Some techniques applied for plutonium measurements in waste drums
B. Autrusson, J.L. Dufour, P. Funk, T. Lambert, N. Pépin, B. Thaurel
Institut de Protection et de Sûreté Nucléaire,
BP 6, 92265 Fontenay-aux-Roses, France
1. Introduction
In the framework of the French domestic
safeguards, nuclear materials such as plutonium
are controlled within France by the Institut de
Protection et de Sûreté Nucléaire on behalf of
the Ministry for Industry. This control, that
concerns any kind of material containing
plutonium, is also performed for waste produced
by operators.
Nuclear materials in waste drums may be in
relatively weak quantities. The localization of the
plutonium in the drum as well as the matrix
composition are unknown. Analysis methods
may also be disrupted by the presence of
"polluting" elements that emit neutrons or
photons (curium, neptunium, americium etc.).
The detection limit of the measurement devices
of as well as the uncertainties on the announced
result are then raised.
For that reason, the French domestic
safeguards attach a particular importance for the
characterization of waste: bad estimations of the
quantities of nuclear materials would result in a
loss of control on these materials.
Operators have to know the quantity and the
quality of the plutonium contained in the waste
they produce and shall declare these values to
the domestic safeguards. In practice, operators
calibrate their measurement devices, passive
neutron
measurement
and
gamma
measurement, using reference material: the
packaging, the matrix and the quantity of nuclear
materials are perfectly known for their
installations. These declared values are
controlled periodically, on a random basis, by
inspectors from IPSN who use their own
equipment to perform measurements. The
operator measurement method is not usable by
IPSN inspectors during inspections: it would be
necessary to have a reference material for every
installation. Subsequently, IPSN has developed
original and complementary methods for that
purposes.
This paper presents the measurement
devices (PLUM and FUNE) and the methodology
used by IPSN inspectors for the control of
plutonium in waste drums.
2. Quantification of the plutonium in
waste drum: FUNE [1]
The device FUNE (picture 1) was specifically
developed by IPSN by using an original
analytical method, explained hereafter. It is a
passive neutron coincidence counting device
with 28 helium 3 tubes. The whole system is
designed to be easily dismantled and to be
transported for on site measurements.
Calibration curves have been established to
relate the neutron coincidence counting to the
equivalent mass of plutonium 240, according to
the absorption of the matrix of the drum. For that
purpose, a correction is done by means of a
transmission measurement using an external
californium 252 source.
Therefore, a preliminary work of qualification
consisted in establishing, for each matrix of
density ρi, a calibration curve using certified
plutonium sources.
In first estimate, the data follows a relation of
the type:
m240 Pueq = aρ i . Rρ i + bρ i
aρ i ,bρ i  are the coefficients of the linear
regression relative to the calibration curve
obtained for a matrix of density ρi, (or for an
empty drum);
Rρ i
corresponds to the doublets counting
rate for a drum with a matrix density equal to ρi
(or for an empty drum).
The
above
equation
supposes
homogeneous matrix and no induced fission.
a
To achieve this study, IPSN made several
matrixes, of vinyl type, with density in the range
of 0.1 and 0.4 g/cm3. A transmission
measurement with a californium 252 source
allows the evaluation of the neutron absorption of
each matrix. With this method, the neutron
absorption correction factor is measured
experimentally, independently of the nature of
the matrix. That allows to establish equation
ρi = f(T). T is the total counting rate measured
during the transmission measurement.
Three measurements are necessary to
proceed to the quantification of the plutonium in
a waste drum:
1°) Neutron coincidence counting for the
drum (schematic 1)
2°) Transmission measurement with the
californium 252 source. For this measurement,
only half of the detectors of the chamber are
used. The drum is set between the half-crown of
detectors and the source of californium
(schematic 2 a). The measurement result, noted
T252Cf+Pu, integrates the contribution of neutrons
emitted from the californium 252 source and
those resulting from the plutonium contained in
the waste drum.
Picture 1: FUNE
Determination of the matrix density
On site, the content of a drum (metal, wood,
concrete, vinyl, etc.) is never directly accessible
to the inspectors. On the other hand, a simple
weighing leads to the evaluation of the density.
However this value does not indicate the real
neutron absorption. Indeed, neutron absorption
depends on the nature of the matrix and not only
on the density. For example, matrix constituted
with light elements are going to produce
significant neutron absorption with regard to
matrix constituted with heavy elements. How, in
these conditions, to estimate a density ρi, which
would be characteristic of neutron absorption?
3°) Measurement of the drum alone, in the
previous configuration. The result is the
contribution of the plutonium (noted TPu) in the
term T252Cf+Pu (schematic 2 b).
The difference between T252Cf+Pu and TPu,
noted T’ 252Cf, allows the calculation of the value
of the matrix density ρvinyl-equivalent using the
equation ρi = f(T).
According to the previous value of ρvinyla
calibration
of
type
240
m Pueq = aρ i . Rρ i + bρ i is chosen.
equivalent,
The domain of application of this method
imposes to have a measured value of ρvinyl3
equivalent always in the range of 0 and 0.4 g/cm .
