ML19344E085
| ML19344E085 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 08/21/1980 |
| From: | Johnson W YANKEE ATOMIC ELECTRIC CO. |
| To: | Check P Office of Nuclear Reactor Regulation |
| References | |
| WYR-80-96, NUDOCS 8008260601 | |
| Download: ML19344E085 (2) | |
Text
I Telephone 617 366-9011 4
TWX 710 390-0739 YANKEE ATOMIC ELECTRIC COMPANY n.a.2.1
- m.
WYR 80-96 fgl%
20 Turnpike Road Westborough, Massachusetts 01581
,Yauxse h*
August 21, 1980 United States Nuclear Regulatory Commission Washington, D. C.
20555 Attention: Office of Nuclear Reactor Regulation Mr. Paul S. Check, Assistant Director for Plant Systems Division of Systems Integration
Reference:
a) USNRC letter to Yankee Atomic Electric Company, dated July 28, 1980 Subjec t: Void Formation in Vessel Head During St. Lucie Natural Circulation Cooldown Event on 6/11/80.
Dear Sir:
M your letter, reference (a), you have solicited our technical response and advice regarding the potential for void formation in our plants under circumstances similar to those which occured at St. Lucie. We have studied the issue and have reached the following conclusions.
For the event described, St. Lucie experienced depressurization rates which are relatively higher than those expected during normal natural circulation cooldown events.
If rapid depressurization rates similar to St. Lucie occur and if stagnation conditions similar to St. Lucie in the upper head exist, void formation may be expected to occur which may lead to l
excessive increases in the pressurizer liquid level. However, the rate of l
depressurization and the degree of consnunication which the upperhead fluid has l
with the rest of the primary system fluid will determine the existence and the l
extent of voiding in the system. As an example, during normal operation at Yankee Rowe about 1% of its total systes flow is bypassed from the downcomer to the upper plenum.
It is expected that even during a natural circulation cooldown, enough flow will be able to reach the upperhead region to keep the upperhead temperature in line with the rest of the system. Additionally, Yankee Rowe does not hsve the capability to direct charging flow to the pressurizer spray system during normal operation. This serves to limit the rate of depressurization to the capability of the pressurizer spray system.
These two features make the possibility of void formation reurote for this l
plant.
8 008260 Goj
I U. S. Nuclear Regulatory Consnission August 21, 1980 Page Two Subsequent to the receipt of your letter, training and operating personnel have been briefed about the St. Lucie event. They have been made aware that high depressurization rates may result in void formation in the upperhead region, and that this may cause erratic pressurizer level behavior.
Additionally, we intend to incorporate a caution in operating procedures with regard to depressurization during natural circulation at our plants.
In conclusion, it is our belief that during excessive cooldown rates on natural circulation void formation in the upperhead region may occur at our plants. Void formation would be limited to the stagnant upperhead region.
Colder fluid, available in the upper plenum, would suppress the growth of voids beyond the upperhead. Therefore, void formation during natural circulation cooldown in itself has no impact on safety. An excessive pressurizer level rise during natural circulation may be misleading to an operator unaware of the phenomena.
It is our belief that the recent information on the issue along with the guidelines cautioning against rapid depressurization
--r-vill enable our plant personnel to effectivnly eliminate void formation
. per head region during natural circulation.
To determine if auf further action is necessary we are working with the TMI owners groups on the analysis of events that could lead to void formation.
In addition, we will be evaluating analytically this phenomena as associated with the RCP question following events calling for a natural circulation cooldown. Specifically, our intention is to utilize a RELAP/RETRAN based methodology to analyze plant response during natural circulation with emphasis on plant cooldown events. Model performance will be compared-to available plant natural circulation data which includes a cooldown to cold shutdown conditions. Various cooldown rates and depressurization rates consistent with equipment performance characteristics and operating procedures will be included in the study. The aim of this study would be to determine the extent of void formation and to identify the conditions under which it would occur and to identify indications that would be available to the optrator for recognition that this pehnomena is occurring.
Very truly yours, YANKEE ATOMIC ELECTRIC COMPANY WM W. P.. Johnson Vice President WPJ/cjp
.