ML19262C487
| ML19262C487 | |
| Person / Time | |
|---|---|
| Issue date: | 01/22/1980 |
| From: | Eisenhut D Office of Nuclear Reactor Regulation |
| To: | Mattson R Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8002140075 | |
| Download: ML19262C487 (4) | |
Text
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- As 2 21980 Roger J. Mattson, Director Division of Systems Safety Ocerating Excerience Memorandum No. 24 BYPASSING AND RESETTING OF ENGINEERED SAFETY FEATURE SIGNALS PROBLEM Several instances have been recorted where automatic closure of the containment ventilation / urge valves would not have occurred because tne safety actuation signals were either manually overridden or biccked during normal plant cperations.
Specifically, on July 25, 1978, Northeast Nuclear Energy Comoany discovered tnat intermittent containment curge coerations had been conducted at Millstone Unit No. 2 with the safety actuation isolation signals to both inlet and outlet redundant centainment isolation valves (43 inen butterfly valves) in the curge inlet and outlet penetrations manually overridden and inoperable. The isolation signals wnich are rer;uirec to autenatically close tne purge valves for containment integrity were manually overridden to alicw purging of containment with a high radiation signal present. The manual override circuitry designed by the plant's architect / engineer defeated the hign radiation signal anc all otner isolation signals to tnese valves. To manually override a safety actuation signal, tne a:erator cycles the valve control switen to tne closed position and tnen to the ocen cosition. This action energized a relay wnicn blocked tne safety signal and allcwec manual cceration indepencent of any safety actuation signal. This circuitry was designed to permit reopening these valves after an accident to allcw manual operation o# certain safety ecui; ment.
On Sectemcer 8,1978, the staff was advised that, as a matter of routine, Salem Unit No. I nas been venting the containment tnrough the c ntainment ventilation system valves to reduce pressure.
In certain instances this venting has occurred with tne containment hign : articulate radiation monitor isolation signal to the purge and pressure-vacuum relief valves cverridden. Override of the containment isolation signal was acccmolished by resetting tne train A and 3 reset buttons. Under these circumstances, six valves in the containment vent and purge systems could be acened wi a a nign particulate isolation signal present.
This override was
- e-for ed after verifying that the actual containment carticulate
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Roger J. Mattsc JAN 2 2 lcE0 levels were acceptable for venting. The licensee, after further investigation of this practice, determined that the reset of the particulate alarm also byoasses the containment isolation signal to tne purge valves and that the purge valves would not have automatically closed in the event of an emergency core cooling system (ECCS) safety injection signal.
In addition, many utilities do not have safety grade radiation monitors in use at their facilities.
It is our view that all instrumentation and control systems provided to initiate containment ventilation isolation should be designed and oualified as safety grade equipment.
Licensees of older plants have objected to the requirement that radiation monitors used to initiate containment isolation te safety grade as this was not a requirement at the time of their licensing review.
PRESUMED CAUSE These events resulted frcm procedural inacequacies, design deficiencies, and lack of proper management controls. These events also brought into question cne mechanical operability of the contair. ment isolation valves themselves.
SAFETY SIGNIFICANCE Isolation signals whicn are required to automatically close purge valves for containment integrity have been manually overridden, reset, or bypassed to allow purging of containment witn high radiation signals present. Manual override of safety actuation signals is a breacn of containment integrity. Containment integrity must be maintained during DBA-LOCA conditions.
In addition, the overriding of certain CVI signals could defeat other safety actuation signals and thus prevent valve closure when otner isolation signals are present. Also, other ESF circuits contain the same types of overrides (e.g., retentive memo ry ).
COR ACTION The avents cescribed above were determined by tne U.S. Nuclear Regulatory Commission (NRC) to be an Abnormal Occurrence (?78-05) and were, accordingly, reported to the U.S. Congress.
Roger J. Mattson JAN 2 21980 As a follow-up on this Abnormal Occurrence, the NRC staff is reviewing the electrical override aspects and the mechanical cperability aspects of containment purging for all operating power reactors. On November 28, 1978, the NRC issued a letter entitled " Containment Purging During Normal Plant Oceration" to all boiling water mactor (BWR) and pressuri:ed water reactor (PWR) licensees, requesting them to either cease all containment purging or to provide justification for continued purging at their facilities. Subsequently, an " Interim Position for Containment Purge and Vent Valve Operation Pending Resolution of Isolation Valve Operability" was forwarded to all licensees by letter dated Octcber 23, 1979.
The NRC staff review criteria for the safety signals to all purge and ventilation isolation valves and other ESF are listed in Enclosure A to this menorandum.
We are currently conducting reviews of licensee submittals regarding containment purge and other Engineered Safety Feature (ESF) Systems whicn have manual override capability.
RECCMMENCAT'CNS COR recommends that an assessment be made of the apolicability of the criteria listed in Enclosure A to this memoranoum to those plants currently under licensing review. It is our view that these criteria should be !=plemented in tne CSS review crocess and that compliance with tnese criteria by :ne licensee he required.
Darrell G. Ei ennut, Acting Director Division of Operating Reactors Conta:t:
R. Kendall, X23218
Enclosure:
Review Criteria
Enclosure A Review Criteria The NRC staff review criteria for the safety signals to all ourge and ventilation isolation valves are listed below:
Criterion No.1 - The overriding
- of one type of safety actuation signal (e.g., radiation) must not cause the blocking of any other type of safety actuation signal (e.g., pressure) to the isolation valves.
Criterion No. 2 - Sufficient physical features (e.g., key lock switches) are provided to facilitate adequate administrative controls.
Criterion No. 3 - The system-level annunciation of the overridden status is provided for every safety system impacted when any overr:de is active.
Criterien No. 4 - Diverse signals should be provided to initiate isciation of the containment ventilation system. Sceci fically, containment high radiation, safety injection actuation, and containment high pressure should autcmatically initiate CVI.
This is in conformance with Branch Technical Position 6.4 of Section 6.2.4 of the Standard Review Plan.
Criterion No. 5 - The instrumentation and control systems provided to initiate CVI should be designed and qualified as safety-grade equipment.
Criterien No. 6 - The overriding or resetting
- of the isolation actuation signal should not cause the autcmatic reccening of any isolation /ourge valve.
'The foiicwing cefinitiens are given for clarity:
Override - The signal is still present, and it is blocked in order to perform a function contrary to the signal.
Reset -
The signal nas come anc gone, and the circuit is being cleared in order to return it to the normal condition.