Fission Rate Research Articles

Reactive Oxygen Species and Mitochondrial Dynamics: The Yin and Yang of Mitochondrial Dysfunction and Cancer Progression.

Mitochondria are organelles with a highly dynamic ultrastructure maintained by a delicate equilibrium between its fission and fusion rates. Understanding the factors influencing this balance is important as perturbations to mitochondrial dynamics can result in pathological states. As a terminal site of nutrient oxidation for the cell, mitochondrial powerhouses harness energy in the form of ATP in a process driven by the electron transport chain. Contemporaneously, electrons translocated within the electron transport chain undergo spontaneous side reactions with oxygen, giving rise to superoxide and a variety of other downstream reactive oxygen species (ROS). Mitochondrially-derived ROS can mediate redox signaling or, in excess, cause cell injury and even cell death. Recent evidence suggests that mitochondrial ultrastructure is tightly coupled to ROS generation depending on the physiological status of the cell. Yet, the mechanism by which changes in mitochondrial shape modulate mitochondrial function and redox homeostasis is less clear. Aberrant mitochondrial morphology may lead to enhanced ROS formation, which, in turn, may deteriorate mitochondrial health and further exacerbate oxidative stress in a self-perpetuating vicious cycle. Here, we review the latest findings on the intricate relationship between mitochondrial dynamics and ROS production, focusing mainly on its role in malignant disease.

Three-dimensional phase-field simulations of intragranular gas bubble evolution in irradiated U-Mo fuel

The evolution of fission gas bubbles in irradiated materials plays a critical role in the microstructural processes that leads to dimensional changes of U-Mo alloy fuels, e.g., fuel swelling. Although the intergranular bubbles-induced fuel swelling has been long-discussed for U-Mo fuel, there are very few computational studies of the formation of intragranular gas bubbles and its impact on fuel swelling. To this end, we develop a three-dimensional phase-field model to investigate the evolution of intragranular gas bubbles in U-Mo fuel. Fission induced defect formation and annihilation processes, such as vacancy-interstitial recombination, fission gas atom resolution, and interactions with dislocations and grain boundaries are incorporated in the model. Simulations show that the intragranular gas bubbles can be stabilized to certain sizes due to the balance between the generation and annihilation of defects. The intragranular gas bubbles induced fuel swelling is predicted to be comparable to experimental measurements. The effects of the irradiation and fuel fabrication conditions (i.e., fission rate, fuel grain size, and mechanical work induced deformation) on the bubble evolution and the resultant swelling are investigated. The current simulations provide a better understanding of intragranular gas bubble-induced swelling and a solid foundation for the future study of the nucleation and growth of intergranular gas bubbles and recrystallization in U-Mo fuel.

Singlet Fission Involves an Interplay between Energetic Driving Force and Electronic Coupling in Perylenediimide Films.

Due to its ability to offset thermalization losses in photoharvesting systems, singlet fission has become a topic of research interest. During singlet fission, a high energy spin-singlet state in an organic semiconductor divides its energy to form two lower energy spin-triplet excitations on neighboring chromophores. While key insights into mechanisms leading to singlet fission have been gained recently, developing photostable compounds that undergo quantitative singlet fission remains a key challenge. In this report, we explore triplet exciton production via singlet fission in films of perylenediimides, a class of compounds with a long history of use as industrial dyes and pigments due to their photostability. As singlet fission necessitates electron transfer between neighboring molecules, its rate and yield depend sensitively on their local arrangement. By adding different functional groups at their imide positions, we control how perylenediimides pack in the solid state. We find inducing a long axis displacement of ∼3 Å between neighboring perylenediimides gives a maximal triplet production yield of 178% with a fission rate of ∼245 ps despite the presence of an activation barrier of ∼190 meV. These findings disagree with Marcus theory predictions for the optimal perylenediimide geometry for singlet fission, but do agree with Redfield theory calculations that allow singlet fission to occur via a charge transfer-mediated superexchange mechanism. Unfortunately, triplets produced by singlet fission are found to decay over tens of nanoseconds. Our results highlight that singlet fission materials must be designed to not only produce triplet excitons but to also facilitate their extraction.

