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Estimation of maximum credible atmospheric radioactivity concentrations and dose rates from nuclear tests
OCQ4--6981~/7910201-0327$02.00x)
Amosphpric Enuironmenr Vol. 13. pp. 327-334. 0 Pergamon Press Ltd. 1979 Printed NI Great IMaIn.
ESTIMATION OF MAXIMUM CREDIBLE ATMOSPHERIC RADIOACTIVITY CONCENTRATIONS AND DOSE RATES FROM NUCLEAR TESTS KSSTA
TELEC~ADAS
Air Resources Laboratories, National Oceanic and Atmospheric Administration, 8060 13th Street, Silver Spring, MD 20910, U.S.A. (First received 15 May 1978 and in.~na~~r~
29 August 1978)
Abstract - A simple technique is presented for estimating maximum credible gross beta air concentrations from nuclear detonations in the atmosphere, based on aircraft sampling of radioactivity following each Chinese nuclear test from 1964 to 1976. The calculated concentration is a function of the total yield and fission yield, initial vertical radioactivity distribution, time after detonation, and rate of horizontal spread of the debris with time. Calculated maximum credible concentrations are compared with the highest concentrations measured during aircraft sampling. The technique provides a reasonable estimate of maximum air concentrations from t to 10 after a detonation. An estimate of the whole-body external gamma dose rate corresponding to the maximum credible gross beta concentration is also given.
1. INTRODUCTION
The injection of radioactive debris into the atmosphere by a nuclear explosion could result in exposure of aircraft to relatively high radioactivity concentrations. Several U.S. government agencies are engaged in a cooperative effort to monitor airborne nuclear debris, project its future path and concentration, and notify responsible authorities and the public of any potential hazards. Air concentrations depend on the amount and initial distribution of radioactivity, the elapsed time since the explosion and the rate of dispersion of the debris as it travels. This paper presents a simple technique for estimating maximum credible concentrations as a function of the time after detonation. The only input information required is an estimate of the energy yield of the explosion. Calculated concentrations are compared to the highest measured concentrations after aircraft sampling. Results are applicable to air bursts near the ground. For surface detonations, local fallout would reduce the airborne concentrations.
x r
Conversion factors used in Equation (1) are: (a) 1 kt is equivalent to 1.45 x 10z3 fissions, and (b) 0.45 picocurie (pCi) is equivalent to 1 disintegration per minute (disjmin). Fission yield
For all lower yield detonations ( I 500kt) the fission yield is assumed to be equivalent to the total yield. For detonations greater than 500 kt it is assumed that the fission yield is one-half the total yield. Gross beta rad~~ctivity The total quantity of gross beta, per fission, and its decay with time, are shown in Fig. 1 based on Harley et al. (1965) and Healy and Baker (1968). This curve very closeiy approximates a t - I” decay.
2. A SIMPLE METHOD FOR RADIOACTIVITY
CALCULATING CONCENTRATIONS
Radioactivity concentrations resulting from atmospheric nuclear explosions can be estimated from
L =
7zr2
f
where L
is the average gross beta concentration,
curies per ambient cubic meter ofair, pCi m-3), at altitude (h) and time (t) after the detonation is the fission yield of the device (kilotons, kt) is the gross beta radioactivity per fission at time (t), (dis/min/fission) is the fraction of radioactivity per meter at altitude (h) is the radius (m) of the debris cloud at time (I).
(pico-
Vertical radioactivity distribution
Telegadas (1974; 1976; 1977) ~timat~ the vertical distributions of radioactivity for each of the first five high yield Chinese tests and presented a mean distribution (Fig. 2 inset). Although the vertical distributions were based on radioactivity m~sur~ents made several weeks or months after each event, the mean distribution, which is a composite of the five events, may be used as a reasonable estimate of the 327
Kosr .A ~rbLLGAI)AS 1
I
v
,I,,,,
I
500
I
KIloton
= 1.45
I
,
--I--_
I,,,,
x 10z3fisskons
1
-
-1 1
1 1
4 Heoly 8. Evoker (t-10
days)
1 i
P
ey et
01. (IO-lOOdays)
Days
Fig. I. Total beta distintegrations
per min per 10h fissions remaining
as a function
of time
-2O-
-25-
KIlOtOnS
Tolol
Fig. 2. Mean cloud
Yield
top and base as a function of total yield. Inset diagram vertical activity distribution within the cloud.
presents
the assumed
initial
Estimation of atmospheric radioactivity concentrations and dose rates from nuclear tests Table 1. Concentrations
I
329
of radioactive debris from Chinese atmospheric nuclear tests, 1964-1976 Approximote Yie,d
c 1966’
Oct.
‘28
20(”
Dec. 28
i 1967
Jun.
17
Dec.
24
8.8
20
300(‘) -_-
300
3OOOt’)
l6OOt*’
’
4.3
A:‘: L_-
-.-;:;;
3000(
29
1970 1971
1972
1974
Oct.
14
Nov.
18
Jon.
7
Mar.
‘“,:
1800”’
/ 30504
20’3) < 20 14’
C
1976
12~,2
20
6.4
0.06
L.2
20
7.6
0.08
2.3
13.7
2.9
20 - 200
2@)-~()0$)20O-tOOO
20.0
I.0
I I .5
CL07 0.10
16.5 Jon. 23
/
3ooo~z~’
18 20-200’“’ 17
I .4
._.,
-- 1400(” Jun.
