IYNC 2006
Stockholm, Sweden – Olkiluoto, Finland, 18 – 23 June 2006
Paper No. 210
Development of a Matrix Correction Method to Improve NonDestructive Assay of Fissile Mass in α-Low Level Waste Drums.
F. Loche, F. Jallu,
French Atomic Energy Commission
CEA/CAD DTN/SMTM/LMN B. 225
F-13108 SAINT PAUL LEZ DURANCE CEDEX
fanny.jallu@cea.fr
ABSTRACT
In the framework of radioactive waste control, Non Destructive Assay (NDA) methods may be
employed. The Active Neutron Interrogation (ANI) method is now well-known and effective in
quantifying low α-activity fissile masses (235U, 239Pu, 241Pu) in low density (i.e. less than about 0.4)
radioactive waste drums of up to 200 l volume.
Difficulties arise when dealing with matrices containing neutron energy moderators (for example
H) and neutron absorbents (for example Cl). These components may have a great influence on the
fissile mass deduced from the neutron signal measured by ANI. Without any information about the
matrix, the fissile mass is often overestimated, due to safety procedures, by considering the most
disadvantageous calibration coefficient corresponding to the most absorbing and moderating
calibration matrix.
The work presented in this paper is performed at the Nuclear Measurement Laboratory, CEA,
France. It concerns the development of a matrix effects correction method which consists in identifying
and quantifying the matrix components by using the prompt gamma-rays following a neutron radiative
capture. The goal is to adjust the value of the adequate calibration coefficient for ANI analysis. The
preliminary results presented are obtained for low α-activity and low density 118 l waste drums.
1
INTRODUCTION
1.1
Non Destructive Assay methods (NDA)
Nuclear Non Destructive Assay (NDA) correspond to the measurement of nuclear radiations to
identify and quantify the components of a sample. NDA methods do not alter the physical or chemical
form of the studied material.
NDA methods can be used in two modes: in the passive mode, the measured radiations are
spontaneously emitted by the radionuclides of the sample ; in the active mode, an external source of
interrogation particles (neutron, photon) is required to induce reactions inside the sample.
Two active NDA methods are of particular interest for our purpose: Active Neutron Interrogation
(ANI) for α-emitting nuclides quantification [1,2], and Prompt Gamma Neutron Activation Analysis
(PGNAA) using the potentialities of neutron capture reactions [3-5]. The first method is applied at the
Nuclear Measurement Laboratory, CEA, France, to low α-activity and low density waste drums of
volume up to 200 litres. The second one is studied to characterize the components of these matrices.
1.2
The Active Neutron Interrogation method
The ANI method can be employed to quantify fissile masses (235U,
waste drums.
210.1
239Pu, 241Pu)
in radioactive
Proceedings of the International Youth Nuclear Congress 2006
The principle of the method is the following one [2]: a pulsed 14 MeV neutron generator
produces neutrons with a given frequency and duration. Those neutrons are moderated within the
graphite of an assay system so that neutron energy falls to the thermal region (0.025 eV). Then the
thermal neutrons induce fissions in fissile nuclides. Finally, prompt and delayed neutrons resulting
from fissions are detected by using 3He proportional counters.
The neutron signal attributed to fissions (Sn) is theoretically proportional to the fissile content
(fissile mass MF) of the waste drum:
MF 
Sn
K
(1)
with K, the most adapted calibration coefficient.
This signal depends on the matrix (composition, density…) which may be composed of neutronmoderating (for example H) and neutron-absorbing (for example Cl) materials.
2
MATRIX EFFECT CORRECTION
Without any information about the matrix, and for safety reasons, the fissile mass measured by
the ANI method is often overestimated by considering the most disadvantageous calibration coefficient
corresponding to the most absorbing and moderating calibration matrix. It is thus necessary to develop
a method to estimate and correct the matrix effects.
2.1
Overview of matrix correction methods
The influence of matrix components on the neutron signal measured by ANI is a recurrent
difficulty in NDA. Many researches have thus been performed to try to develop a correction method.
