Yonghee Kim

Burnable absorber-integrated Guide Thimble (BigT) – I: design concepts and neutronic characterization on the fuel assembly benchmarks

This paper presents the conceptual designs of a new burnable absorber (BA) for the pressurized water reactor (PWR), which is named ‘Burnable absorber-integrated Guide Thimble’ (BigT). The BigT integrates BA materials into standard guide thimble in a PWR fuel assembly. Neutronic sensitivities and practical design considerations of the BigT concept are points of highlight in the first half of the paper. Specifically, the BigT concepts are characterized in view of its BA material and spatial self-shielding variations. In addition, the BigT replaceability requirement, bottom-end design specifications and thermal–hydraulic considerations are also deliberated. Meanwhile, much of the second half of the paper is devoted to demonstrate practical viability of the BigT absorbers via comparative evaluations against the conventional BA technologies in representative 17×17 and 16×16 fuel assembly lattices. For the 17×17 lattice evaluations, all three BigT variants are benchmarked against Westinghouses existing BA technologies, while in the 16×16 assembly analyses, the BigT designs are compared against traditional integral gadolinia-urania rod design. All analyses clearly show that the BigT absorbers perform as well as the commercial BA technologies in terms of reactivity and power peaking management. In addition, it has been shown that sufficiently high control rod worth can be obtained with the BigT absorbers in place. All neutronic simulations were completed using the Monte Carlo Serpent code with ENDF/B-VII.0 library.

Improvement of Nodal Accuracy by Using Albedo-Corrected Parameterized Equivalence Constants

Abstract This paper introduces the albedo-corrected parameterized equivalence constants (APEC) method, a new method for correcting the homogenized two-group cross sections of the pressurized water reactor (PWR) fuel assemblies (FAs) by taking into account the neutron leakage. First, an analysis was performed of the position dependence of the assembly-homogenized two-group cross sections in an actual core. In order to eliminate the two-group cross-section error in the conventional homogenization method, the APEC method is proposed which parameterizes the homogenized two-group cross sections in terms of an integrated albedo information current-to-flux ratio (CFR). Also, small color-set models are introduced to obtain physically meaningful CFR boundary conditions for the APEC method and their characteristic features are discussed. In the case of FAs with neighboring baffle, slightly modified APEC functions are introduced to deal with the strong spectral interaction between the FA and the baffle-reflector region in PWRs. In addition, an improved APEC function is developed by explicitly accounting for the neutron spectrum change in a FA in terms of a spectral index defined as the fast-to-thermal-flux ratio. For the test of the proposed APEC functions, a small modular reactor (SMR) core was chosen and comparative analyses were performed in detail for each type of homogenized two-group cross section. In this work, the transport lattice code DeCART2D was used for the analysis of the benchmark problems. In the comparative analyses, the APEC-corrected cross sections were compared with the conventional two-group constants and reference ones for several representative FAs. The APEC algorithm was implemented into an in-house nodal expansion method code in conjunction with a partial-current CMFD (p-CMFD) acceleration. The nodal analyses of an SMR initial core and a large PWR core were performed to evaluate the performance of the APEC method. In order to show the generality of the APEC functions obtained from lattice calculations, several modified core configurations were also analyzed. In addition, a rodded SMR initial core problem was also analyzed to test the APEC method in an extremely abnormal core configuration. The nodal analyses showed that the APEC method can improve the nodal accuracy significantly with a small amount of additional computing cost.

Burnable absorber-integrated guide thimble (BigT) – II: application to 3D PWR core design

ABSTRACT This paper is a companion to its immediate predecessor in the “Burnable absorber-integrated Guide Thimble” (BigT) series. It aims to demonstrate potential applications of the BigT concepts in a three-dimensional (3D) commercial pressurized water reactor core, which is based on the AP1000 first core design. The study specifically compares neutronic characteristics of the reference core against a BigT-loaded design. In this study, reactivity depletion patterns of nine fuel assembly lattices in the reference core were first evaluated. Corresponding sets of BigT-loaded assemblies that yield neutronically similar characteristics (i.e., initial reactivity suppression and depletion trend) with those of the reference assembly lattices were determined next. These BigT-loaded fuel assemblies were later loaded in place of the reference fuel assemblies for the subsequent high-fidelity 3D Monte Carlo core simulations. Results of the study clearly demonstrate that the BigT-loaded AP1000 first core performs as well as the reference core since all neutronic parameters are comparable, especially in terms of reactivity depletion, power peaking factors and shutdown margin. All simulations were completed using the Monte Carlo Serpent code with ENDF/B-VII.0 library.

