Technologically enhanced naturally occurring radioactive material (NORM): Pathway analysis and radiological impact

Appl. Radiat. lsot. Vol.49, No. 3, pp. 227-239, 1998 ! 1997ElsevierScienceLtd. All rights reserved Printed in Great Britain PII: S0969-8043(97)00244-3 0969-8043/97 $19.00+ 0.00

Pergamon

Technologically Enhanced Naturally Occurring Radioactive Material (NORM): Pathway Analysis and Radiological Impact R. S. O ' B R I E N and M. B. C O O P E R Radiation Health Section, Australian Radiation Laboratory, Yallambie, Vic., 3085, Australia The main pathways by which technologicallyenhanced radioactive materials can impact on human health have been examined. Analytical methods are presented for calculation of the radiation doses for the dominant pathways for external and internal exposure. The application of computer modelling to the assessment of the radiological impact of NORM is also discussed. :~ 1997 ElsevierScience Ltd. All rights reserved

Introduction The earth's crust contains radionuclides which constitute the major source of naturally occurring radioactive material in the environment. Most of these radionuclides are members of the radioactive decay chains beginning with 238U, 235U and 232Th. The subject of this paper is the analysis of some of the pathways by which technologically enhanced naturally occurring radioactive material (NORM) can pass through the environment from its point of origin to man, and a discussion of the radiological impact of NORM on humans, plants and animals. Activities that lead to the technological enhancement of NORM

There are many human activities that lead to the production of NORM. These include activities which can enhance NORM levels directly, such as the mining, milling and processing of uranium ores and mineral sands (UNSCEAR, 1988), fertiliser manufacture and use (UNSCEAR, 1988; Burnett et al., 1996; Ravila and Holm, 1996), phosphate manufacture (Keating et al., 1996), burning of fossil fuels (UNSCEAR, 1988; Hedvall and Erlandsson, 1996; Papastefanou, 1996), metal refining (UNSCEAR, 1988; Baxter et al., 1996), and general underground mining and open-cut mining activities. The use of buildings as dwellings and workplaces can also lead to exposure to NORM (Baxter, 1996). In addition, the use and disposal of waste materials (e.g. mine tailings, phosphogypsum, and fly ash) associated with activities which produce NORM can pose significant radiological problems. Since this paper is restricted to a discussion of the radiological impact of technologically enhanced naturally occurring radionuclides, processes such as nuclear power generation, nuclear

weapons testing, the manufacture and use of radioactive sources (e.g. 6°Co), radiopharmaceutical production and use, and medical uses of radiation can be ignored as the radionuclides involved in these processes do not occur in the natural environment. Each of the processes or activities which produce NORM has associated with it a series, or several series, of pathways by which the radioactive material can reach humans. These pathways depend on the process, but fall into several broad categories (on-site, off-site, airborne, waterborne, etc.). The radiological impact on humans (and plants and animals) can depend strongly on the process which produces the NORM and the pathways by which it is transferred from the source to humans. The production of TechNORM and the use of and disposal of wastes containing NORM are discussed by UNSCEAR (1988), A draft report by the United States Environmental Protection Agency, USEPA (1993) discusses many generic examples of problems associated with the disposal of waste products arising from processes leading to the production of NORM, and also discusses the assessment of the radiologica[ risk associated with these generic processes. As a more specific example, the pathways associated with the use of mining waste as landfill in northern Australia have been discussed by Moroney 11992).

Analysis of Pathways Associated with NORM NORM can reach humans via several pathways, including the food chain, inhalation or ingestion of airborne radioactive dust and the inhalation of radon isotopes and their progeny which reach the atmosphere as a result of the exhalation of radon isotopes from the ground surface or from the surface of building materials. Figure 1 shows the general

227

228

R.S. O'Brien and M. B. Cooper

SO.OE

I Depmlltlon__

1

TRANSPORT J - I

t

Loss

DEPOSITION ON _~ TERRESTRIAL SURFACES AND IN SEDIMENTS

I

PRODUCTS

I

1.,

Ingestion

Ingestlon~

Ingestion

I

1

BIO-

Iln..

GROUND I WATER TRANSPORT

WATER TRANSPORT

|

~NUCUDES INAIR DepoeltlonRoot , _~esulq~r.slon

UPTAKEBYHUtlAN8

Fig. 3. An alternative representation of the environmental cycle for airborne releases of radionuclides.

| ACCUMULATION | | IN FOOD I

I

USAGE RATES DOSE RATE FACTORS HEALTH EFFECTS

Fig. 1. The general environmental contamination problem. problem, while Fig. 2 shows the general environmental pathways for releases of radioactive material to water. These pathways can be extremely complex, so a systematic approach to establishing which pathways are important is essential. Figure 3 shows some of the actual mechanisms for the pathways shown in Fig. 2. This process of building up the complexity of

each part of a general pathway diagram is an important aid to pathway analysis. Figure 4 shows the general environmental pathways for releases of radioactive material to the atmosphere. The naturally occurring cycle of 226Ra in the environment (IAEA, 1990) is shown in Fig. 5, and this figure also shows how human activities can affect this natural cycle, leading to (technological) enhancement of the radionuclide concentrations. The environmental cycles for other naturally occurring radionuclides are similar in principle, but different in detail because of differences in the radioactive decay times and chemical behaviour of the different radionuclides. The pathways by which NORM can move through the environment and impact on humans, animals

RADIOACTIVEMATERIALS

]

RADIOACTIVEMATERIALS

1

/dR

I

SOIL

I"

tH

,lU.S

'

I

C #S I ~R D

PLA

t HUMANS

t

v I

Fig. 2. The general environmental cycle for airborne releases of radionuclides. The dashed lines represent external exposures, the dotted lines represent ingestion, and the broken line represents inhalation.

