Isotopic Characterization of Radioiodine and Radioxenon in Releases from Underground Nuclear Explosions with Various Degrees of Fractionation

Isotopic Characterization of Radioiodine and Radioxenon in Releases from Underground

Nuclear Explosions with Various Degrees of Fractionation

MARTIN B. KALINOWSKI1 and YEN-YO LIAO

1

Abstract—Both radioxenon and radioiodine are possible indi-

cators for a nuclear explosion. Therefore, they will be, together with

other relevant radionuclides, globally monitored by the International

Monitoring System in order to verify compliance with the Com-

prehensive Nuclear-Test-Ban Treaty once the treaty has entered into

force. This paper studies the temporal development of radioxenon

and radioiodine activities with two different assumptions on frac-

tionation during the release from an underground test. In the first

case, only the noble gases are released, in the second case, radioio-

dine is released as well while the precursors remain underground. For

the second case, the simulated curves of activity ratios are compared

to prompt and delayed atmospheric radioactivity releases from

underground nuclear tests at Nevada as a function of the time of

atmospheric air sampling for concentration measurements of 135I,133I and 131I. In addition, the effect of both fractionation cases on the

isotopic activity ratios is shown in the four-isotope-plot (with 135Xe,133mXe, 133Xe and 131mXe) that can be utilized for distinguishing

nuclear explosion sources from civilian releases.

Key words: Nuclear explosion, test ban, CTBT, isotope

activity ratios, radioiodine, radioxenon, fractionation, radioactivity

monitoring, source discrimination.

1. Introduction

Due to their half-lives, fission yields and decay

radiation the iodine radionuclides 135I, 133I and 131I as

well as the xenon radionuclides 135Xe, 133mXe, 133Xe

and 131mXe are relevant for detecting a nuclear

explosion (DE GEER, 2001). Though two of the latter

are metastable isomers, for convenience, this paper

refers to the entities of this quartet as the four relevant

xenon isotopes.

Xenon isotopes are the most likely observable

radioactive signatures of underground nuclear

explosions because xenon is a gas and chemically

inert. Radioiodine is volatile and can also be released

from underground nuclear explosions. Since it is less

likely to escape from deep underground than xenon,

however. it is still much more likely to be released

than its particle bound precursors in the decay chains,

and some degree of fractionation can be expected.

This fractionation influences the temporal develop-

ment of the radioxenon isotopes released from

underground into an atmospheric plume, because it

depends on the amount of its radioiodine precursor

isotopes which are present in the same plume. This

paper studies empirically the degree of fractionation

and theoretically the impact of various fractionation

scenarios on the isotopic activity ratios of radioxenon.

Atmospheric radioiodine and radioxenon are

monitored on a daily basis at the radionuclide stations

(see, e.g., KALINOWSKI and SCHULZE, 2002) of the

International Monitoring System (IMS) that is cur-

rently being established (see, e.g., HOFFMANN et al.,

1999; KALINOWSKI 2006). Though it may be released

in gaseous form, during its transport through the

atmosphere iodine tends to undergo to some degree

chemical reactions and subsequently to attach itself to

aerosol particles. It will be collected on particle filters

at the 80 radionuclide stations of the IMS network.

For radioxenon, suitable sensors will be installed at

40 of these stations with the option of a later

expansion to all 80 sites of the radionuclide particle

monitoring network.

A previous paper described the temporal devel-

opment of prompt and delayed atmospheric

radioactivity releases from underground nuclear

tests at the Nevada Test Site in the United States

(KALINOWSKI, 2011). A subset of this data is used in

this paper to investigate the degree of fraction-

ation that occurred with the reported releases of

1 Carl Friedrich von Weizsacker Center for Science and

Peace Research (ZNF), Beim Schlump 83, 20144 Hamburg, Ger-

many. E-mail: [email protected]

Pure Appl. Geophys.

� 2012 Springer Basel AG

DOI 10.1007/s00024-012-0580-7 Pure and Applied Geophysics

radioiodine at that site. Other previous papers pre-

sented the proof of principle (KALINOWSKI and PISTNER,

2006) and a demonstration (KALINOWSKI et al., 2010)

of a robust method for discriminating between the

source being a nuclear explosion or a civilian reactor

source. The method is based on isotopic activity ratios

of radioxenon in the atmosphere. This paper builds on

those findings and explores the impact of various

fractionation scenarios on the radioxenon isotopic

activity ratios. A companion paper explores the

capability to use activity ratios of radioiodine iso-

topes for source characterization (KALINOWSKI et al.

2012).

