Hazard Evaluation on Geologic Disposal of High-Level Radioactive Waste

For a long-term safety assessment of geologic disposal of high-level radioactivewaste (HLW), it is necessary to evaluate the potential hazard of radionuclides discharged into the biosphere. Therefore, the result of the safety assessement may heavily depend on this evaluation basis for the potential hazard of radionuclides. The most frequently used measure of the potential hazard is the so-called Ingestion Hazard Index based on the Maximum Permissible Concentration (MPC). Recently, however, the International Commission on Radiological Protection gave the Annual Limits of Intake for workers (ALI). The two different bases of MPC and ALI for the potential hazard bring about considerably different results concerning the safety analysis on the geologic disposal. We derived the explicit form of the solution for the migration of the 4-member decay chain through the one-dimensional sorbing media with dispersion. By applying the solution, it was revealed that in a limited condition of geologic formation, the most dangerous radionuclide in the geologic disposal may be altered by a change of the evaluation basis for the potential hazard. It was also pointed out that the importance of the waste form performance may be much increased by the alteration of the evaluation basis from MPC to AL

A Semi-analytical Solution for Advection-dispersion Migration of Radionuclides through Two-layered Geologic Media

A semi-analytical solution was obtained for a one-dimensional migration of radionuclides in four (or less) member-decay chains with dispersion through a two-layered sorbing medium. The Preferential-release model is applied, in which each element can take a different and time-depending release rate from the waste form. The two types of geologic media concerned were characterized by different retardation factors for each element, the dispersion coefficients, and the ground-water velocities between the two layers. In connection with a long-term safety assessment of geologic disposal of high-level radioactive wastes (HLW), the concentration profiles of radionuclides predicted by the solution can be applied to heterogeneous geologic formations, in which radionuclides migrate through e. g. granite of a host rock followed by subsoil bounded by a surface water body

Memory Window in Ferroelectric Field-Effect Transistors: Analytical Approach

A memory window of ferroelectric field-effect transistors (FeFETs), defined as a separation of the HIGH-state and the LOW-state threshold voltages, is an important measure of the FeFET memory characteristics. In this study, we theoretically investigate the relation between the FeFET memory window and the P-E hysteresis loop of the ferroelectric gate insulator, and derive a compact model explicitly described by material parameters. It is found that the memory window is linearly proportional to the ferroelectric polarization for the small polarization regime, and converges to the limit value of 2 x coercive field x thickness when the remanent polarization is much larger than permittivity x coercive field. We discuss additional factors that possibly influence the memory window in actual devices such as the existence of interlayer (no direct impact), interface charges (invalidity of linear superposition between the ferroelectric and charge-trapping hysteresis), and minor-loop operation (behavior equivalent to the generation of interface charges)

Invited; HfZrO-based ferroelectric capacitors and FETs for ultralow-power signal processing

Since the discovery of ferroelectricity in HfO2-based dielectric films in 2011 [1], MFM capacitors and FETs using HfO2-based thin films as dielectrics have attracted strong interest. Thus, active research and developments have been conducted for various applications including memory, logic, and AI computing with extremely low power consumption. In this paper, we introduce our recent research on a variety of HfZrO2 (HZO)-based ferroelectric devices such as FeRAM [2-3], FeFET memory [4-8], anti-ferroelectric FETs [9-10] and reservoir computing devices [11-13], for ultralow-power signal processing.The high polarization reversal voltage associated with the high coercive field of HZO films makes it difficult to achieve the low voltage operation of HZO FeRAM. Here, scaling HZO film thickness is effective in a reduction of the supply voltage of FeRAM with HZO MFM capacitors. It has been found through a systematic study on ferroelectric characteristics of Hf0.5Zr0.5O2 films with a thickness from 9.5 to 2.8 nm [2, 3] that scaling HZO film thickness to 4-5 nm can reduce operating voltage below 1 V (~0.8 V) with sufficient 2Pr by performing 106 cycles of wakeup. Also, the electric field causing dielectric breakdown can significantly increase by HZO scaling. The experimental endurance characteristic of 4-nm-thick HZO has indicated that the maximum cycle times determined by dielectric breakdown is around 1010 and 1012 times at 4 MV/cm and 3 MV/cm (1.2 V), respectively, under a pulse voltage operation of 200 kHz. Please click Download on the upper right corner to see the full abstract

PERFORMANCE EVALUATION OF MULTI-STAGE SOFC AND GAS TURBINE COMBINED SYSTEMS

ABSTRACT Solid oxide fuel cell (SOFC) and gas turbine hybrid power generation systems have gained more and more attention with regard to the development of the high performance distributed energy systems. The SOFC can be combined with a gas turbine because the SOFC operating temperature of about 1000 o C matches the turbine inlet temperature. In this study, we proposed the multi-stage type SOFC/GT combined system and compared the system performance of it with that of other combined systems using the thermal efficiency and exergy evaluation. It is noted that the thermal efficiency of the 3-stage type SOFC/GT combined system can reach more than 70% (HHV) at low pressure ratio

Non-volatile hybrid optical phase shifter driven by a ferroelectric transistor

Optical phase shifters are essential elements in photonic integrated circuits (PICs) and function as a direct interface to program the PIC. Non-volatile phase shifters, which can retain information without a power supply, are highly desirable for low-power static operations. Here a non-volatile optical phase shifter is demonstrated by driving a III-V/Si hybrid metal-oxide-semiconductor (MOS) phase shifter with a ferroelectric field-effect transistor (FeFET) operating in the source follower mode. Owing to the various polarization states in the FeFET, multistate non-volatile phase shifts up to 1.25{\pi} are obtained with CMOS-compatible operation voltages and low switching energy up to 3.3 nJ. Furthermore, a crossbar array architecture is proposed to simplify the control of non-volatile phase shifters in large-scale PICs and its feasibility is verified by confirming the selective write-in operation of a targeted FeFET with a negligible disturbance to the others. This work paves the way for realizing large-scale non-volatile programmable PICs for emerging computing applications such as deep learning and quantum computing