United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-70: Reliance on Water Level Instrumentation with a Common Reference Leg

                                                          SSINS No.: 6835  
                                                          IN 84-70         

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

                             September 04, 1984 

Information Notice No. 84-70:  RELIANCE ON WATER LEVEL INSTRUMENTATION WITH 
                              A COMMON REFERENCE LEG 

Addressees: 

All nuclear power reactor facilities holding an operating license (OL) or 
construction permit (CP). 

Purpose: 

This information notice is provided to alert licensees and applicants of the
potential for degradation of safety associated with operator reliance on 
level instruments that share a common reference leg. Recipients are expected
to review the information for applicability to their facilities and consider
actions, if appropriate, to preclude similar problems occurring at their 
facilities. However, suggestions contained in this information notice do not
constitute NRC requirements; therefore, no specific action or written 
response is required. 

Description of Circumstances: 

At Maine Yankee, on June 12, 1984, while preparing for startup from the cold
shutdown condition following a 10 week refueling outage, the licensee 
discovered that the pressurizer had been inadvertently drained and that a 
bubble of air and nitrogen had entered the vessel head. Although the event 
was primarily caused by inadequate filling and venting operations and 
pressurizer level irregularities, the event highlights the need for 
operators to avoid total reliance on level instruments that share a single 
common reference leg. 

There were three conditions that made the initiation of this event possible.
First, no form of evacuation was used during the initial loop fills. Second,
all the cold-calibrated, wide-range level instruments have a common 
reference leg. Third, the common reference leg was slowly drained because of 
a small leak. The leak was associated with a modification to permit 
wide-range level to be indicated at a remote shutdown panel. 

At approximately 5:00 a.m., 100 psi air and later 150 psi nitrogen was 
applied to the top of the pressurizer as part of the normal filling and 
venting procedures. Operators were relying upon cold-calibrated, wide-range 
level indications to control pressurizer level. The operators did not use or
pay attention to the narrow-range level instruments, which each have 
independent reference legs, because the narrow-range instruments are not 
calibrated for use at power. Thus, because of the slow leak in the single 
reference leg, the operators maintained an apparent level while the actual 
level decreased, 

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                                                        September 04, 1984 
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thereby causing the pressurizer to drain and a bubble to enter the top of 
the head. Sometime during this event, both channels of narrow-range level 
went offscale low. 

At about 7:30 p.m., there was an unexplained rise in the wide-range 
indicated level, and the plant shift supervisor initiated an investigation.  
The unexplained rise in level was subsequently found to be associated with a
routine calibration of one of the wide-range instruments and the previously 
mentioned slow leak. Examination of the shutdown cooling flow and 
temperature readings verified that the level was still above the core, but 
realizing what had happened, operators had to use 8,000 gallons of water to 
refill the vessel head and pressurizer. 

Discussion: 

Situations where a common reference leg can cause multiple level instruments
to have erroneous indication are common to most light water nuclear power 
plants. Boiling water reactors, for example, typically share a reference leg
between narrow-range vessel instruments used for feedwater control and 
wide-range instruments used for emergency core cooling system (ECCS) 
actuations and ECCS trips such as turbine trip of high pressure coolant 
injection. 

For pressurized water reactors that use cold-calibrated, wide-range 
instruments with a common reference leg, the narrow-range, hot-calibrated 
instruments can still provide a valuable source of information to the 
operators during cold conditions. Hot-calibrated instruments should 
generally read higher than cold-calibrated instruments. If the operators at 
Maine Yankee had questioned why cold-calibrated instruments were reading 
higher than the hot-calibrated instruments, or if the operators had followed 
the narrow-range instruments for general trending of pressurizer level as 
opposed to precise measurement of level, perhaps the draining of the 
pressurizer would have been recognized earlier. 

Licensees and applicants should review their system descriptions, operating 
procedures, and operator training programs to ensure that a common reference
leg shared by multiple level instruments is adequately addressed. 
Appropriate operator awareness of the effect of draining a common reference 
leg on multiple level instruments can enhance plant safety. 

No written response to this information notice is required. If you need 
additional information about this matter, please contact the Regional 
Administrator of the appropriate NRC regional office or this office. 


                                   Edward l. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  Eric Weiss, IE 
                    (301) 492-4973 

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