2. 1
Introduction
The USS Independence was a World War II aircraft carrier used as a target vessel for the 1946 atomic bomb
test at Bikini Atoll. Following the irradiation, the vessel was used for study in San Francisco to train
personnel in radiological decontamination. In 1951, the vessel was retired into the deep sea with 55-gallon
drums of low level nuclear waste. In August 2016, a group of scientists embarked on a mission to rediscover
the content of the vessel. This involved more than 40 hours of survey along with video and photographic
documentation as well as sponge sampling to assess the condition of the wreck and its surroundings. The
elemental composition of the sponge holds information pertaining to the radiation levels surrounding the
USS Independence.
Initial dosimeter readings on the sponge samples upon acquisition did not find readings significantly above
ambient levels. However, the ROV did not make any direct contact with the vessel and the dosimeter does
not provide energy information. These readings would not allow us to confirm the lack of radioactive
contamination.
The goal of these measurements is to survey the surrounding area of the USS Independence to determine
radiation levels. Levels of contamination indicate whether or not the shipwreck is a threat to marine life
and if any measurements should be taken to clean up the wreck. This will be determined by collecting
sponge samples from the vessel and measuring them for certain radioisotopes. Measurements deemed
significant will be compared to previous measurements on various other types of samples, including fish
and kelp
Procedures
Seven sponge samples were collected from the
Independence; the locations that these were
gathered from are shown in Figure 1. The locations
were determined by the proximity to a point on the
vessel that had the most damage caused by the
weapons test. Sponges were then selected based on
how removable they were from their location.
Goiter and Barrel sponges were acquired for
measurement and an additional two sponges (one of
each) were collected from Pioneer Canyon to serve
as reference. The sponges collected from Pioneer
Canyon are “clean” since they were not exposed to
radiation from nuclear weapons testing and can
therefore serve as the reference sponges.
Figure 1: Approximate locations of the sponge
samples on the Independence. Multiple sponges
were collected in some of these locations.
The sponges collected are then baked in an oven to
remove the water content. Water content in the
samples cause attenuation in the emitted gamma
3. 1
radiation and therefore causes inaccurate readings in
the measurements. Once the sponges are fully dried,
they are weighed and placed inside a Marinelli
beaker made specifically for Germanium detectors.
Figure 2: A processes sponge sample inside the
Marinelli Beaker.
Each sponge sample is placed on a High Purity
Germanium (HPGe) detector located inside a lead
cave that consists of lead bricks and copper slabs
(for structural support). The purpose of the lead
cave is to prevent as much background radiation as
possible from entering the Germanium detector
while a sample is being measured. The setup of the
lead cave is located in Figure 3. Individual samples
are measured for 24 hours each.
Figure 3: Sponge sample mounted onto an opened
HPGe detector. After the sample is placed, the
entrance is covered with lead bricks to reduce
background radiation from entering the cave.
Each measurement produces an energy spectra
similar to that of Figure 4. Energies below 100 keV
are considered to be within the x-ray region and are
therefore not taken into account in terms of
measurement since many are a product of the lead
shielding. We search for peaks that correspond to
radioisotopes that are not naturally occurring. This
means finding peaks at energies that correspond to
Cs-137 (661 keV) and Co-60 (1173 keV and 1332
keV). Other radioisotopes, such as K-40, Bi-214,
and Tl-208, are also evaluated for comparison to
other samples.
After all samples are measured, a background
spectrum is taken in order to quantify the ambient
gamma radiation in the laboratory. The background
count-rate in each energy of interest is subtracted
from the measurement count-rates, and the
concentration of each isotope is calculated along
with a 95% confidence interval. Concentrations that
are lower than the confidence interval are
considered insignificant and listed as a detection
limit.
4. Figure 4: Energy spectra for NA077-015 with labeled peaks. Cs-137 was not found in this sample. It must
be noted that the Co-60 peaks are part of the laboratory background. Calculations are done using the peaks
and energies corresponding to Cs-137, Co-60, K-40, Bi-214, and Tl-208.
Results The results gathered from each sponge
measurement are located in Table 1.
Table 1: Concentration levels of select radioisotopes for processed sponge samples. The top-most sample
represents an average of four measurements. Detection limits are placed for each sponge in order to
determine what concentrations of the isotopes must be present in order to consider the concentration
significant.
Sample Location Type Mass
(g)
Cs-137
(Bq/kg-dry)
Co-60
(Bq/kg-dry)
K-40
(Bq/kg-dry)
Bi-214
(Bq/kg-dry)
Tl-208
(Bq/kg-dry)
NA077-
012/13 (Avg.)
Independence Goiter 50.8 < 0.23 < 27 383 (2) 6.5 (6) 1.0 (1)
NA077-014 Independence Barrel 65.1 < 18 < 18 390 (2) 10.5 (5) 4.7 (3)
NA077-015 Independence Barrel 31.0 < 0.37 < 38 141 (2) 4.2 (1.0) < 0.68
NA077-016 Independence Barrel 13.1 < 0.87 < 109 238 (5) 18 (2) < 1.6
NA077-017 Independence Barrel 25.9 < 0.44 < 106 113 (3) 8 (1) 2.4 (8)
NA077-018 Independence Barrel 17.9 < 0.63 < 63 169 (4) < 1.6 8.6 (1.2)
NA077-019 Independence Barrel 19.8 < 0.59 < 150 218 (4) 4.8 (1.5) < 1.1
5. 1
NA0777-024 Pioneer
Canyon
Goiter 21.9 < 0.52 < 108 184 (3) 11.1 (1.4) 6.0 (1.0)
NA077-033 Pioneer
Canyon
Barrel 5.9 < 1.9 < 429 222 (11) < 4.9 < 3.6
Table 1 shows there are no significant quantities
of Co-60 in any of the sponge samples. Detection
limits indicate what concentrations would have to
be in order to produce a significant measurement.
The reason for this is the fact that it has been
approximately 65 years since the USS
Independence was sunk to the depths of the ocean
in 1951. The half-life of Co-60 is only 5.3 years
and thus any present amount of Co-60 in 1951 has
gone through roughly 12 half-lives and has
therefore decayed away.
Bismuth and Thallium concentrations were found
to be relatively consistent across all samples with
the exception of slightly higher bismuth
concentrations in NA077-016. Tl-208 is part of
the Uranium-238 decay chain whereas Bismuth is
part of the Thorium-232 decay chain. Both of
these isotopes (Uranium and Thorium), are
present in the Earth’s crust and soil. Therefore,
these decay chain products are most likely
present in sediments that have accumulated in the
sponge samples.
Potassium was found to have the highest
concentrations out of all the measured isotopes.
Potassium-40 is a naturally occurring isotope
with an isotopic abundance of 0.0117%. Most
sponges are filter feeders and tend to gain
nutrients from scraps and sediments that sink
down to the ocean floor. This includes pieces of
fish which are rich in potassium. Comparisons
made with the other measurements are shown in
Figure 5.
Figure 5: Potassium concentrations for sponges
and three other samples. The sponge
concentration represents an average and the
error bars represent the range of concentrations
over the nine sponge measurements. The
potassium concentration, with the given range, is
similar to that of the fish samples.
Only one sponge was measured to have a
significant quantity of Cs-137. Comparisons to
other measurements are shown in the figure
below. The amount of Caesium present in the
sponge is higher than the amount present in a kelp
sample acquired from the Pacific Grove but
similar to that of a Petrale Sole sample bought
from a local fish market. Although the sponge
was collected near the blast area on the
independence, it is not the sponge collected
0
50
100
150
200
250
300
350
400
450
Sponge
Avg.
Pacific
Blue Fin
Tuna
Petrale
Sole
Banana
K-40(Bq/kg-dry)
6. 1
nearest to the blast (this would be NA077-
012/13). This suggests that these sponge samples
arrived on the ship at different times; the sponge
with the significant Caesium content must have
arrived much earlier.
Since the USS Independence was not struck
directly during Operation Crossroads, the
Caesium in the sponge sample most likely
originated from the Low Level Waste stored
inside the vessel. It may also be noted that since
only one out of seven acquired sponges yielded
significant levels of Cs-137, radioactivity from
the waste has decayed to relatively low levels.
Figure 6: Caesium-137 comparison between
NA077-014 and three other measurements. The
sponge concentration is most similar to that of the
Petrale Sole.
Conclusion
Although one sample contains a significant
amount of Cs-137, the concentration is not much
different than that of the Caesium concentration
in a fish from a local market. In addition, no
sponge sample contains a significant amount of
Co-60.
In conclusion, the results demonstrate that the
vessel holds no significant threat since the
obtained radiation levels are not significantly
different from ambient levels.
0
1
2
3
4
5
6
7
8
9
Cs-137(Bq/kg-dry)