In the first section of this paper, the simulations of

various nuclear explosion types and fractionation

scenarios are presented. The second section uses the

empirical release data of underground nuclear tests at

the Nevada Test Site to explore the degree of frac-

tionation by comparing their isotopic activity ratios

with those of the simulations. The third section shows

the effect of fractionation on the radioxenon isotopic

activity ratios for source discrimination.

2. Simulated Radioxenon Isotopic Activities Under

Different Fractionation Assumptions

A nuclear explosion takes place in a very short time

where the nuclear chain reaction and possibly fusion

reactions occur on a time scale of a few millionth of a

second and less. In this short instant, the initial feed of

hundreds of fission products is formed and radioactive

decay dictates the subsequent activity changes with

time. Three different scenarios are studied here with

regard to the combination of fissile material used and

the neutron energy spectrum. Fission of 235U and 239Pu

is simulated at fission neutron energies using the

Bateman equations (BATEMAN, 1910) implemented in

Matlab (KALINOWSKI and PISTNER 2006; LIAO, 2011).

The independent fission yields and the decay branch-

ing fractions are taken from ENGLAND and RIDER

(1994). These were adopted for ENDF/B-VI. Differ-

ences for figures in other libraries are most significant

for the short lived pre-cursors. One day after the fission

process ended, the uncertainty originating from the

library values is negligible for the purpose of this

paper.

The development over time for simulated

radioxenon and radioiodine isotopic activity ratios

with regard to different nuclear explosion scenarios

has been published before (KALINOWSKI, 2011). In the

totally unfractionated case, all fission products are

staying together, perhaps in an underground cav-

ity, such that in-growth from precursors is fully

allowed.

Fractionation changes the isotopic activity

ratios. The temporal development of radioiodine

activities with various fractionation scenarios after

a nuclear explosion is plotted in the companion

paper (KALINOWSKI et al., 2012). For radioxenon, two

different fractionation types are considered in this

paper. In the standard type, radioxenon is fully

removed from all its precursors immediately, or at a

later time, and from then on decays without further

in-growth from the decay chain. The alternative

fractionation type considered here assumes that ra-

dioxenon and radioiodine are fully removed from all

other precursors together and at the same time. The

different impacts of these two fractionation types on

the development over time of radioxenon activities

can be seen in Fig. 1 using 235U with fission energy

neutrons as example. The activity in TBq refers to a

1 kT explosion. The resulting curves for isotopic

activity ratios are presented together with data from

the Nevada Test Site in the following section.

As can be seen in Fig. 1, there is a strong impact

of fractionation that is most pronounced for early

separation of the volatiles from the non-volatile pre-

cursors. The unfractionated case is represented by the

upper curve while the simulated fractionations start at

various delay times (0.01, 0.1, 1, 10 and 100 h) from

that line and stay always below it. The impact gets

less with increased delay between time zero and the

separation time and eventually vanishes at a delay

time related to the relevant half-lives. The impact is

of course strongest for radioxenon separating from

the precursors alone and less pronounced for xenon

and iodine separating together. This can be noted in

Fig. 1b by the lower curves all bending upwards

before going down due to decay in contrast to Fig. 1a.

For 131mXe, the maximum activity can be reduced by

almost five orders of magnitude if separated alone

from all precursors and by more than three orders of

magnitude if iodine and xenon are separated together.

M. B. Kalinowski, Y.-Y. Liao Pure Appl. Geophys.

However, the fractionation of iodine and xenon

together has no significant impact on the radioxenon

activities, if it happens later than 1 h after the

explosion. For xenon separating alone, the impact is

still strong after 10 h and for 131mXe even after

100 h.

Figure 1Different impacts of two fractionation types on the development over time of radioxenon activities using 235U with fission energy neutrons as

an example. a Radioxenon is removed from all precursors, b radioxenon and radioiodine are removed together. The activity in TBq refers to a

1 kT explosion

Isotopic Characterization of Radioiodine and Radioxenon

The parent of the isomer 131mXe, 131I, has a half-

life of 8.02 days. The other isotopes have shorter-

lived precursors with a maximum of 2.19 days

(133mXe) for 133Xe, 20.8 h (133I) for 133mXe and

6.61 h (135I) for 135Xe. Due to the long half-life of its

precursor, the presence of 131mXe in the set of

quantified isotopes provides a good capability for

distinguishing different types of sources by separat-

ing them in a multi-isotopic-ratio plot (KALINOWSKI

et al., 2010).

3. Empirical Radioiodine Release Data

of Underground Nuclear Tests

SCHOENGOLD et al. (1996) report detailed atmo-

spheric radioactivity release information for 433

nuclear tests conducted on the Nevada Test Site

(NTS) from 15 September 1961 through 23 Septem-

ber 1992. An analysis of these data can be found in

KALINOWSKI (2011). Only for 45 of the 433 releases,

two or more radioiodine isotopes were reported. 131I

and 133I were measured in all of them except for one

case each. Only in 24 of the 45 cases the third rele-

vant radioiodine isotope (135I) was measured.

