United States Nuclear Regulatory Commission - Protecting People and the Environment

Natural Circulation Cooldown (Generic Letter No. 81-21)

                               UNITED STATES 
                          WASHINGTON, D. C. 20555 

                                 MAY 5 1981



          (Generic Letter No. 81-21) 

On June 11, 1980, the St. Lucie Plant, Unit No. 1, was forced to cool down 
on natural circulation as a result of a component cooling water malfunction.
During the cooldown process, abnormally rapid increases in pressurizer level
were observed. Subsequent analyses have confirmed that these abnormal level 
increases were produced by flashing of liquid in the upper head of the, 
reactor vessel, forcing water out of the vessel and into the pressurizer. A 
more complete description of the,event and circumstances involved is 
provided in the enclosure which includes a letter sent to the PWR NSSS 
vendors soliciting their opinions and comments on the significance of the 
event and phenomenon in general. 

Based on our review of the event to date, we believe that core cooling was 
never lost during the St. Lucie, Unit No. 1 event. That specific event does 
not constitute a direct safety concern. We have, however, identified two 
areas of concern applicable to all pressurized water reactors requiring 
prompt action: 

1.   The Unacceptability of Vessel Voiding During Anticipated Cooldown 
     Conditions (Natural Circulation Due to Loss of Offsite Power, Loss of 
     Pumps, etc.) 

     Cooldown with a significant steam void in the vessel requires 
     controlling a "two pressurizer" system, which is an undesirable 
     challenge to the operator. In fact, we ate not aware of any training 
     facilities (simulators) today which would allow an operator "hands on" 
     experience in practicing such control. Moreover, it is our opinion that
     any significant vessel voiding produced during controlled cooldown 
     conditions increases the susceptibility of the plant to more serious 
     accidents. For these reasons reactor vessel voiding during controlled 
     natural circulation cooldowns should be avoided. 

                                    - 2 - 

     As described in the enclosure, vessel voiding at St. Lucie, Unit No. 1,
     was caused by the operator reducing system pressure such that the 
     corresponding saturation temperature dropped to the temperature of the 
     relatively stagnant fluid in the reactor vessel upper head. Presently, 
     primary system cooldown rates are based on vessel structural integrity 
     considerations and do not explicitly consider avoiding production of 
     significant steam voids in the vessel. Moreover, cooldown rates are 
     based on fluid temperatures measured in the primary piping. As the St. 
     Lucie Unit No. 1 event has shown, these measured temperatures can in 
     fact be on the order of 100 degrees Fahrenheit or more lower than the 
     upper head fluid temperature, and, therefore, not indicative of the 
     saturation pressure of all fluid in the primary system. 

     Under conditions which require cooldown on natural circulation and when
     rapid depressurization is not necessary there may be a number of ways 
     to avoid reactor vessel voiding. For example, a low cooldown rate can 
     be specified, coupled with "holding" the plant at intermediate 
     conditions to allow the fluid in the upper vessel to equilibrate with 
     the rest of the primary system. However, avoidance of vessel voiding by
     lower primary system cooldown rates can increase the time required to 
     achieve shutdown cooling entry conditions and thus increase the time 
     auxiliary feedwater is depended upon to remove decay heat 
     (specifically, for the loss-of-offsite power case). Thus, supplies of 
     condensate-grade auxiliary feedwater must be considered if cooldown 
     times are extended. 

2.   Failure of the Operator to Have Prior Knowledge and Training for This 

     The cause of initial surges in pressurizer level at St. Lucie, Unit No.
     1, was not immediately recognized or understood by the operator. We 
     attribute this to the fact that long-term natural circulation cooldown 
     under the specific circumstances of the event was never explicitly 
     analysed by the NSSS vendor from the standpoint of trying to recognize 
     a phenomenon such as that which occurred at St. Lucie, Unit No. 1. In 
     the St. Lucie event, the operator ultimately recognized the cause.of 
     the level surges and was able to maintain control of the plant. Our 
     concern, however, is the possibility of an operator taking incorrect 
     action in an effort to correct for an unknown event or unrecognized 

We believe that proper procedures and training can provide the necessary 
guidance to the operators both to avoid reactor vessel voiding as well as 
recognize it when, and if, it occurs during controlled natural circulation 
cooldown.  We are not sure if such procedures and training are in place at 
pressurized water reactor facilities. 

                                    - 3 - 

Consequently, we request that you promptly review your current plant 
operations in light of the St. Lucie, Unit No. 1 event and the discussions 
above and implement, as necessary, procedures and training which will enable
operators to avoid (if possible), recognize and properly react to reactor 
vessel voiding during natural circulation cooldown. 

We conclude that the actions described above should be completed as soon as 
they reasonably can be (i.e., within 6 months for operating reactors). In 
addition, so that we may determine whether your license should be amended to
incorporate these actions as requirements, licensees of operating 
pressurized water reactors are requested, pursuant to SS50.54(f), to 
furnish, within 6 months of receipt of this letter, an assessment of your 
facility procedures and training program with respect to the matters 
described above. Your assessment should include: 

1.   a demonstration (e.g. analysis and/or test) that controlled natural 
     circulation cooldown from operating conditions to cold shutdown 
     conditions, conducted in accordance with your procedure, should not 
     result in reactor vessel voiding; 

2.   verification that supplies of condensate-grade auxiliary feedwater are 
     sufficient to support your cooldown method; and 

3.   a description of your training program and the provisions of your 
     procedures (e.g. limited cooldown rate, response to rapid change in 
     pressurizer level) that deal with prevention or mitigation of reactor 
     vessel voiding. 

Applicants for operating licensees are requested to implement the subject 
procedures and training and provide the requested assessment within 6 months
of receipt of this letter or 4 months prior to the staff's scheduled 
issuance of its operating license Safety Evaluation Report, whichever is 

Please refer to this letter in your response. 

This request for information was approved by OMB under a blanket clearance 
number R0072 which expires December 31, 1981. Comments on burden and 
duplication may be directed to the Office of Management and Budget, Reports 
Management, Room 3208, New Executive Office Building, Washington, D.C. 


                              Darrell G. Eisenhut, Director 
                              Division of Licensing 
                              Office of Nuclear Reactor Regulation 

Enclosure: As stated 
Page Last Reviewed/Updated Friday, May 22, 2015