0.8 370
I .?
3OOO”‘i
/
,43;05
I30
’)
19691 Sep.
6.9 10.7
0.55
z!O(‘)
1968 / Dec. 28
0.48
9.8
<20(S) -_-.-.
Sep.26
2&200(“)
Nov. 17
4000
(6) , /
2000
j I’
2.7
< 20
10.4
20-200 (8)
10.7 20.4
_
8.3
11000 0.2 10.6 0.003
j
1.0
2.9
/
I.6
4.0
5 x lO-6
65
7600
(1) Hardy, 1970; (2) Telegadas, 1974 ; (3) USAEC News Release, O-21 3 ; (4) LJSAEC News Release, P-6; (5) USAEC News Release, P-77; (6) Information received from Office of Public Affairs, USERDA; (7) Telegadas, 1976; (8) Assumed for computation purposes.
distribution at earlier times assuming the effect of vertical diffusion to be small.
mum credible
concentrations,
we will minimize
cr,(m) = 0.2%(s), From a compilation of many lateral dispersion measurements in the troposphere (Heffter, 1965) an expression for the growth, with time, of the horizontal standard deviation, Q”, crH(m) = O.%(s),
(2)
has been suggested (Heffter et al., 1975) as a good approximation for the growth of a cloud during several days travel time in the troposphere. Very few measurements have been made of lateral dispersion in the lower stratosphere. Virtually no measurements after several days travel time have been reported. Equation (1) was used to caiculate u,, from the stratospheric concentration measurements for the six high yield tests given in Table 1. Calculated values of (TVranged from 0.2% to l.lt for travel times from 1.5 to 17 days. This agrees well with the range of uH given by Bauer (1974) for these travel times in the lower stratosphere. Since we are concerned with estimating the maxiA.E.t3i*--*
the
cloud growth using (3)
for travel times of several days in the lower stratosphere. The radius of the growing cloud is then approximated by r =
2a,,
(4)
and for simplicity, it is assumed that the concentration is horizontally uniform, since we are interested in the average concentration along a flight path through the cloud. 3. AIR CONCENTRATION DATA The highest radioactivity concentration measurements from aircraft sampling following each of the Chinese tests from 1964 to 1976 are given in Table 1. The approximate total yield and fission yield of each detonation and the data source are given. For detonations having a total yield of 500 kt or less it was assumed that the fission yield was equal to the total yield.
:.io
KOSTA
1
< 20
TELEfiAflAS
KT * 0.003
-, -Mrpcrtonr-
Kilotons Total
Yttld
Fig. 3. Observed concentrations @G/SCM) adjusted to one day after detonation. Solid curves denote calculated maximum credible concentration at one day assuming tropospheric dispersion (Equation 2) for yields 500 kt or less and stratospheric dispersion (Equation 3) for yields greater than 500 kt. (* See text for
explanation.)
The altitude of sampling is shown and the gross beta radioactivity concentrations at sampling time are given in units ofpic~u~es per standard cubic meter of air (pCi/SCM). The concentration in the last column has been adjusted to its expected value one day (D + 1) after the detonation. This adjustment takes into account the decay of mixed fission products (Fig. f f and also includes an atmospheric dilution factor which will be discussed later. 4.
MAXIMUM CREDIBLE AIR CONCENTRATIONS
The prediction of maximum credible air concentrations is based on Equation (1) using the mean vertical radioactivity distribution and an estimate of the altitude of the radioactivity maximum based on the total yield of the explosion. The cloud thickness as a function of yield is shown in
Fig. 2 which represents a composite of previous cloud height determinations (e.g. Quenneville and Nagler, 1959; Ferber, 1965; and Peterson, 1970) and is intended to represent the vertical extent of nuclear clouds in temperate latitudes. Differing atmospheric conditions produce variability in the height of nuclear clouds of the same yield and height of burst. The variability about the cloud top curve in Fig. 2 is about + 2 km as estimated by Quenneville and Nagler (1959) and Ferber (1965). It is assumed that the mean vertical distribution as given by Telegadas (1974) is valid for all air bursts. This distribution, shown in the inset diagram of Fig. 2, is divided into seven layers of equal thickness and is similar to the distribution proposed by Ferber (1%5) and Peterson (1970). The altitude of the maximum concentration is assumed to be midway between the cloud top and base. This altitude is assumed to be the
Estimation of atmospheric radioactivity concentrations and dose rates from nuclear tests midpoint of a layer containing 38% of the total activity. Since we are concerned with estimating the maximum credible air concentration, we will assume that the vertical diffusion in both the troposphere and stratosphere to be negligible. For horizontal dispersion we use Equation (2) for tropospheric injections (500 kt or less) and Equation (3) for stratospheric injections (> 500 kt). With the above assumptions, a maximum credible concentration at one day after detonation has been estimated from Equation (1) for nuclear yields from 1 kt to 10 Mt as shown in Fig. 3. Since the horizontal standard deviation u,,, is assumed to increase linearly with time, the volume of a nuclear cloud layer (assuming no vertical dispersion) increases as t’. The gross beta radioactivity decreases according to t -I” decay. By combining these two factors, dispersion and decay, one arrives at a reduction factor, shown in Fig. 4, which gives the decrease in concentration with time. The reduction factors in Fig. 4 were applied to the observed concentration data in Table 1 to adjust all observed concentrations to D+ 1. The highest D+ 1 concentration for each detonation is plotted in Fig. 3. Only one point, that for the June 27, 1973 nuclear test, lies above the maximum credible curve. This concentration measurement (1 x lo6 pCi/SCM) at 20 km was made 17 days after detonation and then adjusted