Fig. 4. The environmental cycle for releases of radionuclides to surface or ground water. The dashed lines represent external exposures, while the dotted lines represent ingestion.

229

Technologicallyenhanced NORM

m

J

,-]

-I .....

I

Fig. 5. The -~2~Racycle (IAEA, 1990) and biota can be divided into several broad categories.

On-site pathways These pathways tend to be direct, and usually result in external exposure to gamma radiation, or internal exposure resulting from inhalation of radioactive dust or radon progeny. External exposures can occur due to the build-up of radioactive dust on equipment surfaces and floors, and the build-up of sludges in pipes and storage tanks. Due to the presence of NORM in most soils and rocks, underground mining activities can lead to enhanced levels of radioactive dust, and radon isotopes and their radioactive progeny, unless careful attention is given to the design and use of suitable ventilation systems in these mines. In open-cut mines, NORM can also be produced and, since the ventilation cannot be controlled, work practices have to be carefully controlled to minimise the radiological risk to the on-site work-force. This enhancement can also be a problem in buildings where the ventilation is inadequate and/or where construction materials containing NORM are used. There is also the possibility of radionuclides entering the body directly as a result of injury. Analysis of on-site pathways at a particular site involves a detailed knowledge of the manufacturing or mining process(es) at the site. An example is given in Fig. 6, which shows a typical acid leaching circuit in uranium mills (IAEA, 1990). A second example, a simplified flow diagram for the industrial-scale Merseberg process for converting phosphogypsum to ammonium sulphate (Burnett et al., 1996), is shown in Fig. 7.

Off-site pathways Off-site pathway analysis is involved in the assessment of exposures or potential exposures to

humans living near a site where NORM is produced, and to plants and animals which may be exposed to these technologically enhanced levels of radioactivity. Off-site exposures can result from the transfer of NORM via environmental pathways or from the use of industrial wastes containing NORM. Environmental pathways tend to be much more indirect and complex than on-site pathways, e.g. transfer of radionuclides through the food chain (Dahlgaard, 1996), by river and oceanic transport (McDonald et al., 1996), by atmospheric dispersion, by resuspension of radioactive dust, etc. A careful analysis of the potential sources of exposure and pathways by which inadvertent exposures can occur in any situation is necessary to ensure that unwanted exposures are minimised. This type of inadvertent exposure can arise from the use of waste materials which contain NORM. An example is the use of phosphogypsum as a substitute for natural gypsum in the manufacture of cement, wall-board and plaster (UNSCEAR, 1988). Another example is the use of coal ash in the manufacture of cement and concrete, as a road stabiliser, in road fill, and as fertiliser (UNSCEAR, 1988). Still another example is the use of mining waste as landfill (Moroney, 1992). Again a careful analysis of procedures for the disposal of NORM wastes, and the pathways by which these materials can move through the

1Uranium°re; ~; Mining ; ~ ~

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Fig. 6. A typical acid leaching circuit in uranium mills (IAEA, 1990).

230

R. S. O'Brien and M. B. Cooper

0-1 cm

~ phool:~S~sum

Filtration and

Conwrslon

Wuhlng

'1 '1

1 dilute(NH~SO4~

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I combinatioI-"n

oo.

l

Fig. 7. A simplified flow chart of the industrial-scale Merseberg process for the conversion of phosphogypsum to ammonium sulphate (Burnett et al., 1996). environment, is necessary to ensure that the impact of these activities on the environment is minimised.

Radiological Impact of NORM Assessment of the radiological impact of NORM involves the analysis of exposures or potential exposures to humans and the environment (plants, animals, soils, water, etc.). Each of these exposures can involve different pathways or groups of pathways, depending on the precise scenario under consideration. The radiological impact of NORM on humans can be discussed in terms of the effects of exposure to external radiation or to internal radiation, and in terms of on-site exposures and

RADIONUCUDES INAIR

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Fig. 8. A model of radionuclide transport in pasture.

off-site exposures, e.g. Timmermans and Van der Steen (1996). The radiological impact of external exposures can be assessed by direct calculation if the source terms are known or can be measured. On-site external exposures in industrial or mining situations can be due to the presence of NORM in stock-piles, waste piles, and storage tanks, and can also be caused by the build-up of surface contamination on equipment, or in pipes and storage tanks. External exposures to members of the public (off-site) can result from exposure to gamma radiation resulting from the passage of a cloud of radioactive material through the atmosphere (cloud shine) or exposure to gamma radiation from material deposited on the surface of the ground (ground shine). The radiological impact of internal exposures is usually assessed by means of direct measurement of body burden and calculation of the dose to a particular organ or group of organs, or by the use of faecal or urine analysis or, if the intake is known, by the use of a model which simulates the behaviour of radionuclides in rico. The guiding principle in controlling the radiological impact of NORM is the A L A R A Principle (ICRP, 1977), which states that all exposures should be kept as low as reasonably achievable, social and economic factors being taken into account.