The published documents containing information about the correction of matrix effects mainly
describe techniques using correction coefficients deduced from neutron measurements [6-12]. Those
are based on the use of neutron monitors placed inside of the measurement cavity: a flux monitor
sensitive to the variation of the interrogating neutron flux with the matrix type, and/or a drum monitor
sensitive to the variation of the fission neutron flux emerging from the drum (Figure 1).
Flux monitor
CH2
Neutron
generator
Drum
3He
Drum monitor
proportional
counters embedded in
CH2 shielded by Cd or
B4C
Graphite
Figure 1: Scheme of typical INA set-up.
The combination of some of those signal characteristics (value, lifetime deduced from the signal
decay…) is then studied to determine parameters relevant to the moderating and absorbing properties
of the matrix.
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The main idea is to correct the expression (1) by a correction coefficient C to estimate the fissile
mass MF according to this expression:
MF 
Sn
*C
K
(2)
with C describing the absorption and moderation effects [6-11].
The study published by Caldwell and al. in [6-8], for example, is based on passive (with a lower
252Cf source as passive neutron emitter) and active (with inner fissile samples) measurements of
different assay matrices. The neutron signal is detected by 3He proportional counters shielded, or not,
by cadmium. A flux monitor and a drum monitor are added. The different signals are studied to identify
two parameters describing the absorption and the moderation properties of the matrix. Two correction
algorithms (one for passive measurements, and one for active measurements) have then been
developed.
Moreover, two correction algorithms are proposed by R.J. Estep and al. [12] in function of the
high or low position of the drum in the experimental set-up. They do not explicitly describe the
absorption and moderation effects and are based on the signal measured by 3He proportional
counters shielded by polyethylene (CH2), cadmium (Cd) and boron carbide (B4C).
Besides, D.A. Close and al. [3] suggested to identify and quantify the matrix components
through the use of (n,γ) reactions. They based their work on the use of an electron accelerator to
produce interrogating neutrons.
2.2
Potentialities of the neutron radiative capture reaction for matrix effect correction
Using a neutron generator, the matrix effects correction method described here exploits the
potentialities of the neutron radiative capture to identify and quantify the components of the matrix.
The correction is based on the principle described above (2) in which, instead of a correction
coefficient C, we chose to develop a corrected calibration coefficient K’:
MF 
2.2.1
Sn
K'
(3)
The neutron absorption
The neutron absorption is a nuclear interaction leading to a compound nucleus that can release
the excess of energy of its excited energetic level by different processes:
n 
A
Z
XN
A1
Z
X * N 1
particles : p d 



electromagnetic radiations : 
multiplica tive processes : 2n, induced

(4)
fission
where the induced fission only concerns heavy nuclei.
2.2.2
The radiative neutron capture
The interaction of interest in the framework of the correction method is the radiative capture. As
said above, the absorption of the neutron leaves the compound nucleus in an excited state. It releases
the energy by emitting prompt gamma-rays in a ‘very short time’ (10-17 - 10-16 s) according to:
n 
A
Z
XN 
A1
Z
X * N 1
 AZ1 X N 1

0
0
210.3

(5)
Proceedings of the International Youth Nuclear Congress 2006
The radiative capture cross section, directly correlated to the probability of interaction, is higher
at thermal neutron energies.
The gamma-rays emitted by radiative capture are a characteristic signature of the nucleus: they
have specific energies and intensities. This property could lead to identify the components of the
matrix and to develop the matrix effect correction method.
2.2.3
Application to the identification of the matrix components
As each isotope irradiated by neutrons emits characteristic gamma-rays, identification of the
components becomes possible: the principle is to measure, using gamma-ray spectrometry, gammarays emitted by the matrix during a neutron irradiation. This measurement is performed in a specific
moderating assay system which is built in view to increase the radiative capture probability.
The obtained gamma-ray spectrum contains total absorption peaks of the emitted gamma-ray
and a continuous background (Compton and background noise). It is a combination of elemental
spectra and is thus representative to the components of the matrix. The peaks corresponding to the
components of interest can then be identified according to their energies.
2.2.4
Application to the quantification of the matrix components
To extract information concerning the quantity of the components of interest in a matrix, the
analysis of the gamma-ray spectrum is based on the determination of the net area of the
corresponding peak (after subtraction of the background).
