IEEE Transactions on Nuclear Science, Vol. NS-27, No. 1, February 1980

SOME RADIOLOGICAL ASPECTS OF COAL COMBUSTION

Harold L. Beck and Kevin M. MillerEnvironmental Measurements Laboratory

U. S. Department of EnergyNew York, New York 10014

SummaryTABLE 1

Data on the radionuclide content of coal, bottomash and fly ash are reviewed. Estimates are made ofthe quantities of various radionuclides released tothe environment from coal comoustion and the probableresultant radiation doses to the population. Factorsthat influence these dose estimates, such as particlesize, solubility and radon emanating power arediscussed.

Introduction

The combustion of coal results in the release ofsmall quantities of radionuclides into the atmosphere.Unlike nuclear power, these releases result entirelyfrom a redistribution of already existing radionuclidesrather than the creation of new radionuclides. Whetheror not such releases constitute a potential healthhazard and whether the releases of radioactivity arecomparable to or greater than those from a nuclear fuelcycle has become a matter of some public and scientificconcern. This question is particularly relevant as aresult of recent government policy decisions to in-crease the utilization of coal relative to other fuels.

We have made an extensive review of data on theradioactivity of coal and coal ashes and the releasesof radionuclides from typical power plants, and havealso analyzed the results of a number of publishedassessments of potential health hazards.(l) In thispaper we summarize some of our findings and discusssome of our conclusions as well as some caveats withrespect to other published assessments, and presentsome additional data.

Reference 1 contains a comprehensive list ofliterature citations, and we refer the reader to thatpaper for more complete documentation on some of theresults summarized here.

Radioactivity of U. S. Coals

Measurements of uranium, thorium, and potassium incoal have been reported in the literature for almost1000 different samples obtained directly from the minesthat provide most of the coal used in the U. S. todayas well as from most of the proven reserves. Meanconcentrations were found to be 1.7 ug/g for naturaluranium, 4.5 ,ug/g for thorium and 1700 'g/g forpotassium, equivalent to the mean radioactivity levelsfor U-238, Th-232, and K-40 shown in Table 1. Althougha few coals were found to have fairly high concentra-tions of radionuclides, over 95 percent of the sampleshad activities less than a factor of three times themeans. Contrary to some published reports, western U.S.coals are not, in general, more radioactive than east-ern U. S. coals, nor are lignites, although smallpockets of uraniferous coal do exist in some westernstates.

RADIOACTIVITY OF U. S. COALS IN pCi/g

Nuclide: 238U 232Th 40K

Samples collected mean: 0.60 0.50 1.4from mines (see range: < 0 1-15 < 0.1-5.3 0.02-20Reference 1) #samples:J (910) (910) (983)

Samples collected mean: 0.58 0.24 1.9from power plants range: 0.13-1.8 < 0.1-0.47 . 1 -5.3and analyzed by # samples:J (14) (14) (14)EML

Other reported mean: 0.34 0.56 2.8analyses of samples range: < 0.1-0.8 0.17-2.3 0.73-5.6from power plants* # samples: (15) (10) (10)

Soil '6 mean: 0.70 0.70 10(for comparison) range: ! 0.3-1.4 0.2 -1.3 3-20

*Values for mine samples are for coal as mined, for power plantsfor air-dried samples.

Since a large fraction of the coal burned in U.S.power plants is "cleaned" prior to use to reduce ashand sulfur content, we have compared the activities ofthe mine samples with those of samples obtaineddirectly from power plants. These results are alsogiven in Table 1. Data for samples analyzed by us arereported separately from other data in the literature,since some of the latter were ambiguous with respectto numbers of different samples, origins of thesamples and accuracy of analysis. Our data, most ofwhich have not been previously reported, were obtainedby performing high resolution gamma-ray spectrometryon samples obtained from 14 different power plants,three of which burn lignite.

Uranium-238 ahd thorium-232 have generally beenfound to be in secular equilibrium with their decayproducts in coal. Our own analyses generally confirmthis. Thus, the data in Table 1 for U-238 and Th-232also represent the activities of their respective de-cay products. In Reference 1, we referred to a pre-liminary report from Mound Laboratory which suggestedenhanced Pb-210 and Po-210 levels in some coal samples.In an updated report, however, this group reportssecular equilibrium does appear to exist for theirsamples.(2)

Considering the relatively small number ofsamples from power plants as opposed to mines, theagreement in means and ranges of activities is excel-lent and tends to support our previous conclusion thatthe mean activities inferred from the mine samples arerepresentative of coal burned in actual power plants.

