Tetravalent uranium readily undergoes hydrolysis even in highly acidic aqueous solutions. In the present work, solutions ranging from 0.4 to 19？mM (total U) concentration ( ) are carefully investigated by light scattering technique with special emphasis on polymerization leading to colloid formation. The results clearly indicate that the concentration has significant effect on particle size as well as stability of colloids. With increasing concentration the size of colloids formed is smaller due to more crystalline nature of the colloids. Stability of colloids formed at lower concentration is greater than that of colloids formed at higher concentration. Weight average molecular weight of the freshly prepared and colloidal polymers aged for 3 days is determined from the Debye plot. It increases from 1,800 to 13,000？Da. 40–50 atoms of U are considered to be present in the polymer. Positive value of second virial coefficient shows that solute-solvent interaction is high leading to stable suspension. The results of this work are a clear indication that U(IV) hydrolysis does not differ from hydrolysis of Pu(IV). 1. Introduction Due to high electric charge, tetravalent actinides have an inordinate tendency to undergo hydrolysis leading to formation of polynuclear species of colloidal dimensions even under very acidic conditions [1–4]. Such processes are observed for tetravalent ions such as Th(IV), Pu(IV), U(IV), Np(IV), and Pa(IV), and to a lesser extent for the hexavalent actinyl ions U(VI) and Pu(VI) [5–7]. The initial step in hydrolysis is the formation of mononuclear species. But further hydrolysis may lead to a variety of polynuclear species [8, 9]. Consider, for example, above millimolar concentration and close to the solubility limit, Pu(IV) form polynuclear species [10]. The consequences of formation of polynuclear species include excessive foaming in evaporation operation, clog in transfer lines, interference in ion exchange operations, and emulsification in solvent extraction operations, and mainly it can lead to criticality hazard due to increase in local concentration of Pu [11]. Colloids can also facilitate the transport of actinide elements in the environment. The transport of plutonium from repository to surrounding may increase when the stable colloidal Pu(IV) formed in ground water comes in contact with spent nuclear fuel [12]. Rate of mobility depends on the size of colloids. For instance, colloids of smaller size less than 50？nm have high mobility and surface to volume ratio [13, 14]. Similarly the nature of colloidal polymers also plays a
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