Indeed, the quantity of the component x (%x) and the peak area A are linked according to:
A( E ) 
% x.m
.N A .I ( E ).  ( E ).t.  ( E n ). ( E n ).dE n
MA
(6)
where:
E : Gamma-ray energy (keV)
A( E ) : Area of the peak at the energy E (count)
% x : Proportion in mass of the isotope x in the sample (%)
m : Total mass of the sample (g)
M A : Atomic mass of the isotope (g.mol-1)
N A : Avogadro number (6.02.1023 atoms.mol-1)
I ( E ) : Intensity of the gamma-ray at the energy E
 ( E ) : Detection efficiency of the detector at the energy E
t : Measurement duration (s)
En : Neutron energy (keV)
 ( En ) : Neutron capture cross section at the neutron energy En (cm2)
( E n ) : Incident neutron flux at the energy En (n.s-1.cm-2.keV-1).
Thus, for a given gamma-ray peak, a detector and a neutron flux, the variation of the proportion
of the isotope of interest is proportional to the peak area. Knowing energies and intensities of the
gamma-rays, it becomes thus possible to identify and quantify the components of a matrix.
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Proceedings of the International Youth Nuclear Congress 2006
3
DEVELOPMENT OF THE MATRIX EFFECT CORRECTION METHOD
Our preliminary PGNAA studies are limited to 118 l homogeneous low density drums (i.e.
between 0.05 and 0.4). The absorbing material studied is chlorine (Cl), and the moderation material is
hydrogen (H). Table 1 presents the main gamma-rays emitted by Cl and H.
Table 1: Gamma-rays energies, intensities and thermal neutron radiative capture
cross sections of Cl [13] and H [14]
Nucleus
Cl
Thermal neutron radiative capture
cross section (barn)
33.2
H
0.332
Energy (keV)
Intensity (%)
517.10
786.30
788.40
1164.9
1951.10
1959.40
6110.80
7414.00
2223.25
24.30
10.52
16.32
27.20
19.39
12.56
20.58
10.52
100.00
The first step consists in correction method development using calibration matrices. The
gamma-ray spectra of the calibration matrices are analysed to create graphs linking measured
parameters about the matrix to researched information about its composition.
The development of these graphs is based on an experimental and a simulation work.
3.1
Experimental studies
3.1.1
Experimental set-up
The experimental part of our study uses the REGAIN (in French, REcherche des Gamma
d’Activation et d’Irradiation Neutronique, which means, research of the gamma-rays produced by
activation and neutron irradiation) experimental set-up (Figure 2), which was specifically developed at
the laboratory. The set-up is mainly composed of graphite (neutron moderating material). The
mechanical structure is made of aluminium. Hydrogen was prohibited so as to further the detection of
the matrix hydrogen content. The door and one lateral face (on the left of Figure 2) of the set-up
possess openings to introduce gamma-ray detectors. We used a high purity germanium (HPGe)
detector covered by a cadmium shielding and placed in the opening of the door.
Figure 2: The REGAIN experimental set-up.
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Proceedings of the International Youth Nuclear Congress 2006
The neutron irradiation is obtained by a D-T neutron generator (14 MeV 95%; 2.5 MeV 5%)
emitting about 7.5.108 neutrons per second (4π). It is placed in the graphite wall on the right of
Figure 2.
3.1.2
Measurements
To study Cl and H matrix effects, calibration matrices of wood, EVA and PVC were composed.
The wood matrices are made of hollowed out cylinders, with different inner and outer diameters
relative to the following densities: 0.07 ; 0.144 ; 0.28 et 0.377. The EVA matrix is filled with leaves of
non chlorinated plastic and its density is 0.254. The PVC matrices are constituted of PVC films rolled
around cylinders of polystyrene (Figure 3). Their densities (0.05 ; 0.18 ; 0.253 ; 0.30) are obtained by
varying the thickness of the rolled PVC film.
Figure 3: PVC elements composing a calibration matrix.
A total of 9 matrices were measured in the REGAIN experimental set-up.