10-4
’
’
’
’
2
’
’
3
’
’
4
’
331
to a concentration of 11000 x lo6 pCi/SCM at D + 1. The adjusted concentration is about a factor of 2 higher than the estimated maximum credible concentration. The highest adjusted concentrations for the nuclear tests greater than 2 Mt, other than the June 27, 1973 test, were for the 2 events sampled at the highest altitude, about 20 km. These concentrations were within a factor of 2 of the predicted maximum credible concentration. For the other 3 high yield events, which were sampled at lower altitudes, the adjusted concentrations were a factor of about 20-400 below the predicted maximum credible concentration. The marked over-predictions may be due to the actual atmospheric dispersion being much greater than that assumed for maximum credible concentrations. However, aircraft sampling is limited and may not always encounter the maximum concentration in a debris cloud. There are four events with an estimated yield of20 kt that were sampled at altitudes ranging from 6.4 to 10.7 km (see Table 1) from one to two and a half days after detonation. The adjusted D + 1 concentrations were between 0.8 x lo6 and 6.9 x lo6 pCi/SCM. This illustrates the variability in the maximum concentration encountered for any given yield. The point denoted by an asterisk in Fig. 3 represents radioactive debris encountered over the mid-western
’
5 Doyr
1
’
6
1
’
7
*
1
8
1
I
9
I
IO
Fig. 4. Reduction factor, combining horizontal dispersion and decay, adjusted to D + 1.
332
KOSTATELEGADAS
50.000
-
IO,000
7
5.ooo -
1.000~
2
500-
2 \ 3 a
*
0 I
IOO-
Q
z
50-
0
h
IOr 5b
Kilotons
l -
Toiol
Meqotons
-
Yield
Fig. 5. Calculated gross beta concentrations and corresponding gamma dose rates as a function of yield. Solid curves denote maximum credible conditions at D + 1 day, dashed curves denote maximum conditions at later times. Altitude of cloud center (from Fig. 3) is given for selected yields.
U.S. 4.3 days after a U.S. nuclear test over Christmas Island (List et al., 1964). The detonation occurred on May 4, 1962 and was reported to have a total yield in the intermediate range (200 kt-1 Mt). The highest concentration, measured at 15.2 km, was about lo6 pCi/SCM at collection time. This concentration was adjusted to D + 1 and plotted as another valid data point. The maximum credible concentration as a function of yield as depicted in Fig. 3 seems to be a reasonable estimate. It is felt that the assumed stratospheric dispersion, although an oversimplification of the real dispersion, gives a reasonable approximation of cloud radius growth up to about 10 days. The reduction
factor in Fig. 4 was applied to the D + 1 curve in Fig. 3 to estimate a maximum credible concentration for times up to 10 days as shown in Fig. 5. If desired, Equation (1) and the vertical activity distribution in Fig. 2 may also be used to estimate radioactivity concentrations at altitudes above and below the expected cloud center. 5. VISUAL OBSERVATIONOF A NUCLEA%CLOUD
The atmospheric nuclear test of November 17,1976 conducted at Lop Nor, China (4O”N, 9O”E) at 06W GMT was reported to have a total yield of about
Estimation of atmospheric radioactivity concentrations and dose rates from nuclear tests
4 Mt. Aircraft sampling 4.4 days after this event at 20.4 km obtained a maximum gross beta air concentration of 65 x lo6 pCi/SCM. This concentration was one of the highest measured for any Chinese test. The pilot of the sampling aircraft reported that the debris was identifiable as a rust-colored cloud. It is believed that the rust color was due to the oxides of nitrogen formed by the nuclear explosion. The cloud was described as elliptical in shape with the major axis in a west-east direction approximately 330 km long, the minor axis about 110 km long, with an estimated vertical extent of 230 m. This sighting suggests that a portion of the nuclear cloud hung together for four or five days after detonation, undergoing rather slow dispersion (Telegadas, 1977). 6. GAMMA DOSE RATES FROM NUCLEAR CLOUDS
Air travellers are effectively shielded from beta radiation originating outside the aircraft. The extent of beta exposure from contaminated air in the cabin would depend on the amount of air drawn into the cabin and the effectiveness of the air filtering system. The aircraft provides negligible protection from gamma radiation. Assuming a uniform infinite cloud (i.e. the radioactive cloud is large compared to the gamma ray path-length in air) the whole-body gamma dose rate may be estimated from the gross beta concentration (neglecting possible aircraft contamination). The relation between dose rate in a uniform infinite cloud and cloud concentration at standard temperature and pressure (Healy and Baker, 1968) can be expressed as ?Da: =
E . C(1.6 x 1O-6)(3.7x 10”)~ ’
(1293) (100)
’
(3
where : YD m E, C
1.6 x 1O-6 X 3.7 x 10’0 1293 100
gamma dose rate in an infinite cloud (rad/sec) average gamma energy per disintegration (MeV/dis) the ratio of the electron density in tissue to that in air. This ratio is approximately 1.1 over the energy range of 0.1-5.0 MeV number of ergs per MeV concentration of gross gamma radioactivity in the cloud (Ci/SCM) disintegration rate per curie (dis/s-Ci) density of air at standard temperature and pressure (g/SCM) energy absorbed per gram of absorbing material per rad (erg/g-rad).