A methodology for assessing the radiological consequences of routine releases of radioactivity to the environment has been presented by Simmonds et al. (1995); although many of the radionuclides which were considered in their report are outside the scope of the present work, the principles and method of approach are appropriate to any study of the radiological impact of radioactivity in the environment. The radiological impact of diffuse NORM wastes has been discussed by USEPA (1993). A brief overview of these methodologies and the equations

231

Technologically enhanced NORM recommended for estimating the annual dose resulting from exposure to N O R M waste in several different scenarios is presented in the following paragraphs, not as a definitive exposition, but rather as an indication of a general approach to the problem. Except where otherwise indicated, the discussion follows that presented in U S E P A (1993). The groups of people considered are on-site workers, residents, members of critical groups (where the critical group consists of an actual or hypothetical group of individuals who would be considered to be at maximum risk for the scenario under consideration), and members of the population living near a source or potential source of exposure. The dominant exposure pathways in most situations are external gamma radiation, inhalation of radon gas and its decay products, inhalation of radioactive dust, and ingestion of contaminated food and/or water. The scenarios considered in the following discussion are listed below. I. Workers - direct gamma exposure dust inhalation - indoor radon inhalation. 2. On-site individuals - direct gamma exposure dust inhalation - indoor radon inhalation. 3. Members of the critical group - direct gamma exposure inhalation of contaminated dust - downwind inhalation of radon - gamma exposure from N O R M in building materials - inhalation of radon from N O R M in building materials ingestion of contaminated drinking water - ingestion of food from land irrigated by contaminated water - ingestion of food contaminated by deposition of radioactive dust - ingestion of food grown on repeatedly fertilised soil - exposure to radiation from road pavement and aggregate -- inhalation of contaminated dust from stack releases - downwind exposure from stack releases. 4. General population - downwind exposure to resuspended particulates - downwind exposure to radon ingestion of contaminated river water ground water pathway ingestion of contaminated river water surface runoff pathway ingestion of food grown on repeatedly fertilised soil. -

-

In these scenarios, a worker is an employee working at a site where diffuse N O R M waste is stored, disposed of or reused, an on-site individual is an individual living on an abandoned waste storage or disposal site, a member of the critical group is an individual residing near a waste storage or disposal site who is assumed to receive the largest dose of any member of the public (living near the site) as a result of normal daily activities, and a member of the general population is an individual living in the general vicinity of the site or an individual who may be exposed as a result of the use of phosphogypsum and coal ash in building materials, the reuse of phosphogypsum and water treatment sludges as fertiliser, the use of phosphate slag and copper slag in road construction, and recycling of N O R M - c o n taminated oil production equipment. For analysis purposes these scenarios can be divided into exposure to gamma radiation, exposure to radon and its decay products, exposure to contaminated dust, and ingestion of contaminated food and/or water. U S E P A (1993) provides tables for all the input parameters used in the equations.

External exposure to gamma radiation from a NORM waste or storage pile For an individual exposed to gamma radiation from a waste pile or storage pile containing N O R M the dose received in one year can be expressed in the general form

D = Fp*Ac*S*Tcxp*DCF:

(1)

-

-

-

where D = annual effective dose (mSv a ~), Fp = gamma radiation flux at the top of the pile due to gamma radiation from the pile (J m -~s ~), Ac = attenuation factor due to the presence of overburden or capping material above the pile, S = shielding factor due to the presence of a vehicle or building, t,p = exposure time (s a '), D C F . = dose conversion factor for external gamma radiation (mSv a - ' ( J m-2) - '). The shielding factor S allows for the fact that the individual may be inside a vehicle (e.g. a loader or bulldozer) or a building (e.g. a store room or an office or a house built over an abandoned storage or disposal site). If the exposure occurs at more than one location, the contributions from each location can be summed. For example, for a worker who works at a site where diffuse N O R M waste is being stored, disposed or reused, and spends time on the waste/storage pile in a vehicle which places the exposed individual approximately 1 m above the uncovered waste surface and provides some shielding, the gamma radiation flux at the top of the pile is approximated by

-

-

Fp=

C~*E"*M *( I ~ e- " ~ ~ A' l~wA,, /

(2)

232

R.S. O'Brien and M. B. Cooper

where 6", = average radionuclide concentration in the waste pile (disintegration s - ' k g - ' ) , E = total energy per disintegration (J disintegration-'), M = mass of the waste pile (kg), A = horizontal area of the waste pile (m:), Pw = attenuation coefficient of the waste material (m - '), Aw = thickness of the waste pile (m). This equation overestimates the dose, because it does not correct for radionuclide decay in the waste pile. This correction is easily applied to the factor C, if necessary. The attenuation factor for the cover (if any) can be approximated by A~ = e -'~A~

(3)

where/~c = attenuation coefficient of the cover over the waste (m - '), Ac = thickness of the cover over the waste (m). The same equations can be used for the case of a member of the critical group by multiplying by a (geometric) factor G which allows for the fact that the exposed individual is off-site. For an individual at a distance x (m) from the edge of the pile, G can be approximated by G = 0.5*e- 0005~.