The gamma-ray peak areas are analysed to create a set of abacus linking measured
parameters about the matrix (the peak area of the components of interest and the density of the
matrix) to researched information about its composition (proportion of Cl and H).
3.1.3
Experimental results
Figure 4 presents an example of measured spectrum. Some of the main gamma-rays of interest
are labeled.
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Proceedings of the International Youth Nuclear Congress 2006
Counts
1E+5
1E+4
400
Cd : 808 keV
Cd : 558 keV
1E+6
600
800
1000
1200
1400
1600
1800
2000
H : 2223 keV
1E+5
1E+7
Cl : 1952-1959 keV
Graphe expérimental matrice PVC 0,05
Cl : 1164 keV
Cl : 786-788 keV
Cd : 652 keV
1E+6
2200
2400
1E+4
Cl : 6110 keV
1E+3
1E+2
1E+1
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Energy (keV)
Figure 4: Identification of the main gamma-rays of interest on the spectrum measured by
neutron irradiation of a PVC matrix.
The analysis of such spectra (i.e. extraction of the net signals) leads to the creation of the
abacus mentioned above.
Figure 5 shows the abacus relative to the matrices which does not contain chlorine (cellulose
and EVA). The chosen measured parameter is ( S H  S Ho ) Tact , where SH is the hydrogen net peak
area of the matrix spectrum, SHo is the hydrogen net peak area of the spectrum obtained without
matrix (corrected so that SH and SHo correspond to the same active measurement time T act). The
researched parameter is %H*Dr, where %H is the H weight fraction of the matrix (%) and Dr is its real
density (equal to the ratio of the real mass to the real filling level).
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Proceedings of the International Youth Nuclear Congress 2006
matrice cellulose et EVA
H déconvolué
300
250
(SH-SHo)/Tact
200
y = -12,163x2 + 122,37x
R2 = 0,9994
150
100
50
0
0
0,5
1
1,5
2
2,5
3
3,5
%H*Dr
Figure 5: Abacus for the matrices with H and without Cl.
The uncertainties on the abacus are calculated by taking into account these parameters:
- the statistical error on the determination of the peak areas (SH and SHo),
- the uncertainty on the measured real density (Dr),
- an error of 10% on the active measurement time (T act) announced by the measurement
system and deduced from the real time and dead time.
The weight fractions of calibration matrix components are considered as known.
Similar abacus concerning chlorinated matrices are under development.
3.2
Modeling studies
The aim of the modeling studies is to complete the former abacus with fictive matrices. These
modeling studies are performed in four steps.
In a first time, the REGAIN experimental set-up is described with the Monte Carlo N-Particle
calculation code, MCNP [15]. The description is as detailed as possible including the neutron
generator and the germanium detector. The walls, floor, and top of the room that houses the set-up
are also modeled. In this configuration, calculation to experiment comparisons were made to validate
the model. The lack of some gamma-rays in the MCNP data libraries (ENDFB-VI release 1) was
solved by using the ENDFB-VI release 8 libraries and by creating the gamma-ray libraries for Ge [16]
and for Cd.
In a second time, matrices are included in the model. The calculated spectra and calculated
abacus are then compared with the experimental ones to validate the matrix model .
The figure 6 shows the comparison between the two sets of data for the matrices containing no
chlorine.
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Proceedings of the International Youth Nuclear Congress 2006
matrice cellulose et EVA
H déconvolué PUBLICATION 31/01/06
500
450
400
(SH-SHo)/Tact
350
300
250
200
150
100
50
0
0
1
2
3
4
5
6
%H*Dr
Figure 6: Experimental (blue dots) calculated (pink squares) and fictive (green triangles) abacus
obtained for the matrices without chlorine.
Thirdly, those graphs are completed by modeling fictive matrices (in term of density, %Cl and
%H). The corresponding cellulose results are added on Figure 6 (green dots).
The last step is to correlate these abacus to the corrected ANI calibration coefficients.
4
CORRECTION METHODOLOGY AND EXAMPLE
4.1
Correction methodology
The methodology employed to apply the matrix effect correction method can be summarized by
the correction scheme presented on Figure 7: thanks to the gamma-ray energies and intensities, the
matrix components are identified on the measured gamma-ray spectrum and these information help
us to chose the abacus devoted to the measured drum.
