UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

July 19, 2002

NRC INFORMATION NOTICE 2002-24: POTENTIAL PROBLEMS WITH HEAT

COLLECTORS ON FIRE PROTECTION

SPRINKLERS

Addressees

All holders of licenses for nuclear power, research, and tests reactors and fuel cycle facilities.

Purpose 

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert

licensees to potential problems with using heat collectors on sprinklers and fire detectors

installed to satisfy NRC fire protection requirements. It is expected that recipients will review

the information for applicability to their facilities and consider actions, as appropriate. However,

suggestions contained in the IN are not NRC requirements; therefore, no specific action or

written response is required. 

Background

NRC fire protection engineers and inspectors have recently raised technical concerns regarding

the adequacy of sprinklers that rely on metal plates, commonly referred to as “heat collectors,”

to activate (see Figure 1 in the attachment). When convective heat energy rises above the fire

plume, it terminates its vertical movement at the ceiling and flows past the sprinkler’s heatsensitive element. This flow of heat across the ceiling is called the “ceiling jet.” The ceiling

directs the convective heat energy horizontally towards the sprinklers nearest the fire. If the

sprinkler’s heat-sensitive element is a fusible link, the heat melts the link, releasing water to

control the fire. If the sprinkler’s heat-sensitive element is a glass bulb, the heat expands the

liquid in the glass bulb until the bulb breaks, releasing water to control the fire. 

Heat collectors were intended to reduce the time a fire takes to activate sprinklers located too

far below the ceiling. When sprinklers are too far below the ceiling, most of the heat energy

rises past the sprinklers and heat collectors and the sprinklers are not activated. Locating the

sprinkler close to the ceiling ensures that the sprinkler will be in the hot gas layer, minimizing

activation time and enabling the sprinkler to provide a fully developed water spray pattern to

control the fire. In addition, the water from the sprinkler cools the upper gas layer (preventing

flashover conditions) and cools the structural steel supports of the compartment boundaries

(preventing structural collapse). 

In the late 1970s and early 1980s, some sprinkler system designers and fire protection

engineers supported the opinion that sprinklers could be placed far below the ceiling if heat

collectors were installed above them to help activate them during a fire. They reasoned that 

ML022000376
1

NFPA 13, Section 5-5.5.3.1, 1999 edition, states that “sprinklers shall be installed under fixed

obstructions over 4 ft (1.2 m) wide such as ducts, decks, open grate flooring, cutting tables, and

overhead doors.”

IN 2002-24

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heat from the fire plume can create temperatures or draft conditions that cause water droplets

to evaporate before penetrating and cooling the plume and that large clearances between

sprinklers and floor-level combustibles could aggravate the problem of ensuring that the correct

sprinkler water density is available from sprinklers. Therefore, they installed heat collectors so

that sprinklers could be closer to the combustibles at floor level. 

Other sprinkler system designers and engineers thought that ceilings were too congested with

cable trays, conduits, piping, ductwork, etc., and that sprinklers could be mounted below these

ceiling-level obstructions to develop adequate spray patterns. The 1999 edition of National

Fire Protection Association (NFPA) 13, “Standard for the Installation of Sprinkler Systems,”

provides requirements for the installation of sprinklers under obstructed ceilings. NFPA 13,

Section 5-6.4.1.1, states that “under unobstructed construction, the distance between the

sprinkler deflector and the ceiling shall be a minimum of 1 in. (25.4 mm) and a maximum of 12

in. (305 mm).” In certain cases NFPA 13 allows sprinklers to be placed as far as 22 inches

below an obstructed construction. NFPA 13 also requires sprinklers to be installed below fixed

obstructions and to provide full ceiling-level sprinkler coverage1

. These NFPA 13 requirements

appear to be consistent with the requirements of NFPA 13 from the early 1980s.

