vi is present in a high concentration for example of around 400 g l whereas the element e which is for example neptunium iv or thorium 228 which is also tetravalent is present at a much lower concentration for example of around 1 or less than that of the uranium or else as a trace element with regard to the concentration of nitric acid in the phase a 1 this is for example from 4 to 6 moles l an extraction step per se whose aim is to extract the uranium vi from the phase a 1 by means of an organic phase immiscible with water and containing an extractant capable of extracting the uranium and to a lesser degree the element e and a step for washing the organic phase with a nitric aqueous phase hereinafter denoted phase a 2 whose purpose is to strip from the organic phase the fraction of element e having been extracted with the uranium vi in the preceding step the organic phase is for example composed of tri n butyl phosphate with 30 v v in a dodecane whereas the phase a 2 is for example an aqueous solution with acidity less than 2 moles l of hno 3 for example of around.