Absorber Rods Research Articles

First principles density functional theory study of tritium species adsorption on Ni(111) surface and diffusion in nickel-sublayer for tritium storage.

The nickel-plated zircaloy-4 is used as a tritium (3H) getter in the tritium-producing burnable absorber rods (TPBARs) to capture 3H produced in the 6Li-riched annular γ-LiAlO2 pellet under neutron irradiation. The experimental data and our previous theoretical results showed that the 3H species produced from the γ-LiAlO2 pellet were mainly 3H2 and 3H2O. These 3H species diffuse from the surface of the LiAlO2 pellet across vacuum to the nickel-plated zircaloy-4 getter and then further diffuse into the getter to chemically form metal hydrides. While a number of studies show that oxygen binds strongly as compared to 3H on the nickel (Ni) layer, the detailed mechanism of 3H species absorption and diffusion across the Ni plate and Ni/Zr interface are still unclear. By employing density functional theory calculations, here we explored the 3H2 and 3H2O species adsorption and dissociation on the Ni(111) surface and diffusion into the Ni sublayer. Our results indicated that the 3H2 and 3H2O dissociate on the Ni(111) surface. The NiOx and Ni(O3H)x could be formed in the Ni layer due to the higher oxygen (O) diffusion energy barrier and formation of Ni vacancy defects. The oxygen was found to be retained in the Ni layer from diffusing across the Ni-Zr interface. This was revealed by comparing the diffusion barriers for 3H with O. 3H was found to have nearly three times smaller diffusion barrier than for O, making 3H comparatively easier to diffuse through the Ni layer. The obtained results provide guidelines for experimental measurements on 3H retention behavior in TPBARs and may open further avenues to explore the impurity effects on 3H diffusion and storage at the Ni/zircaloy interfaces.

Searching for optimal reactivity management using a new burnable absorber for SCLWR with (Th, 233U)O2

This work investigates the efficiency of some new burnable absorbers (BAs) suggested to provide possible improvements in the fuel management of a supercritical light water reactor (SCLWR) assembly fueled with (Th, 233U)O2. Four BAs including gadolinium, Erbium, Iridium and Lutetium are suggested as integral burnable absorber (IBA) rods. A three-dimensional model of SCLWR has been modelled using MCNPX version 2.7. A complete neutronic analysis has been carried out for the suggested BAs and compared with the gadolinium vector which is the common BAs. The IBA rods were distributed through the SCWR fuel assembly in a way that provided a flat power distribution. Various concentrations of the suggested BAs have been investigated to obtain the optimum value that can suppress the excess reactivity at the beginning of the cycle and flatten the infinite multiplication factor. The burnup parameters including the fertile, fissile, BAs and most effective fission product concentration as a function of effective full power days (EFPDs) of the SCLWR have been examined for the suggested BAs. The reactivity temperature coefficients have been studied to ensure the viability of the suggested BAs. A further analysis is performed to study the axial, radial power distribution and local power peaking factor.

Modeling and Visualization of Coolant Flow in a Fuel Rod Bundle of a Small Modular Reactor

This article presents the results of an experimental study of the coolant flow in a fuel rod bundle of a nuclear reactor fuel assembly of a small modular reactor for a small ground-based nuclear power plant. The aim of the work is to experimentally determine the hydrodynamic characteristics of the coolant flow in a fuel rod bundle of a fuel assembly. For this purpose, experimental studies were conducted in an aerodynamic model that included simulators of fuel elements, burnable absorber rods, spacer grids, a central displacer, and stiffening corners. During the experiments, the water coolant flow was modeled using airflow based on the theory of hydrodynamic similarity. The studies were conducted using the pneumometric method and the contrast agent injection method. The flow structure was visualized by contour plots of axial and tangential velocity, as well as the distribution of the contrast agent. During the experiments, the features of the axial flow were identified, and the structure of the cross-flows of the coolant was determined. The database obtained during the experiments can be used to validate CFD programs, refine the methods of thermal-hydraulic calculation of nuclear reactor cores, and also to justify the design of fuel assemblies.

