ML20113H336
| ML20113H336 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 07/24/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20113H335 | List: |
| References | |
| NUDOCS 9207290219 | |
| Download: ML20113H336 (5) | |
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SAFETY 13LA.LUATION BY TEL_.Qf_FICf 0F NtKLLAR R.fM10R REGULATIQf!
REULTfD TO AMENDMENT NO 161 TO FACl(IJ10PERATING LL(fNSE NO. DPR-65 FQRTHEAST NUC1LAR ENIRGY.C_0MPANY. ET AL.
tilLLST0!if NUCLEAR PQ_WER STATIONm VNIT NO. 2 QOCKET P. 50-115
1.0 INTRODUCTION
By let+er dated January 30, 1992, Northeast Nuclear Energy Company (NNECO or 'be 'icensee) submitted a proposal to change the Technical Specit%.ations for Hillstone Nuclear Power Station, Unit No. 2.
The change concerns the deletica of the autoclosure interlock (ACI) from the shutdown cooling system (SDCS) suction valves.
Currently the design includes an autoclosure interlock (ACI) and an open permissive interlock (OPI) on each of the isolation valves to reduce the probability of inadvertent connection of the reactor coolant system (RCS) to the Shutdown Cooling System (SDCS) when the RCS pressure is above 280 psia.
The licensee proposed to remove the ACI and add an alarm on these valves to warn the operators whenever a SDCS suction isolation valve is open and the RCS pressure is greater than 280 psia.
The OPl feature would remain and is unaffected by this change.
2.0 BACKGROUf!Q The SDCS is designed to achieve and maintain cold shutdown condition by removing residual heat from the RCS and the reactor core. The RCS has a design pressure of 2500 psia and the SDCS has a design pressure of 500 psig.
To avoid overpressurization of the SDCS with the potential for loss of primary coolar.t outside the containment, it is necessary to isolate the SDCS from the RCS piping when the RCS is operated at high pressure. The isolation of SOCS is achieved by two isolation valves in series. The SDCS pressure boundary is protected against postulated pressurization transients by relief valve 2-SI-468 located on the SDCS suction line. However,.it is not designed to mitigate the overpressurization due to the inadvertent exposure of the full RCS pressure during power operation.
To guard against overpressurization and failure of the SDCS, alarms and two instrumentation interlocks are in place as described in the Reactor Systems Branch Technical Position (BTP) 5-1.
The first interlock is designed to prevent opening the SDCS isolation valves when the RCS pressure is above the SDCS design pressure.
This is an OPI and the 9207290219 920724 PDH ADOCK 05000336 p
l.
. proposed design modification does not involve a change to this interlock.
The second interlock automatically provides a close signal to the isolation valves when RCS pressure exceeds 280 psia.
Removal of this feature is proposed as a way to decrease the probability of a loss of shutdown cooling.
The two valves in series guard against a single failure causing a complete loss in the pressure boundary.
The barrier is established by the operator closing both valves when going from SDCS operation to steam generator cooling during plant heatup.
Failure to establish this double barrier is possible due to a mechanical failure or operator error.
The ACI and alarms are intended to guard against this failure.
When the SDCS is required, tne suction valves are required to remain open.
Failures resulting in valve closure are a safety concern due to the loss of decay heat removal.
In the converse when ACI is required, failures leaving the valves open adversely impact safety by overpressurizing the SDCS.
Since ACI has been a significant contributor
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to loss of SDCS events at other plants, NNEC0 proposes the removal at Hillstone Unit 2.
3.0 EY_A10AT10N The licensee evaluated the impact of ramoving the ACI from the SDCS ba - d on tSe guidelines recommended by the NnC (Referer.ce 2) as discussed below.
The guidelines include:
i-Means Available to Prevent a 1.0CA Outside of Containment Alarms to Notify the Operator that SDCS Suction Valves are Hispositioned Verification of the Adequacy of Relief Valve Capp. city Heans Other than ACI to Ensure Both Isolation Valves are Closed Assurance that the OPl is not Affected by ACI Removal Ass':rance that Valve Position Indication Will Remain Available in tne Control Room After ACI Removal Assessment of the Effect of ACI Removal on SDCS Availability and Low-Temperature Overpressure Protection 3.1 Means Available to Prevent a LOCA Outside of Containment Millstone Unit 2 is designed with a double barrier between the RCS and the SDCS (two isolation valves in series 2-SI-651 and 2-SI-652) providing a high probability that at least one barrier can be established and maintained under a postulated break.
Procedural controls, training, alarms, and OPI function minimize the potential that the operator will fall to achieve double isolation during normal heatup and pressurization of the RCS.
