Radiochemical Dating of Nuclear Weapon Debris ... - ACS Publications

26 Radiochemical Dating of Nuclear Weapon Debris Precipitated from Cyclonic Storms

Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch026

over the California Coast PAUL KRUGER Civil Engineering Department, Stanford University, Stanford, Calif. 94305

Radiochemical analyses of the fission products Ba, Sr, and Sr in precipitation from three Pacific cyclones were examined as functions of time and latitude along the California coast to estimate the fractional contribution of nuclear debris from recent nuclear weapons tests. The estimates are based upon the assumptions that the observed fission products originated solely from announced tests in the atmosphere and were produced with the same relative yields as those reported for U. S. nuclear weapons. The analyses show evidences of fractionation of these fission products in the precipitation along the coast in each of the three cyclone storms studied. In accordance with the given assumptions, the fractionation seems to be associated with the precipitation patterns resulting from the meteorological features of the storms. 140

89

90

adiochemical methods have proved effective for studying fractionation of radionuclides resulting from atmospheric nuclear weapon tests and the deposition of these radionuclides by meteorological processes. The phenomena of fractionation have been studied extensively by many investigators. The extent of fractionation was shown to depend on many parameters, such as the total yield and height of the detonation and the amount and chemical composition of the earth's surface affected by the fireball. Freiling (7) distinguished between primary fractionation, resulting from processes occurring during the condensation of the fireball materials and secondary fractionation, resulting from the further contact of the condensed, debris, with the immediate environmental

American Chemical Society Library

^1155 16th St, W.W. Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.


448

RADIONUCLIDES

I N

T H E

E N V I R O N M E N T

materials. It was further shown (8) that radionuclide fractionation occurs even i n air bursts, where the fireball does not affect the earth's surface. Potential and actual fractionation for airburst debris were distinguished, where potential fractionation was defined to exist when the radiochemical composition varies among the aerosol particles, by size, density, macro scopic composition, or history i n the fireball, even though these particles contain totally among themselves an unfractionated composition of the radionuchdes. Potential fractionation of the debris from air bursts has been shown (2). Freiling and K a y (8) suggested that to achieve an actual fractionation, a particle-separation process must occur—e.g., as a result of unequal rates of sedimentation through the atmosphere over long periods of time. It is well established that i n non-arid regions, precipitation is the primary means by which contaminating aerosols are removed from the atmosphere. M a n y chemical, physical, and meteorological parameters affect the micro, meso, and synoptic scale processes through which pre cipitation transports radioactive aerosols from atmosphere to ground. These parameters include the radioactivity component of the natural aerosols, the processes by which water vapor condenses and grows to raindrops, and the incorporation of the radioactive aerosol into the pre cipitation. Thus, the prediction of specific deposition from fundamental considerations has proved to be difficult because of the many uncertain ties yet prevalent i n these processes. M a n y attempts have been made to evaluate the deposition of these aerosols by empirical studies. The initial distribution of radioaerosols from an atmospheric nuclear explosion depends upon condensation and coagulation during the rise and cooling of the fireball. These processes result i n small particle sizes of the nuclear weapon debris. F o r example, Stewart (24) calculated that for yields of about 20 kilotons, the particles coagulated from vaporized materials w i l l reappear as very small particles (with modal radii of the order of 0.1 to 0.01/A) and remain airborne for long periods. W h e n such particles are carried into the stratosphere by the buoyant lifting of the fireball, it is expected that they w i l l become a quasi-conservative con stituent of the stratospheric air. It has been shown that particles w i t h diameters less than 1μ have an essentially infinite residence i n the atmos phere for sedimentation processes alone (16). The transport of stratospheric air and the contained aerosol particles has been examined by many investigators. For example, the intrusion of stratospheric air into the troposphere has been demonstrated by Danielson (5). Such intrusions occur frequently i n association with cyclonic disturbances (23). In the troposphere, the aerosol particles may be removed effectively by precipitation processes. They may be removed by collisions between the aerosol particles and cloud or rain drops (9).

Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

26.

K R U G E R

Radiochemical Dating of Nuclear Debris

449


Aerosol particles of the size likely to contain the nuclear weapon debris radionuchdes and to come from stratospheric air masses are also more likely to serve efficiently as precipitation nuclei (13). Either of these two precipitation removal processes could result in the actual fractiona tion of potentially fractionated nuclear weapon debris. The possibility of meteorological fractionation of nuclear weapon debris has been suggested. F o r example, Storebo (25) observed more short lived fission products at a mountain station in Norway compared with those at lowland stations. H e suggested that the short lived fission products were on larger particles which should be transported more easily downward by rain than smaller particles. However, another change in the radioactivity composition during a single rain has been explained with the assumption that the first rain fractions collected contained a larger number of very small particles which were potentially fractionated (20). The assumption was attributed to the observation of Greenfield (9) that collection efficiencies of 20-/* diameter raindrops was very high for aerosol particles with radius less than about 0.01/x. Radiochemistry offers an excellent method for further examination of meteorological fractionation of nuclear weapon debris, especially dur ing periods when known single sources of nuclear weapons testing has occurred. Three of the more biologically important fission products are 12.8-day B a , 50.5-day S r , and 27.7-year ^Sr. These alkaline-earth element radionuclides have in common their production as secondary fission products from decay of rare-gas element primary fission products, high solubility in most chemical forms, and biological similarity to cal cium, an element important in man. The characteristics of the decay chains leading to these three fission products are given in Table I. The relative production values for the underlined nuclide i n each chain are for fission yields reported for nuclear weapon explosives (12). Barium and strontium have boiling points over 1100°C. and are considered to be refractory elements. However, their condensation properties during the early history of the fireball are determined by their volatile and rare-gas element precursors. Thus, during the cooling of the rapidly rising fire ball, the formation of the three radionuclides by β-decay of their gaseous precursors takes place at different rates. B a , having the shortest lived precursors condenses first; S r condenses next; S r condenses last. Thus, these three radionuchdes may be potential fractionated by being dis tributed unequally among the aerosol particles contained in stratospheric air masses. 1 4 0

89

140

90

89

The differences in condensation history of the three alkaline-earth element fission products allows examination of their radioactivity ratio as a method for detennining fractionation. The recent atmospheric


450

RADIONUCLIDES

Table I.

Mass No.

First Nuclide

Second Nuclide

I N

T H E


Characteristics of the Third Nuclide

89

—2-sec. Se (17) 89

4.5-sec. B r (37)

3.16-min. K r (53)

90

—3-sec. B r ( 17 )

33-sec. K r ( 57 )

2.7-min. R b ( 26 )

—1.5-sec.

16-sec.

66-sec.

140

90

1 4 0

I (7)

89

90

140

X e (49)

89

90

140

C s (40)

From Freiling and Kay (8); values in parenthesis are the fractional chain yield cal culated by the theory of Present (22). The relative production values are from Harley


β

nuclear weapons tests on the China mainland offered an opportunity to use this method to examine the possibility of meteorological fractionation in precipitation from cyclonic storms affecting the California coast. Radionuclide concentrations in precipitation reaching the ground depend upon many meteorological parameters. The concentration is influenced by the amount of radioactivity incorporated into the precipi tation and the amount of precipitation reaching the ground. These two, in turn, may be divided conveniently into the following six parameters: (1) the height of the precipitation generating level, (2) the precipitation generation and growth mechanisms in the cloud, (3) the amount and characteristics of the radioactive aerosols initially in the air masses par ticipating in the precipitation process, (4) the specific humidity at the generating level, (5) the previous precipitation experience of the air at the generating level, and (6) the descent experience of the precipitation from the cloud in which the precipitation originates to the ground. Some of the major precipitation-producing storm systems include extratropical cyclones, convective showers, severe storms, and orographic lifting. Many of these storm types have been examined for their radio activity deposition characteristics. One of the more important types of storms, both i n duration and areal extent, is the extratropical cyclone, which in the United States, generally may last for days and cover thou sands of miles of trajectory and width. A n examination of deposition from extratropical cyclones in Pennsylvania was given by Kruger, Hosier, and Davis (18). Several meteorological parameters were observed to influence the deposition of radioactive aerosols (i.e., 27.7-year S r ) . These included the effects of frontal and trough passage, the changes i n precipitation growth processes, the gradient of radioactive aerosol i n the cloud and in the different air masses, the amount and rate of rainfall, and the effects of evaporation below the cloud bases. 9 0


