Fission Systems Research Articles

Selective brain hypothermia-induced neuroprotection against focal cerebral ischemia/reperfusion injury is associated with Fis1 inhibition.

Selective brain hypothermia is considered an effective treatment for neuronal injury after stroke, and avoids the complications of general hypothermia. However, the mechanisms by which selective brain hypothermia affects mitochondrial fission remain unknown. In this study, we investigated the effect of selective brain hypothermia on the expression of fission 1 (Fis1) protein, a key factor in the mitochondrial fission system, during focal cerebral ischemia/reperfusion injury. Sprague-Dawley rats were divided into four groups. In the sham group, the carotid arteries were exposed only. In the other three groups, middle cerebral artery occlusion was performed using the intraluminal filament technique. After 2 hours of occlusion, the filament was slowly removed to allow blood reperfusion in the ischemia/reperfusion group. Saline, at 4°C and 37°C, were perfused through the carotid artery in the hypothermia and normothermia groups, respectively, followed by restoration of blood flow. Neurological function was assessed with the Zea Longa 5-point scoring method. Cerebral infarct volume was assessed by 2,3,5-triphenyltetrazolium chloride staining, and apoptosis was assessed by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining. Fis1 and cytosolic cytochrome c levels were assessed by western blot assay. Fis1 mRNA expression was assessed by quantitative reverse transcription-polymerase chain reaction. Mitochondrial ultrastructure was evaluated by transmission electron microscopy. Compared with the sham group, apoptosis, Fis1 protein and mRNA expression and cytosolic cytochrome c levels in the cortical ischemic penumbra and cerebral infarct volume were increased after reperfusion in the other three groups. These changes caused by cerebral ischemia/reperfusion were inhibited in the hypothermia group compared with the normothermia group. These findings show that selective brain hypothermia inhibits Fis1 expression and reduces apoptosis, thereby ameliorating focal cerebral ischemia/reperfusion injury in rats. Experiments were authorized by the Ethics Committee of Qingdao Municipal Hospital of China (approval No. 2019008).

Mass distributions of fission fragments for n+$^{233}$U reaction investigated by GEF model

The General Fission (GEF) model is a semi-empirical model describing fission process, which combines physical concepts from quantum mechanics and statistical mechanics with specific experimental information to adjust a set of parameters suitable for different fission systems and gives reliable predictions for essentially all fission observables of a broad range of fissioning nuclei. In this work, the mass distributions of $^{233}$U neutron-induced fission are calculated using the GEF model. The results show that with the increase of incident neutron energy, the contribution of symmetric fission channel increases gradually, and the asymmetric fission channel decreases. Detailed analysis of the fragment mass distribution is performed by taking multi-chance fission into account. The contribution of (n,f) fission channel gradually decreases with the increase of incident neutron energy, while the other fission channels increases. The variation trend of the yield energy-dependence is related to the position located on the mass distribution.

Tuning Spin Dynamics in Crystalline Tetracene.

Tetracene is an archetypal material undergoing singlet fission-the generation of a pair of triplet excitons from one singlet exciton. Here, using time-resolved electron spin resonance, we show how the spin dynamics in tetracene crystals are influenced by temperature and morphology. Upon cooling from 300 to 200 K, we observea switch between singlet fission and intersystem crossing generated triplets, manifesting as an inversion in transient spin polarization. We extract a spin dephasing time of approximately 40 ns for fission-generated triplets at room temperature, nearly 100 times shorter than the dephasing time that we measure for triplets localized on isolated tetracene molecules. These results highlight the importance of morphology and thermal activation in singlet fission systems.

Enhanced tritium production in fusion-fission hybrids for the external consumption

While the future fusion power plants (FPP) will consume and breed tritium for their own use, an essential amounts of tritium will be needed additionally from external sources for the FPPs initial start-ups in the commissioning periods and after technical interruptions that result in plasma operation shutdowns and the following a year-scale recovery time. Up-to-date predictive estimates of the start-up T-inventories for a single FPP give the values comparable with the commercial tritium resources currently available in the world nuclear industry or even exceed those. The neutron rich fusion-fission hybrid reactors (FFHR) with subcritical blankets controlled by a fusion neutron source are generally considered as possible producers of several product types as nuclear fuel for fission systems, energy and neutrons for waste burning. In this study, the hybrids are regarded as multifunctional systems acting also as tritium breeders. Some intrinsic properties and distinctive features of such systems are noted with emphasis on the nuclear performance variation during the operation periods. It is shown that a tritium surplus of a few kilograms per year may be produced even in a FFHR with a modest (˜50–100 MW) fusion driven neutron source during 3–4 operation years. Such productivity exceeds essentially the tritium production rate of about 0.2 kg/year at a typical commercial CANDU heavy water nuclear reactor.

