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• The compressible governing equations including turbulent mass flux terms were applied. • The property-dependent turbulent Prandtl number was applied. • The proposed method were successful in reproducing highly buoyant upward flowing fluid in tubes. Accurate estimation of thermal-hydraulic characteristics of supercritical flows has long been an attractive but elusive subject to many researchers in spite of tremendous effort devoted to the development of suitable turbulence models. One of the key reasons for the difficulty is a lack of measured turbulence data, which might have been used to formulate adequate turbulence models suitable for highly buoyant fluids. Turbulence models are typically based on the log-law, while the velocity profile in buoyant fluids substantially deviates from the log-law because of significant density variation in a turbulent boundary layer. In this paper, axisymmetric compressible Reynolds-Averaged governing equations were employed together with the property-dependent turbulent Prandtl number to reproduce experimental data representing heat transfer deterioration and consequential sudden temperature increase. The additional turbulence terms associated with turbulent mass flux appeared in the governing equations were modeled using the simple gradient diffusion hypothesis (SGDH). The proposed model successfully reproduced the experimental data. The various turbulence properties are presented and discussed. [ABSTRACT FROM AUTHOR]

Low-Reynolds-number k - ε turbulence models have been successfully used by numerous researchers in various applications. It has been found that the Myong-Kasagi model (MK) among them outperforms in simulations of thermal-fluid fields at supercritical pressures and near the corresponding pseudo-critical temperature. However, they are used as is without a clear understanding of the cause of the good performance. In this paper, several well-known low-Reynolds-number turbulence models, including MK, are critically reviewed against DNS data and RANS calculation results to find the reasons, if any exist, for the superiority of MK model. The most outstanding factor identified may be the fact that MK introduced the Taylor microscale as the near-wall length scale and combined it with the integral length to result in a combined turbulence length scale, which is valid over the entire range of a turbulent boundary layer. The eddy viscosity formula with the incorporation of the turbulence length scale is naturally expected to provide a better representation of flows with strong buoyancy due to wall heating, especially in the near-wall region, where the buoyancy effect mainly occurs. As a result, MK-simulated highly buoyant flows showed excellent agreement with experimental data when applied with the property-dependent turbulent Prandtl number and shear-stress-dependent damping length. A comparison with DNS data of the turbulence data obtained from RANS calculations with MK also showed a good agreement. [ABSTRACT FROM AUTHOR]

When fluids at supercritical pressure enter into a region close to the pseudo-critical temperature, their numerical simulations with constant turbulent Prandtl number, Pr t , and conventional damping function in turbulence modeling are never successful in reproducing the fluid–thermal behaviors, since fluids experience strong physical property variations across the turbulent boundary layer, and their influence has not been properly incorporated into a turbulence model. Several experimental data and numerical studies have indicated that Pr t can be very smaller or larger than unity in a region of strong property variation. Recent research, both numerical and experimental, has also indicated that Pr t is very likely a function of fluid–thermal variables, when the gradients of the physical properties of the fluid are significant. In this regard, the property-dependent turbulent Pr t as a function of physical properties, which has recently been published by the first author, was introduced. Another point deserving attention is that turbulent boundary layer (TBL) deforms so severely, and so does the effective viscous sublayer thickness, A + , such that A + is no longer a constant, but a function of the degree of TBL deformation due to buoyancy. A + is expressed as a function of buoyancy and acceleration parameters when using already-known information. With this introduction of the functional form for A + it is believed that the velocity overshoot and consequential TBL deformation are taken into account. The numerical simulations with property-dependent Pr t and buoyancy-dependent A + of flows under strong buoyancy agreed very well with the experimental data. [ABSTRACT FROM AUTHOR]

The potential of reducing the oxidation of the supporting graphite components during normal and/or accident conditions in the Very High Temperature Reactor (VHTR) design has been studied. In this work efforts have been made to slow the oxidation process of the graphite with a thin SiC coating (∼ 10 μm). Upon heating at ≥ 1173 K in air, the spallations and cracks were formed in the dense columnar structured SiC coating layer grown on the graphite with a functionally gradient electron beam physical vapor deposition (EB-PVD. In accordance with the formations of these defects, the sample was vigorously oxidized, leaving only the SiC coating layer. Then, efforts were made to heal the surface defects using additional EB-PVD with ion beam bombardment and chemical vapor deposition (CVD). The EB-PVD did not effectively heal the cracks. But, the CVD was more appropriate for crack healing, likely due to its excellent crack line filling capability with a high density and high aspect ratio. It took ∼ 34 min for the 20% weight loss of the CVD crack healed sample in the oxidation test with annealing at 1173 K, while it took ∼ 8 min for the EB-PVD coated sample, which means it took ∼4 times longer at 1173 K for the same weight reduction in this experimental set-up. [ABSTRACT FROM AUTHOR]

