Information Notice No. 80-43: Failures of the Continuous Water Level Monitor for the Scram Discharge Volume at Dresden Unit No. 2
SSINS NO.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
December 5, 1980
Information Notice No. 80-43: FAILURES OF THE CONTINUOUS WATER LEVEL
MONITOR FOR THE SCRAM DISCHARGE VOLUME AT
DRESDEN UNIT NO. 2
Description of Circumstances:
On December 2, 1980, following a reactor scram at Dresden Nuclear Power
Station Unit 2, the licensee discovered that the recently installed scram
discharge volume (SDV) continuous water level monitoring system had failed
to respond as expected. The continuous water level monitoring system was
designed and supplied by Nortec Corporation, and utilizes a single transmit/
receive transducer for each of the four level monitoring channels. Following
installation of the continuous level monitoring system, the licensee
verified its operability by tripping each alarm channel while its sensor was
connected to a calibration standard consisting of a section of 4 inch pipe
identical to the SDV piping. No in situ test of the equipment was
The expected high level annunciator alarms were not received in the control
room following scram when the SDV filled with water. An investigation to
determine the cause was initiated and once per shift checks of the SDV level
on both Units 2 and 3 were re-instituted. The reactor scram system
functioned normally and post scram manual ultrasonic tests on the SDV
verified proper system drain.
Investigations by the licensee determined that the ultrasonic detectors were
inadequately coupled to the SDV piping. The coupling was improved and
testing performed to show that the system could detect water flow into the
SDV during single control rod scram tests. The unit was taken critical and
was heating up on December 4, 1980, at approximately 400 psig when a reactor
scram occurred. The expected SDV high level annunciator alarms were not
received in the control room immediately following the scram, but were
received about 10-15 minutes later when the SDV was draining. Following an
additional scram at approximately 20 psig with control rods inserted these
alarms were received properly. Investigation identified level detection
system operability problems as evidenced by a low signal to noise ratio.
Arrangements are being made for replacement components (including
transducers) to improve the signal characteristics and reduce system noise.
Until the improvements are installed and demonstrated adequate by in situ
testing, use of the ultrasonic detection system is being augmented with
manual ultrasonic checks of level in the SDV, using the previously required
once per shift check. A manual check will be performed after each reactor
scram. At Dresden Unit 2 the licensee will
December 5, 1980
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complete the system improvements and required testing within 7 days in
accordance with the Immediate Action Letter issued by Region III on December
The above operating experience indicates the need for a demonstration of the
operability of ultrasonic systems recently installed to continuously monitor
for water in the SDV. Until demonstrated operable by in situ testing, these
systems should not be solely relied on for detection of water in the SDV.
Use of new ultrasonic detection systems should be augmented by manual checks
for water in the SDV each shift and after reactor scram until in situ
testing is performed.
It is important that a high degree of reliability be established in
operability of these continuous water level monitoring systems in order to
support plant operations in the interim until final scram discharge system
modifications are performed.
This Information Notice No. is provided as an early notification of a
possibly significant matter that is still under review by the NRC staff.
Recipients should review the information for possible applicability to their
facilities. No specific response is requested at this time. Pending further
NRC evaluations, we expect licensee actions will be requested or required.
No written response to this Information Notice No. is required. If you have
any questions regarding this matter, please contact the Director of the
appropriate NRC Regional Office.
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