26.

A


K R U G E R

= 89, 90, 140 Decay Chains

15.4-min. 27.7-yr.

9 0

8 9

Rb

Sr

0

Relative Production (HASL-164)

Last Radioactive Nuclide

Fourth Nuclide (10)

(0)

50.4-day 61-hr.

9 ( )

40-hr.

1 4 0

451

8 9

Sr

Y

147

(0)

1

(0)

(1384) 12.8-day

1 4 0

Ba

(3)

La

1170

(0)


(ÎÏ4Ô) et al. (12) for weapon fission yields. below in °C.

The boiling point for the element is given

Pacific cyclones represent the major source of precipitation over the western United States. Therefore, they also represent a major source of deposition of contaminant aerosols in the waters of these states. The examination of radioactive aerosols in precipitation resulting from Pacific cyclones is thus of interest in understanding the transport of atmospheric aerosols to the ground by precipitation. A study of a Pacific Coast cyclone at Santa Barbara, Calif, on Feb. 7, 1962, has been reported (19). The effects of the overturning of the marine layer and the passage of the surface front and the troughline at 500 mbars, on the observed concentrations of S r and S r in the precipitation were noted. Similar effects have been noted at other locations— e.g., a change of radioactivity concentration during the passage of a cold front over the Netherlands ( 3 ) and a marked change in S r concentration in rainfall associated with the presence of a trough at 500 mbars (21). During the winters of 1966-1967 and 1967-1968, cyclonic storms o o curred over the California coast on three occasions shortly after the detonation of a nuclear weapon on the China mainland such that measurable concentrations of 12.8-day B a were present in 6-hour rainfall samples. The deposition radioactivity ratios of B a , S r , and S r were examined in the rainfall along the California coast for each of these three storms and used to examine the fractional contribution of the longer hved strontium radionuclides from the various known Chinese nuclear explosions. 89

90

90

1 4 0

1 4 0

Experimental

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Results

A precipitation-collection network is maintained at 11 sites along the California coast. Each site has an identical stainless-steel funnel with an area of 3.62 sq. meters; rainfall drains through a small valve into polyethylene bottles. Arrangements have been made for collecting rainfall in prearranged collection increments from specific storms. The locations



452

RADIONUCLIDES

-J 130

I N

T H E


L_ 120

I 125 LONGITUDE

CW)

Figure 1. Collection sites along the California coast (rectangles) and the ESSA weather stations (circles) Of the collection sites are shown in Figure 1. The corresponding meteorological data for each storm are obtained from the several E S S A Weather Bureau stations also shown in Figure 1. For each storm, vertical cross sections of the atmosphere are constructed from the available radiosonde data, and the synoptic conditions are taken from the surface, 500 mbars, 300 mbars, and other facsimile charts. Radiochemical analyses for B a , S r , and ^Sr are made for each of the rain samples collected at each site affected by the storm. Standard radiochemical procedures (11) are used. The rainfall is acidified with H N 0 , and barium and strontium carriers are added with mixing to ensure exchange of carrier and radioisotopes. The sample is made basic with NH4OH and run through an ion-exchange column containing Chelex-100 anion exchange resin i n the ammonium form. The barium and strontium carriers are eluted with HNO3 and evaporated to fumes to precipitate B a ( N 0 ) and S r ( N 0 ) . Following dissolution and F e ( O H ) scavenging precipitations, the barium is separated as B a C r 0 . The strontium is precipitated as S r C 0 for chemical yield determination. After reestabhshment of the S r — Y radioactivity equilibrium, Y is removed quantitatively. The radioactivity measurements of the B a — 1 4 0

89

3

3

2

3

2

3

4

3

90

9 0

9 0

1 4 0


26.