For perspective, the activities of typical soilsare also given in Table 1. (3)

U.S. Government work not protected by U.S. copyright 689

Radioactivity of Coal Ash

When coal is burned in power plants, non-combus-tible trace elements are concentrated in the ash.Based on a mean ash content in U. S. coals of 13.4percent, we would expect the specific activity of thisash to average 7.5 times that of the coal, i.e., 4.5pCi/g for U-238 and each of its decay products, 3.7pCi/g for Th-232 and each of its decay products, and10 pCi/g for K-40. Studies have shown, however, thatsome of the more volatile trace elements are preferen-tially recondensed on smaller particles. This resultsin an enrichment of fly ash relative to bottom ash,and particularly on the smaller flyash particles,which are less efficiently removed by the emissioncontrol equipment. Radionuclides which have beendemonstrated to exhibit this enrichment are Pb-210and Po-210, in particular, and to a lesser extentU-238, U-234 and Ra-226.

In Table 2 we summarize available data on radio-nuclides in ash samples obtained from U. S. powerplants. Except for one set of data by Coles et al.,(4) these are all samples of contained rather thanescaping ash. The Coles et al. data are for 4 dif-ferent size fractions of the same stack sample andillustrate the progressive enrichment on smaller sizedparticles for certain radionuclides. Again we havelisted our own data separately from those of the otherinvestigators. Both sets of data clearly indicate the

enrichment of Pb-210 in fly ash relative to bottomash. The enrichment of other uranium and radium nu-clides is too small to be seen in collected ash,particularly for this small number of samples. Our

TABLE 2

RADIOACTIVITY OF COAL ASHES COLLECTED FROM U. S. POWER PLANTSin pCi/g

Nuclide' 238U Ra Pb Th K

Samples analyzed by EML:

own data include 6 lignite ashes and 5 bituminouscoal ashes obtained from 11 different power plants.No significant differences in activity were observedbetween lignite and non-lignite ashes.

Considering the relatively small number ofsamples, the average activities shown in Table 2 arein excellent agreement with those expected based on

the coal data. We thus conclude that the mean en-riched ash activities derived from the coal minesamples can be used as a basis for estimating typicalradionuclide emissions from coal-fired power plants.

Radioactivity Emissions from Power Plants

The amount of fly ash, and thus the amount ofradioactivity released to the atmosphere, depends onthe efficiency of the plant's emission control equip-ment. Although the total amount of ash generated bythe plant depends on the type and amount of coalburned, a typical modern 1000 MWe plant operating 80

percent of the time will consume about 2.3x109 kg ofcoal per year and produce about 3x108 kg of ash peryear. If this plant just meets thg EPA particulateemission standard of 0.1 lb per 10 BTU it will re-lease 3.1 x 106 kg/y of fly ash. Based on the Coleset al. (4) data, as well as other measurements re-viewed in Reference 1, we have estimated fly ash es-caping from such a plant will be enriched in Pb-210and Po-210 by about a factor of five over the concen-tration expected based on the ash content of the fuel,in U-238 and U-234 by a factor of two, and in Ra-226by a factor of 1.5. These values will of course vary

T'ABLE 3

ES'I'lMATED ANNUAL, RADIONUCLIDE EMISSIONS FROM COAL-FIREDPOWER PLANTS*

Modern ilant 1972 Plant

Flyash Annual Flyash AtintnalActivity Emissions Activity Emissions

NtLclide (pCi/g) (mVCi /y) (pCi/y) (m_Cqi /y)

mean:' 4.2 3.6

Flyash range: t2.3-6.6 2.3-6.1

# samples: l (11) (11)

mean: 1 4.2

Bottom ash range: I1.8-8.6# samples: j (10)

Scrubber mean 1 2.9

sludge # samples J (1)

9.5 2.1 10

2-19 1.2-2.8 2-20(6) (11) (11)

4.0 2.21.6-7.6 0.5-5(10) (5)

2.6(1)

