NRC Generic Letter 81-18, BWR Scram Discharge System-Clarification of Diverse Instrumentation Requirements

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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."

."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.

.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