K R U G E R

Radiochemical Dating of Nuclear Debns

453

L a , the S r + S r , and the Y samples are determined in low background (0.2 c.p.m.) anti-coincidence ^-counters having a sensitivity for measurement of ^-activity of < 1 d.p.m. The disintegration rates are extrapolated back from time of measurement to midtime of the rainfall collection period. Repeated analysis of S r standards shows a standard deviation for single measurements of about ± 5 % . For the B a concentrations prevalent during the measurement periods, the standard deviations for single measurements were estimated to be about ± 1 0 to ± 2 0 % . The reproducibility of the analytical procedures was checked with the large 146.4liter sample from Bodega Bay for the January 1968 storm by dividing it into two equal aliquots and analyzing them as two independent samples. Table II shows that the spread in the duplicate values ranged from 1.5 to 4% for these relatively high activity samples. The S r value, obtained from the difference in activity of its S r - j - S r sample free of Y , and the S r value determined from the Y measurement, shows the greater average deviation. In general, the S r activities were sufficiently greater than the S r activity (ratios ranging from 3 to 60) such that the standard deviations of the S r activity were of the order of ± 1 0 to ± 2 0 % . 1 4 0

89

90

9 0

90


1 4 0

89

89

90

90

9 0

9 0

89

90

89

Table II.

Reproducibility Measured for a Split Sample

Concentration,

Percent Deviation, AC

d.p.m./liter ^°Ba 55.8 53.5

Sr

89

25.3 27.4

— Sr

Ba

90

no

1.15 1.12

2.1

Sr

89

4.0

X 100 Sr

90

1.3

During each winter rainy season, meteorological alert was established after each of the Chinese nuclear weapons tests: October 1966, December 1966, and December 1967. The criteria used to alert the collection network of a forecasted storm system included the presence of a well-defined jet stream and a forecast for precipitation in excess of 3 mm. over a majority of the sampling sites. A cyclonic storm satisfying these criteria moved in over the C a l i fornia coast within 25 days after each of these three nuclear explosions affording high probability that the B a from the latest explosion would be measurable with adequate precision. The three Pacific cyclones examined occurred during Nov. 15-17, 1966, Jan. 20-22, 1967, and Jan. 8-11, 1968. Each of these storms exhibited different meteorological characteristics. To assist in evaluating the radiochemical data, each storm was analyzed. 1 4 0



454

RADIONUCLIDES

I N T H E


Storm of N o v . 15-17, 1966. The large-scale circulation pattern leading up to this storm period was characterized by a strong blocking high building over western Canada and extending into the central Pacific. The storm which affected the California coast developed out of a deep low pressure system which moved into eastern Pacific offshore OregonWashington coast. The associated frontal system moved across the Pacific northwest states into northern California and became quasi-stationary i n that posi tion on November 15. The positions of the surface front taken from the surface charts for Nov. 15-17, 1966, are shown i n Figure 2. Subsequent wave developments on this front resulted i n heavy precipitation i n north ern California throughout November 15, and into the morning of Novem ber 16 before the front started to push southward along the California coast.

Figure 2.

The 6-hourly frontal positions for the storm of Νου. 15-17, 1966

The 0400 P S T upper circulation patterns at the 500-mbar level for the Nov. 15-17 period are shown i n Figure 3. A deep cold low remained nearly stationary offshore Vancouver Island during this storm period,


K R U G E R


455


26.

Figure 3.

