.
It would appear unlikely that only 13 plants should have experienced ECCS minimum flow valve failures considering that approximately 80 plants were in operation during the 1981 to 1983 period and that most plants have a number of minimum flow valves. Also, ECCS minimum flow valves are routinely exercised and, as would be expected for most valves, occasionally should require corrective maintenance which would render them inoperable. The fact that only nine licensees submitted an LER involving minimum flow valves appears to suggest, therefore, that although some licensees recognize the importance of minimum flow valves for pump operability, other licensees may not. In summary, the Brunswick and Peach Bottom events coupled with the relatively few licensees reporting any minimum flow valve problems would appear to strongly suggest that a number of licensees may not fully recognize the importance of minimum flow valves for ECCS pump operability.
Minimum Flow Valve Functional and Design Requirements In this section, the control logic and operation of the Brunswick CSS minimum flow valves are compared with the minimum flow valve control logic at another operating plant, LaSalle-1. The purpose of the comparison is to evaluate whether the minimum flow valve control icgic at other LWRs may also be inadequate to perform both the pump protection and containment isolation functions. At Brunswick, the CSS has two trains, each with a separate pump and motor-operated minimum flow valve (see Figure 1). Flow in each pump discharge line is sensed by a differential pressure (d/p) transmitter tapped into orifice plate flanges located immediately downstream of each CSS pump. The variable differential pressure (flow) signal from tne transmitter is compared to a fixed reference flow value (i.e., the low flow setpoint). If the actual flow signal is less than the low flow setpoint, a signal is sent to open the minimum flow valve to ensure that the minimum required flow passes through the pump.
When the actual flow signal is greater than the low flow setpoint, the con-
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troller closes the minimum flow valve. At Brunswick, the CSS pump flow signal is independent of the pump operating status. Therefore, if the pump is not running, the valve will be signaled to open because the actual flow is less than the low flow setpoint.
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l At Brunswick, each of the CSS minimum flow valves also can be remotely controlled by separate switches in the control room. Each switch has three positions:
'open', ' neutral' and 'close' and'will spring-return to neutral from the open or close positions. Although each valve can be opened or closed with its con- -
trol switch at any time, the valve will immediately return to the position pequirp.d by the flow signal. .
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According to the licensee, the Brunswick CSS minimum flow valves must meet the
, requirements of GDC 57 of Appendix A to 10 CFR 50 for closed system isolation valves. GDC 57 states in part that, "...(the applicable system piping) shall have at least one containment isolation valve which shall be either automatic, or locked closed, or capable of remote manual operation." At. Brunswick, the remote switch in the control room is provided to satisfy the containment isola-tion requirement of the minimum flow valves. However, the licensee found that the minimum flow valves would immediately reopen after remote closure if the pumps were not running. This occurred because the control logic opens the valves when no (low) flow is sensed.
The CSS pump operates only during system testing or following an ECCS actuation signal. During normal operation, the CSS minimum flow valve will only close and remain closed when the associated CSS pump is running and the discharge flow is greater than the low flow setpoint. When the pump is not running (i.e., no pump discharge flo.0, the actual flow is less than the low flow setpoint. Therefore, if an operator attempts to close a CSS minimum flow valve using the remote switch when the L:9 pump is idle, the valve will close but will immediately reopen because of the low flow signal. With this logic, the valve cannot be kept closed using the remote switch when the pump is not running and, therefore, valve operation does not satisfy GDC 57.
For comparison, the LaSalle LPCS minimum flow valve control logic was reviewed.
At LaSalle, positioning of the LPCS minimum flow valves is also based on a comparison of the actual flow to a low flow setpoint. The LaSalle minimum flow valves also utilize a remote manual switch to satisfy containment isolation capability requirements. However, at LaSalle, an additional input to the LPCS minimum flow valve control logic is provided from its associated pump breaker. The input cuts out the flow signal when the pump is not running.
