ML20126F393
| ML20126F393 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 12/15/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20126F396 | List: |
| References | |
| NUDOCS 9212300284 | |
| Download: ML20126F393 (4) | |
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- 'n UNITED STATES i '
.i NUCLEAR REGULATORY COMMISSION f
WASHINoTON. o C. 20'7A Q.....f SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.108 TO FACILITY OPERATING LICENSE NO. NPF-12 SOUTH CAROLINA E1ECTRIC & GAS COMPANY SOUlH CAROLINA PUBLIC SERVICE AUTHORITY VIRGIL C. SUMMER NUCLEAR STATION. UNIT N0, 1 DOCKET NO. 50-395 r
1.0 INTRODUCTION
By letter dated April 15, 1992, South Carolina Electric & Gas Company (the licensee), submitted a request for changes to the Virgil C. Summer Nuclear Station, Unit No.1 (Summer Station) Technical Specifications (TS). The requested changes would revise Engineered Safety Features (ESF) response times in Table 3.3-5 to account for sequential stroking of the outlet isolation valves on the refueling water storage tank (RWST) and the volume control tank (VCT).
2.0 EVALUATION Table 3.3-5 in the Summer Station TS, Enqineered Safety Features Responsq Times, provides the interval from when a monitored ESF parameter exceeds its actuation setpoint until the ESF equipment must be capable of performing its safety function.
These response times are measured at least once every 18 months.
In its normal configuration, the chemical and volume control system (CVCS) is aligned such that the charging pumps, which double as the high-head safety injection pumps, take suction from the VCT. When a safety injection (SI) signal is generated by the protection logic, a start signal is sent to the charging pumps (which then begin to function as SI pumps).
In addition, a signal is sent to open the RWST outlet isolation valves, thus lining up this source of borated water to the suction of the high-head SI pumps.
Only af ter the RWST outlet isolation valves are fully open does the logic tell the VCT outlet isolation valves to shut.
The logic is designed this way to ensure that there is always an adequate net positive suction head to the SI pumps.
The entire switchover from VCT suction to RWST suction can take as long as 25 seconds.
Since the RWST contains heavily borated water and the VCT does not, it is conservative to assume that the RWST serves as the source of water to the SI pumps only after the VCT outlet isolation valves are fully shut.
Current TS times take into account only the opening of the RWST outlet isolation valves; the requested change would include the time for the VCT outlet isolation valves to shut, in order to evaluate this change, its impact on large and small break loss-of-coolant accidents (LOCA's), post-LOCA plant performance, steam generator tube rupture (SGTR) accidents, and non-LOCA transients was assessed.
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loss of Coolant Accidents Large Break LOCA Af ter a large break LOCA, the formation of steam voids in the core adds enough negative reactivity to cause the reactor to go subcritical.
Thus, the immediate injection of boron to reduce nuclear generated heat is not-necessary.
This is why large break LOCA analyses assume that the immediate function of the St system is to provide water to the primary in order to cool the core.
Whether or not the water is borated or unborated is not of y
immediate concern. Water will be available from the SI system when the SI pumps reach full speed regardless of the position o_f the VCT outlet isolation valves.
The injection of borated water will be necessary to maintain long-term subcriticality; however, the additional delay due to RWST switchover is not long-term.
Since borated water is not immediately necessary to mitigate a large break LOCA, the proposed change does not have any impact on the Summer Station large break LOCA analyses.
Small Break LOCA Small break LOCA analyses assume that the reactor will become subcritical.due to the insertion of all but the most reactive control rod, and no credit is taken for increased boration due to i jection of water from the RWST.- As with n
large break LOCA, then, the assumption is that the immediate function of the SI system is to provide water to cool the core.
The analyses take credit for the injection of borated water in maintaining long-term subcriticality; however, the additional delay due to_ RWST sitchover is not long-term.
Since borated water is not immediately necessary to mitigate a small break LOCA, the proposed change does not have any impact on the Summer Station small break LOCA analyses.
Control Rod Ejection Accident Since the SI flow following a control rod ejection is modeled under similar.
assumptions as in the LOCA analyses, the delay in. borated water injection will have negligible impact.
The analysis of the control rod ejection accident in the Final Safety Analysis Report (FSAR) will remain valid.