Preequilibrium Asymmetries in the ^{239}Pu(n,f) Prompt Fission Neutron Spectrum.

The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of ^{239}Pu is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.

How do plastids and mitochondria divide?

Plastids and mitochondria are thought to have originated from free-living cyanobacterial and alpha-proteobacterial ancestors, respectively, via endosymbiosis. Their evolutionary origins dictate that these organelles do not multiply de novo but through the division of pre-existing plastids and mitochondria. Over the past three decades, studies have shown that plastid and mitochondrial division are performed by contractile ring-shaped structures, broadly termed the plastid and mitochondrial-division machineries. Interestingly, the division machineries are hybrid forms of the bacterial cell division system and eukaryotic membrane fission system. The structure and function of the plastid and mitochondrial-division machineries are similar to each other, implying that the division machineries evolved in parallel since their establishment in primitive eukaryotes. Compared with our knowledge of their structures, our understanding of the mechanical details of how these division machineries function is still quite limited. Here, we review and compare the structural frameworks of the plastid and mitochondrial-division machineries in both lower and higher eukaryotes. Then, we highlight fundamental issues that need to be resolved to reveal the underlying mechanisms of plastid and mitochondrial division. Finally, we highlight related studies that point to an exciting future for the field.

Delayed gamma determination at the JSI TRIGA reactor by synchronous measurements with fission and ionization chambers

Well characterized neutron and gamma fields inside a nuclear reactor are of key importance for its safe operation and for successful utilization of various research reactor irradiation facilities. In case of high-flux research reactors such as BR2 in Belgium, Maria in Poland and the future Jules Horowitz Reactor in France, the gamma energy deposition rate in reactor structural components and irradiated samples reaches values well over 10W/g. To assure safe reactor operation, the gamma field and associated heating must therefore be thoroughly characterized in order to provide adequate component and sample cooling.The gamma field can be divided into two contributions: prompt gamma rays are emitted almost instantly after neutron interaction with nuclei, while the delayed gamma rays are emitted from nuclei, which become radioactive by neutron absorption, generated from fission and other processes. Most modern Monte-Carlo particle transport codes enable the transport of prompt gamma rays; a few support delayed gamma ray generation and transport as well. The latter have mostly been applied to fusion devices, where detailed shutdown dose-rate measurements have been performed. Although the delayed gamma field can also be simulated in fission devices, significant inaccuracy in the result is to be expected due to the computational complexity arising from the large number of radioactive fission products and incompleteness of nuclear data. Furthermore, the unavailability of experimental delayed gamma measurements in fission systems presents an important challenge for the validation of the experimental results.Previous measurements in several research reactors show that the delayed gamma flux amounts to around 30% of the total gamma flux. However, these evaluations were performed with measurement data obtained during rapid reactor shutdowns (SCRAMs), using a single measurement point per SCRAM.In this paper we propose a new technique to accurately determine the magnitude of the delayed gamma component and its time evolution, based on synchronous acquisition of fission and ionization chamber signals. The measurements were performed at the JSI TRIGA reactor, using fission and ionization chambers placed in several in-core measurement positions. Their signal was acquired synchronously and at the highest possible acquisition rate in order to distinguish between measurement noise and reactor transients. Using the novel delayed gamma extraction technique we were able to estimate the magnitude of the delayed gamma contribution to be: 18.9% ± 2.0% at the reactor core periphery, linearly increasing towards the reactor core center to 31.4% ± 2.8% of the total measured gamma flux signal after 10min of reactor operation.

Transmutation performance of uniform and nonuniform distributions of plutonium and minor actinides in TRIGA Mark II ADS reactor

For long times there has been a sustained international interest in partitioning and transmutation of spent fuel produced by fission reactors, in particular both plutonium (Pu) and minor actinides (MA), which are associated with nuclear proliferation risks and radioactivity hazards. Transmutation requires the Pu and MA to be irradiated in a very intense neutron flux that can only be attained in a high power fission system such as a thermal reactor, a fast reactor or an accelerator driven system (ADS). The present work investigated two proposed models of core design of different Pu and MA distribution (uniform and nonuniform) applied to TRIGA Mark II ADS reactor to maximize fuel utilization, covering energy extraction and Pu/MA mass reduction. Transmutation performance was studied through burnup and mass ratio time evolution calculations of fuel and minor actinides various components including U, Pu, Am and Cm isotopes, up to two years operation cycle using MCNPX Transport code.