Helium 3 neutron
Detector (28)
Polyethylene
moderator
Schematic 1: waste drum alone in the neutron chamber
TPu
T252Cf + Pu
T252Cf
(a)
(b)
Schematic 2: acquisition with and without the 252Cf
3. Quantification of the plutonium in
waste drum: PLUM
3.1. Principle of the measurement
The principle of the measurement is based on a
method, said of "infinite energy extrapolation". The
software, PUMA [2] was developed by the CEA. It
requires the use of a germanium gamma detector,
and analyzes the main gamma rays emitted by the
plutonium between 100 and 500 keV [2].
It is generally difficult to estimate within a waste
drum the corrections for self-absorption and
attenuation by the usual equations that suppose to
know the geometry of the sample. Methods used in
the device PLUM are based on the evaluation of
these corrections from countings observed on
several gamma peaks of different energies in the
region from 100 to 500 keV.
For a plutonium sample, the mass M relates
with the counting rate N(E) measured for the total
absorption peak of energy E by means of the
expression:
M=
N(E)
C(E)
ε(E) × R × Pγ (E)
ε(E) : Total absorption efficiency at the energy
E. This efficiency is determined experimentally by
a measurement at a distance X0 corresponding to
experimental conditions
R : isotopic ratio of the measured sample
(239Pu/Pu or 241Pu/Pu), according to the peak
Pγ(E) : number of photons emitted per second
and per gram of the considered isotope.
C(E) :
Self-absorption
and
attenuation
correction factor due to the nuclear material or due
to the presence of screens.
A curve giving a mass, called apparent mass,
according to the energy E of the various peaks of
total absorption of the plutonium is so obtained
(schematic 3). This curve is plotted in a graph with
the logarithm of the mass according to the
opposite of the energy. The extrapolation at the
infinite energy value (intersection of the curve with
the Y axis) gives a value that allows the calculation
of the real mass of the plutonium. Indeed, C(E)
aims towards the unity when energy aims towards
the infinity. Obtained result is valid only for a
surface mass of the sample of plutonium lower
than a value of the order of 10 g/cm2. Beyond this
value,
plutonium
has
infinite
thickness.
Extrapolation at the infinite energy leads then to a
value lower than the real mass.
3.2. Correction for combustible matrix
It has already been stated [2] that PUMA has a
tendency to underestimate the plutonium mass
value for waste drum with combustible matrix (vinyl
etc.), when the interaction of gamma photons is
essentially due to the Compton effect. An empirical
correction factor has been proposed from series of
measurements using 100-liters simulated waste
drum. The radius of the 100-liters drum is equal to
22 cm [3].
M = MPUMA × e
k
R
c
22
M: corrected plutonium mass
MPUMA: plutonium mass using PUMA
K=1.427 cm3/g (empirical value)
R: drum’s radius (cm)
c: volumic mass of the drum (g/cm3)
3.3. Instrumentation and software
The instrumentation (Picture 2) was realized to
meet the needs of inspectors for whom speed and
flexibility of installation on site, as well as the
geometrical reproducibility of the measurement
conditions, are important. This measurement
device is constituted with a frame on which come
to position two mobile wagons; the first supports
the detector germanium; the second is constituted
with a rotating plateau on which is set the drum to
measure. The instrumentation consists of a
multichannel analyzer and of a high efficiency
germanium detector. Dedicated software has been
developed by the IPSN, linked to commercialized
module, to acquire spectra and to perform the
analyses. The whole devices are transported in
flight cases and are operational in thirty minutes on
site.
Picture 2: PLUM
Schematic 3
4. Isotopic composition measurement
Both PLUM and FUNE need the isotopic
composition value to get quantitative results on the
plutonium mass. FUNE uses data obtained form
MGA code. PLUM uses data obtained from
COMPOS code. The main idea here is to have
independent method to get isotopic data.
Subsequently, PLUM and FUNE use independent
methods to measure the mass of plutonium.
5. Inspection’s organization
IPSN’s inspectors are used to organizing
inspections on duration of a week, measures
beginning on Monday and ending on Thursday.
The material is transported the week preceding the
inspection in order to settle and test the equipment
on the previous Friday. This day allows also doing
the formalities for access. The last day of the
inspection is foreseen to make some additional
measures, to pack the material and to make a
summary meeting of the inspection.
For each drum, several measurements are
performed.
•
Gross weight, that gives an indication of
the density of the drum
•
Passive
FUNE
neutron
measurement
using
•
Gamma spectrometric measurement using
PLUM (and COMPOS for isotopic composition)
∆ = measured plutonium mass (g) – declared
plutonium mass (g) ;
σ: standard deviation of the measurement (g).
For PLUM and FUNE, σ = 0,15 × MPu
MPu: measured plutonium mass (g)
This statistical test allows the comparison of a
measurement result to a reference value (the
operator’s declared value) [4].
