United States Nuclear Regulatory Commission - Protecting People and the Environment

NRC Positions on Certain Requirements of Appendix R to 10 CFR 50 (Generic Letter No. 83-33)

UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555

October 19, 1983

TO ALL LICENSEES AND APPLICANTS OF NUCLEAR POWER REACTORS

Gentlemen:

SUBJECT: NRC POSITIONS ON CERTAIN REQUIREMENTS OF APPENDIX R TO 10 CFR 50 (GENERIC LETTER 83-33)

During our evaluations of exemption requests, we determined that some licensees were interpreting certain requirements of Appendix R in a manner that was not consistent with the position that the staff was using. Where any such differences were discovered, we informed these licensees in the NRC Safety Evaluation Report supporting the granting or denial of an exemption. More recently, we have completed inspections for conformance to Appendix R at four plants, the licensees for which had indicated that all modifications for conformance had been completed or other modifications approved by exemptions had been completed. In these inspections, the NRC inspection team also identified what the staff considers to be non-conformance with requirements of Appendix R, for which exemptions had not been requested of justified.

Therefore, we are transmitting the enclosure to all licensees and applicants for information and use as appropriate. The NRC inspection teams that will be conducting the inspections for conformance to Appendix R at each plant will be using these positions as their criteria for conformance for these particular issues. No written response to this letter is required.

Sincerely,

Darrell G. Eisenhut, Director
Division of Licensing
Office of Nuclear Reactor Regulation

Enclosure:  As stated


(NUDOCS Accession Number 8310200362 )

Enclosure
GL 83-33
October 19, 1983

NRC Staff Positions on Certain
Requirements of Appendix R to 10 CFR 50

Introduction

During our reviews of Appendix R exemption requests and our review of applications for operating licenses, it has become apparent that certain requirements of Appendix R to 10 CFR 50 and the corresponding guidelines in SRP 9.5-1 were not being interpreted correctly by some licensees. On several occasions members of the staff met with representatives of the Nuclear Utility Fire Protection Group (NUFPG), other industry representatives, and individual licensees to disucss clarification of certain requirements. The staff agreed to send the staff positions on these issues to all licensees.

1.  Detection And Automatic Suppression

Staff Position:  Subsections III.G.2.b, III.G.2.c, and III.G.2.e require that fire detectors and an automatic fire suppression system be installed "in" any fire area. To satisfy this requirement, the fire detectors and automatic suppression system need to be installed "throughout" the fire area.

Some licensees have not interpreted "in" the fire area to require full detection and suppression "throughout" the fire area. This interpretation makes the requirement ambiguous.

In some fire areas, however, the installation of a fire detection and a fire suppression system throughout the fire area may not significantly increase the level of fire safety afforded by only partial coverage; or the installation of a fire suppression system throughout the area may he detrimental to overall plant safety. Such areas must be evaluated under the exemption process, along with a fire hazards analysis that shows the installation of fire detection and/or suppression systems in only select locations within the fire area will provide an equivalent level of protection.

2.  Fire Areas

Staff Position:  Section III.G of Appendix R sets forth the requirement for fire protection for safe shutdown capability on the basis of fire areas.

A fire area is defined as that portion of a building or plant that is separated from other areas by boundary fire barriers (walls, floors and ceilings with any openings or penetrations protected with seals or closures having a fire resistance rating equal to that required of the barrier). Open stairwells and hatchways in ceilings and floors are not fire area boundaries.

For boundary fire barriers, using walls, floors, ceilings, dampers, doors, etc. existing prior to Appendix R, the rating required of a boundary fire barrier is based on the guidance in Appendix A to BTP ASB 9.5-1, i.e.,the rating of the barrier or boundary must exceed with margin the fire loading in the area and need not necessarily be a 3-hour rated boundary unless the fire loading warrants such a boundary. For modifications which involve the installation of new boundary fire barriers pursuant to Section III.G.2.a, the fire rating of such boundaries must be three hours, or an exemption must be justified and requested.

The evaluations by some licensees made prior to Appendix R were based on fire zones which do not meet the strict definition of fire areas clarified above. In some cases, the separation of redundant trains under consideration within the "fire zone boundaries" and the separation between fire zones does not comply with the separation, i.e., barrier or distance, requirements of Appendix R. Such configureations need to be evaluated under the exemption process.

The fire protection requirements are intended to provide reasonable assurance that at least one safe shutdown division is free of fire damage after a postulated fire in any fire area. The definition of "fire areas,"noted above, is predicated on sound fire protection engineering principles as they apply to limiting the fire and fire suppressant damage to redundant shutdown equipment and cables. Fire areas defined by non-physical boundaries, such as "logical divisions"or "equipment groupings", may not necessarily restrict fire and smoke spread, and do not necessarily provide reasonable assurance that the limits of fire or fire suppressant damage to shutdown systems have been defined.