Other analyses:*

mean: 1 2.4 2.8 4.5

Flyash range: .4-6.5 0.8-5.0 1.4-11# samples (27) (17) (5)

mean

Bottom ash range:# samples: -

1.9 8.01.1-2.8 0.6-20(10) (10)

2381L2341;

230 ,h

226Ra

232Tt,3.4 0.5 5.6(1) (1) (1)

3.4 140.4-7.5 1.2-29(39) (33)

2.3 1.7 2.0 1.9 6.5

\1.5-2.8 0.6-2.5 0.6-5.6 1.5-2.2 6.2-6.8

(3) (3) (4) (2) (2)

-1 Ia a a &-17 2,8-3.3 7.0-7.4

Stack range: 5.4-12 3. -. 4.3-' -I

sample4

*See Reference I for details.

690

228Ra

210Pb

210P

40K

222Rn

9.0 28

9.0 28

4.5 15

6.8 21

3.7 11

3.7 11

3.7 11

22.5 70

22.5 70

10 31

- L500

4.5 126

4. 5 126

4.5 126

4.5 126

3.7 104

3.7 104

3.7 104

9.0 252

9.0 252

10 280

- 1500

6 2.x17Flyash (kg/y) - 3.lxO _ 2.8xlO

*Modern plant is a plant meeting EPA particulate emissions standardsof 0.1 lb per 106 BTU, 1972 plant is orne releasing at a rate equal to

mean for all U. S. plants operating in 1972.

from plant to plant, and are perhaps slightly greaterfor plants using scrubbers instead of electrostaticprecipitators, since the former plants releasesmaller-size particles for the same total mass re-lease.(5) No enrichment is expected for other radio-nuclides. The final assumed mean activities of es-caping fly ash and the resultant total annual releasefrom a typical modern plant for each long-lived radio-nuclide are given in Table 3.

For comparison, we considered a typical olderpower plant, i.e., one that releases at a rate equalto the average for all power plants operating in theU. S. in 1972. Such a plant, which would release2.8x107 kg of fly ash per year per 1000 MWe, might beconsidered a present-day worst case situation. Since,as the mass of released ash increases, the fractionmade up by small particles decreases, the relative en-richment of volatile elements on escaping fly ash ismuch smaller. We have assumed an enrichment of afactor of two for Pb-210 and Po-210, consistent withthe measurements made on collected fly ash, and nosignificant enrichment for other nuclides. The flyash activities and annual releases for this plant arealso given in Table 3.

The estimated annual emissions of Rn-222 given inthe Table assume that essentially all the availableRn-222 in the coal is released during pulverizing andcombustion.

Releases for actual plants may, in a given year,deviate from the estimates of Table 3 as a result oftemporarily burning coal or releasing ash containingactivities significantly greater or less than our as-sumptions. However} since most plants, particularlythose in populated areas remote from coal mines, willburn coal from a variety of mines over their lifetime,we would expect the results in Table 3 to fairly re-flect the long term average releases to be expectedfrom both older and newer U. S. power plants.

Perturbations on Ambient Natural Radiation Levels

The actual number of curies released of eachnuclide is not in itself particularly significant.The important question is how much do these releasesincrease ambient natural background radiation levelsin air and soil and thus potentially increase theradiation exposure of the surrounding population. Oneimportant factor which influences the extent of theseperturbations is the height at which the effluents arereleased. We therefore calculated expected averageannual ground-level activities resulting from the re-leases given in Table 3, for both a 50 meter stack anda 150 meter stack (see Reference 1 for details). Theresults obtained for the distance of maximum concen-tration are given in Table 4. Even for the older"dirtier" 1972 plant, releasing from a short stack,the calculated maximum ground-level activities are atmost only a few times ambient natural backgroundlevels while for a modern plant releasing from tallstacks the calculated maximum activities are smallfractions of ambient background levels. These max-imum levels for the 5o meter stack occur at about 2 kmfrom the plant but drop off quite rapidly as shown in

Figure 1. Thus only a small area is actually exposedto these levels. Conversely, the maximum for the tallstacks occurs at about 6 km and drops off more slowly.Background levels were estimated by assuming a meanambient air dust loading (resuspended soil) (6) of 100Lg/m3 and the average soil activities given in Table 1.

Figure 1. Mean annual ground level airborne flyash concentration versus distance from power plantfor release rate of 1 kg/s.