Detailed neutronic analysis of a MOX-fueled metal-cooled reactor

With the prominent features of different U-Pu fuel compositions, the MOX-fueled fast spectrum metal-cooled reactors have been of increasing interest for recent years worldwide. Present study invokes a detailed investigation on the calculations for neutronic analysis and it conducts a comparative study for neutron physics parameters of a metal-cooled fast spectrum reactor – the BFS-62-3A. This work is an amalgamation of four tasks. In the first part, a detailed comparative analysis was performed to perform a code-to-code verification using the available results of three Monte Carlo codes including SuperMC, MCNP, and Serpent, and a deterministic one, the DYN3D-MG with the employment of continuous neutron energy cross-sections. The experimental results of BFS-62-3A benchmark were used to assess the potentiality of the aforementioned reactor physics codes. For most of the integral parameters, the SuperMC was found to be on the leading edge. In the second part, the effects of data libraries including ENDF/B-7.1, ENDF/B-7.0, ENDF/B-6.6, JEFF3.2, and HENDL3.0 on the simulations performed using SuperMC code for evaluating k-eff and radial fission rates were all assessed. The third part incorporates the investigation of the fission rates’ deviations in stainless steel radial reflector by changing the density of the reflector. The decrease of density by 5% was found to be in good agreement with the benchmark. In the last part, that has a special importance to the concept pertaining to safety-enhanced Sodium-cooled Fast Reactor (SFR) core, the reactivity of the critical assembly was studied by calculating the sodium void reactivity effect. The simulation results of SuperMC code agreed well with the available experimental and simulation results. The present study has enabled SuperMC code to pass another big milestone on dealing with complex and advanced nuclear systems.

Reaction Rate Benchmark Experiments with Miniature Fission Chambers at the Slovenian TRIGA Mark II Reactor

A series of fission rate profile measurements with miniature fission chambers, developed by the Commisariat á l’énergie atomique et auxénergies alternatives, were performed at the Jožef Stefan Institute’s TRIGA research reactor. Two types of fission chambers with different fissionable coating (235U and 238U) were used to perform axial fission rate profile measurements at various radial positions and several control rod configurations. The experimental campaign was supported by an extensive set of computations, based on a validated Monte Carlo computational model of the TRIGA reactor. The computing effort included neutron transport calculations to support the planning and design of the experiments as well as calculations to aid the evaluation of experimental and computational uncertainties and major biases. The evaluation of uncertainties was performed by employing various types of sensitivity analyses such as experimental parameter perturbation and core reaction rate gradient calculations. It has been found that the experimental uncertainty of the measurements is sufficiently low, i.e. the total relative fission rate uncertainty being approximately 5 %, in order for the experiments to serve as benchmark experiments for validation of fission rate profiles. The effect of the neutron flux redistribution due to the control rod movement was studied by performing measurements and calculations of fission rates and fission chamber responses in different axial and radial positions at different control rod configurations. It was confirmed that the control rod movement affects the position of the maximum in the axial fission rate distribution, as well as the height of the local maxima. The optimal detector position, in which the redistributions would have minimum effect on its signal, was determined.

Determination of the fast-neutron-induced fission cross-section of 242Pu at nELBE

The fast-neutron-induced fission cross section of 242Pu was determined in the energy range of 0.5 MeV to 10MeV at the neutron time-of-flight facility nELBE. Using a parallel-plate fission ionization chamber this quantity was measured relative to 235U(n,f). The number of target nuclei was thereby calculated by means of measuring the spontaneous fission rate of 242Pu. An MCNP 6 neutron transport simulation was used to correct the relative cross section for neutron scattering. The determined results are in good agreement with current experimental and evaluated data sets.

Влияние характеристик делящихся ядер на параметры сглаженной зависимости скорости деления от времени

Влияние характеристик делящихся ядер на параметры сглаженной зависимости скорости деления от времени

Validating COMSOL multiphysics for VVER-1000 whole-core-steady-state via AER benchmark problem

In this work, a full core, three-dimensional, multi-group model of VVER-1000 reactor core is developed using specifications of the Schulz benchmark. This paper presents a new 3-D full core solution, and simulates the neutronic behaviour of the VVER-1000 reactor by predicting the system criticality, power distribution, and the neutron flux distribution. Multi-group constants are applied to the COMSOL model to perform neutronics calculations using finite element method with adaptive mesh refinement. The study found that the calculated effective multiplication factor (keff) compares well with the reference value. Furthermore, the fission rates 3D power distributions and axially averaged 2D power distribution are in good agreements with reported reference results. The thermal neutron spectrum is also calculated by the COMSOL model as presented in this paper. This study allows us to validate the COMSOL calculation schemes for VVER-type reactors and to compare our solutions with reference solutions at steady state.