Post-LOCA Plant Performance Reactor Vessel and loop Blowdown Forces Since the maximum blowdown forces are generated with-in the first few seconds:
of break ~ initiation, the initiation of ECCS flow is not considered in the hydraulic forces modeling.
For this reason. the delay time'~in.the initiation of borated water flow has no impact on these analyses.
Short-and Long-Term Mass and Energy Release For the containment subcompartment analyses the short duration of the transient (<3 seconds) leads to the assumption that there is not yet any S1 i
flow to the reactor coolant system (RCS).
Therefore, any delay in the
= injection of borated water could not have any effect on the analyses.
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3-Post-LOCA Long-Term Core Cooling The Summer Station licensing position regarding long-term core cooling is that, following a postulated LOCA,-the reactor will remain subcritical due to borated ECCS water residing in the sump.
Since no credit is taken for control rod worth, the core must remain subcritical due to the boron concentration of the SI water, assuming it is diluted by all other water that could be in the sump.
In the normal configuration, the charging pumps are aligned-to the VCT.
It is conservative to assume that, following an SI signal, the SI pumps inject only VCT water into the primary until the VCT outlet isolation valves are fully shut. The effect of injecting.non-borated water for the duration of the RWST switchover has been considered in the long-term core cooling evaluation.
Assuming maximum flow from all SI pumps, the amount of unborated water that could be injected before switchover to the RWST is complete has been estimated at about 3363 lbs.
This amount of water would result in a reduction in sump boron concentration of about 2.1 ppm.
Since the boron concentration in the RWST is 2300 - 2500 ppm and the amount of water that would be injected following a LOCA would be about 3,200,000 lbs, the additional delay in RWST switchover has no impact on the Summer Station long-term cooling analysis.
Hot leg Switchover to Prevent Boron Precipitation Post-LOCA hot leg recirculation switchover time is determined to ensure that no baron precipitates in the reactor vessel following boiling in the core.
The time to initiate hot leg switchover is not affected by the delay in RWST switchover; therefore, the proposed TS change does not affect these analyses.
Steam Generator Tube Ruoture For the SGTR, primary to secondary break flow was assumed to terminate 30 minutes after event initiation, and operator action to cool down the plant was not modeled. With no RCS cooldown, sufficient shutdown margin is assumed to be available initially, and long-term shutdown margin is assumed to be maintained with borated water.
The increase in delay time will have-no impact on these assumptions.
Non-loca Transients Steam Line Rupture Accident The only non-LOCA event affected by the increased time delay is the steam line break.
This is the only event which relies on short-term boration.
Based on-the current steam line break analysis, the additional delay time is acceptable since the delay in the availability of borated water occurs early in the-transient when RCS pressure is still high and SI flow rates are low.
A' sensitivity study conducted by Westinghouse shows that delays of the magnitude
.under consideration will result in maximum increases of 0.2% in reactor power, 0.6'F in RCS temperature, and 10 psi in pressure. A Summer Station plant specific analysis has shown that there is sufficient design margin to accommodate these small increases; therefore, the analyses in the FSAR remain valid.
. Sensitivity studies on the steam line break superheated mass / energy release-outside containment show that these releases are not sensitive to large changes in SI flow.
The additional-time delay is small when compared to the large change in total SI flow; therefore, the impact of the delay on superheated mass / energy releases outside containment is insignificant.
The proposed additions of 10 to 15 seconds to the response times given in the Table 3.3-5 of the TS in conjunction with the modifications to the notation at the back of the Table reflect both the opening of the RWST outlet valves and the closing of the VCT outlet valves.
As the above evaluation indicate,
these changes will not invalidate the analyses or subsequent conclusions in the FSAR for any LOCA or non-LOCA transient; therefore, the staff has found-the proposed TS changes acceptable.
3.0 STATE CONSULTATION
in accordance with the Commission's regulations, the State of South Carolina official was notified of the proposed issuance of the amendment.
The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20.
The staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has pre'liously issued a proposed finding that the amendment involves no significant hazards consideration and there has been no public comment on such finding (57 FR 24679).
Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR j
51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
5.0 CONCLUSION
The Commission has concluded, based on the considerations discussed above, that :.
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner,. (2) _ such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and_ security or to the health and safety of the public.
Principal Contributor:
G. Wunder Date: December 15, 1992 i
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