Information Notice No. 79-13 – Indication of Low Water Level in the Oyster Creek Reactor
IN79013
May 29, 1979
MEMORANDUM FOR: B. H. Grier, Director, Region I
J. P. O'Reilly, Director, Region II
J. G. Keppler, Director, Region III
K. V. Seyfrit, Director, Region IV
R. H. Engelken, Director, Region V
FROM: Norman C. Moseley, Director, ROI:IE
SUBJECT: Information Notice No. 79-13 - INDICATION OF LOW
WATER LEVEL IN THE OYSTER CREEK REACTOR
The subject Information Notice is transmitted for issuance on May 29, 1979.
The Information Notice should be issued to all holders of Reactor Operating
Licenses and Construction Permits. Also enclosed is a draft copy of the
transmittal letter for this purpose.
Norman C. Moseley, Director
Division of Reactor Operations
Inspection
Office of Inspection and Enforcement
Enclosures:
1. IE Information Notice
No. 79-13
2. Draft Transmittal Letter
CONTACT: C. J. DeBevec, IE
49-28180
.
(Draft letter to all holders of Reactor Operating Licenses and Construction
Permits)
Information Notice No. 79-13
Addressee:
This Information Notice is provided as an early notification of a possibly
significant matter. It is expected that recipients will review the
information for possible applicability to their facilities. No specific
action or response is requested at this time. If further NRC evaluations so
indicate, an IE Circular or Bulletin will be issued to recommend or request
specific licensee actions. If you have questions regarding this matter,
please contact the Director of the appropriate NRC Regional Office.
Sincerely,
Signature
(Regional Director)
Enclosure:
IE Information Notice
No. 79-13
.
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
May 29, 1979
Information Notice No. 79-13
INDICATION OF LOW WATER LEVEL IN THE OYSTER CREEK REACTOR
Summary
A loss of feedwater transient at the Oyster Creek facility on May 2, 1979,
resulted in a significant reduction in water inventory within the reactor
core shroud area as measured by one set of water level instruments (triple
low level), while the remaining level instruments, sensing from the reactor
annulus area indicated water levels above any protective feature setpoint
(Figure 1). The water level within the core shroud area was reduced below
the "triple low level" setpoint of 4-feet, 8-inches above the top of the
fuel.
Subsequent analysis by the licensee has determined that the minimum
collapsed water level (solid, without steam voids) over the top of the fuel
was 1 to 1-1/2 feet.
Coolant sample analyses and offgas release rates indicate that no fuel
damage occurred.
General
Oyster Creek is a non-jet pump BWR with licensed power of 1930 MWt. The
plant was first made critical May 3, 1969.
Status Before Transient
Operating at near full power with the main parameters at levels as
follows:
1895 MWt power level
79" Yarway (13'4" over top of fuel) reactor water level
1020 psig reactor,- pressure
7.1 x 10 6 #/hr feedflow
14.8 x 10 4 gpm recirculation flow rate (4 pumps)
12 psid core Ap
Equipment Out of Service (OOS)
"D" recirc pump OOS due to seal cooler problem
"B" startup transformer OOS for inspection of associated 4160v cabling
.
Information Notice No. 79-13 May 29, 1979
Page 2 of 4
Initiating Event
The initiating event was a false pressure spike on the reactor high pressure
scram switches. This pressure spike was caused by an instrument technician
who was performing Technical Specification required surveillance testing on
isolation condenser pressure switches.
Sequence of Events
The most significant events following the reactor scram are listed in Table
I, with the time in seconds following the scram. A discussion of these
events is provided for further information.
Thirteen seconds after the reactor scram on a false high reactor pressure
signal, the turbine tripped at 25% low load setpoint. The turbine trip
initiated a transfer of power from the auxiliary transformers to the startup
transformers (Figure 2). Because one startup transformer (SB) was out of
service, two feedpumps and two condensate pumps on that 4160v bus (IB) lost
power (pumps 1B and 1C). The third feedpump (1A) tripped due to low suction
pressure resulting from the feedwater transient. An immediate attempt to
restart IA was unsuccessful because of failure of an auxiliary oil pump to
start. The oil pump is interlocked in the feedpump start sequence.
