Resolution of Generic Issue 101 "Boiling Water Reactor Water Level Redundancy" (Generic Letter 89-11)

 June 30, 1989



This generic letter is being issued to:

(1)  inform you of the staff's resolution of the subject generic issue, and

(2)  request that you continue to maintain appropriate procedures and training 
     for the operators so that they may readily identify and mitigate the 
     consequences of a leak or break in a reactor water-level instrument line.

For resolution of Generic Issue (GI) 101, "Boiling Water Reactor [BWR] Water 
Level Redundancy," instrument line breaks coupled with an additional 
independent single failure in a control or protection system were evaluated.  
For the purposes of this evaluation, all BWR plant designs were placed into one
of five groups, based on their system characteristics.  A plant from each group
was then evaluated.

The reactor water-level measurement systems in BWRs consist of a reference 
water leg, a variable water leg, and a differential pressure measuring 
transmitter.  The reference water leg is connected to a condensing chamber and 
to the reactor vessel steam space.  The variable water leg is connected to the 
reactor vessel at an elevation below the expected normal range of water level. 
The actual water level in the reactor vessel is then determined by measuring 
the differential pressure between the reference water leg and the variable 
water leg.  These pressure sensors provide input to the protection systems and 
to the control systems.

The GI 101 concern is that a leak or break in the instrument sensing line that 
is connected to the constant head condensing chamber could cause the reference 
water leg level to decrease.  This decrease in the reference water leg level 
could cause all the differential pressure instruments connected to that line to
indicate a false high reactor water level.  Under these conditions, the 
feedwater system may automatically reduce the feedwater flow into the reactor 
vessel, causing the actual reactor water level to decrease.  The presence of 
this false high water level signal may also prevent automatic operation of the 
emergency safety systems, such as High Pressure Coolant Injection System/High 
Pressure Core Spray System, Reactor Core Isolation Cooling System, or Automatic
Depressurization System in some designs, and may confuse the operator when 
trying to assess the actual water level in the vessel.  

.Generic Letter 89-11                  -2-              June 30, 1989

The NRC staff has completed its evaluation of GI 101 and has concluded that all
BWR designs, in conjunction with operator training and procedures, provide 
adequate protection in the event of an instrument line break in any of the 
reactor vessel water-level instrument systems.  The staff believes that emer-
gency procedures for the operator to identify and mitigate the consequences of 
instrument line breaks exist at all plants and that the reactor operators are 
being trained to achieve safe shutdown, if needed.  The technical basis for 
this conclusion is documented in NUREG/CR-5112, "Evaluation of Boiling Water 
Reactor Water-Level Sensing Line Break and Single Failure."  NUREG/CR-5112 has 
been mailed to all BWR licensees and construction permit holders.

The NRC is, therefore, not proposing that the BWR licensees and applicants take
any action.  However, the staff is providing to all licensees and applicants of
BWR plants the results of the analysis conducted for this review.  This 
information is presented in NUREG/CR-5112.  Plant-specific design features, 
such as common sensing lines for the water-level instrumentation, automatic 
initiation logic for vital protection systems, inhibition of vital protection 
systems, and additional single failures of safety-related and non-safety-
related systems, were considered.  The results, including the value impact 
analyses of the alternatives considered for plant improvements for BWR plant 
designs, are provided for information.  Each addressee is expected to review 
the information to verify that the design of its facility has been correctly 
represented.  It may also be prudent for some licensees to consider a reassess-
ment of plant procedures and operator training to ensure that plant operators 
can readily detect and mitigate a leak or break of a sensing line.

No written response or specific action is required by this letter.  Therefore, 
no clearance from the Office of Management and Budget is required.

If you have any questions on this matter, please contact the technical contact 
listed below or your project manager.


                                   James G. Partlow
                                   Associate Director for Projects
                                   Office of Nuclear Reactor Regulation

Enclosure:  List of Recently Issued NRC Generic Letters

Technical Contact:  Tim Collins, NRR
                    (301) 492-0897
                    Andrew J. Szukiewicz, RES
                    (301) 492-3914


Page Last Reviewed/Updated Thursday, March 25, 2021