G. Sánchez Sarmiento

Heat flow shape factors for circular rods with regular polygonal concentric inner bore

Abstract The title problem is solved using an approximate conformal mapping approach. The results are in good agreement with those obtained using a finite element code.

Failure internal pressure of spherical steel containments

Abstract An application of the British CEGBs R6 Failure Assessment Approach to the determination of failure internal pressure of nuclear power plant spherical steel containments is presented. The presence of hypothetical cracks both in the base metal and in the welding material of the containment, with geometrical idealizations according to the ASME Boiler and Pressure Vessel Code (Section XI), was taken into account in order to analyze the sensitivity of the failure assessment with the values of the material fracture properties. Calculations of the elastoplastic collapse load have been performed by means of the Finite Element System SAMCEF. The clean axisymmetric shell (neglecting the influence of nozzles and minor irregularities) and two major penetrations (personnel and emergency locks) have been taken separately into account. Large-strain elastoplastic behaviour of the material was considered in the Code, using lower bounds of true stress—true strain relations obtained by testing a collection of tensile specimens. Assuming the presence of cracks in non-perturbed regions, the reserve factor for test pressure and the failure internal pressure have been determined as a function of the flaw depth.

Analytical determination of heat flow shape factors for composite, prismatic bars of doubly-connected cross section

Abstract Cylindrical or prismatic configurations are used in many engineering situations (nuclear, mechanical, etc.). Oddly-shaped, doubly-connected geometries are required in some applications, and generated in general computer-oriented solutions by the research engineer. The title problem is solved in the present paper using an approximate conformal mapping approach. It is shown that the calculated shape factors are in good agreement with those obtained using a finite element code.

Theoretical fuel element modelling at CNEA

Abstract Computer codes and theoretical developments aiming to model nuclear reactor fuel elements at CNEA are reviewed. The codes PELT, VAINA and BACO for the overall fuel behaviour, as well as the finite element systems ELASTEF, PLASTEF and CTR are described. The influence on the BACO code predictions of including a fuel cracking model is discussed. Also, some examples of the calculated fuel cladding contact pressure are shown for different situations. Applications of the finite element systems to calculate the stress concentration at the skids of the Central Nuclear Atucha fuel rods and to predict local thermal effects of PuO 2 particles in a UO 2 fuel are discussed.

Application of conformal mapping to the determination of unsteady temperature distribution in thermally orthotropic plates

Abstract Very few papers and reports are available on the solution of heat conduction problems in anisotropic solids. This situation is of particular interest in some nuclear engineering applications (for instance, fuel elements possess, in general, anisotropic characteristics). The present study deals with the solution of an unsteady heat conduction problem in domains of complicated boundary shape, considering a particular case of anisotropy: a thermally orthotropic material. It is shown that the conformal mapping technique coupled with the Ritz method leads to a unified solution of the title problem for an arbitrary orientation for the axes of orthotropy with respect to the directions of the sides of the plates.

Plastef: A code for the numerical simulation of thermoelastoplastic behaviour of materials using the finite element method

Abstract A general code for solving two-dimensional thermo-elastoplastic problems in geometries of arbitrary shape using the finite element method, is presented. The initial stress incremental procedure was adopted, for given histories of load and temperature. Some classical applications are included.

Thermal stresses at nozzles of nuclear steel containments under LOCA-conditions

Abstract During a loss of coolant accident (LOCA) of a PWR-nuclear power plant, a considerable heating of the containment atmosphere is expected to occur. Transient thermal stresses will appear at the containment as a consequence of a non-uniform rise of its temperature. Applying computer codes based on the finite element method, dimensionless general thermal stresses at nozzles of spherical steel containment have been calculated, varying the principal geometrical parameters and the Biot number for the containment internal surface. Atmosphere temperature and Biot number are assumed constant after the accident. Several plots of the maximum principal stresses are provided, which constitute general results applicable to stress analysis of any particular containment of this kind.

Heat transfer analysis in internally-cooled fuel elements by means of a conformal mapping approach

Abstract The present paper deals with an approximate solution of the steady-state heat conduction problem in internally cooled fuel elements of fast breeder reactors. Explicit expressions for the dimensionless temperature distribution in terms of the governing physical and geometrical parameters are determined by means of a coupled conformal mapping-variational approach. The results obtained are found to be in very good agreement with those calculated by means of a finite element code.

Finite element analysis of local overheating within plutonium enriched UO2 fuel rods caused by PuO2 islands

Abstract Within natural UO 2 fuel elements enriched with plutonium, this last material should form PuO 2 solid solutions inside the UO 2 pellets, in a wide range of concentrations. If the solutions are obtained by mechanical mixing of the oxides, PuO 2 islands are formed in the UO 2 matrix. These islands may be the source of several problems in the fuel behaviour, the most important being the overheating of the matrix in the neighbourhood of the particles. It is caused by the large fission cross section of plutonium compared with that of uranium. A detailed study of the thermal effects produced by PuO 2 particles in the UO 2 matrix and the cladding is then important for the specification of their permissible size. A portion of the fuel rods with spherical particles in the most significant places was studied. In order to obtain the dimensionless overheating of the fuel and cladding produced by the presence of those particles, the spacial distribution of temperature was calculated, solving the stationary and linear bidimensional equation of heat conducting using a finite element code. Several geometrical variables and material properties have been taken as dimensionless parameters. A satisfactory convergence of the numerical results to an asymptotic limit with a wellknown exact solution, has been obtained.

Numerical experiments on the solution of the Helmholtz equation in the case of domains of complicated boundary shape

Abstract Domains of complicated boundary shape are of great practical importance in several fields of technology and applied science; e.g. solid propellant rocket grains, electromagnetic and acoustic waveguides, and certain elements used in nuclear engineering. The technical literature contains very few comparative studies of analytical and numerical solutions when dealing with such rather complex geometries. The present study constitutes an effort in that direction.