Gamma ray scanner systems for nondestructive assay of heterogeneous waste barrels Page: 9 of 16
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two accuracies and precisions is 68.5% and 0.7%, respectively. Therefore, the bias of our
IMPACT system is ~30% low. Representative three-dimensionally rendered ACT and PCT
images of the glass matrix barrel are shown in Fig. 4.
Two additional measurements were made on the glass matrix barrel to better understand the
system bias at shorter integration times. PCT measurements at 10 and 5 s ray-sum integration
times resulted in 69.4 and 69.1 g, respectively. These results agree with the 20 s PCT assay
to within 1%; Thus, for signals well above the system noise our assay results are independent
of ray-sum integration times.
2.6 Summary of LLNL Developed A&PCT systems
There are two A&PCT scanners. One is at LLNL; the other is within a mobile WIT trailer.
These systems have been used to assay a wide range of radioactive waste from 1-70 g of Pu
within matrices from combustibles to sludge. The preliminary performance of LLNL's
IMPACT scanner was determined to have an accuracy of ~70% with a precision to within a
few percent for combustible and glass matrices. To increase system throughput LLNL
completed a preliminary design for the multiple HPGe detector A&PCT system.[9]
Additional R&D is required to better understand the negative 30% bias for good statistical
data sets and to reduce the high positive bias for poor statistical data. New image
reconstruction and assay codes are near completion and hopefully will reduce these biases.
LLNL is also working on automating the isotopic analysis and determining our systematic
uncertainties, as well as determining IMPACT's MDC and its performance at the other 3
QAPP activity ranges.
3. COMBINED SGS-LOW RESOLUTION TOMO SCANNER AT INT
INT's effort is aimed at modifying an existing SGS to employ tomographic imaging
principles to assay heterogeneous waste. An SGS system was put into operation at a
Hungarian Nuclear power plant to characterize solid waste containers of low- and medium-
density matrices compacted into 200-L barrels before shipment to the waste disposal site.
Results of measurements of 144 compacted barrels, 7 segments for each, demonstrated [10]
that transmissions measured at 1275 keV (22Na) are mostly in the range 1-15% (average
density 0.65 g/cm3). Calculations and measurements also demonstrated that a very large error
can be introduced if the transmission is lower than 20% or the average density is above 0.2
g/cm3.[11,12] These high-attenuating cases become difficult when no a priori information is
available because the systematic error is not known. There are some techniques to obtain
some indication of heterogeneity, but for these cases significant improvement can only be
obtained when some imaging principles are used for measurements of unknown barrels. Even
low spatial-resolution imaging can lead to significantly smaller measurement errors. This is
demonstrated by measurement using the modular tomographic arrangement developed by
INT.[10,11,12] A cost-benefit analysis can provide the compromise between the necessary
accuracy level and cost. Lower accuracy is needed for waste management and higher accuracy
is needed for safeguards purposes. In both cases, however, monitoring of the measurement is
needed to prevent unknown errors, sometimes 2-3 orders of magnitude. These large errors are
possible under unfavorable conditions.[10]7
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Martz, H. E.; Roberson, G. P.; Decman, D. J.; Camp, D. C. & Levai, F. Gamma ray scanner systems for nondestructive assay of heterogeneous waste barrels, article, August 1, 1997; California. (https://digital.library.unt.edu/ark:/67531/metadc693863/m1/9/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.