Krypton-85 pollution and atmospheric electricity

Atmospheric Environment Vol. 28, No. 4, pp. 637-648, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352-2310/94 $6.00+0.00

Pergamon

KRYPTON-85

POLLUTION AND ATMOSPHERIC ELECTRICITY R. G. HARRISON

Department of Meteorology, University of Reading, 2 Earley Gate, Reading, RG6 2AU, U.K.

and H. M. APSIMON Air Pollution Group, Imperial College, London, SW7 2BX, U.K.

(First received 15 March 1993 and in final form 8 August 1993) Abst~et--Krypton-85 is a chemically inert radioactive gas present in the atmosphere, concentrations of which have been greatly increased by nuclear reprocessing and weapons testing since 1945. The long half-life (10.7 yr), allows the gas to mix thoroughly in the atmosphere. Ionization caused by krypton-85 increases the electrical conductivity of atmospheric air. Further increases in krypton-85 emissions seem inevitable. The increase in air conductivity due to release of krypton-85 will vary with height, and be larger over the oceans than over the land. Increases in conductivity will produce uncertain effects on atmospheric phenomena, so changes are compared in magnitude with other factors perturbing the conductivity, such as combustion aerosol burdens, volcanic eruptions and nuclear weapons testing. Conductivity changes ar~ expected to have the greatest significance for meteorological phenomena close to the source.

Key word index : Environmental radioactivity, air conductivity.

I. INTRODUCTION Krypton-85 is a long-lived radioactive isotope which is released into the atmosphere in small quantities naturally and in rather larger quantities artificially. It has steadily accumulated in the atmosphere since 1945, when nuclear weapons production and testing and civil nuclear activities began. The nuclear power industry releases krypton-85 during the reprocessing of nuclear fuels: this necessarily ruptures fuel cladding which would otherwise retain the gas. The principal concern with krypton-85 release is not a radiological one, as population doses are small (Boeck, 1976), but that atmospheric release of the gas may disturb the global electrical system (Legasov et al., 1984). The chemical inactivity of the gas and the isotope's 10.7 yr half-life will allow it to become uniformly distributed throughout the atmosphere. It is known from nuclear weapons testing (Huzita, 1966), that atmospheric radioactivity increases air's natural conductivity. Hence it must be expected that existing discharges of krypton-85 will increase the air conductivity; some meteorological consequences are considered here. The Thermal Oxide Reprocessing Plant (THORP) at Sellafield, UoK. is expected to begin operation soon, which will contribute additional quantities of krypton-85 to that already produced by the reprocessing plant at Cap La Hague and the other facilities worldwide. It is with the increased emissions expected that this paper has been written.

2. KRYPTON..g5 IN THE ATMOSPHERE

2.1. Natural production Krypton-85 is produced naturally by two processes (Styra and Butkus, 1991). Atmospheric air contains about 1 ppm by volume of non-radioactive krypton isotopes, of which about half is krypton-84. Reaction with neutrons, i.e. S4Kr(n, ~)8s Kr, produces krypton85, which accounts for about 5.2 x 1013Bq yr -~ of krypton-85 globally. The other natural source of krypton-85 is much smaller and comes from spontaneous fission of the heavy elements uranium and thorium in the Earth's crust, and subsequent atmospheric diffusion. Some physical properties of krypton-85 are listed in Appendix 1. 2.2. Artificial production Artificial production of krypton-85 has varied in intensity during the past forty years. The principal initial discharges were due to nuclear weapons manufacture and testing, but the more recent releases are from civil power programmes. Releases from weapons testing during 1961-1970 have been estimated by Styra and Butkus (1991) as about 3 x 10 la Bq. Civil nuclear power stations release various amounts of krypton-85 depending on the type of fuel employed: an estimate by Styra and Butkus (1991) for all power plant types is 6.9 x 1012 Bq per megawattyear of energy produced. The other major civil source 637

638

R.G. HARRISONand H. M. APS1MON

of krypton-85 is nuclear reprocessing plants, whose purpose is to remove uranium and plutonium from fuel elements for possible re-use. Krypton-85 is released when the spent fuel canister is opened. Plants releasing krypton-85 are operational at several sites, principally Savannah River (U.S.A.), Idaho City (U.S.A.), Sellafield (U.K.), Marcoule (France), La Hague (France), Karlsruhe (F.R.G.) and Tokai-Mura (Japan). The approximate global annual release is (Jacob et al., 1987) 2.5 × 10~7Bq. 2.3. Contemporary krypton-85 concentrations Atmospheric krypton-85 concentrations in 1940 were, according to Boeck (1983), near zero, because of the low natural production rate. Since that time, the concentration in the Northern Hemisphere has risen to about 0.8 Bq m -3, apparently entirely due to civil and military nuclear activity. There was concern that increases in future years could be very large due to predictions of considerable expansion in nuclear energy production (Boeck, 1976; Gentili et al., 1977), but the expected increase has not occurred. In part this is due to less development of nuclear energy than was forecast; however, fears of global warming together with increases in population and energy demand might cause nuclear power programmes to expand. Although pilot techniques are available for trapping krypton-85, these are not in industrial use at the French reprocessing plant at Cap La Hague: they are also not planned to be used when the T H O R P begins operation at Sellafield (Judd, 1991). Consequently atmospheric krypton-85 concentrations will increase. Measurements of krypton-85 concentration have been made infrequently, hence estimates of the global distribution have been made by atmospheric circulation models. A global source-receptor model MOGUNTIA, developed at the Max Planck Institute

