SUBCRITICALITY ESTIMATION BY VIRTUAL NEUTRON CAPTURE METHOD

The criticality safety control technique is required for the fuel debris removal from the Fukushima Dai-Ichi Nuclear Power Station which experienced a severe accident. The subcriticality estimation is expected to be done with only limited information about the fuel debris while the primary containment vessel internal survey work is ongoing. The purpose of this study is to develop the subcriticality estimation method called the virtual neutron capture method. The neutrons from the surface of the fuel debris represent a major portion of detector counts. The method consists of two evaluations: the evaluation at the surface of the fuel debris for which the isotope compositions are known by fuel debris sampling and the evaluation at the region of the fuel debris for which these compositions are unknown. For the unknown composition region, the average isotope composition with arbitrary water content is given. The method surveys the relationship with the detector count and the neutron multiplication factor with any size of the unknown composition region and any ratio of the water content before the on-site evaluation. The method is verified by experiments done in the Kyoto University Critical Assembly. The method shows that the maximum difference from the reference neutron multiplication factor is 4.5 %dk. As a result, the virtual neutron capture method can be adopted to the subcriticality monitoring if the method includes the estimation margin of 4.5 %dk within the neutron multiplication factor range from 0.70 to 0.95.

Atomic-Approach to Predict the Energetically Favored Composition Region and to Characterize the Short-, Medium-, and Extended-Range Structures of the Ti-Nb-Al Ternary Metallic Glasses

Ab initio calculations were conducted to assist the construction of the n-body potential of the Ti-Nb-Al ternary metal system. Applying the constructed Ti-Nb-Al interatomic potential, molecular dynamics and Monte Carlo simulations were performed to predict a quadrilateral composition region, within which metallic glass was energetically favored to be formed. In addition, the amorphous driving force of those predicted possible glassy alloys was derived and an optimized composition around Ti15Nb45Al40 was pinpointed, implying that this alloy was easier to be obtained. The atomic structure of Ti-Nb-Al metallic glasses was identified by short-, medium-, and extended-range analysis/calculations, and their hierarchical structures were responsible to the formation ability and unique properties in many aspects.

Формирование нанокристаллов в аморфной фазе многокомпонентных систем

The effect of the concentration of alloying components on the crystallization of amorphous (Fe_73Si_13B_9)_1 – _ x _– _ y Nb_ x Cu_ y and (Co_70Si_12B_9)_1 – _ x _– _ y Fe_ x Nb_ y alloys has been studied by X-ray diffraction and transmission electron microscopy in a wide composition region. The formation of the bcc structures in both the alloy groups is shown to be substantially dependent on the alloying component concentrations. The bcc phase is found to form in the cobalt-based alloys in the concentration region, where it was not observed before. In the cobalt-based alloys, the bcc phase appears at a niobium concentration higher than 1 at % and the average bcc nanocrystal size varies from 40 nm (at 1 at % Nb) to 14 nm (at 5 at % Nb). In the Fe-based alloys, nanocrystals with the bcc lattice form at the copper concentrations of 0.45–1 at %, and the average nanocrystal size is dependent on the alloy composition and varies in the range 16–24 nm. The causes of the concentration dependence of the formation of nanostructures in these alloys are discussed.

Atomic approach to the optimized compositions of Ni–Nb–Ti glassy alloys with large glass-forming ability

The optimized composition region and the optimum composition with the largest GFA were obtained through atomic simulations.

Combinatrial study for Mg-Si thin films fabricated by RF co-sputtering with Mg and Si targets

ABSTRACTMg-Si thin films were systematically studied using combinatorial approach by co-sputtering with Mg and Si targets. Single phase of Mg2Si appeared around the stoichiometric composition region, and in Mg-rich region (Mg/Si>4) Mg2Si and Mg phases coexisted. The transition of electrical conduction type from n-type to p-type occurred near the stoichiometric composition region where the strongest peak of Mg2Si appeared in the XRD patterns and the Raman scattering spectra. The p-type conduction was observed in Mg-poor region near the stoichiometric composition region. The results of first principle calculation suggest that Mg vacancy may cause p-type conduction.

Combinatorial exploration of newly pseudo-quintenary layered-type Li-Ni-Co-Fe-Ti oxides

ABSTRACTIn order to decrease the use of cobalt element as in LiCoO2,which is mainly used for cathode materials in lithium ion secondary batteries, a newly pseudo-quintenary layered-type Fe-doped Li-Ni-Co-Ti oxides library was created using the combinatorial technology "M-ist Combi system" based on the electrostatic spray deposition method.The starting materials used were LiNO3, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Fe(NO3)3·9H2O and TiO2 nano-slurry. These materials were dissolved or dispersed in a mixture of ethanol and butyl carbitol and mixed in a predefined ratio, respectively. Subsequently, each of the mixtures was sprayed and dried on a grounded reaction plate that was heated to 400˚C. The deposited powder was sintered at 700˚C for 5 hours in air atmosphere. Phase identification of the obtained powder library was evaluated by a combinatorial powder x-ray diffractometer. A reaction phase diagram was established from the structure information and chemical composition by ICP-AES measurement.From all results, single-phase layered-type compounds showed the composition region that included many Co and Ni elements. On the other hand, single phase spinel-type compounds existed in the Ti-rich composition region.