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

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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.Generic Letter 93-06October 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.

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

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Generic Letter 93-06October 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.

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Generic Letter 93-06October 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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