by Zimmerman et al. 11987), has been used to assess the relative inputs from various reprocessing plants in Europe and the U.S.A., as experimental data in many cases are not available (Fig. 1). Recent measurements by Achasov et al. (1989) of krypton-85 concentration give an average surface value for the krypton-85 concentration c~r at ground level in the Northern Hemisphere as 0.8 Bqm -3 (0.6 Bq per kg of air at standard density). Earlier measurements by Weiss et al. (1983), found that the annual increase of concentration was about 0.02 Bq m - 3 yr 1 (measured for five years), with a seasonal variability in the concentration of 0.04 Bq m - 3. There is a similar difference in concentration between the Northern and Southern Hemispheres. It is possible to estimate the average equivalent global source rate during this period, since the time span of emission is now many times longer than the isotope's half-life. If the concentration of the isotope is c, and the discharge rate is dQ/dt, then the instantaneous change in concentration 6c is 6c = P---dO._ ~t - ;~c~t

M dt

i 1t

where M is the mass of the atmosphere (~5.3 × 1018 kg: Newell, 1971), p is the average density of air at ground level (1.293 k g m - 3 ) and 2 is the isotope's radioactive decay constant. On taking limits dc p dQ dt M dt

2c.

(2)

Assuming an equivalent constant emission rate R (= dQ/dt) and integrating, the concentration c(T) a period T after the concentration was co is Rp c(T)=~+ Co--~-~

exp(-- 2T).

Fig. 1. The global distribution of krypton-85 for January 1983, with surface air concentrations (Bqm -3) calculated by the MOGUNTIA atmospheric transport model (from Zimmermann et al., 1987).

(3)

Krypton-85 pollution and atmospheric electricity Since the initial concentration 40 years ago was almost zero, and the concentration is known now, the average source rate can be found by solving equation (3) for R. Inserting the values Co= 0 and c=0.8 Bq m -3, 2=0.06442yr -1 and T = 4 0 y r gives R=2.3 × 1017 Bq yr-1. This agrees with the estimated krypton-85 emissions from reprocessing in Section 2.2. A detailed analysis of the various contributions from international reprocessing plants, Table 1 (from Jacob et al., 1987) gives the annual discharge in 1983 from known reprocessing facilities. The then Soviet Union discharges were estimated. Emissions from Sellafield are expected to increase when THORP begins operation, to become of the order of 1.8x 10 t~ Bq yr- t (Brown, 1991; ApSimon and Harrison, 1992). The total amount of krypton-85 released in the reactor accident at Chernobyl was estimated to be 2 x I0 ta Bq (ApSimon et al., 1989).

3. ION PRODUCTIONAND AIR CONDUCTIVITY 3.1. Small ion production Atmospheric small ion concentration can be expected to increase with increasing radioactive discharges, and consequently air's natural conductivity will also rise: air conductivity is proportional to small ion concentration. (For the purposes here ion will be taken to mean small ion: molecular clusters with a mobility of approximately 1 cm- 2 V - 1s - t.) Ions are formed naturally in atmospheric air at a rate (near the surface) of about 10ion-pairs cm -3 s-1 (Chalmers, 1967). There are three major sources of these ions: airborne alpha radiation, cosmic rays and terrestrial gamma radiation. Equal numbers of positive and negative ions are usually formed. For air at room temperature and pressure, about 35 eV of work is done in forming an ion-pair (Christophoru, 1970), which comes from the kinetic energy of the radioactive particle. Some of the kinetic energy may also go into excitation of the air molecules, when an ion-pair is not formed. Near the Earth's surface, gamma radiation from the soil is the chief source of ionization, due to the decay of thorium and uranium in the Earth's crust. This accounts for about 40% of ionization near the surface. Other decay products of these nuclear decays in the

Table 1. Krypton-85 discharges from reprocessing plants (1983) Savannah River Idaho City Sellafield Mareoule Cap La Hague Karlsruhe Tokai-Mura Soviet Union Total

2.55 x 1016Bq 0 4.18 × 1016 Bq 1.15 x 1016Bq 7.22 x 1016Bq 2.96 x 1015Bq 3.33 x 1015Bq 8.88 x 1016Bq 2.46 x 1017Bq

639

crust include the isotopes of radon, which are alphaemitting radioactive gases, and indeed a very small amount of natural krypton-85. These gases escape into the air through fissures in the surface rocks, and contribute a further 40% of the ionization. The remaining ionization is caused by cosmic rays, whose intensity increases greatly with height. Ionization over the oceans is considerably lower, since there is no gamma contribution and a greatly reduced amount of airborne alpha radiation. The cosmic proportion is consequently very significant. A fundamental physical difference between positive and negative ions is their different mobility (the electrical migration speed for a particle of unit charge in a unit electric field). Natural atmospheric negative ions have a greater mobility than positive ions, due to different levels of hydration, caused by their chemical composition. Average ion mobilities It found by Mohnen (1974) in laboratory air, are

positive ions #+ = 1.14 x 10 -4 m 2 V- 1 s- 1 negative ions It- = 1.25 x 10 -4 m 2 V - 1s- 1. Ions contribute to the quantity of free or space charge present in air. If the ion number concentrations are n + and n_, then the corresponding space charge is p = e(n + -- n_ )

(4)

(assuming that each ion carries a single charge), where e is the charge on a proton. Space charge is related to the potential gradient by Poisson's equation

V'E=p/eo

(5)

hence changes in ion concentration are directly coupled to changes in the electric field. 3.2. Conductivity In atmospheric air, the natural aerosol contains particles across a spectrum of size and charge (Harrison, 1992), but since the mobility of aerosol particles is small, the major direct contribution to air conductivity comes solely from ions. The conductivity is given by Chalmers (1967) as

a=]el[it+n+ + i t - n - ] .

