The effects of postulated accidents, including dynamic effects of pipe ruptures, must be analyzed for licensing of nuclear power plants (NPPs). Applicants and licensees of NPPs have struggled to address U.S. Nuclear Regulatory Commission (NRC) expectations to assess if high energy line break (HELB) jet impingement on structures and components can lead to dynamic amplification, and to accurately simulate blast wave-induced loadings. In this paper, evaluation of the potential for load amplification and occurrence of resonance conclusively demonstrates that the phenomenon does not occur. In a HELB, several physical parameters of jets issuing from a ruptured pipe - such as non-equilibrium condensation of steam, unsteady separation between the jet exit and target, non-orthogonal alignment of jet axis to impingement surface, uneven impingement surfaces, or mismatch of jet excitation frequency and target natural frequency - prevent occurrence of the phase lock conditions needed to initiate and maintain a resonance. The analytical approach to evaluate the blast wave-induced loading applied a pressure vessel burst (PVB) correlation instead of performing time consuming CFD analysis for all break locations. Three dimensional CFD analysis of blast wave transient propagation provided the basis to develop benchmarking factors for use with the PVB correlation. The simplified methodology utilizes shockwave reflection, shape, and environment factors for application to impacted targets, which significantly reduces the amount of time to evaluate all break locations. The modified PVB method is also more appropriate than an explosion-type correlation to model the blast wave pressures from steam pipe breaks.
Unexpected steam generator tube wear has recently been discovered during in-service inspection of newly installed PWR replacement steam generators. The extent of this unexpected tube wear suggests that a new wear mechanism is emerging that is not described by current tube wear models. A key to understanding the emerging unexpected tube wear may be to better understand the effectiveness of the mechanical design for tube support (flat bars with tight clearances) with respect to preventing flow induced tube vibration, and the characteristics of hydraulic forces exerted on tubes. This paper describes the results of a non-linear, dynamic, finite element analysis of steam generator tube in-plane vibration with typical flat bar mechanical supports. Mechanical support conditions such as initial gaps between tube and supports, contact forces between the tube and the supporting flat bars, and the effect of various forces applied to the tubes by fluid flow are investigated. The analysis describes the effect of these conditions on tube in-plane vibration frequency and damping for various tube forcing functions. Computational fluid dynamics calculations that determine the unsteady forces in two-phase flow test geometries are presented. A model describing the emerging unexpected tube wear is also introduced.
The absence of a long-term solution for the storage of spent nuclear fuel prompts utilities in the United States to establish on-site storage for used fuel. The challenges associated with placement of spent fuel in dry cask storage on the power plant’s Independent Spent Fuel Storage Installations (ISFSI’s) include aging management of the stainless steel canisters and monitoring for the possible onset of stress corrosion cracking (SCC). The San Onofre Nuclear Generating Station (SONGS) has initiated a test program to examine the effects of heat generation variations inside a test canister using an electric heater rather than spent fuel on the shell temperatures. The test helps in the evaluation of external environmental factors and shell temperature, and to monitor for SCC. This paper presents the computational fluid dynamics (CFD) modeling developed in support of the test to analyze the air natural circulation in the subgrade enclosure and within the test canister with the electrical heating. The thermal analysis is performed using ANSYS CFX and integrally simulates the flow behavior and heat transfer mechanisms both inside and outside the test canister. Comparison of results from different heat loads that represent the decay heat time-profile, sensitivity to the turbulence model, and modes of heat dissipation are discussed. The CFD results are also compared to in-situ temperature measurements to validate the analysis.
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