Hyun-Chul Lee

From the spectrum to inflation: an inverse formula for the general slow-roll spectrum

We propose a general inverse formula for extracting inflationary parameters from the power spectrum of cosmological perturbations. Under the general slow-roll scheme, which helps to probe the properties of inflation in a model independent way, we invert the leading-order, single-field, power spectrum formula. We also give some physically interesting examples to demonstrate its wide applicability and illuminate its properties.

A New delta N formalism for multi-component inflation

The δN formula that relates the final curvature perturbation on comoving slices to the inflaton perturbation on flat slices after horizon crossing is a powerful and intuitive tool for computing the curvature perturbation spectrum from inflation. However, it is customarily assumed further that the conventional slow-roll condition is satisfied, and satisfied by all components, during horizon crossing. In this paper, we develop a new δN formalism for multi-component inflation that can be applied in the most general situations. This allows us to generalize the idea of general slow-roll inflation to the multi-component case, in particular only applying the general slow-roll condition to the relevant component. We compute the power spectrum of the curvature perturbation in multi-component general slow-roll inflation, and find that under quite general conditions it is invertible.

A computational model for evaluating the effects of attention, memory, and mental models on situation assessment of nuclear power plant operators

Abstract Operators in nuclear power plants have to acquire information from human system interfaces (HSIs) and the environment in order to create, update, and confirm their understanding of a plant state, as failures of situation assessment may cause wrong decisions for process control and finally errors of commission in nuclear power plants. A few computational models that can be used to predict and quantify the situation awareness of operators have been suggested. However, these models do not sufficiently consider human characteristics for nuclear power plant operators. In this paper, we propose a computational model for situation assessment of nuclear power plant operators using a Bayesian network. This model incorporates human factors significantly affecting operators’ situation assessment, such as attention, working memory decay, and mental model. As this proposed model provides quantitative results of situation assessment and diagnostic performance, we expect that this model can be used in the design and evaluation of human system interfaces as well as the prediction of situation awareness errors in the human reliability analysis.

Multiobjective Loading Pattern Optimization by Simulated Annealing Employing Discontinuous Penalty Function and Screening Technique

Abstract The problem of multiobjective fuel loading pattern (LP) optimization employing high-fidelity three-dimensional (3-D) models is resolved by introducing the concepts of discontinuous penalty function, dominance, and two-dimensional (2-D)–based screening into the simulated annealing (SA) algorithm. Each constraint and objective imposed on a reload LP design is transformed into a discontinuous penalty function that involves a jump to a quadratic variation at the point of the limiting value of the corresponding core characteristics parameter. It is shown that with this discontinuous form the sensitivity of the penalty coefficients is quite weak compared to the stochastic effect of SA. The feasible LPs found during SA update the set of candidate LPs through a dominance check that is done by examining multiple objectives altogether. The 2-D–based screening technique uses a precalculated database of the 2-D solution errors and is shown to be very effective in saving the SA computation time by avoiding 3-D evaluations for the unfavorable LPs that are frequently encountered in SA. Realistic applications of the proposed method to a pressurized water reactor reload LP optimization with the dual objectives of maximizing the cycle length and minimizing the radial peaking factor demonstrate that the method works quite well in practice.

Unified Formulation of Nodal Expansion Method and Analytic Nodal Method Solutions to Two-Group Diffusion Equations

Abstract This paper demonstrates that the analytic nodal method (ANM) solution to two-group (2-G) diffusion equations can be formulated in the same way as the nodal expansion method (NEM) solution, and thereby, the two most popular transverse integrated nodal method formulations can be integrated into a unified nodal method (UNM) formulation. For this purpose, the analytic solution, i.e., the combined homogeneous and particular solution, of transverse-integrated one-dimensional, 2-G diffusion equations is represented by an expansion of analytic basis functions while the expansion coefficients are obtained in the same way as the NEM. The advantages of the UNM formulation are then discussed. It is a stable method in itself so that it does not require approximate schemes to avoid the instability at the near-critical nodes. Because it does not introduce any approximate scheme in conjunction with the stability questions at the near-critical nodes, it is more accurate than the conventional ANM formulation in the case where the latter needs to introduce approximations. It is readily incorporated into a number of existing NEM production codes. These advantages are demonstrated in terms of numerical solutions of Nuclear Energy Agency Committee on Reactor Physics pressurized water reactor benchmark problems.