(6)

The effect of aerosol on ion removal is of much greater importance for the ionic conductivity than its own charge and mobility contribution. Large aerosol concentrations substantially reduce ion concentrations and therefore also the conductivity, as considered in Section 3.3. 3.3. Conductivity and ionization Production of small ions within the atmosphere is balanced by ion removal from two mechanisms: ion-ion recombination and ion-aerosol attachment. In the absence of aerosol, recombination of ions is the major removal process. When aerosol is present, ions become attached to the aerosol particles, and the

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R.G. HARRISONand H. M. APS1MON

particles become electrically charged (Fuchs, 1963). In the steady-state, the bipolar ion production rate q per unit volume and the ion loss rates are balanced, given by q - =tn2 - flnZ = 0

(7)

where e is defined as the ion-ion recombination coefficient, approximately 1.6 x 10 6 cm 3 s- ~ (e.g. Gringet et al., 1978), and fl is the attachment coefficient between an ion and aerosol particle, fl depends on the aerosol particle radius and charge (Gunn, 1954). Z is the aerosol particle number concentration per unit volume, and n is the average ion number concentration from assuming n + ~ n = n. From equation (7), the steadystate ion concentration depends on the relative magnitudes of the ion removal terms ~n 2 and flnZ. In the low aerosol concentration limit (when Z~(~/fl)n), the ion concentration is given by n = v'l(q/~)

(8)

and when ion-aerosol attachment dominates, i.e.

Z>>(~IB)n n=q/flZ.

(9)

Since the differences in sign have been neglected, the air conductivity is approximately (assuming an average ion mobility ix) a = 2nep.

( 1O)

If the ion-pair production rate q is changed to some new value q + 6q (by, for example the additional ionization caused by krypton-85), the fractional change in equation (9) is 6n

6q

--=--. n q

(11)

In the high aerosol concentration limit, the fractional change in air conductivity is therefore, from equations (10) and (11) 6a --

a

high aerosol concentrations, equations (9) and (10) give 2#eq ~r(Z)= f l Z

(14)

Hence the fractional change in conductivity with change in aerosol concentration is 6a

6Z

T =T

(15)

which is of the same order as the changes in conductivity from changes in ionization, equation (12). Thus if fractional increases in ionization are met with similar fractional increases in aerosol concentration, the overall change in conductivity will be small. However, this assumes that the aerosol concentration is initially large enough for the expansion to be valid, and also that the representation of the naturally polydisperse atmospheric aerosol by an averaged monodisperse aerosol is adequate. Figure 2 shows the relationship, between change in conductivity and change in aerosol mass concentration, calculated from equation (13). It assumes a monodisperse aerosol of radius 0.2 #m, consisting of solid particles with the same density as ammonium sulphate. Mass concentrations greater than about 10 # g m - 3 lead to an appreciable reduction in conductivity. Such concentrations are typically found over some regions of Europe, and may be compared with natural background concentrations of less than l # g m -3. Figure 3 illustrates model predictions of ammonium sulphate concentrations, implying that air conductivity is already substantially affected. 3.5. Krypton-85 and conductivity Ionization caused by beta radiation from krypton85 increases the natural value of ion production rate q by some small amount qKr. The amount of extra

6q =--.

q

(12)

CONDUCTIVITY

AND AEROSOL

CONCENTRATIC~q

3.4. Conductivity and aerosol concentration Long-term trends of increasing aerosol concentrations may compensate for increases in ionization rate, because of the reduction in conductivity by ion-aerosol attachment. Equation (7) gave the general relation between aerosol and ion concentrations in the steadystate. If equation (7) is combined with equation (10), the conductivity may be found as a function of aerosol concentration Z, equation (13). #e a = - - [ - - f l Z +x/[(flZ)2 +4~q]].

(13)

I.OOE

a4

L

c. : , ClO£

] 5

] .OOE-16 . . 0,I

.

.

. . 1.0

Particle

.

.

.

.

.

i<.o mass

2urn

In the limit when the aerosol concentration Z tends to zero, the conductivity tends to the value given by equation (10), with n given by equation (8): when Z is large the conductivity tends to zero, as expected. For

.

.

.

.

.

]0o.0

concentration

.

.

lO00.O

.

.

i0000,0

/ ~ g . m -3

r a,~i
Fig. 2. Atmospheric conductivity (~-2-1m l) as a function of aerosol mass concentration (#gm-a), calculated from equation (131.The aerosol is assumed to be monodisperse with a radius of 0.2 #m

Krypton-85 pollution and atmospheric electricity

641

tional significance of krypton-85 ion production will fall. The fractional change in ionization estimated at an arbitrary height z is, combining equations (18) and (19)

6q/qlz=7.114 x lO-4 c(z)/q(z).

(20)

Calculations of the fractional change in ionization as a function of height have been made from equation (20). Figure 4 shows the profile found. The maximum fractional change in ionization is found at about 1 km above the surface, where it is about twice as large as at ground level. At this height the effect of ionization from the soil is much reduced, and eddy diffusion of the two radon isotopes is small. Cosmic ray ionization is also small at this height. The ionization due to krypton-85 is at its most significant when ionization from other sources is small. Fig. 3. Model predicted European total particulate sulphate aerosol concentrations (/~gm-3) for 1987-1990 (from EMEP report, 1991).

