Fission Systems Research Articles

Asteroid pairs: Survey of the inner main belt

Context. An asteroid pair forms when an asteroid splits into two unbound fragments because of collision, rotational fission, or binary system decay. The two components of the asteroid pair share similar physical properties and their orbits converge when integrated into the past. Currently, 268 asteroid pairs are known, and new pairs are discovered alongside the continuous discovery of new asteroids. Aims. We conducted a survey in the inner asteroid belt to find new asteroid pairs, estimated their age, and classified their physical properties. As presently no M-type asteroid pairs are known, we also conducted a specialized survey of them. Methods. We preselected asteroid pair candidates based on their distances in the five-dimensional space of osculating orbital elements. We created multiple clones within the uncertainties of their orbital elements and conducted their backtrack integration into the past. We searched for convergence of their clones at close spatial points with small relative velocities, the distribution of which determines the pair formation age. Results. We find 40 new asteroid pairs, thus increasing the total number of known pairs by 15%. One of the newly discovered pairs, 469759 - 2016 QZ 123, with an age of 2.6−0.2+0.7 kyr is now the third-youngest known asteroid pair. We studied the influence of the mutual gravitation of pair components on the process of their evolution and successfully observed the gravitational catching of the two pair members in the past. As a byproduct of pair search, we find eight asteroids connected in a cluster with an age of 76−25+15 kyr that belongs to the Phocaea family and incorporates one previously known asteroid pair. We confirm the convergence of ten asteroid pairs discovered in our previous research and improve their age estimates. We observed a deficiency of M-type asteroid pairs, and therefore conducted a dedicated search for M-type pairs, but found none.

Speciation and Diffusivity of Hydrogen in Molten 2LiF-BeF2 for Nuclear Fission and Fusion Applications

The behavior of hydrogen isotopes in molten 2LiF-BeF2 (FLiBe) is of interest to both advanced fission and fusion reactor designs. Tritium (hydrogen-3) is both a fission product and neutron activation product of lithium-6 fluoride in fission reactors which use FLiBe as a heat transfer fluid. In this case, tritium is an undesirable side product. In fusion reactors, FLiBe may be used as a blanket around the plasma which, upon neutron irradiation, generates tritium by the same neutron–Li-6 reaction. Here, tritium is a necessary and desirable product; the tritium is used to continuously fuel the deuterium-tritium fusion reaction. A thorough understanding of tritium’s behavior and residence time is relevant to managing tritium inventories in fission and fusion systems, which is essential for both the safety cases and functional operation of these reactors. Using hydrogen as a non-radioactive surrogate for tritium, we study the speciation and diffusivity of hydrogen in molten FLiBe at various temperatures relevant to reactor operating conditions.Here we present results of electrochemical investigations of hydrogen in FLiBe using cyclic and square wave voltammetry. We introduce hydrogen to FLiBe through 1) LiH additions and as 2) H2 gas, electrochemically desorbed from graphite. The expected solubility of H2 in FLiBe at 1 atm partial pressure is as low as 1 appm. Previous studies saw a higher apparent solubility of H2 at atmospheric pressure (up to 8000 appm). The possible presence of the postulated quasi-stable intermediate BeH2 is explored to explain the high apparent solubility of H2. The two methods of introducing H (as LiH reacting with BeF2 to form BeH2, which decomposes to H2, and the surface reduction of H2 on graphite) are compared to investigate the different species in which hydrogen may exist in molten FLiBe. Additionally, we attempt to quantify the diffusivity of these hydrogen species in FLiBe at 500ºC and 800ºC and compare findings with predicted diffusivities from computational models of Be and H additions to FLiBe. Overall, these studies seek to elucidate the speciation of hydrogen in FLiBe to aid in the management and operation of fission and fusion systems.