The Effect of Surface Conditions on Friction by Tip Test

In the present investigation, a tip test based on upsetting and backward extrusion was utilized to characterize the effect of surface roughness of the billet and forming tools, and the type of lubricants on friction. For the test, cylindrical specimens made of aluminum alloys of 6061-O and 2024-O, and single punch and two die sets with different surface topologies, were used with four lubricants such as VG32, VG100, corn oil, and grease. The load levels and tip distances were measured for both materials, and compared with each other to determine shear friction factors at the punch and counter punch interfaces separately, depending on the variation in surface topologies and lubrications using finite element simulations. As a result, a linear relationship among the dimensionless load, tip distance, and shear friction factors at the punch and counter punch interfaces was derived for the experimental conditions investigated. The slope change of this linear relationship from negative to positive clearly depends on the variation in surface conditions at the billet/punch and billet/counter punch interfaces. Also, it was demonstrated that the dimensionless tip distance for the frictionless case can be extrapolated from the experimental data. This value can be used for characterizing the relative effect on friction due to surface conditions at the punch and counter punch, and lubrication quality of the lubricant for the given processing conditions.

Unified Nodal Method for Solution to the Space-Time Kinetics Problems

Abstract The two popular transverse integrated nodal methods (TINMs), the nodal expansion method (NEM) and analytical nodal method (ANM), and the analytic function expansion nodal (AFEN) method are integrated into a single unified nodal formulation for the space-time kinetics calculations in rectangular core geometry. In particular, the nodal coupling equations of the conventional ANM and AFEN method are reformulated by the matrix function theory based on the unified nodal method (UNM) principle for the solution to the transient two-group neutronics benchmark problems. The difference between the two transient AFEN formulations by the UNM and the conventional AFEN principles is pointed out. The performance of the UNM formulation is examined in terms of the solutions to the transient light water reactor benchmark problems such as the Nuclear Energy Agency Committee on Reactor Physics pressurized water reactor rod ejection kinetics benchmark problems. Through comparison of several nodal computational options by the UNM formulation, it is shown that one node-per-fuel assembly (N/A) calculations by the AFEN method are superior to those by the NEM and the ANM, but that 4 N/A calculations by the AFEN method are not better than those by ANM, in prediction accuracy at the sacrifice of the computational time. The advantages of the transient UNM formulation over the conventional TINM and AFEN method formulations are discussed.

Unified nodal method formulation for analytic function expansion nodal method solution to two-group diffusion equations in rectangular geometry

Abstract The analytic function expansion nodal (AFEN) method formulation for the solution to two-group diffusion equations in rectangular geometry is reformulated in the principle of the unified nodal method (UNM) formulation. Except for the corner point neutron balance equations, the nodal coupling relations of the reformulated AFEN method are shown to resemble exactly those of the nodal expansion method (NEM) so that they not only can be easily incorporated into the existing NEM production codes but also can enable one to make the most of the well-established numerical solution schemes including the nonlinear coarse-mesh finite difference (CMFD) schemes for speedy AFEN method calculations. A one-node CMFD scheme for the speedy AFEN calculations of the UNM formulation is newly proposed. The effectiveness of the one-node scheme is compared with that of the two-node CMFD scheme in terms of UNM solutions to the International Atomic Energy Agency and Organization for Economic Cooperation and Development L336 neutronics benchmark problems. Advantages of the UNM formulation for the AFEN method calculations over the original AFEN method formulation are discussed.

Measurement of the effect of surface topology on friction by tip test

In the present investigation, tip test based on backward extrusion was utilized to characterize the effect of surface roughness of the billet and forming tools and type of lubricants on friction. For the test, cylindrical specimens made of aluminum alloys of 6061-O and 2024-O with four lubricants such as VG32, VG100, corn oil, and grease were used. Single punch and two die sets with different surface topologies were manufactured in order to investigate the effect of surface conditions on friction and flow behavior. The load levels and tip distances were measured for both materials and compared with each other to determine shear friction factors at the punch and counter punch interfaces separately depending on the variation of surface topologies using the finite element simulations. As a result, a linear relationship among the dimensionless load, tip distance, and shear friction factors at punch and counter punch interfaces was derived for the experimental conditions investigated. The slope change of this linear relationship from negative to positive clearly depends on the variation of the surface conditions at the billet/punch and billet/counter punch interfaces. Also, it was clearly demonstrated that the dimensionless tip distance for the frictionless case can be extrapolated from the experimental data based on the simulation results. The value for the frictionless case can be used for characterizing the relative effect due to surface topologies at punch and counter punch and lubrication qualities of lubricants under various processing conditions.Copyright