(4)

Exposure to gamma radiation .from N O R M in building materials Equations similar to those already given can be derived for the annual dose resulting from the exposure of individuals to gamma radiation from N O R M or N O R M waste used in building materials. The radiating material can either consist of a basement or foundation slab, or walls, constructed of concrete or concrete blocks made of coal ash containing N O R M . For a building containing more than six rooms, the gamma flux from such materials can be approximated by

rb=

b ~' ~k

;b-~b

}

(5)

where Cb = average radionuclide concentration in the building material (disintegration s ]kg ]), E.. = total energy per disintegration (J disintegration-]), #b = attenuation coefficient of the building material (m-I), A b = thickness of the building material (m). In addition, sheets of phosphogypsum plasterboard containing N O R M are used ( U N S C E A R , 1988) as a substitute for normal gypsum as a wall liner. The situation for the plasterboard is different from the waste pile scenario, because of the relatively small thickness of the source material (O'Brien, 1997).

radon decay products, ingestion of contaminated food and/or water and ingestion of food grown on soil which has been contaminated by the deposition of contaminated dust or the repeated application of fertiliser, the estimation of the annual dose to an individual begins with the very general equation D = I*DCF

(6)

where D - - c o m m i t t e d effective dose for one year's intake (mSv a - '), I = intake (Bq a - ' ) , D C F = an appropriate dose conversion factor, or dose per unit intake (mSv B q - '). For inhalation, the intake is given in general terms by I = C~*f*B*4w

(7)

where C~ = radionuclide concentration in the inhaled material (Bq m - 3 ) , f = respirable fraction (applies to dust intake only), B = breathing rate (m ~ s-~), texp = exposure time per year (s a-]). The activity concentration in the inhaled (or ingested) material is given, again in general terms, by C, = ST*TF

(8)

where S T = t h e source term ( B q m 3 or B q s ]), T F = a transfer factor (dimensionless or s m-3), which allows for transport from the point of origin to the exposed individual. For ingestion, equation (7) is modified to I = C,*M,

(9)

where C = activity concentration in the ingested material (Bq kg-~), Mi = m a s s of contaminated material ingested per year (kg a-t). These general equations need to be modified for each specific application. Dust inhalation For a worker inhaling radioactive dust at a storage or disposal site it is usually assumed that the transfer factor is unity and that the dust concentration above the pile is constant, so that the source term ST is given by ST = C*d~*M~.

(10)

where C = average radionuclide concentration in the waste source material (Bq k g - ' ) , dc = a dilution

Ingeetlon

ID

Inhalation and ingestion For analysis of intakes of N O R M by inhalation of contaminated dust, inhalation of radon and

Fig. 10. A simple compartment model for intake and uptake of radionuclides by humans.

Technologically enhanced NORM

Inhalation

y,,

i Skin

<

!I

Lymph nodes

Wound

Ingestion

Exhalation

Respiratory Tract

] Liver

Direct absorption Sweat

.i

233

GI

Body fluids

Tract

(Blood, Lymph)

cutaneous tissue

f

/

I

II

I Urine

Faeces

Fig. 11. A compartment model for the assessment of intake and excretion of radionuclides by humans.

factor which allows for the effect of any covering over the waste, M , = dust loading over the pile (kg m - 3). This equation will also apply to an individual working in a building at such a site, or to an individual living over an abandoned site. For a member of the critical group, i.e. an individual (real or hypothetical) living near the waste pile, the exposure is assumed to occur as the result of inhalation of contaminated dust arising from the downwind transport of resuspended particulates, so the source term is expressed as a release rate. In this case the calculation of the transfer factor is of central importance. The source term is given by ST = C*A*Ew

(11)

where C = average radionuclide concentration in the waste pile or soil (Bq k g - t), A = area of the pile (m2), Ew = resuspension factor (kg m - 2 s - ~). This source term does not allow for the reduction in dust resuspension due to application of moisture to the waste pile (by rainfall or work practice). The resuspension factor used in this model is given by

17,, = ~ R , * f

(12)

where R , = resuspension rate at wind speed s (kg m - 2 s - '), f = frequency of occurrence of wind speed s. In this case a Gaussian plume dispersion model can be used to represent the transport of resuspended material. The transfer factor can be approximated by (USEPA, 1993)

TF -

2*fw a,* V~*X~+-~

(13)

where f , = frequency with which the wind blows in the direction from the waste pile to the exposure point, a~ = atmospheric stability constant (dimensionless), V, = average wind speed (m s ~), X~ = virtual distance (m) from the waste pile to the exposure point (USEPA, 1993); this corrects for the position and orientation of the source relative to the exposure point, b = (2/rt) '/2. Vd/(V~*a,). This equation is not dimensionally correct, because of the presence of the factor b in the term (1/Xt) ~~ "-. This arises from the fact that the Gaussian plume model used is a point source model, so a correction has to be made for the finite size of the source. A more 'correct' expression would possibly be of the form (I/X~)2c(a~)(X/X~)% where c is a dimensionless function of a~, and X is the actual distance from the centre of the source to the point of exposure.