Information for all these releases is collected in

Table 1. Both particulate and gaseous radioiodine

were measured and Table 1 gives the sum of the

activities collected on glass fiber filters and in char-

coal cartridges.

Twelve of the reported releases of radioiodine were

uncontrolled ventings following very quickly after the

explosion but with no more than 1 h delay. Five inci-

dents of measuring radioiodine resulted from cratering

explosions that set the material free without delay. In

six cases, these were operational releases.1 At least

38 h after the explosion and with passing the releases

through a particulate air and charcoal filter combina-

tion, a further 22 cases were operational releases

without filtering occurring between five and 312 h

after the explosion. The duration of uncontrolled

releases is very short, up to half an hour. For opera-

tional releases with filtering, the releases are reported

to last between 10 min and 114 h, without filtering

between 6 and 130 h.

For some tests, more data are available than

reported in SCHOENGOLD et al. (1996). For one of the

prominent examples, the Sedan test conducted on 7

June 1962, PLACAK (1963) contains atmospheric

concentrations observed at several stations. All 18

samples with two or three radioiodine isotopes mea-

sured in the charcoal cartridges are collected in

Table 2.

Measurement uncertainties are reported neither

in SCHOENGOLD et al. (1996) nor in PLACAK (1963)

but all data is given with two significant digits. The

largest uncertainty is related to the time for which

the reported values are valid. The passage of a

plume is often not matching the sampling periods

and uncertainties in fixing a time to which the

measurement is corrected bears uncertainties due

to the change of atmospheric concentrations during

plume passage.

For the operational releases, the sampling times

are identical to the release periods reported in

SCHOENGOLD et al. (1996). For the ventings and cra-

tering tests, the sampling times are different from the

reported release delay and duration. For these tests,

beginning and ending of gas sampling at the sites hit

by the radioactive cloud are gathered from the rele-

vant documents issued by USPHS (U. S. Public

Health Service) and EPA (Environmental Protection

Agency). With this information and the known time

of the explosion, the delay and duration of the sam-

pling period during which the cloud passed were

calculated. These times are plotted in Figs. 3, 4, 5

together with the associated radioiodine ratio values.

The length of the time bar marks the sampling

duration. The reported activity is normally extrapo-

lated to the middle of the sampling period and in a

few cases to the time of the cloud passage. Figure 2

demonstrates this for the example of the test called

Pampas as part of Operation Nougat conducted on 1

March 1962. The 9.5 kt charge was detonated at

11:10 PST. The Gunderson’s Ranch station began

sampling at 12:45 and ended at 17:50 at the same day

(USPHS, 1964). The delay of sampling start can be

calculated to be 2 h 35 min (marked with A in the

1 Operational releases result from purging of tunnels or

sometimes shafts to minimize the exposure to personnel, from drill-

back operations to recover samples for diagnostic purposes, from

gas sampling or from sealing the drill hole with a plug and

cementing it to the surface (SCHOENGOLD et al., 1996).


Table 1

The table lists all historical releases of radioiodine from nuclear explosions at the Nevada Test Site between 1961 and 1992 as reported by

SCHOENGOLD et al. (1996)

Days Month Years Test name Depth (m) Delay of

sampling

start (h)

Sampling

duration (h)

I-131 (Ci) I-133 (Ci) I-135 (Ci) Type Yield

(kt)

