BWR Scram Discharge System; Clarification of Diverse Instrumentation Requirement (Generic Letter 81-18)

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

                               March 30, 1981 

TO ALL LICENSEES OF OPERATING BWRs AND APPLICANTS FOR BWR OPERATING LICENSES
(GENERIC LETTER No. 81-18) 

Gentlemen: 

SUBJECT:  BWR SCRAM DISCHARGE SYSTEM; CLARIFICATION OF DIVERSE 
               INSTRUMENTATION REQUIREMENT (Generic Letter 81-18) 

Reference:     NRC Staff Safety Evaluation Report (SER) on BWR Scram 
                    Discharge System dated December 1, 1980 

We are aware that efforts are underway to implement permanent modifications 
to the BWR Scram Discharge Systems that will comply with the criteria 
proposed by the staff in the referenced SER. 

As a result of our continuing review, the staff has revised its position on 
the requirement for diverse instrumentation. The following information is 
being provided to enable our revised position to be factored into your 
design modification efforts. 

Safety Criterion 3 of the staff SER (Reference) states: 

     "The scram discharge system instrumentation shall be designed to 
     provide redundancy, to operate reliably under all conditions, and shall 
     not be adversely affected by hydrodynamic forces or flow 
     characteristics." 

The "acceptable compliance" section of the staff SER expanded this criterion
to consider not only redundancy but also diversity. Two alternatives for 
acceptable compliance to meet the diversity requirement were given. These 
were: 

     "Alternative 1: 

     (a)  Provide additional (or substitute) level-sensing instrumentation 
          for the automatic scram function to include diversity as well as 
          redundancy. The diversity should, as a minimum, be achieved by 
          level sensors that employ different operating principles foe 
          measuring the water level; 

     (b)  For the instrumentation selected, demonstrate how common-cause 
          failures; such as those identified by operating history and those 
          identified in the Forward to IEEE 379-1977 will be considered." 
          
.

                                     - 2 -

     "Alternative 2: 

     (a)  Utilize the existing type level sensors, i.e., float level 
          switches; 

     (b)  Provide a demonstration that the cause of the damaged floats has 
          been identified and will be corrected or compensated for; 

     (c)  Provide a periodic test procedure (for on-line at power testing as
          well as for testing during and after shutdown) which helps ensure 
          that testing-induced human error and adverse environmental factors
          will be minimized (requirement under surveillance criteria); and 
          
     (d)  Provide either: 

          i.   Additional (or substitute) level switches for automatic scram
               which are made by a different manufacturer; or 

         ii.   Demonstrate that the diverse indication of SDV water 
               accumulation provided to the operator by an alarmed 
               continuous monitoring system can provide adequate backup 
               protection if the level sensors (providing auto scram) should 
               fail (note that operator's response to a fast SDV fill event 
               must be considered, i.e., automatic air dump on loss of 
               air)." 

We have reevaluated these methods of complying with the criteria and find 
them to be appropriate with the exception of alternative 2. with option 
(d)ii, and with the clarification that option (d)i includes the overall 
float assembly, not merely the switch. 

We are deleting the option that allows credit for operator action on the 
basis of an ATWS consideration that allows no credit for operator action for
a minimum of 10 minutes. 

Accordingly, Alternative 2.(d) for Safety Criterion 3 should be revised as 
follows: 

     (d)  Provide additional (or substitute) float assemblies (including 
          level switches) for automatic scram which are made by a different 
          manufacturer. 

We consider divers instrumentation for the automatic level sensing system on
the Scram Discharge Volume to be a necessary and important provision that 
will enhance the overall reliability of the BWR Scram System. 
.

                                    - 3 - 

We have concluded that the two compliance alternatives, as revised, allow 
sufficient flexibility to enable operation BWR licensees to implement the 
criterion within the time allowed for permanent modifications (end of 1982).
Please contact this office if we can be of further assistance. 

                              Sincerely, 


                              Darrell G. Eisenhut, Director 
                              Division of Licensing