Information Notice 2008-06, Instrument Air System Failure Resulting in Manual Reactor Trip
| ML073540243 | |
| Person / Time | |
|---|---|
| Issue date: | 04/10/2008 |
| From: | Michael Case NRC/NRR/ADRA/DPR |
| To: | |
| References | |
| IN-08-006 | |
| Download: ML073540243 (4) | |
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
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 licensees 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.
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.
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