" ~~~~50mStack

10:

15OmS tackck

1 10

Distance From Stack (km)For short periods of time, of course, actual air

activities can be considerably higher than these annualaverages,depending on meteorological conditions. Theannual average values may also vary from year to yeardue to variations in coal burned, annual average mete-orology, and other factors. Clearly even large suchvariations will not result in significant perturbatiorsof ambient radiation levels for modern plants, re-leasing from tall stacks.

The increases in concentration for Rn-222 havenot been given in Table 4 since in all cases they areinsignificant relative to ambient levels. The amountof Rn-222 being released is equivalent to that whichis released from only about 0.1 km2 of soil surface(l)

Assuming a deposition velocity of 1.0 cm/s each,for both dry deposition and wet deposition, we canestimate the average long term deposition onto theground from the air concentrations given in Table 4.(See Reference 1 for a discussion of the appropri-ateness of these deposition velocity estimates.) Thetotal depositions after 50 years of operation aregiven in Table 5. Again, these represent values atthe distance of peak air concentrations and the aver-age deposition in the environs of the plants would beabout a factor of 70 less for releases from 50 meterstacks and a factor of 10 less for 150 meter stacksif averaged out to a distance of 50 km.

As an illustration of the net effect of these de-positions on ambient soil activities in fields where

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TABLE 4

ESTIMATED MAXIMUM MEAN ANNUAL GROUND LEVEL AIR ACTIVITY IN aCi/m RESULTINGFROM COAL-FIRED POWER PLANT EMISSIONS

Modern Plant 1972 Plant

50 meter 150 meter 50 meter 150 meter Natural

Nuclide stack stack stack stack Background

238U 234U 70 6 310 27 70

230Th 35 3 310 27 70

226Ra 52 4.5 310 27 70

232Th, 228Th, 228Ra 29 2.5 250 22 70

210Pb 180 15 620 54 14000

210Po 180 15 620 54 3300

K 78 6.7 690 60 1000

Ash (,ug/m ) 7.8 0.67 69.4 6.0 100

crops might be grown, we have assumed that the activ-ities given in Table 5 are mixed uniformly down to adepth of 30 cm due to periodic plowing and have com-pared the resultant increase in soil specific activ-ity with the mean ambient soil activities given inTable 1. The results are shown in Table 6. Even forthe 1972 plant releasing from a short stack, theincrease over background is only about 3 percent.For modern plants, releasing from tall stacks, it isclear that the increase in soil specific activitywould be negligible even for a much smaller pene-tration or mixing of the fly ash into the soil.

TABLE 5

ESTIMATED MAXIMUM DEPOSITION IN nCi/m FROM 50 YEARS OFPOWER PIANT OPERATION

Modern Plant 1972 Plant

50 meter 150 meter 50 meter 150 meterNuclide stack stack stack stack

8I, U 2.25 0 .19 10 0.85

230Th 1.35 0.10 10 0.85

226Ra 1.7 0.14 10 0.85

232Th, 228Tt, 228Ra 0.9 0.08 8 0.70

20Pb, 210Po 5.5 0.50 20 1.7

K 2.5 0.22 22 1.9

TABLE 6

INCREASE IN SOI.L ACTIVITY IN fCi/g FOR PLOWED FIELDSFROM 50 YEARS CUMULATIVE DEPOSITION

Modern Plant 1972 Plant150 Meter 50 Meter Natural Soil

Nuclide Stack Stack Background

U, U 0.42 22 700

230Th 0.21 22 700

226 Ra 0.30 22 700

232Th 228Th, Ra 0.18 18 700

Pb, Po 1.1 44 .1,000

K 0.49 49 10,000

Dose to Individuals

The major potential pathways which might resultin increased radiation doses to humans from coal firedpower plant emissions are inhalation of fly ash, in-gestion of food grown in contaminated soil, or directradiation exposure from the increased deposited radio-activity. We concluded in Reference 1 that the lattertwo pathways could be completely neglected and thatthe only potentially significant pathway is direct in-halation, and the only organ potentially at risk isthe lung.

