Sample thickness effect on nuclear material quantification with NRTA for
Transcription
Sample thickness effect on nuclear material quantification with NRTA for
Sample thickness effect on nuclear material quantification with NRTA for particle like debris of melted fuel H. Tsuchiya1, H. Harada1, M. Koizumi1, F. Kitatani1, J. Takamine1, M. Kureta1, H. Iimura1, B. Becker2, S. Kopecky2, K. Kauwenberghs2, A. Moens2, W. Mondelaers2, P. Schillebeeckx2 1 Japan Atomic Energy Agency (JAEA) Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan E-mail: [email protected], [email protected] 2 EC-JRC-IRMM Unit D.4. Standards for Nuclear Safety, Security and Safeguards Retieseweg 111, B-2440 Geel, Belgium E-mail: [email protected] Abstract: A method to quantify the amount of uranium and plutonium in melted fuel derived from a nuclear accident such as the one occurred at the Fukushima Daiichi nuclear power plant has not been established yet. For this reason, neutron resonance densitometry, combining neutron resonance transmission analysis and neutron resonance capture analysis, is proposed and its feasibility study has been defined in a collaboration between Japan Atomic Energy Agency (JAEA) and Joint Research Center, Institute for Reference Materials and Measurements (JRCIRMM). Within this contribution, transmission experiments using three Cu metal disks with different thickness of 0.125 mm, 0.25 mm, and 0.7 mm were made between November 2012 and February 2013 at the Geel Electron LINear Accelerator (GELINA) to investigate sample thickness effect on neutron resonance transmission analysis. We experimentally derived the areal density for the individual Cu samples with the resonance shape analysis code REFIT, and then compared them with the declared areal density. It was found that the REFIT-evaluated areal density was consistent with declared ones for each sample. Keywords: neutron resonance transmission analysis; neutron time-of-flight; melted fuel; Cu sample 1. Introduction The earthquake and the subsequent gigantic tsunami, which occurred on March 11, 2011 in Japan, resulted in a failure of electricity of the Fukushima Daiichi nuclear power plants. As a consequence, the cooling system for the nuclear fuel installed at the reactors Units 1−3 stopped its operation. Consequently, the nuclear fuel melted in the pressure vessels. In addition, a certain part of melted fuel (MF) most likely penetrated through the pressure vessel. Therefore, it is possible that melted fuel dropped on the concrete floor and solidified together with other surrounding materials. At present, it is planned that MF will be removed from the reactors after a cooling time of at least 10 years. From the viewpoint of nuclear safeguards and security, special nuclear materials (SNM) of uranium and plutonium in the MF should be accurately quantified after its removal. A possible 1 technique for the measurement is neutron resonance transmission analysis (NRTA). A detailed description of its application for various materials as well as principle of NRTA is given in Refs. [1, 2]. With respect to its application for nuclear fuel, Bowman et al. [3] and Behrens et al. [4] applied NRTA to quantify SNM in fresh and spent fuel pins and determined the abundance of 239,240,242 Pu and 235,236,238U in the pins with an accuracy better than 4%. Furthermore, a feasibility study with Monte Carlo simulations was carried out to investigate how NRTA can quantify the amount of SNM in spent fuel from commercial light water reactors [5]. This showed the great potential of NRTA to assay intact spent fuel assemblies. However, unlike such fresh and spent fuel assemblies, a method to assess SNM in MF caused by a severe accident like the Fukushima case has not been established yet. It is expected that the accuracy of NRTA for MF will be significantly affected by the characteristics of the sample to be measured: thickness, inhomogeneity, and presence of impurities such as 10B and concrete. It is most likely that particle-like debris of MF were formed due to e.g. a steam explosion [6]. Moreover, debris will also be produced during the removing process of the MF from the reactors. Therefore, such particle-like debris will have a wide variety of size and composition, which complicates the measurement. Accordingly a new technique that considers these difficulties is required. For this reason, neutron resonance densitometry (NRD), which is based on NRTA combined with a kind of neutron resonance capture analysis (NRCA), is proposed and under development. A description of NRD can be found in Ref [7,8]. In addition, its feasibility studies considering 10B contamination in MF were made with numerical calculations [9,10]. Also, particle size effect on NRTA was examined in Ref [11]. In this paper, we focus on a NRTA experiment conducted at GELINA. The experiment utilized three Cu-metal disk samples with different thickness in order to investigate the sample thickness effect on NRTA. We first present the basic of NRTA. Then, experimental transmissions for the three Cu samples are shown. Lastly, we compare the declared areal density for the Cu samples with the ones determined using the resonance shape analysis code REFIT [12]. 