The heat transfer through the prismatic fuel blocks in a Very High Temperature Reactor (VHTR) is of crucial importance for the proper operation of the reactor under normal operating conditions and upset conditions. It is therefore vital that the heat transfer should be modelled properly for the design and analysis of the reactor and the proper understanding of the performance of the reactor and the associated thermalflow systems. The heat transfer and fluid flow can be simulated using a detailed CFD model. This requires extensive computational resources and is typically restricted to parts of a block or a single block and precludes the simulation of the entire reactor and the associated thermal-flow systems. An approach is therefore required that can model the heat transfer and fluid flow in a prismatic block in a representative manner while allowing the simulation of the entire reactor and the associated thermal-flow systems. This paper discusses one-dimensional (1D) network models of three levels of complexity that simulate the heat transfer and fluid flow in a prismatic fuel block and a prismatic fuel block assembly in a representative manner. The models consist of a collection of one-dimensional solid conduction heat transfer, convection heat transfer and pipe elements that are arranged in such a manner to represent the heat transfer and fluid flow in the prismatic block using a network approach. The validity of the prismatic fuel block models was investigated by comparing the temperature distribution in the block with the corresponding values obtained using a detail 3D CFD model of a prismatic block. Similarly, the validity of the prismatic fuel block assembly models was evaluated by comparing the axial profile for the maximum fuel temperature with the corresponding values obtained using a coupled 3D unit cell heat transfer and 1D fluid flow model. In all cases the corresponding temperatures were found to be in good agreement. The least complex 1D network model was also used to study the transient thermal behaviour of the prismatic block fuel assembly during a forced shut down, whilst maintaining the coolant flow.