Therefore, when the LPCS pump is idle, the remote switch can close the minimum flow valve and the valve will remain closed. If the LPCS pump is running, the actual flow signal is compared to the low flow setpoint and the minimum flow
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valve opens or. closes depending on the pump discharge flow. By including an input from the pump breaker, the minimum flow valves at the LaSalle plant satisfy the GDC requirement for containment isolation and the pump protection' function.
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. o Due to the different nuclear power plant designs and the corresponding system-unique containment isolation requirements, not all ECCS minimum flow valves may ~ currently be required to satisfy GDC-57. For exanple, both Hatch
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units involved the same NSSS (General Electric) and were designed by the same architectengineer;(Bechtel). Howev are listed as containment isolation,9r, the CSS yaTyes minimum in the flow valves Unit 1 F.SAR, at Unit while the same 1 ,
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valves a~t Unit 2 arE not listed as containment isolation valves. Therefore, a plant-specific review would be required to determine if other plants have minimum flow valves installed that have inadequate control logic to perform their containment isolation functions.
FINDINGS AND CONCLUSIONS An evaluation of several recent BWR events involving closed minimum flow bypass valves associated with low pressure emergency core cooling system pumps has resulted in a number of significant findings and conclusions. The study found that pump damage or possible failure could occur within a few minutes of pump start if a pump were operated without minimum flow bypass with the pump i deadheaded. Additionally, a prolonged high pressure condition in the reactor l coolant system which would attend a small break LOCA would be expected to result in the low pressure ECCS pumps operating deadheaded for at least several i minutes if minimum flow protection was unavailable. The study concludes, ,
therefore, that the minimum flow bypass feature provides an essential l protection function for the low pressure ECCS pumps. 4 In spite of their importance, the study found that the CSS purrp minimum flow valves at Brunswick Units 1 and 2 and the RHR pump minimum flow valves at Peach Bottom Unit 3 were closed and deactivated for extended periods of plant operation. Furthermore, the affected ECCS trains were not declared inoperable during the periods when the valves were inoperable even though functional I capability of the trains during a postulated small break accident would be substantially degraded with the valves closed. The study found the valves were inappropriately. closed at both plants due to an apparent failure by the opera- ;
ting staffs to recognize the importance of the minimum flow valves to system operability and a failure by the plant licensing staffs to perform a written safety evaluation for plant operation with the minimum flow valves closed and deactivated. The results of an LER search for events involving ECCS minimum flow valves also suggests that a number of other licensees may not fully recognize the relationship between pump operability and inimum fles valve operability.
Finally, at Brunswick it was found that the CSS minimum flow valves had been f designed with control logic which did not permit the valves to fulfill a required containment isolation function whenever their associated pumps were not running. In view of the number of years both Brunswick units had been operating, the study concluded that other LWRs could be operating with the 1 same type of undetected control logic problem.
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SUGGESTIONS It,is suggested-that the Office of Inspection and Enforcement (IE) consider
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issuing an IE Information Notice to all LWR licensees concerning
. the Brunswick apd, Peach Bottom events involving the closure and deactiva-
_ tion of ECCS mihimum flow valves. Emphasis should be placed';pp. alerting licensees to the importance of the minimum flow bypass line as an essential j pump protection feature and the dependency of ECCS pump operability on l minimum flos valve operability. It is also suggested that the information notice u ge that licensees review the control logic of the ECCS minimum flow l valves to ensure that it is adequate to satisfy the containment isolation !
function, if applicable. {
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I REFERENCES l
- 1. Licensee Event Report 84-009, Brunswick Steam Electric Plant Unit 1, Docket No. 50-325, July 27, 1984.
, 2. NRC, Inspection Report No. 50-324/84.27 and 50-325/84-27, October 15, j 1984. -
g- 3. NRC, Inspection Report No. 50-277/8p~-24and50-278/8:4-20, pctober17, 1984. i i
- 4. Letter from S. L. Daltroff, Vice President, Electric Production, Philadelphia Electric Company to R. W. Starostecki, NRC,-
Subject:
i Response to Violation Identified in Inspection Report No. 50-277/84-24 l and 50-278/84-20, November 15, 1984. i e
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