ML20136D791
| ML20136D791 | |
| Person / Time | |
|---|---|
| Site: | Saint Lucie  |
| Issue date: | 02/02/1993 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20136C539 | List:
|
| References | |
| FOIA-96-485 NUDOCS 9703130012 | |
| Download: ML20136D791 (7) | |
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l SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REGARDING THE REMOVAL OF THE AUTOCLOSURE INTERLOCK FOR I
THE SHUTDOWN COOLING SYSTEM SUCTION VALVES l
FLORIDA POWER & LIGHT COMPANY ST. LUCIE UNIT 1 DOCKET NO. 50-335 1
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1.0 INTRODUCTION
The Florida Power and Light Company submitted a proposal on August 19, 1992 to amend Facility Operating License DPR-67 for St. Lucie Unit 1. The modification involves the removal of the auto-closure interlock (ACI) on the shutdown cooling system (SDCS) to minimize the potential for a loss of shutdown cooling capability during cold shutdown and refueling operations.
Currently, the ACI and an open permissive interlock (OPI) exist to prevent overpressurization of the SDCS. These interlocks ensure closure of two isolation valves on each of the two SDCS suction lines when the reactor coolant system (RCS) pressure exceeds an established setpoint. The OPI is unaffected by the proposed changes.
2.0 BACKGROUND
The SDCS is a relatively low pressure system (< 500 psig) designed to remove heat from the RCS at low RCS pressure. During power operation the SDCS is isolated from the RCS via two motor-operated _ valves in series on each SDCS suction leg (V3480 and V3481 for suction line 1A, V3651 and V3652 for suction line 1B).
.To guard against overpressurization resulting from postulated transients under normal operating conditions, relief valves are 9703130012 970301 PDR FOIA BINDER 96-485 PDR
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t employed on the suction legs. These valves are not designed to relieve pressure in the event that this system is exposed to RCS pressure during power operation. Inadvertent overpressurization by exposure to RCS pressure during power operation of the SDCS could result in a rupture outside of containment resulting in an interfacing system loss-of-coolant accident (ISLOCA).
i However, to minimize the potential for such a scenario, two interlocks are currently employed to ensure SDCS isolation via closure of the two isolation valves on each suction line. The N.I automatically closes the valves when the SDCS is in operation and the RCS pressure exceeds the ACI setpoint pressure of 267 psig.
i In addition, the OPI prevents opening of these suction line isolation valves when RCS pressure exceeds the design pressure of l
i the SDCS. The proposed changes for the removal of the ACI do not affect the OPI.
J The industry has experienced a number of spurious valve closures resulting in a loss of shutdown cooling capability caused at least in part by the presence of ACI. Although the removal of the i
ACI may increase the potential for an ISLOCA, it can be expected to reduce the potential for a loss of shutdown cooling. The proposed changes stem from a desire to minimize the potential for an ISLOCA and the loss of shutdown cooling.
3.0 SAFETY ASSESSMENT The NRC has recommended several guidelines for ACI removal
[ reference 1). The following is a list of the specific points addressed by the licensee:
1.
Means to prevent LOCA outside containment.
2.
Alarms to alert the operator of an improperly positioned SDCS suction valve.
3.
Verification'of the adequacy of relief valve capacity.
4.
Means other than ACI to ensure that both isolation valves are closed.
5.
Assurance that the OPI is not affected by ACI removal.
6.
Assurance that valve position indication will remain available in the control room after ACI removal.
7.
Assessment of the effect of ACI removal on interfacing systems loss-of-coolant accident (ISLOCA), SDCS unavailability, and low temperature overpressurization protection (LTOP).
3.1 Means to prevent LOCA outside containment.
The isolation valves on the SDCS suction line provide a double barrier between the SDCS and the RCS thus providing a high probability that at least one barrier can be established to prevent overpressurization of the SDCS. Appropriate training and procedures as well as the installation of alarms will be implemented to decrease the chances that the operator will fail to achieve closure of at least one of the two isolation valves.
Once isolation is achieved and the RCS pressure exceeds the OPI setpoint pressure separation will be maintained by the OPI. The i
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OPI is unaffected by the proposed removal of the ACI.
e 3.2 Alarms to alert the operator of an improperly positioned l
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SDCS suction valve.
j Nq Annunciator type alarms in the control room, training, and
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procedural controls will be implemented to minimize the potential of RCS heatup and pressurization above the SDCS design pressure while the suction line isolation valves are in the open position.