692

Ingestion and direct radiation doses are negli-gible since, as we have shown, the probable increasesin soil specific activity are very small, even overthe small area of maximum deposition. Even if directdeposition on foliage is considered, the availableevidence indicates fly ash is very insoluble (1,7) andthus its activity is not likely to be incorporated in-to the vegetation, or into body tissues and organs ifingested. Also it is unlikely any individual wouldobtain more than a small fraction of his diet fromcrops grown in the small area of maximum contaminatiornand even less likely he would do so for the extendedperiod necessary to build up an increased body burdenof naturally occurring radionuclides.

The dose-equivalent to the lung from inhalation offly ash particles can be estimated from the airborneactivity levels given in Table 4. These estimates,which are described in more detail in Reference 1, aresummarized in Table 7. They assume, conservatively,that all the airborne fly ash is respirable and in-soluble, and are for the maximum exposed individual,i.e., one who is exposed continuously at the levelsgiven in Table 4 for a sufficient period of time tobuild up equilibrium levels in the lung. Even for theolder 1972 plant with a 50 meter stack, the maximumdose equivalent is less than 10 percent of that fromnatural background. The dose equivalent for the older1972 plant is additionally conservative in that, aslarger quantities of ash are released, the relativefraction in the respirable range decreases. Even forplants with the most efficient emission controlsystems, a considerable fraction of the released flyash is not respirable. (5, 7-9)

TABLE 7

ESTIMATES OF DOSE EQUIVALENT TO LUNG OF MAXIMUM EXPOSEDINDIVIDUALS FROM INHALATION OF COAL PLANT EMISSIONS

Dose Equivalent (mrem/y)

50 Meter 150 MeterType Plant Stack Stack

Modern Plant 1.6 0.14

1972 Plant 8.7 0.75

(Natural Background)6 100

The dose-equivalents to other organs from inhaledradionuclides are considerably smaller than those tothe lung, since the insolubility of most of the flyash results in very little transfer of radionuclidesfrom the lung to other organs or to soft tissue. Wethus conclude that the effluents from even "dirty"coal-fired power plants releasing from short stackswill result in negligible increases in radiation ex-posure to the general population or to any individual.

Radioactivity of Waste Materials

Only a small fraction of the ash produced in coalplants is released through the stack. The remainderis usually sluiced to holding ponds. About 20 percentis currently sold for use as land fill or in manufac-turing building materials. The use of scrubbers toremove sulfur from flue gases results in an additionalwaste product in the form of sludge. As shown inTable 2, these wastes have radioactivities severaltimes that of the average soil. Because of the in-solubility of the ash, leaching of radionuclides intoground water does not appear to present a seriousproblem, although there have been few studies of lorg-term weathering effects. The use of ash in buildingmaterials can result in potential increases in directexternal radiation exposure to occupants of the re-sultant buildings depending on the activity of thematerials with which the ash is mixed. There havebeen only a few scattered measurements of the activityof building materials made with ash and slag and nosystematic studies have been made of the potentialincrease in population exposure.

One serious concern has been the potential re-lease of Rn-222 from ash waste piles and ash products.We reported previously that radon did not appear toemanate significantly from fly ash and bottom ash.Since then, we have analyzed a number of additionalsamples for radon emanation. These results are sum-marized in Table 8. As shown, the emanation from ashaverages less than 2 percent. This compares to about15 percent for the average soil (3) and 20 percent foruranium mill tailings. (10) Bituminous coal appearsto emanate at a rate comparable to soil, however, thethree lignite coal samples we have evaluated emanatedat much higher rates. Unless long term weatheringsomehow modified the physical composition of the im-permeable glassy fly ash particles, we would concludethat emanation of radon from ash does not constitutea significant potential perturbation to ambient Rn-222levels.

TABLE 8

Rn EMANATION* FROM COAL AND ASH SAMPLES

No. of Emanation (9)Type of Sample Samples Mean Range

bituminous coal 6 20 11-37

lignite coal 3 53 44-63

flyash 11 < 2 < 1-3

bottom ash, slag 10 < 2 < 1-5

scrubber sludge 1 4 -

*Ratio of escape to production rates.