Inhalation o f radon and radon decay products from a N O R M storage or disposal site For an individual working or living in a building over a storage or waste pile, the source of exposure is radon gas which enters the building by diffusion through the floor (this includes any concrete slab which may be used as a support for the building). The source term is the number of radon atoms exhaled from the pile per second beneath the building. Provided the thickness of the waste is greater than approximately 1 m, the source term is given by ( U N S C E A R , 1988)

234

R. S. O'Brien and M. B. Cooper ST = A * ( C R ~ * p w * E * ~ )

(14)

where A = the floor area of the room or building (m2), Caa = 226Ra concentration in the waste pile (Bq kg L), p~ = waste density (kg m-3), E = radon emanation coefficient (dimensionless), 2 = radon decay constant (s-~), D~ = radon diffusion constant in the waste pile material (m 2 s-~). The transfer factor is approximated by e

TF -

dbV'2~'Db

V*2~

(15)

where V = volume of the room or building (m~), 2~ = average air ventilation rate (building/room air changes s-~), Ab = thickness of the building foundation material (m), Db = radon diffusion constant in the building foundation material (m 2 s ~). The exponential term accounts for radon attenuation through the floor of the building. The term l/(V*2v) converts the total rate at which radon enters the building/room to building/room concentration. This equation does not allow for the presence of cracks in the building foundation material. For members of the critical group, and other individuals living in the general vicinity of a waste or storage pile, the dose calculations for dust and radon inhalation are more complex because of the need to include a factor in the equations which will account for the airborne transport of the radionuclide(s) from the point of origin (the waste pile) to the point of exposure. USEPA (1993) used a Gaussian plume dispersion model to represent the transport of radon and radon decay products in the atmosphere. Their transfer factor is given by

Inhalation /

[ i

Respiratory tract

TF -

2%

ai* V~*X~

(16)

where the notation is the same as in equation (13). The difference between this and equation (13) is that the radon is exhaled directly into the atmosphere, and there is no resuspension.

Inhalation of radon and radon decay products from NORM in building materials In this case it is only necessary to consider exposure to radon which originates in the building materials. Again the annual effective dose from radon and its decay products is given by equation (7). The source term is given by ST = ZAj*Jj

(17)

J

where Aj is the area of the j t h exhaling surface (m2), and Jj is the exhalation rate of radon from the j t h exhaling surface (Bq m-2 s - '). The exhalation rate from a thin slab of building material is given by (UNSCEAR, 1988)

J = Caa*pb*Eb*2*Ab

(18)

where CRa = 226Ra concentration in the building material (Bq kg ~), Pb = density of the building material (kg m 3), Eb = emanation coefficient of the building material, 2 = decay coefficient of radon (s- ~), Ab = thickness of the building material (m). The transfer factor is given by TF-

1

V*2v

(19)

where the notation is the same as in equation (15).

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Ingestion

II Transfer compartment (Body fluids)

~/ a 1 Tissue 1 Compartment 1

:•

fu Urinary excretion

•!/

• a

a2

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Tissue I C°mp~rtment I [

. . . . .

. . . .

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Excretion Systemic faecal excretion Fig. 12. An alternative model for assessment of intake and excretion.

235

Technologicallyenhanced NORM

Ingestion of contaminated drinking water from a well

Ingestion of food contaminated by well water

Rain water can soak through a waste or storage pile, leach the material from the pile, flow downward into an aquifer beneath the pile, and contaminate the drinking water drawn from the aquifer. If dispersion and delay effects due to the (slow) transport of water through the soil and aquifer beneath the pile are ignored, the activity concentration of a radionuclide, C~, in the drinking water is given by

For an individual who eats food irrigated with contaminated water drawn from a well, the quantity of radioactivity ingested is given by an equation similar to equation (20), but modified to account for the uptake of radioactivity from the water by the plants and animals which are subsequently consumed.

C i - Cw*Mw*2L*f0 qw

(20)

where C~ = radionuclide concentration in the waste pile (Bq kg-~), M, = mass of the waste pile (kg), 2L = fraction of the radionuclide leached from the pile in one year (a-~), )Co= the fraction of the radionuclide in the pile which traverses the soil and aquifer under the pile and arrives at the well (dimensionless), q~ = the dilution volume of the well (m 3 a '). For a mixture of radionuclides the doses resulting from ingestion of individual radionuclides are additive. The fraction of the radionuclide leached from the pile in one year (a-~) is given by W2 2t-

Kd*pw*A~*K~

(21)

where W = annual water infiltration rate through the waste (m a ~), K~ = equilibrium distribution coefficient of the waste/soil matrix (m 3 kg- ~) (assumed to be the same as that of the aquifer), Pw = density of the waste (kg m - 3), d~ = thickness of the waste (m), K~ = saturated hydraulic conductivity of the waste/ soil matrix (m a - ' ) . The fraction of the radionuclide (in the pile) which reaches the well is given by

f0-

V~ (1 - e-;-LRL'V0e-~'~ I*R*2L

(22)

where V,~= horizontal velocity of the water in the aquifer (m a - ~), L = length of the waste site parallel to the aquifer flow (m), R = 1 + (p,/ p)*K~ = retardation factor, p~ = aquifer density (kg m-3), p = aquifer porosity (dimensionless), 2 = d e c a y constant of the radionuclide (s-t), t~ = time of the peak concentration of the radionuclide in the ground water at the well concentration. The dilution volume of the well is assumed to be the annual rainfall multiplied by the area of the waste pile, and the time of peak concentration in the ground water at the well is given by R t~ = -~ *(L + Xw)

(23)

where Xw is the distance of ground water flow from the nearest edge of the waste pile to the well (m).