Generic Letter (GL) 86-10, “Implementation of Fire Protection Requirements,” provides

guidance on sprinkler location. Section 3.4.3, “Sprinkler Location,” and Section 3.4.5, “Sprinkler

Head Location,” state that sprinkler system designs should meet NFPA 13 and be located at the

ceiling. The GL also states that “in order to achieve complete area wide coverage, sprinklers

should be located at the ceiling, with additional sprinklers provided below significant

obstructions such as wide HVAC ducts and ‘shielded’ or solid bottom stacked cable trays. To

the extent that an existing or proposed sprinkler system deviates from this concept, the design

would have to be justified by a fire hazards analysis.” GL 86-10 provides no heat collector

guidance.

NFPA 13 contains no guidance or requirements regarding the installation of heat collectors in

lieu of a ceiling. To understand the origins of the heat collector concept, the NRC staff

examined previous editions of NFPA 13. The staff discovered that NFPA 13 has never

permitted, required, or taken a formal position on the installation of heat collectors installed

above sprinklers in lieu of locating the sprinklers close to the ceiling. The term “heat collector”

was first used in the NFPA 13 standard up to the 1963 edition and was listed in the index of

NFPA 13 up to the 1974 edition. The term first appeared in the NFPA 13 section on “Guards &

Shields” for protecting sprinklers. The staff discovered no references to heat collectors in any

editions of the NFPA 13 section that address the distance of sprinklers from the ceiling. “Heat

collectors” was changed to “baffles” in the 1963 edition during the NFPA document revision. 

NFPA 13 was completely revised again in 1973, and “baffles” became “shields.” 

NFPA 13 makes it clear that there are acceptable uses of shields. Shields are very similar in

design to heat collectors. Shields are metal plates installed over sprinklers in midair in fire

areas where full ceiling-level sprinkler protection is also provided. Shields function solely to

protect the midair sprinklers from “cold-soldering.” The cold-solder effect occurs when an
2

This also applies to fire detectors with heat collectors. Dead air spaces under heat collectors

can prevent fire detectors from activating during the incipient stages of a fire.

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operating sprinkler (usually at the ceiling) wets midair sprinklers, delaying or preventing their

activation. The wetting prevents the midair sprinklers from fusing (opening) to release water. 

Sections 6-4 and 7-4 of NFPA 231C, “Rack Storage of Materials,” state that “water shields shall

be provided directly above in-rack sprinklers, or listed sprinklers equipped with water shields

shall be used where there is more then one level, if not shielded by horizontal barriers.” 

NFPA 13 also provides guidance on the use of shields to prevent mechanical injury to

sprinklers and protect sprinklers under open gratings. 

The first tests of heat collectors were apparently the tests conducted for the U.S. Atomic Energy

Commission (AEC, the predecessor to the NRC) by Union Carbide at the Oak Ridge Y-12 Plant

and documented in a 1973 report. Union Carbide tested sprinklers with and without canopies

(i.e. heat collectors) for response above kerosene fires. In the spring 1989 Sprinkler Quarterly,

Russell P. Fleming contended that heat collector testing at the Oak Ridge Y-12 plant was

flawed. Fleming concluded that the test scenario placed the heat collector and standard

response sprinkler 3 feet above the kerosene fire, so that the sprinkler mainly activated by

radiant heating. In a more realistic fire scenario the sprinkler would be activated primarily by

convective heating.

Similar findings were reported in a January 1990 report, “A Study of the Utility of Heat

Collectors in Reducing the Response Time of Automatic Fire Sprinklers Located in Production

Modules of Building 707,” prepared for the U.S. Department of Energy Rocky Flats Plant by

Hughes Associates, Inc. (HAI). HAI conducted small-scale and large-scale testing for the

Rocky Flats Plant to determine if heat collectors directed enough of the convective heat of the

fire plume past sprinklers to activate them. HAI also studied the effect of using a quickresponse sprinkler in lieu of a standard-response sprinkler with a heat collector. The results of

the HAI tests for the Rocky Flats Plant are summarized below: 

• Heat collectors with the edges turned down around the side produced a dead air space

and the sprinklers had longer response times than sprinklers with a flat heat collector2

(see Figure 2 in the attachment).