4. CONDUCTIVITY CHANGES FROM THE SELLAFIELD THORP DISCHARGES

ionization caused by the beta radiation can be found from the data in the Appendices, by using the average beta energy for the isotope. The approximate number of ion pairs produced by each disintegration is the ratio of the average beta-particle energy to the ionpair production energy, if it is assumed that all the beta particle energy is used in ionization. For a krypton-85 concentration of ctr Becquerels per unit volume of air, the ionization rate qrr per unit volume is qKr =(2.49 x 105/35)cKr.

Small global changes in conductivity due to krypton-85 emission are eventually likely from the Sellafield reprocessing plant, although larger increases will clearly occur near the plant. For the mass of the atmosphere M, the added concentration 6c at a height z due to an additional global burden of krypton-85 6Q is

6c(z) = t~Q/M p(z)

(16)

Thus the fractional change in ion production over land at the surface due to krypton-g5 is, assuming a surface ionization rate qo of 10 ion-pairs c m - a s-1

6q/qo = 7.114 x 10 -4 Ctr.

4.1. Global

(17)

Over the oceans, where qo is about one-fifth of its continental value, the fractional change will be correspondingly larger. At considerable heights above the surface, the natural variation of ionization with height must be considered. An expression for the variation of ion production with altitude was given by Clement and Harrison (1990) as q(z) = 106. [4.5e-°.°°15~ + 3.5e-O.OOStz+ 2e -o.ooo4sz]

(21)

for an air density p(z) at the same height. Thus the percentage change in global concentration from its present value may be estimated, due to the operation of T H O R P alone. Equation (21) shows that, during the 20 yr life of the plant, after which a net discharge (allowing for decay) of 2 x 10 is Bq may

VERTICAL

I~ISATION

Krypton-85

ionisation

PERTURBATION over

land

0,00151

0.001o1

(18) where the altitude z is in metres. For the variation in krypton-related ionization, it is usually assumed (Boeck, 1976) that the concentration of krypton-85 falls with the density of air, and therefore with increasing height. From the altimeter equation, the concentration of krypton-85 at a height z is

c(z) = c(0) e-:/as61

(19)

where c(0) is the surface concentration. As the quantity of cosmic ray ionization in the upper atmosphere increases with height, so the frac-

0.00051

0.0000 i000

2000 3000 vertical height / metres

4000

5000

Fig. 4. Fractional change in ion production rate dq/q as a function of height, according to equation (20). A surface concentration of 0.8 Bq m- 3 has been assumed.

642

R.G. HARRISONand H. M. APSIMON

have occurred, the present surface krypton-85 concentration would increase by 0.48 Bqm -3, assuming a surface air density of 1.29 kg m - 3. Consequently, the krypton-85-related increase in average global surface conductivity, calculated from equations (13) and (18), is about 0.03% over land, for the 20-yr discharge, and about five-times greater over the sea with lower natural ionization.

FRACTIONAL CHANGE IN C O N D U C T M T Y (over land) 10000

T

.

.

.

.

.

.

.

.

,000+

.

.

.

.

.

.

.

.

.

.

:

100

'

,j

o

/

4.2. Medium scale

100~ ,I

For national and European-scale effects, the krypton-85 dispersal may be evaluated using a long-range transport model. The MESOS model simulates the release of radioactivity from a source into atmospheric weather systems (ApSimon and Goddard, 1983), using a meteorological database to include the effects of atmospheric processes on pollutant transport. Results from the model (for release at a site close to Sellafield) have been used to estimate the mediumscale krypton-85 releases from THORP. Figure 5 shows the corresponding percentage changes in surface conductivity expected, from the krypton-85 concentration patterns predicted by the model.

[

,! /

0.I0]

/

i

° °~o-. . . . . . . .

looo

distance downwind

--

height

= Om

-

height

=

lore

of

10000

plant / m height

Fig. 6. Fractional change in conductivity downwind of a reprocessingplant assuming a release rate of 2.5× 101°Bqs -1, at 0, 10 and 100m above the land surface, from a Gaussian plume model and equation (20).

4.3. Local scale Changes in ionization rate and conductivity will be at their greatest near to a reprocessing plant. These changes have been simulated using a Gaussian plume model (Clarke, 1979) to calculate krypton-85 concentrations in the plume downwind of the plant, and then the fractional changes calculated. Figure 6 shows the changes expected, for a release rate of 2.5 x 101° Bqs -1 A Pasquill stability category D has been assumed, together with a surface roughness length of 0.1m, 10 m '"

g

wind speed of 5 m s- 1 and a release height of 92.5 m. Fractional changes in conductivity at heights of 0, 10 and 100m above the surface are of the order of 10-times at about 5 km downwind.

5. CONSEQUENCESFOR THE ATMOSPHERIC ELECTRICALSYSTEM

5.1. Summary of atmospheric electrical system Electric fields are produced naturally by charge separation in thunderstorms, between the Earth and the upper atmosphere. Background ionization gives atmospheric air a slight conductivity, thus a sniall current of atmospheric ions flows continuously between these regions (Chalmers, 1967). Figure 7a shows a simplified atmospheric electrical system. The conduction current lc consists of small ions which move under the Earth's electric field. In general, positive charge is carried to the upper atmosphere from the top of clouds, and negative charge is brought to Earth by the combined effects of lightning and point discharge currents. At the surface, the fair weather charge flux is related to the potential gradient E and air conductivity tr by Ohm's Law, for currents less than saturation. J = aE.

