United States Nuclear Regulatory Commission - Protecting People and the Environment

Reactor Vessel Water Level Instrumentation in BWRS (Generic Letter No. 84-23)

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                           WASHINGTON. D.C. 20555 

                              October 26, 1984 

TO ALL BOILING WATER REACTOR (BWR) LICENSEES OF OPERATING REACTORS EXCEPT 
LACROSSE, BIG ROCK POINT, HUMBOLDT BAY AND DRESDEN-1) 

Gentlemen: 

SUBJECT:  REACTOR VESSEL WATER LEVEL INSTRUMENTATION IN BWRs (GENERIC LETTER
          NO. 84-23)  

The water level instrumentation in a BWR is relied upon for controlling 
feedwater, actuating emergency systems, and for providing the operators 
information which is used as basis for actions to assure adequate core 
cooling. Many of the actions in the emergency procedures guidelines are 
keyed to reactor water level. 

The NRC staff has reviewed the S. Levy, Inc. report SLI-8211, "Review of BWR
Reactor Vessel Water Level Measurement System," which was commissioned by 
the PWR Owners Group. The report provides a generic evaluation of water 
level instrumentation adequacy of BWR/2 through BWR/6 plants and identifies 
several improvements in BWR water level measurement and instrumentation 
systems which will improve the reliability and accuracy of those systems. 
The staff has concluded that changes identified in the emergency procedure 
guidelines are adequate for the short term, but permanent physical 
improvements should be made on a deliberate schedule to reduce the burden on 
the operator. Three potential improvement categories are presented below: 

o    Improvements to plant(s) that will reduce level indication errors 
     caused by high drywell temperature. These improvements include 
     prevention of reference leg overheating or reduction of the vertical 
     drops in the drywell. (Vertical drop should be measured. from the 
     condensation pot to the drywell exit point. Maximum drop would allow an 
     indicated level at the bottom of the normal operating range when actual 
     level is just above lower tap for worst flashing condition.) Those 
     plants for which the vertical drop in the drywell has already been 
     minimized will not have to make additional changes for the drywell 
     heating effect. 

o    Review of plant experience relating to mechanical level indication 
     equipment. Plant experience shows mechanical level equipment is more 
     vulnerable to failure or malfunction than analog equipment. A number of
     plants have already connected analog trip units to their level 
     transmitters to improve reliability and accuracy. Those plants that use
     mechanical level indication should replace the mechanical level 
     indication equipment with analog level transmitters unless operating 
     experience confirms high reliability. 

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o    Changes to the protection system logic that may be needed for those 
     plants in which operator action may be required to mitigate the 
     consequences of a break ir a reference leg and a single failure in a 
     protection system channel associated with an intact reference leg. 
     Changes will generally result in additional transmitters to satisfy the
     single failure criterion. This improvement, under evaluation by NRC, 
     may be needed in plants where an analysis has demonstrated a 
     vulnerability. 

Implementation of the first two categories of improvements will give 
increased assurance that the level instrumentation will provide the 
inadequate core cooling instrumentation required by NUREG-0737, Item II.F.2 
and thereby satisfy this requirement. Please submit within 30 days a 
description of your plans to implement these improvements and your proposed 
schedule. 

The last improvement is still being evaluated by the staff; hence it is not 
a requirement at this time. However, should the continuing evaluation show 
that there is a significantly high priority. It will be identified as a new 
generic letter. 

This request for information was approved by the Office of Management and 
Budget under clearance number 3150-0065 which expires September 30, 1985. 

                              Sincerely, 


                              Darrell G. Eisenhut, Director
                              Division of Licensing
                              Office of Nuclear Reactor Regulation
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