15 9 1961 Antler 400 2.52 4.32 4.2 102 450 1 2.6

22 12 1961 Feather 250 7.79 0.79 – 27 94 1 0.15

1 3 1962 Pampas 360 4.125 2.54 0.012 0.29 0.47 1 9.5

5 3 1962 Danny Boy 34 2.542 2.46 73,000 290,000 470,000 4 0.43

14 4 1962 Platte 190 4.25 1.42 11.4 75 120 1 1.85

13 6 1962 Des Moines 200 2.25 2.92 33,000 38,0000 5,400,000 1 2.9

6 7 1962 Sedan 195 3.03 3.58 8,80,000 2,400,000 11,000,000 4 100

11 7 1962 Johnnie Boy 1 11.75 2 70,000 130,000 320,000 4 0.5

19 10 1962 Bandicoot 240 28.25 0.083 9,000 – 73,000 1 12.5

14 11 1963 Anchovy 260 24 6 2.5 11 350 3 Low

23 1 1964 Oconto 265 38 26 0.001 0.0005 – 2 10.5

13 3 1964 Pike 114 7.17 0 360 13,000 – 1 \20

19 8 1964 Alva 166 85 129.6 0.041 0.055 – 3 4.4

5 12 1964 Crepe 220 217.75 38.4 1 1 – 3 3.4

18 2 1965 Wishbone 179 20 216 1.3 15 26 3 \20

3 3 1965 Wagtail 750 168 6 0.03 0.02 – 3 20–200

20 3 1965 Suede 143 31.37 15.5 0.1 1.6 0.89 3 \20

26 3 1965 Cup 537 79.27 89 0.3 0.7 – 3 20–200

5 4 1965 Kestrel 447 112.58 16.5 0.029 0.09 0.00029 3 \20

14 4 1965 Palanquin 85 11.02 0 9,06,000 3,500,000 7100000 4 4.3

21 5 1965 Tweed 281 96.87 120 0.014 0.0068 – 3 \20

23 7 1965 Bronze 531 150.17 6 0.23 0.23 – 3 20–200

21 8 1965 Ticking 208 47.75 16.25 0.16 0.13 – 3 \20

27 8 1965 Centaur 172 45.5 1.5 0.0022 0.0065 – 3 \20

12 11 1965 Sepia 241 96 6 0.0011 0.0035 0.0001 3 \20

18 1 1966 Sienna 275 46.42 6 0.0016 0.0056 0.0002 3 \20

3 2 1966 Plaid II 270 52.5 74.4 0.019 0.076 – 3 \20

23 4 1966 Fenton 167 5.08 115.2 0.056 2.4 7.3 3 1.4

12 9 1966 Derringer 255 0.83 47.1 1.5 41 152 1 7.8

5 11 1966 Simms 198 48.33 47.7 0.009 0.018 – 3 \20

25 10 1967 Cognac 301 24 6 0.0049 0.027 0.003 3 \20

26 1 1968 Cabriolet 52 2.33 4.66 6,000 90,000 – 4 2.3

12 3 1968 Buggy a-e 41 1.55 5.13 22,000 200,000 970,000 1 5.4

27 8 1968 Diana Moon 242 5 13 0.1 2.1 3.6 1 \20

12 9 1969 Minute Steak 264 0.083 3.92 0.05 3.4 34 1 \20

18 12 1970 Baneberry 278 0.5 259 80,400 1,200,000 – 1 10

25 7 1972 Atarque 294 144 6 1.7E - 06 1.4E - 06 – 3 \20

9 8 1972 Cebolla 287 312 48 2.4E - 07 2.4E - 08 – 3 \20

25 4 1973 Velarde 277 48 6 0.027 0.14 0.016 3 \20

14 8 1974 Puye 430 168 6 2.1E - 06 8.6E - 07 2.5E - 09 2 \20

3 8 1983 Laban 326 48 0.8 0.000011 0.000025 – 2 \20

22 3 1986 Glencoe 610 120 6 8.9E - 06 9.6E - 06 – 3 29

21 5 1986 Panamint 480 55 0.0617 0.0001 0.0009 – 2 \20

13 5 1988 Schellbourne 463 56.9 113.5 0.000032 0.00011 – 2 20–150

10 3 1990 Metropolis 469 48 48 0.000088 0.00019 – 2 20–150

Only those tests are included for which at least two radioiodine isotopes were measured. The activities include both particulate and gaseous

radioiodine. A dash (–) means that the concentration was below the detection limit. The types of the releases are noted with the following

numbers: type 1 uncontrolled rapid venting, type 2 operational release with filtering, type 3 operational release without filtering, type 4

cratering


figure) after the explosion and the sampling duration

is 5 h 5 min (marked with B).2

4. Further Comparisons of Observed and Simulated

Data

The simulated radioiodine isotopic activity ratios

are compared to the values of reported releases from

nuclear tests at the Nevada Test Site. Figure 3 applies

for the ratio between 135I and 133I, Fig. 4 for 135I and131I and Fig. 5 for 133I and 131I. The data are picked

from SCHOENGOLD et al. (1996) to show all releases

from Nevada tests in the three plots labeled a) and

from PLACAK (1963) to show all data from the Sedan

test only in the three plots labeled b). The simulations

use 235U (a) and 239Pu (b) with fission energy neu-

trons and assume complete fractionation at various

time steps.

In each figure, the unfractionated case is repre-

sented by the lower curve while the simulated

fractionations start at various delay times (0.001,

0.01, 0.1, and 1 h) from that line and stay always

above it, except for some cases at separation times in

the order of minutes and shorter. The simulated

curves range over many orders of magnitude and

show the largest spread in the time frame between an

hour and a day after the explosion. However, the

dynamic range is dominated by early fractionations.

Obviously, a separation between radioiodine and its

precursors later than 1 h after the explosion has little

impact on the isotopic activity ratio and remains close

to the unfractionated case.