Temperature-Dependent Growth and Fission Rate Plasticity Drive Seasonal and Geographic Changes in Body Size in a Clonal Sea Anemone.

The temperature-size rule is a commonly observed pattern where adult body size is negatively correlated with developmental temperature. In part, this may occur as a consequence of allometric scaling, where changes in the ratio of surface area to mass limit oxygen diffusion as body size increases. As oxygen demand increases with temperature, a smaller body should be favored as temperature increases. For clonal animals, small changes in growth and/or fission rate can rapidly alter the average body size of clonal descendants. Here I test the hypothesis that the clonal sea anemone Diadumene lineata is able to track an optimal body size through seasonal temperature changes using fission rate plasticity. Individuals from three regions (Florida, Georgia, and Massachusetts) across the species' latitudinal range were grown in a year-long reciprocal common garden experiment mimicking seasonal temperature changes at three sites. Average body size was found to be smaller and fission rates higher in warmer conditions, consistent with the temperature-size rule pattern. However, seasonal size and fission patterns reflect a complex interaction between region-specific thermal reaction norms and the local temperature regime. These details provide insight into both the range of conditions required for oxygen limitation to contribute to a negative correlation between body size and temperature and the role that fission rate plasticity can play in tracking a rapidly changing optimal phenotype.

Controlling Long-Lived Triplet Generation from Intramolecular Singlet Fission in the Solid State.

The conjugated polymer poly(benzothiophene dioxide) (PBTDO1) has recently been shown to exhibit efficient intramolecular singlet fission in solution. We investigate the role of intermolecular interactions in triplet separation dynamics after singlet fission. We use transient absorption spectroscopy to determine the singlet fission rate and triplet yield in two polymers differing only by side-chain motif in both solution and the solid state. Whereas solid-state films show singlet fission rates identical to those measured in solution, the average lifetime of the triplet population increases dramatically and is strongly dependent on side-chain identity. These results show that it may be necessary to carefully engineer the solid-state microstructure of these "singlet fission polymers" to produce the long-lived triplets needed to realize efficient photovoltaic devices.

Transmutation of uranium and thorium in the particle field of the Quinta sub-critical assembly

The fission rates of natural uranium and thorium were measured in the particle field of Quinta, a 512 kg natural uranium target–blanket sub-critical assembly. The Quinta assembly was irradiated with deuterons of energy 4 GeV from the Nuclotron accelerator of the Joint Institute for Nuclear Research (JINR), Dubna, Russia. Fission rates of uranium and thorium were measured using Gamma spectroscopy and fission track techniques. The production rate of 239Np was also measured. The obtained experimental results were compared with Monte Carlo predictions using the MCNPX 2.7 code employing the physics and fission–evaporation models of INCL4–ABLA, CEM03.03 and LAQGSM03.03. Some of the neutronic characteristics of the Quinta are compared with the “Energy plus Transmutation (EpT)” subcritical assembly, which is composed of a lead target and natU blanket. This comparison clearly demonstrates the importance of target material, neutron moderator and reflector types on the performance of a spallation neutron driven subcritical system.As the dimensions of the Quinta are very close to those of an optimal multi-rod-uranium target, the experimental and Monte Carlo calculation results presented in this paper provide insights on the particle field within a uranium target as well as in Accelerator Driven Systems in general.

