IE Circular No. 81-10, Steam Voiding in the Reactor Coolant System During Decay Heat Removal Cooldown
SSINS No.: 6835
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
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
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,
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.
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
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.
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.
2. Each licensed operator should be informed of the matters discussed in
3. Consider including this information in your operator training and
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.
Recently issued IE Circulars
Page Last Reviewed/Updated Tuesday, September 01, 2015