A series of experiments on low enriched HTR fuel was started in 1970 at Cadarache on the MARIUS reactor at room temperature and on the CESAR reactor at tempera­ture up to 450°C. The first MARIUS experiments have been performed on a configura­tion with a lattice of hexagonal cells of 61 mm diameter with a 90-mm triangular pitch. The programme covered buckling and spectrum measurements at 3.5% enrichment, measurements with the PCTR method at 1.1% enrichment and measurements on teledial fuel/32"35’ The agreement with calculations performed with the APOLLO code was very satisfactory with differences in reactivity of the order of 0.1—0.3%/36’ Oscillations of Pu samples in the CESAR reactor have been performed in 1973 at room temperature, 200°C and 400°C.<37’38’ A new series of experiments on fuel blocks of General Atomic design with Th fuel cycle is planned in MARIUS for 1975/391