Information Notice No.80-30 – Potential for Unacceptable Interaction Between the Control Rod Drive Scram Function and Non-Essential Control Air at Certain GE BWR Facilities
SSINS No.: 6870
Accession No.:
8006190057
IN 80-30
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
August 19, 1980
Information Notice No. 80-30: POTENTIAL FOR UNACCEPTABLE INTERACTION
BETWEEN THE CONTROL ROD DRIVE SCRAM
FUNCTION AND NON-ESSENTIAL CONTROL AIR AT
CERTAIN GE BWR FACILITIES
Continued NRC evaluation of possible failure modes of the control rod drive
system related to the Browns Ferry event (IE Bulletin 80-17), has identified
a potentially significant single failure mechanism which could degrade the
scram function. The slow loss of non-essential control air pressure could
degrade the scram function at certain facilities which have relatively poor
communication between the scram discharge volume (SDV) and the scram
discharge instrumented volume (SDIV) such that the SDIV drains more rapidly
than the SDV drain rate into the SDIV.
The significance of such a single failure warrants transmission of an
interim assessment by the NRC Office for Analysis and Evaluation of
Operational Data as an early notification of a possibly significant matter
that is still under review by the NRC staff. It is expected that recipients
will review the information for possible applicability to their facility. No
specific action or response is requested at this time. If you have any
questions regarding this matter, please contact the Director of the
appropriate NRC Regional Office.
Enclosure:
Memo from C. Michelson to
H. Denton, dated August 18,
1980
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UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555
AUG 18 1980
MEMORANDUM FOR: Harold R. Denton, Director
Office of Nuclear Reactor Regulation
FROM: Carlyle Michelson, Director
Office for Analysis and Evaluation of
Operational Data
SUBJECT: POTENTIAL FOR UNACCEPTABLE INTERACTION BETWEEN THE
CONTROL ROD DRIVE SYSTEM AND NON-ESSENTIAL CONTROL AIR
SYSTEM AT THE BROWNS FERRY NUCLEAR PLANT
Since completing its analysis of the Browns Ferry 3 partial failure to scram
event, AEOD has been taking a closer look at the added (temporary) scram
discharge volume instrument arrangement in terms of its acceptability for
continued operation pending completion of the recommended system
modifications. Our evaluation is still ongoing, but one established
conclusion is that an immediate, in-depth evaluation is needed of the
potential for unacceptable interaction between the control rod drive system
and the non-essential (nonsafety) control air system.
At the recent meeting with General Electric relating to the Browns Ferry
event, we questioned GE concerning the scram inlet and outlet valves and how
they might 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 pressure decreased. 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 fill rate, the time
for operator action, and the alarms and indications to guide his actions.
We are already aware that the scram discharge volume drain rate is less than
the scram instrument volume drain rate so any substantial inleakage 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 loss of control air. In
fact, the alarm signaling low air pressure would undoubtedly cause the
operator's attention to be distracted from the control rod drive system.
Enclosure
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Harold R. Denton - 2 -
Thus, failures in the non-essential control air system which result in a
degraded 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, 1978
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 inward 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 control air depressurization and the timely manual scram
might have been the saving factors.
It appears essential that we obtain an adequate understanding of the control
rod drive unit response to a loss of control air pressure. It is an
immediate safety concern if the degradation of the nonsafety-related control
air system can lead to 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, at the same time, any
significant portion of the full power heat removal capability
(turbogenerator, main steam or feedwater systems) was lost for any reason
such as the pressure reduction or its consequences. Recirculation pump trip,
where available, could be timely and 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 protect 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 temporary basis with a
single ultrasonic 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 initiation. It is read locally by a
roving operator every
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Harold R. Denton - 3 -
30 minutes. We believe that these temporary scram discharge volume level
monitoring measures do not adequately protect against a loss of scram
function within a few minutes due to degraded air pressure.
In summary, we recommend that the margin of safety inherent in the licensed
design basis should be promptly restored, although improved temporary
measures and arrangements may be acceptable until permanent modifications
can be completed. For example, improved temporary measures might be
redundant level sensors on the scram discharge volume with control room
alarms and readout, or a dedicated operator might be used locally with
adequate control room communication and plant procedures covering the manual
scram function. The adequacy of any modifications, temporary or permanent,
should be confirmed by appropriate analysis, testing, and onsite evaluation.
Although our interim assessment and recommendation applies only to Browns
Ferry 3, it is likely that it also applies to the other units at Browns
Ferry and to many other BWRs. We will submit a full report for your
consideration when our evaluation work is completed.
/signed/
Carlyle Michelson, Director
Office for Analysis and Evaluation
of Operational Data
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