United States Nuclear Regulatory Commission - Protecting People and the Environment

Bursting of High Pressure Coolant Injection Steam Line Rupture Discs Injures Plant Personnel

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                                August 16, 1993

                               STEAM LINE RUPTURE DISCS INJURES PLANT          


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to remind addressees of the hazards associated with steam driven
coolant injection systems such as the high pressure coolant injection (HPCI)
system and the reactor core isolation cooling (RCIC) system at BWRs and the
auxiliary feedwater system at PWRs.  It is expected that recipients will
review the information for applicability to their facilities and consider
actions, as appropriate, to avoid similar problems.  However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.  


The purpose of HPCI steam exhaust rupture discs is to protect the HPCI turbine
casing and associated exhaust piping from an overpressure event if the normal
exhaust path to the suppression pool becomes blocked.  At Quad Cities Station,
two 40.6-cm [16-inch] stainless steel discs are installed in series with a
6.4-cm [2.5-inch] spacer between them to allow for a pressure sensor
instrumentation line.  The pressure sensor, while serving no control function,
actuates an alarm in the control room at 68.9 kPa [10 psig] to alert the
operators that the inner disc is leaking.  The normal range of the HPCI
exhaust steam pressure is 172 to 207 kPa [25 to 30 psig].  The HPCI high
exhaust pressure turbine trip setpoint is 689 kPa [100 psig] and is sensed by
pressure sensors located downstream from where the section of piping
containing the rupture discs branches off from the exhaust piping.  The
section of piping containing the rupture discs is mounted vertically over the
turbine exhaust and vents directly to the HPCI room.  Pressure greater than
1034 kPa [150 psig] in the exhaust line will cause the inner disc to rupture
and impact the outer disc, releasing steam into the HPCI room.

Description of Circumstances

During a quarterly inservice test of the HPCI pump at the Commonwealth Edison
Quad Cities Station, Unit 1, on June 9, 1993, the exhaust steam line rupture 


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discs burst, releasing steam into the HPCI room, burning, and slightly
contaminating, five workers.  The rupture discs burst within one second after
the turbine was started.  Fire doors between the Unit 1 and Unit 2 HPCI rooms
were blown off their hinges into the Unit 2 HPCI room.  Both sets of double
doors that are part of the secondary containment boundary were also blown
open.  The floor latch on the inner containment door was bent; the outer doors
were also slightly damaged, but were repaired, closed and sealed approximately
35 minutes after the event.  The steam release was terminated by automatic
isolation of the steam supply line on high HPCI turbine area temperature about
20 seconds into the event.


Upon investigating the event, the licensee determined that water had
accumulated in the turbine casing because the drain system level switches for
the Unit 1 HPCI system had failed.  In April 1992, the licensee performed a
reliability-centered maintenance study which recommended the level switches be
included in the preventive maintenance program but the recommendation had not
been acted on at the time of the event.  The Unit 2 HPCI drain system level
switches were also found to be inoperable.

The slug of water created during the HPCI turbine roll passed from the turbine
casing to the vertical exhaust line and compressed the air in the 40.6 cm 
[16 inch] line containing the rupture discs.  The resulting pressure pulse
caused the inner rupture disc to burst, which impacted the outer disc as
designed and caused it to burst as well.  The exhaust line pressure sensors,
located in the horizontal 61 cm [24 inch] exhaust line that tees off from the
vertical 40.6 cm [16 inch] rupture disc line, did not detect a high pressure
(see Figure 1).  The pressure switches were within tolerance and should have
immediately isolated the steam supply upon sensing a high exhaust pressure
before the rupture discs burst.  The fact that a high exhaust line pressure
was not detected indicates two possible causes for the disc rupture.  Either
the inner rupture disc was degraded and burst at lower-than-design pressure,
or the pressure pulse actually exceeded the design pressure and caused the
disc to burst, relieving the exhaust line pressure before the downstream
pressure sensors detected a high pressure condition.

While inspection of the rupture discs did not reveal any degradation from
corrosion or aging, the vendor, Black Sivalls & Bryson, Inc, stated that the
discs are warranted for one year of service under normal conditions.  The HPCI
and RCIC rupture discs at Quad Cities Station had been in service for 20 years
and were not part of any scheduled inspection or preventive maintenance
program.  When consulted about the event, the vendor advised against using the
spare discs in the plant storeroom because they were purchased at the same
time as the failed discs.  The licensee later replaced the failed discs with
new units.  Before this event, the licensee recently replaced a HPCI rupture
disc during a refueling outage at the Dresden Station, a plant similar to Quad
Cities, after finding a crack in the disc during an inspection.  The rupture
discs were inspected at the Dresden station as part of an enhancement to the
preventive maintenance program that was recommended by a reliability-centered

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The steam injured five workers, four of whom were participating in the HPCI
pump surveillance test.  The fifth, and most severely injured worker was a
health physics technician in the room on routine rounds, who was not aware of
the danger posed by the surveillance test.  The test procedure contained no
specific guidance on room occupancy.  Interviews with the workers revealed
that they had performed the surveillance in the past and were familiar with
the process.  This familiarity may have led to a relaxed attitude toward
personal safety.  The workers stated that during previous surveillances they
sometimes evacuated the room before rolling the turbine, or stood near the
doors to be ready to escape.  The licensee made no routine announcement over
the plant paging system to alert plant personnel before the HPCI turbine
start, and the workers were not prepared for the turbine to start.

A lack of constant communication between the workers and the control room may
have contributed to the personnel injuries.  If a control room operator had
been in contact with the workers in the HPCI room during the turbine roll, the
workers would have been aware of the impending turbine start and, after start,
the steam supply might have been manually isolated or the turbine manually
tripped before the automatic isolation caused by the high area temperature in
the HPCI room.  

When the Unit 1 HPCI rupture disc failure caused the fire doors to be blown
off their hinges and into the Unit 2 HPCI room, one of the doors impacted a
pipe hanger in the room and moved it 5.1 centimeters [2 inches].  If the steam
release had resulted from a high energy line break from the inlet side of the
HPCI steam line instead of the exhaust line, the damage to the Unit 2 HPCI
system could have been more severe.  While high energy line breaks have been
analyzed for safety systems at all plants, the doors to rooms containing high
energy steam lines may not have been included in the analysis.  For example,
at Duane Arnold Energy Center, the licensee discovered that while the HPCI and
RCIC rooms were qualified for pressures of 26.2 kPa [3.8 psig], the doors
between these rooms and the reactor building would yield at 6.9 kPa [1 psig].

The licensee for the Quad Cities Station will implement a preventive
maintenance schedule for both the HPCI and RCIC rupture discs and the level
switches.  The licensee will review the surveillance test procedure and
evaluate the missile hazard created by the fire doors.  The licensee replaced
the rupture discs for the Unit 2 HPCI and RCIC systems with new ones after the
event at Unit 1 and is considering burst testing the removed discs to
determine if they are degraded..

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This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.

                                    /s/'d by CIGrimes/for

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Clark Vanderniet, RIII
                     (708) 790-5594

                     David Skeen, NRR
                     (301) 504-1174

1.  Figure 1, "Simplified Layout of Quad Cities Station HPCI Turbine Exhaust   
    Steam Line"
2.  List of Recently Issued NRC Information Notices.
Page Last Reviewed/Updated Tuesday, November 12, 2013