United States Nuclear Regulatory Commission - Protecting People and the Environment

Research Results on Generic Sagety Issue 106, "Piping and the Use of Highly Combustible Gases in Vital Areas" (Generic Letter 93-06)


October 25, 1993


TO:       ALL HOLDERS OF OPERATING LICENSES OR CONSTRUCTION PERMITS FOR 
          NUCLEAR POWER REACTORS

SUBJECT:  RESEARCH RESULTS ON GENERIC SAFETY ISSUE 106, "PIPING AND THE USE    
          OF HIGHLY COMBUSTIBLE GASES IN VITAL AREAS" (GENERIC LETTER 93-06)

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter to
inform addressees about technical findings resulting from the NRC resolution
of Generic Safety Issue 106 (GSI-106), "Piping and the Use of Highly
Combustible Gases in Vital Areas."  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 generic letter are not NRC requirements; therefore, no specific action or
written response is required.

DISCUSSION

The basic regulatory requirement dealing with the storage, distribution, and
use of combustible gases at nuclear power plants is General Design Criterion
(GDC) 3, "Fire Protection," Appendix A, Part 50, Title 10 of the 
Code of Federal Regulations (10 CFR Part 50).  This criterion states, in part,
that "structures, systems, and components important to safety shall be
designed and located to minimize, consistent with other safety requirements,
the probability and effect of fires and explosions."  Additional discussion of
the regulation of this subject is provided in NUREG-1364, "Regulatory Analysis
for the Resolution of Generic Safety Issue 106:  Piping and the Use of Highly
Combustible Gases in Vital Areas," Section 1.2 (Enclosure 1).*  

Reviews of plant literature, site visits, and discussions with licensees have
indicated large differences in individual plant characteristics that could
affect risk from failures of hydrogen system lines or components.  These
differences include the hydrogen storage and distribution system design
features and relative locations of hydrogen components and safety-related
equipment.  On the basis of generic evaluations, the NRC staff has concluded
that several possible methods to reduce risk, involving equipment
modifications and administrative controls, could provide cost-effective safety
benefits at some plants.  However, the NRC staff also concludes, based on a
small sample of plants, that the safety benefit of recommended actions for
 
                                      
*Copies of this document are enclosed for addressees.  For other readers, a
copy of this document is available for inspection and copying in the NRC
Public Document Room, 2120 L Street NW, Washington, DC 20037.


9310200286.

Generic Letter 93-06               - 2 -          October 25, 1993


some or all licensees or applicants is marginal.  The reviews indicated that a
number of plants have system design characteristics, operating procedures, and
other mitigating features that would be responsive to some or all of the
concerns of this generic issue.  While the staff analysis indicates that the
industry-wide risk is small, it cannot preclude the possibility of larger risk
at some plants.  The NRC is aware that information relevant to 10 CFR Part 21,
has been made available to licensees and applicants with General Electric
boiling-water reactor (BWR) plant designs, emphasizing the need for individual
licensees and applicants to determine the safety hazard of a postulated
generator coolant hydrogen explosion in their plants (Enclosure 2).  In
addition, in March 1993, a turbine fire, which may have been caused by turbine
blade failure, vibration, and hydrogen seal leakage, occurred in a nuclear
power plant in India.

In view of the observed large differences in plant-specific characteristics
affecting the risk associated with the use of hydrogen, and the marginal
generic safety benefit that can be achieved in a cost-effective manner, the
NRC intends to resolve this generic issue by making these results available in
this generic letter.  This information may help licensees in their plant
evaluations recommended by Generic Letter 88-20, Supplement 4, "Individual
Plant Examination of External Events for Severe Accident Vulnerabilities,"
June 28, 1991.

As part of the NRC evaluation of GSI-106, the risk from potential hydrogen
system failures was analyzed by the Idaho National Engineering Laboratory
(INEL).  The technical findings are reported in NUREG/CR-5759, "Risk Analysis
of Highly Combustible Gas Storage, Supply, and Distribution Systems in
Pressurized Water Reactor Plants," July 1991; EGG-SSRE-10198, "Risk Analysis
of Highly Combustible Gas Storage, Supply, and Distribution Systems in
Pressurized Water Reactor Plants--Supplementary Cost/Benefit Analysis,"  
March 1992; and EGG-NTA-9082, "Scoping Risk Analysis of Highly Combustible Gas
Storage, Supply, and Distribution Systems in Boiling Water Reactor Plants,"
November 1991.  In addition, the NRC staff evaluated the safety benefits and
costs of implementing various alternatives to reduce generic risk in  
NUREG-1364.  This regulatory analysis includes discussion of several precursor
events involving the storage, distribution, and use of hydrogen (the
combustible gas of principal concern) at nuclear power plants.**   

The scope of GSI-106 included evaluation of the risk from (1) the storage and
distribution of hydrogen for the volume control tank (VCT) in PWRs and the 
main electric generator in BWRs and PWRs; (2) other sources of hydrogen such
as battery rooms, the waste gas system in PWRs and the offgas system in BWRs;
and (3) small, portable bottles of combustible gases used in maintenance,
testing, and calibration.  The risk from large storage facilities outside the
reactor, auxiliary, and turbine buildings is being addressed separately and is
not within the scope of GSI-106.


