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Optimisation of gamma assay techniques for the standard quality checking of nuclear waste packages and samples

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European Commission
Community Research
Project report
Nuclear Science and Technology
Optimisation of Gamma Assay Techniques for the
Standard Quality Checking of Nuclear Waste
Packages and Samples
EURATOM EUR 19127 EN EUROPEAN COMMISSION
DG Research/D. 11.3 - R & Τ programme 'Nuclear fission safety 1994-98'
Contad: Mr G.A. Cottone
Address: European Commission, rue de la Loi/Wetstraat 200 (MO 75 5/43),
B-1049 Brussels - Tel. (32-2) 29-5)589; fox (32-2) 29-54991 European Commission
Optimisation of Gamma Assay Techniques for the
Standard Quality Checking of Nuclear Waste
Packages and Samples
' 'H-J. SANDEN, G. GASPARY, FZJ (DE)
2'M. BRUGGEMAN, SCK-CEN (BE)
3'L van VELZEN, NRG (NL)
41 A. LEWIS, NNC/WQCL (UK)
5 'G. PINA, CIEMAT (ES)
6 'C. VICINI, ANPA (IT)
7 'R. REMETTWJniv. Rome (IT)
8'T. BÜCHERLJUM/RCM (DE)
Contract No FI4W-CT96-0036
FINAL REPORT
Work performed as part of the European Atomic Energy Community's R&T Specific Programme
"Nuclear Fission Safety 1994-1998"
Area C: Radioactive Waste Management and Disposal and Decommissioning
Directorate-Genera I
Science, Research and Development
1999 EUR19127EN LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information
A great deal of additional information on the European Union is available on the Internet
It can be accessed through the Europa server (http://europa.eu.int)
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1 999
ISBN 92-828-8186-5
© European Communities, 1999
Reproduction is authorized provided the source is acknowledged
Printed in Belgium
PRINTED ON WHITE CHLORINE-FREE PAPER TABLE OF CONTENTS
1 Introduction 1
1.1 Background and Motivation
1.2 Objectives: Improvement of'conventional'gamma scanning 2
2 Basic Gamma Scanning Techniques 3
2.1 Determination of activity inventories by conventional gamma scanning
('volumetric scanning')
2.2 Attenuation correction by transmission measurement 6
2.3 Determination of activity inventories by the method of 'Swivel Scanning' 9
. 2.4n of activitys with the SRWGA system (ANPA) 11
3 Planned Working Programme 15
3.1 Structure of the work programme and description of main work areas 1
3.1.1 Definition Phase
3.1.2 System design, modelling and performance calculations (Area A) 17
3.1.3 System set-up for improved gamma scanning techniques on intermediate and
high density waste (Area B)
3.1.4 Testing and performance assessment of improved scanning techniques (Area
C) 18
3.1.5 Results and Conclusions9
3.2 Scheduled work programme breakdown 20
4 Work Performed and Results Achieved1
4.1 Modelling Software SOLIDANG
4.2 Parameter Studies and Performance Calculations with SOLIDANG 25
4.2.1 Validation of SOLIDANG by NRG 2
4.2.2 Performance Calculations and Experimental Validation by WQCL 28
4.3 Development of correction procedures 30
4.3.1 Angular Scanning (AS)1
4.3.2 Swivel scanning (SWS)8
4.3.3 Transmission corrected segmented gamma scanning (TC-SGS) 44
4.4 System Layout for Transmission Corrected Segmented Gamma
Scanning (TC-SGS) 49
111 4.5 Set-up of prototype systems 54
4.5.1 Set-Up of the GERNOD II system at FZJ
4.5.2 The SRWGA System (ANPA)7
4.5.3 System layout and set-up of a prototype system by TUM 59
4.6 Proficiency tests of prototype systems 6
4.6.1 Testing of the GERNOD II system at FZJ
4.6.2 Calibration and Proficiency testing of Eu-152 transmission source 68
4.7 Design and Fabrication of Test Drums 71
4.8 Specification of Benchmark Test Procedures5
4.9 Measurements on 'mock-up' drums and Performance Assessment of
improved scanning techniques7
4.9.1 Test measurements on 'mock-up' drums with the GERNOD II system 7
4.9.2 Test Measurements performed by TUM 89
4.9.3 Measurements on 'mock-up' drums by ANPA 96
4.10s on real waste packages and Performance assessment 108
4.10.1 Field Measurements and Performance Assessment at FZJ 108
4.10.2 Fields performed by ANPA 120
4.10.3 'Field" measurementsd by TUM/RCM4
5 Summary of Results and Achievements8
5.1 List of Deliverables generated within the Project 129
5.1.1 Summary of Milestones 13
5.1.2y of Reports issued1
5.2 Technology Implementation Plan (TIP)4
5.3 Conclusions of partners5
5.4 Recommendations for the Application of the Improved Scanning
Techniques 142
6 References
Key Words / List of abbreviations8
Annex: List of Final Progress Reports of Participating Partners
IV INTRODUCTION
1.1 Background and Motivation
Conditioned radioactive waste has to meet the specifications and acceptance criteria defined by
national regulatory and management authorities. Conventional gamma scanning is one of the most
widely used nondestructive testing method for quality checking of radioactive waste drums in
routine operation /PAR77/. Its range of application covers raw waste prior to conditioning as well
as conditioned waste prior to disposal or interim storage.
The accuracy and reliability of the results of conventional gamma scanning measurements depends
strongly on the fulfilment of the assumption that the gamma attenuation in the waste matrix can l;c
corrected properly. Such corrections are usually performed on the basis of 'a priori' knowledge >f
the attenuation properties of the waste or by simply weighing the drum and calculating the average
density. These assumptions are only fulfilled for well defined waste packages. In the case of
heterogeneous activity or matrix distributions uncertainties will be introduced that can lead to
significant errors in the activity determination especially for waste with higher matrix density or
large inhomogeneities.
