ML20056D896
| ML20056D896 | |
| Person / Time | |
|---|---|
| Issue date: | 09/08/1992 |
| From: | Mccracken C Office of Nuclear Reactor Regulation |
| To: | Charemagne Grimes Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20056D898 | List: |
| References | |
| NUDOCS 9308180240 | |
| Download: ML20056D896 (4) | |
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September 8, 1992 MEMORANDUM FOR:
Christopher I. Grimes, Chief Technical Specifications Branch Division of Operational Events Assessment FROM:
Conrad E. McCracken, Chief Plant Systems Branch Civision of Systems Technology
SUBJECT:
CLARIFICATION OF IMPROVED STS CONTAINMENT REQUIREMENTS In response to your memorandum dated August 14, 1992, we are hereby providing our position on two issues that have been raised as part of the standard technical specification (STS) improvement program.
The issues concern containment air locks and drywell vacuum breakers.
Containment Air Locks Your question involves a note in the required actions for inoperable containment air locks which allows passage (entry and exit) through an air lock with an inoperable door for up to 7 days.
The required action for an inoperable air lock door is to lock the operable door closed, whereupon plant operation may continue indefinitely.
Normally, the operable door would have to remain locked closed to assure containment integrity, but the note allows an exception when both air locks (most plants have two) are inoperable and containment entry would otherwise not be possible.
For a limited period (7 days), the note allows passage through an airlock with certain restrictions.
The problem is, what should those restrictions be? The proof-and-review version said "to perform activities related to Technical Specification j
systems." We agree with your assessment that this is too vague to effectively implement.
The revised words, "to perform activities requireo by Technical Specifications," is certainly restrictive enough, but the Owners Groups think it is too restrictive.
We should remember that nearly all PWRs operating today are not allowed to open the locked-closed operable door at all, and those few that may are only allowed to open it if it is the outer door that is locked closed and they must open it in order to get to the inner, inoperable j
door to make repairs. The improved STS also allow this via a separate footnote.
Some operating BWRs are allowed by their TS to enter containment through an inoperable air lock for up to 7 days with no other restrictions, presumably to facilitate completion of actions necessary to allow continued plant operation until the air lock is fixed.
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h e Opening the operable door, even momentarily, breaches containment integrity with a potentially very large hole.
For this reason, we want to minimize door opening.
Licensees should not be allowed to use an inoperable air lock as if it were business as usual.
Therefore, some kind of restriction is necessary.
We believe that the revised words men u. 3d above, that is, "to perform activities required by Technical Spec.fio. ions," are not too restrictive and should be used.
We consider these words consistent with the safety significance of maintaining containment integrity.
Also, it is true that, once a containment entry has been made in accordance with this restriction, personnel may remain inside containment as long as they wish and conduct any other desired activities before finally exiting containment, without i
compromising safety.
Thus, this STS is perhaps not as restrictive as the Owners Groups think.
Drywell Vacuum Breakers Before we can talk about actions and the timing of these actions, we must clearly understand the safety concern. To do this, let us briefly go through the valve's purpose and the event of an SRV firing and how it affects the vacuum breaker operation.
The concern is restricted to the vacuum breakers between the drywell and wetwell. They are necessary to return the drywell atmosphero that was transferred during a blowdown from the drywell.
For most events, all of the original air would have been carried over to the wetwell.
However, after the blowdown has ceased, continued steam condensation on the drywell heat sinks will reduce tne drywell pressure to less than the wetwell pressure. At this point, the breakers will open to replace the condensed steam with air and maintain near equal pressures.
However, if one or more breakers were open at the beginning of the blowdown, tne steam would bypass the suppression pool and quickly overpressurize the containment.
It is for this reason that we are sensitive to any transient that has the potential of opening the vacuum breakers.
SRV operation is one of these events.
Steam discharge into the suppression pool has the potential of pressurizing the wetwell due to general heatup of the wetwell atmosphere from the heated pool or direct steam leakage into the wetwell atmosphere.
Of course, the latter would only result due to some sort of piping failure, but cannot be discounted.
i i Therefore, if there is an SRV discharge into the suppression pool, the j
immediate concern is to check to determine if there are any vacuum breakers open or not securely seated.
This should occur almost immediately.
- However, recognizing that the opening of the breaker could occur long after the SRV has reseated, a two hour TS time limit is reasonable.
If the position indicators (limit switches) show one or more breakers to be j
off their seats, then we allow 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to demonstrate via drywell pressurization that it is an instrumentation problem and not a breaker problem.
If it is a breaker problem, the plant should be shutdown in an orderly fashion to make the necessary repairs.
Having established that all vacuum breakers are properly seated, the need for an operability check is in order.
This is called for in case the event caused damage to one or more vacuum breakers.
Operability is demonstrated if the valve passes the stroke or cycle test. There is no need to perform this test in haste since it is more precautionary than to detect an immediate concern.
This is because the plant was designed to accommodate at least one failed closed breaker.
Therefore, for a plant like Fermi to perform this test within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> is unnecessarily rapid.
However, since the operability test can be performed remotely, the tests can easily be accomplished within even this short time interval.
As for the Mark Ills, they should also be included in the need for the tests.
The only difference is the potential of the event to open the valves.
The larger wetwell volume will result in a smaller pressure response.
- However, i
one cannot completely discount valve movement.
Therefore, we think the surveillance requirements should be restored to the improved STS, both BWR/4 and BWR/6, as shown in Enclosure 5 to your memorandum (including Options 1, 2, and 3), with Option I changed to read as follows:
Within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> after any discharge of steam to the suppression chamber from the safety-relief valves or any operation that causes the drywell-suppression chamber differential pressure to be reduced by > [0.5] psid.*
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- (Footnote) If position indicating instruments indicate that one or more vacuum breakers are not closed, verify by alternate means that each vacuum breaker is closed within the following 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />.
Originalsignedby Conrad E. McCracken, Chief Plant Systems Branch Division of Systems Technology cc: C. Moon l
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