ML073540243

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

OFFICE OF NUCLEAR REACTOR REGULATION 

WASHINGTON, DC 20555-0001 

April 10, 2008 

NRC INFORMATION NOTICE 2008-06: INSTRUMENT AIR SYSTEM FAILURE RESULTING 

IN MANUAL REACTOR TRIP 

ADDRESSEES 

All holders of operating licenses or construction permits for nuclear power reactors, except 

those who have permanently ceased operations and have certified that fuel has been 

permanently removed from the reactor vessel. 

PURPOSE 

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert 

addressees of an event involving an instrument air system failure that resulted in a manual 

reactor trip. The NRC expects that recipients will review this information for applicability to their 

facilities and consider actions, as appropriate, to avoid similar problems. Suggestions in this 

information notice are not NRC requirements; therefore, no specific action or written response is 

required. 

DESCRIPTION OF CIRCUMSTANCES 

On June 20, 2007, San Onofre Nuclear Generating Station Unit 2 (SONGS-2) was operating at 

about 96 percent power when a line in the instrument air system separated at a soldered 

connection. The resulting loss of instrument air pressure caused a loss of control of the steam 

generator feedwater regulating valves. The water level in one of the steam generators rose in 

an uncontrolled manner necessitating action by the control room operators to trip the reactor 

manually. Subsequently, the operators tripped the main feedwater pumps to stop excess 

feedwater to the steam generators and actuated the auxiliary feedwater system. The loss of air 

also prevented the use of the steam dumps to the main condenser (i.e., the normal heat 

removal method) and; therefore, operators controlled the steam generator pressure and decay 

heat removal using the steam generator atmospheric dump valves. As designed, a loss of 

instrument air system pressure will cause the isolation of cooling water to the normal 

containment coolers; therefore, the operators manually started the containment emergency 

cooling units as a conservative measure. 

The licensee at SONGS-2 reported this occurrence in Licensee Event Report (LER) 

50-361/2007-001-01, dated August 24, 2007 (Agencywide Documents Access Management 

System (ADAMS) Accession No. ML072400033). The LER stated that the cause of this event 

was the loss of instrument air system pressure when the instrument air system separated at a 

soldered connection in a section of 3-inch diameter piping. The connection failed because of a 

weak solder joint and corrosion of the solder joint. This section of pipe with the failed solder 

joint was installed in about 1980 during original plant construction. The solder connection was 
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weak because the gap between the tube and the coupling was too large. The larger gap did not 

allow for the capillary action necessary to provide an even distribution of the melted solder and 

the solder pooled at the bottom of the coupling. The larger gap also allowed solder flux to 

remain in the solder, which led to slow-acting corrosion of the solder joint. When corrosion had 

sufficiently reduced the solder joint strength, the connection separated. 

As corrective actions, the licensee inspected the instrument air piping and replaced the 

instrument air line solder joint that had separated and the leaking joint adjoining it. The licensee 

also inspected all soldered joints in the instrument air system on piping with a diameter greater 

than 1 inch and installed pipe clamps to strengthen pipe joints, where needed, to supply 

additional margin. 

At SONGS-2, the instrument air system is a shared, nonsafety-related system. However, the 

system is equipped with certain protective features (e.g., excess flow check valves) to ensure 

that a failure in the piping system on one unit does not significantly affect instrument air 

pressure on the other unit. In this incident, the SONGS-2 instrument air pressure dropped 

significantly from 110 psig to about 43 psig while the back-up system and check valves 

sustained service to Unit 3 until the break could be isolated. The loss of instrument air pressure 

caused the feedwater control valves to stop functioning and the water level in the steam 

generators increased in an uncontrolled manner necessitating a manual trip of the reactor by 

the control room operators. The licensee has assessed the safety significance of the event and 

performed a risk assessment based on the reported actual component unavailability, system 

alignments, and operating conditions. The assessment of the conditional core damage 

probability and conditional large early release probability found the safety significance of this 

event to be very low. 

Based on the conditions reported in the LER, as supported by the licensee’s risk analysis, the 

NRC staff found that although this event challenged plant operations, it did not affect the health 

and safety of either plant personnel or the public. 

DISCUSSION 

Although no specific requirements or standards were unmet, the failure of the instrument air 

system at SONGS-2 illustrates the importance of understanding the potential failure 

mechanisms associated with the fabrication/installation process, monitoring the 

fabrication/installation of the systems, and performing inspections that check for potential failure 

mechanisms. 
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CONTACT 

This information notice does not require any specific action or written response. Please direct 

any questions about this matter to the technical contact listed below. 

/RA/

Michael J. Case, Director 

Division of Policy and Rulemaking 

Office of Nuclear Reactor Regulation 

Technical Contact: Edward Smith, NRR/DSS/SBPB 

301-415-1548 

Email: ets1@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 
IN 2008-06 

Page 3 of 3 

CONTACT 

This information notice does not require any specific action or written response. Please direct 

any questions about this matter to the technical contact listed below. 

/RA/

Michael J. Case, Director 

Division of Policy and Rulemaking 

Office of Nuclear Reactor Regulation 

Technical Contact: Edward Smith, NRR/DSS/SBPB 

Phone: 301-415-1548 

Email: ets1@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 

Distribution: 

IN Reading File 

ADAMS ACCESSION NUMBER: ML073540243 

OFFICE SBPB:DSS TECH EDITOR BC:SBPB:DSS D:DSS 

NAME ESmith HChang DHarrison JWermiel for WRuland 

DATE 04/03/2008 04/02/2008 04/03/2008 04/04/2008 

OFFICE LA:PGCB:DPR PGCB:DPR BC:PGCB:DPR D:DPR 

NAME CHawes DBeaulieu MMurphy MCase 

DATE 04/10/2008 04/08/2008 04/10/2008 04/10/2008 

OFFICIAL RECORD COPY