For the time being, the development of the correction method allows us to study 3 cases (with
Cl and no H, with Cl and H, with H and no Cl).
The uncertainties on the final result (weight fraction of H and Cl) are calculated by taking into
account the same parameters as in § 3.1, plus the dispersion of the points on the abacus
(uncertainties on the coefficients resulting from a linear or a polynomial regression).
The connection with the fissile mass correction is under development.
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Gamma-rays identification
Presence of Cl
Yes
No
Presence of H
Presence of H
Yes
No
Yes
Abacus
Cl+H
Abacus
Cl+H for
%H=0
Abacus
H
No
Search for other
moderating or
absorbing
isotopes
Corrected calibration coefficient
Figure 7: Scheme of the matrix effect correction method.
4.2
Blind test of the abacus
A blind test was conducted to validate the correction scheme and the gamma-ray abacus. An
inactive drum (without fissile material or radiative source) was composed in this aim. Unknown
parameters are matrix density and composition.
First of all, the matrix was weighted and, considering a full drum, its calculated density is (0.114
+/-0.005) (1σ).
Then the unknown matrix was measured in the REGAIN set-up, using similar experimental
conditions to those employed in the calibration phase. The analysis of the gamma-ray spectrum
showed the absence of chlorine in the matrix. The chosen abacus is then adapted to the matrices
without chlorine and expressing (SH-SHo)/Tact as a function of %H*Dr (Figure 5).
The measurement leads to (SH-SHo)/Tact = 155 counts per second. According to the abacus
(Figure 5), the value of the parameter %H*Dr is then (1.49 +/- 0.04). This value, thus divided by the
density, leads to deduce %Hmeasured= (13.05 +/- 0.69) % (1σ).
The real composition of the matrix, given afterwards, is: 8.55 kg of polyethylene (13.4 %H),
4.25 kg of cellulose (6.5 %H) and 0.30 kg of EVA (12.2 %H), thus %Hreal = (11.1 +/- 1.9)% (1σ).
The relative difference between the estimated and the real proportion of H is about 17% but
remains within the statistical uncertainty. Furthermore, the unknown matrix appears to be
heterogeneous whereas the abacus were developed with the hypothesis of homogeneous matrices.
Considering these points, this first test leads to good results.
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5
CONCLUSION
The work presented in this paper mainly concerns the development of a matrix effect correction
method which consists in identifying and quantifying the components of a matrix by using the prompt
gamma-rays following a neutron radiative capture. The preliminary PGNAA studies are limited to 118 l
homogeneous low density drums (i.e. between 0.05 and 0.4) composed of hydrogenated and
chlorinated materials.
To study Cl and H matrix effects, calibration matrices of wood, EVA and PVC were composed
and their gamma-ray spectra were measured. The gamma-ray peak areas were analysed to create a
set of abacus linking measured parameters about the matrix (the peak area of the components of
interest and the density of the matrix) to researched information about its composition (proportion of Cl
and H).
For example, in the case of hydrogenated matrices, the weight fraction of H is determined from
the H peak area and the matrix density.
A blind test was conducted with an hydrogenated matrix to validate the created abacus and
conducted to a good agreement (~17 %) between the estimated and the real weight fraction of H.
For the time being, the abacus developed for the correction method allows us to study 3 cases
(with Cl, with Cl and H, with H). Similar abacus to the hydrogenated matrices ones are under
development for chlorinated matrices.
Finally, the correction of the fissile mass deduced from ANI by using our method will end this
study by integrating the whole set of abacus in a general correction algorithm [5].
ACKNOWLEDGEMENTS
We wish to thank Mrs. A. Lamarche, M. J-L. Ma, M. J-C. Sublet for their advice and help in the
calculation work, experimental work and nuclear data research and processing approach, respectively,
M. A. Lyoussi for his review, M. R. Brissot as PhD supervisor, and AREVA NC for its support.
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Proceedings of the International Youth Nuclear Congress 2006
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210.12