United States Nuclear Regulatory Commission - Protecting People and the Environment

Inadvertent Boron Dilution Events (Generic Letter No. 85-05)



                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                           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. 

8502010366 
.

                                    - 2 - 

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: 
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