Behavior of impurities in a cryogenic krypton removal system

Two radiolytic reactions which may take place in the cryogenic columns of a Kr-85 removal system in the off-gas cleaning system of reprocessing plants were simulated in the inactive test unit KRETA: formation of ozone from O2 and of nitrogen oxides from O2 and N2. The reactions were simulated by adding O3 and NO + O2 to the feed gas, respectively. Whereas accumulation of O3 in the first column is not critical, it can reach concentrations in the percent range between the krypton and xenon sections of the second column. NO reacts with O2 rapidly at low temperatures to N2O3 and N2O4. The solubility of N2O4 in liquid xenon has been determined to be about 1 mole-% at 200 K in a laboratory study. Crystallization of moderate amounts of N2O4 in the sump does not cause column malfunctions. However, if N2O4 crystallizes in the feed gas section of the column, plugging of sieve plates may occur. It was found that CH4 and other hydrocarbons can be removed from the off-gas by oxidation with H2O vapor even in an atmosphere containing excess H2 for the reduction of O2 and NO/sub x/. For complete removal of CH4 temperatures of greater than or equal to5000C must be maintained