To a good approximation (Peterson, 1977) the number of beta disintegrations are equivalent to the number of gamma disintegrations and ,!?, = & = 0.68 MeV/dis,
(6)
333
from 1 to 10 days after detonation. Using this value for E and converting dose rate to m/rad per h, Equation (5) becomes ,DW(m/rad/h) = (1.2 x 10e6)x,
(7)
where x is pCi/SCM of gross beta. Using this relationship, the right-hand scale in Fig. 5 gives the external gamma whole-body dose rates corresponding to the gross beta concentrations on the left side. Also shown in Fig. 5 is the altitude of the maximum credible concentration as deduced from Fig. 2. For example, a 1 Mt detonation (50”/, fission) would have a maximum credible gross beta concentration of 2 x lo9 pCi/SCM at 13.4 km at D+ 1 day which corresponds to an external gamma whole-body dose rate of 2400 mrad/h. At D +4 days the concentration at this altitude is estimated to be 2.5 x 10’ pCi/SCM with a corresponding gamma dose rate of about 30 mrad/h The total whole-body gamma dose received is the product of the dose rate and the transit time through the cloud. The time spent in-cloud can be estimated from the aircraft speed and the estimated cloud diameter at time of transit. In the example given above the dose rate was reduced by about a factor of 80 between D + 1 and D +4 days. The estimated diameter of a cloud at D + 4 days (Equation 4) is a factor of 4 longer than at D+ 1. Therefore, the total dose at D + 4 days would be about a factor of 20 smaller than at D+ 1. Knowing the aircraft speed one can use Equation (4) with Fig. 5 to estimate a maximum credible whole-body gamma dose. Figure 5 can be used to ascertain maximum credible air concentrations and gamma dose rates at the level of the assumed cloud center for any nuclear yield. However, the actual distribution of radioactivity in air will differ from the assumed uniform infinite cloud and the aircraft geometry may significantly affect the actual whole-body dose rate. It is strongly recommended that efforts be made, in the event of future atmospheric nuclear detonations, to obtain simultaneous measurements of gross beta concentrations in the air and onboard gamma dose rates to verify the calculations. 7. CONCLUSION
Many of the parameters used to determine the maximum credible concentration as a function of yield and time after detonation, such as vertical extent of the cloud, radioactivity distribution with altitude, and assumed fission-fusion ratio are based on limited data. The use of linear growth of cloud radius with time oversimplifies real atmospheric dispersion. It must be stressed that the initial altitude of the nuclear cloud will vary with meteorological conditions. In spite of the uncertainties involved, the calculated curves of maximum credible concentration are consistent with the limited data on maximum concentrations measured in nuclear clouds from 1 to 17 days after detonation. Tine procedures outlined here
334
KOSTA TELEGADAS
can be used to estimate maximum potential exposures to aircraft, as a function offlight altitude and time after detonation. Simultaneous measurements of in-cloud radioactivity concentrations and gamma dose rates onboard aircraft are needed at various aircraft altitudes to check the calculations, which should be regarded as order-of-magnitude estimates. Acknowledgements - The author wishes to acknowledge the Air Force Technical Applications Center for its assistance in technical discussions as well as providing observational data on nuclear debris concentration in the atmosphere. The author also wishes to express his thanks to Dr. L. Machta, G. J. Ferber and J. L. Heffter for their helpful suggestions and encouragement during the preparation of this report. Support of the U.S. Department of Energy is gratefully acknowledged.
REFERENCES Bauer E. (1974) Dispersion of tracers in the atmosphere and ocean: survey and comparison of experimental data. J. geophys. Res. 79, 789-794. Ferber G. (1965) Distribution of radioactivity with height in nuclear clouds. In Radioactive Fallout from Nuclear Weapons Tests. pp. 629-645. U.S. Atomic Energy Commission Publ. 5, Symposium Series. Hardy E. (1970) Sr-89 Fallout from Atmospheric Testing. U.S. Atomic Energy Commission, Rept. HASL-227 (Nat’l. Tech. Information Service, U.S. Dept. of Commerce, Springfield, VA (22161).
Harley N., Fisenne I., Ong L. D. Y. and Harley J. (IYb5) Fission Yield and Fission Product Decay. U.S. Atomic Energy Commission, Rept. HASL-164 (Nat? Tech. Information Service, U.S. Dept. of Commerce, Springfield. VA 22161). Healy J. A. and Baker R. E. (1968) Radioactivity cloud-dose calculations. In Meteorology and Atomic Energy (Wited b) D. H. Slade), (TID-24190, Nat?. Tech. Information Service, U.S. Dept. of Commerce, Springfield, VA 22161). Heffter J. L. (1965) The variation of horizontal diffusion parameters with time for travel periods of one hour or longer. J. appl. Met. 4, 153 156. Heffter J. L., Taylor A. D. and Ferber G. J. (1975) A regionalcontinental scale transport, diffusion and deposition model. NOAA Tech. Memo. ERL ARL-50, NOAA Environmental Research Laboratories. Air Resources Laboratories, Silver Spring, MD 20910. Peterson K. R. (1970) An empirical model for estlmatmg world-wide deposition from atmospheric nuclear
Amosphpric Enuironmenr Vol. 13. pp. 327-334. 0 Pergamon Press Ltd. 1979 Printed NI Great IMaIn.
