ML20070A039
| ML20070A039 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 01/10/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20070A034 | List: |
| References | |
| NUDOCS 9101170072 | |
| Download: ML20070A039 (4) | |
Text
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UMITED STATES l
8 NUCLEAR REGULATORY COMMISSION -
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- I WA$HINGTON. D. C. 20556 ENCLOSURE 2 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO.187 TO FACILITY OPERATING LICENSE NO. DPR-52 TENNESSEE VALLEY AUTHORITY BROWNS FERRY NUCLEAR PLANT, UNIT 2 DOCKET NO. 50-260
1.0 INTRODUCTION
Tennessee Valley Authority (TVA or the licensee) submitted a request on July 13, 1990 to change the Browns Ferry Nuclear Plant (BFN) Technical Specifications (TS). The NRC staff reviewed the TS amendment request (TS 290) and concluded that additional information was necessary to complete our evaluation. By letter dated Avaust 17, 1990, the NRC requested that TVA furnish the additional information. TVA responded to the NRC's request by letter dated September 17, 1990.
2.0 BACKGROUND
The Reactor Core isolation -Coo?ing (RCIC) and High Pressure Coolant Injection (HPCI) turbines use steam supplied from the reactor.
Excessive steam flow and local area high temperature measuring instruments are used to detect a steam line break and ensure automatic closure of each system's primary containment isolation valves. This closure prevents excessive loss of reactor coolant _and the release of significant amounts of radioactive material from the nuclear system process barrier. The requested TS changes only affect the temperature measuring instrument channels, not the flow measuring instruments, used to detect a steam line break in the RCIC or HPCI systems. The licensee does not propose to change the existing surveillance intervals for either of the functional or calibration tests.
3.0 EVALUATION The licensee has utilized computer modeling techniques to predict the tempera-ture response of various reactor building zones during a steam line break. The licensee's study identified temperatures below the present TS setting of 200'F that could be present for various RCIC and HPCI line break scenarios. There-fore, the licensee proposas to change the temperature setpoint in the TS.
The RCIC and HPCI systems each has four sets of four bimetallic temperature measuring instruments located in the areas along the path of the steam supply piping of each system. The sixteen switches from the temperature instruments for each system are arranged into two divisional trip logic schemes with eight temperature switches in each division.
The trip logic fcr each division is i
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arranged in a "one-out-of-two taken twice" logic configuration for each of the four reactor building areas being monitored.
The following are the creas and new temperature setpoints:
Number of Instruments o
RCIC pump room 180*F or less 4
o RCIC torus 3 areas 155'F or less 12 (4 per area) o HPCI pump room 200*F or less 4
o HPCI torus 3 areas 180*F or less 12 (4 per area)
The licensee does not propose to chanor the existing surveillance frequency of functional test (which is once a mont..) or calibration test (which is once every threemonths). However, there is a proposed change regarding the action to be taken for an inoperable instrument / channel. The present action statement is:
A.
Repair in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
If the function is not operable in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, take action B.
B.
Declare the system or component inoperable.
The new minimum nunter of operable channels required would be "two" for each of two trip systems.
TVA proposes to replace A and B (above) for HPCI and RCIC instrument temperature channels with a new note, 1.E for Table 3.2.B which states: "Within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> restore the inoperable channel (s) to OPERABLE status or place the inoperable channel (s)inthetrippedcondition." The NRC requested that TVA justify the impact on safety for not having a time limit on how long an inoperable channel is placed in the tripped condition. TVA's response to the NRC's concerns stated:
l Placing a channel in the tripped position restores the isolation function to a single failure tolerant condition.
Although tripping the channel does result in a condition where a spurious actuation in another channel could prevent the function of the associated system, l
this condition is consistent with the design of the HPCI and RCIC systems since neither of these systems are themselves single failure proof.
The NRC staff agrees that neither the HPCI or RCIC systems have redundant counterparts; however, HPCI and RCIC systems can furnish high pressure coolant to the reactor vessel; furthermore, the HPCI s which is the Automatic Depression System (ADS)ystem has a diverse counterpart, and Low Pressure Coolant Injec-tion System (LPCI) or Core Spray System (CSS). Placing a safety system in a less reliable condition for an extended time period cannot be justified by the fact that the system is diverse rather than redundant.
