Haematophagous biting midges of the genus Culicoides are pests of humans, livestock, and wildlife, and some also serve as vectors of bluetongue virus (BTV) and epizootic hemorrhagic disease virus (EHDV) worldwide. In North America, there are only two Culicoides spp. proven to transmit BTV and/or EHDV-Culicoides (Hoffmania) insignis Lutz (Diptera: Ceratopogonidae) and Culicoides (Monoculicoides) sonorensis Wirth and Jones (Diptera: Ceratopogonidae). Culicoides sonorensis is considered the primary vector due to its wide distribution across much of North America, whereas C. insignis has a neotropical distribution historically limited to peninsular Florida. However, Culicoides surveys conducted 2007 to 2015 have detected the presence of C. insignis in five southeastern states (Florida, Georgia, Alabama, Mississippi, and Louisiana), suggesting C. insignis has or is currently experiencing a northwestward range expansion in the southeastern United States. Because C. insignis has a neotropical distribution and is a known vector of BTV serotypes exotic to North America, an expanding range could pose an introduction risk of virus serotypes new to the region and/or increased transmission of circulating endemic serotypes.
Two dimensional (2-D) axisymmetric finite element models (FEMs) are often used as a simplification to modeling cylindrical nozzles that intersect a cylindrical pressure vessel. However, an axisymmetric model has the effect of representing the vessel as a spherical shell rather than a cylindrical shell. Previous work has been done to determine 2-D axisymmetric to three dimensional (3-D) stress correction factors (CFs) for the total stress at the nozzle blend radius to account for this inconsistency. The present paper expands on that work to investigate the effects of the 2-D axisymmetric modeling simplification on the through wall stress distribution at the nozzle corner. The through-wall stress distribution is necessary for some fracture mechanics analyses performed for corner cracked nozzles and for using the simplified elastic-plastic analysis given in NB-3228.5. A simplified method is proposed which can be used to obtain a nozzle specific correction factor, rather than a bounding correction factor, that can be applied to 2-D finite element analysis stress results to correct for the inaccuracy introduced by modeling the intersection as an axisymmetric section.
Two dimensional (2-D) axisymmetric finite element models (FEMs) are often used as a simplified means of modeling cylindrical nozzles that intersect cylindrical pressure vessels. An axisymmetric model represents the vessel as a spherical shell rather than a cylindrical shell. Therefore, analysts must correct the stresses predicted by the 2-D model to account for the three dimensional (3-D) effects caused by the true nozzle geometry, which are not represented by the 2-D axisymmetric simplification. This paper presents total stress correction factors for the nozzle blend radius region of Boiling Water Reactor (BWR) Feedwater, Core Spray and Recirculation Inlet nozzle geometries. The correction factors are defined by taking the ratio of the total hoop stress from the 3-D nozzle FEM to the total hoop stress obtained from the 2-D nozzle FEM. Eighteen (18) separate nozzle designs were evaluated. These cases are considered to span the range of dimensions expected for General Electric type BWR-2 through BWR-6 Feedwater, Core Spray, and Recirculation Inlet nozzles.
As part of the license basis of a nuclear boiling water reactor pressure vessel, a sudden loss of coolant accident (LOCA) event needs to be analyzed. One of the loads that results from this event is a sudden depressurization of the recirculation line. This leads to an acoustic wave that propagates through the reactor coolant and impacts several structures inside the reactor pressure vessel (RPV). The authors have previously published a PVP paper (PVP2015-45769) which provides a survey of LOCA acoustic loads on boiling water reactor core shrouds. Acoustic loads are required for structural evaluation of core shrouds; therefore, a defensible load is required. The previous research compiled plant-specific data that was available at the time. Since then, additional data has become available which will add to the robustness of the bounding load methodology that was developed. Investigations are also made regarding the shroud support to RPV weld, which was neglected from the previous study. This will allow a practitioner a convenient method to calculate bounding acoustic loads on all shroud and shroud support welds in the absence of a plant-specific analysis.
Cracking in boiling water reactor (BWR) core shroud welds has been identified in operating nuclear plants worldwide. The Boiling Water Reactor Vessel and Internals Project (BWRVIP) has published several reports providing inspection and evaluation (I&E) guidance for intergranular stress corrosion cracking (IGSCC) in the core shroud of BWRs. This guidance is predominately focused on evaluating crack stability. Calculating through-wall leakage was not previously a focus of the existing BWRVIP I&E guidelines for the core shroud; however, there is some guidance in the current documentation. In recent years there has been some evidence of through-wall indications in the core shroud where the through-wall indications were aligned in an array of parallel, short, flaws. BWRVIP-158-A contains rules for treating parallel flaws with respect to calculation of structural margin for both net section collapse (limit load) and brittle fracture (linear elastic fracture mechanics (LEFM)) failure modes. There is currently no BWRVIP document or other open literature, to the authors’ knowledge, that provides insight into whether the crack opening displacements (CODs) for an array of parallel through-wall cracks are larger than that calculated for a single through-wall crack. Developing an understanding of the effect of parallel cracks on the CODs and subsequent crack opening areas (COAs) of each crack is important in augmenting the existing guidance on how to appropriately disposition through-wall cracking in reactor internal components. Specifically, it is important to know if multiple parallel cracks can lead to individual COAs that are larger than for a single crack of the same length, in order to perform accurate leakage rate calculations. This paper documents linear-elastic finite element analyses (FEA) performed to study the effect of a parallel crack configuration on the resulting COA for the set of cracks compared to the COA calculated if each crack was treated as an individual crack, without adjacent cracking present. Various separation distances, number of crack cases and crack lengths are considered. While the object of this work is to provide criteria for the evaluation of reactor internals, the results can be applied to evaluate COD and COA in any component for which the cracking configuration and inherent assumptions of LEFM are applicable.
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