ESTIMATION OF MAXIMUM CREDIBLE ATMOSPHERIC RADIOACTIVITY CONCENTRATIONS AND DOSE RATES FROM NUCLEAR TESTS KSSTA
TELEC~ADAS
Air Resources Laboratories, National Oceanic and Atmospheric Administration, 8060 13th Street, Silver Spring, MD 20910, U.S.A. (First received 15 May 1978 and in.~na~~r~
29 August 1978)
Abstract - A simple technique is presented for estimating maximum credible gross beta air concentrations from nuclear detonations in the atmosphere, based on aircraft sampling of radioactivity following each Chinese nuclear test from 1964 to 1976. The calculated concentration is a function of the total yield and fission yield, initial vertical radioactivity distribution, time after detonation, and rate of horizontal spread of the debris with time. Calculated maximum credible concentrations are compared with the highest concentrations measured during aircraft sampling. The technique provides a reasonable estimate of maximum air concentrations from t to 10 after a detonation. An estimate of the whole-body external gamma dose rate corresponding to the maximum credible gross beta concentration is also given.
1. INTRODUCTION
The injection of radioactive debris into the atmosphere by a nuclear explosion could result in exposure of aircraft to relatively high radioactivity concentrations. Several U.S. government agencies are engaged in a cooperative effort to monitor airborne nuclear debris, project its future path and concentration, and notify responsible authorities and the public of any potential hazards. Air concentrations depend on the amount and initial distribution of radioactivity, the elapsed time since the explosion and the rate of dispersion of the debris as it travels. This paper presents a simple technique for estimating maximum credible concentrations as a function of the time after detonation. The only input information required is an estimate of the energy yield of the explosion. Calculated concentrations are compared to the highest measured concentrations after aircraft sampling. Results are applicable to air bursts near the ground. For surface detonations, local fallout would reduce the airborne concentrations.
x r
Conversion factors used in Equation (1) are: (a) 1 kt is equivalent to 1.45 x 10z3 fissions, and (b) 0.45 picocurie (pCi) is equivalent to 1 disintegration per minute (disjmin). Fission yield
For all lower yield detonations ( I 500kt) the fission yield is assumed to be equivalent to the total yield. For detonations greater than 500 kt it is assumed that the fission yield is one-half the total yield. Gross beta rad~~ctivity The total quantity of gross beta, per fission, and its decay with time, are shown in Fig. 1 based on Harley et al. (1965) and Healy and Baker (1968). This curve very closeiy approximates a t - I” decay.
2. A SIMPLE METHOD FOR RADIOACTIVITY
CALCULATING CONCENTRATIONS
Radioactivity concentrations resulting from atmospheric nuclear explosions can be estimated from
L =
7zr2
f
where L
is the average gross beta concentration,
curies per ambient cubic meter ofair, pCi m-3), at altitude (h) and time (t) after the detonation is the fission yield of the device (kilotons, kt) is the gross beta radioactivity per fission at time (t), (dis/min/fission) is the fraction of radioactivity per meter at altitude (h) is the radius (m) of the debris cloud at time (I).
(pico-
Vertical radioactivity distribution
Telegadas (1974; 1976; 1977) ~timat~ the vertical distributions of radioactivity for each of the first five high yield Chinese tests and presented a mean distribution (Fig. 2 inset). Although the vertical distributions were based on radioactivity m~sur~ents made several weeks or months after each event, the mean distribution, which is a composite of the five events, may be used as a reasonable estimate of the 327
Kosr .A ~rbLLGAI)AS 1
I
v
,I,,,,
I
500
I
KIloton
= 1.45
I
,
--I--_
I,,,,
x 10z3fisskons
1
-
-1 1
1 1
4 Heoly 8. Evoker (t-10
days)
1 i
P
ey et
01. (IO-lOOdays)
Days
Fig. I. Total beta distintegrations
per min per 10h fissions remaining
as a function
of time
-2O-
-25-
KIlOtOnS
Tolol
Fig. 2. Mean cloud
Yield
top and base as a function of total yield. Inset diagram vertical activity distribution within the cloud.
presents
the assumed
initial
Estimation of atmospheric radioactivity concentrations and dose rates from nuclear tests Table 1. Concentrations
I
329
of radioactive debris from Chinese atmospheric nuclear tests, 1964-1976 Approximote Yie,d
c 1966’
Oct.
‘28
20(”
Dec. 28
i 1967
Jun.
17
Dec.
24
8.8
20
300(‘) -_-
300
3OOOt’)
l6OOt*’
’
4.3
A:‘: L_-
-.-;:;;
3000(
29
1970 1971
1972
1974
Oct.
14
Nov.
18
Jon.
7
Mar.