Microscopic description of fission in odd-mass uranium and plutonium nuclei with the Gogny energy density functional

The parametrization D1M of the Gogny energy density functional is used to study fission in the odd-mass Uranium and Plutonium isotopes with A=233,\ldots,249 within the framework of the Hartree-Fock-Bogoliubov (HFB) Equal Filling Approximation (EFA). Ground state quantum numbers and deformations, pairing energies, one-neutron separation energies, barrier heights and fission isomer excitation energies are given. Fission paths, collective masses and zero point rotational and vibrational quantum corrections are used to compute the systematic of the spontaneous fission half-lives t$_{SF}$, the masses and charges of the fission fragments as well as their intrinsic shapes. Although there exits a strong variance of the predicted fission rates with respect to the details involved in their computation, it is shown that both the specialization energy and the pairing quenching effects, taken into account fully variationally within the HFB-EFA blocking scheme, lead to larger spontaneous fission half-lives in odd-mass U and Pu nuclei as compared with the corresponding even-even neighbors. It is shown that modifications of a few percent in the strengths of the neutron and proton pairing fields can have a significant impact on the collective masses leading to uncertainties of several orders of magnitude in the predicted t$_{SF}$ values. Alpha-decay lifetimes have also been computed using a parametrization of the Viola-Seaborg formula.

Model for radiation damage-induced grain subdivision and its influence in U3Si2 fuel swelling

The swelling mechanisms of U3Si2 from neutron irradiation under power reactor conditions are not unequivocally known. The limited experimental evidence that is available suggests that the main driver for swelling in this material would be fission gas accumulation at crystalline grain boundaries. The stages that lead to the accumulation of fission gases at these locations are multiple and complex. However the main mechanism is that, gradually, the gaseous fission products migrate by diffusion. Upon reaching a grain boundary, which acts asa trap, the gaseous fission products accumulate thus leading to formation of bubbles and hence to induced swelling. Therefore, a quantitative model of swelling requires a correct accounting for the presence of grain boundaries and for phenomena and changes that increase their numbers, thus creating sites for bubble formation and growth while concurrently decreasing grain sizes, thus decreasing the effective distance for gas species migration to gas accumulation sites.A model for the subdivision, or recrystallization, of crystal grains is presented. It is assumed that new grain boundary formation results from the conversion of stored energy from accumulated dislocations into energy for local matter rearrangements that create the new grain boundaries hence subdividing the initial grain. The model is applied to grain subdivision in U3Si2, albeit using highly uncertain parameters. Then the implications of the model are assessed using a swelling modeling code to evaluate the total swelling of a fuel pellet during its lifetime in the I2S-LWR reactor, accounting for the influence of grain subdivision.The present model relaxes assumptions from previous models that limit their applicability in the case of the power levels and temperature conditions of the I2S-LWR concept. In particular, the new model assumes fully time-dependent conditions and does not postulate equilibrium or steady-state conditions for any population of radiation induced damage structures or the products of their evolution, such as vacancies, interstitials, and dislocations. Unlike others, the present model explicitly accounts for a population of di-interstitials. The model predicts that when a critical fission density is reached, the crystal grains subdivide and new, smaller grains are formed.It is shown that for some plausible, though uncertain, sets of physical parameters, a critical fission density can be reached at which recrystallization can occur in U3Si2, which can then result in the onset of breakaway swelling. It is found that the critical fission density depends strongly on the fission rate density and that it decreases with increases in the fission rate density. The critical fission density is found to display a minimum at intermediate temperatures, which suggest possible fuel performance advantages in operating the reactor in ways that imply higher fuel temperatures.

A modelling study of the inter-diffusion layer formation in U-Mo/Al dispersion fuel plates at high power

Post irradiation examinations of full-size U-Mo/Al dispersion fuel plates fabricated with ZrN- or Si- coated U-Mo particles revealed that the reaction rate of irradiation-induced U-Mo-Al inter-diffusion, an important microstructural change impacting the performance of this type of fuel, transited at a threshold temperature/fission rate. The existing inter-diffusion layer (IL) growth correlation, which does not describe the transition behavior of IL growth, was modified by applying a temperature-dependent multiplication factor that transits around a threshold fission rate. In-pile irradiation data from four tests in the BR2 reactors, including FUTURE, E-FUTURE, SELEMIUM, and SELEMIUM-1a, were utilized to determine and validate the updated IL growth correlation. Irradiation behavior of the plates was simulated with the DART-2D computational code. The general agreement between the calculated and measured fuel meat swelling and constituent volume fractions as a function of fission density demonstrated the plausibility of the updated IL growth correlation. The simulation results also suggested the temperature dependence of the IL growth rate, similar to the temperature dependence of the inter-mixing rate in ion-irradiated bi-layer systems.