Evaluation of tritium production capability of the 5 MWe gas-cooled graphite-moderated reactor at Yongbyon using tritium-producing burnable absorber rods

The 5 MWe graphite-moderated gas-cooled reactor located at Yongbyon, North Korea, is a major facility for producing nuclear materials. The objective of this study is to evaluate the tritium production capability of the 5 MWe reactor when using a tritium-producing burnable absorber rod (TPBAR). This paper presents burnup analysis results for TPBAR-embedded core models performed by McCARD, a Monte Carlo neutron transport simulation code. For three types of cores with TPBARs, the numerical results show that the 5 MWe reactor can produce 1.83–7.51 g of tritium per year depending on the number of TPBARs in the core. The numerical results can serve as a basis for assessing the feasibility of producing tritium using graphite-moderated gas-cooled reactors.

Reactor core design with enriched gadolinia burnable absorbers for soluble Boron-Free operation in the innovative SMR

Effective strategies for maintaining reactor reactivity at a consistent level are indispensable to enable soluble boron-free (SBF) operation in innovative small modular reactor (i-SMR) operation. This paper presents a reactor core design tailored for SBF operation in i-SMR, utilizing concentrated gadolinia burnable absorber rods. This investigation evaluates the concentration range of 155,157Gd and its reactivity characteristics, which are crucial for sustaining excess reactivity. The viability of employing enriched 155,157Gd for the i-SMR SBF operation is established through comprehensive analysis and simulations. Moreover, integrating 155,157Gd with integral gadolinia burnable absorber rods enhances the reactivity control and operational adaptability. Leveraging the spatial self-shielding effect of gadolinia extends the reactivity management capabilities and ensures the operational performance of the i-SMR as optimizing the cycle length. Core design considerations encompassing FA configurations and control rod patterns are investigated to ensure safety and performance across the initial and reload cycles. The feasibility of the SBF operation is verified by assessing compliance with the top-tier requirements of the i-SMR, thus underlining its potential for practical implementation.

COOLING OF CONTROL RODS AND SPECIFICATION OF CRITICAL TEMPERATURES TO JUSTIFY SAFE OPERATION

The paper analyzes the cooling conditions of the control rods in the mixed core of the WWER-1000 reactor containing fuel assemblies of different vendors (TVSA, WFA/RWFA). The coolant heat-up in the guide thimbles due to own energy release in control rod materials and control rod temperatures required to justify safe operation were calculated for the most conservative case, when the core consists only of TVSAs with a standardized guide thimble inlet plug of NCCP. The energy release was obtained using the calculation method for the boron carbide absorber (B4C), dysprosium titanate (Dy2O3·TiO2), control rod cladding (42CrNiMo), water gap between the cladding and the guide thimble and in the guide thimble (zirconium alloy), fully immersed and 70% immersed RCCA 10th control group. The highest calculated energy release will be observed in fully immersed control rods with boron carbide material at the height position corresponding to the zone of maximum neutron fluence and will be 43.8 W/cm3. The maximum energy release of the 70% immersed (from the bottom of the core) 10th control group is 29.4 W/cm3 for fresh B4C. For the condition of the reactor facility at which the temperatures in FA guide thimbles are maximum, for a "fresh" control rod characterized by maximum energy release in the neutron-absorbing material, the maximum coolant heat-up in the guide thimbles does not exceed 28.9 °C, with the maximum coolant heat-up in FAs being 44 °C, and the maximum control rod cladding temperature not exceeding 335.6 °C at a coolant saturation temperature of 345.8 °C, which indicates that there will be no coolant boiling in the guide thimbles under operation. The maximum temperature typical for the central part of the control rod absorber and for the most conservative calculation for a "fresh" absorber is 529.6 °C, which is significantly lower the melting point of boron carbide, which is ~2350 °C. The maximum calculated temperature in the contact zone of the B4C absorber with control rod cladding is 348 °C. The performed calculations and further analysis of the results confirm that during operation of the RCCA control rods with both “fresh” and “burned-up” B4C, there will be no boiling of the coolant and melting of the absorber in the central part of the control rod. This means that the VVER-1000 core with TVSAs and WFAs/RWFAs and the RCCA rods are reliably cooled.

