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| Fig. 1: Annual radiation dose to individuals living near a model 1,000 MWe coal-fired power plant versus model 1,000 MWe nuclear power plants (BWR and PWR), from McBride et al. (1978). U.S. natural background (~315 mrem/yr) is included for comparison. Logarithmic vertical scale. [1,2, 5] |
A well-known argument in energy policy holds that coal-fired power plants release more radioactivity into the environment than nuclear power plants do. The argument is arithmetically correct. In 1978, McBride et al. showed that the maximum individual bone dose near a model 1,000 megawatt-electric (MWe) coal plant was approximately 18 millirem per year (mrem/yr), which is equivalent, in modern units, to 0.18 millisieverts (mSv) using a weighting factor of 1 for gamma rays. For reference, the sievert measures biological radiation dose, and equals the absorbed dose in grays (joules per kilogram of body weight) multiplied by a weighting factor that accounts for the type of radiation. For gamma rays, the dominant radiation in this context, the weighting factor is 1, making the dose in sieverts numerically equal to the dose in grays. In its turn, nuclear plants of the same capacity were measured to deliver inferior doses of 0.03 - 0.06 mSv/year. [1] These numbers have been cited for nearly five decades, most often as evidence that nuclear power is not dangerous.
Let us examine whether the comparison is meaningful. Three questions structure the analysis. First, are the coal-plant radiation doses large enough to matter? Second, does the comparison between normal-operation doses capture the actual source of nuclear risk? Third, does the regulatory asymmetry between coal and nuclear reflect a rational assessment of risk, or is it partly a product of public fear?
Coal contains 1& - 4 parts per million (ppm) uranium and 1 - 4 ppm thorium. [2] These concentrations are comparable to those found in common rocks and soils. But coal is burned in enormous quantities. When it combusts, the radioactive elements do not volatilize. They remain in the solid residue. Because the average ash yield of U.S. coal is approximately 10% by weight, uranium and thorium concentrate by a factor of roughly 10 in the fly ash, producing uranium concentrations of 10 - 30 ppm. [2] The Electric Power Research Institute measured typical fly ash activity at approximately 7 picocuries per gram (pCi/g) from uranium, 4 pCi/g from thorium, and 15 pCi/g from K-40. [5] This is equivalent, in SI units, to roughly 260, 150, and 555 becquerels per kilogram (Bq/kg) respectively, where becquerel is the quantity of radioactive material in which one nucleus decays per second.
In 1982, the U.S. coal fleet burned 616 million short tons of coal and, in doing so, released approximately 801 tons of uranium (containing 11,371 pounds of fissile U-235) and 1,971 tons of thorium into ash ponds, landfills, and the atmosphere. The total associated radioactivity was quoted to be approximately 2,630,230 millicuries, or 97.32 terabecquerels (TBq). [3]
Mass, however, is not dose. What matters for human health is how much radiation actually reaches people living nearby. McBride et al. modeled this in 1978 for a 1,000 MWe coal plant and comparable nuclear plants, assuming 1 ppm uranium and 2 ppm thorium in coal with a 1% ash release to atmosphere (the EPA standard at the time). The result: the maximum whole-body dose to an individual near the coal plant was 1.9 mrem/yr (0.019 mSv/yr), and the maximum bone dose was 18 mrem/yr (0.18 mSv/yr). For nuclear plants of the same capacity (both BWR and PWR types), bone doses were 3 - 6 mrem/yr (0.03 - 0.06 mSv/yr) (Fig. 1). [1]
The arithmetic is not disputed. Under normal operating conditions, coal combustion releases more radioactivity per unit of electricity than nuclear power does. The question is whether either quantity is large enough to matter.
The USGS concluded that the maximum radiation dose to an individual living within 1 kilometer of a modern coal plant is equivalent to a 1 - 5% increase above natural background radiation. [2] The average population dose attributed to coal burning falls under the "consumer products" category in the U.S. dose breakdown, accounting for less than 1% of total radiation exposure. [2] Natural sources - radon, cosmic rays, terrestrial radiation, and internal radionuclides - constitute roughly 50%, or 315 mrem (3.15 mSv), of the average American's annual dose of approximately 624 mrem (6.2 mSv). [5]
Nuclear plant doses are smaller still. Under NRC regulations (10 CFR 20.1301), the annual dose limit to any member of the public from licensed nuclear facilities is 100 mrem (1 mSv), and the EPA standard (40 CFR 190) further limits the uranium fuel cycle contribution to 25 mrem/yr (0.25 mSv/yr). [4] In practice, doses from U.S. nuclear plant operations are far below these limits. In 2019, 96 commercial reactors at 57 sites reported effluent data to the NRC; resulting doses were typically well below 1 mrem/yr. [4]
Comparing 18 mrem (coal, bone) with 3 - 6 mrem (nuclear, bone) against a natural background of 315 mrem is comparing two small fractions of a small fraction. The coal-versus-nuclear comparison, while arithmetically real, is a comparison between two quantities that are both small relative to the doses people already receive from natural sources.
