Information Notice No. 91-63: Natural Gas Hazards at Fort St. Vrain Nuclear Generating Station

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                               October 3, 1991

                               NUCLEAR GENERATING STATION


All holders of operating licenses or construction permits for nuclear power 


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to problems that could result from the presence 
of natural gas at nuclear facilities.  It is expected that recipients will 
review the information for applicability to their facilities and consider 
actions, as appropriate, to avoid similar problems.  Although this notice 
specifically describes hazards associated with natural gas, licensees should 
note that similar hazards may be imposed by other toxic, flammable, or 
explosive materials which may be brought into close proximity to licensee's 
facilities.  However, suggestions contained in this information notice are 
not new NRC requirements; therefore, no specific action or written response 
is required.


When the Fort St. Vrain facility was licensed in 1973, no natural gas 
pipelines were located nearby; thus, neither the licensee's final safety 
analysis report nor the NRC staff's safety evaluation report addressed the 
hazards associated with natural gas present in geological formations in the 
area.  In 1974, a 16-inch low pressure natural gas collection pipeline was 
constructed nearby by a natural gas company.  This pipeline crossed a corner 
of the licensee's property about 0.9 mile from the reactor building.  At its 
closest point, this pipeline came within 0.85 mile of the reactor building.  
Although the pipeline was not constructed by the Public Service Company of 
Colorado (PSC), the Fort St. Vrain licensee, PSC was informed of its 

Between 1981 and 1983, 12 natural gas wells were installed within about one 
mile of the Fort St. Vrain reactor building.  Seven of these wells were 
drilled on land owned by the Fort St. Vrain licensee by a company that had 
acquired the mineral rights from the licensee.  Some of these wells were 
located within the Exclusion Area Boundary, but all were outside of the 
protected area.  Nine of these wells were connected to the 16-inch 
collection pipeline by a 6-inch pipeline.  The closest well was located 1524 
feet from the reactor building, and the 6-inch pipe passed within 1340 feet 
of the reactor building.  Personnel involved in the drilling of these wells 
and the licensee's manager of nuclear production concluded that possible 
accidents at the well sites would not 


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produce adverse effects further than 300 feet from the wellheads.  The 
licensee did not prepare any written analyses for any of the wells or 

Late in 1987, PSC allowed the drilling of a gas well within 1184 feet of the 
Fort St. Vrain reactor building, which was within the Exclusion Area 
Boundary and 300 feet from the protected area fence.  The pipeline 
associated with this well passed within 560 feet of the Fort St. Vrain 
switchyard.  PSC prepared a 10 CFR 50.59 safety evaluation that concluded 
that in the event of a well fire caused by a blow out, the area affected 
both by the fire and the equipment needed to control it would not be larger 
than the existing drillsite location.  The licensee also concluded that the 
air temperature beyond a radius of 200 feet would not be elevated above the 
ambient temperature and therefore, the construction and operation of the 
closest well did not create the possibility of a new type of reactor plant 
accident or constitute an unreviewed safety question.  The safety analysis 
did not evaluate the consequences of a rupture of either the 6-inch or 
16-inch pipeline and did not postulate the release of a cloud of natural gas 
which might drift toward safety-related structures or equipment and ignite 
and either burn or detonate.

Description of Circumstances

On August 18, 1989, the licensee shut down the Fort St. Vrain facility and, 
by letter of August 29, 1989, informed the NRC that the plant would be shut 
down permanently.  In November 1990, the licensee submitted a proposed 
decommissioning plan.  While reviewing the proposed decommissioning plan, 
the NRC became concerned that plans to introduce natural gas at Fort St. 
Vrain as part of a proposed repowering of the facility could lead to an 
accident that had not been reviewed.  During this review, the NRC further 
determined that the licensee had not adequately addressed the natural gas 
already on site.  The licensee responded by preparing analyses that 
addressed the limiting failures of all natural gas pipelines existing on 
site.  Before completing these analyses, the licensee took prompt corrective 
actions to limit the amount of natural gas that could be released from a 
large rupture of the 6-inch collection pipeline.  These actions consisted of 
closing a 6-inch valve and opening a 1 1/2-inch bypass valve in the line 
that carried gas from the wells to the 16-inch collection pipeline.  This 
configuration would reduce the gas leakage from a rupture in the 6-inch 
pipeline by reducing the flow of gas back from the 16-inch pipeline.  

The NRC staff reviewed the licensee's analyses including those regarding 
flammability and explosive hazards associated with the gas cloud that could 
be released by a postulated worst-case rupture of a wellhead or pipelines.  
Both the licensee and the NRC determined that, with the compensatory 
measures taken, postulated accidents associated with the natural gas on site 
would not affect the safe storage of spent fuel or other radioactive 
components in the reactor building.  After reaching this conclusion, the NRC 
issued the Fort St. Vrain possession only license (POL) amendment on May 21, 
1991.  The POL amendment included a license condition that requires the 
licensee to obtain the NRC's approval before making changes to the 
collection system that would involve permanently opening the 6-inch valve.

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In August 1991, the licensee completed additional analyses which indicated 
that postulated explosions or deflagrations resulting from natural gas line 
ruptures with the 6-inch line open would not have resulted in unacceptable 
consequences at the Fort St. Vrain reactor building or at the switchyard.  
Nevertheless, PSC indicated that redundant check valves would be installed 
in the 6-inch pipeline to reduce the possibility that natural gas could flow 
back from the 16-inch pipeline, if the 6-inch line ruptured.

Discussion of Safety Significance

The natural gas pipelines and wells completed between 1973 and 1983 
introduced additional unanalyzed external hazards that could have affected 
the safe operation of the Fort St. Vrain facility.  These additional hazards 
were not evaluated by the licensee prior to their introduction to the site 
to determine the impacts on the safe operation of the plant and whether 
these hazards exceeded those evaluated during the initial licensing of the 
facility.  For the gas well drilled in 1987, the licensee's 10 CFR 50.59 
evaluation was too narrowly focused and did not consider additional possible 
malfunctions before concluding that an unreviewed safety question was not 

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 project manager.

                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation

Technical contacts:  Richard Dudley, Jr., NRR 
                     (301) 492-1116 

                     Stephen Koscielny, NRR 
                     (301) 492-0726

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