0400 PST upper circulation patterns at the 500-mbar level and location of the jet stream for Nov. 15-17,1966


456

RADIONUCLIDES

I N

T H E



while the jet stream at the 300-mbar level shifted southward from southern Oregon to near Vandenberg i n southern California. The southward shift of the jet stream core can be seen in more detail on the atmospheric cross sections presented i n Figure 4. The cross sections extend from Medford ( M F R ) southward across the coastal range to Oakland ( O A K ) and onward to Vandenberg ( V B G ) , San Nicolas Island ( N S I ) , and San Diego ( S A N ) . The partial soundings from Los Angeles ( L A X ) were used when available.

Figure 4.

0400 PST cross sections through California for Nov. 15-17, 1966

A t 0400 PST, November 15, the main polar front cut across the coastal range between Medford and Oakland with the jet stream core of strength in excess of 110 knots near the 250-mbar level. This front, which remained nearly stationary until the 16th, was located between Oakland and Vandenberg with the jet stream core immediately south of Oakland showing no discernible change i n its height and strength. H o w ever, during the subsequent 24 hours, the jet stream apparendy weakened and lost its well-defined core as it shifted southward to near Vandenberg. This change i n strength and position of the jet stream is also reflected in the rainfall distribution observed along the coast during the


26.

457

Radiochemical Dating of Nuclear Dehns

K R U G E R

southward movement of the front. Figure 5 shows the latitudinal distribution of 6-hourly precipitation amounts relative to the time of the frontal passage. FRONTAL POSITION

CRESCENT CITY ARCATA


FORT BRAGG BODEGA BAY

PESCADERO PACIFIC GROVE PIEDRAS BLANCAS VANDENBERG GOLETA LOS ANGELES LA JOLLA NOV.

15

NOV.

16

NOV. 17

Figure 5. Time section of 6-hourly precipitation amount and frontal position along the California coast for Nov. 15-17,1966 The isohyets show that precipitation rates in excess of 30 mm. per six hours fell i n northern California near the time of the frontal passage, but as the front accelerated southward through central California the precipitation amounts decreased substantially. Thus, south of Vandenberg, where the jet stream was observed to lose its strength, the frontal precipitation was mostly i n form of drizzle and light intermittent rain. Precipitation collection was initiated at 1200 PST, November 15 and continued i n 6-hour increments thereafter. F r o m Monterey northward the collection was terminated at 1800 P S T , November 16, but collections continued through southern California until 1200 P S T , November 17. The rainfall and radiochemical data for this storm are given in Table III. Storm of Jan. 20-23, 1967. The synoptic meteorological situation leading to this storm period was similar to that for the Nov. 15-17, 1966 storm. The large scale pressure distribution was again characterized by a blocking high over northwestern Canada and another strong high i n the central Pacific just east of the International Date Line. A deep low pressure center developed i n the Gulf of Alaska on January 19. T h e


458

R A D I O N U C L I D E S IN

Table III.

T H E ENVIRONMENT

Rainfall Collection Storm of November

Sr

89

Collection Data Site, Times Crescent C i t y 15:1200-15:1800 15:1800-15:2400 16:0000-16:0600

Cone,

Dep., Precip., mm. d.p.m./m.

2

Cone,

Dep.

d.p.m./I. d.p.m./m.

2

d.p.m./

—

518 90 237

7.5 5.1 80.9

50 334

5.4 13.1

45 182

4.8 7.1

—

—

—

—

12.9 15.8 0.2

178 487

13.7 30.8

6.5

—

—

84 L 40

Bodega B a y 15:1200-15:1800 15:1800-15:2400 16:0000-16:0600

5.8 7.2 3.2

152 272

26.1 37.8

—

—

Pescadero 15:1200-15:1800 15:1800-15:2400 16:0000-16:0600

10.9 20.0 4.6

310 457 117

28.4

Pacific Grove 15:1200-15:1800 15:1800-15:2400 16:0000-16:0600

3.9 7.1 8.7

Piedras Blancas 15:1200-15:1800 15:1800-15:2400 16:0000-16:0600

0.05 3.1 6.6

69.0 17.5 2.9

998 214

14.4 12.2

—

9.3 25.5

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