Unusual nanoscale amorphization of metallic chromium interfacing with SiC under high energy irradiation

Layered metal/silicon carbide (SiC) materials systems are becoming increasingly relevant to solve materials challenges in advanced fission and fusion nuclear energy systems, necessitating a deeper understanding of how interfaces between dissimilar materials behave under high energy irradiation. In this work, we use a Cr-SiC bilayer system as a model to study the behavior of such interfaces under high energy ion irradiation (80 MeV Xe26+ ions) at elevated temperatures (∼350 °C). Through high resolution characterization of the interface, we observed the formation of a nanoscale Cr-rich amorphous layer adjacent to crystalline SiC. We explain this phenomenon through a multi-scale computational approach incorporating ballistic mixing simulations, density functional theory calculations, and CALPHAD-based non-equilibrium modeling that shows the localized amorphization of Cr to be driven by the synergistic action of irradiation-induced point defects within Cr and transport of Si and C atoms across the interface. In particular, the accumulation of radiation damage results in the thermodynamic destabilization of the point-defect containing, metastable Cr-Si-C solid solution with respect to an amorphous phase of identical composition. This study advances the understanding of how metal/SiC interfaces behave under irradiation and establishes a modeling framework that can be applied to interfacial systems to understand irradiation-induced amorphization.

Compositionally complex carbide ceramics: A perspective on irradiation damage

Extensive experimental and computational studies have demonstrated outstanding physical and chemical properties of the novel materials of compositionally complex carbides (CCCs), enabling their promising applications in advanced fission and fusion energy systems. This perspective provides a comprehensive overview of radiation damage behavior reported in the literature to understand the fundamental mechanisms related to the impact of multi-principal metal components on phase stability, irradiation-induced defect clusters, irradiation hardening, and thermal conductivity of compositionally complex carbides. Several future research directions are recommended to critically evaluate the feasibility of designing and developing new ceramic materials for extreme environments using the transformative “multi-principal component” concept. Compared to the existing materials for nuclear applications including stainless steels, nickel alloys, ZrC, SiC, and potentially high-entropy alloys, as well as certain other compositionally complex ceramic families. CCCs appear to be more resistant to amorphization, growth of irradiation defect clusters, and void swelling.

Comparative analysis of different FeCrAl alloys in pressurized water reactors

Discussing and researching issues related to their safety is essential to guarantee the continuity of fission systems in generating energy for existing and new electrical matrices. Commercial fission reactors currently in operation have undergone a series of redesigns. One aspect much discussed in recent years is related to research about new fuels and cladding materials. Therefore, in this work, the feasibility of replacing Zircaloy-4 with FeCrAl-based alloys (C06M, C35M, C36M, Kanthal APMT) was evaluated considering these materials' physical–chemical, thermal, and neutronic properties. Moreover, thermal–hydraulic and neutronic simulations have been presented to compare the cladding behavior in a PWR core in steady state and transient operation conditions. The results showed, in a general way, that the analyzed alloys have equivalent properties from a physical–chemical and thermal point of view. Despite the similarity, the Kanthal APMT alloy presented the worst performance, and the C35M alloy showed more advantageous characteristics. Considering the similarities in the composition of the C06M, C35M, and C36M alloys, which have the same elements in their constitution, the alloy with the best thermal performance has the lowest aluminum content. Considering the neutronic behavior, the Kanthal APMT alloy, with more Cr percentile, presents the lowest neutron effective multiplication factor, keff. Axial and radial power distribution did not change significantly after the cladding replacement. Nevertheless, modifications must be made to keep the criticality during the fuel cycle burnup for all FeCrAl alloys.

Excitation energy dependency of the low-energy fission dynamics: Probing through prompt gamma-ray spectroscopy

Fission Fragment Spectroscopy (FFS) technique has been utilized to make simultaneous measurements of both the relative charge and mass yield distributions of even-even fission fragments produced from the fission of 236U at two different excitation energies, Eex. The necessary analysis has been carried out on the measured yield distributions by employing the Multimodal Random Neck Rupture Model (MM-RNRM). The relative contribution of two different types of asymmetric mode of fission – Standard I and Standard II has been extracted at Eex = 6.5 and 21.5 MeV. An attempt has been made to study the evolution of these two asymmetric fission modes with respect to the values of Eex of the concerned fissioning system at low-excitation regime. The probable evidence, although at the primitive stage, for an oscillatory behavior of the relative contributions of different low-energy fission modes with increasing values of Eex has been presented.

Molecular Control of Triplet-Pair Spin Polarization and Its Optoelectronic Magnetic Resonance Probes.

