How to choose lights for nuclear applications

Those responsible for specifying lighting for nuclear power stations face myriad challenges: how to select the most effective, safe designs, while factoring in issues like reducing radwaste, considering effects on performance due to exposure to fuel, and staying compliant with new post-Fukushima protocols. While it is interesting to consider new technologies to enhance lighting options inside containment, there are inherent cautions to heed in adopting any new system into some of the most extreme environments on the planet. In fact, in these demanding nuclear applications, utilizing the right lighting sources can mean the difference between long and short outages, effective fuel movement and wasted effort, and most importantly, additional hazards and enhanced safety in nuclear work environments.

Underwater illumination

There are many applications for underwater nuclear lighting, each critical to facilitate major operations such as fuel movement, inspection and maintenance. For instance, in choosing any underwater lighting systems, it is important to find a solution that considers the big picture: one that can provide intense light, is engineered to last for years, and factors in minute detail, such as avoiding systems with non-captivated small parts. The tiniest screw or O-ring that falls off could result in costly and time-consuming pool draining, and put part of a plant offline.

There are some obvious considerations: all underwater nuclear lights should be crafted from stainless steel with rounded and smooth surfaces for easy decontamination, and have no sharp or jagged edges to reduce the risk of workers tearing safety gloves or clothing. Despite the sophistication of these types of lighting systems, they should be developed in such a way that does not require tools for relamping—again, anything that can be done by hand, simply, with rounded corners, reduces the risk of tearing protective gear. Other considerations include power cables with true underwater connectors, and the choice of material for segments like lamp protectors. Lexan polycarbonate, for example, is the most impact-resistant of all thermoplastics—more than 30 times the impact resistance of safety glass—and provides more than 300% the radiation tolerance of acrylic.

Impact of radiation on glass

Glass is an amorphous (non-crystalline) inelastic solid material often used for luminaire lenses; its transmittivity is typically ~92%. Although glass lacks the long-range order characteristics of a crystal, its basic short-range order structural unit is the silica tetrahedron, making it particularly susceptible to radiation-induced discolouration (‘browning’), which can be visibly recognizable at doses as low as 10 Gy (103 R). Optical density of glass is almost always increased by irradiation and reaches saturation at ~108 Gy (1010 R). At 100 Gy, glass transmittivity (at 550nm wavelength) can drop to 70%; at 105 Gy (107 R), it can be as low as 25%—eliminating 75% of a lighting system’s luminance at the source. Translucent thermoplastics, for example Poly(methyl methacrylate) or PMMA, Plexiglas [(C5O2H8)n], also discolour under radiation. Transmittivity in PMMA drops from 91% to 56% at 5.5 x 104 Gy (550 R) despite only slight discolouration, while higher doses of gamma radiation (cobalt-60) turns PMMA dark yellow and, ultimately, brown. Thus, for reactor cavity (and other high-radiation area) lighting, it is important to select luminaires with radiation-resistant lenses. These lenses can be made of crystalline or near-crystalline materials like quartz (SiO2) or fused silica—certain varieties of which remain transparent at exposures of 106 Gy (108 R)—or be doped with additives (for example CeO2) to increase their radiation resistance.

ROI

The most important reason for choosing high performance nuclear lighting is to increase plant operation efficiency. A fuel pool that has dark spots where light doesn’t reach means additional time and effort during large-scale activities that negatively impact the bottom line. Most nuclear power plants replace fuel on average every 18-24 months [1]. Storage and use of the fuel underwater is necessary because the water acts as a radiation shield. Of nuclear electric power production costs, fuel typically represents 31%, and operation and maintenance, 69% [2]. Approximately 15% of a commercial plant’s total shutdown time (outage) is directly attributable to fuel handling; this operation is the single largest component of downtime and takes the average plant more than six days, costing $2.27 million. Thus, a mere 3% increase in fuel movement efficiency due to improved underwater illumination can save the NPP $68,800 during every outage.

Determining the lighting fixtures needed for underwater lighting requires consideration of the task requirements, the water clarity, the pool size, and the total amount of illumination needed for individual tasks. The Illuminating Engineering Society of North America (IESNA) provides recommended illuminance categories [3] and values for various types of work. The IESNA categorizes nuclear power plant illuminances as follows:

• Fuel Handling Building, Operating Floor: Illuminance Category D
• Reactor Building, Operating Floor: Illuminance Category D

Illuminance Category D specifies illuminance levels of 200, 300, and 500 lux for “Performance of visual tasks of high contrast or large size”. Selection of which of the three lighting levels is most appropriate depends on weighting factors including workers’ ages, importance of work speed and/or accuracy, and the reflectance of the task background.

Calculating illuminance

Predicting the radiative transport of luminous flux from a source to a receiving surface is fundamental to all lighting calculations. The flux transport is through air, which is assumed to be non-absorbing and non-scattering, so we also need to calculate for attenuation in the water medium and reflection from and absorption by suspended particulate matter (the total beam attenuation is the sum of the absorption coefficient and the total scattering coefficient). Flux transfer is categorized into six types by geometry and emitter type. Transfer type 1 (point source to a point or differential receiving area) is conceptually the simplest and is the easiest to formulate, so it is the type considered in this article. We assume the luminaires act as point sources, given the ‘five times’ rule (which allows the luminaire to be considered a point source if the distance between the source and the target is more than five times the maximum dimension of the emitter). Under these conditions the illuminances always vary as the inverse of the distance squared. The “five times” rule permits a computational accuracy of at worst 2% for diffuse emitters.

Light reaching a point (or an area) is described by illuminance—a measure of flux density, the incident flux per unit area. The illuminance E produced on an area A centred at a point P is related to the luminous intensity of a light source I in the spherical coordinates (?,?). The illuminance E is defined in terms of the flux F incident on an area A:

That flux can also be analyzed directionally in terms of the solid angle ?. If the origin of a spherical coordinate system is located at the source, and the area A is small with respect to the distance D, then:

Where D = distance between the source and point P; ? = angle between the normal to the surface A and direction of the distance D.

The definition of the luminous intensity from the source is used to relate the two equations above, as follows:

Or:

Substituting this into the illuminance definition gives:

And substituting this into the flux equation with respect to ? gives:

This is the fundamental equation of flux transfer, the ‘inverse-square cosine law’.

When figuring for attenuation in the water medium and reflection from and absorption by suspended particulate matter, adding Lambert’s Law to the inverse-square cosine law, and by simplifying for a 3-dimensional pool, one obtains:

For instance, assume that between the luminaire and the task surface (work plane), the vertical elevation distance is 7m (22 ft) and the pool width is 8m (~25 ft), and that the luminaire’s initial intensity is 50,000 Cd (16,000 lumen luminous flux, a 4,000 hour lamp life, and medium-flood beam). At 550 nm, the spectral transmittance of distilled water is ~0.95/m. Individual plant water transmittance is variable, but it will be assumed that it is clear and clean, to be nearly that of distilled water. (NPPs with lesser water clarity should perform calculations based on reduced water transitivity.) Given these assumptions, a single luminaire can provide an illuminance of ~520 lux (centre-beam) on the target surface. However, a mere 5% reduction in water clarity (that is, 0.90/m water transmittance) will result in a 35% drop in illuminance; the same luminaire at the same location but in 0.90/m water will provide illuminance of ~340 lux.

Choosing a source

Two major field-proven lighting sources for applications that require either large floodlights or low-profile droplights are tungsten-halogen fixtures and high-pressure sodium vapor (HPSV) fixtures. Both types have advantages and disadvantages (see Table 1). If HPSV lamps are chosen, care should be taken to select one with low mercury—approximately 0.01 mg—as in extreme circumstances, should the arc tube break and mercury be released, a risk exists of contaminating fuel pools, fuel storage racks, and core support structures. In the event of arc tube rupture, water could enter the arc tube, reacting with the sodium in the amalgam and releasing metallic mercury to the environment. The total mercury available to be released to the environment can be as much as 45 mg in non-low Hg lamps, but the number and size of the released particles would be difficult to predict. High levels of Hg, on the order or 1-2 ppm in solution during continuous power operation, could produce corrosion due to characteristics of fuel cladding. Forty-five mg of mercury is approximately 1000 times as much mercury required to create a pit size capable of initiating a stress corrosion crack. However, routine use of a light fixture assembly which receives minor impacts from equipment would not likely result in arc tube breakage.

LED lighting for underwater nuclear applications is still in its early development stage, and there are numerous unknowns in this field: how LED technology will perform in prolonged close contact with fuel; what increases in radwaste will emerge in implementing LED systems without thorough and lengthy testing; how to fix maximum power into lamps, given the importance of thermal management in high-power LEDs; and whether radiation-induced discolouration of LED glass might cause significant lighting attenuation.

Post-Fukushima emergency preparedness

The use of LED lighting for in-air applications has been very successful in nuclear stations. For lighting fixtures that are used to illuminate large rooms, like reactor halls, requirements for maximum light output combined with minimum power draw make LED technology a viable choice. These powerful systems can have up to 100,000 lumens, with 85,000-hour lamp lives, with power draws of just 7 amps, while withstanding radiation tolerances of 105 Gy (107 R) (see also NEI November 2012, p45).

The newest emergency lighting fixtures (ELFs) also have LED options. These options have the same bright light output, and also low power requirements. They must conform to new post-Fukushima regulatory requirements. The USNRC has changed the minimum duration of emergency lighting in case of a loss of offsite power, or station blackout (in which onsite emergency diesel generators). The rule requires each plant to prove that it can successfully cope with the blackout for a period from eight to now up to 24 hours. With LEDs, BIRNS has been able to increase the battery burn time from eight hours to nearly 40, with an increase in actual illuminance. It should be noted that BIRNS emergency lighting fixtures only draw appreciable load during post-emergency fast-battery charge; during emergency loss of power the lighting fixtures operate on their own internal batteries. ELFs should be seismically qualified to IEEE-344, (IEEE Recommended Practice for Seismic Qualification of Class 1E Equipment for Nuclear Power Generating Stations), be constructed of stainless steel, with the same rounded corners and safety features in the design to make it user friendly to station employees, as well as robust and high-performance.

Today’s nuclear stations are taking aggressive measures to both conform to new NRC safety requirements, and to set their own additional standards of excellence and safety in the case of emergencies. BIRNS was recently asked to develop a comprehensive illumination study to determine the number of emergency lighting fixtures that would be required to maintain sufficient operational illumination in case of main and remote control room power/lighting loss (Figure 1).

Conclusion

Creating a safe, highly effective work environment inside containment is a complex undertaking—and the lighting systems chosen to enhance productivity for both large and small operations must all be strategically chosen. However, with the advancements in technology in manufacturing and illumination sources over the past several decades, those choices can be made in a highly informed, systematic fashion, with measurable data and success rates to make selections that will provide maximum benefit in the short and long term.

Amy Brown and Eric Birns, BIRNS, Inc, 1720 Fiske Place, Oxnard, CA 93033-1863