Radiation-enhanced diffusion and fission gas release from recrystallised grains in high burn-up hbox{UO}_{2} nuclear fuel

The effective diffusion coefficient that gives a steady-state xenon concentration of 0.2-0.3wt% in the recrystallised grains of high burn-up UO 2 fuel is calculated to lie in the range 10 m 24 to 10 m 22 m 2 s m 1 . These values are one to three orders of magnitude lower than the value currently accepted for the radiation-enhanced diffusion coefficient. The time required to reach the steady-state concentration depends on the local fission rate, the grain size distribution and the precise magnitude of the radiation-enhanced diffusion coefficient, and can take from 2 to 10 years. Additional calculations reveal that substantially less than 10% of the fission gas inventory is released from the original UO 2 grains in the outer region of the fuel prior to recrystallisation. In contrast, with a diffusion coefficient of 10 m 22 m 2 s m 1 more than 80% of the fission gas is released from the recrystallised grains of the high burn-up structure in one year.     

Theory of nuclear fission by stopped pions

Nuclear fission by stopped pions is analyzed in a two-step model in which the pion is absorbed on a nucleon pair, followed by the prompt emission of one nucleon and compound nucleus formation by the other. This simple picture yields agreement with observed prompt neutron spectra and average nucleon multiplicities. The analysis of nuclear fission in a statistical framework is shown to provide valuable corroboration of the expected properties of a highly excited compound nucleus and of our understanding of the pion absorption mechanism. The ratio of fission to neutron level density parameters is shown to exceed unity in all cases studied.     

Ion-scattering structure studies of UO2 surfaces

Planar (111) and stepped (553) surfaces of single crystalline UO₂ have been studied by low-energy He⁺ ion scattering spectroscopy, (ISS). Angular dependent ISS and comparison data from oxide-coated, polycrystalline U indicate that the outermost layer of UO₂(111) is oxygen occupying, to a first approximation, the lattice position expected from a simple termination of bulk structure. A similar study of UO₂(553) indicates that ledge U is covered with excess oxygen.     

Atomistic simulation of a superionic transition in UO2

The results of the atomistic simulation of a superionic transition and melting of uranium dioxide are presented. The temperature dependences of the concentration of defects in the oxygen sublattice and changes in the heat capacity and isothermal compressibility upon the superionic transition are calculated. It is shown that the curve of the superionic transition in the PT diagram can be described by the Ehrenfest’s equation. The possibility of describing the superionic transition within the framework of the theory of second- order phase transitions is discussed. Based on the results obtained, it is considered that this structural transformation can occur in other materials.     

自辐射场下UO2分子光谱研究

铀原子和氧原子分别使用相对论有效原子实势（Relativistic Effective Core Potential）和6-311+G(d)基组,采用优选的密度泛函B3P86方法,研究了铀本身产生自辐射场（-0.005～0.005a.u.）作用下UO2基态分子的能隙Eg和谐振频率ν.结果表明：能隙Eg＝2.0028eV、1.9974 eV,接近实验值2.1eV,UO2在自辐射场中具有半导体性质;反对称伸缩振动频率ν3(σg)、弯曲振动频率ν2(πu)和对称伸缩振动频率ν1(σg)与实验值776.1 cm-1、225.2 cm-1、765.4cm-1基本吻合.     

IR-Spectra of M2UO2F4.H2O Complexes

This work is devoted to the study of and Rb₂UO₂F₄.H₂O and Cs₂UO₂F₄. H₂O IR-spectra with the aim of obtaining a set of vibration frequencies characterising M₂U0₂F₄.H₂O complexe elucidating the role and nature of water bonds in the structures of the above-mentioned compounds; and receiving preliminary information on the structure of M₂UO₂F₄.H₂O. The investigated compounds were synthesized in accord with our previous paper¹. M₂UO₂D₂O and M₂UO₂F₄.HDO were obtained by recrystallizing M₂UO₂F₄.H₂O from D₂O and HDO respectively     

Adsorption of UO2 2+ from Aqueous Solutions by Inorganic Exchangers

Various natural and treated clays as well as some synthetic exchangers have been used to remove UO₂ ²⁺ from aqueous solutions. Different factors which affect the adsorption process such as time, pH, competing ions … etc were studied. The mechanism of adsorption was found to be under film diffusion control. It was concluded that the naturally occuring clays are suitable as adsorbent for UO₂ ²⁺ and their capacities are comparable, if not better than those treated clays or synthetic exchangers.

Es werden unterschiedliche natürliche und vorher behandelte Tonarten als auch einige synthetische Anstauschcr zur Abtrennung von UO₂ ²⁺ aus wäβrigen Lösungen eingeselzt. Verschiedene Faktoren, die den Adsorptionsprozeβ beeinflussen, wie die Zeitdauer, der pH-Wert, andere konhurrierende Ionen u. ä. m. wurden untersucht. Es stellte sich heraus, daβ der Adsorptionsmechanismus durch eine Filmdiffusion kontrolliert wird. Dienatürlich vorkommenden Tonesind durchaus als Adsorbentien fur UO₂ ²⁺ geeignet und ihre Aufnahmekapzitäten sind vergleichbar, wenn nicht mit unter besser als jene, die bei behandelten Tonen oder synthetischen Austauschem gefunden werden.     

