G. Jossens a, C. Deyglun a, P. Gironès b, C. Mathonat a *, and A.Godot a

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1 INMM 2015, NDA quantification of Nuclear Waste Containers G. Jossens a, C. Deyglun a, P. Gironès b, C. Mathonat a *, and A.Godot a a KEP Technologies SETARAM, 7 rue de l Oratoire, Caluire, 69300, France b CEA Marcoule, DEN/DPAD - B.P Bagnols sur Cèze Cedex *Corresponding author ABSTRACT The precise determination of the amount of nuclear material within a container has long been a challenge, and especially within drums containing nuclear wastes due to complex matrix and lack of homogeneity. A measurement approach combining calorimetry, gamma spectrometry and radiological modeling will be presented as an integrated waste characterization system. Calorimetry is one of the best solutions to estimate the overall quantity of nuclear material on a wide range of mass, from a few milligrams up to kilograms of radionuclides. Modern technological developments within the calorimetry field give the promise of faster measurement times and precise measurements with low levels of uncertainty. Gamma spectrometry allows to the estimate precisely of isotopic abundance of each radionuclide. I. INTRODUCTION Two methods are used for the nondestructive assay (NDA) of plutonium; the neutron coincidence counting and the calorimetric assay. These two methods need the isotopic composition of the item to interpret the absolute plutonium mass. Passive or active neutron coincidence uses the isotopic fraction to calculate the effective 240 Pu fraction ( 240 Pu eff ) and calorimetry uses the isotopic fraction to interpret the total power of the sample. The gamma spectrometry is a well-developed method and allows determining the isotopic distribution on a plutonium and uranium sample. Calorimetry is considered to be the gold standard for NDA measurements in the safeguards domain but the long measurement times required can be a hindrance to rate of production. While passive neutron multiplicity counting is the preferred method for fast assays of plutonium, the interaction of neutrons with the matrix, especially with low-z impurities (beryllium, bore, oxygen etc.), erodes the precision, and increases measurements uncertainties. Conversely, calorimeters are immune to these matrix effects. Many measurement strategies are developed in order to balance accuracy, precision and measurement times combining the different characterization technics: passive neutrons counting and gamma-ray spectrometry. Each of them has their advantages and drawbacks. The strategy, presented hereafter, consists to develop a solution to use a calorimeter with an integrated gamma-ray spectrometer in a complete device to characterize nuclear material or waste. This instrumentation strategy must take into account different limitations: 1. Calorimeter needs thick walls (made of aluminum or insulating material such as polyethylene) for insulation against external perturbation coming from the environment and for internal thermal homogeneity. 2. Thus, the combination of calorimeter with another measurement technic in only one system imposes compromises, which can erodes performances of each technic. One preferred solution in order to obtain the best accuracy and efficient rate of production is to have the different technics in line, so called characterization measurement line approach (Figure 1), instead of simultaneous calorimetric and gamma-ray spectrometric assay. Containers are handled by means of manual or robotized manipulators and conveyor, bringing them in sequence to a RX or 1

tomographic station (to have an overview of what is inside), to a gamma ray spectrometer (to determine isotopic ratio), to a calorimeter (to determine sample power).

2 tomographic station (to have an overview of what is inside), to a gamma ray spectrometer (to determine isotopic ratio), to a calorimeter (to determine sample power). Each container can be identified before storage with reliable equipment (RFID or bar-code system) and a weighing station if necessary. This automatic container manipulation reduces operator radiation exposure and repeatability uncertainty. Radiography, Tomography Gamma-ray spectrometry Automatic manipulator Calorimeter LVC Figure 1. The design of a complete measurement line. Moreover some constant developments are done in order to decrease the calorimetric measurement time required to reach the thermal equilibrium inside the calorimeter and to improve repeatability [i,ii]. With these design advances measurement times of less than 8 hours can be achieved for a US 3013 storage container [iii]. Measurement time can also be reduced to 3-4 hours using prediction calculation. This paper will present the recent improvement in the calorimetric assay and the possible combination of gamma-ray isotopic measurement and calorimetric techniques. II. Calorimetry-gamma spectrometry combination A. Alpha and beta emitters heating First of all, it is important to understand how heat is produced by alpha and beta emitters. In the case of beta emitter, the total reaction energy is shared between the beta particle, neutrino, the excitation energy and recoil energy of the daughter. The characteristic distance of 2 MeV electrons is a few millimeters in the matrix and all of the kinetic energy of the beta particle is absorbed by the sample. One 200 kev or 2 MeV electron in polyethylene produces respectively ~0.15 kev and ~11 kev X-ray due to bremsstrahlung radiation.. The neutrino is not absorbed, so its energy is lost. At last, the thermal power measured by a calorimeter from beta emitters is the product of the activity and the average beta-particle energy, plus the de-excitation gamma-ray energy deposit taking into account the decay scheme. The gamma-ray contribution can be calculated with Monte Carlo code, e.g. MCNPX [iv]. 2

