United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-34: Hydrogen Gas Ignition During Closure Welding of a VSC-24 Multi-Assembly Sealed Basket

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

May 31, 1996


NRC INFORMATION NOTICE 96-34: HYDROGEN GAS IGNITION DURING CLOSURE WELDING OF
                              A VSC-24 MULTI-ASSEMBLY SEALED BASKET 


Addressees  

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

Independent spent fuel storage installation designers and fabricators.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a hydrogen gas ignition event that occurred
during the welding of the shield lid on a spent fuel storage cask at the Point
Beach Nuclear Plant.  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 May 28, 1996, a hydrogen gas ignition occurred during the welding of the
shield lid on a ventilated storage cask (VSC-24) multi-assembly sealed basket
(MSB).  The gas ignition displaced the shield lid (weighing about 2898
kilograms [6,390 pounds]), leaving it in place but tipped at a slight angle,
with one edge about 7.6 centimeters [3 inches] higher than normal.

The VSC-24 multi-assembly transfer cask (MTC), a shielded lifting device used
to transfer the MSB loaded with spent fuel to the ventilated concrete cask,
had been placed in the cask decontamination work area in the auxiliary
building.  Approximately 114 liters [30 gallons] of spent fuel pool water had
been drained from the MSB to facilitate welding of the shield lid, creating an
air space below the lid.  The hydrogen gas ignition occurred during the
initiation of the shield lid welding, approximately 11 hours after the loaded
MTC had been removed from the spent fuel storage pool.  

Discussion

Following the event, gas and water samples collected from the MSB internals
showed detectable levels of hydrogen both in the air space beneath the shield
lid and dissolved in the MSB water.  The licensee then continuously purged the
air space beneath the lid with nitrogen to prevent the accumulation of
combustible gases and returned the shield lid to its original position.   

9605310132.                                                            IN 96-34
                                                            May 31, 1996
                                                            Page 2 of 2


The MSB was then fully flooded to eliminate the air space under the shield lid
and returned to the spent fuel storage pool.  The licensee unloaded the spent
fuel assemblies and placed them in the spent fuel pool storage racks.

The MTC/MSB was subsequently moved back to the decontamination work area for
further inspection as part of the licensee's investigation of the combustible
gas ignition.  A visual examination of the MSB, the MTC, and the spent fuel
assemblies showed no evidence of damage as a result of the combustible gas
ignition. 
  
The investigation into the possible sources of hydrogen is focusing on a zinc-
based coating applied to the internal surfaces of the MSB.  The zinc may have
reacted chemically with the acidic borated water from the spent fuel storage
pool to produce hydrogen.  Borated water is used for criticality control in
the spent fuel storage pool water at plants with pressurized water reactors
and during fuel loading operations with this cask design.  

An NRC Augmented Inspection Team (AIT) has been formed and is on site to
investigate the event.  The objectives of the AIT are to identify and
communicate both the facts of the event and any generic safety concerns and to
document the findings and conclusions of the on site inspection.

Related Generic Communications

IN 95-29, "Oversight of Design and Fabrication Activities for Metal Components 
           Used in Spent Fuel Dry Storage Systems."

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.


      signed by                                       signed by
                                                      Charles J. Haughney
Brian K. Grimes, Director                       William D. Travers, Director
Division of Reactor Program Management          Spent Fuel Project Office
Office of Nuclear Reactor Regulation            Office of Nuclear Material
                                                  Safety and Safeguards

Technical contact:  A. Hansen, NRR
                    (301) 415-1390
                    Internet:agh@nrc.gov  

Attachment:  List of Recently Issued NRC Information Notices


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