Pointwise cross-section-based on-the-fly resonance interference treatment with intermediate resonance approximation

AbstractThe effective cross sections (XSs) in the direct whole core calculation code nTRACER are evaluated by the equivalence theory-based resonance-integral-table method using the WIMS-based library as an alternative to the subgroup method. The background XSs, as well as the Dancoff correction factors, were evaluated by the enhanced neutron-current method. A method, with pointwise microscopic XSs on a union-lethargy grid, was used for the generation of resonance-interference factors (RIFs) for mixed resonant absorbers. This method was modified by the intermediate-resonance approximation by replacing the potential XSs for the non-absorbing moderator nuclides with the background XSs and neglecting the resonance-elastic scattering. The resonance-escape probability was implemented to incorporate the energy self-shielding effect in the spectrum. The XSs were improved using the proposed method as compared to the narrow resonance infinite mass-based method. The RIFs were improved by 1% in 235U, 7% in 239Pu, and >2% in 240Pu. To account for thermal feedback, a new feature was incorporated with the interpolation of pre-generated RIFs at the multigroup level and the results compared with the conventional resonance-interference model. This method provided adequate results in terms of XSs and k-eff. The results were verified first by the comparison of RIFs with the exact RIFs, and then comparing the XSs with the McCARD calculations for the homogeneous configurations, with burned fuel containing a mixture of resonant nuclides at different burnups and temperatures. The RIFs and XSs for the mixture showed good agreement, which verified the accuracy of the RIF evaluation using the proposed method. The method was then verified by comparing the XSs for the virtual environment for reactor application-benchmark pin-cell problem, as well as the heterogeneous pin cell containing burned fuel with McCARD. The method works well for homogeneous, as well as heterogeneous configurations

Implementation of the Sanchez-Pomraning Double Heterogeneity Treatment Method in DeCART

The Sanchez-Pomraning method to resolve the double heterogeneity (DH) in the method of characteristics (MOC) transport calculation, which we call DH MOC, is implemented into the DeCART code for the application of the DeCART code to the gas cooled reactors involving particulate triso fuels. Since the DH MOC requires essentially no modification of the existing MOC calculation module other than providing the effective macroscopic cross section and the effective source constructed from the stochastic material properties and micro regional source, implementation is focused on the variable structure changes to incorporate micromaterials and the pre and post processing for the DH MOC calculation. The DH MOC solver is incorporated first in the shielded resonance cross section determination and then as in the normal MOC transport solution. As a prerequisite for the implementation of the DH MOC solver, a method to determine the volume to volume collision probabilities (CP) for a sphere is derived in detail. The verification of the CP routines and other code modifications are performed using various analytical solutions. The final verification of the DH MOC is performed by comparing the DeCART DH MOC results with the MCNP results obtained with an explicit representation of the particulate fuels. The effect of double heterogeneity on the resonance cross section and on the normal MOC transport calculation are assessed separately

Derivation of Analytic Solution and MOC Calculation Procedure for Double Heterogeneity Treatment

The Sanchez-Pomraning method to resolve the double heterogeneity problem in the MOC transport calculation is described in detail. This method is founded on the collision probability method concepts which involves the collision and escape probabilities, and formally derived by using the statistical treatment of the neutron balance equation along a path. The statistical approach bring the concept of the chord and segment length distributions in the formulation of the integral equation for the grain surface and matrix fluxes, which later turns into collision and escape probabilities. For the analytic solution, a boundary layer of a grain thickness, in which no grain is present, is assumed within each flat source region. The analytic solution of the coupled integral equation which involves the convolution integral is derived first by Laplace transform, but finally by substitution. This solution introduces an effective cross section which represents the homogenized mixture of the matrix and grain. With observation that the resulting analytic solution for the matrix is the same as the MOC solution for the homogenized medium, the equivalent source is constructed so that the MOC calculation can be performed for the homogenized mixture. The assumption of the boundary layer causes, however, a problem in the neutron conservation which should be corrected by renormalization. This method requires very little modifications to the existing MOC code to implement the double heterogeneity treatment. Starting from the very basic collision probability relation, the exhaustive derivation and explanation of the all the solution and terms needed to establish the MOC calculation sequence with the double heterogeneity treatment are provided for complete understanding of the reader who might not have sufficient background on this subject

High-Fidelity Light Water Reactor Analysis with the Numerical Nuclear Reactor

The Numerical Nuclear Reactor (NNR) was developed to provide a high-fidelity tool for light water reactor analysis based on first-principles models. High fidelity is accomplished by integrating full physics, highly refined solution modules for the coupled neutronic and thermal-hydraulic phenomena. Each solution module employs methods and models that are formulated faithfully to the first principles governing the physics, real geometry, and constituents. Specifically, the critical analysis elements that are incorporated in the coupled code capability are a direct whole-core neutron transport solution and an ultra-fine-mesh computational fluid dynamics / heat transfer solution, each obtained with explicit (subfuel- pin-cell level) heterogeneous representations of the components of the core. The considerable computational resources required for such highly refined modeling are addressed by using massively parallel computers, which together with the coupled codes constitute the NNR. To establish confidence in the NNR methodology, verification and validation of the solution modules have been performed and are continuing for both the neutronic module and the thermal-hydraulic module for single-phase and two-phase boiling conditions under prototypical pressurized water reactor and boiling water reactor conditions. This paper describes the features of the NNR and validation of each module and provides the results of several coupled code calculations.This work was supported by the I-NERI program jointly funded by the Ministry of Science and Technology of Korea and the DOE. Additional activities related to the BWR version of the NNR were supported by the DOE