Ingestion of food contaminated by deposition of radioactive dust In this case the radionuclide concentration in the food will be equal to the deposited dust concentration per unit area multiplied by factors which allow for the mixing of dust in the soil surface layer and the uptake of the radionuclide being considered by plants and animals. The source term is given by equation (11), while the transfer factor is given by V0*Ur TF = TF(I 3)* 2p,dr

(24)

where T F ( 1 3 ) = transfer factor from equation (13), lid = dust deposition velocity (m s - ~), p~ = soil density (kg m-3), do = soil mixing depth (m), Uf = a dimensionless factor which allows for uptake of radioactivity by the plants (or animals) and also for the loss of radioactivity by washing, etc.

Ingestion of.food grown on repeatedly fertilised soil When fertilisers are spread on agricultural soil the fertiliser is diluted by mixing with the soil surface (plough depth) layer. Radionuclides in the fertiliser are removed by mechanisms such as plant uptake, downward leaching by infiltration of surface water, and wind and water erosion. If fertiliser is repeatedly applied the radionuclide concentration should reach an equilibrium value, which can be estimated (or measured). The dose to an individual who consumes food grown on such land can then be estimated. The same procedure is used for a member of the critical group or a member of the general population.

Exposure to radiation J?om road pavement and aggregate The use of industrial slag (e.g. phosphate slag and slag from copper smelting) as road bed material and as aggregate in paving can lead to external exposures, If the member of the critical group is assumed to be an individual who spends their working time in a vehicle on a highway where such material is used, the annual dose can be considered to be the sum of the doses from paving and roadbed. The radiation fluxes from these two sources can be approximated by equations similar to equations (1)-(3), with appropriate corrections applied for attenuation by the paving (which acts as a cover) and for the finite width of the roadbed.

236

R. S. O'Brien and M. B. Cooper

Inhalation o f contaminated dust from stack releases

Modelling the behaviour o f N O R M

In this case a Gaussian plume dispersion model can be used to estimate the radionuclide concentration in air downwind of the stack from which the material is released (e.g. a stack at a steel mill that recycles NORM-contaminated piping from oil production facilities). The maximum dose will be received by an individual positioned at the point at which the radionuclide concentration in air at ground level is a maximum. The transfer factor can be approximated by

As the foregoing discussion suggests, detailed study of the behaviour of NORM lends itself naturally to the use of computer models. These models, of necessity, greatly simplify the complex environmental processes and pathways involved but can still be extremely useful tools (Moroney, 1992). Examples (Simmonds et al., 1995; McDonald et al., 1996) of models which have been used for the analysis of environmental transport of radioactive material include models of transport in oceans, rivers, lakes and estuaries, soils, plants and animals. Similar models are also used for the transport of radionuclides and calculation of radiation dose in the human body (French et al., 1986; Jarvis et al., 1996). First-order compartment models are generally used in this work because of their mathematical simplicity. These models describe the processes involved in environmental and in vivo transport of radionuclides in terms of transport between and first-order decay within a series of compartments, which represent the concentration of a particular radionuclide in, for example, a storage tank or stock-pile, or a particular organ (for example the kidney) in the human body. The use of these models therefore necessitates a knowledge of the coefficients describing the rate at which radionuclides are transferred between compartments and decay within compartments. Estimating these transport coefficients can be difficult and involve both a wide range of experimental work and the use of much more complex models of the components of the system being studied. An example of this is the development of the ICRP respiratory tract model (ICRP, 1994), in which the respiratory tract is modelled by a relatively small number of compartments, but the deposition, transport and clearance parameters used in this relatively simple compartment model are derived from extensive experimental and modelling studies of the different parts of the respiratory tract. The values of many of the parameters given in the equations in the foregoing discussion are derived from other models such as PATHRAE (USEPA, 1987). The use of such models involves a trade-off between computing efficiency and realistic dynamics (Moroney, 1992). The models should contain the minimum number of compartments and transport coefficients needed to give a realistic simulation of the dynamic, physical and chemical processes being modelled. A major advantage of such models is that they are extremely flexible and can easily be expanded to allow a more detailed study of a particular process. This is shown in Fig. 8, which shows the plant compartment in Fig. 3 split into four compartments, two for crops and two for pasture. Figure 9 shows how the soil and plant compartments in Fig. 8 can be expanded further (Simmonds et al., 1995) for the case of undisturbed pasture. This process can be continued to any desired level of complexity.