2. NRTA experiment 2.1. Basic of NRTA experiment NRTA utilizes an intense pulsed white neutron source as a diagnostic beam. The probability that a neutron beam is transmitted through a sample is measured as a function of neutron energy. The neutron energy is obtained by the time-of-flight (TOF) technique. That is by measuring the time difference between a start signal and a stop signal, provided by the neutron detector and accelerator, respectively. The transmitted spectrum has characteristic dips resulting from resonance structures in neutron-induced reaction cross sections of nuclides in the sample. In an actual measurement, the observed quantity is the fraction of the neutron beam that traverses the sample without any interaction. For a parallel neutron beam that is perpendicular to a sample material, the transmission T is represented by T = 𝑒 ! ! !! !!"!,! , (1) where 𝜎!"!,! is Doppler broadening total cross section and nk is the number of atoms per unit area of nuclide k, which is also denoted as areal density. 2 Experimentally, the transmission Texp is computed from the ratio of the counts of a sample-in measurement Cin and a sample-out measurement Cout, after subtraction of the background contributions Bin and Bout, respectively, 𝑇!"# = 𝑁! 𝐶!" − 𝐵!" , 𝐶!"# − 𝐵!"# (2) where NT is a normalization factor that is the ratio of the total intensities of the incident neutron beam during the sample-out and sample-in cycles. The background (Bin and Bout) is determined by an analytical expression applying the black resonance technique [13]. Eq. (2) indicates that the experimental transmission is independent of both the detection efficiency and neutron flux incident to the sample. Therefore, we can consider that NRTA provides an absolute measurement that does not require additional calibration experiments. 2.1. Experiments at GELINA Thickness (mm) 0.125 0.25 0.7 Diameter (mm) 80 80 80 Mass (g) 5.25 11.1 31.4 Areal density (at/b) 9.80×10-4 2.09×10-3 5.92×10-3 Table 1: Characteristics of individual Cu samples. The declared areal density is derived from a measurement of the weight and the area. To investigate the applicability of NRD, the Japan Atomic Energy Agency and the Institute for Reference Materials and Measurements of the Joint Research Centre (EC-JRC-IRMM) started collaboration in 2012. Within the collaboration various experiments are scheduled at the neutron TOF facility Geel Electron LINear Accelerator (GELINA) of the EC-JRC-IRMM. The experiments focus on the influence of the characteristics of debris on the accuracy of NRD, in particular, the impact of the thickness of the sample, distribution of the particle size, the presence of neutron absorbing impurities, and the radioactivity of the sample. The sample thickness effect on NRTA was investigated between November 2012 and February 2013 at GELINA of the EC-JRC-IRMM. Here, we give a brief description on the neutron TOF facility of GELINA. A linear electron accelerator delivers a very short electron pulse with energies up to 150 MeV and a maximum repetition frequency of 800 Hz. Electron bunches, with peak currents of 12 A in a 10 ns time interval, are compressed by a compression magnet to a width of less than 1 ns. Neutrons are produced via photonuclear and photofission reactions by electrons impinging on a rotating target consisting of an U-Mo alloy. To produce a white neutron spectrum from thermal energy up to a few MeV, two 4-cm thick beryllium containers filled with water, placed beneath and above the target, were used as moderators. Produced pulsed neutrons travel through vacuum flight paths to measurement stations in which detectors and samples are installed. There are 10 flight paths with a length ranging from 10 m up to 400 m. Several measurement stations are arranged at various nominal distances of 10, 30, 50, 60, 100, 200, 300, and 400 m. A detailed description of GELINA including the accelerator and its neutron-producing target can be found in Ref. [14]. 