In many plant areas, however, the erection of physical barriers between redundant shutdown systems is precluded by the location of cable trays, HVAC ducts and other plant features. In such situations, the staff has accepted, in concept, the use of an automatic fire suppression system which discharges a "water curtain" across the boundary areas separating the redundant systems. The design of such "water curtains" has not been codified, i.e., the National Fire Protection Association Standards do not address the use of fire suppression systems for such applications. However, the staff is currently working with several applicants and licensees to define design requirements which will satisfy mutual concerns. The staff's present position is that such systems should feature close-spaced, open-head sprinklers with water discharge initiated by tripping a deluge valve activated by cross-zoned smoke detectors. Where smoke propagation does not represent a hazard to redundant shutdown systems, a close-space, close-head sprinkler system may be deemed acceptable. Where such "water curtains" are used, the operation of such systems should not endanger safety systems on either side of the "water curtains."

3.  Structural Steel Related To Fire Barriers

Staff Position:  Appendix R requires that structural steel forming a part of, or supporting a fire barrier separating redundant trains shall be protected so as to have a fire rating equivalent to the fire resistance required of the barrier.

The protection of structural steel is required because steel loses strength when subjected to temperatures that may be attained in a fire. 1100 degrees Fahrenheit is normally considered to be the critical temperature. At this temperature the yield stress in steel has decreased to about 60 percent of the value at room temperature. This is approximately the level normally used as the design working stress. Because steel has a high thermal conductivity, and heat is transferred away from a localized heat source rather quickly, a relatively long period of time is required to reach the critical temperature. However, an exposure fire that distributes heat over a greater area may reduce this time considerably. Structural steel need only be rated to the level of the barrier of which it is a part, based on the combustible loading in the area. If protection is required to achieve such a rating, then the steel would have to be protected. In cases where the structural steel is not protected and has a lower fire rating than the required rating of the fire barrier, an exemption must be requested and justified by a fire hazards analysis which shows the temperature the steel will reach during fire, and the ability of the steel to carry the required loads at that temperature.

4.  Fixed Suppression System

Staff Position: A fixed fire suppression system shall have discharge heads and the distribution piping for such heads installed. Hose stations do not satisfy this requirements.

The majority of areas for which a fixed fire suppression system is required contain large concentrations of cables and, therefore, have high fire loadings. In addition, access for fire fighting may be hampered by congestion and smoke. A fixed fire suppression system should be capable of controlling a fire in such areas even under limited access conditions.

5.  Intervening Combustibles

Staff Position: Section III.G.2.b requires the "separation . . . with no intervening combustibles . . ." To meet this requirement, plastic jackets and insulation of grouped electrical cables, including those which are coated, should be considered as intervening combustibles.

Numerous comprehensive flammability tests conducted by the Electric Power Research Institute (EPRI NP-1200, EPRI EL-1263), Factory Mutual (Contract RP-1165-l), and Sandia National Laboratories (NUREG/CR-2431, among others) have shown that burning plastic cable insulation represents a significant fire hazard. These tests were conducted on both IEEE-383 qualified and unqualified cable. While the qualified cable exhibited a tendency to ignite and propagate flame less rapidly, combustion of grouped cables continued at significant levels. In particular, grouped vertical cables which are not protected by a fire propagation retardant, such as metal tray covers or fire retardant coatings, can result in rapidly developing fires with high heat release rates.

6.  Transient Fire Hazards

Staff Position:  When addressing transient combustibles in exemption requests, the fire hazards analysis should consider the conservative bounding value for a transient fire hazard that could reasonably be expected over the life of the plant.

Over the past few several years, several attempts have been made by the NRC and industry to develop criteria for establishing "design basis transient combustibles." These have been suggested:

  1. The maximum amounts permitted by the plant's administrative controls or some multiple of that amount.

  2. Selected amounts (e.g., 1 pint, 1 quart, or 1-20 gallons) of a combustible liquid (e.g., acetone, heptane, lube oil, or solvents).

However, none of these criteria have stood up to critical evaluation as to why they are bounding conditions in all possible circumstances. During the life of the plant, transient combustible materials may be located in, or pass through safety related areas. These hazards arise from activities associated with operation, maintenance, repairs or modifications. They may arrive deliberately under approved work permits or inadvertently asa temporary expedient. Usually, a fire involving such materials would not overpower the fire protection features provided in accordance with Section III.G and, therefore, are only of concern when exemptions or deviations are requested.

In reviewing "transient combustibles" when evaluating exemption requests, the staff considers, among other things:  (1) the physical attributes of the area that will tend to limit the amounts of transient combustibles, e.g., restricted access due to the environment within the area and location of the area or physical access limitations; (2) whether the fire area is required by the plant Technical Specifications to be manned continuously; (3) the physical attributes of the fire area and configuration of the systems of concern which apply to their capability to limit fire and fire suppressant damage; and (4) the safety significance of the systems of concern.

Page Last Reviewed/Updated Tuesday, May 27, 2014