At low temperatures (<200°C), spent fuel from power reactors oxidizes from its UO2 lattice to a U4O9 lattice but with an oxygen-to-uranium (O/U) ratio of ∼2.4. Also, the weight gain time response has a plateau as the O/U approaches 2.4. Part of this response results from a geometrical dependency as a U4O9 oxidation front propagates into grain volumes of UO2. It may also be indicative of a metastable, non-stoichiometric U4O9 phase whose existence may inhibit the transition kinetics to the next expected phase of U3O8. To gain a mechanistic understanding and to plan future oxidation tests, lattice kinematic and thermodynamic models are developed for lattice deformations and energetics of lattice phase changes (UO2 → U4O9 → U3O7 → U3O8) that include zeroth order influences on oxidation kinetics due to interstitial oxygen atoms and vacancies plus interstitial and substitutional actinides and fission decay products in spent fuel.

Poroelastic materials are two phase material systems consisting of a porous linear elastic solid phase filled with a Newtonian viscous fluid. Analytical investigations have demonstrated that poroelastic structures can, depending on loading and geometry, exhibit elastic response or the creep and relaxation response associated with various models of linear viscoelastic materials. This paper examines the dynamic response to harmonic loading of a disk or slab of poroelastic material. Biot's dynamic theory for deformable poroelastic media is applied to derive expressions fro; a complex modulus of the material in terms of poroelastic material coefficients. Both a quasistatic analysis, accounting for dissipation but neglecting inertia, and a dynamic analysis, which neglects dissipation, are presented. For a choice of poroelastic coefficients roughly appropriate to water filled sandstone or compact bone, the poroelastic layer is shown to exhibit rubber-to-glass transition in a low-frequency range. Resonance effects arising due to inertia are shown to take place at much higher frequencies than those at which changes due to dissipation take place.

Traditional pressure testing models for naturally fractured reservoirs assume that the rock formation is heavily fractured to an extent that the matrix blocks are completely surrounded by fracture surfaces. Thus, the matrix blocks are unconnected and fluid transport over macroscopic distances can take place only in the fracture network. The model presented here overcomes this deficiency by allowing for matrix and fracture network permeabilities over macroscopic distances; it is thus able to represent moderately fractured formations as well as the limiting cases of unfractured and heavily fractured formations. Also, a new unsteady interporosity fluid flow formulation, which accounts for independent variation of the matrix and fracture system pressures is employed in this model. This feature has not been included in previous interporosity flow formulations. Analytical solutions and type curves for single-well pressure tests in one, two and three dimensional fluid flow configurations in unbounded domains are presented. Interporosity fluid transfer is modelled using the standard unsteady flow models and the new unsteady interporosity flow formulation. The effects of well bore storage and of the matrix and fracture system skin factors on constant rate drawdown well tests are illustrated through numerical examples. The significance of the special features of this model, namely, that of modelling the macroscopic permeability of the matrix system in addition to that of the fracture system and of using the new unsteady interporosity flow model is demonstrated. The numerical results show that, for moderately fractured formations, significant errors are incurred in the predicted formation properties when matrix block connectivity and the resulting direct fluid transport between matrix blocks are neglected. Also, the results show that the dependence of interporosity flow on the independently varying matrix and fracture system pressures must be accounted for in the interporosity flow model.

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Hydraulic fractures created in oil and gas bearing rock formations can be made to propagate for a limited time at approximately constant height if favorable stress, deformation modulus or fracture toughness barriers to height growth exist and if the fracture design is suitably optimized to exploit these favorable conditions and reduce height growth. In this report, a unified theoretical formulation for the Perkins-Kern-Nordgren (PKN) and Christianovitch-Geertsma-De Klerk-Daneshy (CGDD) constant height fracture models is first presented. For a fracture fluid injection rate that varies as an arbitrary power of time, growth laws for fracturing fluid pressure, fracture width, and flow rate are rigorously derived for PKN and CGDD types of fractures. These similarity solutions account for nowNewtonian power-law fluid flow, transient fluid storage and generalized power-law fluid leak-off to the rock formation. They include and extend the results currently available in the literature for PKN and CGDD fractures. The results an approximate constant height for PKN and CGDD fractures are then generalized to obtain hybrid CGDD-PKN fracture model that can be applied to fractures of arbitrary length/height aspect ratio and