United States Nuclear Regulatory Commission - Protecting People and the Environment

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 
                                                           Accession No.   
                                                           8008220279      
                                                           IN 80-43        

                               UNITED STATES 
                       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 
accomplished. 

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  
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                                                           IN 80-43        
                                                           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
4, 1980. 

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. 

Page Last Reviewed/Updated Tuesday, November 12, 2013