Evaluation of missing pellet surface geometry on cladding stress distribution and magnitude
Abstract Missing pellet surface (MPS) defects are local geometric defects in nuclear fuel pellets that result from pellet mishandling or manufacturing. The presence of MPS defects can cause significant clad stress concentrations that can lead to through-wall cladding failure for certain combinations of fuel burnup, and reactor power level or power change. Consequently, the impact of MPS defects has limited the rate of power increase, or ramp rate, in both pressurized and boiling water reactors (PWRs and BWRs, respectively). Improved three-dimensional (3-D) fuel performance models of MPS defect geometry can provide better understanding of the probability for pellet clad mechanical interaction (PCMI), and correspondingly the available margin against cladding failure by stress corrosion cracking (SCC). The Consortium of Advanced Simulations of Light Water Reactors (CASL) has been developing the Bison-CASL fuel performance code to consider the inherently multi-physics and multi-dimensional mechanisms that control fuel behavior, including cladding stress concentrations resulting from MPS defects. This paper evaluates the cladding hoop stress distributions as a function of MPS defect geometry with discrete pellet radial cracks for a set of typical operating conditions in a PWR fuel rod. The results provide a first step toward a probabilistic approach to assess cladding failure during power maneuvers. This analysis provides insight into how varying pellet defect geometries affect the distribution of the cladding stress, as well as the temperature distributions within the fuel and clad; and are used to develop stress concentration factors for comparing 2-D and 3-D models.
A new phenomenological criterion for pellet–cladding interaction rupture
Abstract This study is concerned with structural integrity assessment of Pressure Water Reactor's (PWR) fuel rods under pellet–cladding interaction (PCI) loading condition. An important experimental and research cooperative program between EDF, AREVA-NP and the Atomic Energy Commission CEA is achieved in order to get a better understanding of the mechanisms possibly leading to PCI failure, as well as to qualify a PCI resistant rod design. The objectives of this work are: first, to improve the understanding of the pellet mechanical properties impact on cladding local loading with 3D simulations results, and second, to propose a new phenomenological rupture criterion for a better assessment of the failure risk. In this study fuel behaviour modelling under nominal and transient loading conditions is achieved with a multi-dimensional simulation tool called ALCYONE, included in the new fuel software PLEIADES currently co-developed by the CEA and EDF. Cladding loading due to mechanical interaction during power transient stage is first analysed through pellet–cladding interfacial stresses computed in the 3D simulation. Then, a 2D model is proposed in order to establish a correlation between interfacial loading and stress concentration in the cladding. In order to assess the failure risk under PCI a phenomenological criterion based on the membrane circumferential stress in the cladding and shear stresses at pellet–cladding interface is proposed. To compute the shear loading at pellet–cladding interface a new parameter (called W rθ ) is introduced. Based on 3D calculations of PCI, it is shown in this paper that pellet fracture properties can have a significant effect on PCI loading.
Mechanical interaction between fuel and cladding
Abstract Because UO 2 expands more than the zircaloy cladding tube, is hotter, cracks and swells, the UO 2 will interact with the cladding. How much, is dependent upon the gap, length of pellet, whether it has flat ends or not, the depth of the dish, the width of the shoulder, heat load, rate of power increase and cladding properties. The stress-strain pattern can be very complicated. It usually has a changing multiaxiality with consequent varying strains to fracture, and the strains may be concentrated over small lengths of the cladding tube. All this adds up to a threat to the integrity of the zircaloy, particularly since the properties of this material are so adversely affected by irradiation. To avoid difficulties and failure, the pellet shape, dimensions and density and cladding properties should be chosen with due regard to reactor system conditions, the way the reactor is loaded and operated. The ideal cladding should have good ductility and high strength at the same time. Such a combination seems impossible, unfortunately. Many of the processes described we know only qualitatively. We have, therefore, a definite need for many more precisely defined and executed experiments, preferably in-pile. Precisely dimensioned, shaped and formed hardware has but little use, if flux shape along a rod and heat production cannot be determined with commensurable accuracy. So, while on this end of the fuel design spectrum we toil to sharpen out tools, may I express the hope that our colleagues on the other end of the spectrum, the physicist and the thermohydraulicist, will make equal progress.