NRC Generic Letter 81-21, Natural Circulation Cooldown

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WASHINGTON, D. C. 20555

MAY 5 1981

TO ALL LICENSEES OF OPERATING PRESSURIZED WATER NUCLEAR POWER REACTORS AND APPLICANTS FOR OPERATING LICENSES (EXCEPT FOR ST. LUCIE, UNIT NO. 1)

Gentlemen:

SUBJECT: NATURAL CIRCULATION COOLDOWN (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.

.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 Event

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 phenomena.

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.

.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 later.

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. 20503.

Sincerely,

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

Enclosure: As stated