ML20054C977
| ML20054C977 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 03/11/1982 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20054C964 | List: |
| References | |
| NUDOCS 8204220191 | |
| Download: ML20054C977 (4) | |
Text
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UNITED STATES NUCLEAR REGULATORY COMMISSION h
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO A_MENDMENT NO. 73 TO FACILITY OPERATING LICENSE NO. DPR-39 C0410NWEALTH EDISON COMPANY ZION STATION, UNIT 1 DOCKET N0. 50-295 Introduction In October 1981, it became apparent to the Commonwealth Edison Company (CECO) that the primary to secondary leakage in the Zion Station Unit No. 1 steam generator (designated 1B) was increasing.
During November 1981, the rate of increase leveled off at abo'it 200 gallons per day (the Technical Specifications limit is 500 gallons per day from any one steam generator).
In December 1981, l
the rate again began to increase to just over 400 gallons per day in January.
On January 21, 1982, CECO submitted a request to remove the 500 gallon per day limit until the unit was shutdown for refueling in late Feuruary or early March 1982. The authorization to exceed the 500 gallons per day was issued by telephone on February 9,1982 and a new limit of 700 gallons per day from steam generator 1B was imposed until the Unit No.1 is shutdown for refueling about March 4, 1982.
This authorization was confirmed by letter dated February 9,1932.
dvaluation The 500 gallon per day (gpd) Technical Specification limit on primary to secondary leakage is intended to ensure that leaking steam generator tubes will maintain an acceptable level of tube integrity during normal operating and accident con-ditions.
500 gpd corresponds to the expected leakage, based upon test data, from a crack which is about 0.5 inches in length under 1500 psi differential pressure (AP) between the primary and secondary. The 500 gpd is a conservative limit for the case of single cracks in straight span portions of tubing.
Test data indicates that through wall cracks of lengths less 0.66 inches will not fail under postulated main steam line break (MSLB) conditions (differential pressure of 2560 psi). The leakage rate corresponding to a 0.66 inch long single crack is approximately 1 gpm (1440 gpd) under normal operating AP (1500 psi), again based on test data (July 29, 1977 submittal for Carolina Power and Light Company, Docket No. 50-261).
The licensee's proposal in his January 21, 1982 request for temporary change to the Technical Specifications would relieve the 500 gpd limit on the IB Steam generator through March 4,1982; the date for the scheduled refueling outage.
The licensee has proposed a 1 gpm or 1440 gpd limit on all steam generator leaks before taking the next action. With the time remaining between this licensing action and the March 4, 1982 date, the rate of increase in leakage to reach the 1440 gpd is unacceptable.
Based upon our projection of the leak rate increase which has been gradual and so as not to pennit steam generator tube degradation to proceed unchecked, we believe an upper limit of 700 gpd for steam generator 1B to be appropriate. To assure that the leak rate does not g4220191 820311 p
ADOCK 05000295 PDR
, increase at an unacceptable rate and the 700 gpd limit is not exceeded, the licensee has agreed to make iodine measurements in the primary and secondary coolant daily and to use a correlation of plotted leakage against the measured iodine to establish the new indicated leakage rate.
This correlation is accurate to within 50 gpd and requires only one hour to complete.
The licensee will also set the condensor air ejector monitor to the radiation level corresponding to 700 gpd. Whenever the alarm has sounded for one hour continuously (to avoid any peaks in the monitor detection), the operator will request that the iodine samples be pulled as-soon-as-possible.
In addition, the normal activity measurements requiring steam blowdown isolation for 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> will be performed twice a week rather than the usual once a week. This latter method of measuring leakage is considered more accurate but requires longer to accomplish. With these steps in place, we find the licensee's ability to follow the leakage in the 1B steam generator and to take the necessary action to be acceptable.
A possible cause for the current leakage at Zion is cracks in the inner row (s)
U-bends, based upon the past operating experience at this Unit.
Laboratory study of U-bend cracks in similar tubes removed from Trojan Unit 1 indicates that the Trojan U-bend phenomenon involves an array of crack segments, rather than isolated cracks, located at the transition between the U-bend and the straight span portion of tubing. The test data which supports the 500 gpd limit does not specifically address the integrity of tubing degraded by this type of cracking, which was only first observed during the examination of the Trojan tube specimens in 1980. We have requested that Westinghouse and the licensee develop a program to evaluate the leakage rate limit in the context of U-bend cracks.