Initially, water inventory was decreasing due to steam flow through the
turbine bypass valves to the main condenser. This loss together with the
void collapse associated with decreased feedwater temperature and the
subsequent loss of feed flow, resulted in a rapid reactor water level
reduction to the low water level alarm setpoint of 11-feet, 5-inches above
the top of the fuel at 13.6 seconds. At 25 seconds, an emergency diesel
generator (DG-2) had automatically fast-started following the power loss to
bus 1D and picked up emergency loads as designed. This loading included the
second control rod drive (CRD) pump which with the other CRD pump (powered
through the retained startup transformer) provided the only coolant makeup
during this time. The operator manually initiated, a main steam line
isolation valve (MSIV) closure at about 43 seconds into the transient to
conserve water inventory. The minimum indicated water level in the annulus
area was 9-feet, 8-inches above the top of the fuel (the low low setpoint is
7-feet, 2-inches above the top of the fuel). After closure of the MSIV the
isolation condenser was condensing steam from the core and returning the
condensate to the reactor annulus (downcomer) region through connection to a
recirculation loop pump suction line (Figure 3). Reactor water level began
to increase shortly after MSIV closure.
Subsequently, at 76 seconds, an isolation condenser was manually placed in
service for core decay heat removal. At approximately this time the
discharge valves in "A" and "E" recirculation loops were closed in
accordance with a Standing Order which was in effect at that time to prevent
inadvertent isolation condenser isolation due to forced flow from operating
recirculation pumps being
.
Information Notice No. 79-13 May 29, 1979
Page 3 of 4
sensed as a line break. This Standing Order was no longer appropriate since
a plant modification had been made which tripped the recirculation pumps on
any scram. The necessary procedure change had not been performed following
the plant modification. The "D" loop discharge valve had been closed prior
to the event since the associated pump was out of service. The "B" and "C"
loop discharge valves were apparently closed in anticipation of restarting
the recirculation pumps.
The reactor triple low water level (4-feet, 8-inches above the top of the
fuel) setpoint was reached at 172 seconds into the transient. Shortly after
this the "Events Recorder," which records the time certain events occur
following a reactor trip, was turned off.
The triple low level in the core shroud area resulted from restriction of
the isolation condenser return flow path to the core region due to closure
of the recirculation pump discharge valves. With the recirculation pump
discharge valves closed, the flow path back to the core shroud was via the
2-inch bypass lines around the discharge valves. The effect was to reduce
the water level in the core shroud area and to increase the level in the
reactor annulus area.
The core shroud area of the reactor was also receiving water from the CRD
pumps.
Reactor pressure was controlled during the transient by intermittent manual
operation of the two isolation condensers. Reactor pressure and reactor
annulus water level variations with time are shown on Figure 4.
At 1922 seconds, a recirculation pump was started. It was manually tripped
90 seconds later because of a rapid decrease in the annulus level.
At 2208 seconds, a feed pump was started, following which at 2340 seconds, a
primary recirculation pump and subsequently a reactor feedpump were operated
for continued cooldown of the reactor. From this time on, water level within
the core shroud area was normal. The plant was brought to a cold shutdown
condition to analyze the cause of the triple low level and to evaluate the
possibility of fuel damage.
Conclusion
Review of the occurrence by the licensee and NRC established that although
the water level in the core shroud area went below the triple low level
setpoint, no fuel was uncovered and no fuel damage is expected to have
occurred.
.
Information Notice No. 79-13 May 29, 1979
Page 4 of 4
This Information Notice provides the details of a significant occurrence
that is still under review by the NRC staff. If further NRC review
indicates, an IE Circular or Bulletin may be issued to recommend or request
specific licensee actions.
No written response is required. If you desire additional information
regarding this matter, please contact the Director of the appropriate NRC
Regional Office.
Enclosure:
List of Information Notices
Issued in 1979
.
Table 1
SEQUENCE OF EVENTS
TIME (SEC.) EVENT
0 REACTOR SCRAM/RECIRC. PUMP TRIP
13 TURBINE TRIP/LOSS OF FEEDWATER/CONDENSATE PUMPS
13.6 LOW WATER LEVEL SCRAM POINT
16.8 #2 RPS MOTOR GEN. SET TRIP
31 #2 DIESEL GENERATOR BREAKER CLOSED/2ND CRD PUMP
STARTED
43 MSIV CLOSED*
76 B ISO. CONDENSER PLACED IN SERVICE*
90 LOW WATER LEVEL ALARM CLEARED
172 LOW LOW LOW WATER LEVEL TRIP POINT
186 RECIRC. LOOP DISCHARGE VALVES NOTED CLOSED
1914 C RECIRC. PUMP STARTED THEN TRIPPED*
2208 FEEDWATER PUMP STARTED*
2304 RECIRC. PUMP STARTED/LOW LOW LOW WATER LEVEL ALARM
CLEARED
2700 RPS #2 RESTARTED/SCRAM RESET* 3
600 STARTUP TRANSFORMER SB RETURNED TO SERVICE
* By Operator Action
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