Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Finite element modelling of multilayer Advanced Gas-cooled Reactor bricks and creep interaction M. Fahad a, ⁎ , K. McNally b , T. Yates b , N. Warren b , B.J. Marsden a , P.M. Mummery a , G.N. Hall a a NGRG, School of MACE, University of Manchester, Manchester, M13 9PL, UK b Health and Safety Laboratory, Harpur Hill, Buxton, Derbyshire SK17 9JN, UK ARTICLE INFO Keywords: Finite element modelling of AGR bricks Creep interaction Ovalisation ABSTRACT The structural integrity of nuclear graphite bricks is important in Advanced Gas-cooled Reactors (AGRs) as they not only provide moderation but channels for fuel, cooling and control rods. AGR graphite moderator bricks are subjected to fast neutron irradiation and radiolytic oxidation during reactor operation, leading to component dimensional and material properties changes. With irradiation ageing the graphite components deform in the axial and radial directions causing changes to bore diameter and brick length. Furthermore, deformation at the brick end faces (called dishing) can lead to the formation of axial gaps between the bricks which can then lead to fuel channel bowing. However, it is believed that irradiation creep reduces the potential size of these axial gaps and retains bricks in contact to some extent. Therefore, as the reactors age it is important to understand the nature of this contact behaviour between bricks in the fuel channels. This paper focuses primarily on the finite element modelling of contact behaviour between bricks and the effects of irradiation-induced creep on contact conditions and hence, ovality of the brick bore. Multilayer fuel brick models and a brick with a rigid body on the top surface have been modelled. The results show that multilayer models are required to understand the contact conditions between the bricks throughout the life of a reactor. 1. Introduction The core of a typical graphite moderated reactor is constructed from a large number of nuclear graphite bricks (Haiyan et al., 2004). These bricks act as a moderator and form channels for fuel, control rods, and coolant (Apu et al., 2016; Berre et al., 2008). It is important that the graphite bricks maintain their dimensional stability during the lifetime of the reactor in order to maintain fuel cooling and control rod entry. Nuclear graphite bricks which form fuel channels in graphite moder- ated reactors exhibit non-uniform properties; material and dimensional changes which are mainly a function of the fast neutron irradiation and temperature (Muhammad et al., 2013) and are further altered by radiolytic oxidation (Brocklehurst et al., 1990). Dimensional changes in AGR moderator bricks differ in the axial and radial directions, due to the semi-anisotropic nature of Gilsocarbon graphite from which the bricks are manufactured. The radial de- formation within the brick is non-uniform along the height of the brick and hence produces bowing, end-barrelling and tilting of bricks (the latter being caused by brick bowing) (Wang and Yu, 2008; McLachlan et al., 1995). The radial deformation is a function of height within a brick as well as height within the core (Neighbour, 2010). The axial deformation may affect the brick end faces (at the top face this is called dishing) (Aiden and Reed, 2007) and can lead to axial gaps between bricks which have the potential to cause the fuel channels to bow and kink. Furthermore, axial deformation may lead to coolant bypass (Haiyan et al., 2004). The general dimensional change behaviour described above is common to the moderator bricks in all graphite moderated reactors. This paper considers a typical generic AGR design. In this reactor design the moderator bricks have ovalised with increasing irradiation due to the presence of a brick-end feature known as the rocking flat in the moderator fuel brick design, which influences the contact region be- tween the bricks in the column (Aiden and Reed, 2007). Observations and modelling have shown that the major semi-axis of ovality at mid- height of the brick is perpendicular to the major axis of the rocking feature whereas at the brick-ends it is aligned with the rocking features (Arregui-Mena et al., 2014). The ovalisation is a second-order de- formation compared with the larger radial shrinkage of the bricks: the observed difference in cross brick diameters (ovality) is of the order of 1–2 mm compared with radial shrinkage of approximately 8 mm. Under operating conditions dimensional changes vary between brick-to-brick within a fuel channel and this may produce complex http://dx.doi.org/10.1016/j.nucengdes.2017.09.014 Received 16 March 2017; Received in revised form 22 August 2017; Accepted 19 September 2017 ⁎ Corresponding author at: Room C17, Pariser Building, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester M13 9PL, UK. E-mail address: muhammad.fahad@manchester.ac.uk (M. Fahad). Nuclear Engineering and Design 324 (2017) 390–401 0029-5493/ © 2017 Elsevier B.V. All rights reserved. MARK