Radionuclides in the Environment

19 The Specific Activity of Nuclear Debris from Ground Surface Bursts as a Function of Particle Size

Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch019

M. W. NATHANS Trapelo Division of L F E Corp., 2030 Wright Ave., Richmond, Calif. 94804

Cloud samples from five coral surface bursts and one silicate surface burst were subjected to a size separation by means of a density gradient column. The fractions were analyzed for Sr, Pm, and uranium. One sample was also analyzed for Cs and Ce. Fraction weights were obtained by direct weighing or by matrix element analysis. Mean di ameters of the particles in the size fractions were determined from size distribution measurements by optical and electron microscopy. The specific activities are varying functions of the radioactive species. No systematic differences between the behavior of Pm and Sr are apparent as a function of yield. Uranium behaves almost identically to Sr. There is evidence of a strong decrease in the specific activity at particle sizes above 60μ. 90

137

147

144

147

90

90

C o m e years ago Freiling proposed that the specific activity (activity per ^ unit weight or volume) of individual fission products i n nuclear debris, particularly from air bursts, be expressed as an inverse power function of the particle size (2, 3). Thus: A«> = A

( i )

(r=l)-™

(1)

In this expression A is the specific activity of radionuclide i , A (r — 1) is the specific activity of nuclide i at r — 1, r is the radius of a debris particle, and m is a number between 0 and 1. A value of m — 0 implies that the activity is proportional to the volume of the particles. Such a volume dependence results for nuclides which combine with the matrix material in the cooling fireball and applies to the so-called refractory chains, such as the 95-, 11-, and 147-chains. A value of m = 1 or a surface( i )

( 1 )

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

19.

N A T H A N S

Specific Activity of Nuclear Debns

353


area dependence results for nuclides which are still in the gas phase or whose precursors are still in the gas phase when solidification is essentially complete. In this latter case, the last radioactive member or members of the chain condense on the surface of the particles, provided these end members are not volatile. Examples are the 89- and 137-chains. If both volatile and nonvolatile members of a mass chain are present during con densation and solidification, volume as w e l l as surface deposition occurs. For such cases a fractional value of m between 0 and 1 is found. Exam ples are the 90- and 140-chains. The over-all result is a fractionation of fission products to an extent which depends upon the particle size. The "radial distribution model" sketched above has been found to be a good approximation for airbursts i n the particle size range above about 3μ (1 ). No data were available for particles below 3μ i n diameter. [Recently, some strong evidence was obtained in our laboratory that the specific activities i n airburst debris increase with decreasing size below about 0.5/x]. The model is inadequate, however, for describing the specific activities of debris particles from ground surface and near-surface bursts. For such events, the underlying hypothesis certainly breaks down. In airbursts one has to consider only the condensation and subsequent solidification of material which has been i n the gas phase exclusively. In ground surface bursts a great quantity of soil material is drawn into the fireball and later into the rising cloud. Only a small portion of this material fuses, and even less vaporizes. Thus, the greater part of even the refractory chains condense on the surface of the particles only and may migrate into the particles primarily by diffusion. Thus, a more complicated size dependence of the specific activities is expected to be present in the debris from surface and near-surface bursts. The deter mination of this size dependence is important for predicting dose rates in fallout fields. Therefore, a systematic investigation was carried out to determine the size-activity relationships for several representative nuclides i n debris resulting from four coral surface bursts and one silicate surface burst. Experimental Samples from the following events were analyzed: Castle Bravo, Castle Koon, Redwing LaCrosse, Redwing Zuni, Redwing Tewa, and from Johnie Boy. Koon samples from three different altitudes were analyzed but only one of the Johnie Boy samples (842L). Additional Johnie Boy samples have been analyzed by Russell (6). Details of a l l events and samples are reported elsewhere in this volume (5), except for LaCrosse and for Tewa. LaCrosse was a coral surface burst of about 40 kilotons whose cloud topped out at 12,000 meters. The sample ana lyzed (054) was collected at 6500 meters and at 2.6 hours after the event.