Given that there is some likelihood that the current leakage at Zion 1 involves U-bend cracks, an increase in the current limit cannot be justified simply on the basis of tube integrity considerations. From an overall safety viewpoint, however, an increase in the leakage rate limit is justified for a short term period as is discussed below.
The primary concern to be addressed is the potential for a main steam line break (MSLB) with concurrent tube rupture (s). More specifically, the concern is for a MSLB occurring outside containment on the steam line from the 1B steam generator out to the first isolation valve. A break anywhere else is not so much a concern because the IB steam generator could be isolated from the break or the break inside containment would be contained there.
It has been our contention that the MSLB for pipes and valves would be preceeded by a leak (s).
Therefore, in order to assure integrity of the steam line and valves in question, the licensee was requested to review their inservice inspection (ISI) records and to periodically walk down this line and observe any indication of leaks or failures of mechanisms required to close the valves.
The licensee has reported that their ISI records indicate no problems on the piping and that their walk downs have revealed no indication of leakage or valve failures. The licensee has agreed to continue the walk downs daily. On this basis, we find that a MSLB in the 1B steam generator steam line out to and including the first isolation valves'outside containment to be a very remote possibility, particularly during the very short time interval (less than 4 weeks) prior to the scheduled shutdown.
N
3-Another concern to be addressed is the anticipated off site radiological releases in the highly unlikely event a tube should fail during the current operating condition of the plant.
In discussions with the licensee, CECO has detemined l
from the reactor coolant and secondary coolant activity that should a tube break, l
the exposure at the site boundary would be no more than 4.5 mrem whole body.
This exposure was determined by the equation from the Zion Technical Specification concerning steam generator tube rupture.
This Technical Specification limit on i
j coolant activity for Zion was developed on the basis of limiting exposure to less than 1 Rem whole body exposure at the site boundary. The CECO calculations would.
indicate that there is considerable margin between the Technical Specification exposure basis and the expected exposure should a tube rupture occur at this time.
We have also been informed by the licensee that the iodine activity in the coolant at this time is approximately.005 uc/gm of I-131 equivalent.
The limit from the Westinghouse Standard Technical Specifications is 1.0 pc/gm, therefore, the iodine levels in the Zion coolant are also of little concern.
On the basis of potential l
consequences from a steam generator tube rupture in the Zion Station Unit No.1 during this fuel cycle, such a failure would not be desirable but potential exposure j
would be well within the levels for which the plant has been designed and the limits i
upon which the Technical Specifications are based. The operators have also familiarized themselves with the emergency operating procedures to assure any action necessary will limit any offsite releases.
The Technical Specification limits 4
associated with main steam line break are not affected by the proposed one time l
change. This limit is 1 gallon per minute from all steam generators.
The staff has determined that based upon the very short time interval to which the authorization to exceed the 500 gpd limit would apply, the improbability i
of an accident (e.g., MSLB) and concurrent tube rupture (s), and the limited l
consequences should a tube rupture we find that continued operation for a limited period with leakage rates in steam generator 1B in excess of 500 gpd i
but limited to 700 gpd does not represent an undue risk to public health or safety.. The measures being taken by the licensee to assure the integrity of of the steam lines and isolation valves, to measure the leakage rate daily by i
iodine measurements and twice weekly by blowdown isolation, and to alarm the condensor air ejector monitor at the 700 gpd indicated activity are acceptable to limit tube degradation.
Environmental Consideration We have determined that the amendment does not authorize a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact. Having made this' determination, we have further concluded that the amendment involves an action'which is insignificant from the standpoint of environmental impact and, pursuant to 10 CFR 551.5(d)(4),
that an environmental impact statement or negative declaration and environmental impact appraisal need not be prepared in connection with the issuance of this amendment.
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, Conclusion We have concluded, based on the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the probability or consequences of accidents previously considered and do not involve a signifi-cant hazards consideration, (2) there is reasonable assuranc. that the health and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in compliance with the Commission's regulations and the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
MAR 11 1982 Date:
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