How would you……describe the overall significance of this paper? This paper describes the analytical modeling and simulation of light water reactor fuel behavior. This technology emphasizes highly detailed large-scale threedimensional computation and introduces to the traditional engineering-scale simulation the higher-order meso-scale and atomistic-scale modeling and simulation.…describe this work to a materials science and engineering professional with no experience in your technical specialty? The modeling of nuclear fuel systems involves complex physical phenomena that operate over time and geometric scales that vary by many orders of magnitude. These phenomena involve temporally and spatially dependent interactions between several material constituents that include ceramic UO 2 fuel, metallic cladding and two-phase water coolant, and take place under continuously changing irradiation and temperature conditions. The behavior of this system is simulated in multi-dimensional computer codes that constitute analytical analogs for the nuclear reactor core as a whole.…describe this work to a layperson? This paper describes the complex behavior of a nuclear reactor fuel system, which is composed of thousands of fuel rods, assembled in precise patterns and immersed in water in a pressure vessel. A typical fuel rod is a thin-walled metal tube, that contains several hundred uranium oxide pellets which generate heat through the nuclear fission process that takes place in a precisely controlled manner. The heat is transferred to the surrounding water in the vessel and is subsequently converted to mechanical energy and ultimately to electricity generation.Light water reactor fuel is a multicomponent system required to produce thermal energy through the fission process, efficiently transfer the thermal energy to the coolant system, and provide a barrier to fission product release by maintaining structural integrity. The operating conditions within a reactor induce complex multi-physics phenomena that occur over time scales ranging from less than a microsecond to years and act over distances ranging from inter-atomic spacing to meters. These conditions impose challenging and unique modeling, simulation, and verification data requirements in order to accurately determine the state of the fuel during its lifetime in the reactor. The capabilities and limitations of the current engineering-scale one-dimensional and two-dimensional fuel performance codes is discussed and the challenges of employing higher level fidelity atomistic modeling techniques such as molecular dynamics and phase-field simulations is presented.

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Lifeline safety structures, such as, for example, concrete dams, nuclear power plants, and highway bridges, are designed to high levels of safety using traditionally conservative methods. Events of recent years, however, have raised public concerns about the degree of vulnerability of these structures to deliberate attacks involving large-airplane crash or close-proximity blast loading. This paper presents recent development in the state of the art of finite-element-based constitutive modeling and computational methodology of reinforced concrete with emphasis on severe damage modeling and failure evaluation. Verification and validation of the developed methodology is illustrated using high-velocity impact tests conducted in the U.S. and Japan. This involves explicit finite element computations for high velocity rigid missiles impacting reinforced concrete walls. Application of the methodology to nuclear fuel facilities is discussed.

The numerous indications recently found by UT-inspection in the reactor pressure vessel shell forgings at two Belgian nuclear power plants have raised some concerns about the effects of such indications on the vessel integrity and fitness for continued service. The UT indications have been attributed to hydrogen flaking, and preliminary estimates give a density of ∼40 indications per liter, with diameter of about 10–14 mm, oriented at a shallow ∼10° angle to the vessel inner surface. This type of high-density indications would not be characterized as geometric flaws with well defined crack-tip geometry that permits high-fidelity application of traditional fracture mechanics methods. An alternative analysis approach, with higher fidelity simulation of this type of “distributed discontinuities”, is proposed, as described in this paper. From a behavioral standpoint, the UT indications at Doel 3 and Tihange 2 represent material discontinuities whose mechanical effect can be evaluated using a damage-mechanics-based constitutive model. Previously, a special multiphase damage model was developed for cladding with zirconium hydrides, of similar morphology to the Doel 3 indications, in which the metal matrix and the hydride platelets are treated as separate material phases interacting at their interfaces with appropriate constraint conditions between them to ensure strain and stress compatibility. The hydride precipitates are represented as a brittle material and the metal matrix is modeled as a ductile elastic-plastic material. This damage model was implemented in a finite element computer program, and was validated using ring-tension and ring-compression tests of cladding specimens with various hydride morphologies. The model was able to predict specimens complete stress-strain curves and failure states with very high accuracy. The above described damage model is adapted to the high-density UT indications, morphology and distribution similar to the conditions of the Doel 3 vessel. The “hydrogen flakes” are characterized in the model as distributed damage of known orientation and volume fraction. A vessel of typical geometry and radiation-dependent mechanical properties is analyzed for various values of volume fraction of hydrogen flakes, and considering a transient loading scenario that conservatively simulates pressurized thermal shock. Interlinking of the “hydrogen flakes” and propagation of damage through the wall under the specified loading condition are part of the model’s capability of directly predicting whether or not vessel failure will occur. Thus, vessel susceptibility to failure and failure margin are judged by the degree of damage propagation through the wall.