This comparison, however, concerns only normal operations. Nuclear power also has a failure mode that coal does not: the very low-probability, high-consequence accident. Such events are, undoubtedly, exceedingly rare. The NRC's probabilistic risk assessments estimate core damage frequencies for U.S. reactors on the order of 10−5 per reactor-year, or roughly one event per 100,000 years of reactor operation. [7] But when they do occur, the scale of release is qualitatively different from anything coal combustion can produce.
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| Table 1: Radioactive releases and consequences of the two major nuclear power plant accidents. Release figures from the NAS. [6]. All other data are from UNSCEAR. [8] (1 PBq = 1015 Bq.) |
The Chernobyl accident in 1986 released approximately 1,760 PBq of I-131 and 85 PBq of Cs-137 into the environment. [6] Approximately 116,000 people were evacuated in 1986, and a further 220,000 were relocated between 1989 and 1992, bringing the total displaced population to roughly 340,000. [8] Of the approximately 600 emergency workers at the site, 134 were diagnosed with acute radiation syndrome, and 28 of those died (Table 1). [8] Chernobyl involved an RBMK reactor design with no containment structure and a positive void coefficient, features absent from all Western commercial reactors. [6] In the years following the accident, a significant increase in childhood thyroid cancer was documented in Belarus, Ukraine, and Russia, attributed to I-131 exposure. [9] The long-term cancer burden remains a subject of ongoing epidemiological study, and latency periods mean that the full consequences cannot be assessed from acute mortality figures alone.
The Fukushima Daiichi accident in 2011 released approximately 100 - 500 PBq of I-131 and 6 - 20 PBq of Cs-137, on the order of 10% of Chernobyl's release, depending on the estimates, and required the evacuation of approximately 118,000 people (Table 1). [6][8] UNSCEAR's 2020 report found that no worker was diagnosed with acute radiation syndrome, but long-term health effects are notoriously difficult to track, particularly in a politically charged situation like this. [8] Large exclusion zones remain in place around both accident sites.
Coal combustion has no equivalent failure mode. A coal plant can burn more coal, or burn dirtier coal, but the resulting radioactive release scales linearly with input. There is no mechanism by which a coal plant can release a thousand times its annual output in a single event.
The probability of a core damage event is on the order of 10−5 per reactor-year. [7] Whether the current level of nuclear regulation is appropriately calibrated to the actuarial risk, including the delayed health effects that may follow a major release, or is partly driven by public perception of catastrophic scenarios, is a question the numbers alone do not resolve.
It is worth noting, nonetheless, that coal ash receives no radiological scrutiny at all. Coal ash was classified by the EPA as nonhazardous solid waste. The USGS noted that standardized leaching tests address toxic trace elements such as arsenic and mercury, but the regulatory framework does not specifically address radioactivity in coal combustion products. [2] The hundreds of tons of uranium and thorium released annually by the U.S. coal fleet (see above) entered the environment under no radiological monitoring or limits. [3] Nuclear plant operators file annual radionuclide-by-radionuclide effluent reports to the NRC for releases that are typically below detection limits. [4] Coal operators file nothing. Whether those quantities warrant regulation is a policy question, but the absence of any measurement framework is a gap worth noting.
Coal plants release more radioactivity than nuclear plants in normal operation. [1,3] Both releases are small fractions of natural background radiation. [2] The argument that this comparison demonstrates nuclear safety is misleading because it restricts the analysis to the one operating regime in which nuclear power already poses no measurable risk. It says nothing about accident scenarios, which involve releases measured in petabecquerels, large-scale evacuations, and - in the case of Chernobyl - documented long-term health effects including childhood thyroid cancer. [8] The coal-versus-nuclear radioactivity comparison is an answer to a question nobody informed should be asking. The harder questions are about the tails of distributions and the price of regulating against them.
© Felipe Leite Teixeira. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. P. McBride et al., "Radiological Impact of Airborne Effluents of Coal and Nuclear Plants," Science 202, 1045 (1978).
[2] R. A. Zielinski and R. B. Finkelman, "Radioactive Elements in Coal and Fly Ash: Abundance, Forms, and Environmental Significance," U.S. Geological Survey, FS-163-97, October 1997.
[3] A. Gabbard, "Coal Combustion: Nuclear Resource or Danger," Oak Ridge National Laboratory, ORNL Review, 26, No. 3-4, 18 (1993).
[4] J. Davis, "Radioactive Effluents from Nuclear Power Plants, Annual Report 2019," U.S. Nuclear Regulatory Commission, NUREG/CR-2907, Vol. 25, September 2021.
[5] K. Ladwig, "Assessment of Radioactive Elements in Coal Combustion Products," Electric Power Research Institute, Technical Report 3002003774, August 2014.
[6] Lessons Learned from the Fukushima Nuclear Accident for Improving Safety of U.S. Nuclear Plants (National Academies Press, 2014).
[7] "Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants," U.S. Nuclear Regulatory Commission, NUREG-1150, Vol. 1, December 1990.
[8] "Sources, Effects and Risks of Ionizing Radiation, UNSCEAR 2020/2021 Report," Volume II, Annex B, United Nations Scientific Committee on the Effects of Atomic Radiation, 2022.
[9] "Sources and Effects of Ionizing Radiation, UNSCEAR 2008 Report," Volume II, Scientific Annexes C, D, and E, United Nations Scientific Committee on the Effects of Atomic Radiation, 2011.