For these inspections where two devices
(FUNE and PLUM) independent and additional are
implemented, an anomaly is declared significant
when the test described above is verified at the
same moment on both PLUM and FUNE and when
the values of ∆ are of the same sign.
7. Complementarity of FUNE and PLUM
FUNE, as well as PLUM are devoted to the
measurement of plutonium mass in waste drums.
The interest of these two systems is to be
complementary. They are based on two methods:
measurement of the spontaneous neutron
emission of even isotopes of the plutonium
(plutonium 238, 240 and 242) for FUNE,
measurement of the photon emission of odd
isotopes of the plutonium (plutonium 239 and 241)
for PLUM. It is necessary however to note that the
two methods require the knowledge of the isotopic
composition of the plutonium.
Regarding detection limits for both PLUM and
FUNE, a typical value is on the range of a few
dozen of milligram of plutonium. These detection
limits depend on many factors like radioactive
background conditions, matrix: every inspection is
a special case.
To illustrate the interest of those two systems
used together, let us suppose that the plutonium
waste drum is "polluted" with the emission of
neutrons from other material than the plutonium.
The mass of plutonium measured by FUNE will be
overestimated. This overestimation will be
revealed by measures with the device PLUM.
Another example is the case of a drum that
contains plutonium with a lot of self-absorption or
metallic
screen.
In
that
case,
PLUM
underestimates the mass of plutonium, contrary to
FUNE, which is not sensitive to these phenomena.
The joint use of the two systems allows to enhance
the capacity of measurement and to explain some
discrepancies between values announced by the
operator and those measured by inspectors.
6. Criteria to declare an anomaly
8. Results of EQRAIN n°3 exercise
An anomaly on the operator’s value is declared
∆
is
for PLUM or FUNE whenever the difference
IPSN participated in EQRAIN n°3 intercomparison exercise, organized by CEA/CETAMA
for the characterization of 4 plutonium waste drum,
A, B, C and D. A, B and C drums were specifically
made for this exercise. They are constituted with a
•
Isotopic composition, using MGA code.
A typical duration for FUNE and PLUM counting
and analysis is 30 min. This means an average
duration of a drum’s expertise in 45 min and an
average of 25 to 30 drums’ expertise for the total
inspection week.
σ
superior to a chosen value " u " equal to 3, where:
combustible matrix of a given density (0.17g/cm3)
and various plutonium sources, a priori punctual.
137
Cs were added to the contents of B and C
drums. D is a "real" drum with a density of
approximately 0.27g/cm3.
developed and/or tested by the people who will
perform inspections.
Table 1 shows the results obtained on these
drums, using FUNE and PLUM. The measurement
of A drum using FUNE was under the detection
limit of the equipment and there is no reference
value for the real D drum.
[1] Mesures des quantités de plutonium dans
des fûts de déchets par comptage neutronique
passif (FUNE). T. Lambert & al, ESARDA 19th
annual symposium, 1997 Montpellier, France
N°
A
B
C
D
CETAMA
Reference
Value (mg)
8,3
76
204,6
No value
Gamma
measurement
PLUM
Pu
mass
(mg)
6
74
197
91
± σ(mg)
±
±
±
±
0,9
11
29
14
Neutron
measurement
FUNE
Pu
mass
(mg)
*******
65
204
86
± σ( mg)
*******
±
10
±
31
±
13
Table 1
9. Conclusion
IPSN has a long expertise in measuring
plutonium waste drums. For instance, for the last
five years, IPSN’s inspectors, using on-site
measurement devices, have controlled most
French facility producing plutonium waste drums.
This represents about 250 measurements on
waste drums. From this feedback, we think that
there is not a unique method that can solve every
kind of measurement on waste. The use of passive
neutron and gamma spectrometry techniques,
which are complementary, is a powerful
methodology to assess most cases.
The methods presented in this paper are rather
user’s independent. The variety of physical forms
and containers that can be met during inspection
requires to have really well trained people. This is
especially true for waste that may always lead to a
special case analysis, where some non-scheduled
radionucleide may be detected. For those cases,
IPSN’s inspectors have other tools to do their
expertise: for instance PC/FRAM (isotopic
composition analysis), or ISOTOPIC (quantitative
measurement). IPSN’s inspectors must be
capable, during the inspection, of analyzing the
results and of understanding the reasons of a
possible anomaly with the operator declared value.
One can understand all the interest of the
current organization: measurement devices are
10.
Reference
[2] Adaptation of the gamma spectrometry
method based on the infinite energy extrapolation
to the measurement of small amounts of plutonium
in wastes. J. Morel & al, ESARDA 15th annual
symposium, 1993 Rome, Italy
[3] Note interne. Etude de la correction
d’atténuation matricielle pour le calcul de la masse
de plutonium dans un fût de déchets. J. L. Dufour
& al.
[4] Statistique appliquée à l'exploitation des
mesures. CETAMA, Ed. MASSON, 1986 - 2ème
éditio