Multiobjective loading pattern optimization by simulated annealing employing discontinuous penalty function and screening technique

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.This work was supported by the project funded by the Ministry of Knowledge Economy of Korea to develop primary design codes for nuclear power plants

Methods and performance of a three-dimensional whole-core transport code DeCART

DeCART is a three-dimensional whole-core transport code capable of performing direct core calculations at power generating conditions without involving a priori homogenized few-group constant generation. In this paper, the methods of DeCART, which are characterized by the planar method of characteristics (MOC) solutions, the cell based coarse mesh finite difference (CMFD) formulation, the subgroup method for resonance treatment and subpin level thermal feedback, are presented as a whole. The performance of the code in the aspect of solution accuracy and computing speed is then examined using the applications to the C5G7MOX benchmark and its modified rodded variation problems and also to a three-dimensional core case involving thermal feedback. The examination indicates that accurate direct whole core calculations with subpin level thermal feedback for practical PWR problems are quite possible on affordable LINUX clusters within a time span of a few hours.This work was supported by the International Nuclear Energy Research Initiative (I-NERI) program jointly funded by the Ministry of Science and Technology of Korea and the Department of Energy of the United States

CTCF cooperates with CtIP to drive homologous recombination repair of double-strand breaks

The pleiotropic CCCTC-binding factor (CTCF) plays a role in homologous recombination (HR) repair of DNA double-strand breaks (DSBs). However, the precise mechanistic role of CTCF in HR remains largely unclear. Here, we show that CTCF engages in DNA end resection, which is the initial, crucial step in HR, through its interactions with MRE11 and CtIP. Depletion of CTCF profoundly impairs HR and attenuates CtIP recruitment at DSBs. CTCF physically interacts with MRE11 and CtIP and promotes CtIP recruitment to sites of DNA damage. Subsequently, CTCF facilitates DNA end resection to allow HR, in conjunction with MRE11-CtIP. Notably, the zinc finger domain of CTCF binds to both MRE11 and CtIP and enables proficient CtIP recruitment, DNA end resection and HR. The N-terminus of CTCF is able to bind to only MRE11 and its C-terminus is incapable of binding to MRE11 and CtIP, thereby resulting in compromised CtIP recruitment, DSB resection and HR. Overall, this suggests an important function of CTCF in DNA end resection through the recruitment of CtIP at DSBs. Collectively, our findings identify a critical role of CTCF at the first control point in selecting the HR repair pathway

Omega-3 index and smoking in patients with acute ST-elevation myocardial infarction taking statins: a case-control study in Korea

<p>Abstract</p> <p>Background</p> <p>n-3 fatty acids and lifestyle also are closely related to risk of CVD. Most Koreans have higher fish consumption than people of Western populations. However, little is known about the recommended value of omega-3 index in Korean patients with acute ST-elevation myocardial infarction (STEMI) taking statins. Here, we tested the hypothesis that lower omega-3 fatty acids and/or smoking are associated with acute STEMI, even though patients with dyslipidemia who were taking statins and who attained their LDL-C goals.</p> <p>Methods</p> <p>We conducted a case-control study in which omega-3 fatty acids and lifestyle factors were determined in 24 consecutive Korean patients taking statins with angiographically confirmed acute STEMI and 68 healthy controls without acute STEMI. The omega-3 index was calculated by the sum of eicosapentaenoic acid and docosahexaenoic acid in erythrocyte membranes. Multivariable adjusted regression analysis was used to assess independent associations between acute STEMI, omega-3 index, and lifestyle factors after adjusting for age, sex, and body mass index (BMI).</p> <p>Results</p> <p>The mean age of total subjects was 59.9 years, and 57.6% of the subjects were male. The omega-3 index was significantly lower in cases (8.83%) than controls (11.13%; P < 0.001); however, total <it>trans</it>-fatty acids were not different between the two groups. The omega-3 index was inversely associated with odds for being a case (OR 0.16 (95% CI 0.03-1.14); P = 0.047), while smoking was positively associated with odds for being a case (OR 6.67 (95% CI 1.77-25.23); P = 0.005) after adjusting for all confounding variables.</p> <p>Conclusion</p> <p>This study shows that relative to controls, acute STEMI cases are more likely to be smokers and to have a lower omega-3 index, even though the cases were taking statins. An omega-3 index of at least 11% and abstinence from smoking are associated with cardioprotection for Koreans.</p