Abstract: Atomistic structure change in a nuclear grade graphite irradiated at 353 K to 3.4×1017 ion/cm2 with 3 MeV H+ was characterized by measuring positron lifetime and Raman spectrum at room temperature. It is evident from the positron lifetime results that the pre-existing structural defect is disoriented crystalline boundaries, and vacancy clusters ranging from di- to quadruple-vacancies were newly formed after ion irradiation. The relative intensity ratio of the Raman D and G peaks increased from 0.25 to 0.67 after ion irradiation. The concentration of radiation-induced vacancies was reasonably estimated by the Raman intensity ratio. [ABSTRACT FROM AUTHOR]

• We obtained various optimal thicknesses of the coating layers of an 800-μm UO 2 TRISO. • Failure fraction and fission product release are evaluated for the optimal TRISO. • The optimal thicknesses of buffer, IPyC, SiC, OPyC are 110/40/57/35, 120/35/60/27 μm. • They are suitable for a 10-MWth block-type micro modular HTGR within 7600 days. The study evaluates the optimal thicknesses of the coating layers of an 800-μm UO 2 tri-structural isotropic coated fuel particle (TRISO) for a 10-MW th block-type micro modular high temperature gas-cooled reactor (HTGR) using a response surface methodology (RSM) and an HTGR fuel performance analysis code, COPA. The objective function to be minimized in the optimal design of a TRISO consists of the product of the packing fraction of TRISO particles in a compact, the fractional releases of major metallic fission products from a TRISO and the failure probability of the silicon carbide (SiC) layers. A constraint on the sum of the coating layer thicknesses was made using a maximum packing fraction. Two fuel performance analyses such as a failure fraction analysis and a fission product release analysis are applied to the optimal designs of a TRISO that the RSM produced. An 800-μm UO 2 TRISO having buffer, inner high-density pyrocarbon (IPyC), SiC, outer high-density pyrocarbon (OPyC), each with a thickness of 110, 40, 57, 35 μm, or 120, 35, 60, 27 μm is suitable for the operation of a 10-MW th block-type micro modular HTGR within 7600 days when both the SiC and through-coatings failure fractions do not occur and the fractional release of silver, cesium, strontium and krypton are below 1 × 10−9. The packing fraction of the optimal TRISOs in a compact is 40%. [ABSTRACT FROM AUTHOR]

This study deals with the high temperature low cycle fatigue failure properties of Alloy 617 base metal and weldments at 950 °C. Comparative study on a wide range of strain controls and its microstructural heterogeneity within base metal and weldments during high temperature fatigue is discussed. Due to the fact that the weldment structures have a degradation impact on the specimens' fatigue lifetime against the base metal. Results are provided for some baseline data about the prediction of the Alloy 617 and its joint service lifetime. The high temperature low cycle fatigue properties of the Alloy 617 and its joint are presented and comparison of other existing literatures are given. Statistical analysis through the strain-life and strain-energy density as a suitable fatigue parameter for characterizing the Alloy 617 base metal and weldments is described. Metallographic studies are presented for understanding the initiation and propagation of fatigue cracks, including effects of sequential brittle oxide layer formation. [ABSTRACT FROM AUTHOR]

To obtain high tritium breeding performance with limited blanket thickness, most of solid breeder blanket concepts employ a combination of lithium ceramic as a breeder and beryllium as a multiplier. In this case, considering that huge amount of beryllium are needed in fusion power plants, its handling difficulty and cost can be a major factor to be accounted for commercial use. Korea has proposed a Helium-Cooled Ceramic Reflector (HCCR) breeding blanket concept relevant to fusion power plants. Here, graphite is used as a reflector material by reducing the amount of beryllium multiplier. Its feasibility has been investigated in view point of the nuclear performance as well as material-related issues. In this paper, a nuclear analysis is performed under the fusion reactor condition to address the feasibility of graphite reflector in breeding blanket, considering tritium breeding capability and neutron shielding and activation aspects. Also, the chemical stability of the graphite is investigated considering the chemical stability under accident conditions, resulting in that the adaptation of graphite reflector in breeding blanket is intrinsically safe and plausible under fusion reactor condition. [ABSTRACT FROM AUTHOR]