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

354

RADIONUCLIDES

I N

T H E

E N V I R O N M E N T


T e w a was detonated on a barge on a shallow portion of the Bikini lagoon. Its yield was about 4.6 megatons, and its cloud topped at about 32,000 meters. The sample analyzed (501) was taken at 16,000 meters at 2.2 hours after the event. Prior to radiochemical analysis the samples were ashed and separated into size fractions by means of procedures described by Nathans et al. The determinations of the fraction weights and of the mean diameters of the particles i n the fractions have also been described extensively i n the same paper. A n aliquot of each size fraction was dissolved and subjected to a separation procedure to isolate Sr, R u , Sb, Cs, Ce, Pm, U , and P u fractions. The procedure is sketched in Figure 1. Further decontamination of R u and Ce was carried out only with the Johnie Boy sample. The Sb and P u fractions were set aside for later analysis. After complete analysis of the Cs fractions, anomalies were found in the data for the coral samples. These samples had been ashed at about 475°C. Apparently some Cs had volatilized at this temperature. Such a behavior explained the anomalies, and this was confirmed by Heft by more extensive experimentation (4). Thus, Cs data are reported only for the Johnie Boy sample, which was ashed at low temperature in a Tracerlab low temperature asher. The methods used for analyzing the activities are summarized in Table I. The > p tracer was prepared at the Oak Ridge National Laboratory by cyclotron bombardment of enriched N d with protons. The S r was counted within 24 hours of final separation so that the growing S r activity could be neglected. Most size fractions contained sufficient S r and C s activity such that the counting statistics could be kept within ± 3 % without the need for excessive counting times. The P m and C e data include a possible counting error of ± 6 % or less, except for the fractions containing the largest particles, for which the counting error was as high as ± 2 0 % . The uranium data are associated with errors not exceeding 3 % . 143

144

m

1 4 3

90

90

90

1 4 7

137

1 4 4

Specific activities were calculated by dividing the fraction weights into the counting results corrected for chemical yield. Consideration was given to the question of whether the weights and the radiochemical data were obtained from the same portion of the filter containing the particle fractions or from different portions as described by Nathans et al. Radiochemistry and matrix element analysis on the same particles were performed on the Zuni sample and on Koon samples 1086 and 7269. Thus, for these samples corrected weights based on the matrix element analyses were certainly indicated. In regard to the remaining samples, fraction weights from matrix analyses were used for Zuni, LaCrosse, and Koon sample 051 because any errors introduced by the inhomogeneity of the filter surface were deemed less than the errors in the directly measured fractions from the Bravo sample, so that for this sample the directlysults indicated that the relation between these errors was reversed i n the fractions from the Bravo sample, so that for this sample the directlymeasured fraction weights were used. Since no matrix analyses were performed on the fractions of the Johnie Boy sample, in this case again the directly measured weights were used.


19.

Specific Activity of Nuclear Oebris

N A T H A N S

355

Sr, R u , Sb, C s , C e , P m , U , P u HN0

3

HC10

4

Distill Ru Sr, Sb, C s , C e , P m , U , P u HF HC10

4

HOH Sb 0


9

4

Sr, C s , C e , P m , U , P u NH OH 4

Ce, Pm, U , Pu Sr, C s

2 ml. ION H N 0

3

KBrO , :

Sat.

(NH ) C 0 4

2

2

Sat. N H N O

4

4

s

Extract with hexone SrC Q 2

4

U , Pu

Cs

2 ml. IN HNO

Ce, Pm

H

Extract with 0 . 2 M T T A benzene

A d d L a carrier HN0

Pu U

3

9 ml. 0.35M H I O , :

NaBr0

8

ce(icy 4

Pm

Figure 1. Table I.

Flow diagram of separation procedure

Methods of Analyzing Some Radionuclides

Nuclide

Yield Tracer

Sr

Stable Sr

9 0

106 1 3 7

1 4 4

i 4 7

U

R u

Cs Ce Pm

Stable R u Stable C s Stable C e 143.144

L'33U

Pm

Chemical Form SrC0 R u metal 3

CsC10 Ce 0 Metal plated 4

2

8

on Pt disc Determined by

Activity Counted Beta Beta Gamma Beta Beta, x-ray

mass spectrometry


356

RADIONUCLIDES

Table II.