3 The heat produced by the decay of alpha emitter as for 239 Pu, in case of complete stopping of particles is distributed as follows [v]: - Alpha particle: 98.17% - Recoil: 1.67% - Conversion electrons: 0.1% - Auger electrons: 0.035% - X-rays: 0.01% - Gamma-rays: 0.001% The range of 5-6 MeV alpha particles is around 5 µm in common materials. Thus, all of the energy released by charged particles during the alpha decay will remain within the item as heat. Low-energy gamma-rays and X-rays can be assumed to be totally absorbed in the matrix and the specific power of alpha emitter is directly deduced from the Q-value. The gamma-rays and X-rays contribution can also be calculated with MCNPX [4]. Because the heat-measurement result is independent of material and matrix, it can be used for the inspection of any material form or matrix. The specific powers and the associated uncertainties are listed in Table 1 for all plutonium isotopes and 241 Am. Table 1. Specific powers and uncertainties (gamma-rays and X-rays are assumed to be totally absorbed in the matrix). PANDA database [Erreur! Signet Isotopes non défini.] specific power Relative Standard [mw/g] Deviation (%) 238 Pu Pu Pu Pu Am In the case of alpha emitter study, the assumption that all particles are completely absorbed by the matrix is acceptable; photons represent less than 0.01% of the total release energy. Whereas in the case of beta emitter, or mixing alpha/beta emitters, we have to calculated separately the charged particles energy deposit and gamma-ray energy deposit after the daughter nuclei de-excitation using MCNPX code. The F6 tally in MCNPX gives the energy deposition in the item. In the case of waste from the decommissioning of spent fuel reprocessing facilities, 238 Pu and 241 Am are main power contributors [5]. However in case of high activities beta emitters, the contribution of the gamma-heating and interference with the heat flow measurement cannot be neglect. B. Calorimeter SETARAM Large Volume Calorimeters (LVC) developed for more than 40 years are active differential isothermal calorimeter. By optimizing contact between the sample and the thermal block, and using optimized power supply and heat flow sensor it is possible to achieve high accuracy and faster stabilization of large-volume sample and to reduce the measurement time for nuclear applications. The LVC can work on a wide range of power and volume as presented below. Table 2 Measurement characteristics of different Large Volume Calorimeters versions 3

4 Volume (liter) Measurement range (W) Precision (repeatability) Detection limit (µw) Measurement time (h) with predictive calculation LVC3013 LVC270 3W LVC270 15W LVC300 LVC580 LVC390 LVC680 LVC % 0.2% 0.2% 0.2% 0.3% 0.5% 0.5% 1% This is a differential calorimeter with a measuring cell for a container loaded with an active product to be analyzed, and a reference cell for a container that is either empty or loaded with an inactive product. Principle is based on measuring the heat (heat flow rate) between the block maintained at a constant temperature and the containers containing the sample and the reference. The differential calorimeter design is more precise because residual fluctuations caused by environmental changes (external temperature control variation) are cancelled out in the net signal formed between the measurement and reference. The calorimetric block is insulated by alternating layers of insulating and conducting materials to protect the blocks from external disturbances (external temperature variations). The insulating layers constitute a thermal barrier designed to filter short term and long term (between day and night) disturbances. To increase calorimeter temperature stability, and therefore its performance, thermal leakage between the sample and the environment must be perfectly controlled and reproducible. This is achieved by having the optimized insulation with a well regulated external temperature. C. Gamma-rays spectrometry The estimation of the isotopy of a plutonium and uranium sample is a well-developed technics. To calculate the amount of nuclear materials, the global system needs to well-known the isotopic fractions and the specific power (mg/w) for each isotope. A planar HPGe detector is used commonly. With a front surface area of 2000 mm 2 and a thickness of 20 mm, this type of detector gives a good compromise between resolution and efficiency (figure 3). 4