Percentage increases in conductivity over land and sea Fig. 5. Percentage increase in average annual surface conductivity due to expected krypton-85 discharges from the SellafieldThermal Oxide ReprocessingPlant, assuming an average daily emission rate of 2.5 x 101° Bq s-1. Changes over the sea are about five-timesgreater due to the lower natural ionization.

(22)

For fair weather conditions, E is about + l o o V m - 1, j is ~ 2 pA m - 2 at the surface: other quantities of the fair weather electrical system are listed in Appendix 2. Figure 7b shows an equivalent electrical circuit for the atmospheric system. Essentially the two conducting regions in the atmosphere form a spherical capaci-

Krypton-85 pollution and atmospheric electricity (a) SCHEMATIC THUNDERSYOP~S S~ARATE

643

Hence the total resistance is 1 ~ o dz Rx=T~-~ aiz)

(25)

where R is the Earth's radius. Taking the upper limit of the integral as 50km gives Rx=235fi. From RC = z = 667 s, the magnitude of the global capacitor, Cx is about 2.8 Farads. 5.2. Conductivity changes and electrification

(b) EQUIVALENTCIRCUIT IONOSPHERE / t"

I

Ic

THUNDERSTORMCURRENT

1800A

EARTH

Fig. 7. (a) Spherical capacitor conceptual model of the Earth's atmospheric electrical system, R is the Earth's radius and h is the effective electrical atmospheric height used in equation (25). (b) Equivalent electrical circuit for the atmospheric electrical system. The spherical capacitor CT is in parallel with the leakage resistance of the air, RT. Thunderstorms generate a total current IT.

tor with a total capacitance Cx, in parallel with the leakage resistance of the air, RT. Production of charge by thunderstorms is represented by a battery of emf equal to the potential difference between the Earth and the ionosphere, passing a current Ix (a sum of all the charge carrying processes in the atmosphere). The RC time constant z for a capacitor of any geometry with a weakly conducting dielectric is ~=e/a

(23)

where e is the permittivity of the dielectric and a its conductivity. Taking a = 1.3 x 10-14 f l - 1 m - 1, and e = to gives z ~ 667 s; in the absence of charge generation the Earth's electrical system would discharge in about an hour. Calculation of the value of the "global resistor" Rx is found by integration of the conductivity as a function of height tr(z), which is given by Volland (1984) as

a(z)=l/[ple-'~Z+p2e-~2Z+pae-'3~ ] where the constants are pl =4.69 x 1013 r i m p2=2.22x1013f~m pa=5.90x1013flm

cq =4.527 x 10-3m -1 ~ 2 = 3 . 5 7 x 1 0 - 4 m -1 ~ t 3 = l . 2 1 x 1 0 - 4 m -1.

(24)

Increasing the global conductivity by krypton-85 pollution will clearly increase a, but all the quantities above (E, tr, IT, Rx) are coupled. Global electrical models (Makino and Ogawa, 1985; Hays and Roble, 1979) have shown that a change in one quantity is accompanied by changes in all the other quantities: the spherical capacitor model is therefore necessarily simplistic. Globally, any increase in the conduction current lc can be cancelled by (i) long-term increases in aerosol concentration, or (ii) by an increase in the mean global charge transfer. However, if long-term changes in conductivity were also to influence the behaviour of thunderstorms directly (such as in reducing the rate of lightning flashes), then the effective positive feedback might jeopardise the global electrical balance. This might be expected if the charge separation necessary in thunderstorms was, for example, inhibited by the intervening air space becoming more conductive. However, it might equally be argued that existing charge separation processes would simply separate the larger quantities of charge, increasing the electrical intensity of thunderstorms. An increase in lightning activity may have been observed in Sweden following the Chernobyl release of radioactivity and surface deposition (Israelsson et al., 1986): this is at present hard to explain (Saunders, 1993). Local effects close to the reprocessing plants are likely to be more significant. The effects to be considered include charge separation at the Earth's surface, the electrode effect, and turbulent motion of charge. Consequently the following sections consider the local and global effects separately. 5.3. Global effects 5.3.1. Thunderstorms. The thunderstorms which sustain the global electrical system occur continuously, mainly within the intertropicai convergence zone: half of all lightning activity is within I0 ° of the equator (Orville and Spencer, 1979). These storms are of considerable vertical extent. Estimates of the mean number of lightning flashes per storm and the average quantity of charge transferred vary, depending on their location. Tropical storms are likely to have greater flash rates than mid-latitude storms and Williams (1985) has shown that the number of flashes is proportional to the depth of a storm. For electrical balance, the average rate of thunderstorm charge exchange must equal the fair weather

644

R.G. HARRISONand H. M. APSIMON

leakage current between the upper atmosphere and Earth, which is of the order of 2000 A (Chalmers, 1967). The global current 17 can be written as IT = N [ fQf + Ip]

