ML20079N872
| ML20079N872 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 10/24/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20079N871 | List: |
| References | |
| NUDOCS 9111120061 | |
| Download: ML20079N872 (4) | |
Text
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UNITED STATES
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENPMENT NO. 165 TO FACILITY OPERATING LICENSE NO. NPF-3 TOLEDO EDISON COMPANY CENTER 10R SERVICE COMPANY
$!E THE CLEVELAND ELECTRIC ILLUMINATING COMPANY DAY 1S-BESSE NUCLEAR POWER STATION. UNIT N0d DOCKET NO. 50-346 1.0 INTRODULTION By letter dated August 1,1989, the Toledo Edison Company (the licensee) requested changes to Technical Specification (TS) 3/4.4.10 Integrity, which would revise the surveillance interval for, Structural.
inyection and operability testing of reactor vessel internals vent valves (RVVYs) from "at least once per 18 months, during shutdown" to "during-shutdown for refueling." The proposed changes also delete the applicability of TS 4.0.2 from Surveillance Requirement (SR) 4.4.10.1.b. and delete an outdated Cycle Five specific footnote.
By letter dated February 1,1990, after further evaluation of the subject a sp11 cation, the licensee requested that the proposed-surveillance interval c1ange be revised to "at least once per 24 months during shutdown."' This revised wording is consistent with Amendment Number 108,-dated Septen6er 26,1988, issued by the NRC staff to Florida power Corporation's Crystal' River,. Unit 3(CR-3). The licensee's application of Augu:t 1,1989 also proposed a foctnote which deleted the applicability of TS 4.0.2.
The licensee has determined that this portion of the origina1' request no. longer applies and hereby withdraws it from the application. The. applicability of-TS 4.0.2 to this surveillance requirement.is also consistent with Amendment Number 108 for CR-3.
2.0 EVALVATION In 1975 the NRC revised 10 CFR 50.55a to require an Inservice Inspection (ISI) of various safety-related cumponents, including pumps and valves, to.
be performed in accordance with the ASME Boiler and Pressure Yessel Code, ON 6'
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-2 Section XI, "to the extent practical within the limitations of design, geometry, and materials of construction." The existing Davis Be:;se Technical Specification Section 4.4.10.1.b requires that the RVVVs be demonstrated operable at least once per 18 months with a provision that an extension of 251 may be granted for the 18-month period. The proposed amendment would change the surveillance intervr.1 for inspection and operability testing of the RYVVs from 18 months to 24 months.
The RVVVs are large swing check valves mounted vertically between the inlet and outlet sides of the core support shield. The core support shield directs cold leg (inlet) flow downward into the annular space ju;t inside the vessel and contains core outlet flow in the central portion, directing it upward to the hot leg nozzles.
The vent valve assemblies are installed 50 t1ey can swing outward into the cold leg water space should pressure on the outlet side of the core exceed inlet pressure.
Under nomal operating conditions, the vent valves are closed.
The RVVV materials were selected on the basis of their corrosion resistance, surface hardness, anti-galling characteristics, and compatibility with mating materials in the reactor coolant environment. The valve disc shaft, shaf t journals (bushings), disc journal receptacles, and valve, hinge body journal receptacles have been designed to withstand, without failure, the internal and external differential pressure loadings resulting from a loss-of-coolant accident. These valve materials are non-destructively tested and accepted in accordance with the ASME Code 111 requirements for Class A vessels as a reference quality level.
The design criteria for these valves included (1) functional intagrity, (2) structural integrity, (3) individual part-capture capability, (4) functional reliability, (5) structural reliability, and (6) leak integrity through the design life.
The RVVV hinge assenbly provides eight loose rotational clearances and two end-clearances to minimize any possibility of impairment of disc-free motion in service.
In the event that one rotational clearanc should bind in service, seven loose rotational clearances would remain to allow unhampered disc-free motion.
In the worst case, at least three clearances must bind or seite solidly to adversely affect the valve disc-f ree motion.
In addition, the valve disc hinge loose clearances permit disc self-alignment so that the external differential pressure adjusts the disc seal face to the valve body seal face. This feature minimizes the possibility of increased leakage and pressure-induced deflection loadings on the hinge parts in-service.
