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."
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"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.
.
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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
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