United States Nuclear Regulatory Commission - Protecting People and the Environment

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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