Searching for optimum design and burnable absorber for controlling the reactivity of U.S. Supper critical water reactor (SCWR)

Numerous research endeavours are currently underway to advance supercritical water reactors (SCWR), acknowledged as a cornerstone among Generation IV nuclear reactor designs. Evaluation and managing the reactivity of the reactor is a vital issue in the reactor operation. This research seeks to identify efficacious burnable absorber (BA) materials and determine an optimal spatial distribution within the fuel assembly to regulate reactivity levels effectively. Gadolinium, erbium and Lutetium have been suggested as BA in the form of integral burnable absorber (IBA) rods ((UO2 + Gd2O3), (UO2 + Er2O3) and (UO2 + Lu2O3)). Two SCWR assembly models, each featuring varied quantities and distributions of BA, have been examined. Various concentrations of the suggested BAs have been examined in the suggested models to verify the optimum cases. Burnup analyses have been conducted to evaluate the proposed cases. Different alloys of BAs including B4C+Dy2O3 and B4C+Sm2O3 have been investigated in the control rod and compared with the standard alloy B4C.

Laser-driven acceleration of a 94.8 MeV uniform proton beam enhanced by a thin axial absorber rod

Laser-based proton accelerators offer promising advantages for many sensitive applications due to their compact size. However, achieving uniform beams with a narrow energy spread remains a challenge. Here, a beamline simulation is performed using the racein and codes to optimize the spectral and spatial characteristics of a laser-driven proton beam. This beamline, comprised of four magnetic quadrupoles and equipped with a thin axial absorber rod in conjunction with an energy selection aperture for narrowing the energy spread, a carbon scatterer foil for beam spot smoothing, and an angular selection aperture for beam collimation, effectively transports a selected proton beam energy within the range of 50–250 MeV. In this work, laser-accelerated protons with a wide energy spread were transported through the proposed beamline, which was optimized for 109 MeV protons. The protons emerged well-collimated with a significantly narrowed energy spread of 94.8±3.9 MeV (a reduction from the intended 109 MeV due to passage through the scatterer) and a homogenized, 11 mm diameter beam spot while maintaining a transmission efficiency above 2.2%. Published by the American Physical Society 2024

Research on spherical harmonics method based on the MOC neutron transport code OpenMOC

In the Method of Characteristics (MOC) neutron transport code OpenMOC, the scattering source term is processed with an isotropic scattering approximation. However, in the actual process of neutron transport, the probability of neutron scattering in various directions is not uniform. By expanding the scattering cross-sections with Legendre polynomials and the angular flux terms with spherical harmonics, anisotropic scattering treatment under the flat source approximation is incorporated in the MOC code OpenMOC. This study selects VERA benchmark problems 1–3, which include different temperatures and burnable poison absorber rods, as well as several problems containing MOX fuel assemblies to validate the higher-order scattering functionality. Additionally, a sensitivity analysis is conducted about the impact of the number of particle histories on simulation time and keff calculation accuracy in OpenMOC. The results show that, compared to the zeroth-order approximation and the transport-corrected approximation, anisotropic scattering treatment achieves higher calculation accuracy in most cases.

Application of artificial neural network for assembly homogenized equivalence parameter generation

The development of a new reactor core and optimizations for fuel assembly (FA) structures necessitates the generation of homogenized equivalence parameters, including macroscopic cross-sections (XS), for numerous FA configurations. A feed-forward convolutional neural network, XSNET, has been created to streamline this process. Currently, XSNET can produce these parameters for fresh and burned UO2 fuel, both with and without Gadolinia (Gd) burnable absorber rods. In this study, we integrated XSNET with our in-house nodal diffusion code, RAST-K, to create a conventional two-step approximation code system. This system was tested on various tasks, including analyzing a 3-dimensional pressurized water reactor employing 16 × 16 FA types with Gd rods. Over 1,000 unique loading patterns were considered for nodal calculation, which involved steady-state depletion calculations and pin power reconstruction. The results obtained using XSNET/RAST-K demonstrated good agreement with the reference STREAM/RAST-K code system.