I N

T H E


Relative Specific Activities in a Bravo Cloud Sample Specific Activity Sr


Event

90

Bravo

69 57 41 24 19 15 12.3 8.1 (5) (1.5) 1.0

Tewa

41 32 25 11.8 8.2 a a a a a

m a a

Zuni

Koon 051

3.78 6.39 13.0 14.0 24.0 43.1 48.7 57.3 49.6 40.5 49.4 38.3 25.1

2.7 1.86 14.8 13.7 20.2 37.2 43.4 38.9 55.4 22.3 29.1 35.0 31.9

(90) (60) 54 43* 33 32.5 23 23 (18) 15.8

0.19 0.16 0.19 0.27 0.38 0.54 0.77 0.95 0.90 0.95

6

6

17

Ce

lu

1.33 3.56 5.18 5.08 6.77 7.15 8.98 5.46 13.50 15.02 47.07

0.80 0.79 0.99 1.16 1.46 2.03 1.79 1.66 2.33 2.47 4.27 5.58

6

Cs

1S7

1.20 1.48 1.94 1.85 2.32 2.25 3.11 3.64 4.76 4.03 5.62

58 42 36 22 18 11.5 7.5 5.4 3.1 2.4 1.2 0.58

6

Ru

106

0.67 1.10 2.49 2.24 2.87 2.71 2.71 3.92 2.03 2.26 2.55 1.05 — — 0.11 0.037 0.29 0.20 0.81 0.71 — 0.82


1.59 1.78 2.74 2.98 3.58 3.45 4.02 4.18 3.40 3.18 2.28 0.61 — 0.17 0.46 1.8 3.01 3.22 4.48 3.21 (10.4) 3.34 3.69 3.51 13.1 9.6 11.2 13.7 14.5 17.8 18.6 13.1 43.3 — 65.6 13.9 1.73 2.47 — — 2.25 — 5.12 — 6.92 —

19.

357

Specific Activity of Nuclear Oebris

N A T H A N S

Table II.

Continued Specific Activity

Sr

Event

90

14 10.7 9 8.3 (5.3) (3) 2.75 (2) 1.70 (1) 0.91 (0.6) 0.6 6

6


6

6

b

1.15 1.07 1.50 1.35 1.32 3.12 2.79 1.42 2.65 3.99 6.57 4.67 6.72

Koon 1086

16 11 1.4 0.98 0.45 0.47 0.30 0.28

42.1 124 130 161 146 126 135 95.6

Koon 7269

68 42 27 23 21 20 6.8 5.2 4.3 3.6 1.2 0.74 0.39

2.23 6.05 11.3 15.1 (11.4) (31.2) (61.8) (6.16) 49.2 72.1 63.7 80.3 79.7

(50) 30 18 (13) 9 4.8 (3) (2) (1.5) (0.9) 0.50

0.050 2.09 9.14 14.7 14.3 17.6 18.8 19.3 19.0 (11.1) (13.2)

LaCrosse

Ru

106

Cs

1S7

Ce

lu

* Pm

U

— 1.18 1.22 0.93 — 0.83 1.49 1.24 1.34 2.46 3.94 — 4.73

8.08 — 8.97 — 8.22 8.06 — 19.0 — 39.0 — 74.5 —

2

7

2.15 21.7 28.9 16.7 44.9 11.7 15.0 13.3

(24.0) (21.1) 30.5 6.41 9.60 15.0 21.4 0.015 0.58 2.57 4.25 3.44 2.94 3.14 3.45 3.45 (1.63) (1.72)


61 220 261 325 238 238 309 255 4.34 8.30 13.4 16.8 (14.5) (30.4) (70.0) (98.3) 93.7 116 76.5 98.5 124 1.83 2.35 1.71 1.05 0.88 1.02 1.08 1.11 1.14 (0.85) (0.86)

358

RADIONUCLIDES

Table II.

