Importance of radiolytic decomposition of uranium hexafluoride in the course of uranium-235 enrichment under the separation nozzle method

In U-235 enrichment under the separation nozzle method gaseous uranium hexafluoride is used in a mixture with H2 or He. To predict the longterm behavior of the separation elements the question was of interest whether the radioactive radiation of uranium causes UF6 to undergo significant decomposition while forming solid uranium compounds. In the operation of an apparatus in which the ratio of uranium inventory to slit length of the separation elements was greater by several orders of magnitude than in a commercial scale separation nozzle stage, significant depositions of dusty uranium compounds in the separation element were observed already after one day. Within the limits of error, the effect was independent of the kind of auxiliary gas used. In case of helium, dust deposition could be avoided by addition of not more than 0.06 mol% of F2. Careful elimination of all other possible dust sources and an estimate of the conceivable radiation effect permit to conclude that the dust results from the radiolytic decomposition of UF6. Dust deposition in the separation element greatly depends on the mean residence time of UF6 in the reservoir. The importance ofthe residence time can be explained by the fact that this parameter determines the degree of agglomeration, i.e., the particle size of the dust produced and that the line connecting the reservoir with the separation nozzle system acts as a filter whose efficiency is dependent on the particle size. From an analysis of the transport processes and from corresponding experiments it can be expected that a filter with a low pressure drop will keep away all solid products resulting from radiolytic decomposition of UF6 from the separation elements of a commercial separation nozzle stage. (orig.)