ConspectusPreparing and manipulating pure magnetic states in molecular systems are the key initial requirements for harnessing the power of synthetic chemistry to drive practical quantum sensing and computing technologies. One route for achieving the requisite higher spin states in organic systems exploits the phenomenon of singlet fission, which produces pairs of triplet excited states from initially photoexcited singlets in molecular assemblies with multiple chromophores. The resulting spin states are characterized by total spin (quintet, triplet, or singlet) and its projection onto a specified molecular or magnetic field axis. These excited states are typically highly polarized but exhibit an impure spin population pattern. Herein, we report the prediction and experimental verification of molecular design rules that drive the population of a single pure magnetic state and describe the progress toward its experimental realization.A vital feature of this work is the close partnership among theory, chemical synthesis, and spectroscopy. We begin by presenting our theoretical framework for understanding spin manifold interconversion in singlet fission systems. This theory makes specific testable predictions about the intermolecular structure and orientation relative to an external magnetic field that should lead to pure magnetic state preparation and provides a powerful tool for interpreting magnetic spectra. We then test these predictions through detailed magnetic spectroscopy experiments on a series of new molecular architectures that meet one or more of the identified structural criteria. Many of these architectures rely on the synthesis of molecules with features unique to this effort: rigid bridges between chromophores in dimers, heteroacenes with tailored singlet/triplet-pair energy level matching, or side-group engineering to produce specific crystal structures. The spin evolution of these systems is revealed through our application and development of several magnetic resonance methods, each of which has different sensitivities and relevance in environments relevant to quantum applications.Our theoretical predictions prove to be remarkably consistent with our experimental results, though experimentally meeting all the structural prescriptions demanded by theory for true pure-state preparation remains a challenge. Our magnetic spectra agree with our model of triplet-pair behavior, including funneling of the population to the ms = 0 magnetic sublevel of the quintet under specified conditions in dimers and crystals, showing that this phenomenon is subject to control through molecular design. Moreover, our demonstration of novel and/or highly sensitive detection mechanisms of spin states in singlet fission systems, including photoluminescence (PL), photoinduced absorption (PA), and magnetoconductance (MC), points the way toward both a deeper understanding of how these systems evolve and technologically feasible routes toward experiments at the single-molecule quantum limit that are desirable for computational applications.

Examination of how properties of a fissioning system impact isomeric yield ratios of the fragments

Examination of how properties of a fissioning system impact isomeric yield ratios of the fragments

Raw Material Effect on the Characteristics of the Fission and Fusion Reactor System

Raw Material Effect on the Characteristics of the Fission and Fusion Reactor System

The lifetime distribution of finite fission chain group around the prompt criticality

In order to describe the behavior of finite fission chains in a fission system initiated by a δ neutron source, we present the conception of the finite fission chains group (FFCG). The group is the collectivity of the fission chains initiated by plenty of neutrons at the same time. Based on the result of the solving generating function, we deduced the lifetime distribution formula of FFCG based on the point kinetics model. Finally, the formula is validated by the experiment with a single pulse neutron source in CFBR-Ⅱ.

Formation of Transient Protein Aggregate-like Centers Is a General Strategy Postponing Degradation of Misfolded Intermediates

When misfolded intermediates accumulate during heat shock, the protein quality control system promotes cellular adaptation strategies. In Schizosaccharomyces pombe, thermo-sensitive proteins assemble upon stress into protein aggregate-like centers, PACs, to escape from degradation. The role of this protein deposition strategy has been elusive due to the use of different model systems and reporters, and to the addition of artificial inhibitors, which made interpretation of the results difficult. Here, we compare fission and budding yeast model systems, expressing the same misfolding reporters in experiments lacking proteasome or translation inhibitors. We demonstrate that mild heat shock triggers reversible PAC formation, with the collapse of both reporters and chaperones in a process largely mediated by chaperones. This assembly postpones proteasomal degradation of the misfolding reporters, and their Hsp104-dependent disassembly occurs during stress recovery. Severe heat shock induces formation of cytosolic PACs, but also of nuclear structures resembling nucleolar rings, NuRs, presumably to halt nuclear functions. Our study demonstrates that these distantly related yeasts use very similar strategies to adapt and survive to mild and severe heat shock and that aggregate-like formation is a general cellular scheme to postpone protein degradation and facilitate exit from stress.

