A. Sagara

A. Sukegawa

H. Motota

I. "Murata

M. Manabe

N. Hesham

R. Nakamura

S. Ohnishi

S. Sato

T. Muroga

T. Obata

T." Tanaka

National Institute for Fusion Science (NIFS-Repository)

    Liquid metal lead lithium alloy (Pb17Li) is candidate to be liquid breeder in fusion blanket system. The compatibility of Pb17Li with structural materials is critical issue. Corrosion characteristics of reduced activation ferritic martensitic (RAFM) steels in Pb17Li have been investigated by a series of corrosion tests. The corrosion behaviors were classified into two types, i.e. liquid metal corrosion and oxidation corrosion. The occurrence of either corrosion may be determined due to an oxygen potential of liquid Pb17Li. These corrosions cannot be caused in the same time, and can be sequentially caused when the oxygen potential changed during the exposure. Liquid metal corrosion or oxidation corrosion caused just before the end of corrosion tests might affect the result of the corrosion test. For example, a trace of oxidation corrosion cannot be easily detected on the specimen tested if a liquid metal corrosion occurred after the oxidation corrosion during the exposure. The interface between a specimen and a liquid metal has impedance when the specimen surface is oxidized in the liquid metal. Then, the interface condition can be known by the online measurement of the impedance. In the present study, experimental study on an insitu impedance measurement of a corrosion interface in liquid metal Pb17Li was performed. The purpose is to investigate the basic corrosion kinetics in the liquid metals. Figure 1 shows the experimental apparatus. Rectangular specimen (10 mm x 15 mm x 1 mm) of an unalloyed iron (Fe) were immersed in the liquid metals Pb17Li (30 cc) at 773 K. The reason for the use of Fe specimen is that Fe is the base metal of RAFM steel. An electro impedance of the specimen surface contact with Pb17Li was measured by an impedance measurement system (Princeton applied research, VERSASTAT 3). Some oxide was initially formed on a free surface of Pb17Li by heating procedure in cover gas with air. Therefore, the oxygen concentration in Pb17Li must be fixed at the oxygen saturated condition. The surface cross section of the specimen tested was analyzed by an electronprobe micro analyzer (EPMA) after the corrosion test. An electro resistance R [] of an oxide layer formed on a specimen surface in Pb17Li is expressed with an electrical resistivity ρ [·m], an area of wetted part A [m2] and a thickness of the oxide layer t [m],         (1).  Here, the electro resistance of Pb17Li and a leading wire is negligibly small. An electro resistivity of Fe oxide (Fe3O4) and lead oxide (PbO) is known as approximately 1 ·m and 3x103 ·m, respectively. The wetted area A is 2.3x104 m2 in the present work. When the thickness of the oxide layer increased according to the growth, the electrical resistance also increased based on Eq. (1). The measured results were obtained as Nyquist plot. Then, the electro resistance was evaluated and summarized in Fig. 2. Initially, the electrical resistance was not obtained possibly because an oxide layer was not formed at low oxygen potential in liquid Pb17Li. After 150hour exposure, the electro resistance increased up to approximately 260 , and kept constant. The result indicated the formation of the oxide layer, which had small impedance. Fig.3 shows the results of EPMA analysis on the surface cross section of the specimen, which was exposed to Pb17Li for 311 hours. There was double layer consisted of lead lithium oxide and iron oxide. The thickness of the iron oxide is approximately 20 m. Then, the electro resistance by Fe3O4 is expected to be 0.2 . The electro resistance of the PbO layer is approximately 260 when the layer thickness is 20m, and agrees with that obtained in the experiment. The PbLiO layer formed due to oxidation of Pb17Li, and had an electro resistivity.  These results indicated that the insitu impedance measurement method can be applied to the corrosion monitoring in Pb17Li when the lead oxide does not form or the impedance of the corrosion product is much larger than that of the lead oxide.       Fig. 1 Experimental apparatus   Fig. 2 Electro resistance of the corrosion products   Fig. 3 Result of crosssectional EPMA analysis  on specimen surface       There are a lot of integral experiments carried out so far using DT neutrons. In the experiments, elastic scatterings, especially forward scattering events are dominant because the forward scattering reaction cross section is larger than that of backward angle scattering. However, in deeper places contribution of the backward angle scattering can increase.1,2) In order to validate large angle scattering cross sections, we have started investigating how integral experiments should be performed with a DT neutron source. The aim of this work is to design a new integral benchmark experiment with activation foils, in which the contribution of forward scattering neutrons to activation would be reduced as low as possible in order to emphasize that of backward scattering neutrons. For this purpose, we have proposed a shadow-bar experiment.3) The basic geometry is shown in Fig. 1. By using this arrangement, neutrons enter the detector foil after a few times scatterings at the target plate which are more-or-less backward scattering.            Fig. 1 Conceptual arrangement of experiment.  We examined several configurations for the experiments as shown in Figs. 2 and 3. To clarify which angle of scattering is frequent and to estimate the detection efficiency in these configurations, neutron transportation calculations were performed and tracks of neutrons were analyzed. In the simulation, Monte Carlo transport calculation code, MCNP5 was used and all the neutron tracks and events were recorded by PTRAC option. Also, the reaction rate of 93Nb(n,2n)92mNb reaction was estimated. As the results, in C configurations in Fig. 2, the large angle scatterings occurred more frequently and the reaction rates became larger than in E configurations in Fig. 3, if the target angles are the same. In the configuration where the target plate was set with a slant against a shadow-bar, the scattering angles of the tallied neutrons become shifted to the forward direction.  The contribution of large angle scatterings varies according to configurations of the shadow bar and the target plate. It is emphasized best in the case of E-22.5 in Fig. 3, though the reaction rate is very small. It is thus implied that different configuration can validate different angle part of scattering cross sections. The validation of large angle scattering cross section would be performed by comparing C/Es obtained from these experiments.  1) S. Ohnishi, K. Kondo, S. Sato, K. Ochiai, K. Takakura, I. Murata, C. Konno, “Implementation of A DT Neutron Beam at the 1st Target Room of JAEA/FNS for New Integral Benchmark Experiments,” J. Korean Phys. Soc., 59[2] (2011) 1949-1952. 2) S. Ohnishi, K. Kondo, T. Azuma, S. Sato, K. Ochiai, K. Takakura, I. Murata, C. Konno, “New integral experiments for large angle scattering cross section data benchmarking with DT neutron beam at JAEA/FNS,” Fusion Engineering and Design, 87 (2012) 695-699. 3) S. Ohnishi, I. Murata, “Design of A New Integral Benchmark Experiment for Large Angle Scattering Using Shadow Bar and DT Neutron Source, “ to be appeared in Progress in Nucl. Sci. and Technol. (2013). DT neutron sourceShadow-bar Neutron detector Target plate           Fig. 2 Centered configuration.          Fig. 3 Off-centered configuration.  353§34. Study on Benchmark Experiment for Backward-Angle Scattering Reaction Cross Section at 14 MeVMurata, I., Ohnishi, S., Manabe, M., Obata, T., Nakamura, R. (Osaka Univ.),  Sato, S., Sukegawa, A. (Japan Atomic, Energy Agency), Motota, H., Hesham, N. (MHI Nuclear Engineering Company),  Sagara, A., Muroga, T., Tanaka, T.

Study on Benchmark Experiment for Backward-Angle Scattering Reaction Cross Section at 14 MeV

https://nifs-repository.repo.nii.ac.jp/?action=repository_action_common_download&item_id=7480&item_no=1&attribute_id=25&file_no=1

Study on Benchmark Experiment for Backward-Angle Scattering Reaction Cross Section at 14 MeV

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)