(26)

where N is the number of storms continuously active globally each contributing a point discharge current Ip, with an average charge Qf exchanged per cloudto-ground flash at a flash frequency f. This direct proportionality may be used to show what a 0.1% change in global air conductivity is in atmospheric electrical terms. If there are about 2000 thunderstorms continuously active globally (Schonland, 1953), this change corresponds to the electrical output of two continually active thunderstorms. An understanding of thunderstorm electrification is essential to understanding the complete effects of krypton-85 on the global electrical system, but some fundamental aspects are controversial (e.g. |llingworth and Latham, 1975). Aspects of microscale charge exchange are poorly understood, giving different theories for the macroscopic behaviour of thunderclouds: the convective mechanism, the inductive mechanism, and the ice crystal mechanism. Recent reviews of thunderstorm electrification are given by Williams 0988), and Saunders (1988, 1993). Changes in conductivity are not likely to change thunderstorm electrification processes on the ice crystal mechanism, since it is independent of existing electrical parameters (Jayaratne et al., 1983). Furthermore, the local space charge is not important: electrification depends solely on solid state properties. Both the alternative convective and inductive mechanisms could be affected, however. Since the convective hypothesis requires ion separation to occur, changes in ion concentration could change the local ion balance in the region from which charge convection was occurring. Consequently the final cloud charge distribution would change. The inductive electrification method depends on the initial fair weather electrification of soft hail: any change in the fair weather field would therefore also change the amount of subsequent cloud charge exchange. On the assumption of the inductive mechanism, Spangler and Rosenkilde (1979) found that an ionization rate of 1012 ion-pairs m - 3 s-1 would completely inhibit cloud electrification, although there is some freedom in choosing the parameters. There is of course no suggestion here that a thunderstorm as a convective system would not occur: the vast energy supply to a storm cell by condensation would not be affected. However, it should be observed that in the absence of lightning, the local energy distribution of a storm would change. For a comparison of the convective and electrical energies, see Vonnegut (1960). 5.3.2. Volcanoes. Eruption of volcanoes is a natural event which usually discharges vast quantities of volcanic ash and dust into the atmosphere, reducing air conductivity by the usual process of ion-aerosol

attachment. Since large eruptions have occurred frequently during the period during which the atmospheric electrical parameters have been studied, it is clear that volcanic eruptions produce global changes in conductivity which can be absorbed by the electrical system on a long timescale, and it is therefore useful to estimate their magnitude. The eruption of Mount St Helens, U.S.A. on 18 May 1980 was closely studied. Vertical profiles of the aerosol concentration had been taken before the eruption by Kondo et al. (1982), from both balloon ascents and laser backscatter measurements. These vertical soundings recorded the mean value of ion-aerosol attachment rate ( f l Z ) . Conductivity profiles found showed a decrease around the tropopause, and a further, sharp decrease between 14 and 17 km. Table 2 gives values of ( f l Z ) found at Garmisch-Partenkirchen before and after the eruption at about 12 km height, where most of the volcanic dust plume was concentrated. Hence the fractional change in ion depletion by aerosol attachment from the eruption is about a factor of two, and a corresponding change from equation (15) would be expected in conductivity measurements at the same height. 5.3.3. Long-term pollution. Long-term reduction in conductivity in the Northern Hemisphere would be expected because of increased average aerosol concentrations from anthropogenic pollution. A study of historical conductivity measurements made originally by Wait (1946) showed a downwards trend in conductivity in the North Atlantic. However, subsequent measurements are far less conclusive, and Van der Hage (1991) has shown that during this century there has been little overall change in atmospheric conductivity. Figure 8 shows the measurements of positive ion Table 2. Mean ion-aerosol attachment rates before and after eruption Mean attachment rate Before eruption After eruption

(flZ) s- 1 2.5 x 10 3 4.5 x 10-3

? E o

800

I

600

400

""

200

"7, o o.

0

t

i

i

I

1920

1940

1960

1980

Year

Fig. 8. Variation in positive ion concentration (directly proportional to conductivity), measured during the 20th Century (from Van der Hage, 1991).

Krypton-85 pollution and atmospheric electricity

645

sphere, which can directly reduce the air conductivity, by ion removal. A direct consequence is the production of increased potential gradients nearby (Jennings and Jones, 1976), up to - 6 kVm-1. Manohar et al. (1989) reported measurements under plumes found downwind of the Sarni thermal power plant in India. They observed the point discharge current (conduction current found at a corona point), potential gradient and droplet charge. Large changes in the potential gradient were observed, up to an order of magnitude greater than the fair weather value, and of opposite sign. The accompanying increases in the point discharge current were also considerable. Concerns expressed in this work included the risk of ignition of particles present at otherwise nonexplosive concentrations, and the premature failure of lightning arresters. 5.4.3. Weapons testing. Nuclear weapons testing, which was at its most intense in the fifties and sixties, caused considerable concern about long term changes in the atmospheric electrical parameters. Pierce (1957) suggested that there could be a connection between observed falls in the annual average potential gradients at Eskdalemuir and increased ionization rates during the same period. However, potential gradients at Lerwick, during the same period, were fairly constant. In a later publication, Pierce (1959) showed that the fall in conductivity at Eskdalemuir had continued and explained the differences between the data taken at Eskdalemuir and Lerwick in terms of their relative positions. Pierce (1972) subsequently showed a correlation between the quantity of nuclear debris from weapons testing, and the value of the potential gradient. The approximate two-year residence time of debris in the stratosphere correlated with an increase in the poten5.4. Local effects tial gradient (measured at six sites worldwide) with the test-ban treaty of 1963. This paper also showed that 5.4.1. Natural variations. In addition to variations in conductivity due to pollution, there are natural the difference between the Eskdalemuir and Lerwick variations in all the average atmospheric electrical measurements was due to different quantities of aeroparameters: standard fair weather conditions are sol present: at Lerwick (low aerosol concentration) the examples of ideal values found only in stable condi- field was proportional to x/~, and at Eskdalemuir tions. Considerable departures from these values are (relatively polluted), proportional to q. However, found in disturbed weather (Clement and Harrison, weapon testing released radioactive isotopes into the 1990), which usually lead to a greatly increased poten- stratosphere; hence, it is hard to draw conclusions for tropospheric effects, since exchange rates between the tial gradient. There are regular variations in the natural fair two regions are small. Huzita (1966), vividly demonstrated the increase weather parameters. All the Ohm's Law variables (potential gradient, conductivity and air-Earth cur- in conductivity after a nuclear explosion. Following rent) show a diurnal variation, influenced by time of the detonation of a 50 Megaton device at Novayasunrise (Law, 1963) and the onset of local convection. Zyemlya, U.S.S.R., on 30 October 1961, a large inThe extent of daily variations in the fair-weather para- crease in conductivity was observed at Osaka, Japan meters gives some estimate of changes which may be (600km distant), between 8 and 11 November 1961. regarded as "normal", and hence a comparison with In this case, the airborne activity was low, but the conductivity perturbations due to krypton-85 pollu- surface contamination due to rain-out and dry fallout was substantial. This caused the background ioniztion. 5.4.2. Industrial effects. Emission of plumes from ation to rise to 42 ion-pairs c m - 3 s - 1 at 1 m above power stations and industrial plants contribute con- the ground, and the conductivity approximately siderable local quantities of aerosol to the atmo- doubled. This change is consistent with the assump-