Fuel element microreactor integrating a square UO2 fuel rod with an internal heat pipe

In this study we propose the Fuel Element Micro Reactor (FEMR), with 308 square UO2 fuel rods, each one containing inside a heat pipe. We selected Mercury as the working fluid for the heat pipes and operate the microreactor at average temperature around 900 K or 637 °C. Heat pipes introduce empty regions into the core that increase neutron leakage and severely reduce core reactivity. Square fuel lattices allow greater fuel volume fractions and favor increasing core reactivity. The adoption of a 20 cm BeO reflector and Zircaloy as fuel cladding further increases core reactivity. The FEMR core thermal power is 2.3 MW, the power density distribution is rather flat with peaks occurring at the core-reflector interface and the peak fuel temperature is 1160 K. The reactor has a negative temperature isothermal coefficient of reactivity. The reactivity control system with 8 rotational drums and a central cruciform absorber rod meets the criteria of stuck rod and diversity of engineering principles. The average fuel discharge burnup is 8.96 MWd/kgU and the fuel cycle length without refueling is 8.7 years. The ratio between thermal power and total Uranium mass at beginning of life is 2.46 kW/kgU.

Reactor core design with practical gadolinia burnable absorbers for soluble boron-free operation in the innovative SMR

The development of soluble boron-free (SBF) operation in the innovative Small Modular Reactor (i-SMR) requires effective strategies for managing excess reactivity over extended operational cycles. This paper introduces a practical approach to reactor core design for SBF operation in i-SMR, emphasizing the use of gadolinia burnable absorbers (BA). The study investigates the feasibility of Highly Intensive and Discrete Gadolinia/Alumina Burnable Absorber (HIGA) rods for controlling excess reactivity sustainably. Through comprehensive analysis and simulations, the reactivity behavior with varying quantities of HIGA rods is examined, leading to the development of optimized fuel assembly designs. Furthermore, the integration of HIGA rods with integral gadolinia BA rods is discussed to enhance reactivity control and operational flexibility further. This approach utilizes the spatial self-shielding effect of gadolinia for extended reactivity management, crucial for stable and efficient reactor performance. The paper thoroughly addresses core design considerations, including fuel assembly configurations and control rod patterns, to ensure safety and performance in initial and reload cycles. This research advances the development of SBF operation in i-SMR by offering practical reactivity management solutions.

Cluster dynamics simulations of tritium and helium diffusion in lithium ceramics

Tritium (T) and He diffusion in LiAlO2 and LiAl5O8 phases influences the performance of tritium producing burnable absorber rods (TPBARs) by affecting the gas release, swelling and thermal conductivity of Li-bearing ceramic pellets. Frenkel pair defects and clusters created by irradiation can attract T and He interstitials and form clusters of the type HeixLi,HeixAl,HeixO,TixLi,TixAlandTixO,1≤x≤4 in a Li, Al or O vacancy site (notation denotes x He or T atoms in a 1 Li, 1 Al or 1 O vacant site). The concentration and mobility of each of these clusters collectively contribute to the diffusion of the He and T gases in LiAlO2 and LiAl5O8. In this work, free energy cluster dynamics simulations implemented in the Centipede code, are used to obtain the concentration and diffusivities of these clusters which are then used to calculate the total diffusivity of T and He gases in LiAlO2 and LiAl5O8. The results show that diffusivity of T is at least one order of magnitude higher in LiAlO2 as compared to that in LiAl5O8 whereas He diffusion is 2–13 orders of magnitude higher in LiAlO2 as compared to that in LiAl5O8. There is a higher concentration of highly diffusive species (T interstitials and Ti03Li for the case of tritium and Hei01Li, Hei02Li and Hei03Li for the case of He) in LiAlO2 than in LiAl5O8 which increase the total diffusion of T and He in LiAlO2.

Feasibility Study on the Application of Boron Carbide for Long Term Reactivity Control in the LOTUS Small Fast Reactor

