Effects of SiC nanofluid on reflood heat transfer and hydrodynamic cavitation on its crud-like deposition for light water reactors

Nuclear Science & EngineeringEfficiency and safety have been considered as the one of the important factors in the nuclear power plant. To enhance the efficiency and safety of the nuclear power plant, the heat transfer between fuel rods and coolant is important. For that reason, many researchers have been studying the heat transfer phenomena of the fuel rod in view of the coating layer. To analyze and resolve the coating effect in the fuel rods, this study consists of two parts of reflooding using nanofluids and removal test of crud using the cavitation. Nanofluids were engineered by dispersing nanoparticles having high thermal conductivity into conventional heat transfer fluids. Because of these nanoparticles, the heat transfer performance of the fuel rod having coated surface is improved. This attractive property ensures that nanofluids can play an important role as a coolant of the heat removal by quenching the fuel rod during LOCA (Loss of Coolant Accident). Generally, the quenching experiments have been performed using the heated sphere and rodlet in the water and nanofliud (2-6). The results show the enhanced cooling performance like shifted cooling curve and reduced cooling time. Several quenched specimens that were exposed to high temperature in the working fluid have been gradually reducing the quenching time. Enhanced cooling performance was due to the specimen???s structure with both dispersed and coated nanoparticle. This result shows the feasibility of nanofluid to adapt in ECCS (Emergency Core Cooling System) and primary system. During LOCA, the heated fuel rod is rapidly contact with coolant and particles of nanofluid will be attached on the fuel rod surface. The surface coated these nanoparticle shows porosity. It is similar with crud. In nuclear power plant, crud is known as composition of corrosion and oxidation materials. It has a porous structure which is combined with boron that is injected into the coolant for control of the power levels. The buildup of corrosion products on the fuel cladding surface has proven to be particularly significant for both BWRs and PWRs. The high temperature of the cladding surface attracts impurities and chemical additives in the reactor coolant that were deposited on the fuel rod surface in the process. The deposits on a fuel rod, known as crud, can be tenacious, insulative that one compounds capable of increasing the local clad temperature and accelerating clad corrosion???sometimes to the point of fuel failure. Crud composition on fuel cladding surfaces causes uneven heating of the reactor core. The situation is exacerbated by boron and added to the coolant to control power levels. In Part I, to observe the quenching phenomenon of nanofluids, the test facility with a long vertical tube was designed and made. The experiments were conducted by using 0.01% volume fraction SiC/water and GO (Graphene Oxide)/water nanofluids as a coolant. By performing the experiments, the heat transfer performance of nanofluids were observed and compared to water. Compared with deionized water, the cooling time of nanofluids was reduced 20 sec from and approximately 20 degree of contact angle on the tube was smaller. In Part II, the possibility of removing the crud was studied using deposition like crud which is coated by SiC nanofluid and is exposed in the cavitation at each cavitation number. To optimize the removal effect, two type of flow was conducted as like common orifice flow and swirl orifice flow. In the case of experiment, the maximum shock pressure was gained during swirl flow at low cavitation number. It shows the concentrated pressure on pressure film than common flow. And, it shows maximized erosion effect at cavitation number 0.6. Adoption of nanofluid to research the probability of SiC/water nanofluid and GO/water nanofluid can attribute to enhance the fuel performance in point of efficiency caused by enhanced cooling performance. Undesired specimens, as like crud, can be removed by cavitation phenomenon. In order to adopt cavitation, length of cavitation cloud should be considered to optimize the removal effect, because collapsing cavitation bubble is following the cavitation cloud length and the cavitation number. And SiC cladding material can bear the cavitation flow at above the cavitation number 0.7.ope

Temperature-dependent evolutions of excitonic superfluid plasma frequency in a srong excitonic insulator candidate, Ta2_2NiSe5_5

We investigate an interesting anisotropic van der Waals material, Ta$_{2}$NiSe$_{5}$, using optical spectroscopy. Ta$_{2}$NiSe$_{5}$ has been known as one of the few excitonic insulators proposed over 50 years ago. Ta$_{2}$NiSe$_{5}$ has quasi-one dimensional chains along the $a$-axis. We have obtained anisotropic optical properties of a single crystal Ta$_{2}$NiSe$_{5}$ along the $a$- and $c$-axes. The measured $a$- and $c$-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the $a$-axis optical conductivity, a sharp peak near 3050 cm$^{-1}$ at 9 K, with a well-defined optical gap ($\Delta^{EI} \simeq$ 1800 cm$^{-1}$) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around $\sim$325 K($T_c^{EI}$). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap ($\Delta^{EI}(T)/\Delta^{EI}(0)$) is $1-(T/T_c^{EI})^2$. The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta$_{2}$NiSe$_{5}$ has been determined from measured optical data. Our findings may be useful for future research on excitonic insulators.Comment: 17 pages, 5 figure

Study on fatigue experiment for transverse butt weldsunder 2G and 3G weld positions

ABSTRACTAlthough the transverse butt weld method with ceramic backing strip has been widely used in various industrial fields for its fabricational convenience, it is rarely used in offshore industries since the fatigue strength of the weld joint has not been proved sufficiently. This study conducted fatigue tests for series of butt weld specimens with horizontal (2G) and vertical (3G) welding positions in order to verify the fatigue strength compared to S-N curve by DNV (Det Norske Veritas), IIW (International Institute of Welding) and Eurocode 3. The difference of the 2G specimens and the 3G specimens are investigated in terms of angular distortion and the effect on the fatigue strength are analyzed