Variance Estimation in Monte Carlo Eigenvalue Simulations Using Spectral Analysis Method

Monte Carlo (MC) simulation is widely used to solve the eigenvalue form of the Boltzmann transport equation that mathematically represents the neutron transport process through complex multiplying (fissionable) systems. Monte Carlo eigenvalue simulation starts with an assumed fission source distribution and uses the fission sites from the previous iteration (cycle) as the starting source in the current iteration. Important system parameters (MC tallies) such as fuel pin-power distribution are estimated over several cycles after the convergence of the fission source distribution to a stationary distribution. However, the MC fission source iteration algorithm that uses fission source sites from the previous cycle introduces a cycle-to-cycle correlation. Monte Carlo simulations that do not account for the cycle-to-cycle correlation systematically underestimate the variance of the estimated system parameters (sample mean). This paper presents the relationship between the spectral density in the frequency domain at frequency zero and the variance of the sample mean. This paper introduces a novel method in the frequency domain for the MC variance estimation. For the three test problems used in this paper, researchers have observed that the new method results in an improvement of more than one order of magnitude to the standard deviation of the sample mean. The new method also compares favorably with the previously introduced batch, bootstrap, and covariance-adjusted methods when applied to the three test problems investigated in this paper. This new method does not require modification of the MC eigenvalue algorithm (power iteration), is code agnostic, and is therefore easy to use when implementing in any existing MC code. The new estimate can be calculated without saving tally results of all active/stationary cycles.

Variable charge transfer state energies at nanostructured pentacene/C60 interfaces

While it has recently been recognized that organic donor-acceptor charge transfer (CT) state energies can vary substantially under different interfacial morphologies, this behavior is under-appreciated in the context of organic singlet fission solar cells where a specific alignment between the triplet state of the fission material and the CT state of the donor-acceptor interface is necessary to the function of the device. In this work, we demonstrate that the CT state energy of a prototypical pentacene-C60 singlet fission system is around 1 eV in most systems, but can vary over 300 meV depending on the composition and morphology of the interface. Moreover, we show that the inclusion of a poly(3-hexylthiophene-2,5-diyl) underlayer, which commonly serves as a triplet blocker and hole collector in pentacene/C60 solar cells, helps promote active layer morphologies with stabilized, low energy CT states. These trends in the interfacial energetics are correlated with structural characterization of the films by atomic force microscopy and x-ray diffraction.

Revisiting the residual temperature distribution in prompt neutron emission in fission

A new triangular form of the residual temperature distribution P(T), entering the prompt emission models in which the sequential emission is globally taken into account (e.g., the Los Alamos model of Madland and Nix with subsequent improvements and the Point-by-Point model), is proposed. A deterministic treatment of the successive emission of prompt neutrons, which is based on recursive equations of the residual temperatures, was developed. This modeling was validated by the good description of many and different experimental data of prompt emission (e.g., $\overline{\nu}(A)$ , $\langle\nu\rangle$ (TKE), $\langle\varepsilon\rangle (A)$ , $\langle\varepsilon\rangle$ (TKE), $\overline{E}_{\gamma} (A)$ , etc.) and the good agreement with the results of other prompt emission models. To see a possible systematic behaviour of P(T) as a function of energy and fissioning nucleus, the deterministic treatment of sequential emission was applied to 11 nuclei undergoing fission (spontaneously or induced by thermal and fast neutrons with energies up to the threshold of the second chance fission) for which reliable experimental fission fragment distributions Y(A, TKE) exist. The shapes of all P(T) distributions for the light and heavy fragment groups and for all fragments resulting from this modeling can be approximated with a triangular form. To make possible the use of this form into the prompt emission models with a global treatment of sequential emission, a connection between the average residual temperature $\langle \mathrm{Tr} \rangle$ and the temperature of initial fragments $\langle \mathrm{Ti} \rangle$ is needed. An important finding of this study concerns the ratio $\langle \mathrm{Tr} \rangle / \langle \mathrm{Ti} \rangle$ , which is $ \approx 0.6$ for all studied fissioning systems. This result allows to obtain a new triangular form of P(T) defined only as a function of initial temperature, which is applicable to any fissioning system at any energy, in the frame of prompt emission models with a global treatment of sequential emission.

