05000298/LER-1993-001
| ML20056C066 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 03/18/1993 |
| From: | Gardner R, Reeves D NEBRASKA PUBLIC POWER DISTRICT |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| CNSS933054, LER-93-001, LER-93-1, NUDOCS 9303260087 | |
| Download: ML20056C066 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| 2982093001R00 - NRC Website | |
text
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.e CNSS933054 March 18, 1993 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555
Dear Sir:
Cooper Nuclear Station Licensee Event Report 93-001, Revision 0, is forwarded as an attachment to this letter.
Sincerely,
. L. Gardner Plant Manager RLG/ju Attachment ec: J. L. Milhoan ;
G. R. Horn J. M. Meacham i
R. E. Wilbur V. L. Wolstenholm l D. A. Whitman l
INPO Records Center NRC Resident Inspector R. J. Singer CNS Training ,
CNS Quality Assurance j i
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t Potential for Incufficient Component Cooling During A Design Basis Accident Duc To l Desien Discremancles With The Service Water And Reactor Equipment Cooling Systems ;
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. NO l l' l Am R ACT u ,. ,m ., c . . . % ,s -,-. ,-, ua , nei i During ongoing efforts associated with the Design Basis Reconstitution Program for !
) Cooper Nuclear Station (CNS), several design discrepancies in the Service Water (SW) '
and Reactor Equipment Cooling (REC) Systems were identified. These discrepancies l affected the ability to achieve the minimum required SW and REC flows to essential equipment requiring cooling during and following a design basis accident (DBA). On ,
February 25, at 12:30 pm, both systems were declared inoperable and the 2411our LCO requiring plant shutdown specified in Technical Specifications was entered. At the time of entry into the LCO, the plant was operating at approximately 85 percent {
power (nearly 700 MWe) on end of cycle coastdown with all rods fully withdrawn.
d 1 Review of design and construction documentation revealed that a SW piping (
configuration error was made during original plant construction, resulting in
' While subsequent evaluations were performed by the Architect / Engineer resulting in system operational ch'nges to compensate for the as-built condition, apparently the i analyses were not sufficiently thorough to identify and resolve these concerns.
Compensatory measures taken to restore the SW and REC Systems to an operable status !
included interim procedure changes providing operator guidance and system valve '
alignment restrictions. Additionally, operators were briefed regarding the scenarios of concern and the necessary compensatory actions requircd. These actions were in effect until the plant achieved cold shutdown for the 1993 Refueling Outage which began on March 5. Prior to startup from the outage, pertinent mechanical and '
electrical modifications will be made to resolve the identified concerns.
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A. Event Description I 1
During ongoing efforts associated with the Nebraska Public Power ';
District's Design Basis Reconstitution Program for Cooper Nuclear !
Station (CNS), several design discrepancies in_the Service Water _(SW)- !
and Reactor Equipment Cooling (REC) Systems were identified. These i discrepancies involved the ability to a.:hieve the minimum required'SW ; j and REC flows to the essential equipment requiring cooling during and ;
following a design basis accident (DBA). On February 25, at 12:15 pm, !'
an evaluation of the discrepancies was conducted by the Station Operations Review Committee (SORC). A recommendation was then inade to i the Shift Supervisor to declare the SW and REC Systems inoperable and l enter the 2411our LCO requiring plant shutdown specified in s4ctions i 3.12.B.4 (REC System) and 3.12.C 3 (SW System) of the Technica.1 ;!
Specifications. Both systems were subsequently declared inoperable at !
12:30 pm and the 24 Ilour LCO was entered. !
DBA conditions include a loss-of-coolant accident (LOCA) with a concurrent loss-of-offsite power (IAOP) and a worst case single failure.
The postulated worst case single failure is a failure of one emergency diesel generator (DC), since this causes the failure of the most SW and REC components. Under these conditions, if both REC IIeat Exchangers are -
in service prior to the accident, _It may not be possibic to remotely isolate the heat exchanger in the loop that serves the Core Standby.
Cooling Systems (CSCS) equipment in the failed electrical division. For example, if DG No. I fails and all off-site power is lost, valve REC-MOV-711MV (the South Critical Loop Supply Valve) will not;open since it is powered by Division 1. Thus, only REC lleat Exchanger B is available for critical loop component cooling. With both heat exchangers in service at the start of the event and both SW outlet' valves open, it would not be possible without manual operator action to isolate the SW flow to llent Exchanger A since the REC A Heat Exchanger SW System outlet valve (SU-MOV-651MV) is a Division I valve.
