Wigner Energy Release Research Articles

The Wigner energy and defects evolution of graphite in neutron-irradiation and annealing

The annealing process of nuclear graphite accompanying the release of Wigner energy was studied in this research. IG-110U and ETP-10 graphite were irradiated at 200 °C by neutrons with low-dose (1.38 × 1019 n/cm2) and high-dose (1.92 × 1020 n/cm2), respectively. Their Wigner energy release at 100–550 °C was measured based on a high-precision differential scanning calorimeter. Characteristics of Wigner energy release spectrum were investigated. The activation energy of the energy release peak (170–230 °C) for low-dose graphite was 1.48 eV, which was related to the decomposition of interstitial clusters and the behaviour of subsequent interstitial pairs. Furthermore, a comprehensive analysis of Wigner energy release combined with thermal conductivity and multi-scale structural parameters during the neutron irradiation and annealing process is given.

Analysis of Heat Release Processes inside Storage Facilities Containing Irradiated Nuclear Graphite

The article is dedicated to the safety assessment of mixed storage of irradiated graphite and other types of radioactive waste accumulated during the operation of uranium-graphite reactors. The analysis of heat release processes inside storages containing irradiated nuclear graphite, representing a potential hazard due to the possible heating and, accordingly, the release of long-lived radionuclides during oxidation was carried out. The following factors were considered as the main factors that can lead to an increase in the temperature inside the storage facility: corrosion of metallic radioactive waste, the presence of fuel fragments, and also the random exposure of irradiated graphite to local sources of thermal energy (spark, etc.). It was noted in the work that the combined or separate influence of some factors can lead to an increase in the temperature of the onset of the initiation of Wigner energy release in graphite radwaste (Tin ≈ 90–100°C for the “Worst-case” graphite). The model of heat generation in the storage was developed based on the analysis of the features of graphite radioactive waste storage and Wigner energy release. The layered location of different types of waste (graphite and aluminum) and the local character of the distribution of heat sources were adopted in this model. The greatest heating is achieved if graphite radioactive waste is located near the concrete walls of the storage facility, as well as in direct contact with irradiated aluminum radioactive waste, which was shown in this paper.

Combustion of a Dust/Gas Mixture Consisting of Particles of Irradiated Nuclear Graphite

The paper describes the problem of the formation of dust/gas mixtures during the work related to the dismantling of graphite stack during the decommissioning of a uranium-graphite reactor. The amount of stored energy in the irradiated graphite elements of the reactor was experimentally determined. The influence of the Wigner energy release process on the combustion of a dust/gas mixture consisting of particles of irradiated nuclear graphite is shown.

Metastable interlayer Frenkel pair defects in black phosphorus

The low formation energy of atomic vacancies in black phosphorus allows it to serve as an ideal prototypical system for exploring the dynamics of interlayer interstitial-vacancy (I-V) pairs (i.e. Frenkel defects) which account for Wigner energy release. Based on a few-layer model of black phosphorus, we conduct discrete geometry analysis and investigate the structural dynamics of intimate interlayer Frenkel pairs from first-principles calculations. We reveal a highly metastable I-V pair state driven by anisotropic dipole-like strain fields which can build strong connections between neighbouring layers. In the 2D limit (monolayer), the intimate I-V pair exhibits a relatively low formation energy of 1.54 eV and is energetically favoured over its isolated constituents by up to 1.68 eV. The barrier for annihilation of the Frenkel pair is 1.46 eV in the bilayer, which is remarkably higher than that of similar defects in graphite. The findings reported in this work suggest that there exist rich bridging pathways in black phosphorus, leading to stable dimensional reduction and structural condensation on exposure to moderate electron excitation or thermal annealing. This study paves the way for creating novel dimensional-hybrid polymorphs of phosphorus via the introduction of such metastable interlayer I-V pair defects.

