Dimensional and microstructural analysis of coextruded UO2-stainless steel cermet pins

Coextrusion has been used to prepare UO2 (35 vol.-%)-stainless steel cermet fuel-pins (5·6 mm dia. × 1000 mm long), which could be used as fuel elements in fast-flux test reactors. The procedure required to obtain a nearly homogeneous distributionof the fissile phase in the matrix is described. The influence of billet design on the dimensions of the fuel core and cladding is analysed with a view to its optimization. The extent of particle deformation as a function of extrusion parameters has been measured quantitatively by means of image analysis on particles having different (12·7%; 5·8%) inner porosities. Thermal expansion and conductivity as well as tensile strength of the cermets are reported and compared with existing data.