Consequently, a flow diversion proportional to the flow established !
prior to the accident would be directed to the "A" REC Heat Exchanger ,
l which would be non-functional; i.e., providing no useful accident I response purpose. With this flow diversion, it may not be possible to-provide the minimum required flow to the operable DG, the functional !
REC liest Exchanger, the Residual lleat Removal (RHR) SW Booster Pump and j other. cooling loads. l The REC System is also not single failure proof in the accomplishment of l its safety function. The automatic isolation valves that isolate the i noncritical header of REC from the critical header are all powered from ~
i Division I electrical power. In the case of a Design Basis LOCA, IDOP and failure of DG No. 1 (DG No. 1 only), the noncritical header does not automatien11y isolate. _As a result, REC flow would be diverted to the nonessential evoling loads.
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With only one REC pump in operation, the essential CSCS area coolers and the RHR Pump seal coolers might not receive the required cooling flow.
In the event of a LOCA, LOOP and loss of DG No. 2, REC flow will be diverted to the noncritical header since REC-MOV-702MV, the Noncritical Header Drywell Supply Isolation Valve, does not receive the low header pressure isolation signal that is received by the other two noncritical ,
loop isolation valves. Thus, remote manual action from the Control Room i must be taken to isolate the valves on the suction side of the REC Pumps to prevent diversion of the REC flow through the noncritical header. ;
i B. flant Status Operating at approximately 85 percent power, at nearly 700 MWe, on ,
end of cycle coastdown with all rods fully withdrawn.
4 C. Basis for Report 1
A condition alone that could have prevented the fulfillment of the j safety function of systems needed to mitigate the consequences of an accident, reportable in accordance with 10CFR50.73(a)(2)(v). :
D. Cnuse A review of documentation associated with the design and construction of ;
the associated systems revealed that a SW piping configuration error (
during original plant construction occurred, resulting in Division I SW being supplied to the Division II REC Heat Exchanger, and vice-versa.
While subsequent failure modes and effects analyses performed by the ,
Architect / Engineer resulted in system operational changes to compensate for the as-built condition, apparently the analyses were not ;
sufficiently thorough to identify and resolve these concerns. t E. Safetv Sinnificance ,
The safety objective of the SW System is to provide a heat sink for the REC, RHR, and DG cooling systems under transient and accident ;
conditions. However, adequate SW flow to all of the required cooling l loads was not assured under postulated LOCA, LOOP and concurrent worst '
case single f allure conditions, without operator action from the Control ,
Room. Existing operating procedures direct the operator to start a !
second SW pump in the event that the SW header pressure.is low, as it !
could be under the postulated accident scenario. Calculations show'that adequate power is available from a DG to support operation of this second pump. Also, calculations show that the operator has at least ten ;
minutes to accomplish this remote manual action from the' Control Room.- I which is within the.CNS licensing basis- and is a reasonable. response !
time. Consequently, the safety objective of the SW System would have been achieved.
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E. Safety Sirnificance - (continued)
The safety objective of the REC System is to provide cooling to the CSCS area coolers and the RER pumps seal coolers, liowever, in the event of a LOCA, LOOP and worst caso single failure condition, the noncritical header will not be automatically isolated from the critical header as a result of low header pressure, an expected condition with only one pump in service. Adequate system flow to the CSCS area coolers and the RllR pumps seal coolers still might not be sufficient even if a second REC pump were started. Therefore, the safety objective of the REC System was jeopardized. {
i F. Safety Imnlications I l
The event which must occur in order for the SW and REC Systems to be of concern is a Design Basis LOCA, which assumes a concurrent LOOP and a ;
simultaneous failure of one DC. The probability of these-three i unrelated events occurring simultaneously is extremely remote (on the .. i order of 10-8 per year of reactor operation).
C. Corrective Actiqn Compensatory measures taken to restore the SW and-REC Systems to an operable status included interim procedure changes which provided ;
operator guidance and valve alignment restrictions associated with the ;
SW and REC Systems, along with operator briefings regarding the scenarios of concern and necessary compensatory actions that would be ;
required upon their occurrence. At approximately 10:00 pm on February 25, SORC reviewed and approved the basis for continued aparation uithin the hour, the required compensatory measures were in place and both the SW and REC Systems were declared operable. Thesc 1 compensatory actions were in effect until the plant achieved cold !
shutdown for the 1993 Refueling Outage which commenced on March 5.
Prior to startup from the refueling outage, necessary mechanical and ;
electrical modifications will be made to resolve the identified ;
concerns. !
11 . Similar Eventa j Nono ;
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