Most of these data in the plots of Figs. 3a, 4, 5a

(Nevada releases as reported by SCHOENGOLD et al.,

1996) lie at the lower end of the simulated curves

indicating either no fractionation of radioiodine from

its precursors at all or a fractionation later than 0.1 h

after the explosion. However, some data are found

outside the range of the theoretical curves, most of

them show a lower than expected isotopic activity

ratio. In general, the agreement of low lying data

points is better with the simulation for 239Pu and this

Table 2

The table lists all observations of radioiodine from the cratering nuclear explosion with a yield of 100 kt TNT equivalent that was called

Sedan and conducted at a depth of 1 5 m on 6 July 1962

Delay of sampling

start (h)

Sampling

duration (h)

Time activities

apply to (h)

I-131

(llCi/m3)

I-133

(llCi/m3)

I-135

(llCi/m3)

0 6.62 3.03 260 13,000 60,000

6.75 3.52 8.367 28 780 –

0 21.83 3.033 7.2 240 500

21.83 23.167 33.42 11 170 –

45 24 57 7.1 34 –

0 21.5 9.5 0.27 4.4 –

21 24.5 33.25 19 860 –

0 10 10 5.2 250 –

10 12 10 70 2,400 –

0 11 3.83 0.22 4.2 –

0 26.25 12.17 6.5 220 510

0 21 9 – 5.0 6.8

3.25 24 5.5 49 200 –

1.083 15.67 2.25 75 3,900 9,300

23.5 7.167 27.083 10 260 –

0 20 4.3 12 79 –

20 24 32 6.9 190 –

0.67 24 12.67 0.23 3.3

Only those measurements are included which are based on iodine collected on charcoal (gaseous fraction) and for which at least two

radioiodine isotopes were quantified as reported by PLACAK (1963). A dash (–) means that the concentration was below the detection limit

2 The data point used in Fig. 2 was deliberately selected to fit

well with the simulation in order to demonstrate how delay and

duration are graphically represented.


is reasonable because most underground tests in

Nevada were dominated by plutonium fission. The

assumption of 235U being the fission material catches

a few data points that lie above the highest simulation

line for 239Pu. Only two single outliers on the high

end of activity ratios cannot be understood by the

simulations. For the test called ‘‘Anchovy’’ on 14

November 1963 at 8 o0clock local time, the 135I to133I activity ratio is as high as the simulation is only

within a few hours after the test explosion (see

Fig. 3a, highest data point at the 1-day delay line).

However, for the delay reported, it is one order of

magnitude higher than the case of instantaneous

fractionation immediately after the time of the

explosion. Even though the time of the release is not

reported by the hour, it definitely happened on the

calendar day after the test, i.e., with a 16-h delay for

the unrealistic assumption that the drill-back opera-

tion was conducted right at midnight. More likely, the

delay was 24 h. For the Puye test on 14 August 1974,

the activity ratio (see Fig. 3a, lowest data point at the

1-week delay line) could be understood, if the drill-

back took place 5 days after the test instead of the

reported delay of 7 days. Two further data have their

x mark (for the time the decay corrected concentra-

tion is valid) to the right of the simulation curves

while the respective sampling periods start left to the

curves, i.e., earlier. A possible explanation might be a

too large delay reported for the plume passage.

On the low end of activity ratios, the deviations of

reported data from the simulation curves cannot be

considered as spurious outliers and need to be

explained. The picture is less serious for the 135I to133I activity ratio (5 out of 27), a little worse for the135I to 131I activity ratio (6 out of 26) and most pro-

nounced for the 133I to 131I activity ratio (14 out

of 43).

The likely explanation for isotopic activity ratios

lying below the theoretical curves are inconsistencies

in the measurements of the different isotopes. The

activity of 131I is often reported as the sum of the

activity on the filter and in the charcoal. However,

due to the measurement method (gross-beta counting

over several days after allowing the short-lived

background to decay) and because of their short

half-lives, the activity of 135I and 133I on the filter is

in most cases below the detection limit at count

time. The activity on the charcoal is measured by

gamma spectroscopy soon after the sampling ended.

Accordingly, only the gaseous part of their activity is

reported, and the atmospheric concentrations of 135I

and 133I are underestimated. As result, their isotopic

Figure 2Model and experimental (x) isotopic activity ratio between 135I and 131I for the 1962 test ‘‘Pampas’’ in a shaft at Nevada Test Site. A is the

delay between the explosion time and the start of the sampling period. B is the duration of the sampling period


Figure 3Comparison of simulated and reported radioiodine isotopic activity ratios between 135I and 133I released from nuclear tests at the Nevada Test

Site. The plot a shows all releases as reported by SCHOENGOLD et al. (1996), the b part applies to all observations made in the wake of the Sedan

test and reported by PLACAK (1963). The simulations use 235U (a) and 239Pu (b) with fission energy neutrons and assumes complete

fractionation at various time steps. The b part (Sedan test) is based on gaseous radioiodine only. For venting and cratering tests, the horizontal

bar indicates the sampling duration, for controlled releases it marks the reported duration of the release (see also Table 1). The X is put at the

time to which the reported activities apply to. These are normally calculated for the mid-point of the collection time but apply to the cloud

passage time, if that was determined


activity ratios with respect to the 131I are shifted

towards too low values.