• The heat collector must be in the plume to be effective. If the centerline of the fire is

more than 1 to 2 feet from the edge of a flat heat collector, a standard-response

sprinkler may take longer to respond, regardless of its thermal sensitivity.

• If a fire is midway between two sprinklers, the sprinklers may not respond at all

(regardless of the size of the heat collector) because the sprinklers are not exposed to

the convective heat flow of the ceiling jet.
3

The thermal sensitivity of a sprinkler operating element is called the “response time index”

(RTI). The RTI provides an indication of how fast the element can absorb enough heat from its

surroundings to activate the sprinkler. Standard-response sprinklers have higher RTIs than quickresponse sprinklers. The lower the RTI, the faster the sprinkler activates. However, even a quickresponse sprinkler with a low RTI will not activate unless it is immersed in the ceiling jet. 

IN 2002-24

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• Quick-response sprinklers activate more quickly than standard-response sprinklers only

if the fire was directly underneath the heat collector.3 

Description of Circumstances

The primary objective of the fire protection programs at U.S. nuclear power plants (NPPs) is to

minimize the probability and consequences of a fire in accordance with General Design

Criterion 3 of 10 CFR Part 50, Appendix A. To meet these objectives, the fire protection

programs at NPPs must provide reasonable assurance, through defense-in-depth, that a fire

will not prevent the performance of necessary safe-shutdown functions and that radioactive

releases to the environment will be minimized. Part 10 to the Code of Federal Regulations

(CFR), Section 50.48, “Fire protection,” and 10 CFR Part 50, Appendix R, “Fire Protection

Program for Nuclear Power Facilities Operating Prior to January 1, 1979," require that

automatic fire suppression systems be installed in certain fire areas to protect separated or

redundant trains of safe-shutdown equipment and circuits or to protect against a fire hazard.

NRC is concerned about the adequacy of sprinklers with heat collectors because NRC

inspectors have found a lack of technical documentation, tests, or calculations to support the

installation of sprinkler systems with heat collectors in fire areas. In addition, NRC inspectors

have found that some licensees have not performed an engineering evaluation to justify the

installation of heat collectors at their facilities. 

NRC plant walkdowns have identified other heat collector concerns. One concern is the

location of sprinklers with heat collectors located below the primary combustible source (i.e.,

cables installed in cable trays). In this configuration, if a cable fire occurred, the sprinklers

would not activate. Other concerns relate to the configuration and orientation of heat collectors

over sprinklers. Inspectors have discovered some heat collectors tilted at an angle over the

sprinklers or even installed sideways. The technical concern is that tilted or vertical heat

collectors over sprinklers could obstruct or deflect the spray pattern of the sprinklers (provided

the sprinklers activate), preventing the sprinkler from effectively controlling the fire. Figure 3 of

the attachment shows how an improperly placed heat collector can obstruct the sprinkler spray

pattern. 

With respect to the NFPA code requirements, the heat collector concerns raised by NRC

inspectors usually involve one of two issues: 

 • In fire areas where the licensee commits to NFPA 13, the distance of the sprinkler from

the ceiling does not comply with the NFPA 13 spacing and location requirements (i.e., a

sprinkler with a heat collector below cables installed in cable trays or a midair sprinkler

for transient fire control). 
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• Water spray systems have been installed under heat collectors in some fire areas where

the licensee designed the system to the requirements of NFPA 15, “Water-Spray Fixed

Systems For Fire Protection.” In accordance with NFPA 15, water spray systems

require directional water spray nozzles, which are open nozzles. Open nozzles do not

have a heat-sensitive element (such as a fusible link). This allows water to flow from all

nozzles at the same time to protect the hazard once the detection system receives an

alarm and trips the deluge valve. Since water spray systems typically are open nozzles

(with no heat-sensitive element), there are no NFPA 15 requirements for nozzles to be

located within a certain distance from a ceiling or overhead beam. This is based on the

fact that the principal design consideration for a water spray system, is the spray pattern

the open nozzles will develop in protecting the hazard. 