Investigation of the heavy nuclei fission with anomalously high values of the fission fragments total kinetic energy

Binary fission of 232Th and 238U induced by fast neutrons were under intent investigation in the IPPE during recent years. These measurements were performed with a twin ionization chamber with Frisch grids. Signals from the detector were digitized for further processing with a specially developed software. It results in information of kinetic energies, masses, directions and Bragg curves of registered fission fragments. Total statistics of a few million fission events were collected during each experiment. It was discovered that for several combinations of fission fragment masses their total kinetic energy was very close to total free energy of the fissioning system. The probability of such fission events for the fast neutron induced fission was found to be much higher than for spontaneous fission of 252Cf and thermal neutron induced fission of 235U. For experiments with 238U target the energy of incident neutrons were 5 MeV and 6.5 MeV. Close analysis of dependence of fission fragment distribution on compound nucleus excitation energy gave us some explanation of the phenomenon. It could be a process in highly excited compound nucleus which leads the fissioning system from the scission point into the fusion valley with high probability.

Elucidation of Excitation Energy Dependent Correlated Triplet Pair Formation Pathways in an Endothermic Singlet Fission System.

Singlet fission is the spin-allowed conversion of a photogenerated singlet exciton into two triplet excitons in organic semiconductors, which could enable single-junction photovoltaic cells to break the Shockley-Queisser limit. The conversion of singlets to free triplets is mediated by an intermediate correlated triplet pair (TT) state, but an understanding of how the formation and dissociation of these states depend on energetics and morphology is lacking. In this study, we probe the dynamics of TT states in a model endothermic fission system, TIPS-Tc nanoparticles, which show a mixture of crystalline and disordered regions. We observe the formation of different TT states, with varying yield and different rates of singlet decay, depending on the excitation energy. An emissive TT state is observed to grow in over 1 ns when excited at 480 nm, in contrast to excitation at lower energies where this emissive TT state is not observed. This suggests that the pathway for singlet fission in these nanoparticles is strongly influenced by the initial sub-100 fs relaxation of the photoexcited state away from the Franck-Condon point, with multiple possible TT states. On nanosecond time scales, the TT states are converted to free triplets, which suggests that TT states might diffuse into the disordered regions of the nanoparticles where their breakup to free triplets is favored. The free triplets then decay on μs time scales, despite the confined nature of the system. Our results provide important insights into the mechanism of endothermic singlet fission and the design of nanostructures to harness singlet fission.

Mitochondria help in dead cell clearance

Cell Biology When cells in our body die, it is important that they are cleared from the system so that they do not cause inflammation or further death among neighboring cells. Dead cells are cleaned up by phagocytic cells such as macrophages. Wang et al. studied the mechanisms involved as macrophages take up and degrade multiple cell corpses from the circulation. Interestingly, they found that the macrophages needed to have an intact mitochondrial fission system in place if they were to be able to take up more than one cell corpse. Mitochondrial fission was important to ensure that cytosolic calcium levels remained high enough to drive further rounds of phagocytosis. In its absence, mice exhibited compromised clearance of dead cells, which exacerbated pathology in mouse models of atherosclerosis. Cell 10.1016/j.cell.2017.08.041 (2017).

Deuterium-lithium plasma as a source of fusion neutrons

The concepts of deuterium-tritium (D–T) fusion neutron source are currently developed for hybrid fusion–fission systems and the waste transmutation ones. The need to use tritium technologies is a deterrent factor in this promising direction of energy production. Potential possibilities of using systems that do not require tritium developments are of a significant interest. A deuterium-deuterium (D–D) reaction is considered for the use in demonstration fusion neutron sources. The product of this reaction is tritium, which will burn in the plasma with the emission of fast neutrons. D–D reaction is significantly slower then D–T reaction. Present study shows an increase in neutron yield using a powerful injection of the beam of deuterium atoms. The reactions of the deuterium with lithium isotopes are considered. In some of these reactions, fast neutrons can be obtained. The results of the calculation of the neutron yield from the deuterium lithium plasma are discussed. The estimates of the parameters needed for the realization of a source of fusion neutrons are presented.

