130
Health Physics
Table 2. Estimatedeffective half-lives (y) of '*’Cs ordered (top to
bottom) by increasing annual average rainfall.
Estimated effective
Atoll or island
half-life (y)
Utirik
Rongelap Atoll: Rongelap Island
Rongerik Atoll: Eniwetak Island
Ailinginae
Taka
Ailuk
Mejit
Wotho
Jemo
Erikub
Ujelang
Wotje
Likiep
Kwajalein (northern half of atoll)
Ujae
Kwajalein (southern half of atoll)
Lae
Maloelap
Lib
Aur
Namu
Alinglapalap
Majuro
Arno
Jaluit
Knox
Namorik
Kili
Ebon
Mili
16
16
16
20
15
15
14
15
14
14
20
14
15
14
14
14
14
14
13
14
13
13
12
12
12
11
11
12
12
11
effective half-life of 12 y is still somewhat smaller than
the values we estimate for the northern atolls, the
Robisonet al. (2003) estimates are only for a few sites on
Bikini and Enewetak Atolls. Our analyses, based not
only on '°’Cs to “Sr ratios, but also on a comparison of
'’Cs measured in soils years apart, indicated that the
degree of environmental loss varied considerably from
island to island on each atoll, as well as from atoll to
atoll, depending on the depth of the soil layer and type of
vegetation.
The effective half-life generally decreases with increasing average annual rainfall rates, as is to be expected
because of precipitation-driven downward migration of the
radioactivity in the soil. We estimate that the uncertainty in
our estimates of effective half-life ranges from +1 to +2 y
in the northern Marshall Islands, where the estimated
half-lives are based on '*’Cs to ”’Sr soil measurements and
comparisonsof '*’Csinventories in different years, to +2
to +3 y in the southern Marshall Islands, where the
estimates are based primarily on relative precipitation
rates. While the environmental half-life strongly depends
on long-term precipitation rates, it is not solely a function
of average annualrainfall since the depth of the soil layer
varies among islands, depending on their size and stage
of vegetative development. Our estimated uncertainty in
August 2010, Volume 99, Number 2
the effective half-life for '*’Cs in soils of the northern
Marshall Islands is based on the variations in the loss
rates calculated from '°*’Cs to *’Sr ratios at atolls with
similar rainfall. We assigned a larger uncertainty to the
effective half-life at southern atolls where '*’Cs to ”’Sr
ratios were not measured and the estimate of effective
half-life was based on extrapolation. Average precipita-
tion rates in the Marshall Islands are knownto increase
from north to south with some references suggesting they
level off just south of Majuro, consistent with the
observed remaining inventories of '*’Cs in soil at atolls
south of Majuro.
Influence of fractionation
The radionuclides created during the explosion of a
nuclear device are usually classified as either refractory
(R) or volatile (V) according to whether their melting
point is higher or lower than 1,500°C (Hicks 1982). For
example, isotopes of iodine and cesium are volatile and
isotopes of zirconium and cerium are refractory.
The mixture of radionuclides in the fallout cloud can
change with time after the test (due to factors other than
radioactive decay) as part of a phenomenon termed
fractionation. A radionuclide mix is fractionated when
the activity ratio of the refractory to the volatile radionuclides in deposited fallout differs from what would be
expected from the composition in the initial debris cloud.
Fractionation is due to the tendency of refractory radionuclides to be distributed throughout fallout particles and
volatile nuclides to be distributed preferentially on the
surface of particles. This phenomenongivesthe ratio of
the refractory to the volatile radionuclides a dependence
on particle size, with smaller particles, because of their
relatively larger surface to volumeratio, typically enriched in volatile nuclides and larger particles typically
enriched in refractory nuclides. Large particles, 1.e., those
greater than ~50 um in diameter, deposit quickly because of their higher gravitational settling velocity. They
also tend to be enriched in refractory nuclides because
those nuclides have higher volatilization temperatures
and condense more quickly as the fireball cools. Conversely, the volatile nuclides have lower volatilization
temperatures and tend to remain gaseous longer and are
adsorbed onto smaller particles that remain aloft longer.
The so-called “unfractionated” radionuclide composition varies only slightly from test to test according to
the fissionable material and construction characteristics
of the nuclear device. Fractionation is very sensitive to
the explosive yield, type of soil, height of burst and other
factors (Freiling 1961, 1962, 1963; Freiling et al. 1965).
On average, the unfractionated activity ratio of the
refractory to the volatile radionuclides in fallout is about
1.6 at H+12.