Fusion Propulsion: Field of the Future

As part of a mini-course on plasma for space propulsion IEEE ICOPS 2022, Seattle, WA, the potential use of controlled fusion is surveyed. Brief reviews are provided on space mission analysis and fusion technology for the respective nonspecialists. Notions for fusion propulsion are described for magnetic-, inertial-, and magneto-inertial confinement schemes. A system framework for a generic fusion rocket is provided, allowing comparison of possible fusion propulsion approaches with fission-driven electric rockets. The higher specific power offered by controlled fusion ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&gt;$</tex-math> </inline-formula> 5 kw/kg) versus <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 100 w/kg for fission or solar-powered systems suggests fusion can reduce trip times from years to months thereby enabling crewed missions to the outer planets.

Structural Properties of He-Irradiated Zr/Nb Multilayer Investigated by Grazing Incidence X-ray Diffraction

Zr/Nb nanoscale multilayers are regarded as one of the important candidate materials used in next-generation reactors. Understanding structural evolution induced by ion bombardment is crucial for the evaluation of lifetime performance. Magnetron sputter-deposited Zr/Nb multilayers with a periodicity of 7 nm were subjected to 300 keV He ion irradiation with three different fluences at room temperature. The depth-resolved strain and damage profiles in the Zr/Nb multilayers were investigated by grazing incidence X-ray diffraction. The tensile strain was found in the deposited Zr/Nb films. After He ion irradiation, the intensity of diffraction peaks increased. The change in diffraction peaks depends on He fluence and incident angle. Irradiation-induced pre-existing defect annealing was observed and the ability to recover the microstructure was more significant in the Zr films compared to the Nb films. Furthermore, the efficiency of defect annealing depends on the concentration of pre-existing defects and He fluence. When the He fluence exceeds the one for pre-existing defect annealing, residual defects will be formed, such as 1/3&lt;12¯10&gt; and 1/3&lt;11¯00&gt; dislocation loops in the Zr films and 1/2&lt;111&gt; dislocation loops in the Nb films. Finally, introducing deposited defects and interfaces can improve the radiation resistance of Zr/Nb nanoscale multilayers. These findings can be extended to other multilayers in order to develop candidate materials for fusion and fission systems with high radiation resistance.

Multi-group analysis of Minor Actinides transmutation in a Fusion Hybrid Reactor

New nuclear technologies are currently being study to face High Level Waste treatment and disposal issues. Generally, GEN-IV fission Fast Reactors (FR) are considered the waste-burners of the future. In fact, a fast flux turns out to be the best choice for actinides irradiation in critical reactors because of favorable cross section conditions. Differently, Fusion Fission Hybrid Reactors (FFHR) are futuristic devices based on the combination of fusion and fission systems and could represent an alternative to FRs. In such systems, the choice spectrum of the neutron flux that irradiates HLW may be non-obvious due to some operational constraints which have to be considered. To design and optimize these systems as waste-burners, one should fully understand the transmutation dynamics occurring into the fission region. A multi-energy-group analysis by FISPACT-II code has been set to analyze the conversion processes in scenarios characterized by different neutron energy spectra and fluences. The results of this study show that, despite fast fluxes are characterized by better behaviors in terms of radiotoxicity treatment, the difficulties of reaching high reaction yields may require solutions involving moderators or broadened neutron fluxes to increase the reactions probabilities and, consequently, actinides mass conversion yield.

Fragmentation-induced fission reactions of 236U in inverse kinematics to investigate the pre-fragment angular momentum parameterizations

In the last decades, measurements of spallation, fragmentation and Coulex induced fission reactions in inverse kinematics have provided valuable data to accurately investigate the fission dynamics and nuclear structure at large deformations of a large variety of stable and non-stable heavy nuclei. The collected data were used to constrain dynamic and nuclear structure parameters of different de-excitation models, such as ABLA and GEF, but the data can also be used to constrain the parameterizations describing the pre-fragment properties after the nuclear collision, such as the angular momentum gained by the pre-fragment. In this work, the fissioning system yields are compared to calculations assuming different parameterizations for modeling the angular momentum gained by the compound nuclei. Our findings indicate that the parameterizations utilized by abrasion models clearly underestimate the angular momentum, resulting in the underestimation of the production of lighter fissioning systems.