Standard gamma scanning techniques /FIL89-1/ together with advanced destructive testing
methods /EIF98/ have been developed in many European Quality Checking facilities for more than
10 years. For example, from the experience gained at FZJ Jülich by applying these techniques in
extensive waste inspection programmes for national authorities the following rough classification
could be derived by analysing more than 1000 conditioned waste drums (produced between 1975
and 1988):
Partly Significant
Fairly homogenous inhomogeneous inhomogeneous
activity and density activity and/or activity and/or density
distribution density distribution distribution
Application of application of
application of standard SGS advanced methods
standard SGS possible only with (radiography,
possible additional correction tomography) or
techniques destructive testing
necessary
Conditioned waste from
nuclear power plants
25% 60% 15%
produced in Germany
fromca. 1975 to 1988
Table 1.1: Classification of ca. 1000 conditioned waste drums due to the experience at FZJ
From this experience it is evident that one of the major problems of waste characterisation by
nondestructive means in practice is related to the unknown distribution of the matrix material and
the radioactive substances in the drum.
To optimise conventional gamma scanning it is therefore necessary to estimate and to reduce these
sources of bias as far as possible.
1 1.2 Objectives: Improvement of 'conventional' gamma scanning
The scope of the work programme was focused on the optimisation of nondestructive gamma
scanning techniques applied to the quality checking of radioactive waste packages. The
improvement of the performance is based on the following two methods:
Attenuation correction by transmission measurements •
('transmission corrected segmented gamma scanning' - TC-SGS).
Correction for non homogeneous distribution of activity by appropriate scanning •
techniques
(swivel scanning and angular scanning).
The method of attenuation correction is based on simple and fast transmission measurements
through the object at selected heights with an 'external transmission source'. This correction
technique has been already developed for fissile material in raw waste. It has been proved that the
method leads to significantly improved accuracy for the determination of activity inventories in
heterogeneous waste matrices of low density /PAR77/. The research work included all necessary
steps to extend the application of attenuation correction techniques for the characterisation of
intermediate and medium density waste (0.4 to 2.2 g/cm^).
The developed methods will decrease systematic uncertainties and bias, caused by unknown
variations of the activity and matrix distributions. The results of the work programme will be of
interest for nearly all applications where nondestructive gamma assay systems are used to measure
volumetric radioactive waste packages.
Furthermore the intense collaboration between the respective testing facilities in the development
of improved gamma scanning techniques contributes to the harmonisation and standardisation of
nondestructive testing methods used within the European Community. This will lead to more
reliable results that can be exchanged among the individual testing laboratories. The
standardisation of gamma measurement methods will also improve the quality of the
documentation of waste packages which are conditioned and inspected in foreign installations and
shipped back to the waste producers. BASIC GAMMA SCANNING TECHNIQUES
2.1 Determination of activity inventories by conventional gamma
scanning ('volumetric scanning')
One standard procedure1 which is used routinely for the determination of radioactive inventories is
conventional gamma scanning of waste drums.
Measurement of the activity of certain key-nuclides in a waste drum can be achieved by the
method of gamma-scanning /PAR77/. This method usually involves a collimated HPGe-detector
moving along the drum axis, while the drum itself rotates on a turn table (Fig. 2.1). For some
specific applications even simpler versions exist where no detector movement takes place and no
collimator is used. The standard measurement time is 30-120 minutes for one scan.
measured
volume V
collimated
Ge-detector
Fig. 2.1 : Sketch of one 'standard' method of gamma-scanning
The activity inventory can be calculated for all detected nuclides from the evaluation of the
integral gamma-spectrum with sufficient accuracy if the activity and the waste are distributed
uniformly throughout the drum. For homogeneous fillings the following relation between the
measured net peak count rate Ζ and the activity concentration a = A/m of a volumetric source
holds /FIL89-1/:
>-l
with Z: measured count rate Ζ = const .(l-r). Ü - (i.i) A: activity of one nuclide
m Τ : gamma transmission
Τ = exp (-μα)
m : mass of the matrix
μ/ρ: mass attenuation
Usually the factors μ/ρ and Τ are estimated on the basis coefficient
of weighing the drum and making assumptions about(orknowing) the composition of the matrix.
1 In the following, those scanning techniques where the attenuation correction for gamma rays is based primarily
on 'a priori' assumptions on the distribution of matrix and/or activity in the waste package (in many cases
homogeneity of matrix and activity distribution is assumed) will be called 'conventional or standard'
techniques. Problems associated with standard nondestructive gamma scanning
For real waste packages the above conditions (homogeneous distribution of activity and matrix
inside the drum) are only fulfilled in rare cases (see table 1.1). As a consequence, the results
obtained by gamma scanning can be affected by large, in general unknown and possibly non-
conservative, errors due to the following effects or difficulties:
- non-uniform activity distribution,
- inhomogeneous matrix composition and distribution.
- internal shielding structures of unknown design.
Figure 2.2 illustrates one of the simple effects which occur in standard volumetric gamma scanning
of a drum with unknown filling height, if only the overall spectrum is analysed under the
assumption of homogeneity of matrix and activity distribution.
A (conevntional) /
Activity measured by conventional scanning / True activity in pellet
A (SGS)
- SpiralscarvSGR
filling height of drum [cm]
Fig. 2.2: Influence of filling height on Performance of Standard Volume Scanning
(μ/ρ=0,12 cm2/g, M=200 kg)
A more systematic and detailed presentation of the influence of varying distributions of activity
and density is shown in figure 2.3 for a 'two pellet drum' configuration. The following parameters
have been fixed for this example: