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Radiological impact analyses were carried out for a near-surface radioactive waste repository at Gyeongju in South Korea. The RESRAD-ONSITE code was applied for the estimation of maximum exposure doses by considering various exposure pathways based on a land area of 2,500 m2 with a 0.15 m thick contamination zone. Typical influencing input parameters such as shield depth, shield materials’ density, and shield erosion rate were examined for a sensitivity analysis. Then both residential farmer and industrial worker scenarios were used for the estimation of maximum exposure doses depending on exposure duration. The radiation dose evaluation results showed that 60Co, 137Cs, and 63Ni were major contributors to the total exposure dose compared with other radionuclides. Furthermore, the total exposure dose from ingestion (plant, meat, and milk) of the contaminated plants was more significant than those assessed for inhalation, with maximum values of 5.5×10−4 mSv‧yr−1 for the plant ingestion. Thus the results of this study can be applied for determining near-surface radioactive waste repository conditions and providing quantitative analysis methods using RESRAD-ONSITE code for the safety assessment of disposing radioactive materials including decommissioning wastes to protect human health and the environment.

This study evaluates the radioactivity of concrete waste that occurs due to large amounts of decommissioned nuclear wastes and then determines the surface dose rate when the waste is packaged in a disposal container. The radiation assessment was conducted under the presumption that impurities included in the bio-shielded concrete contain the highest amount of radioactivity among all the concrete wastes. Neutron flux was applied using the simplified model approach in a sample containing the most Co and Eu impurities, and a maximum of 9.8×104 Bq·g−1 60Co and 2.63×105 Bq·g−1 152Eu was determined. Subsequently, the surface dose rate of the container was measured assuming that the bio-shield concrete waste would be packaged in a newly developed disposal container. Results showed that most of the concrete wastes with a depth of 20 cm or higher from the concrete surface was found to have less than 1.8 mSv·hr−1 in the surface dose of the new-type disposal container. Hence, when bio-shielded concrete wastes, having the highest radioactivity, is disposed in the new disposal container, it satisfies the limit of the surface dose rate (i.e., 2 mSv·hr−1) as per global standards.

Currently, radioactive waste for disposal has been restricted to low and intermediate level radioactive waste generated during operation of nuclear power plants, and these radioactive wastes were managed and disposed of the 200 L and 320 L of steel drums. However, it is expected that it will be difficult to manage a large amount of decommissioning waste of the Kori unit 1 with the existing drums and transportation containers. Accordingly, the KORAD is currently developing various and largesized containers for packaging, transportation, and disposal of decommissioning waste. In this study, the radiation exposure doses of workers and the public were evaluated using RADTRAN computational analysis code in case of the domestic onroad transportation of new package and transportation containers under development. The results were compared with the domestic annual dose limit. In addition, the sensitivity of the expected exposure dose according to the change in the leakage rate of radionuclides in the waste packaging was evaluated. As a result of the evaluation, it was confirmed that the exposure dose under normal and accident condition was less than the domestic annual exposure dose limit. However, in the case of a number of loading and unloading operations, working systems should be prepared to reduce the exposure of workers.

Numerical model was developed that simulates radionuclide (3 H and 14C) transport modeling at the 2nd phase facility at the Wolsong LILW Disposal Center. Four scenarios were simulated with different assumptions about the integrity of the components of the barrier system. For the design case, the multi-barrier system was shown to be effective in diverting infiltration water around the vaults containing radioactive waste. Nevertheless, the volatile radionuclide 14C migrates outside the containment system and through the unsaturated zone, driven by gas diffusion. 3 H is largely contained within the vaults where it decays, with small amounts being flushed out in the liquid state. Various scenarios were examined in which the integrity of the cover barrier system or that of the concrete were compromised. In the absence of any engineered barriers, 3 H is washed out to the water table within the first 20 years. The release of 14C by gas diffusion is suppressed if percolation fluxes through the facility are high after a cover failure. However, the high fluxes lead to advective transport of 14C dissolved in the liquid state. The concrete container is an effective barrier, with approximately the same effectiveness as the cover.

We have generated a system concept for a non-destructive assay (NDA) system to certify identified isotopes and their activity concentrations for radioactive waste in a large container. Various optimization studies have evaluated the system performance in terms of the MDA (minimum detectable activity) results using the Monte Carlo simulation in conjunction with experimental studies. The proposed system consists of a total of eight HPGe (high-pressure germanium) detectors: four detectors on the top and the other four detectors on the bottom. The top and the bottom detector arrays are aligned and face each other. This detector arrangement has advantages in employing the attenuation correction as proposed by T. Chang [IEEE Trans. Nucl. Sci. NS-25, 638 (1978)]. We also found that the UFOV (useful field of view) of the detector highly influences the system efficiency, which leads to an improvement in the MDA performance. However, while the wider FOV of the detector improves the detection efficiency by allowing incoming radiation from other segmented volumes, it may suffer from non-uniform performance or increased errors in estimating an activity concentration for each segmented volume. To prevent such potential errors in employing the wider FOV, we have proposed an activity estimation algorithm, a so-called ‘fine volume reconstruction’, based on a back-projection method that estimates the activity concentration of each segmented volume. We demonstrate the feasibility of the conceptual system for use as a free release assay system for ISO containers up to a size of 2.4 × 6.0 × 1.3 m3 (W x L x H). The detector efficiency was about four times higher than those of typical commercial systems. Future studies include fine-tuning of the activity reconstruction algorithm and a validation study on various materials and non-uniform activity concentrations.

We developed a method to generate optical complex signals based on optically injection-locked (OIL) semiconductor lasers and numerically evaluated their performance depending on the injection-locking parameters, injection ratio, and detuning frequency. We first determined the stable/unstable locking regions of the OIL system as a function of the injection-locking parameters through steady-state analysis. We then mapped the optical complex signal in both the stable injection-locking region and the complex signal plane. Based on the mapped relationship between the positions in the conventional injection-locking map and the complex signal plane, we theoretically evaluated the optical trajectories and constellation diagrams for the optical phase-shift keying signals and calculated the error-vector-magnitude with a data rate of 10 Gbaud/s using a time-domain calculation. We found that the complex signal performance could be improved using a higher injection ratio under a large negative detuning frequency.

We developed an AlGaN/GaN heterojunction phototransistor with a recessed detection area to enhance the photoresponsivity. The recessed-AlGaN/GaN phototransistor exhibited a maximum photoresponsivity of 1.6 × 107 A/W at 375 ㎚, which was ~30% higher than that obtained with a conventional AlGaN/GaN phototransistor. A comparable photoresponsivity was also achieved at 260 ㎚ in UV-C range due to the dual absorption process in conjunction with the polarization induced built-in electric field characteristics of AlGaN/GaN heterojunction.

We have developed a silicon oxynitride (SiON) deposition process using a plasma-enhanced chemical vapor deposition system for the gate dielectric of GaN-on-Si metal–insulator–semiconductor field-effect transistors (MISFETs). The optimized SiON film had a relative dielectric constant of 5.3 and a breakdown field of 12MV/cm. A normally-off GaN-on-Si MISFET fabricated with a 33-nm SiON gate dielectric exhibited a threshold voltage of ~2 V, an ON-resistance of $7.85~\mathsf {m}\Omega \cdot \mathsf {cm}^{{\mathsf {2}}}$ , and a breakdown voltage of ~640 V at the OFF-state current density of $1~\mu \text{A}$ /mm. The extracted interface trap density was $\mathsf {1} \times \mathsf {10}^{{\mathsf {12}}}$ cm $^{{\mathsf {-2}}}\cdot \mathsf {eV}^{\mathsf {-1}}$ at ${E}_{c}- {E}_{t}= \mathsf {0.442}$ eV, which resulted in negligible hysteresis and excellent dynamic characteristics.

This study investigates the effects of a monolithic gate current booster integrated with an AlGaN/GaN-on-Si power-switching device. The integrated gate current booster was implemented by a single-stage inverter topology consisting of a recessed normally-off AlGaN/GaN MOS-HFET and a mesa resistor. The monolithically integrated gate current booster in a switching FET eliminated the parasitic elements caused by external interconnection and enabled fast switching operation. The gate charging and discharging currents were boosted by the integrated inverter, which significantly reduced both rise and fall times: the rise time was reduced from 626 to 41.26 ns, while the fall time was reduced from 554 to 42.19 ns by the single-stage inverter. When the packaged monolithic power chip was tested under 1 MHz hard-switching operation with V DD = 200 V, the switching loss was found to have been drastically reduced, from 5.27 to 0.55 W.

A pair of identical grating couplers can be used to transmit/receive optical signals between vertically separated optical waveguides. We propose a silicon-based grating coupler with an integrated distributed Bragg reflector (DBR) to improve the light coupling bandwidth and efficiency. The optimized grating coupler structure and DBR position were developed using the finite-difference time-domain method. We numerically analyzed the coupling efficiency and bandwidth performance, and the proposed grating coupler structure achieved a theoretical peak coupling efficiency of 0.824 and a 1 dB bandwidth of 157.2 nm at close to 1550 nm, both of which represent improvements of more than two-fold compared with those of conventional grating couplers without a DBR layer.

Domestically developed concrete storage casks include an internal canister to maintain the confinement integrity of radioactive materials. In this study, we analyzed the depth of flaws caused by loads that propagate canister weld cracks under normal, off-normal and accident conditions, and evaluated the maximum allowable weld flaw depth needed to secure the structural integrity of the canister weld and to reduce the welding time of the internal canister lid of the concrete storage cask. Structural analyses for normal, off-normal and accident conditions were performed using the general-purpose finite element analysis program ABAQUS; the allowable flaw depth was assessed according to ASME B&PV Code Section XI. Evaluation results revealed an allowable canister weld flaw depth of 18.75 mm for the concrete storage cask, which satisfies the critical flaw depth recommended in NUREG-1536.

We report on the enhancement of the static extinction ratio by using a dual-section distributed feedback laser diode integrated with an electro-absorption modulator. A directly- modulated dual-section laser can provide improved modulation performance under a low bias level (i.e., below the threshold level) compared with a standard directly-modulated laser. By combining the extinction ratio from a dual-section laser with that from an electro-absorption modulator section, a total extinction ratio of 49.6. dB are successfully achieved.

A high quality SiO 2 deposition process using a plasma enhanced chemical vapor deposition system has been developed for the gate insulator process of normally-off recessed-gate AlGaN/GaN metal–oxide–semiconductor-heterostructure field-effect transistors (MOS-HFETs). SiO 2 films were deposited by using SiH 4 and N 2 O mixtures as reactant gases. The breakdown field increased with increasing the N 2 O flow rate. The optimum SiH 4 /N 2 O ratio was 0.05, which resulted in a maximum breakdown field of 11 MV/cm for the SiO 2 film deposited on recessed GaN surface. The deposition conditions were optimized as follows; a gas flow rate of SiH 4 /N 2 O (=27/540 sccm), a source RF power of 100 W, a pressure of 2 Torr, and a deposition temperature of 350 °C. A fabricated normally-off MOS-HFET exhibited a threshold voltage of 3.2 V, a specific on-resistance of 4.46 mΩ cm 2 , and a breakdown voltage of 810 V.

This paper reports a new method to enable the normally-off operation of AlGaN/GaN heterojunction field-effect transistors (HFETs). A capacitor was connected to the gate input node of a normally-on AlGaN/GaN HFET with a Schottky gate where the Schottky gate acted as a clamping diode. The combination of the capacitor and Schottky gate functioned as a clamp circuit to downshift the input signal to enable the normally-off operation. The normally-off operation with a virtual threshold voltage of 5.3 V was successfully demonstrated with excellent dynamic switching characteristics.

Monolithically integrated devices are strongly desired in next generation power ICs to reduce the chip size and improve the efficiency and frequency response. Three examples of the embedment of different functional diode(s) into AlGaN/GaN heterojunction field-effect transistors are presented, which can minimize the parasitic effects caused by interconnection between devices.

Epoxy nanocomposites were synthesized in the presence of hydroxyapatite with the aid of an ultrasonicator. In general, as the amount of hydroxyapatite increased from 0wt% to 10wt%, the mechanical properties of the hydroxyapatite-containing nanocomposite were enhanced. The mechanical properties of the nanocomposite were significantly enhanced by the simple addition of 10 wt% of hydroxyapatite. Specifically, the storage modulus of the 10 wt% hydroxyapatite-containing nanocompsote was 3.2GPa, which is 46% higher compared to that of the pristine epoxy nanocomposite. The glass transition temperature of hydroxyapatite-containing nanocomposites generally decreased by few degrees in Celsius. To investigate the effect of additives on the mechanical properties of the epoxy-based nanocomposite, nanocomposites were synthesized using both montmorillonite and tellurium dioxide instead of hydroxyapatite. Intrestingly, both additive-based nanocomposite materials resulted in an increase in the storage modulus while the glass transition temperature decreased. These results demonstrate that the addition of few wt% of all three additives (hydroxyapatite, montmorillonite, and tellurium dioxide) can enhance the mechanical properties of epoxy-based nanocomposites.

We performed numerical analysis and design parameter optimization of a silicon-based grating waveguide refractive index (RI) sensor. The performance of the grating waveguide RI sensor was determined by the full-width at half-maximum (FWHM) and the shift in the resonance wavelength in the transmission spectrum. The transmission extinction, a major figure-of-merit of an RI sensor that reflects both FWHM and resonance shift performance, could be significantly improved by the proper determination of three major grating waveguide parameters: duty ratio, grating period, and etching depth. We analyzed the transmission characteristics of the grating waveguide under various design parameter conditions using a finite-difference time domain method. We achieved a transmission extinction improvement of >26 dB under a given bioenvironmental target change by the proper choice of the design procedure and parameters. This design procedure and choice of appropriate parameters would enable the widespread application of silicon-based grating waveguide in high-performance RI biochemical sensor.

In general, conventional criticality analysis for spent fuel transport/storage systems have been performed based on the assumption of fresh fuel concerning the potential uncertainties from number density calculations of actinide nuclides and fission products in spent fuel. However, these evaluation methods cause financial losses due to an excessive criticality margin. In order to overcome this disadvantage, many studies have recently been conducted to design and commercialize a transportation and storage cask applied to the Burnup Credit (BUC). This study conducted an assessment to ensure criticality safety for reactor operating parameters, axial burn-up profiles and misload accident conditions, which are the factors that are likely to affect criticality safety when the BUC is applied to the dual-purpose cask under development at the KOrea RADioactive waste agency (KORAD). As a result, it was found that criticality resulting from specific power, changed substantially and relied on conditions of low enrichment and high burn-up. Considering the end effect in the case of high burn-up produced a positive-definite result. In particular, the increment of maximum effective multiplication factors due to misloading was 0.18467, confirming that misload is a factor that must be taken into account when applying the BUC. The results of this study may therefore be utilized as references in developing technologies to apply the BUC to domestic models and operational procedures or preventing any misload accidents during the process of spent fuel loading.