I N

T H E


Continued Specific Activity

Event

a," μ


Johnie Boy

Sr

120 114 102 70 46 27.5 21.5 13 9.3 5.5 3.4 (2.0) 1.1 0.62

Ru

90

106

1.89 0.27 0.11 0.11 0.085 0.11 0.15 0.21 0.23 0.25 0.49 0.76 1.35 0.58

0.045 0.10 0.12 0.36 0.33 0.40 0.59 0.68 (2.52) 0.58 0.32 1.00 1.97 0.67

Cs

1.48 0.16 0.054 0.070 0.076 0.13 0.28 0.50 0.68 0.83 1.33 1.51 1.84 0.65

U

7

» Ρτη

1S7

0.32 0.55 0.46 0.41 0.37 0.35 0.54

0.67 0.63 0.51 0.50 0.42 0.43 0.67

0.33 0.18 0.18 0.26 0.83 1.16

0.41 0.51 0.56 0.91 0.92 1.21

1.25 1.59 1.46 1.40 0.95 0.93 1.02 1.11 0.88 0.75 1.26 1.84 3.10 1.34

Not measured. * From analysis of a second portion of the same filter sample.

β

Results and

Discussion

The experimental results are shown i n Table II. The indicated d i ameters, a, are the mean diameters of the size fractions. The data show that the specific activity is generally not a simple function of particle size, confirming the composite nature of the samples. The sharp decrease of the P m specific activities i n coral burst samples toward large particle sizes is particularly significant. Systematic differ ences as a function of yield or soil type between refractory P m be havior and semivolatile S r behavior are not apparent. Uranium behaves very much like " S r . 1 4 7

1 4 7

90

O f particular importance is the evidence of strong geometric frac tionation effects i n the Koon cloud. Not only do order of magnitude differences exist between the average specific activities of samples taken from different portions of the cloud, but there are also differences i n the relation between the specific activity and the particle size in those sam ples. One cannot draw the inference, however, that similarly large effects are the rule i n clouds from ground bursts. Indeed, Russell d i d not find any significant geometric effects i n the Johnie Boy cloud. Yet our results pose strongly the question of the uniqueness of the specific activity as a function of particle size and therefore of the representative ness of any one sample. The question of representativeness is also posed by the specific activities of S r and P m relative to each other i n Bravo, for example. 90

1 4 7


19.

N A T H A N S

Specific Activity

359

Debris

Integration of the specific activities over the particle size yields a much lower amount of S r than of P m produced, even i f estimates from analyses of particles greater than ΙΟΟμ are included ( 7 ) . Such a result suggests that the sample was not representative of the true radioactive cloud distribution. Finally, it should be noted that the samples may have been altered by weathering. Oxide is at least partly converted to carbonate and hydroxide as a result of moist storage conditions. In addition, surfacedeposited activity may have been redistributed since moisture could have provided a path between particles. Such an effect would be more sig nificant for S r than for P m but is probably not very large. It was found that the 2-propanol used for the size separations was capable of leaching some activity from the particles. However, the amount of P m leached was only about 1 % ; the amount of S r was never more than about 5 % of the total, so that only a small fraction of these nuclides must be considered capable of migrating. 90

90


of Nuclear

1 4 7

1 4 7

1 4 7

90

Conclusions (1) The specific activities of radionuclides i n debris from ground surface bursts are generally complicated functions of the particle size, in accordance with the composite nature of the debris. (2) The specific activities of radionuclides in samples from surface burst debris are not necessarily unique functions of the particle size. The uniqueness is determined by the degree of mixing in the cloud, and is, therefore, expected to be a function of the time elapsed between the shot time and the sampling time. Literature Cited (1) Benson, P., Gleit, C. E., Leventhal, L., Proc. Conf., Radioactive Fallout Nucl. Weapons Tests, 2nd, Germantown, Md., 1964, 108 (1965). (2) Freiling, E. C., U. S. Naval Radiological Defense Laboratory, Rept. USNRDL-TR-680 (Sept. 12, 1963). (3) Freiling, E. C., Science 139, 1058 (1963). (4) Heft, R. E., private communication, February 1968. (5) Nathans, M. W., Thews, R., Russell, I. J., ADVAN. CHEM. SER. 93, 360 (1969). (6) Russell, I. J., Operation Sunbeam, Shot Johnie Boy, Rept., POR-2291 (WT-2291) (May 5, 1965). (7) Tompkins, R. C., Krey, P. W., Rept. WT-918 (Feb. 1956). RECEIVED February 26, 1969. Work supported by Defense Atomic Support Agency under Contract DA-49-146-XZ-484.


Radionuclides in the Environment

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