Use of Er Burnable Absorber for Improvement of CANDU6 Safety Parameters

Abstract This paper reports on the improvement of the power coefficient of reactivity (PCR) and minimization of the coolant void reactivity (CVR) of a CANDU6 reactor. A burnable absorber of Er2O3 (erbia) was mixed homogeneously with UO2 fuel in the central fuel element to maximize the Doppler broadening and minimize the CVR of the CANDU6 reactor. In this study, recovered uranium (RU) with 0.9 wt% 235U enrichment was utilized in the advanced CANFLEX fuel bundle instead of natural uranium (NU). First, the optimal loading of erbia was investigated through lattice-based analysis, and its impact on the lattice characteristics was examined. In particular, both the fuel Doppler effect and CVR were evaluated for the RU-loaded lattice. For a more reliable analysis, a three-dimensional (3-D) equilibrium core was determined based on the standard time-average methods for erbia-loaded CANDU6 cores using the Serpent-COREDAX/CANDU code system. The core analysis was based on a hybrid two-step method in which the lattice analysis was performed by the Serpent Monte Carlo code, and the 3-D whole-core analysis was done using a diffusion theory–based nodal code named COREDAX. For the derived equilibrium cores, the core performances were evaluated in terms of the fuel burnup and power profile. Additionally, the safety parameters, including the PCR and CVR, were evaluated for the equilibrium core conditions. The safety parameters of the 3-D whole core were compared with those obtained with simple lattice-based analysis. It was observed in the analysis that Er-loaded CANFLEX-RU fuel provides a 60% more negative fuel temperature coefficient than standard CANDU-NU fuel.

Measurement and Analysis for Determination of PCR of the CANDU6 Core

Abstract The power coefficient of reactivity (PCR) needs to be negative to achieve the inherent safety of a reactor. However, the possibility that the PCR of CANada Deuterium Uranium (CANDU) reactors can be positive has been raised in recent studies. In such circumstances, there was an experimental approach on evaluating the PCR of CANDU in 2012 at an in-operation CANDU reactor, Wolsong Unit 2. In the evaluation, the PCR was indirectly measured by a method that required estimating the reactivity variation due to Xe, liquid zone controllers (LZCs), and fuel depletion based on the measurement data. In this study, the PCR of a CANDU was reevaluated by the same methodology with more proper and detailed methods to estimate all the factors in addition to some minor reactivity corrections. The estimation of Xe and LZC reactivity was performed by an in-house three-dimensional code and Serpent2 in addition to RFSP-IST. Furthermore, several short studies regarding the factors that result in uncertainty of the Xe/LZC reactivity estimation were done in detail. First, a method to determine 14 LZC levels at a certain time based on the measurement data was appropriately selected through determining the features of the measurement data. The influence of the power transient scheme and the impact of local refueling transients due to daily refueling of CANDU reactors on xenon reactivity estimation were also analyzed briefly. Finally, the PCR of the CANDU in operational conditions was evaluated to be ~0.5 pcm/%P on average at a measurement time of 5 to 20 min after the power perturbation.

Diffusion-Based Finite Element Method to Estimate the Reactivity Changes due to Core Deformation in an SFR

Abstract Thermomechanical effects, irradiation, and structural restrictions result in very tangled behavior of assemblies in sodium-cooled fast reactors (SFRs). Reactivity feedback caused by the assembly behavior (deformation or distortion) is one of the key parameters in the inherent safety analysis of fast reactor systems. However, to date there has been no accurate and efficient deterministic way to compute directly the reactivity changes by actual local perturbation. This paper evaluates the feasibility of applying the Galerkin finite element method (GFEM) based on linear shape functions to estimate reactivity changes due to local core deformations in SFRs. Assessment of reactivity changes is conducted for six types of deformation scenarios of the two-dimensional prototype Gen-IV SFR. Uniform expansions and local deformations are included in the scenarios. The results from the multigroup diffusion equation based on the GFEM are compared with references calculated by MCNP5. The study shows that diffusion analysis based on the GFEM with linear shape functions can properly estimate reactivity changes by core deformation in the fast reactor with ~13% relative error of Δρ.