‘“,:
1800”’
/ 30504
20’3) < 20 14’
C
1976
12~,2
20
6.4
0.06
L.2
20
7.6
0.08
2.3
13.7
2.9
20 - 200
2@)-~()0$)20O-tOOO
20.0
I.0
I I .5
CL07 0.10
16.5 Jon. 23
/
3ooo~z~’
18 20-200’“’ 17
I .4
._.,
-- 1400(” Jun.
0.8 370
I .?
3OOO”‘i
/
,43;05
I30
’)
19691 Sep.
6.9 10.7
0.55
z!O(‘)
1968 / Dec. 28
0.48
9.8
<20(S) -_-.-.
Sep.26
2&200(“)
Nov. 17
4000
(6) , /
2000
j I’
2.7
< 20
10.4
20-200 (8)
10.7 20.4
_
8.3
11000 0.2 10.6 0.003
j
1.0
2.9
/
I.6
4.0
5 x lO-6
65
7600
(1) Hardy, 1970; (2) Telegadas, 1974 ; (3) USAEC News Release, O-21 3 ; (4) LJSAEC News Release, P-6; (5) USAEC News Release, P-77; (6) Information received from Office of Public Affairs, USERDA; (7) Telegadas, 1976; (8) Assumed for computation purposes.
distribution at earlier times assuming the effect of vertical diffusion to be small.
mum credible
concentrations,
we will minimize
cr,(m) = 0.2%(s), From a compilation of many lateral dispersion measurements in the troposphere (Heffter, 1965) an expression for the growth, with time, of the horizontal standard deviation, Q”, crH(m) = O.%(s),
(2)
has been suggested (Heffter et al., 1975) as a good approximation for the growth of a cloud during several days travel time in the troposphere. Very few measurements have been made of lateral dispersion in the lower stratosphere. Virtually no measurements after several days travel time have been reported. Equation (1) was used to caiculate u,, from the stratospheric concentration measurements for the six high yield tests given in Table 1. Calculated values of (TVranged from 0.2% to l.lt for travel times from 1.5 to 17 days. This agrees well with the range of uH given by Bauer (1974) for these travel times in the lower stratosphere. Since we are concerned with estimating the maxiA.E.t3i*--*
the
cloud growth using (3)
for travel times of several days in the lower stratosphere. The radius of the growing cloud is then approximated by r =
2a,,
(4)
and for simplicity, it is assumed that the concentration is horizontally uniform, since we are interested in the average concentration along a flight path through the cloud. 3. AIR CONCENTRATION DATA The highest radioactivity concentration measurements from aircraft sampling following each of the Chinese tests from 1964 to 1976 are given in Table 1. The approximate total yield and fission yield of each detonation and the data source are given. For detonations having a total yield of 500 kt or less it was assumed that the fission yield was equal to the total yield.
:.io
KOSTA
1
< 20
TELEfiAflAS
KT * 0.003
-, -Mrpcrtonr-
Kilotons Total
Yttld
Fig. 3. Observed concentrations @G/SCM) adjusted to one day after detonation. Solid curves denote calculated maximum credible concentration at one day assuming tropospheric dispersion (Equation 2) for yields 500 kt or less and stratospheric dispersion (Equation 3) for yields greater than 500 kt. (* See text for
explanation.)
The altitude of sampling is shown and the gross beta radioactivity concentrations at sampling time are given in units ofpic~u~es per standard cubic meter of air (pCi/SCM). The concentration in the last column has been adjusted to its expected value one day (D + 1) after the detonation. This adjustment takes into account the decay of mixed fission products (Fig. f f and also includes an atmospheric dilution factor which will be discussed later. 4.
MAXIMUM CREDIBLE AIR CONCENTRATIONS
The prediction of maximum credible air concentrations is based on Equation (1) using the mean vertical radioactivity distribution and an estimate of the altitude of the radioactivity maximum based on the total yield of the explosion. The cloud thickness as a function of yield is shown in
Fig. 2 which represents a composite of previous cloud height determinations (e.g. Quenneville and Nagler, 1959; Ferber, 1965; and Peterson, 1970) and is intended to represent the vertical extent of nuclear clouds in temperate latitudes. Differing atmospheric conditions produce variability in the height of nuclear clouds of the same yield and height of burst. The variability about the cloud top curve in Fig. 2 is about + 2 km as estimated by Quenneville and Nagler (1959) and Ferber (1965). It is assumed that the mean vertical distribution as given by Telegadas (1974) is valid for all air bursts. This distribution, shown in the inset diagram of Fig. 2, is divided into seven layers of equal thickness and is similar to the distribution proposed by Ferber (1%5) and Peterson (1970). The altitude of the maximum concentration is assumed to be midway between the cloud top and base. This altitude is assumed to be the
Estimation of atmospheric radioactivity concentrations and dose rates from nuclear tests midpoint of a layer containing 38% of the total activity. Since we are concerned with estimating the maximum credible air concentration, we will assume that the vertical diffusion in both the troposphere and stratosphere to be negligible. For horizontal dispersion we use Equation (2) for tropospheric injections (500 kt or less) and Equation (3) for stratospheric injections (> 500 kt). With the above assumptions, a maximum credible concentration at one day after detonation has been estimated from Equation (1) for nuclear yields from 1 kt to 10 Mt as shown in Fig. 3. Since the horizontal standard deviation u,,, is assumed to increase linearly with time, the volume of a nuclear cloud layer (assuming no vertical dispersion) increases as t’. The gross beta radioactivity decreases according to t -I” decay. By combining these two factors, dispersion and decay, one arrives at a reduction factor, shown in Fig. 4, which gives the decrease in concentration with time. The reduction factors in Fig. 4 were applied to the observed concentration data in Table 1 to adjust all observed concentrations to D+ 1. The highest D+ 1 concentration for each detonation is plotted in Fig. 3. Only one point, that for the June 27, 1973 nuclear test, lies above the maximum credible curve. This concentration measurement (1 x lo6 pCi/SCM) at 20 km was made 17 days after detonation and then adjusted