Abstract 200 MWth lead-cooled fast reactor (LOTUS) reactor core is a small modular lead-cooled fast reactor with designed power of 200 MWth under development at Vietnam National University (VNU) University of Science, Hanoi for a floating nuclear power plant application. For that purpose, advanced passive safety features and no refueling requirement are the priorities in the core design process. To endure the continuous operation over a long lifetime, the startup core exhibits excess reactivity to cover the reactivity loss due to burnup. The reactivity control system includes burnable poison and absorber rods and layers made of B4C which are employed in the reactor to minimize the excess reactivity of the core to about 1 $ to enhance the safety features of the core. The burnable poison is fixed inside the reactor while absorber rods/absorber layers are withdrawn or inserted in sequence to achieve the required excess reactivity of about 700 pcm. The reactivity control was arranged into ten steps to achieve the operating time of 15 effective full-power years without refueling. Good neutronic behavior of the core was observed with negative fuel temperature coefficient and coolant void reactivity and maximum radial power peaking factor of 1.32. However, a quite large residual absorption caused by fixed burnable poison inside fuel assemblies was revealed. In further study, to increase the neutron absorption efficiency of burnable poison in the fast spectrum as well as the reactor lifetime, a neutron moderator will be considered to add to the burnable poison rods.

Mechanical and thermal properties evaluation of Eu2O3-Gd2Zr2O7 ceramics served as potential neutron absorber rod

Rare earth compounds (RE2O3), typically Eu2O3 and Gd2O3, are considered as absorbing materials due to their long chain neutron absorbing ability and high efficiency of neutron absorption. In this paper, a specifically designed xEu2O3-(1-x)Gd2Zr2O7 (x = 0.6, 0.7, 0.8, 0.85) composite ceramics with excellent thermal properties were synthesized through direct thermal decomposition reactions followed a high-temperature sintering method under vacuum conditions. X-ray diffraction (XRD) and Raman spectroscopy analysis indicated that the obtained ceramics consisted of monoclinic Eu2O3 and fluorite Gd2Zr2O7 phases. The hardness of xEu2O3-(1-x)Gd2Zr2O7 ceramics (x = 0.8, 0.85) have no significantly changes in the test temperature range (25–350 °C). The thermal expansion coefficients (TECs) (7.3–10 ×10-6 K-1) are lower than Dy2TiO5 (9.3–10.4 ×10-6 K-1), and the thermal expansion rate changes linearly with the increase of temperature, which indicates that the xEu2O3-(1-x)Gd2Zr2O7 ceramics exhibit excellent high temperature phase stability. Furthermore, the range of thermal conductivities is from 0.96 W·m-1·K-1 to 2.02 W·m-1·K-1 (25–800 °C), which is higher than Dy2TiO5 (0.27–0.55 W/m·K, 220–650 °C). Compared with the state-of-art Dy2TiO5 ceramics, xEu2O3-(1-x)Gd2Zr2O7 have superior thermal properties and are therefore expected to be the next generational of neutron absorbent materials used in ash rod.

Performance evaluation and qualification of hard coatings used in absorber rod drive mechanism of Indian fast breeder reactor

Prevention of surface damage at various guiding and bearing locations assumes high importance in ensuring reliable operation of Absorber Rod Drive Mechanisms (ARDM). Contact at an unintended location during any of the operating conditions, including seismic conditions, may cause unacceptable damage, reducing the availability of the ARDMs. Based on international and in-house experiences, the materials and surface coatings for mating surfaces and guiding locations were selected in the ARDMs of Indian Fast Breeder Reactors (FBR).Various mechanical and metallurgical investigations followed by integral assembly tests with full-scale components on hard face coatings were done to evaluate the performance of the hard coatings and to qualify them for reactor operation. In spite of detailed investigations, surface damage-related problems were faced during the initial stages of integral assembly testing, which severely delayed the qualification testing. Key lessons learned from the experimental program are that a holistic approach involving complete kinematics is necessary in the design of the systems to prevent unwanted surface damage-related issues. All these details are discussed in the present paper.From the qualification testing, it can be concluded that the performance of hard coatings for the ARDMs of Indian FBRs is satisfactory. The material and design selections made have formed the basis for the given applications in future FBRs in India.

Non-equilibrium molecular dynamics studies of thermal diffusion of hydrogen isotopes in low concentration zirconium hydrides

Tritium permeability in zirconium-based tritium getter critically impacts tritium storage and environmental safety during operation of tritium-producing burnable absorber rods (TPBARs). Previous experiments indicated that during irradiation operation, the hydrogen equilibrium pressured is increased. Further experimental and modeling studies suggested that the enhanced tritium release observed for reactor scale assemblies might be related to a thermal diffusion known as the Soret effect. A direct measurement of the Soret factor, however, has not been performed. To improve TPBAR and other nuclear applications, here we have applied two non-equilibrium molecular dynamics methods to study thermal diffusion of hydrogen isotopes in low-concentration zirconium hydrides. One of the methods produces sufficiently converged results to distinguish crystal orientation, isotope type, and concentration effects. With this method, crystal orientation, isotope type, and concentration effects are discussed.