3.2 Alarms to Notify the Operator that SDCS Suction Valves are Hispositioned I
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. The licensee will install visual and audible alarms (annunciator type) in the main control room to inform the operator if either of the SDCS t
suction valves is not fully closed when RCS pressure is above 280 psia.
The alarm is designed to alert the operator of alarm circuit failure and the alarm setpoints will be tested at least every 18 months.
3.3 Verification of the Adequacy of Relief Valve Capacity During the design of the original system, Combustion Engineering completed calculations to ensure that the relief valve in the SDCS suction line had adequate capacity to prevent overpressurization of the t
SDCS.
These calculations were reviewed to verify that ACI was not credited in the selection of limiting events or mitigation of the resulting transients. The calculations remain applicable with the ACI removed.
The SDCS relief valve remains applicable in all events except for the ovcrpressure transient where one or more safety injection (SI) pumps may actuate.
These events are mitigated by the low temperature overpressure protection system (LTOPs).
3.4 Heans Other than ACI to Ensure Both Isolation Valves are Closed The proposed modifications include alarms, position indication, procedures, and training to ensure that the double barrier is established upon heatup.
3.5 Assurance that the OPI is not Affectcd by ACI Removal The OPI function will be maintained in its present form, and this interlock will be tested at least once every 18 months to verify operability.
3.6 Assurance that
,1ve Position Indication Will Remain Available in the Control Room After ACI Removal Valve position is indicated on the main control board and on the computer display located in the main control room. The indication will be present even when valve operation is locked-out during power operation. Another indication that the valve is closed is by the lack of alarm when the pressure excetds the alarm set point.
3.7 Assessment of the Effect of ACI Removal on SDCS Availability and low-Temperature Overpressure Event A plant specific risk-based analysis was completed to determine the impact of removing the ACI from the Millstone Unit 2 SDCS suction valves 2-51-651 and 2-SI-652. The analysis is based on a safety assessment of the effect of ACI removal on SDCS availability, LTOP, and interfacing system LOCA (ISLOCA) potential.
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Based on industry experience, under normal plant operating procedur6s the removal of the ACI reduces the frequency of loss of SDCS by 28%. At Hillstone Unit 2 the SDCS isolation valves are de-energized in tho OPEN position during midloop operation to preclude an inadvertent automatic closure.
The plant specific analysis also determined that LTOP plays a significant role in overpressure transient mitigation.
In the case of an inadvertent Si actuation without ACI, there is an increased potential for overpressurizing the SDCS should the alarm and operator intervention fail.
However, the overall risk remains low because Hillstone 2 has two indeperdent trains to mitigate LTOP events that may occur during SDC operations.
Based on the liceasee's plant specific analysis the removal of the ACI has a negligibic impact on ISLOCA frequency.
TF!c is because the open i
permissive interlock is not affected by the remaval of the ACI and an alarm will be = installed in place of the 101 to alert the operator that the suction valves are not closed when the RCS prosaure is above the alarm set point.
The.licensec has adequately addressed the staff's guidelines outlined in reference 2.
They have shown that the remosal of the autoclosure interlock will not appreciabl3 effect ISLOCA frequency or the mitigation capability of the LTOP system.
In light of this, the staff finds the licensee's prnposal to remove the ACI and take compensatory actions to be acceptable.
3.0 11a}LCONSULTAT108 i
in accordance_with the-Commission's regulations, the Connecticut State official was notified of the proposed issuance cf the amendment.
The State official had no connents.
4.0 ' 13y_IRONMENEL(Wi1QIM11M The amendment changes surveillance requirements.
The NRC 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 offsito, and that there is no significant increase in individual or cumulative accupational radiation exposure.
The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration, and there has been no public comment on such finding (57 FR 9446);
Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR St.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 tha amendment.
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3-5.0 COElRS10.8 The Commission has concludd, based ori 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 regulation:,, and (3) the issuance of the amendment will not be inimical to the common defen a and security or to the health and safety of the public.
i 6.0 REFERENCE 1.
i.etter froni J.F. Opeka, Northeast Nuclear Energy Company to the NRC,
' Millstone Nuclesr Power Statinn, Unit 2 Shutdown Cooling System Autocicsure Interlock Deletion." January 30, 1992.
2.
"NRC Safety Evaluation Relating to Removal of Autoclosure Interlock function at Diablo Canyon," February 17, 1988 Docket Nos. 50-275 and 50-313.
Principal Contributor:
S. Brewer 2
Date: July 24, 1992 I
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