Validation of SuperMC code by simulating a Metal-cooled fast reactor – BFS-62-3A

Appraising the credibility and competency of a computer simulation can be done by verification and validation procedure. This study has been performed to validate SuperMC for precise simulation of metal-cooled fast reactors using a high profile heterogeneous critical core, the BFS-62-3A. This critical assembly – being attractive because of its multi-zoned structure with MOX fuel, high axial and radial heterogeneity, diversity of materials and their compositions, and eligibility/feasibility to benchmark the experiments and calculations – is a full scale mock-up of BN-600 hybrid core with different fuel regions and stainless steel reflector. Modeling of complex core design, calculation of desired parameters, and visualization of the complete model were all performed using SuperMC. The deviations of less than 0.4% and 0.8% appeared in the calculated values of multiplication factor and spectral indices respectively while average discrepancies for radial fission rate distributions in the fuel region and control rod worths were found to be 2.7% and less than 5% respectively that showed a good agreement of the simulation results with benchmark’s measured data. Present work thus validated SuperMC’s capability to precisely model a complex heterogeneous reactor core and to accurately calculate various neutron physics parameters that ultimately rendered a high level of confidence on its use in fast-spectrum and MOX-fueled nuclear fission reactors.

Neutronic analysis for accident tolerant cladding candidates in CANDU-6 reactors

The neutronic penalty and advantages are quantified for potential accident tolerant claddings in a CANDU-6 (Canada Deuterium Uranium) reactor. Ferritic-based alloy (FeCrAl and APMT), steel-based alloy (304SS and 310SS) and silicon carbide (SiC) claddings are compared with Zircaloy-II. High thermal capture in nickel-59 and iron-56 imply a neutronic penalty in iron and steel-based bundles. A minimum enrichment of 1.0% and 1.1% is required for ferritic and steel-based claddings, respectively, to achieve the CANDU-6 burnup average criticality. An average increase of 0.35% in reactivity is introduced when SiC is considered, while keeping a natural enrichment condition. Average thermal neutron absorption rate is found to be 203, 8 and 6 times higher in UO2pellets than in silica, iron and steel-based claddings, respectively. A spectral hardening is observed in fuel, cladding and coolant for all enriched cells. Neutron flux is 30.5% higher at 0.0253 eV in SiC and Zr bundles. For FeCrAl, APMT, 304SS and 310SS, less 239Pu is produced during all fuel residence time and a minimum of 50% more 135Xe poison is being produced at equilibrium. As well, a minimum of 60% more 235U is left at end-of-life. For bundles cladded with SiC and Zr, 7.6% higher fission rates are found on the pellet periphery. Decreasing the cladding thickness by 200 μm made it possible to satisfy the criticality requirements for all claddings with an enrichment below 1%. Moderator and coolant temperature coefficients are found higher in SiC and Zr bundles. At mid-burnup, the Doppler effect and the voiding effect are higher in FeCrAl, APMT, 304SS and 310SS cladded cells.

Fuel cycle analysis of Advanced Burner Reactor with breed-and-burn thorium blanket

The Seed-and-Blanket (S&B) Sodium-cooled Fast Reactor (SFR) core concept was proposed for generating a significant fraction of the core power from thorium fueled breed-and-burn (B&B) blankets without exceeding the presently verified radiation damage constraint of 200 Displacement per Atom (DPA). To make beneficial use of the excess neutrons from fast reactors, the S&B core is designed to have an elongated TRU transmuting (or “TRU burner”) seed from which over 20% of the fission neutrons leak into a subcritical thorium blanket that radially surrounds the seed. The seed fuel is recycled while the blanket operates in a once-through breed-and-burn (B&B) mode. The objective of this paper is to compare the fuel cycle performance of the S&B reactor against an Advanced Burner Reactor (ABR) and a conventional Pressurized Water Reactor (PWR). For the fast reactors (SFR: ABR and S&B) the fuel cycle performance is evaluated based on a 2-stage PWR-SFR energy system while the reference nuclear system is made of once-through PWRs.It was found that relative to the ABR, the S&B core has a lower fuel cycle cost, higher capacity factor, and comparable short-term radioactivity. The discharged seed fuel from the S&B core features lower fissile Pu-to-Pu ratio, higher 238Pu-to-Pu ratio, higher specific plutonium decay heat, higher spontaneous fission rate, and lower overall material attractiveness for weapon use. Due to the significant amount of 233U discharged from the breed-and-burn thorium fueled blankets, the S&B core has much higher long-term radioactivity and radiotoxicity. Since the thorium fueled blanket operates in the breed-and-burn mode and requires no fuel reprocessing, the discharged blanket fuel is unattractive for weapons application.Compared with a PWR, the S&B core has a lower fuel cycle cost, much lower short-term radioactivity and radiotoxicity but higher long-term values, and higher proliferation resistance for the discharged plutonium. The natural uranium utilization of the 2-stage PWR-S&B system is approximately 60% higher than that of present PWRs; it is few percent higher than that of the 2-stage PWR-ABR system. Approximately 7% of the thorium fed to the blanket is converted into energy, which makes the thorium fuel utilization approximately 12 times the utilization of natural uranium in PWRs.A comprehensive fuel cycle evaluation performed with the methodology developed by the recent U.S Department of Energy’s Nuclear Fuel Cycle Evaluation and Screening campaign concludes that the PWR-S&B system has similar fuel cycle performance characteristics as the PWR-ABR system. The S&B concept may potentially feature improved economics and resource utilization relative to the ABR.