A Au-free ohmic contact process for fabricating AlGaN/GaN heterostructure field-effect transistors (HFETs) on Si substrates was developed by using Mo-based metallization. We investigated Si/Ti/Al/Mo metal stacks for ohmic metallization where the Ti/Al thickness ratio and the annealing temperature were varied. The optimized metal stack and annealing conditions were a Si/Ti/Al/Mo stack with 5/40/60/50 nm thicknesses and rapid thermal annealing in a N2 ambient at 900 °C for 30 sec, which resulted in a contact resistance of 1.24 Ω·mm, a sheet resistance of 410 Ω/sq and a specific contact resistivity of 3.76 × 10−5 Ω·cm2. Devices fabricated using the optimized Mo-based, Au-free ohmic contact process exhibited comparable characteristics with higher breakdown voltage to those of devices fabricated using a conventional Au-based ohmic contact process.

Gallium oxide (Ga2O3) based vertical Schottky barrier diodes (SBDs) were designed for high voltage switching applications. Since p-type Ga2O3 epitaxy growth or p-type ion implantation technique has not been developed yet, a field plate structure was employed in this study to maximize the breakdown voltage by suppressing the electric field at the anode edge. TCAD simulation was used for the physical analysis of Ga2O3 SBDs from which it was found that careful attention must be paid to the insulator under the field plate. Due to the extremely high breakdown field property of Ga2O3, an insulator with both high permittivity and high breakdown field must be used for the field plate formation. Keywords: Gallium oxide, Field plate, Breakdown voltage, High permittivity, Schottky barrier diode

In this work, we performed an extensive experimental study on temperature dependence of the piezoresistive behavior of silicon. For the precise measurement of stress, we derived the expressions for the relationship between the pi-coefficients versus temperature. Also, the relationship was observed to be in good agreement with the calibration results over the temperature range -133 to 100 °C for test chips fabricated on the (111) silicon surface.

We have fabricated p- and n-type resistor stress sensors on (111) silicon surface and investigated the strain effects, which were generally ignored in previous works, for the precise stress measurements. We obtained the corrected values of the pi-coefficients for p- and n-type silicon by considering the strain effects, without which more than 25% discrepancies may be induced for small pi-coefficients. We observed that ignoring the strain effects was one of the potential errors which induce large discrepancies in stress measurements, especially for n-type stress sensors on (111) silicon.

As a part of an effort to determine the ideal storage solution for pressurized water reactor (PWR) spent nuclear fuel, a cost assessment was performed to better quantify the competitiveness of several storage types. Several storage solutions were chosen for comparison, including three dry storage concepts and a wet storage concept. The net present value (NPV) and the levelized unit cost (LUC) of each solution were calculated, taking into consideration established scenarios and facility size. Wet storage was calculated to be the most expensive solution for a 1700 MTU facility, and metal cask storage marked the highest cost for a 5000 MTU facility. Sensitivity analyses on discount rate, metal cask price, operation and maintenance cost, and facility size revealed that the system price is the most decisive factor affecting competitiveness among the storage types.

We investigated the effects of various field plate and buried gate structures on the DC and small signal characteristics of 4H-silicon carbide (SiC) metal-semiconductor field-effect transistors (MES-FETs). In comparison with the source-connected field plate, the gate-connected field plate exhibited superior frequency response while having similar DC characteristics. In order to further enhance the output power, dual field plates were employed in conjunction with a buried gate structure.

Young’s modulus ( E ) and Poisson’s ratio ( ν ) are dependent upon the direction on the silicon surface. In this work, E and ν of silicon have been calculated analytically for any crystallographic direction of silicon by using compliance coefficients ( s 11 , s 12 , and s 44 ), and the values of E are confirmed experimentally by using a “beam deflection” method with a four-point bending fixture. Experimental results for E as a function of temperature from −150 °C to +150 °C are presented for (0 0 1) and (1 1 1) silicon wafers.