concentrations given by this latter work, to illustrate the variability in the data. There is no clear trend in the data, and no increase from krypton-85 production is apparent. 5.3.4. Paleo-atmospheres. Despite the fact that during geological time the composition of the atmosphere has clearly altered, it is helpful to observe that there is fossil evidence (Collinson and Thompson, 1982) for lightning in earlier geological eras, when terrestrial and atmospheric ionization from radioactivity would have been much greater than at present. The oldest fulgurites (fossilized lightning strikes) found are in rock strata in Arran from 250 million years ago (Harland and Hacker, 1966), when surface radioactivity from 23su alone (half-life 4.51 x 109 yr) would have been about 4% greater than today. Thus it could at least be concluded that the small increases in air conductivity from krypton-85 (which would be globally rather less than 4%), would not be sufficient to short out thunderstorm electrification. 5.3.5. Global electrical models. The global electrical model of Makino and Ogawa (1985) was used to consider the effects of (i) a global increase in condensation nuclei, and (ii) a decrease in cosmic ray production. Neither situation is directly relevant to the case of increasing ionization, but both calculations are of some interest here. Makino and Ogawa found that a 20% increase in global condensation nuclei would lead to a global increase in conduction current of 1.3%, with increase in global resistance of 6.6%. A 20% decrease in cosmic rays, however, also led to increases in all the variables: global resistance + 13.9% and global current +0.8%. Hence it seems that the resistance is a more sensitive quantity to .perturbation than the current.

646

R.G. HARRISONand H. M. APSIMON

tion made in equation (8), that ion concentration is proportional to x/q. Harris' measurements (Harris, 1955) are particularly interesting as they show the exponential fall in conductivity with rapid radioactive decay of the emitted isotope (something which would not be expected with releases of krypton-85). These measurements were made at Tucson, Arizona near the Nevada test site. A thunderstorm occurred on 2 June 1952~ which brought down radioactive debris from a recent nuclear explosion. The increase in ionization near the ground was found to increase the conductivity and reduce the potential gradient. Figure 9 shows the relative variation in the positive and negative ion conductivity and the potential gradient. The conductivity increased 10-times, and the potential gradient fell by a factor of 6. Surface deposition was significant, with 1.7 x 103 ion-pairs cm 3 s 1 due to surface beta emission found in the lowest 50 cm. The contribution from gamma radiation in the first few metres above the surface was measured as 11.5 ion-pairs cm-3 s-1. Harris also reported a later nuclear test at Albany on 27 April 1953, which produced substantially more debris, and caused the potential gradient to drop an order of magnitude from its fair-weather value, with no observed effects on the weather• 5.4.4. Electrode effect. The electrode eff~,ct occurs because of negative charge on the Earth's surface. Consequently negative ions are repelled, and a surface layer of positive ions will form in still weather close to the surface. Measurements by, for example, Ruhnke

(t962) and Mfihleisen (1961) support the existence of the electrode effect for low wind speeds and no surface radioactivity (one experiment was over an ice sheet, the other over the sea), as does the more comprehensive study by Law (1963). For the case of increased ionization due to radioactivity, in the absence of turbulence, Hoppel (1967) produced a model to predict ion and potential gradient profiles. This work found that the thickness of the layer of positive ions was greatly reduced by the ionization, and that the positive ion concentration was severely depleted in the first few metres. This has apparently been observed by Crozier (1965). 5.4.5. Scavengin# and deposition. Scavenging of aerosol by raindrops has been shown to increase with aerosol charge (e.g. Byrne, 1991), and found numerically to depend strongly on electric fields and aerosol charge by Wang et al. (1978). Harvey and Edwards (1991) also calculated the electric collection kernel, and showed that the cloud lifetime of charged aerosol particles would be reduced by strong external fields. Such an enhancement in scavenging would be expected in thunderclouds, but Rosenkilde and Serduke (1983) predict it to be reduced by self-discharge of radioactive aerosols: however, self-neutralization does not always occur (Clement and Harrison, 1992). Differential ion uptake from krypton-85 into convective systems leading to large aerosol charge could therefore also affect aerosol scavenging rates. Laboratory aerosol experiments by McMurray and Rader (1985) showed that wall deposition of aerosol is dominated by electrical effects for particles of radii

i

Potential gradient

3

2 I I 1St

I 2nd

I 3rd

A 4th

I 5th

6th

June 1952 Fig. 9. Changes in the surface electrical parameters at Tucson, Arizona, following a nuclear test explosion at Nevada in June, 1952. A thunderstorm late on 2 June brought radioactivity to the surface, increasing the conductivity six-fold (from Harris, 1955).