5 Figure 3: Gamma spectrum of waste drum. The multi peak analysis of all gamma emitted in the keV energy range allows to determine the isotopic ratios with MGA [7] or PC/FRAM [8] software with accuracy better than 1 %. The measurement time is less than 1h. The system can use a rotation platform for the sample and scanning equipment in the case of a tall container. The design allows being applicable to samples of arbitrary size, geometry, age, chemical composition and isotopic composition. The precise determination of isotopic fractions depends of a good energy resolution (less than 0.6 kev at 122 kev) and of the quality shields positioned between detector and canister. D. Uncertainty budget For the uncertainty budget, the different contributions are dependent of the characteristics and construction of the calorimeter, and in general, cannot be accurately predicted. Each contribution is evaluated from measurements on the calorimeter with an electrical calibration heater. In 5

6 Table 3, we present the uncertainty components in the measurement of the power emitted by a radioactive item containing Special Nuclear Material (SNM). Reference [9] gives a complete discussion about uncertainty components in calorimetry. The propagation of variances is used to combine the uncertainty components. Type A uncertainties are independent and normally distributed or can be measured directly: Random uncertainty The random error will be given as the relative standard deviation of the n measurements taken at different heat power. Estimated standard deviation: [10] S 1 n 1 n x i x i 1 2 Equation 1 Where: n: Number of measurements at each heat power x i : Value of the measurement (difference in power control between measuring and reference cell) x : Mean of the n measurements for each power level Relative standard deviation (random error): S RSD (%) 100 Equation 2 x Systematic uncertainty The systematic error will be given as the ratio between the real injected power and the fit point, obtained with the calibration curve. This ratio is referred to as Percent recovery and is given in %. Fit point: P fit ( mw ) HeatFlow Calibratio n Coefficien t Equation 3 Percent recovery (Systematic error): Pfit Percent Recov ery (1 ) 100 Equation 4 Preal The Type B uncertainties are evaluated by any other means, e.g. estimation based on prior knowledge of the measurement procedure, uncertainties in a datasheet; references to reported values in the scientific literature; modeling estimates etc. 6

7 Table 3. Uncertainties for complete characterization line based measurements at 1σ (most of the values were measured with an LVC3013). Origin Type Description Uncertainty Calorimeter hardware A Systematic error 100 mw : 0.1% 1000 mw : 0.2% mw : 0.1% Calorimeter hardware A Random error 100 mw : 0.1% 1000 mw : 0.02% mw : 0.16% Measured Item Properties A Heat source position 0.20% uncertainty assuming an equiprobavble distribution of the heating source Measured Item Properties B Specific power <<1% Measured Item Properties B Gamma heating Measured Item Properties B Energy lost assuming that the fraction of lost heat after neutrons escape (produced by (α,n) reaction or fission) is small and usually neglected. Measured Item Properties B No-radioactive heat source: Endo- and exothermic reaction like oxidation or water radiolysis negligible <<1% negligible Spectrometry measurement Gamma B The count rate, count time, absorbers, sample geometry, sample mass, sample isotopic composition, and instrument Stability can affect the measurement. (k=1) 238 Pu : 1 to 16% 239Pu : 0.1 to 0.5% 240Pu : 0.8 to 5% 241Pu : 0.2 to 0.8% 241 Am : 1 to 10% E. Total uncertainty Total measurement uncertainty (at 1σ) between 100 mw and mw was calculated: 100 mw : 0.24% 1000 mw : 0.28 % mw : 0.27% The gamma-heating contribution, the specific power uncertainty and the no-radiative sources contribution only affect the effective specific power calculation (P eff ), and so, the isotopes mass calculation. The mass, M, of nuclear material in an item is the ratio between the total power, W, and the effective specific power of the item (, where R j and P j are respectively the abundance and specific power of the j th element in the inspected item). Hence, the mass of the i th element in the inspected item, M i is: Equation 5 The usual uncertainty propagation rules [9] note that the variance in the uncertainty of the i th mass 7