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to a concentration of 11000 x lo6 pCi/SCM at D + 1. The adjusted concentration is about a factor of 2 higher than the estimated maximum credible concentration. The highest adjusted concentrations for the nuclear tests greater than 2 Mt, other than the June 27, 1973 test, were for the 2 events sampled at the highest altitude, about 20 km. These concentrations were within a factor of 2 of the predicted maximum credible concentration. For the other 3 high yield events, which were sampled at lower altitudes, the adjusted concentrations were a factor of about 20-400 below the predicted maximum credible concentration. The marked over-predictions may be due to the actual atmospheric dispersion being much greater than that assumed for maximum credible concentrations. However, aircraft sampling is limited and may not always encounter the maximum concentration in a debris cloud. There are four events with an estimated yield of20 kt that were sampled at altitudes ranging from 6.4 to 10.7 km (see Table 1) from one to two and a half days after detonation. The adjusted D + 1 concentrations were between 0.8 x lo6 and 6.9 x lo6 pCi/SCM. This illustrates the variability in the maximum concentration encountered for any given yield. The point denoted by an asterisk in Fig. 3 represents radioactive debris encountered over the mid-western
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Fig. 4. Reduction factor, combining horizontal dispersion and decay, adjusted to D + 1.
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Fig. 5. Calculated gross beta concentrations and corresponding gamma dose rates as a function of yield. Solid curves denote maximum credible conditions at D + 1 day, dashed curves denote maximum conditions at later times. Altitude of cloud center (from Fig. 3) is given for selected yields.
U.S. 4.3 days after a U.S. nuclear test over Christmas Island (List et al., 1964). The detonation occurred on May 4, 1962 and was reported to have a total yield in the intermediate range (200 kt-1 Mt). The highest concentration, measured at 15.2 km, was about lo6 pCi/SCM at collection time. This concentration was adjusted to D + 1 and plotted as another valid data point. The maximum credible concentration as a function of yield as depicted in Fig. 3 seems to be a reasonable estimate. It is felt that the assumed stratospheric dispersion, although an oversimplification of the real dispersion, gives a reasonable approximation of cloud radius growth up to about 10 days. The reduction
factor in Fig. 4 was applied to the D + 1 curve in Fig. 3 to estimate a maximum credible concentration for times up to 10 days as shown in Fig. 5. If desired, Equation (1) and the vertical activity distribution in Fig. 2 may also be used to estimate radioactivity concentrations at altitudes above and below the expected cloud center. 5. VISUAL OBSERVATIONOF A NUCLEA%CLOUD
The atmospheric nuclear test of November 17,1976 conducted at Lop Nor, China (4O”N, 9O”E) at 06W GMT was reported to have a total yield of about
Estimation of atmospheric radioactivity concentrations and dose rates from nuclear tests
4 Mt. Aircraft sampling 4.4 days after this event at 20.4 km obtained a maximum gross beta air concentration of 65 x lo6 pCi/SCM. This concentration was one of the highest measured for any Chinese test. The pilot of the sampling aircraft reported that the debris was identifiable as a rust-colored cloud. It is believed that the rust color was due to the oxides of nitrogen formed by the nuclear explosion. The cloud was described as elliptical in shape with the major axis in a west-east direction approximately 330 km long, the minor axis about 110 km long, with an estimated vertical extent of 230 m. This sighting suggests that a portion of the nuclear cloud hung together for four or five days after detonation, undergoing rather slow dispersion (Telegadas, 1977). 6. GAMMA DOSE RATES FROM NUCLEAR CLOUDS
Air travellers are effectively shielded from beta radiation originating outside the aircraft. The extent of beta exposure from contaminated air in the cabin would depend on the amount of air drawn into the cabin and the effectiveness of the air filtering system. The aircraft provides negligible protection from gamma radiation. Assuming a uniform infinite cloud (i.e. the radioactive cloud is large compared to the gamma ray path-length in air) the whole-body gamma dose rate may be estimated from the gross beta concentration (neglecting possible aircraft contamination). The relation between dose rate in a uniform infinite cloud and cloud concentration at standard temperature and pressure (Healy and Baker, 1968) can be expressed as ?Da: =
E . C(1.6 x 1O-6)(3.7x 10”)~ ’
(1293) (100)
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(3
where : YD m E, C
1.6 x 1O-6 X 3.7 x 10’0 1293 100
gamma dose rate in an infinite cloud (rad/sec) average gamma energy per disintegration (MeV/dis) the ratio of the electron density in tissue to that in air. This ratio is approximately 1.1 over the energy range of 0.1-5.0 MeV number of ergs per MeV concentration of gross gamma radioactivity in the cloud (Ci/SCM) disintegration rate per curie (dis/s-Ci) density of air at standard temperature and pressure (g/SCM) energy absorbed per gram of absorbing material per rad (erg/g-rad).