Kinetics solution with iteration scheme of history-based neutron source in accelerator driven system

In the time-dependent neutronic analyses of the accelerator driven system (ADS), one of the technical issues is the investigation method of reactor transients for validating why detector responses or fission rates are varied in functions of space and time. In previous studies, numerical analyses for ADS have been conducted by using forward transport methods, focusing on reconstruction of the time behavior. In this study, a source history-based kinetics equation, which is superior to cause analysis of time transients comparing with general forward transport methods, is derived by using the time contributions from each source into responses. For the validation of the applicability, through the re-evaluation of ADS experiment at the Kyoto University Critical Assembly, the time transients of fission rates and detector responses are quantitatively analyzed with the time contributions directly obtained by the proposed method. The expanded analysis ability provided by the proposed method will contribute to investigate the reactor transient of ADS including verification of reactor experiments, analysis of space-time detector responses and establishment of estimation strategy for obtaining the kinetics parameters.

Acceleration of within group iteration for pin-by-pin calculations

In thermal reactor physics fine mesh neutron diffusion calculations in which homogenization is only at the pin cell level are of considerable interest. However, the slow convergence of iterations on fine-mesh spatial grids limits their applicability to practical problems. We present here an efficient way to accelerate the within-group iterations for pin-by-pin calculations based on a two-dimensional heterogeneous variational nodal method (VNM). Response matrix (RM) equations are formulated that incorporate multiple pins within each node. Within the nodes, finite elements in the x-y plane are employed to describe the piecewise constant heterogeneous geometry. On the nodal interfaces orthogonal polynomials are employed to approximate current distributions. The RM equations are solved using the Red-Black Gauss-Seidel (RBGS) iteration. Investigations on the coarse nodes acceleration (CNA) by combining homogenized pin cell nodes into larger heterogeneous nodes are performed. A series of meshing schemes are examined with a 2D small modular reactor core problem. With sufficient interface expansion order, CNA do not cause significant errors to either eigenvalues or fission rate distributions compared with fine mesh calculations. It is demonstrated that CNA accelerate the RBGS iteration significantly and achieves favorable accuracy-efficiency trade-off.

Computational validation of the fission rate distribution experimental benchmark at the JSI TRIGA Mark II research reactor using the Monte Carlo method

A fission rate profile benchmark experiment has been performed at the Jožef Stefan Institute TRIGA Mark II reactor. The measurements were made using absolutely calibrated miniature fission chambers developed and manufactured by the Commissariat à l’Énergie Atomique et aux Énergies Alternatives. The aim of the paper is to describe the experimental set-up, fission rate measurements and to present the detailed Monte Carlo computational model of the TRIGA reactor, which was constructed with as used to compute absolute fission rate distributions in the core at a fixed control rod position, taking into account the detailed description of the experimental configuration. The paper focuses on the extensive evaluation of experimental and calculational uncertainties and biases following the International Reactor Physics Experiment Evaluation Project methodology. A comparison between the measured and computed absolute reaction rates concludes the paper, with the agreement being within one sigma standard uncertainty.