The unique nature of the burial area for contaminated material known as the Aomon Crypt called for
special sampling designs. Because the boundary of the buried material was known only in general,
the entire area was core sampled on a 5 m grid in two-foot increments to a maximum depth of
28 feet. The sampling data were used to estimate horizontal and vertical boundaries of the region
with TRU activity exceeding 400 pCi/g. After the soil removal was complete but before backfilling,
samples were collected of the material at the bottom of the exeavation, which had filled with
water. Soil samples were collected and IMP measurements were taken to characterize the area
after backfilling. Details on the sampling and excision methods used for the cleanup of the Aomon
Crypt are in Section 6.8.
In preparation for recontouring of the PACE area on the island Sally, the soil that was to be used as
fill was sampled to estimate the TRU activity. Subsurface sampling methods were used, with slight
modifications to take into account the proposed depth for the fill. In several cases, elevated TRU
activity was found on the surface.
Extra IMP measurements were taken and handheld instruments
were used to verify that the higher activity was confined to a small area and was within acceptable
limits.
5.2.6 Influence of Unique Project Aspects
Detector Field-of-View. There were a number of distinctive and unusual aspects in the Enewetak
Cleanup project that had to be taken into consideration when choosing statistical methods. One of
the most important of these considerations was the field-of-view of the in situ detector.
Even
though the detector is collimated, the detector response does not drop to zero at the nominal angle
of the collimator. The cutoff angle at which gammascease to enter the crystal is approximately 60°
for the 60 keV gamma ray from 241am. One eonsequence of the lack of a clear "edge" of the
field-of-view is that its diameter could reasonably be defined as anything from 21 to 26 m with the
detector at full height (7.4 m). About 95 percent of the total activity detected originates in a circle
of diameter 21 m, so that could be considered the "field-of-view."
On the other hand, about 99
percent of the detected activity comes from a 25 m circle. Thus 25 mis also a reasonable value for
the diameter of the detector field-of-view. (See Section 3.2.8.)
The sampling plan for surface soil samples was designed using a diameter of 21 m for the
field-of-view. Initially, the pattern of the subsamples (see Procedure No. 4) was chosen so that
different areas in the detector field-of-view were soil sampled with approximately the same
probability as that in which radiation in the same areas will be detected by the in situ detector.
However, the design was based on incorrect information about how the detector response changes as
a function of angle, so that the composites overrepresent the center of the field-of-view. Because
the primary purpose of the surface soil sampling was to obtain estimates of the ratio of TRU to
Am, which is not affected by this error, the sampling design was not corrected. However, the
statistical analyses comparing IMP data and soil sample data were adversely affected, because this
error makesit moredifficult to identify a real difference.
The field-of-view of the detector is also a factor in selecting methods for estimating area averages.
The kriging programs used numerical integration methods which were based on the assumption that
the data were point values, or at least represented a small proportion of the total area. This
assumption was valid for data at 50 m or larger spacing, but not for 25 m data. At 25 m spacing,
adjacent detector fields-of-view actually overlap, although the commonarea represents only a small
fraction (less than one percent) of the total activity detected. Thus it would not have been properto
use kriging on 25 m data, while the arithmetic mean of adjacent data values is a good estimate of
the area average. The arithmetic mean was used for all cleanup boundary estimates, post-cleanup
characterization, and certification estimates involving 25 m data.
Field Limitation. Another important set of considerations in performing statistical analyses was the
limitations and difficulties inherent in a field project such as the cleanup. For example, the IMP
system could only measure a limited numberof points each day and the laboratory could only process
a certain number of samples at a time. Also, although the lab had a wide range of analytical
capabilities, it was not equipped for some types of analyses, and could only handle a few samples for
some other types. In light of these limitations, it was important to use methods that made the best
possible use of the amount and type of data available.
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