In order to test this hypothesis and investigate the

isotopic activity ratios without any disturbance by

inconsistent measurements, the plots in Figs. 3b to 5b

display data for the gaseous fraction of radioiodine

only that is trapped on charcoal. All are measure-

ments taken in wake of the Sedan explosion on 6 July

Figure 4Same as in Fig. 3 but for the radioiodine isotopic activity ratios between 135I and 131I


1962 as reported by PLACAK (1963). Only the sample

with the highest concentration is shown in the plot a)

as well. This is the sample taken at Diablo, Nevada,

including the plume cloud passage 3 h after the

explosion. For this sample SCHOENGOLD et al. (1996)

reported a 135I/131I activity ratio for total radioiodine

Figure 5Same as in Fig. 3 but for the radioiodine isotopic activity ratios between 133I and 131I


of 12.5. This is entered in plot of Fig. 4a and lies

below the ensemble of simulated lines. If for both

isotopes only the gaseous fraction is counted as

reported by PLACAK (1963), the activity ratio is 231

and the entry is found within the range of the theo-

retical curves as seen in the plot of Fig. 4b. For the133I/131I the same correction would change the value

from 2.7 (SCHOENGOLD et al., 1996) to 50 according to

PLACAK (1963). This would shift the low entry in

Fig. 5 into the simulated range.

These examples confirm the hypothesis that

inconsistent activity measurements for the different

isotopes could explain why isotopic ratios are found

too low in comparison to the simulated curves. In fact,

the data in the plots of Figs. 3b and 4b are all found

close to or within the range of theoretical lines. In the

plot of Fig. 5b, the majority of data points lie within

the simulation curves, but again two entries are below.

What is more striking however is the fact that the

isotopic ratios are spreading over one order of mag-

nitude even for delay times differing by only 3 h.

Even for samples taken at the same site and at the

same time, the activity ratios differ by a factor of up to

3.3. It is unlikely that any physical or chemical pro-

cess in the cloud causes these differences. Apparently,

the activity measurements are not as reliable as indi-

cated by a two digit reporting precision.

A further strange finding is that the plume has

passed the most distant station (Ely, Nevada) 10 h

after the explosion, but 7 out of 10 data points in

Fig. 5b are determined for times with a delay beyond

10 h and up to 57 h after the explosion. This obser-

vation at Ely is a bit doubtful, because it was a

cratering test that releases most of its activity

instantaneously. Even stranger is a detection of

radioiodine at Lockes, Nevada, determined for about

7 h before the explosion time. These shifted delays

between explosion and detection hint at the possi-

bility that the associations of activity observations to

certain times bear some uncertainties and may in

some instances be off by a few hours for early

detections (before one day has passed) and by a day

for later detections (with delays of several days).

Except for the explained deviations and discussed

outliers, the overall agreement between the mea-

surements at the Nevada Test Site and the model is

reasonable. Most data lie within the ensemble of

simulated curves and their major trends are also

followed by the observational data. These agreements

give confidence in further conclusions drawn from

comparing the reported data and the theoretical cal-

culations. The spread of radioiodine activity ratios of

the observational data centers close to the two lowest

simulation curves, the unfractionated case and the

line that indicates a fractionation time of 1 h after the

explosion. Almost all data are consistent with a

fractionation no earlier than 0.1 h after the explosion.

Accordingly, the radioiodine isotopes and their pre-

cursors remained basically non-fractionated before

the releases took place.

5. Separation of Radioxenon and Radioiodine

in Operational Releases at the Nevada Test

Site

Figure 6 shows the change over time of four

different isotopic activity ratios between radioxenon

and its precursor radioiodine. All ratios are chosen for

the isotopes and isomers that are relevant for nuclear

explosion monitoring and are in the same decay chain

even if no data are available for comparison. The

reported Nevada test release data are shown together

with the theoretical curves for three nuclear explosion

scenarios (the different combinations of fissile iso-

tope and neutron energy spectrum introduced in

section 1) and no fractionation is assumed. Figure 7

shows the same as in Fig. 6 but only for 235U and

fission neutron energy with fractionation of xenon

and radioiodine together from all other precursors at

delays of 0.001, 0.01, 0.1 and 1 h after the explosion.