693

Other sources of wastes from the coal fuel cycle,in particular coal mining refuse and coal cleaningresidues, have been reported to result in contamin-ation of streams and rivers and may constitute a moresignificant potential problem area. (1)

Discussion

The preceding discussion illustrates that if oneuses realistic, yet conservative, estimates of activ-ities in coal and fly ash, power plant release rates,atmospheric diffusion and subsequent deposition, oneconcludes that the combustion of fossil fuels, evenin the less controlled "older" plants that releasewaste products from short stacks, will not result insignificant perturbations on natural environmentalradiation levels or significant increases in doses toany individual. One can also see, however, that bycompounding worst cases or unrealistic assumptions,one arrives at fairly large estimates of dose. Un-fortunately,numerous recent publications containingsuch estimates has resulted in unnecessary public andpress concern regarding this topic. This is regret-table since it tends to divert attention from theother potential health hazards of coal combustion,some of which are likely to be far more consequentiaL

Another controversial, although not particularlyrelevant, question has been the relative radiologicalimpact of coal versus nuclear power. We examinedthis question in Reference 1 and concluded that, whenthe entire fuel cycles are considered, the radio-logical impact of coal is far smaller than that ofnuclear. Most noteworthy, however, is that routinereleases from both cycles produce negligible in-creases in radiation exposure compared to backgroundor other sources of exposure. The public has beenconfused with widely differing published comparisons.Some of these simplistically compared the totalactivities (in curies) released, a very misleadingexercise, since, obviously, different radionuclideswith different half-lives, radiations, radiation

energies', ecological transport properties, dose path-ways to man, and biological effectiveness are in-

volved. Others compared the effects of emissionsfrom power plants only, ignoring the fact that most

of the routine radiological impact of nuclear powerderives from other parts of the fuel cycle.

Although some further research on the long termfate of certain waste products might be beneficial,we feel that it is clear that the radioactivity re-

leased by the combustion of coal in modern plantsmeeting EPA's particulate emission standards is not

a matter of concern.

2. Styron, C. E , Casella, V. , Farmer, B. M. ,Hopkins, L. C., Jenkins, P. H., Phillips, C. A,and Rooinson, B., "Assessment of the RadiologicalImpact of Coal Utilization," Mound LaboratoryReport MLM-2514, Monsanto Research Corp , MoundFacility, Miamisburg, Ohio (1979).

3. "Sources and Effects of Ionizing Radiation,"Report of the United Nations Scientific Com-mittee on the Effects of Ionizing Radiation,United Nations, New York (1977).

4. Coles, D. G., Ragaini, R. C., and Ondov, J. M.,"Behavior of Natural Radionuclides in WesternCoal-Fired Power Plants,?" Environ. Sci. Technol.12, 442 (1978).

5. Ondov, J. M., Ragaini, R. C., and Bierman, A. H.,"Characterization of Trace Element Emissions fromCoal-Fired Power Plants," in Nuclear Methods inEnvironmental and Energy Research, USDOE Pro-ceedings CONF-771072, U. S. Department of Energy(1978).

6. "Natural Background Radiation in the UnitedStates,?" NCRP Report No. 45, National Councilon Radiation Protection and Measurements,Washington, D. C. (1975).

7. Parungu., F., Ackerman, E., Proulx, H., andPueschel, R., "Nucleation Properties of Fly Ashin a Coal-Fired Power Plant Plume," Atmos.Environ. 12, 929 (1978).

8. Hobbs, P. V., Hegg, D. A., Eltgrowth, M. W. andRadke, L. F., "Evaluation of Particles in thePlumes of Coal-Fired Power Plants-1. Deduc-tions from Field Measurements,," Atmos. Environ.12, 935 (1979).

9. Ragaini, R. C. and Ondov., J. M., "Trace ElementEmissions from Western U. S. Coal-Fuel Power

Plants,)t in Proc. Int. Conf. Modern Trends inActivation Analysis, Munich, Germany, I, 654k1976).

10. Swift,p J. J., Hardin, J. M., and Calley, H. W.,"Potential Radiological Impact of Airborne Re-leases and Direct Gamma Radiation to IndividualsLiving Near Inactive Uranium Mill Tailings Piles,"USEPA Report EPA-520/1-76-001, U. S. Environ-mental Protection Agency, Washington, D. C.(1976).

References

1. Beck, H. L., Gogolak, C. V., Miller, K. M.,. andLowder, W. M., "Perturbations on the NaturalRadiation Environment due to the Utilization ofCoal as an Energy Source," Proceedings ofSymposium on the Natural Radiation Environment-III, CONF-780422 , U. S . Department of Energy

(1979).

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