LEED studies of UO2(∼111) vicinal surfaces

The ordering/disordering of terraces, ledges and kinks on three UO₂(~ 111) vicinal surfaces were examined by LEED. Optical transforms were used as an indication of the types of diffraction displays expected. On surfaces with high kink and ledge densities and unheated above 600 °C, the ledges on the average are equally spaced and parallel, but the kink-site positions are random. At higher temperatures of 700 < T < 900 °C, the ledges rotate and decompose irreversibly into {553} and {311} microfacets. By transform simulations it was inferred that ledges on the UO₂{553} surfaces are highly ordered and contain less than 7% kink-sites.     

Hybrid density-functional theory and the insulating gap of UO2

We report the first calculations carried out with a periodic boundary condition code capable of examining hybrid density-functional theory (DFT) for f-element solids. We apply it to the electronic structure of the traditional Mott insulator UO2, and find that it correctly yields an antiferromagnetic insulator as opposed to the ferromagnetic metal predicted by the local spin density and generalized gradient approximations. The gap, density of states, and optimum lattice constant are all in good agreement with experiment. We stress that this results from the functional and the variational principle alone. We compare our results with the more traditional approximations.     

Theory of mean primary charge distribution in low energy fission of even-even nuclei

A molecular model of fission is introduced in which the compound nucleus before scission is described. The ground state wave function is assumed to be a BCS wave function. The single particle wave functions are expanded in terms of eigenfunctions of the unperturbed spherical fragments. The BCS wave function is determined from the minimum condition for the total energy. The fragment masses, the centers of mass, and the total proton and neutron numbers are kept constant. The resulting BCS and Hartree-Bogoliubov equations are solved approximately within the framework of an extended Nilsson model. The numerical results for the charge distribution in low energy fission of²³⁶U are in agreement with experiments. The heavier (lighter) fragment has on the average 0.5 protons less (more) than expected on the basis of the socalled unchanged charge distribution (UCD). At magic configurations the charge distribution shows characteristic deviations from the average value due to the shell structure of the fragments. The charge distribution seems to result mainly from three competing effects:

1. a)
   from the level density of the unperturbed fragments in the vicinity of the Fermi energy.     
   
   

Theory of fluctuations in non-equilibrium Fermi gas

We present the equations for the equal-time two-particle correlation functions in degenerate electron gas. In association with the Onsager-type equation for the time-displaced correlation function, the equations constitute the basis of the theory of fluctuations in non-equilibrium degenerate electron gas. Thanks to the prevalence of the small-angle inter-electron scattering, the theory takes a rather simple form.     

Theory of laser-induced gas ionization

The mechanism of laser-induced gas ionization is analyzed in the context of the theories presently available, namely multiphoton and cascade theory, and their predictions are shown to be in serious divergence from experimentation. A novel hypothesis is then formulated which considers the classical photon energy expression ε=hν as the limit of a general expression ε=hν/[1?β_ν f(I)],f(I) being a function of light intensity and β_ν a coefficient such that β_ν f(I) significantly differs from zero only for high-intensity laser light. Starting from the new formulation of the photon energy expression, a theory is developed for the laser ionization phenomenon and a few simple relations are found. The successful use of these relations in the verification of the experimental results suggests that the new photon energy expression has a real physical meaning rather than an empirical significance, and some considerations on the physical meaning are presented.     

Radiation enhanced diffusion in UO2 and (U,Pu)O2

Abstract

The radiation enhanced diffusion (coefficient D*) of U-233 and Pu-238 in UO₂ and (U, Pu)O₂ with 2.5 and 15% Pu was measured during fission in a nuclear reactor. Normal diffusion sandwiches with a thin tracer layer were used. A radio-frequency furnace allowed the temperatures to be varied between 130 and 1400°. Neutron fluxes (7 × 10¹² to 1.2 × 10¹⁴ n cm^?2 s^?1) and irradiation times (56 to 334 h) were also varied to cover ranges of fission rates [Fdot] between 7× 10¹¹ and 6.4 × 10¹³ f cm^?3 s^?1 and of doses F between 4.2 × 10¹⁷ and 3.1 × 10¹⁹ f cm³. Below ～1000°, D* was completely athermal and increased linearly with [Fdot]. It was described by D* = A[Fdot] with A = 1.2× 10^?29cm⁵. A possible temperature dependence was indicated between ～1000and 1200°. The results are explained in terms of thermal and pressure effects of fission spikes and are related with other studies of radiation damage as well as with technologically interesting processes occurring in UO₂ during irradiation.