Again, the effect of fractionation can be seen with the

curve ending its further build-up through the decay

chain and start decaying according to the effective

half-life of the respective radioxenon to radioiodine

activity ratio.

The theoretical curves in Figs. 6 and 7 do not

exhibit a wide spread, in particular not in the time-

frame for data of emissions at the Nevada Test Site

that are available. All data are found below the

simulation curves spreading over many orders of

magnitude. This indicates that radioiodine is in most

cases strongly depleted in the air samples compared

to radioxenon. This depletion does not result from the


Figure 6Change over time of four different isotopic activity ratios between radioxenon and its precursor radioiodine. The simulations with three

nuclear explosion scenarios are compared to reported Nevada test releases. No fractionation is assumed

Figure 7The plots show the same as in Fig. 6 but only for 235U and with fractionation of radioxenon and radioiodine together from the other precursors


common fractionation of iodine and xenon. It is due

to a fractionation between radioxenon and all its

precursors including radioiodine. Partly this may take

place on the way from the explosion cavern to the

surface; partly it can be explained by the filtering of

air prior to its operational release. The data points for

these filtered releases (noted as type 3 in Table 1) are

found to be at least two orders of magnitude lower

than all other entries for the masses 135 and 133 with

the metastable state of radioxenon. For the activity

ratio of 133I and 133Xe, the operational releases with

and without filtering are mixed but most of the fil-

tered releases have a stronger depletion in radioiodine

than most of the unfiltered releases. The strong effi-

ciency of the filtering procedure is obvious with

reduction of the radioiodine content by two to eight

orders of magnitude compared to the theoretical

values.

It remains to be understood why the unfiltered

releases are depleted in radioiodine by one to four

orders of magnitude. The previous section and

Figs. 3, 4, 5 reveal that the radioiodine isotopes and

their precursors remained basically non-fractionated

before the releases took place. This is consistent with

the result of KALINOWSKI (2011) according to which

no significant fractionation between the radioxenon

isotopes and the precursors occurs prior to release on

any of the relevant pathways for operational releases.

The depletion in radioiodine is most likely due to the

passing of the release through a ventilation system

with a filter without this being explicitly reported.

This would imply that the distinction made here

between filtered and unfiltered operational releases is

not strictly applicable. The clear distinction visible in

the figures and described above gives evidence that at

least a difference in the extent of filtering exists.

There are two less plausible explanations for the

depletion in radioiodine which both cannot be veri-

fied from the available data and information. Either

the released radioiodine is not fully accessible to the

measurement or the gas stream that is released by

some kind of operation at the test site is passing

through a natural filtering material prior to the release

and keeping part of the iodine back while fully

releasing xenon.

Since xenon is a noble gas, its release from

an underground explosion depends only on the

availability of an unplugged path from the explosion

cavity to the surface. For iodine, the mobility depends

on its presence in the air stream at the moment a path

opens up. It may be present either in gaseous form or

attached to aerosol particles. Iodine that has con-

densed on or been adsorbed on non-mobile surfaces

will remain underground. On its way out, aerosol

particles may deposit on surfaces and gaseous iodine

may be partially held back by adsorption on surfaces

and by chemical reactions transforming it into a non-

volatile compound. The former is particularly of

relevance, if the air stream happens to be filtered

through a soil column or is purposely pumped

through a particulate filter and activated charcoal

before being released to the atmosphere. According

to CLEMENT et al. (2007) adsorption of iodine has a

significant impact on its volatility and gaseous iodine

can react with radiolysis products of humid air to

form non-volatile oxides or iodate ions.

The conclusion from the investigation of activity

ratios of radioiodine and radioxenon of the same mass

number is that practically for every operational

release radioiodine was to some extent removed from

the gas stream and less radioiodine was released than

the theoretically predicted activity based on the ra-

dioxenon activity.

6. Effect of Fractionation on Source Discrimination

Based on Radioxenon Isotopic Activity Ratios

A robust method for source discrimination has

been demonstrated earlier based on the relationship

of two different isotopic activity ratios (KALINOWSKI

et al., 2010). This requires three or four radioxenon

isotopes to be quantified. In some cases the method

works with the detection limit by substituting a

missing concentration value, if certain isotopes are

not detected. Hence, it is possible to use this method

if only two isotopes are detected. A special advantage

of this new method is its independence on the time

periods elapsed between generation and release as

well as between release and detection, i.e., it is

invariant to dilution and decay.