To a good approximation (Peterson, 1977) the number of beta disintegrations are equivalent to the number of gamma disintegrations and ,!?, = & = 0.68 MeV/dis,
(6)
333
from 1 to 10 days after detonation. Using this value for E and converting dose rate to m/rad per h, Equation (5) becomes ,DW(m/rad/h) = (1.2 x 10e6)x,
(7)
where x is pCi/SCM of gross beta. Using this relationship, the right-hand scale in Fig. 5 gives the external gamma whole-body dose rates corresponding to the gross beta concentrations on the left side. Also shown in Fig. 5 is the altitude of the maximum credible concentration as deduced from Fig. 2. For example, a 1 Mt detonation (50”/, fission) would have a maximum credible gross beta concentration of 2 x lo9 pCi/SCM at 13.4 km at D+ 1 day which corresponds to an external gamma whole-body dose rate of 2400 mrad/h. At D +4 days the concentration at this altitude is estimated to be 2.5 x 10’ pCi/SCM with a corresponding gamma dose rate of about 30 mrad/h The total whole-body gamma dose received is the product of the dose rate and the transit time through the cloud. The time spent in-cloud can be estimated from the aircraft speed and the estimated cloud diameter at time of transit. In the example given above the dose rate was reduced by about a factor of 80 between D + 1 and D +4 days. The estimated diameter of a cloud at D + 4 days (Equation 4) is a factor of 4 longer than at D+ 1. Therefore, the total dose at D + 4 days would be about a factor of 20 smaller than at D+ 1. Knowing the aircraft speed one can use Equation (4) with Fig. 5 to estimate a maximum credible whole-body gamma dose. Figure 5 can be used to ascertain maximum credible air concentrations and gamma dose rates at the level of the assumed cloud center for any nuclear yield. However, the actual distribution of radioactivity in air will differ from the assumed uniform infinite cloud and the aircraft geometry may significantly affect the actual whole-body dose rate. It is strongly recommended that efforts be made, in the event of future atmospheric nuclear detonations, to obtain simultaneous measurements of gross beta concentrations in the air and onboard gamma dose rates to verify the calculations. 7. CONCLUSION
Many of the parameters used to determine the maximum credible concentration as a function of yield and time after detonation, such as vertical extent of the cloud, radioactivity distribution with altitude, and assumed fission-fusion ratio are based on limited data. The use of linear growth of cloud radius with time oversimplifies real atmospheric dispersion. It must be stressed that the initial altitude of the nuclear cloud will vary with meteorological conditions. In spite of the uncertainties involved, the calculated curves of maximum credible concentration are consistent with the limited data on maximum concentrations measured in nuclear clouds from 1 to 17 days after detonation. Tine procedures outlined here
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KOSTA TELEGADAS
can be used to estimate maximum potential exposures to aircraft, as a function offlight altitude and time after detonation. Simultaneous measurements of in-cloud radioactivity concentrations and gamma dose rates onboard aircraft are needed at various aircraft altitudes to check the calculations, which should be regarded as order-of-magnitude estimates. Acknowledgements - The author wishes to acknowledge the Air Force Technical Applications Center for its assistance in technical discussions as well as providing observational data on nuclear debris concentration in the atmosphere. The author also wishes to express his thanks to Dr. L. Machta, G. J. Ferber and J. L. Heffter for their helpful suggestions and encouragement during the preparation of this report. Support of the U.S. Department of Energy is gratefully acknowledged.
REFERENCES Bauer E. (1974) Dispersion of tracers in the atmosphere and ocean: survey and comparison of experimental data. J. geophys. Res. 79, 789-794. Ferber G. (1965) Distribution of radioactivity with height in nuclear clouds. In Radioactive Fallout from Nuclear Weapons Tests. pp. 629-645. U.S. Atomic Energy Commission Publ. 5, Symposium Series. Hardy E. (1970) Sr-89 Fallout from Atmospheric Testing. U.S. Atomic Energy Commission, Rept. HASL-227 (Nat’l. Tech. Information Service, U.S. Dept. of Commerce, Springfield, VA (22161).
Harley N., Fisenne I., Ong L. D. Y. and Harley J. (IYb5) Fission Yield and Fission Product Decay. U.S. Atomic Energy Commission, Rept. HASL-164 (Nat? Tech. Information Service, U.S. Dept. of Commerce, Springfield. VA 22161). Healy J. A. and Baker R. E. (1968) Radioactivity cloud-dose calculations. In Meteorology and Atomic Energy (Wited b) D. H. Slade), (TID-24190, Nat?. Tech. Information Service, U.S. Dept. of Commerce, Springfield, VA 22161). Heffter J. L. (1965) The variation of horizontal diffusion parameters with time for travel periods of one hour or longer. J. appl. Met. 4, 153 156. Heffter J. L., Taylor A. D. and Ferber G. J. (1975) A regionalcontinental scale transport, diffusion and deposition model. NOAA Tech. Memo. ERL ARL-50, NOAA Environmental Research Laboratories. Air Resources Laboratories, Silver Spring, MD 20910. Peterson K. R. (1970) An empirical model for estlmatmg world-wide deposition from atmospheric nuclear