VF -

2*fw ai V,~ Xm h+2

(25)

where the notation is the same as for equation (13), and the virtual distance Xm is given by

x.1

h~ -

~

(26)

a'*x/b + 2 where a~ and b are defined following equation (13), and h' is the effective stack height, given by h' = h + (l.5v*d + 0.45Q~i'3)/V,

(27)

where h = actual stack height (m), v = stack gas velocity (m s-~), d = inside stack diameter (m), QH = heat emission from the stack (J s-~), V~= average wind speed (m s - ~). The effective stack height allows for plume rise caused by momentum and thermal effects. Downwind exposure to radon In this case the source term is given by equation (14), and the transfer factor can be approximated by equation (16). The same procedure is used for both members of the critical group and members of the general population. Ingestion o f river water contaminated via the ground water pathway In this case the river water is assumed to be contaminated by the same process as that discussed earlier for contamination of well water via the ground water pathway. The radionuclide concentration in the river water is therefore given by equations (20) - (22), with the well dilution factor replaced by the river flow rate (m 3 s-~). Ingestion o f river water contaminated by surface runoff River water can also be contaminated by surface runoff of rainwater transporting leached radionuclides from a waste or storage pile. When estimating the radionuclide concentration in the river water, equations similar to equations (20)-(22) can be used, with corrections made for the dilution of the radionuclide concentration by surface water transport and river flow.

237

Technologically enhanced NORM

Ingestion

1

I SmallIntestine ~-~

BodyFluids

1 Intestine[

Iupper large

1 Intestin~

Lower Large ~l

Excretion

Fig. 13. A compartment model for the GI tract (ICRP, 1979).

Similar models can be used to represent particular processes within the human body which are important for assessing the radiological impact of inhaled or ingested radionuclides, either by providing a means of estimating dose (as in the case of the ICRP respiratory model), or providing tools for use with measurements of urinary and faecal output. For example, the computer model INDOS (French et al.,

GI ~_.~ a IDN-~ Tract

Body Fluids

1986) has been widely used for predicting urinary and faecal output resulting from a known intake, but the same model can be used to estimate intake from measurements of urinary and faecal output. Figure 10 shows an extremely simple compartment model for the intake of radionuclides by humans. As in the case of environmental transport processes, this simple model can be expanded to any required level of complexity. Figs 11 and 12 show models designed to facilitate the assessment of intake and excretion of radionuclides. Figure 13 shows the model of the GI tract currently adopted by the ICRP (ICRP, 1979). Figure 14 shows the old ICRP respiratory tract model (ICRP, 1979), while Fig. 15 shows the new ICRP respiratory tract model (ICRP, 1994). Figure 16 shows an example of a model for use with particular radionuclides, in this case the alkaline earth elements and lead (ICRP, 1993). Some processes, such as the atmospheric transport of radon, its radioactive decay products, and resuspended dust do not lend themselves easily to this type of modelling. Many of the processes associated with atmospheric transport are highly non-linear, and are not easily simulated by first-order models. The Gaussian plume dispersion model is very widely used in modelling the dispersion of atmospheric pollutants because of its relatively simple mathematical formulation, and there is a substantial body of scientific literature on the background, construction and application of such models. However, the Gaussian plume model derived from the formal solution of the atmospheric diffusion equation is an approximation which is only valid under idealised conditions (see, for example, Seinfeld, 1986), and the use of this and other models of environmental transport processes is strongly dependent on the availability of reliable data sets against which the models can be validated and calibrated.

Summary

J Lymph Nodes Fig. 14. The old ICRP respiratory tract model (ICRP, 1979).

The study of NORM, its production, environmental behaviour and radiological impact, is a multi-disciplinary effort, drawing on the work of engineers and scientists in a wide range of fields (physics, chemistry, biology, etc.). NORM can occur in a wide range of mining and industrial processes, and its radiological impact on members of work-forces and the public is governed by the pathways by which the NORM moves from its point of origin to its point of uptake by humans. These pathways and the processes by which NORM moves can be extremely complex and can vary considerably with the site and method of production. The assessment of the radiological impact of NORM therefore lends itself readily to computer modelling. These models have to be carefully developed, as they can rapidly become unwieldy if an attempt is made to incorporate all the pathway and process details. The most widely used type of model

238

R. S. O'Brien and M. B. Cooper

,5 f

I ~r

N~-

I

IEalrJmt~xie

I I I I I I

L S~lWin.

I.~

O.Ol

0.00002 In~fim

Fig. 15. The new respiratory tract model adopted by the ICRP (ICRP, 1994).

l O'mER

--]

IsoFr I~.VlAT~ I~"D~T~ I ~ D ~ ' T ~ i I TU~OV~I I T ~ O V ~ | I ~ O ' ~ JI

,.so rt ;2i CORTICAL ~ _ ~

LIVER2

SURFACE

LIVER1

,

xRAmlcta.AR V~y..U~ ~ION*EXCHI EXCH

I [s~[

= "rR~FI2UL~ SURFACE

___~

! ~I

, GITRACT 1

CONTENTS

It-..

I XmNEYS '1

I OTHER I [ ~NEY

I I T~SSUE

~

URINARY~ - ~

URINARY~...~

Fig. 16. The ICRP biokinetic model for alkaline earth elements and lead (ICRP, 1993).

