On the modelling and simulation of a nuclear graphite brick: effects of cracks and interactions

The graphite core of an Advanced Gas-cooled Reactor (AGR) consists of a complex assembly of a large number of graphite components, such as fuel bricks, interstitial bricks and keys. Fast neutron damage to the graphite causes significant dimensional and material properties changes. Furthermore, in a component, such as a fuel brick, there are significant thermal gradients. The dimensional and property changes which vary across a typical fuel brick as a result of the fluence profiles coupled with the thermal gradients lead to the generation of internal stresses. As the reactor ages, the stresses particularly at keyway corners may lead to the development of cracks in the fuel bricks. In this paper, the behaviour of a cracked brick is investigated and compared to the behaviour of an intact brick. Moreover, the effects of interactions of a cracked or an intact fuel brick with its surrounding components are evaluated. Various numerical models are presented to provide useful information for understanding the behaviour of cracked bricks. These models also give an insight into the effects of the interactions between a fuel brick and its neighbouring components, which have significant implications on the levels of stresses sustained by a graphite brick.