ML20247M941
| ML20247M941 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 05/14/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20247M939 | List: |
| References | |
| NUDOCS 9805260421 | |
| Download: ML20247M941 (4) | |
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. SAFETY EVALUATION BY THE OFFICE OF NUCI FAR REACTOR REGULATION i
l RFI ATED TO AMENDMENT NO.161TO FACILITY OPERATING LICENSE NO. DPR-28 VERMONT YANKFF NUCI FAR POWER CORPORATION VERMONT YANKEE NUCI FAR POWER STATION DOCKET NO. 50-271
1.0 INTRODUCTION
The Vermont Yankee Nuclear Power Station is a boiling water reacL (BWR), model BWR-4, with a Mark I containment. By letter dated March 20,1998, the Vermont Yankee Nuclear.
Power Corporation, the licensee for the Vermont Yankee Nuclear Power Plant, submitted for Nuclear Regulatory Commission (NRC) staff review a proposed change to the licensing bases described in the Final Safety Analysis Report (FSAR). This change involves a licensee-
' imposed limit on opening certain primary containment isolation valves. The valves would be open for no more than 90 hours0.00104 days <br />0.025 hours <br />1.488095e-4 weeks <br />3.4245e-5 months <br /> per calendar year in order to minimize the possibility of overpressurizing the standby gas treatment system (SGTS) if a large break LOCA should occur while purging the primary containment.
Vermont Yankee Licensee Event Report (LER) 97-005-01, dated September 5,1997 reported l
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that the potential existed at Vermont Yankee Nuclear Power Station to overpressurize the standby gas treatment system (SGTS) filter train housings during inerting or deinerting operations at power in the event of a loss-of-coolant accident (LOCA). Although the SGTS is designed to isolate automatically on a primary containment. isolation signal, the stroke times of
- the isolation valves were such that overpressurization of the SGTS was predicted. The licensee has established administrative controls to prohibit the system alignments which could result in overpressurizing the SGTS, given a LOCA. However, these restrictions reduce operational flexibility. The licensee points out in the March 20,1998 submittal that there are important advantages to maintaining the capability to vent and purge the containment during modes of operation in which containment isolation is required. Providing the capability to purge j
and vent (inert and deinert) the primary containment during these periods enables personnel a
entry for maintenance and inspection of equipment and the capability to verify that "all RCS boundaries that have been interrupted during the refueling outage have been retumed to an Y
operable condition."
j in accordance with existing NRC guidance (Standard Review Plan Section 6.2.4, " Containment I
isolation System"), the licensee proposed to limit vent and purge operations to no more than 90
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hours per year. This limit would be located in the technical specifications bases and FSAR.
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-2.0 EVALUATION
. Vermont Yankee LER 97-005-01 reports that on March 24,1997, the licensee determined that the potential existed to overpressurize the SGTS filter train housings, should a LOCA occur l
while the primary containment was being inerted or deinerted using the SGTS. Amendment 91, dated October 28,1985, specifically included a consideration of a LOCA occurring during inerting and deinerting activities in the licensing basis. The licensee performed a radiological dose calculation which demonstrated that all regulatory limits would be satisfied given this event.
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However, although the SGTS is designed to isolate automatically from the primary containment on a LOCA signal, LER 97-005-01 reported that the licensee determined that the valve stroke times were such that there was a potential for SGTS overpressurization because the LOCA may generate a pressure pulse of such a magnitude and velocity that the isolation valve could not prevent overpressurization of the SGTS. The design pressure of the SGTS housing is 2 psig.
. Vermont Yankee technical specifications permit inerting and deinerting of the primary containment for no more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> following startup and prior to a shutdown, Thus, the
. time period during which this situation could occur is limited. The licensee put in place administrative controls to prohibit opening of the torus and drywell 18-inch valves.
Further analyses by the licensee, as discussed in LER 97-014-0, Revision 1, showed that over-pressurization of the SGTS housings could also occur during purging and venting activities utilizing a smaller (3-inch) drywell vent bypass valve (AC-6A).
In order to regain operational flexibility to inert and deinert the primary containment at power, the licensee proposes to limit the allowable time during a calendar year that these operations can be performed to 90 hours0.00104 days <br />0.025 hours <br />1.488095e-4 weeks <br />3.4245e-5 months <br />.