The alarms will sound in the event that isolation is not acheived and the RCS pressure exceeds the alarm setpoint. Upon activation of these alarms, operating procedures will instruct the operators to halt RCS heatup and close the isolation valves. These alarms will be tested every 18 months as per Technical Specification i
4.5.2.d.1 surveillance requirements.
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i 3.3 Verification of the adequacy of relief valve capacity.
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A review of the postulated events challenging the SDCS relief
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devices as described in FSAR section 6.3.2.2.6.d indicates that the ACI was not credited in the selection of limiting events or mitigating the ensuing transients. Therefore, this proposed change has no bearing on the results of the analysis.
3.4 Means other than ACI to ensure that both isolation valves are closed.
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In addition to the means to ensure isolation valve closure described in section 3.2, the circuitry will be modified to alarm t
for valve position independent of the valve controls and position indication. Procedures will instruct operators to verify valve i-position and take appropriate actions in the event of alarm activation. Cautions will be provided to direct the operator to I
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not pressurize the RCS above the SDCS design pressure prior to closure of both isolation valves.
3.5 Assurance that the OPI is not affected by ACI removal.
The OPI function will be unaffected by these proposed changes.
This interlock will be tested at least once per 18 months to verify operability according to TS 4.5.2.d.1.
3.6 Assurance that valve position indication will remain available in the control room after ACI removal.
Valve position indicators within the control room will be powered via a safety grade power supply. Position indication will be available even during power operation.
3.7 Assessment of the effect of ACI removal on interfacing systems loss-of-coolant accident (ISLOCA), SDCS unavailability, and low temperature overpressurization protection (LTOP).
An analysis to determine the impact of ACI removal was completed to quantify the changes in: 1) ISLOCA frequency, 2) The change in SDCS unavailability, and 3) The impact on mitigating LTOP events due to the removal of the ACI.
The ISLOCA frequency is governed by catastrophic failure of both f
l isolation valves. Therefore, removal of the ACI results in only a negligible increase (0.098%) in ISLOCA fregency.
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The analysis of SDCS unavailability includes a comparison based
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on both the failure to start the system and failure to operate given that the system has started. Removal of the ACI results in l
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i a 39% decrease in shutdown cooling unavailability during refueling operations.
In order to mitigate LTOP events that may occur during low RCS
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j temperature operation, St. Lucie Unit No. 1 employs two pressurizer power-operated relief valves (PORVs). Although the 1
SDCS suction relief valves would be available during such an event, these valves are not credited in the LTOP analysis.
J Consequently, LTOP mitigation is unaffected by removal of,the i
ACI. Inadvertantant closure of the SDCS suction valves has been j
identified to be a source for a potential LTOP challenging event under some conditions. However, since ACI removal increases the availability of the SDCS the probability of LTOP challenges i
subsequently decreases.
4.0 CONCLUSIQH I
In addressing the removal of the ACT, the licensee considered all staff guidelines outlined in reference 1 and instituted appropriate compensatory measures. Although the proposed changes result in a slight increase in ISLOCA frequency (0.098%), this is offset by the large decrease in SDCS unavailability (39%). The staff finds the licensee's proposal to remove the ACI and take compensatory actions to be acceptable.
5.0 REFERENCES
1.
"NRC Safety Evaluation Relating to Removal of Autoclosure Interlock Function at Diablo Canyon," February 17, 1988, Docket No. 05-000275/323.
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ENCLOSURE 2 SYSTEMATIC ASSESSMENT OF LICENSEE PERFORMANCE 1
l FACILITY NAME St. Lucie Unit 1 1
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SUMMARY
OF REVIEW i
j This safety evaluation addresses the implications involving the proposed removal of the automatic closure interlock from the shutdown cooling system suction isolation valves. We find the j
proposed changes acceptable based on a review of the licensee's i
analysis of this issue.
3 NARRATIVE DISCUSSION OF LICENSS PERFORMANCE - SAFETY l
8 ASSESSMENT /OUALITY VERIFICATION l
The licensee's application for the amendment change was complete l
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and contained all necessary technical content to support their proposal.
AUTHOR:
Phillio Rush DATE:
12/23/92 l
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