High temperature gas-cooled reactor core

A high temperature gas-cooled reactor core comprising a plurality of elongate prismatic fuel elements, each fuel element having a hexagonal cross-section in a plane parallel to bases of each respective prismatic fuel element, the plurality of fuel elements being formed in a contiguous arrangement of lobes to define an annulus of fuel elements which share a common contiguous border with one another, wherein each lobe is defined by multiple fuel elements and each lobe forms a projection which projects radially beyond an adjacent recessed region of the contiguous border, with one recessed region separating adjacent projections, and wherein each prismatic fuel element comprises an elongate prismatic body and a plurality of elongate fuel channels located within each prismatic body together with a plurality of cooling channels interspersed among the fuel channels, wherein a cross-sectional area of each prismatic fuel element in the plane parallel to the bases of the respective prismatic fuel element is no more than 800 cm2, and wherein a ratio of a height of each prismatic body to its greatest across-the-flats width is greater than or equal to 3.0, and wherein the annulus of fuel elements surrounds a central trunk of moderator material comprising a plurality of columns of moderator material.