Iodine-129 process control monitor for evaporator off-gas streams

Continuous monitoring of gaseous 129I in evaporator off-gas at nuclear fuel reprocessing facilities is important from both regulatory and process control viewpoints. Accordingly, continuous monitoring of evaporator overheads would supply the necessary information to retain 129I in the evaporator bottoms should a process control mechanism for the evaporator be developed. Although a continuous 129I monitor for nuclear fuel reprocessing plant off-gas streams has already been developed, monitoring of evaporator overheads presents unique problems for 129I monitoring, including 129I in the form of liquid aerosols, high concentrations of nitric acid mist and organics, and high concentrations of other radionuclides such as 137Cs. The off-gas is also predominantly water vapor. A two-phase laboratory study was completed. First, the proper configuration to successfully detect 129I was examined. Second, a 24 factorial design experiment was performed to determine the monitor's response to aerosols, organics, HNO3, and 137Cs. Both response and variability for the monitor were determined. Major conclusions were: the presence of HNO3 had no statistically significant effect on the response of the monitor; the presence of TBP significantly increased the response of the monitor; the use of a filter, corresponding to low aerosol conditions, significantly increased the response of the monitor. A filter should be used because the decreased monitor response if the filter is not used (high aerosol conditions); under conditions of the experiments, 137Cs remained in the evaporator bottoms; and no effect of 137Cs on the monitor was observed; and the variability of the monitor was not significantly affected by the filter, TBP, HNO3, or 137Cs. The monitor when used with a filter, was considered suitable for use in evaporator off-gas streams