When the channel is placed in the tripped condition, an inadv~ tent trip of any one of eight temperature detectors of two other channels will cause isolation of the steam supply to the respective turbine drive, thereby making the system inoperable. The probability of a steam line pipe rupture in which isolation would be required is far lower than of plant transient conditions in which it
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may be necessary to provide water to the reactor core. Although TVA's proposed TS change does not place a time limit on when or how long a channel is placed in the tripped condition, it is ia agreement with the GE Standard Technical Specification (STS) for Boiling Water Peactors (BWR/4) and provides for reliable isolation. Extended time in trip, however, could reduce the reliability of HPCI or RCIC. We note that this situation is generic and reconnend that TVA evaluate this issue further. We expect that should this situation develop, there would be sufficient time to evaluate plant conditions and determine whether alternative actions are necessary. However, we recognize that this observation is beyond the scope of this safety evaluation.
The licensee discussed spurious actuation is follows:
"The setpoints are established above the maximum expected room temperatures to avoid spurious action due to ambient conditions and below the analytical limits to ensure timely pipe break detection and isolation." The 'icensee's analysis did not address the effect of normal cooling lost in th+ ;nonitored areas, rate of temperature rise, or area temperature alarm setpoints to preclude spurious steam line supply line isolation. The staff requested that the licensee address this concern. TVA provided the following response:
Substantial margin (at-least 35"F) exists between the maximum abnormal temperature expected in each area and the minimum actuation temperature detennined for each temperature switch. The maximum temperature expected could occur as a result of outside temperature excursions; temporary, greater than design heat loads;- or degraded environmental control system operation. With the substantial margin between maximum abnormal tempera-tures for the areas and the minimum actuation temperature of the switches,
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the maximum abnormal temperatures cannot result in actuation of the switches.
TVA has changed-the column of Table 3.2.B from " Trip Level Setting" to " Allow-able Value" for the temperature values. This type of change is consistent L
with the future STS; however a definition for allowable value will also be in I
the future STS. TVA will establish trip settings in plant instructions to ensure that the allowable values are not exceeded. The trip settirgs will also reflect a margin for instrument drift and inaccuracies.
4.0 ENVIRONMENTAL CONSIDERATION
This amendment involves a change to a requirement with respect to the installation or use of a facility component located within the restricted area as defined in 10 CFR Part.20 and changes.to the surveillance requirements.. The staff has l.
determined that the-amendment involves no sig'nificant increase in the amounts, and no significant change in the _ types, of any effluents that may be released offsite, and that.there is no si occupational radiation exposure.gnificant increase in individual or cumulative The Connission has previously issued a proposed finding that this amendment involves no significant hazards considera-tion and there has been no public comment on such finding.
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 impact statement nor environmental assessment need be prepared in connection with the L
issuance of the amendment.
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5.0 CONCLUSION
The staff finds the lwer isolation system temperature detector setpoints and allowable TS values for HPC1/RCIC turbine supply steam line rupture are acceptible. These settings were also selected to r eclude inadvertent action from transient conditions not associated with a steam line rupture.
It is recomended that TVA evaluate its practice of placirg 6 inoperable channel in the tripped condition for extended periods, be:aust of the potential for a temperature detector failure to adversely affect RC'C/HPCI system relia-bility.
The Comission made a proposed determination that the amendment involves no significant hazards consideration which was published in the Federal Register (55FR36352)onSeptember5 1990 and consulted with the State of Alabama.
No public coments were received and the State of Alabama did not have any coments.
The staff has concluded, based on the considerations discussed.bove, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation In the proposed manner, (?) such activities will be conducted in compliance with the Comission's regulatiuns, and (3) the issuance of the amendments will not be inimital to the comon defense and security nor to the health and safety of the public.
Principal Contributor:
F. paulitz Dated:
January 10, 1991 o
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