An NRC letter to the' Yankee Atomic Electric Company, referring to docket 50-271' (the Vermont Yankee Nuclear Power Station), dated November 29,1978f provided guidance on the use of containment purge and vent systems during modes of operation in which containment integrity is required. This letter discussed operating reactor problems with purge and vent operations that could compromise containment integrity and specified actions licensees should take to avoid these problems. For licensees proposing limited purging of containment, as an attemative to a radiological dose analysis, this guidance limited purging and venting to 90 hours0.00104 days <br />0.025 hours <br />1.488095e-4 weeks <br />3.4245e-5 months <br /> per calendar year, as prescribed in Standard Review Plan Section (SRP) 6.2.4, Containment isolation System and Branch Technical Position 6-4, Containment Puroina Durina Normal Operations By limiting use of the purge and vent paths to 90 hours0.00104 days <br />0.025 hours <br />1.488095e-4 weeks <br />3.4245e-5 months <br /> per calendar year, the licensee is complying with this guidance.
- The November 29,1978, letter also requested that justification for limited purging include a j
demonstration (by test or analysis similar to that specified in SRP Section 3.9.3) of the ability of the purge and vent valves to close under postulated accident conditions, in a May 22,1984, NRC letter to the Vermont Yankee Nuclear Power Corporation, the staff concluded that the t'.
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licensee had demonstrated the ability of Vermont Yankee's containment purge and vent valves to close against the buildup of containment pressure in the event of a design basis LOCA. The conclusion was based on valves V16-19-7 and V16-19-7a being blocked at 50 degrees open.
This restriction on opening is specified in Vermont Yankee technical specification Table 4.7.2a and discussed in Basis section 4.7.D. The licensee has not proposed changing these technical specificat'ons.
The November 29,1978, letter also requested that the 90-hour limit on purging be included in the plant's technical specifications. The licensee has proposed instead that the 90-hour limit be placed in the technical specifications bases (3.7.A and 3.8.L) and FSAR. The staff finds this an acceptable location for the limit. The improved Standard Technical Specifications for BWR-4s, NUREG-1433, Revision 1, do not specify a time limit for purging operations. Rather, permissible reasons for purging are specified.
In the NRC staff's Final Safety Evaluation Report for the Advanced BWR (ABWR) design
- (NUREG-1503, Volume 1, July 1994) the staff stated that an analysis demonstrating that the -
. use of tne SGTS during inerting, deinerting, pressure control or purging of the primary containment during normal plant operation will not impair its functional capability during a
' design basis accident was not necessary provided the use of the SGTS during power operation is limited to no more than 90 hours0.00104 days <br />0.025 hours <br />1.488095e-4 weeks <br />3.4245e-5 months <br /> per year. No technical specifications were needed to address this limit.
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Therefore, the staff finds the licensee's proposal to include a limit in the FSAR (and technical specifications bases) to be consistent with staff guidance and positions with respect to technical specifications and is, therefore, acceptable.
As noted above, Vermont Yanken LER 97-014-01 pointed out that not only could the SGTS be.
damaged by a pressure pulse and flow through the 18-inch purge valves, but also by flow through the drywell 3-inch vent valve. The bases state that when this valve (AC-6A) is open, other valves will be closed by procedure to prevent overpressurization of the SGTS or the reactor building ductwork should a LOCA occur. The staff finds this to be acceptable since the procedural controls will keep SGTS isolated when the dryv!all 3-inch vent valve is open.
In summary, the licensee has taken appropriate steps to ensure that the risk of a large LOCA with concomitant overpressurization and failure of the standby gas treatment system is acceptably low. The licensee used previous NRC staff guidance to limit the time the
~ containment purge and vent valves may be open. Therefore, the NRC staff finds the licensee's proposal acceptable.
3.0 STATE CONSULTATION
in accordance with the Commission's regulations, the Vermont State official was notified of the proposed issuance of the amendment. The State official had no comments.
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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 NRC staff has i
determined that the amendment involves no significant increase in the amounts, and no
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- significant change in the types of any effluents that may be released offsite, and that there is no
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significant increase in individual or cumulative occupational 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 (63 FR 14976). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b),'no environmental 3
impact statement or environmental assessment need be prepared in connection with the l
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: Richard Lobel 1
Date: May 14, 1998
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