Study of Tritium Diffusivity in Pure and Sn-Defective Zr: A First-Principles Density Functional Theory Approach

Zirconium alloys (e.g., zircaloy-4) are used as tritium (3H) getter materials in tritium-producing burnable absorber rods (TPBARs) owing to their ability to capture 3H and chemically convert them into metal hydrides. Understanding of 3H diffusion mechanisms in zircaloy is crucial for the optimal design of material performance in nuclear technology. Here, we perform first-principles density functional theory calculations to study the 3H diffusion mechanism in pure and impure Zr with a low concentration of tin (Sn) atoms to determine the impact of the presence of Sn on the movement of 3H atoms through the material. First, we calculated the diffusion barriers for 3H in pure Zr by taking different migration pathways. We then introduced a low concentration of Sn impurity and systematically explored the impurity effect on the diffusion barriers for 3H. Using our calculated diffusion energy barriers, we further obtained the diffusion coefficients and analyzed the results by comparing them with the experimental and previously calculated values. A diffusion coefficient of the order of 10–8 m2/s is predicted. We also found that the presence of a Sn impurity could reduce the diffusivity up to 4 orders of magnitude. Our results could serve as guidelines for further experimental investigations.

Ab Initio Simulations of Tritium Diffusion in Al-Rich Iron Aluminide Coating Phases

Surface coatings of steels used in extreme conditions and corrosive environments generally aim to provide protection and increased durability. In the case of tritium-producing burnable absorber rods (TPBARs) used in nuclear reactors, a 316 stainless steel has been coated with Al. Scanning transmission electron microscopy (STEM) characterization of the coating found three Al-rich (>60 atom % Al) iron aluminide alloys identified as hexagonal FeNiAl5, monoclinic Fe4Al13, and orthorhombic Fe2Al5. Density functional theory simulations using nudged elastic band have been performed to investigate the diffusion of interstitial tritium in each Al-rich iron aluminide phase. While FeNiAl5 and Fe4Al13 can be viewed as the stacking of two layers, the structural peculiarity of Fe2Al5 is that channels of variable Al vacancy content are present along the c-axis. Therefore, three stoichiometries for Fe2Alx phase, namely, Fe2Al4, Fe2Al5, and Fe2Al6, have been considered to evaluate the impact of Al vacancy concentration on tritium diffusion behavior. Altogether, we found that at 600 K, tritium diffusion decreases from a faster rate in the channels of Fe2Alx phases (DT ≤ 10–11 m2·s–1) to Fe4Al13 (DT ≈ 10–12 m2·s–1), and finally in FeNiAl5 (DT ≈ 10–13 m2·s–1). We also find that interstitial tritium generally diffuses faster in Fe–Al coating phases than in the tritium breeding material γ-LiAlO2 (DT ≈ 10–14 m2·s–1) but slightly slower than in 316 stainless steel (DT ≈ 10–10 m2·s–1).

Towards optimized designs for control rod absorbers with a high-fidelity simulation method implemented in the RMC code

A high-fidelity simulation method of the control rod absorbers is implemented in the Monte Carlo code RMC. For rare-earth absorbers and B4C, detailed calculations and analyses are conducted for neutronics and depletion characteristics. Three kinds of optimized control rod absorbers are proposed with higher reactivity worth and favorable burnup stability. In these new designs, the composite absorbers possess higher reactivity worth and only average 20% loss of reactivity worth at the end of cycle (EOC). The enriched absorbers have better neutron absorbing capacity and more stable radiation resistance, and the loss of reactivity worth of enriched B4C reduces from 90% to only 7.42% at EOC. For mixed absorbers, the best properties are found for the Dy and B4C mixture when the proportion of Dy is about 60%. The coupling of ZrH2 and Hf achieves a better control efficiency, but unfortunately the loss of reactivity worth increases at EOC.