1.of technical uranium and certain alloys of interest f o r use in naturaluranium fuelled reactors are described, .technical purity uranium a r e first discussed and the time-temperaturetransformation data resented.Results on binary alloys comprising U/&, U/Cr, Uhe, Uho, UA, U/Ti, U/V and U/Zr, contahing up t o about 2 atomic per cent of the a l l o y i n g element, a r e described. C r , Fe and Z r were found t o be the most d f e c t i v e grain refining agents.Al and V were satisfactory under some conditions whereas Mo, N% and T i were ineffective. 3.curves indicated that grain refinement i s associated w i t h depression of the temperature of transformation on heat treatment, the extent of' this depression for a given cooling-rate being dependent on the alloy content.i t was found that alloys such a s U/zr and u b e responded t o water quenching thermal treatment a t 500 to 6OO0C.Metallographic and dilatometric studies of the heat treatment behaviourThe micro-structure and secondary phases present i n high purity and