Development of a numerical methodology to simulate the roller expansion forming process

A distinguishing design feature of CANDU nuclear reactors is the use of horizontal fuel channels housed in a horizontal vessel called a calandria, which is made of stainless steel 304L. Each channel consists of a Zr-2.5%Nb alloy pressure tube and an externally concentric Zr-2 calandria tube. The calandria tubes are joined to the end plates (tubesheets) of the calandria vessel by joints formed by roller expansion. The bores in the tubesheets are grooved. Roller expanded joints provide a cost effective means of joining dissimilar materials, require minimal space and no maintenance. The quality of these roller expanded joints is important from a sealing, strength and stress corrosion point of view. The roller expansion process consists of expanding the calandria tubes to deform them plastically against the bores and into the grooves of the tubesheets. Therefore, understanding the effect of the number, geometry and the pitch of the grooves on the quality of a roller expanded joint is very important. The objective of this paper is to present a numerical methodology developed to optimize the design of such roller expanded joints. The numerical techniques required to simulate the behaviour of calandria tubes during roller expansion involve the time dependent evaluation of large deformations, large plastic strains and stresses and rapidly changing contacts. The LS-DYNA general-purpose finite element computer code was selected for such simulations. A three-dimensional finite element model was developed so that the sweeping of the rollers while under outward radial movement is properly modelled and, the plastic deformation and flow of the calandria tube material into the tubesheet grooves is accurately simulated. The paper discusses this numerical methodology, presents results of general interest, and makes comparisons with associated experimental data and other available results. (author)