Fully Ceramic Microencapsulated fuel in prismatic high-temperature gas-cooled reactors: Sensitivity of reactor behavior during design basis accidents to fuel properties and the potential impact of the SiC defect annealing process

We conducted reactor performance calculations to assess the potential design basis accident performance of HTGR fuel designs. Three Fully Ceramic Microencapsulated (FCM) fueled HTGR designs were developed in a previous work (Lu et al., 2018). The maximum fuel temperature in the cores fueled by these three FCM fuels was predicted to be higher than that in the reference 350-MWt mHTGR core in both normal operating conditions and during representative design basis accidents (Lu and Brown, 2019). To better understand the potential safety margins in mHTGR design basis accidents, we performed thermal-hydraulics sensitivity studies to investigate how maximum fuel temperature varies considering various parameters, e.g. thermal properties, within the ranges corresponding to the differences between the FCM-fueled prismatic mHTGR cores and the reference core with conventional fuel compacts. We found that the difference in the steady-state axial power distribution contributed the most to the difference in the maximum fuel temperature, in both normal operation and design basis accidents.Experimental data suggested that the annealing process of irradiation defects in SiC would be rapid at mHTGR relevant fuel temperatures. The bounding potential impact of the SiC annealing on the maximum fuel temperature was analyzed considering both the thermal conductivity recovery and the Wigner energy release due to the annealing of SiC. We found that the defect annealing process in SiC would at most increase the peak maximum fuel temperature of an FCM-fueled core by 40 K in loss of forced cooling accidents and by 10 K in a control rod withdrawal accident. Additional experiments on the SiC defect annealing kinetics and Wigner energy release in more relevant conditions are needed.

Dynamics of temperature fields during Wigner energy release in bulk graphite irradiated at low temperature

This paper is concerned with determination of parameters associated with release of the energy accumulated in irradiated graphite. These parameters define the conditions and possible consequences of the release – the heat effects, both in the course of the all types of uranium-graphite reactors (UGR) decommissioning work and during the entire irradiate graphite management process, including treatment, storage, and disposal of the graphite radioactive waste (GRW).Results of analysis of the magnitude and dynamics of maximum possible heat effects because of Wigner energy release in the graphite stack and in the volume of graphite parts loaded into large containers were presented. Results of modeling showed the most critical conditions from the standpoint of the potential heat effect values because of Wigner energy release in large volumes of the graphite irradiated at low temperature. Those conditions were achieved when the temperature of the entire graphite volume reached the temperature of self-induced stored energy release (the initiation temperature). In the case, the energy was released within the entire volume simultaneously. This path of accumulated energy release was characterized by the high annealing intensity, and under the heat transfer conditions with the air environment the temperature of graphite could reach the values inherent to adiabatic conditions, ∼800oС.

Analysis of Wigner energy release process in graphite stack of shut-down uranium-graphite reactor

Data, which finding during thermal differential analysis of sampled irradiated graphite are presented. Results of computational modeling of Winger energy release process from irradiated graphite staking are demonstrated. It's shown, that spontaneous combustion of graphite possible only in adiabatic case.

The evolution of mechanical and structural properties at the fiber/matrix interphase of SiC/SiC composites

Fiber/matrix interfacial strength is known to play an important role in the mechanical properties of SiCf/SiC composite. The understanding of the deformation mechanism of the fiber/matrix interphase under stress is important in application of SiCf/SiC composite. In the present work, we employ molecular dynamics simulations to investigate the shear strength and structural evolutions of fiber/matrix interphase of SiCf/SiC composite in atomic scale. It is revealed from our computational study that the interphase of graphite-like pyrolytic carbons are the weakest part in the interphase, which determines the interfacial mechanical properties. It is also found that an obvious drop of shear strength occurs at high temperature, owing to the Wigner energy release in the graphite-like carbons.

Electron-irradiation-induced reinforcement of reduced graphene oxide papers

Here we show that the graphene sheets in reduced graphene oxide papers (rGOPs) can be cross-linked by electron-irradiation-induced sp3 carbon atoms, resulting in significant enhancements in both the mechanical and electrical properties compared to unirradiated rGOPs. We find that the residual oxygen-containing functional groups in rGOPs are removed during irradiation. We also observe that the Wigner energy release temperature in rGOPs is 102–188°C, which is lower than that (∼200°C) in pure graphite. The mechanisms to form sp3 cross-links and the evolutions of sp3 carbon atoms under irradiation are revealed through molecular dynamics simulations.

The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite

Models for radiation damage in graphite are reviewed and compared, leading to a re-examination of the contribution made by vacancies to annealing processes. A method based on density functional theory, using large supercells with orthorhombic and hexagonal symmetry, is employed to calculate the properties and behaviour of lattice vacancies and displacement defects. It is concluded that annihilation of intimate Frenkel defects marks the onset of recovery in electrical resistivity, which occurs when the temperature exceeds about 160 K. The migration of isolated monovacancies is estimated to have an activation energy of Ea ≈ 1.1 eV. Coalescence into divacancy defects occurs in several stages, with different barriers at each stage, depending on the path. The formation of pairs ultimately yields up to 8.9 eV energy, which is nearly 1.0 eV more than the formation energy for an isolated monovacancy. Processes resulting in vacancy coalescence and annihilation appear to be responsible for the main Wigner energy release peak in radiation-damaged graphite, occurring at about 475 K.