3 For the purpose of assessing the performance of NRTA for the determination of the areal density, transmission experiments using Cu samples were performed at a 25 m neutron flight path with the accelerator operating 800 Hz. The Cu samples consisted of metal disks with a different nominal thickness of 0.125, 0.25, and 0.7 mm. Table I gives the characteristics of the Cu samples. Each sample was placed at 9 m from the neutron target in a multiposition sample changer. A 6.35 mm thick and 101.6 mm diameter Li-glass scintillator (NE912) enriched to 95% in 6Li detected neutrons penetrating through the samples. The Li-glass scintillator was equipped with a boron-free quartz windowed photomultiplier (RMI9823-QKB). The detector was placed at 25 m from the face of the moderator viewing the flight path. In all measurements, we used a 10B overlap filter and black resonance filters of sulphur, bismuth, and cobalt. 3. Results Figure 1 (Top panel) shows experimental transmission [Eq. (2)] for the different Cu samples, together with uncertainties only due to counting statistics. For comparison, the total cross sections for 63 Cu and 65Cu taken from JENDL-4.0u [15] are plotted in the bottom panel. It is found that several dips are seen in the experimental transmission, corresponding to resonance structures in the Cu cross sections (bottom). Especially, three pronounced dips at TOF of 39 𝜇𝑠, 72 𝜇𝑠, and 115 𝜇𝑠 exist in all the transmission, corresponding to resonance energies of 2038 eV, 579 eV, and 230 eV, respectively. In this study, we used the 2038-eV resonance dips to derive the areal density of each sample. Figure 2 shows the transmission at the 2038-eV resonance region. It clearly shows that the dip structure in the transmission is stronger for a thick sample. We analyzed the transmission data (Fig. 2) with the resonance shape analysis code REFIT [12], to derive the areal density for the 0.125-mm, 0.25-mm, and 0.7mm thick Cu samples. Figure 3 compares the experimental transmission with the transmission evaluated by REFIT. The evaluated values are found to reproduce well the measured transmission (𝜒 ! /𝑑. 𝑜. 𝑓= 1228/989). Figure 4 presents the ratio of the areal density derived from the transmission data and the 4 Figure 1: Experimental transmission (top) and corresponding total cross sections (bottom). Horizontal axis in both panels corresponds to TOF of neutrons in ns. (Top) Black, red, and green lines show transmission with Cusample thickness of 0.125, 0.25, and 0.7 mm, respectively. (Bottom) Black and red lines represent total cross sections of 63Cu and 65Cu, respectively. Numbers in the panel show resonance energies. Figure 2: Experimental transmission close to the 2038-eV resonance region. Black, red, and green lines show transmission through a Cu-sample thickness of 0.125, 0.25, and 0.7 mm, respectively. declared ones. We may conclude that the derived areal densities for the individual samples are in very good agreement with the declared ones. 4. Summary Figure 3: Comparison between the measurement for 0.7-mm thick Cu sample (black points) and the REFIT (red line). Errors quoted are 1 𝜎 statistical ones. Horizontal axis shows neutron energy in eV. In order to quantify SNM in MF caused by a severe accident such as the Fukushima case, Neutron Resonance Densitometry is under development. NRD combines neutron resonance transmission analysis and neutron resonance capture analysis. One of activities within the R&D programme of NRD concerns the assessment of the sample thickness effect on NRTA. In the present work, using three Cu disk samples with different thickness of 0.125 mm, 0.25 mm, and 0.7 mm, we performed NRTA experiments at the GELINA facility of the EC-JRCIRMM as part of collaboration between the JAEA and EC-JRC-IRMM. Consequently, we found that the areal density for the individual samples was, within 2%, consistent with the declared values. 5. Acknowledgements Figure 4: Ratio of the derived areal density to the declared ones, plotted as a function of thickness of the Cu sample. Errors originate from count statistics only. This work was done under the agreement between JAEA and EURATOM in the field of nuclear materials safeguards research and development. This work is supported by JSGO/MEXT. We are very grateful for the technical assistance of J.C. Drohe, D. Vendelbo, and R. Wynants during the measurements at GELINA. References [1] Postma H. and Schillebeeckx P., “Neutron Resonance Capture and Transmission Analysis”, Encyclopedia of Anal. Chem. (John Wiley & Sons Ltd), 1-22 (2009). [2] Schillebeeckx P., Borella A., Emiliani F., Gorini G., Kockelmann W., Kopecky S., Lampoudis C., Moxon M., Perelli Cippo E., Postma H., Rhodes N.J., Schooneveld E.M., Van Beveren C., “Neutron resonance spectroscopy for the characterization of materials and objects”, JINST 7, C03009, (2012). [3] Bowman C.D., Schrack R.A., Behrens J.W., and Johnson R.G., “Neutron resonance transmission analysis of reactor spent fuel assemblies”, Neut. Radiography 503-511 (1983). 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