Rρ corresponds to the doublets counting

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1 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, 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 ρ i, a calibration curve using certified plutonium sources. In first estimate, the data follows a relation of the type: 240 m Pu = aρ. Rρ + bρ eq i i i aρ, b i ρi are the coefficients of the linear regression relative to the calibration curve obtained for a matrix of density ρ i, (or for an empty drum); Rρ corresponds to the doublets counting i rate for a drum with a matrix density equal to ρ i (or for an empty drum). The above equation supposes a homogeneous matrix and no induced fission.

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.

2 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 ρ i, which would be characteristic of neutron absorption? To achieve this study, IPSN made several matrixes, of vinyl type, with density in the range of 0.1 and 0.4 g/cm 3. 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 ρ i = 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 T 252 Cf+Pu, integrates the contribution of neutrons emitted from the californium 252 source and those resulting from the plutonium contained in the waste drum. 3 ) Measurement of the drum alone, in the previous configuration. The result is the contribution of the plutonium (noted T Pu ) in the term T 252 Cf+Pu (schematic 2 b). The difference between T 252 Cf+Pu and T Pu, noted T 252 Cf, allows the calculation of the value of the matrix density ρ vinyl-equivalent using the equation ρ i = f(t). According to the previous value of ρ vinylequivalent, a calibration of type 240 m Pu = aρ. Rρ + bρ is chosen. eq i i i The domain of application of this method imposes to have a measured value of ρ vinylequivalent always in the range of 0 and 0.4 g/cm 3.

3 Polyethylene moderator Helium 3 neutron Detector (28) Schematic 1: waste drum alone in the neutron chamber T 252 Cf + Pu T Pu T 252 Cf (a) (b) Schematic 2: acquisition with and without the 252Cf

4 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: N(E) M = 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 ( 239 Pu/Pu or 241 Pu/Pu), according to the peak P γ (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/cm 2. Beyond this value, plutonium has infinite thickness. Extrapolation at the infinite energy leads then to a value lower than the real mass 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 = M PUMA e R k 22 M: corrected plutonium mass M PUMA : plutonium mass using PUMA K=1.427 cm 3 /g (empirical value) R: drum s radius (cm) c: volumic mass of the drum (g/cm 3 ) c 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.

5 Picture 2: PLUM Schematic 3

6 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 neutron measurement using FUNE Gamma spectrometric measurement using PLUM (and COMPOS for isotopic composition) 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. 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. 6. Criteria to declare an anomaly An anomaly on the operator s value is declared for PLUM or FUNE whenever the difference σ is superior to a chosen value " u " equal to 3, where: = measured plutonium mass (g) declared plutonium mass (g) ; σ: standard deviation of the measurement (g). For PLUM and FUNE, σ = 0,15 MPu M Pu : 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. 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. 8. Results of EQRAIN n 3 exercise 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

7 combustible matrix of a given density (0.17g/cm 3 ) 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/cm 3. 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. N CETAMA Reference Value (mg) Gamma measurement PLUM Pu mass (mg) ± σ(mg) Pu mass (mg) Neutron measurement FUNE ± σ( mg) A 8,3 6 ± 0,9 ******* ******* B ± ± 10 C 204,6 197 ± ± 31 D No value 91 ± ± Conclusion Table 1 developed and/or tested by the people who will perform inspections. 10. Reference [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 [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, ème éditio 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

WASTE DRUMS MEASUREMENTS

WASTE DRUMS MEASUREMENTS J.L. Dufour, B.Autrusson, P. Funk Institut de Radioprotection et de Sûreté Nucléaire-Radioactive Material Security Department B.P. 17-92262 Fontenay-Aux-Roses Cédex - France. 1.