Technologically enhanced NORM the links between c o m p a r t m e n t s represent the d y n a m i c transfer processes. F o r m a x i m u m efficiency the models should involve the smallest n u m b e r o f c o m p a r t m e n t s a n d pathways for the dynamics to be realistic, This type of model can be used in c o n j u n c t i o n with a n atmospheric dispersion model such as the G a u s s i a n plume model to study a wide variety of e n v i r o n m e n t a l t r a n s p o r t problems involving N O R M , a n d the same type of c o m p a r t m e n t model is also used in assessing the radiological impact o f N O R M o n h u m a n s a n d the e n v i r o n m e n t .

References Baxter M. S. (1996) Technologically enhanced radioactivity: an overview. Journal of Environmental Radioactivity 32, 3-18. Baxter M. S., MacKenzie A. B., East B. W. and Scott E. M. (1996) Natural decay series radionuclides in and around a large metal refinery. Journal of Environmental Radioactivity 32, 115-134. Burnett W. C., Schultz M. K. and Hull C. D. (1996) Radionuclide flow during the conversion of phosphogypsum to ammonium sulphate. Journal of Environmental Radioactivity 32, 33-52. Dahlgaard H. (1996) Polonium-210 in mussels and fish from the Baltic-North Sea estuary. Journal of Environmental Radioactivity 32, 91-96. French, C. S., Skrable, K. W. and La Bone, T. R. (1986) INDOS: Internal dosimetry computer programs. Skrable Enterprises. Hedvall R. and Erlandsson B. (1996) Radioactivity concentrations in non-nuclear industries. Journal of Environmental Radioactivity 32, 19-32. IAEA (1990) The Environmental Behaviour of Radium, Technical Reports Series No. 310, Vol. 2. International Atomic Energy Agency, Vienna. ICRP (1977) Recommendations of the International Commission on Radiological Protection, Publication 26. Annals of the ICRP 1(3), 1 53. ICRP (1979) Limits for intakes of radionuclides by workers, Publication 30, Part 1. Annals of the 1CRP 2(3/4), 1-116. ICRP (1993) Age-dependent doses to members of the public from intake of radionuclides: Part 2 - Ingestion dose coefficients, Publication 67. Annals of the ICRP 23(3-4), 1 167. ICRP (1994) Human respiratory tract model for radiological protection, Publication 66. Annals of the ICRP 24(I-3), 1-482.

239

Jarvis, N. S., Birchall, A., James, A. C., Bailey, M. R. and Dorrian, M.-D. (1996) LUDEP 2.0: Personal computer program .['or calculating internal doses using the ICRP Publication 66 respiratory tract model. Technical Report NRPB-SR287, National Radiological Protection Board. Chilton. Keating G. E., McCartney M. and Davidson C. M. (1996) Investigation of the technological enhancement of natural decay series radionuclides by the manufacture of phosphates on the Cumbrian coast. Journal o] Enl'ironmental Radioactivity 32, 53-66. McDonald P., Baxter M. S. and Scott E. M. (1996) Technological enhancement of natural radionuclides in the marine environment. Journal ~?f Environmental Radioactivity 32, 67--90. Moroney. J. R. (1992) Pathway analysis concepts tbr radiological impact assessment, Australian Radiation Laboratory Technical Report ARL/TR106 (Invited paper presented at the Workshop on the Land Application ~/ Effluent Water.l?om Uranium Mines in the Alligator Rivers Region, Jabiru, N.T., September 1991). O'Brien R. S. (1997) Gamma doses from phosphogypsum plasterboard. Health Physics 72, 92-96. Papastefanou C. (1996) Radiological impact from atmospheric releases of Ra-226 from coal-fired power plants. Journal q/" Environmental Radioactivity 32, 105- 114. Ravila A. and Holm E. (1996) Assessment of the radiation field from radioactive elements in a wood-ash-treated coniferous forest in southwest Sweden. Journal g] Environmental Radioactivity 32, 135. 156. Seinfeld, J. H. (1986) Atmospheric Chemistry and Physics g] Air Pollution. John Wiley and Sons, New York. Simmonds, J. R., Lawson, G. and Mayall, A. (1995) Radiation Proteclion: Methodology jor Assessing the Radiological Consequences ~?/'Routine Releases ~[ Radionuclides to the Environment. Report EUR 15760 EN, European Commission, Luxembourg. Timmermans C. W. M. and Van der Steen J. (1996) Environmental and occupational impacts of natural radioactivity from some non-nuclear industries in The Netherlands. Journal ~?["Em'ironmental Radioactil~ity 32, 98-104, UNSCEAR (1988) Sources, Effects and Risks gf hmising Radiation, United Nations Scientific Committee on the Effects of Atomic Radiation Report to the General Assembly with Annexes. United Nations, New York. USEPA (1987) PA THRA E-EPA : A per/brmanee assessment code .[br the land disposal g/ radioactive wastes documentation and user's manual. Office of Radiation Programs, EPA 520/1-870-28, United States Environmental Protection Agency, Washington, DC. USEPA (1993) Diffuse NORM wastes waste characterization and preliminary risk assessment, Vol. 1. Draft Report RAE-92321-2, United States Environmental Protection Agency, Washington, DC.