Information Notice No. 94-53: Hydrogen Gas Burn Inside Pressurizer During Welding

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.  20555

July 18, 1994


NRC INFORMATION NOTICE 94-53:  HYDROGEN GAS BURN INSIDE PRESSURIZER DURING
WELDING


Addressees

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

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for the ignition of hydrogen gas
mixtures during welding operations while the reactor coolant system is
depressurized.  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.

Description of Circumstances

On February 3, 1994, a control room operator at Surry Power Station, Unit 1,
observed pressure spikes on the pressurizer level instruments.  At the same
time, a loud rumbling sound was heard in the containment.  The containment
vent radiation alarm was received, and as a precaution, the containment was
evacuated.

At the time of the event, the pressurizer was drained and vented to both the
pressurizer relief tank and the containment atmosphere through a filtered
covering over the piping where the safety valves had been removed.  The
reactor vessel head had just been detensioned.  There was fuel in the reactor
vessel.  A modification to eliminate the pressurizer loop seals was in
progress.

The pressure fluctuations inside the pressurizer caused the expulsion of
radioactive gases from the pressurizer into the containment causing alarms on
both the containment radiation monitor and the ventilation radiation monitor
where the ongoing filtered containment purge was directed.  Licensee personnel
calculated the release rate through the monitored ventilation purge path to be
approximately 30 percent of the technical specification limits.  One worker
received an estimated internal exposure of 70 Sv [7 mrems] and a total dose
of 160 Sv [16 mrems].

The licensee investigation showed that the screens for excluding foreign
material that were taped over the pressurizer side of the piping where the

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three safety valves were removed were discolored and appeared burned.  The
licensee concluded that a hydrogen gas burn had occurred inside the
pressurizer.  Welding activities associated with the pressurizer loop seal
modification had ignited the hydrogen gas that had come out of solution and
accumulated inside the pressurizer and associated piping.  There were no
personnel inquiries.

Additional details pertaining to this event can be found in Licensee Event
Report 50-280/94-001 dated March 4, 1994, and NRC Inspection Report
50-280/94-002 dated March 7, 1994.

Discussion

During refueling operations, the reactor coolant system was depressurized and
the pressurizer level was drained while a nitrogen blanket was provided via
the pressurizer relief tank.  Before the coolant system was depressurized, the
system was degassed and the hydrogen concentration was reduced to less than
5 cc/kg in accordance with recommendations from Westinghouse, the nuclear
steam system supplier.

The source of the hydrogen gas inside the pressurizer was investigated by the
licensee's root cause team.  The team concluded that pockets of trapped gas
either in the reactor coolant system or in the pressurizer relief tank had
moved as a result of the effects of changing containment pressure on the
pressurizer.  The pressure changes had acted through the removed safety valve
openings.  Because the licensee had maintained the containment at a slight
negative pressure during most of the refueling shutdown, opening and closing
of the equipment hatch caused changes in the containment pressure.

The licensee investigation showed that a similar event had occurred at the
Wolf Creek Nuclear Plant in October 1987.

The licensee imposed additional controls for welding on the primary system.
These require that samples be taken and analyzed before initiating an arc to
determine if explosive gasses are present.  The licensee also evaluated
pressurizer stresses associated with the rapid hydrogen burn and completed a
piping support inspection and internal inspection of the pressurizer.  The
evaluation showed that no unacceptable stresses had resulted, and no damage
was noted during the inspections.

Related Generic Communications

In Information Notice 82-28, "Hydrogen Explosion While Grinding in the
Vicinity of Drained and Open Reactor Coolant System," the NRC described a
hydrogen gas explosion that took place when maintenance personnel at Arkansas
Nuclear One, Unit 1, were grinding a recently cut high-pressure injection
pipe.  The cut was approximately 46 centimeters [18 inches] from the nozzle
connecting the injection pipe to the reactor coolant system piping.
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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
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

/s/'d by BKGrimes


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

Technical contacts:  Morris Branch, RII
               (804) 357-2101

               Mark A. Caruso, NRR
                     (301) 504-3235

Attachment:
List of Recently Issued NRC Information Notices