ML18025B883
| ML18025B883 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 08/18/1980 |
| From: | Michelson C NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML18025B884 | List: |
| References | |
| AEOD-E007, AEOD-E7, NUDOCS 8210120129 | |
| Download: ML18025B883 (5) | |
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UNITED STATES N~
LEAR REGULATORY COMMISSION WASHINGTON. O. C. 20555 AUG 18 1SBO AEOD/E007 Qo
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SUBJECT:
Harold R. Denton, Director Office of Nuclear Reactor Regulation Carlyle Nichelson, Director.
Office for Analysis and Evaluation of Operational Data POTENTIAL FOR UNACCEPTABLE INTERACTION BETWEEN THE CONTROL ROD DRIVE SYSTEl~i AND NON-ESSENTIAL CONTROL'IR SYSTEM AT THE BROMNS FERRY NUCLEAR PLANT Since completing its analysis of the Browns Ferry 3 partial failure to scram
- event, LEOD has been taking a closer look at the added (temporary) scram dis-charge volume instrument arrangement in terms of its.acceptability for con-tinued. operation pending completion of the recommended system modifications.
Our evaluation is still ongoing, but one established conclusion is ihat an imIIediate, in-depth evaluation is needed oi the potential for unacceptable interaction between the control rod drive system and the non-essential (non-;
safety) control air system.
At.he recent meeting with General Electric relating to.the Browns Ferry event,
.we questioned GE concerning the scram inlet and outlet valves and how they mi:ght respond to a slow loss of control air pressure.
Their answer indicated that these valves would drift open slowly, 'but without early indication, as the air
'ressure decreas'ed.
In other words, the loss of air pressure would lead to a significant scram discharge volume inleakage, but the control rods might not move until the air pressure decreased substantially (with even greater inleakage).
The immediate concern is associated with this degraded air pressure situation and is focused on the scram discharge volume fillrate, the time for operator action, and the alarms and indications to guide his actions.
Me are. already aware that the scram discharge volume drain rate is less than the scram instrument volume drain rate so any substantial in Ieakage may go undetected by the existing (permanent) alarm or 'scram instruments.
- Further, there is no position indication for the scram valves, other than full-open, nor is there direct scram protection on partial los's of control air.
In fact, the alarm signaling low air pressure would undoubt-edly cause'the operator's attention to be distracted from the control rod drive system.
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..Harold R. Denton Thus, fail~res in the non-essential control air system which result in a
deoraded air pressure could result in a significant and undetected increase in the scram discharge volume inleakage.
In fact, an inleakage rate of 3
gpm per drive may exist before control rod movement is experienced.
With:
this. flow rate, the operator may have only a few minutes before the scram volume has filled to such an extent that the scram function could be adversely
- affected, and eventually even prevented..We believe the one alarm which might actuate (control rod drive high temperature alarm) would not be a
sufficiently timely or good indication of what is happening to assure proper operator action.
An event related to this concern occurred at Browns Ferry on August 18, 197S when a massive loss of control air pressure to the entire plant was experienced (PNO-78-147).
Units 1
and 2 were operating at full power.
Their control. rods drifted'nward as a result of low air pressure..
Both units were manually
.(and successfully) scrammed in accordance with emergency procedures.
This might have been a close call since the scram discharge volume was certainly filling as the rods drifted in before the manual scram.
The actual rate of cootrol air dept essurization and the timely manual scram might have been the saving factors.
r It appears essential that we obtain an adequate understanding of the control rod drive unit response to a loss of control air pressure.
It i's an irim'ediate safety concern if the degradation of the nonsafety-related control air system can lead b) a loss of scram capability before the situation is diagnosed by the operator
'and manual scram achieved.
It. should be apparent that. the situation would be precarious if a loss of 'control air pressure lead to an inability to scram from
. full.power and, a0 the same time, any significant portion of the full power heat removal capability (turbogenerator, main steam or feedwater systems) was lost for any reason sucb as the pressure reduction or its consequences.
Rec'irculation pump trip, where available, could be timely a'nd helpful, but not adequate.
Boron injection would probably be too slow.
In any case, this is an unacceptable
..interaction between a safety and nonsafety system.
The licensed design basis for Unit 3 was redundant level sensing capability.,
and appropriate provision for scram initiation, to prospect against excessive water in the scram, discharge volume.
The AEOD recommended modifications contained in our report on the BF-3 partial failure to scram would restore this design basis and would protect against degraded air system pressures (provided the control rod drives are not adversely affected by any heatup prior to scram actuation).
However, Unit 3 is being allowed to operate on a'emporary basis with a'ingle'.
ultrasoric'detector to indicate accumulated inleakage to the scram discharge volume This single detector does not indicate or alarm in the control room and provides no scram iniiiation. It is read locally by a r oving operator"every
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Harold R. Denton e
30 minutes.
lie believe that these te~iorary scram discharge volume lovel monitoring measures do not adequately protect agains't a loss of.sera<
function i ithin a few minutes due to degraded air pressure.
1n sugary, we recomi-:end that the margin of safety inherent in the licensed design basis should be promptly restored, although improved te;;;porary measures and arrangements may be acceptable until pemanent.modifications can be co.;deleted.
For exa-.pie, improved te-;porary m asures'ight be redundant level sensors on the scram discharge volume with control room alarms and
- readout, or a dedicated operator might be used locally
> ith a'dequate. control rom ccrmnication and plant procedures covering the manual scrar;. function.
The adequacy of any modifications, temporary or permanent, should be confiri;,ed
'y appropriate analysis, testing, and onsite evaluation:
4ltho>>gh our interim assessment and recommendation applies only to Bro>es
.Fel"ry, it is likely that it also applies to.he other units at Browns Ferry and to nany other B!!Rs.
He will submit a full report for your consideration
<ken our evaluation wort: is completed.
Orig~ Signed by Qer3yle)d.chelson Carlyle llichelson, Director OTTice for Analysis and Evaluation of Operational Data cc0 C. Ber1inger R. Bernero E. Jordan p
Dllcks J.
Shea Y. Stello D. Eisenhut N. Hoseley