_______________
**Copies of these reports are available for inspection and copying in the NRC
Public Document Room, 2120 L Street NW, Washington DC 20037. 
.

Generic Letter 93-06            - 3 -                 October 25, 1993


Screening studies described in NUREG/CR-5759 and EGG-NTA-9082 indicated small
risk for the battery rooms, waste gas and offgas systems, and portable
bottles.  The assessment for the generic risk associated with the hydrogen
distribution system to the electric generator at BWRs involved a vital area
analysis for an actual plant configuration (a BWR-4 with a Mark I
containment), supplemented by information obtained from visits to five other
plants.  The scoping analysis based on this sample of BWRs (two BWR-3s, two
BWR-4s, and two BWR-5s) indicates a small generic risk, but cannot preclude 
the possibility of a larger plant specific risk because of the possible
presence of safety-related equipment in the turbine building.  In 
addition, this scoping analysis did not consider the effect of hydrogen
explosions on barrier walls and on penetrations such as doors between the
turbine building and the adjoining reactor, control, or auxiliary buildings
for these six BWR plants.  

The findings of a more detailed generic risk analysis for the distribution
systems for the VCT and electric generator at PWRs are reported in  
NUREG/CR-5759.  The hydrogen distribution systems to the VCT and generator are
not located near the reactor and primary coolant system piping.  Hence,
hydrogen fires or explosions would not lead to such events as pipe break loss
of coolant accidents (LOCAs), anticipated transients without scram, and steam
generator tube ruptures.  INEL divided the remaining transient-induced core
damage events into transients with failure of decay heat removal systems
(T/DHR) and transient-induced loss of coolant accidents (T/LOCA).  The
initiating event is either a random or seismically induced leak or break in
the hydrogen system that releases hydrogen.  This released hydrogen creates
the potential for a fire or explosion that could cause loss of equipment and
lead to either a T/DHR or a T/LOCA.  The T/DHR events involve scenarios with
loss of all forms of core cooling and coolant release at high pressure from
the pressurizer safety and relief valves.  The T/LOCA events involve failure
of reactor coolant makeup or recirculation systems following a loss of reactor
coolant pump seal cooling or stuck-open safety or relief valves.  In its
generic analysis of GSI-106, INEL addressed risks associated with the T/DHR
and T/LOCA events and considered such plant functional characteristics as
feed-and-bleed cooling capability and relative locations of hydrogen
distribution systems and pertinent equipment (e.g., auxiliary feedwater,
normal and emergency ac power, essential service water, and component cooling
water).  

For the auxiliary building, which may contain most of the safety-related
systems at the plant, the following alternatives were found to be cost
effective:  (1) use of restricting orifices or excess flow valves to limit the
maximum flow rate from the storage facility to the postulated break and 
(2) use of a smaller storage facility normally connected to the VCT to limit
the maximum hydrogen release in a single event.  An alternative involving use
of a normally isolated supply with intermittent manual makeup was somewhat
less cost-effective.  These approaches include preoperational testing and
subsequent retesting of excess flow valves and measures to prevent buildup of
unacceptable amounts of trapped hydrogen and inadvertent operation with the
safety features bypassed.
.

Generic Letter 93-06            - 4 -                   October 25, 1993


For the turbine building, which may also contain safety-related equipment, two
cost-effective alternatives were found for protection against breaks in the
hydrogen supply line up to the hydrogen control station below the generator,
including any branch lines from this line to other buildings.  These involve
limits on the maximum flow rate or operation with a normally isolated supply. 
Isolation of the large quantities of hydrogen (up to about 700 standard cubic 
meters [25,000 standard cubic feet]) contained in the generator probably is
not possible for most breaks downstream of the hydrogen control station.  The
only alternative considered applicable to breaks at or near the generator 
involved structural modifications to prevent fire or blast damage to affected
safety-related equipment; this alternative was not found to be cost-effective.

Additional general measures for risk reduction, such as the use of color
coding, warning signs and training to handle events in the auxiliary and
turbine buildings were considered.  Of these, training to stop hydrogen flow
(e.g., isolation of the storage facility or venting and purging of the
generator) and training to prevent associated large oil fires in the turbine
building were deemed most important.

BACKFIT DISCUSSION

In this generic letter, the NRC is only communicating information on results
of government-sponsored research to resolve a generic safety issue and is not
recommending that licensees or applicants take particular courses of action or
requesting that licensees communicate information back to the NRC on this
matter.  Consequently, this generic letter does not represent a backfit.

If you have any questions about this information, please call one of the
technical contacts listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.

                                    Sincerely,


                                    James G. Partlow 
                                    Associate Director for Projects
                                    Office of Nuclear Reactor Regulation

Enclosures:
1.  NUREG-1364, "Regulatory Analysis for
    the Resolution of Generic Issue 106:
    Piping and the Use of Highly Combustible 
    Gases in Vital Areas" (for addressees)
2.  Letter from J. P. Riley, General Electric 
    Company, to S. E. Scace, Millstone Nuclear 
    Power Station, on "Postulated Hydrogen 
    Explosion in a Non-United States Reactor 
    Turbine Building Mezzanine," December 23, 1992
3.  List of Recently Issued NRC Generic Letters 

Technical Contacts:  Gerald Mazetis, RES  Vern Hodge, NRR
                     (301) 492-3906     (301) 504-1861 



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