The RVVVs are fnstalled in the core support shield to prevent a pressure unbalance whi.
might delay or interfere with emergency core cooling following a poe.alated inlet pipe rupturei The arrangement consists of four 14-inch it. side diameter valve assemblies installed in the cylindrical wall of the internal core support shield. The RVVVs provide a direct path to vent steam in the upper plenum through the break following a postulated
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3 cold-leg rupture. The valves are required because the arrangement of the reactor coolant system (RCS) could delay the venting of steam generated in the core af ter the system is depressurized, if significant quantities of coolant remain in the reactor inlet piping at the end of the blowdown period. Without venting of the steam, the pressure in and above the core region could be greater than the pressure in the reactor vessel inlet annulus where emergency coolant is injected. This pressure differential could retard flow into the core. The RVVVs provide a flowpath from the region above the core directly to the pipe rupture location. This flowpath allows the pressures to equalize and permits emergency coolant water to reflood the core rapidly.
The licensee has stated that surveillance testing of each of the 4 RVVVs at Davis-Desse has been completed nine times since 1977, inspection after an resulting in a total of 36 exercises of the valves. The results of the last extended surveillance interval of 42 months demonstrated satisfactory operability.
The NRC staff has taken into consideration the record of the past tests of similar RVVVs, which represented about 480 inspections and exercises at other B&W facilities. The data presented by the licensee indicated that the RYVVs have demonstrated a high degree of reliability and no failures were found. The only degradation of the RVVVs was noted in November 1978 at two other B&W facilities. At that time, jackstrew locking mechanism wear was discovered in the RVVVs adjacent to the reactor vessel outlet nozzle.
The only problem found at Davis-Besse occurred in 1973 prior to operation, and involved the seizing of one jackscrew, which was attributed to an excessive thickness of electrolyze which spalled off the screw threads.
This problem was corrected and no further jackscrew problems have occurred.
The RVVV components that are vulnerable to corrosion include the shaf t bushing and the body. These parts are constructed of Type 431 martensltic stainless steel, stellite Num>er 6, and Type 304 austentic stainless steel, respectively. Data on the RCS hot operating conditions indicate that the general corrosion rates of these materials are in the range of.05 mils / year or less. The NRC staff has also evaluated the informatica relating to the RCS chemistry, the compatibility of the mating materials, their corrosion resistance, and the reactor coolant environment.
Since the corrusion deposit is about three times the rate of corrosion, the expected thickness of the deposited material is about.15 mils / year.
It is estimated that the minimum cold clearance gap dimensions vary from 3 to 60 mils, thus there would be sufficient gap so as not to hinder valve operation prior to the next RVVV test.
Information is available from TMI-1 and Davis-Besse that indicates that the.
RVVVs are capable of remaining operable for unusually long intervals between tests.
THI-1 experienced one test interval of 37 months, and Davis-Besse experienced an interval of 42 months, and in each case the valves remained operable. Using the corrosion rates and clearance gap ineasurements discussed previously, corrosion would not close the gap and hinder valve operation even during such extended test intervals.
4 B
The operating experience with the RVVVs in the industry, supported by the past performance of these valves at Davis-Besse, has shown that they will remain operable for periods greater than 18 months.
The staff has reviewed the proposed change to TS 4.4.10.1.b and finds that it would be acceptable to extend the surveillance interval for inspection and l
o>erability testing of the RVVVs at Davis.Besse to 24 months.
In addition tie application proposes the deletion of an outdated footnote in TS 4.4.10.1.b.
The footnste allowed a one-tine extension of the RVVis' Surveillance Requirement which was to be perforced no later than the fifth refueling outage.
Since this requirement has been complied with and the time f rame has expired, the footnote is no longer applicable. The staff i
finds this deletion to be acceptable.
3.0 STATE CONSULTATION
in accordance with the Commission's regulations, the Ohio State official was notified of the proposed issuance of the amendment.
The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
This amendment changes a surveillance requirenent. The staff has detennined that the anendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released off-site, and that there is no significant increase-in individual or cumulative occupational radiation exposure. The Commission has previously issued a.
proposed finding that the amendnent involves no significant hazards consider-ationandtherehasbeennopubliccommentonsuchfinding(55FR6121).
Accordingly, the amendment neets the-eligibility criteria for categorical
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exclusion set forth in 10 CFR 51.22(c)(9).
pursuantto10CFR51.22(b),
no environnental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
5.0 CONCLUSION
The staff has' concluded, based on the considerations-discussed abo've, thatt (1) there is reasonable assurance that the health and-safet not be endangered by operation in the stuposed manner, (2) y of the public Wil such activities i
. will be conducted in compliance with tie Connission's regulations, and (3) the 4
issuance of this amendment will not be inimical to the' common defenso and security or to the health and safety of the public, principal Contributor: 4.-Lombardo Date: October 24, 1991 i
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