To prevent inadvertent actuation of water spray systems onto components such as

sensitive electrical components, some licensees have installed automatic directional

water spray nozzles, which are closed nozzles. Closed nozzles can only operate when

the heat-sensitive element is activated. So an automatic directional water spray nozzle

operates on the same principle as an automatic sprinkler. They both have heatsensitive elements which rely on the heat of a fire to fuse (open), to release water on to

the fire. As previously discussed, the closed nozzle and automatic sprinkler both require

proper placement in the ceiling jet so that the heat-sensitive element is exposed to the

convective flow of heat from a fire.

Although NFPA 15 does not require nozzles to be located within a certain distance from

the ceiling or overhead beam, the 2001 Edition of NFPA 15, Section 6.2.1.2 does state

that “automatic nozzles shall be permitted when positioned and located so as to provide

satisfactory performance with respect to activation time and distribution.” Based on the

operation of the heat-sensitive element, the same concerns with respect to automatic

sprinkler placement and response time are also applicable for automatic directional

water spray nozzles.

The ceiling is widely recognized as a fundamental means of directing heat to the sprinklers

located closest to the fire. The use of sprinklers with heat collectors installed far below the

ceiling has not been demonstrated to be effective and may impair sprinkler system response. 

Sprinklers are also used to prevent flashover and the collapse of structural steel. Furthermore,

fire areas with large amounts of combustibles above the sprinkler, may not be adequately

protected in accordance with General Design Criterion 3. 
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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 contacts listed below

or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

/RA/ /RA/

Robert C. Pierson, Director William D. Beckner, Program Director

Division of Fuel Cycle Safety Operating Reactor Improvements Program

 and Safeguards Division of Regulatory Improvement Programs

Office of Nuclear Material Safety Office of Nuclear Reactor Regulation

 and Safeguards

Technical Contacts: Tanya Eaton, NRR Mark Henry Salley, NRR

301-415-3610 301-415-2840

Email: tme@nrc.gov Email: mxs3@nrc.gov

Peter Lee, NMSS

301-415-8111

Email: psl1@nrc.gov

Attachments:

1. Figures 1, 2, and 3

2. List of Recently Issued NRC Information Notices

3. List of Recently Issued NMSS Information Notices
References:

1. Fleming, Russell P., “High-On Sprinklers,” Part II, Sprinkler Quarterly, Spring 1989, pp 21.

2. McCormick, J.W., and DeMonbrun, J.R., “Experiments With Sprinkler Head Canopies for

Fire Protection, Oak Ridge Y-12 Plant,” U.S. Atomic Energy Commission Y-JA-96 July 2,

1973.

3. NFPA 13, “Standard for the Installation of Sprinkler Systems,” National Fire Protection

Association, Quincy, Massachusetts. 

4. NFPA 15, “Water-Spray Fixed Systems for Fire Protection,” National Fire Protection

Association, Quincy, Massachusetts.

5. NFPA 231C, “Rack Storage of Materials,” 1998 edition, National Fire Protection

Association, Quincy, Massachusetts.

6. Shanley, J. H. , and Budnick, E.K., “A Study of the Utility of Heat Collectors in Reducing

the Response Time of Automatic Fire Sprinklers located in Production Modules of

Building 707,” RFP-4874, Hughes Associates, Inc., January 1990.
Attachment 1

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Figure 1 An upright sprinkler with a heat

collector

Figure 2 Heat collector with an edge and without an edge

Figure 3 A heat collector obstructing sprinkler spray pattern