4 - Computational modeling of in-reactor deformation in CANDU fuel channels

Abstract

Fuel channels (FCs) are one of the most critical components in a CANDU reactor. A FC comprises a pressure tube (PT), a calandria tube (CT), and four garter springs and it acts as the pressure boundary between the “hot” heavy water reactor coolant and the “cold” moderator. The structural integrity of FCs is affected by in-reactor deformation caused by irradiation-induced creep, irradiation growth and thermal creep. Excessive FC deformation may result into a PT contacting the CT, which in turn can lead to the development of hydride blisters and ultimately to delayed hydride cracking of PTs, therefore compromising PT integrity. To ensure a reliable operation and to predict the future dimensional changes of the FC, a comprehensive understanding of the nature of in-reactor deformation and a physically based model are necessary. Semiempirical constitutive equations have been developed to predict the dimensional changes occurring in CANDU FCs, namely, the observed axial elongation, wall thinning, and diametral expansion of PTs, and the sag of CTs as a function of operating variables. However, these models have not been developed to directly predict the sag of PT, the temporal evolution of PT−CT gap, and the time at which contact of the PT with the CT occurs. The parameters describing PT sag, PT−CT gap, and the time to contact are essential for maintaining FC integrity and for developing a successful life cycle management program for FCs. These parameters can be predicted mainly using a computational tool such as a finite element model (FEM). This chapter discusses the three-dimensional (3D) modeling that has been developed to simulate the in-reactor deformation of a CANDU FC using the finite element analysis (FEA) package ABAQUS, in which the constitutive models describing the deformation of both the PT and the CT have been implemented as user subroutines using UMAT. The material presented next is based on: