atw 2019-03

atw Vol. 64 (2019) | Issue 3 ı March
DECOMMISSIONING AND WASTE MANAGEMENT 166
E [keV] Center Drum wall
activity for a given gamma line which
can be detected with a high degree of
certainty in the gamma scan. In ASGS,
the evaluation of the decision threshold
and the detection limit is performed
based on the individual spectra
and not on the averaged gamma
spectrum as in conventional SGS.
When a ‘hot spot’ of localized activity
is present in the drum, the evaluation
of the characteristic limits applied in
SGS then becomes invalid whereas
the variation in the count rate in
different measurement positions is
accounted for in the ASGS analysis of
the measurement data. In terms of
­increasing the detection efficiency,
ASGS uses a larger aperture than the
typical collimator geometry used for
SGS which results in a photopeak
­efficiency which is by a factor 50
higher. Assuming the background
emanates from the activity within the
waste drum to be measured, the decision
threshold for detection of radionuclides
scales with the square root of
the efficiency, such that a significant
reduction by an order of magnitude
can be reached for the ASGS system as
compared to the SGS method within
the same time for the measurement.
Summary
ASGS offers a measurement method
for characterization of radioactive
waste which significantly reduces the
model uncertainty based on a spatially
resolved reconstruction. The
ASGS software is designed to permit
the automated operation of the
gamma scanning system which
includes the analysis of the data. The
dedicated ECIAD software module is
developed for the calculation of
Bottom Middle Bottom Middle
SGS 122 0.02 0.04 2.76 4.08
344 0.19 0.37 1.78 2.40
779 0.59 1.13 1.26 1.99
964 0.70 1.33 1.09 1.72
1112 0.78 1.50 0.98 1.63
1408 0.97 1.89 0.92 1.52
ASGS 122 - 1408 0.82 0.99 1.56 1.79
344 - 1408 0.85 1.03 1.16 1.41
779 - 1408 0.83 1.01 1.02 1.27
964 - 1408 0.80 0.97 1.01 1.25
1112 - 1408 0.79 0.95 1.00 1.25
122 - 779 - 1408 0.78 1.37 1.02 1.60
all Lines 0.81 1.17 1.01 1.38
| | Tab. 1.
Ratios of true to reconstructed activities for simulated point source activities located at four different
positions within the waste drum for SGS and ASGS.
­mathematical efficiencies for a partitioned
source model, the reconstruction
of spatially resolved activities,
and the uncertainty calculation. The
ECIAD software operates without
user­ guidance in an automated fashion
using a priori information on the
waste drum. With a suitable interface,
this information can be retrieved by
the software prior to the analysis from
a database. As a result, lower conservative
estimate can be reached
than in conventional gamma scanning
systems, since the spatial information
on the activity distribution is used for
the evaluation of the measurement
data. Therefore, ASGS provides a far
more accurate characterization of the
true activity which facilitates a better
use of the allowed activity limits. With
ASGS, the evaluation is performed
in a consistent manner and will be
coupled with the calculation of uncertainties
according to the current
norms and guidelines for the evaluation
of uncertainties. The evaluation
model for the activity is based on a
reconstruction algorithm which precludes
the propagation of uncertainties
using the general law of error
propagation. Therefore, the propagation
of uncertainties is calculated
­using Monte-Carlo based methods for
the determination of characteristic
limits according to the requirements
of the current guidelines. An experimental
validation of the measurement
method for various measurement
­configurations for the active matrix
compositions and density and for
different activity distributions is
planned for the near future using the
newly designed gamma scanning
system.
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Authors
M. Dürr
K. Krycki
B. Hansmann
T. Hansmann
A. Havenith
Aachen Institute for Nuclear
Training GmbH
M. Fritzsche
D. Pasler
T. Hartmann
Mirion Technologies (Canberra)
GmbH
Decommissioning and Waste Management
Advanced Sectorial Gamma Scanning for the Radiological Characterization of Radioactive Waste Packages ı M. Dürr, M. Fritzsche, K. Krycki, B. Hansmann, T. Hansmann, A. Havenith, D. Pasler and T. Hartmann