Photofission of nuclei with allowance for wriggling vibrations of fissioning nuclei

It is confirmed that one source of the large relative orbital momenta L of fragments in spontaneous and stimulated low-energy nuclear fission is quantum transverse zero-point wriggling vibrations of the fissioning system near its scission point. The angular distributions of fragments of low-energy photofission of actinide nuclei, calculated using the quantum theory of fission, are compared. Vibrations are allowed for by using parameter C w determined by Nix and Swiatecki. Agreement between the experimental and theoretical angular distributions for 234U, 236U, 238U, 238Pu, 240Pu, 242Pu nuclei is observed. The strong sensitivity of the theoretical angular distributions for 238Pu, 240Pu, 242Pu nuclei toward the choice of parameters of transient fissioning states at the external and internal fission barriers is demonstrated.

Fission in Inverse Kinematics: A path to new experimental observables

Historically, experimental fission studies were based on reactions in direct kinematics with fixed target-like fissioning systems. Besides its advantages, this technique suffers from some drawbacks such as the difficulty of producing exotic fissioning systems and the seldom measurement of the fragment atomic number. Inverse kinematic provides an alternative to ease these issues and offers a new set of experimental observables that improves our level of information about the fission process, including an unprecedented access to the scission point. In this document, we review some of the observables obtained from the experimental campaign based on inverse kinematics, performed at VAMOS/GANIL.

Parallel theoretical study of the two components of the prompt fission neutrons: Dynamically released at scission and evaporated from fully accelerated fragments

Prompt fission neutrons (PFN) angular and energy distributions for the reaction 235 U( n th , f ) are calculated as a function of the mass asymmetry of the fission fragments using two extreme assumptions: 1) PFN are released during the neck rupture due to the diabatic coupling between the neutron degree of freedom and the rapidly changing neutron-nucleus potential. These unbound neutrons are faster than the separation of the nascent fragments and most of them leave the fissioning system in few 10 −21 sec. i.e., at the begining of the acceleration phase. Surrounding the fissioning nucleus by a sphere one can calculate the radial component of the neutron current density. Its time integral gives the angular distribution with respect to the fission axis. The average energy of each emitted neutron is also calculated using the unbound part of each neutron wave packet. The distribution of these average energies gives the general trends of the PFN spectrum: the slope, the range and the average value. 2) PFN are evaporated from fully accelerated, fully equilibrated fission fragments. To follow the de-excitation of these fragments via neutron and γ-ray sequential emissions, a Monte Carlo sampling of the initial conditions and a Hauser-Feshbach statistical approach is used. Recording at each step the emission probability, the energy and the angle of each evaporated neutron one can construct the PFN energy and the PFN angular distribution in the laboratory system. The predictions of these two methods are finally compared with recent experimental results obtained for a given fragment mass ratio.

Effect of carbon and alloying solute atoms on helium behaviors in α-Fe

Helium bubbles could strongly degrade the mechanical properties of ferritic steels in fission and fusion systems. The formation of helium bubble is directly affected by the interactions between helium and the compositions in steels, such as solute atoms, carbon and irradiation defects. We thereby performed systematical first-principles calculations to investigate the interactions of solute-helium and carbon-solute-helium. It is found that substitutional helium is more attractive than interstitial helium to all the considered 3p, 4p, 5p and 6p solutes. The attraction between carbon and substitutional helium suggests the carbon-solute-helium complex can be formed stably. By examining the charge density difference and thermal stability, it is found that the ternary complex shows stronger attraction with He than that of solute-helium pair for some solutes (S, Se, In, Te, Pb and Bi) and the complex could existed in iron stably at 700 K. The present theoretical results may be helpful for exploring alloy additions to mitigate the formation of large helium bubbles.

Mechanical behavior of copper containing a gas-bubble superlattice

Helium implantation can cause the formation of a helium gas-bubble superlattice in crystalline materials. Experimental studies of the mechanical response of a material hosting such a superlattice are lacking, especially at the microstructural level. By employing a novel, high-throughput, high-precision nanoscale helium implantation technique, in combination with in-situ transmission electron microscopy nanomechanical testing, we find that the helium-bubble superlattice structure in copper is disordered or maintained depending on whether the material's plasticity occurs by multiple ordinary full dislocations or by deformation twinning. In addition, helium implantation can harden the material significantly: the higher the dose, the larger the yield stress. The measured hardening behavior agrees well with the trend predicted by the Friedel-Kroupa-Hirsch model. Our findings shed new light on understanding the mechanical and microstructural properties of materials subjected to intense helium generation by neutron and alpha-particle bombardment in nuclear fusion, fission or spallation systems.