Raman study of correlation between defects and ferromagnetism in graphite

The variation of ferromagnetism induced by 12C+ ion implantation in highly oriented pyrolytic graphite was systematically studied by using Raman spectroscopy in conjunction with magnetic moment measurements and annealing treatments. It was found that the magnetization of the implanted sample was closely correlated with the density of the defects, which was characterized by the Raman spectra, produced by the implantation. It is clear that by using consecutive implantation steps at different energies to increase the vacancy defects in the implanted layer, the magnetization of the sample increases with the number of the implantation steps until the fourth step of implantation, which causes the near surface layer to be highly disordered or amorphous, weakening the magnetic coupling and thus resulting in the decrease in magnetization. The annealing treatments of the sample indicate that the ferromagnetism induced by the implantations is stable at room temperature. However, when the sample is annealed at 473 K (the Wigner energy release temperature), the density of vacancies and interstitials is abruptly decreased and the magnetism induced by the implantations is extinguished. This finding gives a clear indication of the key role of the defects produced by C+ ion implantation in graphite.

In SituObservation of Thermal Relaxation of Interstitial-Vacancy Pair Defects in a Graphite Gap

Direct observation of individual defects during formation and annihilation in the interlayer gap of double-wall carbon nanotubes (DWNT) is demonstrated by high-resolution transmission electron microscopy. The interlayer defects that bridge two adjacent graphen layers in DWNT are stable for a macroscopic time at the temperature below 450 K. These defects are assigned to a cluster of one or two interstitial-vacancy pairs (I-V pairs) and often disappear just after their formation at higher temperatures due to an instantaneous recombination of the interstitial atom with vacancy. Systematic observations performed at the elevated temperatures find a threshold for the defect annihilation at 450-500 K, which, indeed, corresponds to the known temperature for the Wigner energy release.

A method of calculating the rate of release of Wigner energy in heat conduction problems for irradiated graphite

A method is developed for the calculation of the rate of release of stored energy when annealing stored graphite. The method is based on the utilization of regularities of the sample temperature rise under adiabatic heating. The algorithm of calculation of internal heat sources, which is based on this method, is realized in the case of numerical solution of the problem of heat conduction in irradiated graphite under conditions of Wigner energy release. The method is verified using the adiabatic heating of a cylindrical sample as an example. An investigation is performed of the effect of the coefficient of intensity of heat transfer between a graphite sample and environment on the process of variation of temperature fields in the sample with release of stored energy.

Metastable Frenkel pair defect in graphite: source of Wigner energy?

The atomic processes associated with energy storage and release in irradiated graphite have long been subject to untested speculation. We examine structures and recombination routes for interstitial-vacancy (I-V) pairs in graphite. Interaction results in the formation of a new metastable defect (an intimate I-V pair) or a Stone-Wales defect. The intimate I-V pair, although 2.9 eV more stable than its isolated constituents, still has a formation energy of 10.8 eV. The barrier to recombination to perfect graphite is calculated to be 1.3 eV, consistent with the experimental first Wigner energy release peak at 1.38 eV. We expect similar defects to form in carbon nanostructures such as nanotubes, nested fullerenes, and onions under irradiation.

Fine structure of Wigner energy release spectrum in neutron irradiated graphite

Stored energy release spectra have been measured using the linear-rise method for pyrolytic graphite samples, neutron-irradiated to a fluence of 4 × 10 17 n/cm 2 at about 80°C. Seven heating rates were chosen in the range of 1°C/min to 100°C/min. Total stored energy release is (8±1)J/g. The spectra are composed of three peaks. The reactions of three peaks follow first-order kinetics, and their respective frequency factors and average activation energies are 2.2 × 10 12 s −1 and 1.34 eV; 8.5 × 10 12 s −1 and 1.50 eV; and 1.5 ×10 14 s −1 and 1.78 eV. The initial activation energy spectrum is approximated by the Gaussian distribution. The fraction of each peak varies with the heating rate. The mechanism of the Wigner energy release is discussed on the basis of these kinetic results.