Comprehensive investigation of fission yields by using spallation- and (p,2p)- induced fission reactions in inverse kinematics

In the last decades, measurements of spallation, fragmentation and Coulex induced fission reactions in inverse kinematics have provided valuable data to accurately investigate the fission dynamics and nuclear structure at large deformations of a large variety of stable and non-stable heavy nuclei. To go a step further, we propose now to induce fission by the use of quasi-free (p,2p) scattering reactions in inverse kinematics, which allows us to reconstruct the excitation energy of the compound fissioning system by using the four-momenta of the two outgoing protons. Therefore, this new approach might permit to correlate the excitation energy with the charge and mass distributions of the fission fragments and with the fission probabilities, given for the first time direct access to the simultaneous measurement of the fission yield dependence on temperature and fission barrier heights of exotic heavy nuclei, respectively. The first experiment based on this methodology was realized recently at the GSI/FAIR facility and a detailed description of the experimental setup is given here.

Effect of a Hybrid Fusion Plant Operating in a Fission and Fusion Reactor System on the Fuel Cycle

Effect of a Hybrid Fusion Plant Operating in a Fission and Fusion Reactor System on the Fuel Cycle

Lognormal Bayesian estimation of fission product yield covariance matrix

Covariance matrix of fission product yields (FPYs) is important in propagating FPYs' uncertainty to burnup calculation of reactors, however, it is absent in the current release of ENDF/B-VII.1. Bayesian updating method is used to estimate this covariance matrix from the physical constraints among FPYs. Conventional normal Bayesian method adopts normal distribution to describe FPY probability density, which violates FPYs’ inherent positive physical nature. In order to tackle such issue, lognormal Bayesian method is introduced in this work and FPYs from thermal neutron induced U-235 fission system are considered. By comparing between normal and lognormal Bayesian estimated covariance matrices, it is found that large relative difference (larger than 30%) is observed in the updated FPY standard deviations. Lognormal Bayesian method would preserve the non-negativity of FPY. This would further constraint the estimated strong anti-correlations among FPYs compared with normal Bayesian method. From this work, it is suggested that the non-negativity of FPY should be underlined in FPY covariance matrix estimation process. And lognormal Bayesian method should be used in FPY covariance estimation because of its mathematical self-consistency in preserving the non-negativity of FPYs.

Exploring optimal multimode vibronic pathways in singlet fission of azaborine analogues of perylene.

The development of new singlet fission chromophores is a vibrant area of research to explore the possibility of efficient photovoltaic devices. Using high-level ab-initio density matrix renormalization group calculations, we present a systematic analysis of BN-doped perylenes for their potential application as singlet fission candidates. Four singlet fission chromophores are identified considering the monomer-based properties and their excitonic characters are further analyzed in a dimer configuration optimized in a six-dimensional space for local maxima of fission rates. Furthermore, a multistate multimode vibronic Hamiltonian is employed to identify intra- and interstate vibrational pathways for excitation energy modulation. Several photophysical properties such as Davydov splitting, activation energy and vibronic admixture of multiexcitonic and charge-transfer states are calculated for physically accessible dimers. The optimal dimer packing results in appropriate vibrational relaxation of singlet fission states and promotes significant population transfer which would be more attenuated without such couplings. This work not only identifies potential singlet fission systems with favorable electronic properties but also highlights the sensitivity of dimer packings with respect to the substitution patterns in singlet fission chromophores.

Accurate Simulation of Spectroscopic Signatures of Cavity-Assisted, Conical-Intersection-Controlled Singlet Fission Processes.

A numerically accurate, fully quantum methodology has been developed for the simulation of the dynamics and nonlinear spectroscopic signals of cavity-assisted, conical-intersection-controlled singlet fission systems. The methodology is capable of handling several molecular systems strongly coupled to the photonic mode of the cavity and treats the intrinsic conical intersection and cavity-induced polaritonic conical intersections in a numerically exact manner. Contributions of higher-lying molecular electronic states are accounted for comprehensively. The intriguing process of cavity-modified fission dynamics, including all of its electronic, vibrational, and photonic degrees of freedom, together with its two-dimensional spectroscopic manifestation, is simulated for two rubrene dimers strongly coupled to the cavity mode.