Radiolytic oxidation and reduction of plutonium

It has been shown on several occasions that all other things being equal, the rate of migration of plutonium through an aquifer is greatly influenced by its oxidation state. It is a reasonable expectation that other actinides exhibiting multiple oxidation states will behave similarly. By and large the more highly charged ions in solution move through rock strata more slowly than those of lesser charge. Thus for comparative experiments on the relative migration rates of Pu(IV) and Am(III) one would expect Pu(III) to move about 10 times faster than Pu(IV) and it has been shown that PuO22+ moves about 250 times more rapidly than Pu(IV). Since one of the more important parameters of safety assessment is radionuclide migration rate it can be seen that it becomes important to know the oxidation states of radionuclides (actinides for the purpose of this report) existing in possible leachates from the material originally emplaced in a repository. It was with this in mind that a study of the intrinsic stability of plutonium oxidation states in ground water solutions was undertaken. One of the most prominent proposals for a waste repository in WIPP (Waste Isolation Pilot Project) which intends to utilize bedded salt strata as the isolating medium. Ground water in that context is saturated brine and is made up to specifications issued by Sandia Laboratories and designated as Solution A. The net result of these series of reactions is that plutonium may undergo a cycle of oxidation and reduction. This is a composite effect since it is the sum of radiolytic oxidation and reduction and disproportionation and reproportionation reactions. The period of the cycle must depend on concentration of Pu, radiation field intensity, pH of solution, temperature, and formation of complexes. Thus the migration characteristics of all of the oxidation states of Pu (with the possible exception of Pu(VII)) will have to be considered