8 may be estimated to first order according to: Equation 6 W measurement, P j and R j are each independent variables, so, we neglected the covariance term. The evaluation of each partial differentials leads to the expression: Equation 7 Where and. We calculated the isotopes mass uncertainties based on typical uncertainties described in ref. [11] and plutonium spectra described in table 3. 8

9 Table 4. Uncertainties propagation. Isotope Sample isotopic composition (wt.%) (%) 238 Pu E E E Pu E E E Pu E E E Pu E E E Pu E E E Am E E E As we can see in Table 4, the main uncertainty comes from the isotopic fraction measurement. II. Conclusions A complete solution using a calorimeter with an integrated gamma-ray spectrometer is a very precise and well adapted device for the classification of drums containing nuclear waste depending on their level of activities (low, medium, high). Thanks to the high accuracy of the calorimetric technology it is possible to know the amount of each radionuclide with a low uncertainty. The major contribution for the uncertainty determination is the isotopic measurement by gamma spectrometry. Using a complete measurement line fully automatized, it is possible to optimize the flow of several containers in accordance of the rules for storage or transportation. In addition, the automatic containers manipulation reduces operator radiation exposure and repeatability uncertainty. KEP Technologies [13] is able to propose adapted solution for a characterization measurement line. The specific design takes into account overall treatment process: container size and number, isotopic fractions, matrix characteristics, nuclear and non-nuclear risks, traceability. III. REFERENCES [i] G. Jossens, C. Mathonat, G. Etherington, Time measurement reduction on large volume and sensitive calorimeter for nuclear applications, INMM 2012, July 15-19, Orlando, USA. [ii] G. Jossens, C. Mathonat, and F.Bachelet, Waste management nuclear calorimeter for very large sample, INMM 2013, July 14-19, Palm Spring, USA. [iii] G. Jossens, C. Mathonat, G. Etherington, Large volume and sensitive calorimeter for nuclear applications: latest developments for greater accuracy and shorter measurement times, INMM 2011, July 17-21, 2010, Desert Springs, USA. [iv] [5] D. Relly, N. Enselin, H. Smith, Jr. and S. Kreiner, Passive Nondestructive Assay Manual PANDA, [6] G. Bortels, What information can alpha spectrometry provide to calorimetric assay of mixed alphaparticle emitters? Proceedings of the international workshop on calorimetry, Ispra, Italy March [7] R.Gunnink, MGA : A Gamma-Ray Spectrum Analysis Code For Determining Plutonium Isotopic Abunbances, LLNL report UCRL-LR , April

10 [8] Thomas E. Sampson, Thomas A. Kelley and Duc T. Vo, Application Guide to Gamma-Ray Isotopic Analysis Using the FRAM software, Report LA-14018, Los Alamos National Laboratory, September [9] Morag K. Smith, David Bracken, Cliff Rudy, and Peter Santi, An Analysis of the Systematic Components of Calorimetry Uncertainty, Proceedings of the 46th Annual Meeting of the Institute of Nuclear Materials Management, Phoenix, AZ, July (2005), LA-UR [10] Evaluation of measurement data Guide to the expression of uncertainty in measurement, GUM 1995 with minor corrections, JCGM 100:2008. [11] Standard Test Method for Determination of Plutonium Isotopic Composition by Gamma-Ray Spectrometry, ASTM Designation: C [12] 10

Evaluation of Nondestructive Assay Characterization Methods for Pipe- Over-Pack Containers

Evaluation of Nondestructive Assay Characterization Methods for Pipe- Over-Pack Containers S.B. Stanfield, J.R. Wachter, D.L. Cramer Canberra Instruments Inc. 800 Research Pkwy, Meriden, CT 06450 USA ABSTRACT