Krypton-85 pollution and atmospheric electricity between 0.05 and 1 #m. If the electrode effect separated the charges from increased surface ionization near to a reprocessing plant, it is clearly possible that aerosol particles could be locally inhomogeneously charged, leading to changes in surface deposition rates (Willet, 1985). 6. CONCLUSION There is no doubt that emission of radioactive materials from any source will increase the conductivity of atmospheric air. The conductivity increase from the background value is proportional to the air concentration of the emitted isotope. Emissions of krypton-85 from nuclear reprocessing plants are contributing to a small but steady increase in global air conductivity. This must be expected to continue, and operation of the new reprocessing plant at Sellafield will contribute further to the rise in krypton-85 in the atmosphere over the U.K., in addition to existing discharges on the north coast of France and elsewhere. Krypton-85 will become uniformly distributed throughout the atmosphere, due to its long radioactive half-life and chemical inertness. Fractional increases in conductivity are expected to vary with height, and will be larger over the oceans due to greater natural g a m m a and radon ionization in the latter case. Over the oceans, at the surface, the proportional conductivity increase would be typically fivetimes greater than over land. At a height of 1-2 km, where other sources of ionization are at a minimum, the conductivity increase due to krypton-85 will be at its greatest. Such increases are small, however, when compared to the reduction in conductivity attributable to increased sulphate aerosol burdens in the Northern Hemisphere over recent decades. It is not possible to predict global meteorological effects, as present numerical models do not consider the mechanism of thunderstorm electrification processes explicitly. However, an attempt has been made to consider as many possible areas of significance for further research. Small-scale effects are more likely, as it is close to reprocessing plants that the changes will be greatest. Acknowledoements--We are grateful to Mr K. Spiers of the Meteorology Department for his help with the diagrams, and for stimulating discussions with Dr C. F. Clement of Intera Information Technologies. Some of the work described here was undertaken as part of an assessment for British Nuclear Fuels Ltd. REFERENCES

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the presence of high rates of ion generation, d. geophys. Res. 84, 3184--3190. Styra B. and Butkus D. (1991) Geophysical problems of Krypton-85 in the Atmosphere. Hemisphere, Washington D.C. Van der Hage J. C. (1991) Background aerosol and electric conductivity of marine atlantic air. Atmospheric Environment 25A, 597q~0. Volland H. (1984) Atmospheric Electrodynamics. Springer, Berlin. Vonnegut B. (1960) Electrical theory of tornadoes. J. geophys. Res. 65, I. Wait G. R. (1946) Some experiments relating to the electrical conductivity of the lower atmosphere. J. Wash. Acad. 36, 312-343. Wang P. K., Grover S. N. and Pruppacher H. R. (1978) On the effect of electric charges on the scavenging of aerosol particles by clouds and small raindrops. J. atmos. Sci. 35, 1735-1743. Weiss W., Sittkus A., Stockburger H., Sartorius H. and Munnich K. O. (1983) Large scale atmospheric mixing derived from meridonal profiles of Krypton-85. J. geophys. Res. 88, 8574-8578. Willet J. (1985) Atmospheric-electrical implications of 222Rn daughter deposition on vegetated ground. J. geophys. Res. 90, 5901-5908. Williams E. R. (1985) Large scale charge separation in thunderclouds. J. geophys. Res. 911, 6059-6070. Williams E. R. (1988) The electrification of thunderstorms. In Proceedings 8th international conference on atmospheric electricity, Uppsala University. Zimmermann P. H., Feichter J., Rath H. K., Crutzen P. J. and Weiss W. (1987) A global three-dimensional sourcereceptor model investigation using 85-Kr. In EUROSAP International Workshop on Methodologies of Air Pollution Emission Inventories, Paris, June 1987. APPENDIX I Physical data for krypton-85 Atomic number Relative molecular mass Radioactive half-life Radioactive decay constant Maximum beta-particle energy, Emax Average beta-particle energy, Ear Beta-particle range in air Density (0°C, 105 Pa) Solubility (water, 15'C)

36 0.08380 kg mol 10.76 yr 2.0427× 1 0 - 9 s - I

=6.442 x 10-2 yr -t 0.672 MeV 0.249 MeV 1.2 m 3.733 k g m 3 6.2 x 1 0 - 6 %

APPENDIX 2 Fair weather average atmospheric electrical parameters Potential gradient E Charge flux (down) J Air conductivity (sea level) a Total atmospheric resistance RT Surface charge density Global air-Earth current Earth's surface charge Potential of electrosphere Total charge in atmosphere Space charge at sea level

100 Vm -1 (sea level) _ 2 x 10-12Am-2 2 X 10-14D~lm-I 230 D 10-9C m--2 1800 A --5 x 105C 2.4 x 105 V 700 kC 4.0x 10-12Cm -3

Icompiled from Chalmers. 1967: Volland, 1984; Styrus and Butkus, 1991).