Stress sensing test chips are widely utilized to investigate integrated circuit die stresses arising from assembly and packaging operations. In order to utilize these test chips to measure stresses over a wide range of temperatures, one must have values of six piezoresistive coefficients for n- and p-type silicon over the temperature range of interest. However, the literature provides limited data over the desired range, and even the data at room temperature exhibit wide discrepancies in magnitude as well as sign. Thus, this work focuses on an extensive experimental study of the temperature dependence of the fundamental piezoresistive coefficients, pi11, pi12, and pi44, for both p- and n-type silicon from -150degC to +125degC, as well as a number of useful combined coefficients. Measurements were performed using stress sensors fabricated on (001) silicon. In order to minimize errors associated with misalignment with the crystallographic axes on (001) silicon wafers, anisotropic wet etching was used to accurately locate the axes. Four-point bending (4PB) was used to generate the required stress in strip-on-beam samples, and finite-element simulations were used to determine the states of stress in the silicon material.

Stress sensing test chips fabricated on (001) and (111) silicon surfaces are capable of measuring die stresses arising from assembly and packaging operations. The chips incorporate resistor or transistor sensing elements that are used to measure stresses via the observation of the changes in their resistivity/mobility. The piezoresistive behavior of such sensors is characterized by three piezoresistive (pi) coefficients, which are electro-mechanical material constants. Stress sensor rosette elements are often designed with serpentine layouts in order to achieve acceptable resistance for measurement and to eliminate area lost to multiple contacts. The transverse sensitivity of such designs induces errors in the values of piezoresistive coefficients extracted from the sensor elements, and two cases are important. First, in order to determine values of the three fundamental piezoresistive coefficients of silicon, any transverse sensitivity dependencies must be corrected for, or eliminated from, the calibration elements. In contrast, however, application of the resistors as stress sensors requires the use of uncorrected values of the piezoresistive coefficients in the stress data reduction. This work focuses on a study of transverse sensitivity effects in resistors fabricated in silicon and corrected values of the piezoresistive coefficients are confirmed experimentally by comparing pairs of resistive stress sensors (transverse and transverse-free) on the (001) silicon wafer plane. The stress sensitivity for transverse-free cases was observed to be approximately 25–30% larger than that for transverse cases. The effects of crystallographic misalignment and lateral diffusion that occur during the fabrication process are also included in the discussion.

Piezoresistive stress sensors on the (111) surface of silicon offer the unique ability to measure the complete stress state at a point in the (111) material. However, four-point bending or wafer-level calibration methods can measure only four of the six piezoresistive coefficients for p- and n-type resistors required for application of these sensors. In this work, a hydrostatic test method has been developed in which a high-capacity pressure vessel is used to apply a triaxial load to a single die over the -25degC to+100 degC temperature range. The slopes of the adjusted resistance change versus pressure plots yield pressure coefficients for p- and n-type silicon that provide the additional information necessary to fully determine the complete set of piezoresistive coefficients.

A novel AlGaN/GaN-on-Si rectifier with a gated ohmic anode has been proposed to reduce the turn-on voltage without breakdown-voltage degradation. The combination of an ohmic anode and a recessed Schottky gate is responsible for the low turn-on voltage and thus increases the forward current. In comparison with conventional Schottky diodes, the forward current at 1.5 V was increased by 2 to 3 times, whereas no breakdown-voltage degradation was observed. The proposed rectifier with an anode-to-cathode distance of 18 μm exhibited a turn-on voltage of 0.37 V, a forward current density of 92 mA/mm at 1.5 V, and a breakdown voltage of 1440 V.

In this work, easy to fabricate edge-fed, single-substrate, slotted, square-patch antennas were designed, fabricated, and measured. Effects of slots on patch antenna, such as the size reduction effect and the multi-resonance effect, are demonstrated by examples. Measurements show that up to four additional resonant frequencies below the original patch antenna’s TM020 frequency are obtainable and that the size of the antenna can be reduced to less than approximately 40% its original size. The size reduction property of the proposed antenna may be suitable for miniaturized planar and conformal antennas that are needed for personal communication devices, and the multifrequency property of the proposed antenna has the advantage of serving multiple frequencies with one antenna.