IE Circular No. 81-10, Steam Voiding in the Reactor Coolant System During Decay Heat Removal Cooldown

                                                          SSINS No.: 6835  
                                                          Accession No.:   
                                                          8103300400       
                                                          IEC 81-10        

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                   OFFICE OF INSPECTION AND ENFORCEMENT  
                           WASHINGTON, D.C. 20555 
                                     
                                July 2, 1981

IE Circular No. 81-10:   STEAM VOIDING IN THE REACTOR COOLANT SYSTEM DURING 
                         DECAY HEAT REMOVAL COOLDOWN  

Description of Circumstances: 

On April 21, 1981, Florida Power Corporation's Crystal River Unit 3 facility
(a 2452 MWT Babcock & Wilcox reactor) was in Mode 5 (Cold Shutdown). The 
Decay Heat Removal (DHR) system was in operation taking a suction on the "B"
hot leg and injecting cooled liquid back into the vessel downcomer. The 
Reactor Coolant System (RCS) water temperature was approximately 106F 
as indicated by the DHR pump suction temperature. RCS pressure was about 50 
psig and being maintained by use of pressurizer heaters and auxiliary spray 
from the DHR system. 

The plant had been cooled down from 520F to approximately 270F 
(measured by the cold leg RTDs) over a 13 hour period, at which point DHR 
cooling was initiated and reactor coolant pumps (RCPs) were shut off. The 
plant was on OHR for 15 hours before the 106F, 50 psig conditions were 
reached. 

At this point, when the operators commenced spray of the pressurizer via the
auxiliary spray line to decrease RCS pressure, the pressurizer level began 
to increase from about 82 inches to 180 inches indicating that a void of 
approximately 300 cu.ft. existed in the reactor coolant system. Upon seeing 
the level increase, the operator concluded there was a steam void in the 
system. He also noticed the "A" loop wide range hot leg RTDs were reading 
approximately 300F, which is slightly above the 50 psig saturation 
temperature. Spray was terminated and emergency feedwater flow was initiated
to the "A" Once Through Steam Generator (OTSG) cooling the "A" loop hot leg.
About seven hours later sufficient cooling was achieved to drop pressurizer 
level. The plant depressurized without void formation. 

On June 2, 1981, a similar event occurred at McGuire Unit 1. McGuire had not
achieved criticality but had heated up using RCPs. While reducing RCS 
temperature and pressure to achieve a cold shutdown condition, a steam void 
was apparently formed in the reactor vessel head area when the system was 
vented. 

The plant had initiated Residual Heat Removal (RHR) cooling and turned off 
RCPs at about 318F. At a RCS loop temperature of 160F and a 
pressure of 60 psig, the vessel head was vented. The operators observed a 
pressurizer level increase of three to four percent, indicating the presence
of a steam void in the system. At this time, the reactor vessel upper 
internals showed a temperature of 250F. Since the reactor had not 
achieved initial criticality,  
.

                                                            IEC 81-10     
                                                            July 2, 1981  
                                                            Page 2 of 3   

there was no decay heat to drive natural circulation as there would have 
been for an operating plant. The system was repressurized, the reactor 
coolant pumps were restarted to uniformly cool the system and the plant was 
taken to cold shutdown.  

Discussion: 

These events were apparently caused by insufficient cooling of the large 
masses of hot metal in regions such as the reactor vessel head, upper "J" 
leg (B&W), steam generator walls, and reactor pressure vessel nozzles prior 
to initiating DHR cooling. Local stagnation and stratification of the fluid 
in the upper head region while on DHR cooling may also have contributed to 
the problem. 

While the coolant passing through the core was being maintained relatively 
cool by the DHR system, coolant in the RCS hot legs and in the upper head 
region was essentially stagnant. This allowed the coolant to be heated to 
saturation temperature, or to remain at relatively high temperatures because
of the stagnant conditions. This resulted in steam void formation when the 
system was depressurized. The operators correctly diagnosed the system 
voiding at both Crystal River 3 and McGuire 1 and took appropriate 
corrective actions to bring the plants to cold shutdown. 

While these events were not a concern due to the availability of the DHR 
system at both Crystal River and McGuire and the lack of core heat 
production at McGuire, the NRC believes transmittal of this information is 
appropriate, since voiding in the RCS is a concern if operators fail to 
recognize this conditions. Also, normal natural-circulation flow is reduced 
by voids in hot legs and sufficiently large reactor vessel voids. 

IE Circular No. 80-15, June 1980, and NRR Generic Letter No. 81-21, May 
1981, were directed at the possibility of voids being formed in the reactor 
vessel head region during natural circulation cooldown. It is important that 
reactor operators recognize that voiding can occur in other portions of the 
RCS (e.g., in the "J-leg" of B&W reactors) and under conditions other than 
natural circulation cooldown so that timely and correct action is taken. 

Recommended Actions for Holders of an Operating License and Near Term 
Operating Licensees (NTOL)*: 

1.   Review your operating procedures dealing with plant cooldown and 
     emergency and/or abnormal procedures that address natural circulation 
     to assure that sufficient information is available for operators to 
     recognize the symptoms of RCS voiding and take appropriate actions to 
     recover from a voided condition. Special attention should be directed 
     to the information provided regarding the Crystal River and McGuire 
     events in order to ascertain if they bring to light any conditions you 
     did not consider during your review 


*NTOL is defined, for the purpose of this circular, as a plant currently 
scheduled to receive an OL prior to January 1, 1983. 
.

                                                           IEC 81-10      
                                                           July 2, 1981   
                                                           Page 3 of 3    

     and revision of natural circulation and shutdown cooling procedures 
     that were required in IE Circular 80-15, and NRR Generic Letter No. 
     81-21. 

2.   Each licensed operator should be informed of the matters discussed in 
     this circular. 

3.   Consider including this information in your operator training and 
     retraining classes.  

No written response to this circular is required.  If you need additional 
information with regard to this matter, contact the Director of the 
appropriate NRC Regional Office. 

Attachment:
Recently issued IE Circulars