Information Notice No. 86-51: Excessive Pneumatic Leakage in the Automatic Depressurization System

                                                          SSINS. No. : 6835 
                                                          IN 86-51         

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

                                June 18, 1986

Information Notice No. 86-51:   EXCESSIVE PNEUMATIC LEAKAGE IN THE 
                                   AUTOMATIC DEPRESSURIZATION SYSTEM 

Addressees: 

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

Purpose: 

This notice is being provided to alert recipients to a potentially 
significant problem pertaining to possible pneumatic supply leakage through 
pathways near the accumulators serving automatic depressurization system 
(ADS) safety relief valves. It is expected that recipients will review this 
notice for applicability to their facilities. However, suggestions contained
in this notice do not constitute NRC requirements; therefore, no specific 
action or written response is required. 

Past Related Documents: 

IE Bulletin No. 80-01: "Operability of ADS Valve Pneumatic Supply," 
January 11, 1980 

IE Bulletin No. 80-25: "Operating Problems with Target Rock Safety-Relief  
Valves at BWRs," December 19, 1980 

Information Notice No. 85-35: "Failure of Air Check Valves to Seat," 
April 30, 1985 

IE Inspection Report 50-458/84-18: "Integrated Design Inspection of the 
River Bend Station," Section 2.4, "Automatic Depressurization System 
Design,"  August 16, 1984 

Description of Circumstances: 

On March 19, 1986, after noting that the booster compressors were operating 
more than was normal, the licensee at Grand Gulf determined that excessive 
instrument air flow was required to maintain the ADS's normal operating 
pressure of 183 psig. The licensee isolated the air supply and observed 
header pressure to decrease from 183 to 155 psig in 1 hour. Based on 
licensee measurements, the air system was leaking at a rate of 600-1200 
standard cubic 




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                                                            IN 86-51     
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feet per hour (SCF/hr). This pressure decrease and the previously observed 
excessive air makeup indicated that the ADS receiver and accumulator 
combined leakage exceeded the long-term requirements for ADS safety relief 
valve actuation specified in the final safety analysis report (FSAR). The 
FSAR assumes a leak rate of 1.0 SCF/hr for each valve on the system. 
Technical Specifications (TS) require that eight ADS valves be operable, but 
they do not specify leakage requirements. 

The air supply to the ADS valves was reinstated and the instrument air 
pressure remained above the minimum operating pressure of 150 psig. However,
if there had been a failure of the non-safety portions of the instrument air
system (the compressors or the piping upstream of the receiver), the ADS 
system would have not remained operable. Therefore, the licensee declared 
the ADS inoperable and shut down the reactor for repairs. Investigation of 
the cause of the high leak rate showed that the accumulator relief valves in 
the drywell were defective. It was determined that 16 of 17 were leaking and
required rework and these valves were repaired. Normal operating pressure 
was reduced from 183 psig to 165 psig. This increases the margin between the
operating pressure and the relief set point (190 psig). The operating 
pressure is now below the pressure (171 psig) at which seat leak tightness 
is tested yet sufficiently above the minimum required pressure (110 psig) 
for meeting FSAR commitments. 

Discussion: 

Previous generic correspondence has identified problems in pneumatic systems
that could affect ADS operability. IE Bulletins 80-01 and 80-25 noted 
problems with the control air system in maintaining operability of the ADS. 
In the case of Bulletin 80-01, the ADS was not operable for all events 
because of combined misapplication of the accumulator inlet check valve, 
lack of accumulator testing, and seismic considerations. For Bulletin 80-25,
excessive pressure in the pneumatic supply system caused an ADS safety 
relief valve to stay open when it should have closed. The reactor coolant 
system was then depressurized while the reactor was operating at power. 

The problem at Grand Gulf described above is analogous to the situation 
described in Information Notice 85-35 and the referenced inspection report 
on the River Bend plant. In that case, back leakage through the inlet check 
valve (rather than accumulator relief valves) resulted in reduced 
accumulator pressure. The Grand Gulf ADS accumulators are designed as 
Section III, Class 3 components, according to the ASME Boiler and Pressure 
Vessel Code. Relief valves are attached to the accumulators for overpressure 
protection. Such relief valves may not be found at all plants, depending on 
the accumulator design. This event indicates that such leaking relief valves 
and any other leaking mechanical joints provide pathways for air leakage 
that compromise the operability of the ADS during accident conditions. 

Only portions of pneumatic systems in nuclear power plants are designed as 
safety systems. In general, the air compressors are not powered from 
emergency buses; therefore, the supply of air is dependent on the 
availability of off-site power. Further, piping between the air compressors 
and the safety grade 

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                                                            IN 86-51       
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portions of the system is not designed to withstand major seismic events. 
Thus the components that require compressed air to perform their safety 
function are heavily dependent on the leak-tightness of the seismic 
resistant portions of the pneumatic systems, including appurtenances such as 
relief valves and check valves. Although the TS do not require monitoring of 
the ADS pneumatic supply system makeup rate, lack of monitoring the makeup 
rate subjects the ADS to potentially undetected excessive leakage. This 
could result in the ADS being unable to meet its design bass requirements 
upon loss of the non-safety-related instrument air system. 

No written response to this information notice is required. If there are any
questions regarding 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:  Vern Hodge, IE 
                    (301) 492-7275 

Attachment:    List of Recently Issued IE Information Notices