In Fig. 8, this discrimination method is shown for

the case that all four radioxenon isotopes are mea-

sured and made use of. Two isotopes are used on the


abscissa; the other two are taken for the activity

ratio on the ordinate. The simulation curves entered

here separate the plot area into two distinctive

domains. The trajectories for three operational cycles

of a nuclear reactor follow a circular pattern in

the left half of the plot (KALINOWSKI and PISTNER,

2006). The simulation curves for nuclear explosion

scenarios remain in the right half of the plane. In

order to illustrate the power of this source discrimi-

nation method, the observational data of the

International Noble Gas Experiment (INGE) are

shown in the same plot as used in the similar plot by

KALINOWSKI et al. (2010). This demonstrates that the

method is robust against the typical measurement

errors and a separation line can be defined that leaves

most observations of ambient air in the reactor

domain.

The change in the isotopic ratios is shown for

complete fractionation involving only the indepen-

dent yields (pure radioxenon curve) and without any

fractionation allowing for in-growth (unfractionated

decay chain curve). Depending on the amount of

fractionation of the radioxenon isotopes from their

parents, the actual ratios will fall between the two or

precisely on one of the lines.

Figure 9 shows the same as Fig. 8 but with the

assumption that radioxenon and radioiodine are

together separated from the precursors.

In each plot, a separation line is drawn between

the reactor and the explosion domain. Any isotopic

activity ratio combination on the separation line

moves down along that line with the radioactive

decay. Any isotopic activity ratio combination off

that line remains on the same side of the line forever.

Radioactive decay changes the isotopic activity ratios

along a line that is parallel to the separation line.

In Fig. 9, the left lines bounding possible isotopic

activity ratios of the three nuclear explosion scenarios

remain the same as in Fig. 8. These are valid for the

case without any fractionation. Accounting for radi-

oiodine separating from the non-volatile precursors

together with radioxenon affects all other lines, in

Figure 8Time-invariant source discrimination based on radioxenon isotopic activity ratio relationship with reactor emission data for the case that all

four isotopes are measured (KALINOWSKI et al., 2010). The dashed line marks the time-invariant screening separation. All isotope ratio relations

found above (i.e., left to) this line can be screened out, i.e., the related samples are definitely irrelevant for CTBT verification purposes,

because they cannot be explained by a nuclear explosion. All samples that have ratio dependencies found below (i.e., right to) the line might

be relevant for CTBT monitoring purposes. The exact location of the separation line is subject to further studies. The expected isotopic ratios

present at t = 0 are denoted with the ‘‘closed circle’’ symbol, and different lines styles are used to show the change in the isotopic ratios for

the different fractionation scenarios. Time steps of the first four full days are marked on these lines with ‘‘?’’ symbols. For 235U, the daily time

steps for 1–10 days of delay after the explosion are marked with thin solid lines to indicate were other fractionation scenarios would be at the

daily time steps


particular the position of the right boundary lines.

They are bent towards the separation line in Fig. 8.

The conclusion of this comparison is that no kind of

fractionation compromises the clear separation of the

nuclear explosive domain from emissions of nuclear

reactors.

7. Conclusions and Implications for Monitoring

for Nuclear Explosions

This paper studies the temporal development of

radioxenon and radioiodine activities with two dif-

ferent assumptions on fractionation. In the first case

only the noble gases are released, in the second case

radioiodine is released as well while the precursors

remain underground. For the second case, the simu-

lated curves of activity ratios are compared to prompt

and delayed atmospheric radioactivity releases from

underground nuclear tests at Nevada as a function of

the time of atmospheric air sampling for concentra-

tion measurements of 135I, 133I and 131I. Most of the

data from the Nevada Test Site lie at the lower end of

the simulated curves indicating either no fraction-

ation of radioiodine from its precursors at all or a

fractionation not earlier than 0.1 h and more likely

later than 1 h after the explosion. Accordingly, the

radioiodine isotopes and their precursors remained

basically non-fractionated before the releases took

place.

The conclusion from the investigation of activity

ratios of radioiodine and radioxenon of the same mass

number is that practically for every operational

release iodine was to some extend removed from the

gas stream and less radioiodine was released than the

theoretically predicted activity based on the radiox-

enon activity.

The final conclusion is that no kind of fraction-

ation can compromise the clear separation of the

nuclear explosive domain from emissions of nuclear

reactors that can be used for source discrimination

based on the relationship of two different radioxenon

isotope activity ratios.

Acknowledgments

This work was funded by the German Foundation for

Peace Research (DSF) and the University of Ham-

burg. The authors are grateful to the two reviewers

for their questions and suggestions that helped to

improve on the manuscript.

Figure 9This plot shows the same as in Fig. 8 but with the assumption that radioxenon and radioiodine are together separated from the precursors


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Documents

Isotopic Characterization of Radioiodine and Radioxenon in Releases from Underground Nuclear Explosions with Various Degrees of Fractionation