Effect of stochastic deformation on the vibration characteristics of a tube bundle in axial flow

Abstract

Design evaluation of nuclear components using numerical methods has typically been focused on nominal geometries and conditions. In reality, the geometry of reactor components and operating conditions can differ from the design on the drawing table. Increasing understanding and safety of nuclear energy systems requires investigating these more realistic conditions as well. An example is the bow of fuel assemblies, a deformation that occurs after some residence time as a consequence of thermal and irradiation effects. A paradigm shift from purely deterministic simulations toward simulations involving stochastic processes can help to meet this demand.One of the aspects in the design assessment of nuclear reactors is flow-induced vibration (FIV). A lot of experimental effort has already been allocated to FIV phenomena in the past, and more recently also numerical approaches have been applied. However, it was not clear to what extent vibration characteristics would change for a geometry different from the design drawing. In this study the effect of a random bow deformation on the vibration behavior of a flexible tube in a rigid bundle is investigated using fluid–structure interaction simulations. As the (stressless) geometry presumably has a stochastic nature, the modal characteristics should be treated accordingly.For this uncertainty quantification of the modal characteristics, the generalized Polynomial Chaos method was applied. First, only the steady flow through the bundle was considered, without simulating the vibration. This sufficiently reduced the computational cost to apply the Monte Carlo method, which was used to validate the more efficient Polynomial Chaos method. This latter method was found adequate to predict the stochastic modal characteristics of the deformed, flexible tube.