NRC Generic Letter 85-05, Inadvertent Boron Dilution Events
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WASHINGTON, D. C. 20555
January 31, 1985
TO ALL PRESSURIZED WATER REACTOR LICENSEES
Gentlemen:
SUBJECT: INADVERTENT BORON DILUTION EVENTS (Generic Letter 85-05)
The purpose of this letter is to inform each licensee of operating pressurized water reactors of the staff position resulting from the evaluation of Generic Issue 22, "Inadvertent Boron Dilution Events" regarding the need for upgrading the instrumentation for detection of boron dilution events in operating reactors.
A boron dilution event is considered as an anticipated operational occurrence which may occur at moderate frequency. The staff has performed analyses of unmitigated boron dilution events for a typical plant for each pressurized water reactor (PWR) vendor. The staff determined that while power excursions during boron dilution events are possible if the operator does not take any action and sufficient volume of dilution water is available, the excursion should be self-limiting. The staff analyses indicate that these type of boron dilution transients should not exceed the staff's acceptance criteria. However, our analyses also show that a few plants may experience slight , overpressurization in excess of the 110%
overpressure limit in the Residual Heat Removal system if the event occurs during a particular mode of operation.
In addition, the staff recognizes that many operating plants do not have distinct, positive alarms to alert the operators to boron dilution events but rely on other devices such as audible count rate meters. Other problems include lack of alarm redundancy and lack of technical specifications which would prevent operators from taking alarming devices out of service. The staff also does not consider it prudent to credit operators with the ability to recognize a boron dilution event and take the proper mitigative action within specified time limits in the absence of positive boron dilution alarms.
Considering all of the above factors and possible consequences of boron dilution events, the staff has concluded that the criteria in Section 15.4.6 of the Standard Review Plan are adequate and should continue to be applied to plants currently undergoing licensing review. However, the consequences are not severe enough to jeopardize the health and safety of the public and do not warrant backfitting requirements for boron dilution events at operating reactors. The staff will continue to review the analyses of the Boron Dilution Event in reload applications to assure that reasonable confidence is provided that operators can be expected to take the right corrective action using the installed systems.
In summary, while the NRC will not require operating plant backfits for boron dilution events at this time, the staff would regard an unmitigated boron dilution event as a serious breakdown in the licensee's ability to control its plant, and strongly urges each licensee to assure itself that adequate protection against boron dilution events exists in its plants.This generic letter is provided for information only, and does not involve any reporting requirements. Therefore, no clearance from the Office of Management and Budget is required.
Darrell G. Eisenhut, Director Division of Licensing
Enclosure:
List of Generic Letters