Showing papers by "J. J. Grudzinski published in 2014"

Fuel Assembly Bowing and Core Restraint Design in Fast Reactors

Abstract: The reactivity of a fast spectrum nuclear reactor core is sensitive to changes in the fuel position. The core is formed by a hexagonal array of fuel assemblies which contain the fuel elements. The main structural components of the assemblies are thinwall hexagonal ducts. The fuel elements represent negligible stiffness in the fuel assembly compared to the ducts such that the ducts determine the location of the fuel. Thermal gradients across the fuel assembly cross sections create differential thermal expansion which causes the assemblies to bow. This bowing is proportional to the power to flow ratio such that it can become an important part of the reactivity change during reactor transients such as during reactor start-up, transient overpower (TOP), and unprotected loss of flow without scram (ULOF). In addition to these short term transients, thermal and fast neutron flux gradients within the core cause the assembly ducts to swell and bow over time due to irradiation creep and swelling. These latter effects produce permanent bowing of the ducts which change the reactivity over time and more importantly affect the mechanical forces required to refuel the core as the bowing is greater that the duct-to-duct clearance. Understanding these bowing responses is important to the understanding of both the transient behavior of a fast reactor as well as the refueling loads. Through proper design of the core restraint system, the bowing response can be engineered to provide negative feedback during the above mentioned transients such that it becomes part of the inherent safety of a fast reactor. Similarly, the opposing effects of creep and swelling can be manipulated to reduce the permanent core bowing deformations. We provide a discussion of the key features of analyzing and designing a core restraint system and provide a brief survey of the past work.Copyright © 2014 by ASME

Evaluation of Mechanical Properties at the Knit Line Interface in a Complex Multi-Cell PVC Extrusion

Abstract: In order to form internal cells in PVC extrusions, the die requires an insert or internal member that the material flows past. These inserts are supported to the outer die structure by so-called spiders that pass through the extrusion wall. The extruded material must separate and the recombine as it passes over the spider. The material must then form a bond at this interface shortly before exiting the die. These interfaces are referred to as knit-lines. In a recent project involving a large and complex PVC extrusion, difficulty was encountered in developing these knit lines within the webs of the extrusions. Upon visual inspection, these interfaces appeared to be without bond over portions of the cross section. However, mechanical testing in the worst knits revealed that bonding had occurred with the knit providing 85% or the bulk material strength although without support any significant ductility. At the same time, knits at different parts of the extrusions showed ductility comparable with the base material. Altering the process variables showed a means for improvement in the webs but this was limited by other constraints. While these knits were sufficiently strong for short duration loading, the real question was the time dependent strength such as creep rupture over long times and how the behavior of the knit compared with the bulk material. In this work we describe the character of the knitlines and the resulting mechanical properties. We then discuss the preliminary results of accelerated creep rupture tests which indicate that the creep rupture behavior of the partially interfaces follow a time temperature superposition relationship allowing accelerated prediction of the knit-line rupture. We propose a model for

A Novel Method for the Implementation of Contact Including Energy Dissipation in Finite Element Methods

Abstract: A new method for implementing contact/impact in an implicit finite element formulation has recently been developed. The method uses the idea of buoyancy and fluid drag forces to enforce the contact constraint as well as provide a method of dissipating energy due to the contact. Additionally, the method provides a simplified means for accounting for contact friction. The method has potential usefulness in a certain class of problems such as contact with bodies having soft and viscous foundations as well as problems with sliding. In this work, we introduce the method and illustrate it with an example that shows the benefits and utility of the method.Copyright © 2014 by ASME