Functionalization of the Discontinuity Factor in the Albedo-Corrected Parameterized Equivalence Constants (APEC) Method

Abstract The Albedo-corrected Parameterized Equivalence Constants (APEC) method, a new leakage correction method for two-group nodal analysis of light water reactors, has been extended to discontinuity factor (DF) correction. First, the error of nodal calculations induced by an inaccurate assembly discontinuity factor (ADF) is evaluated using the reference two-group cross section (XS) and DF calculated from heterogeneous core transport calculations. Functionalization of DF is performed by finding relationships between surfacewise current-to-flux ratio and change of DF from ADF. The least-squares method is used to fit several candidate functions to various core calculation results. The coefficients of APEC XS and DF correction functions are determined considering several color-set models. In this work, the two-dimensional method of characteristics–based lattice code DeCART2D is used for reference core calculations and lattice calculations. The extended APEC method is implemented in an in-house NEM nodal code using the partial-current coarse mesh finite difference acceleration. A small modular reactor (SMR) initial core benchmark is analyzed to evaluate the performance of the extended APEC method. In addition, the extended APEC method is applied to several variants of the SMR core and large variants to assess its general applicability.

A Study on Reconstruction of Intrapin Power Distribution in Pinwise Two-Group Diffusion Analysis

Abstract Detailed pin-by-pin core calculations are under development to replace the conventional assembly-based nodal methods. This research investigates a novel intrapin reconstruction procedure coupled with these pinwise calculations to obtain a detailed power profile within a fuel rod. The reconstruction process is based on the well-established form function (FF) method. In this paper, the fuel rod is geometrically divided into 40 equi-volume subsections where the intrapin power is reconstructed with corresponding heterogeneous FF. The intrapin homogeneous flux distributions are approximated by using the analytical solution of the two-group neutron diffusion equation with pinwise boundary constraints. Four types of constraints are considered to determine the flux shapes: surface-average net current, surface-average, corner-point, and volume-average cell fluxes. Therefore, six different combinations of the boundary constraints are separately evaluated for the intrapin power profile. All necessary information, including burnup-dependent FFs, homogenized group constants, reference power distribution, and pinwise boundary constraints, are predetermined from a high-fidelity Monte Carlo calculation. The numerical results demonstrate that the intrapin power can be retrieved for enriched and Gd-loaded fuel pins with reasonable accuracy, even at rodded conditions and in highly burned conditions of 10 and 30 GWd/tonne U. In addition, a sensitivity analysis is also performed to assess the feasibility of the proposed method when it is coupled with a pinwise calculation.

Convergence Studies on Nonlinear Coarse-Mesh Finite Difference Accelerations for Neutron Transport Analysis

Abstract Application of partial current–based coarse-mesh finite difference (pCMFD) acceleration to a one-node scheme is devised for stability enhancement of the parallel neutron transport calculation algorithm. Conventional one-node coarse-mesh finite difference (CMFD) allows parallel algorithms to be more tractable than two-node CMFD, but it has an inherent stability issue for some problems. In order to overcome this issue, pCMFD is modified to be fitted into the one-node scheme and is tested for both sequential and parallel calculations. The superior stability of the one-node pCMFD is shown by comparing results from analytic and numerical approaches. To investigate the convergence behavior of the acceleration methods in an analytic way, Fourier analysis is applied to an infinite homogeneous slab reactor configuration with the monoenergetic neutron flux assumption, and the spectral radius is calculated as a convergence factor. This paper carefully describes the process of the Fourier analysis on the parallel algorithm for neutron transport and compares it to that of the conventional sequential algorithm.

PASSIVE REACTIVITY CONTROL OF NUCLEAR THERMAL PROPULSION REACTORS

This paper explores the possibility of passively controlling the reactivity of a nuclear thermal propulsion (NTP) reactor. The objective of this study is to limit the use of the radial control drums to start-up and shutdown procedures and ensure that the exact same operation is performed for each full-power burn. To achieve the goal, this work considers several design measures, which include a low-density burnable absorber in the tie-tube components of the core, the use of variable hydrogen density in the moderator element coolant passages, and the judicious selection of a modified mission profile to maximize the decay of 135Xe after operation. In addition, the improved stability from the enhanced fuel temperature feedback due to the implementation of low-enriched-uranium fuel is also exploited for the realization of passive reactivity control. In this work, a passive reactivity control system is implemented in the Superb Use of Low Enriched Uranium (SULEU) NTP core and analyzed in terms of its ability to fulfill a NASA Mars Mission Design Reference Architecture 5.0–style mission. It is concluded that the use of the control drums can be limited to start-up and shutdown operations only, eliminating operator input in order to maintain a constant power level in the core.