HWVP Iodine Trap Evaluation

This report details our assessment of the chemistry of the planned Hanford Waste Vitrification Plant (HWVP) off-gas system and its impact on the applicability of known iodine removal and control methods. To predict the gaseous species in the off-gas system, we completed thermodynamic calculations to determine theoretical equilibrium concentrations of the various potential chemical species. In addition, we found that HWVP pilot-plant experiments were generally consistent with the known chemistry of the individual elements present in the off gas. Of the known trapping techniques for radioiodine, caustic scrubbing and silver-containing sorbents are, in our opinion, the most attractive methods to reduce the iodine concentration in the HWVP melter off gas (MOG) after it has passed through the high-efficiency particulate air (HEPA) filter. These two methods were selected because they (1) have demonstrated retention factors (RFs), ratio of amount in and amount out, of 10 to 1000, which would be sufficient to reduce the iodine concentration in the MOG to below regulatory limits; (2) are simple to apply; (3) are resistant to oxidizing gases such as NOx; (4) do not employ highly hazardous or highly corrosive agents; (5) require containment vessels constructed or common materials; (6) have received extensive laboratory development;more » (7) and the radioactive wastes produced should be easy to handle. On the basis of iodine trapping efficiency, simplicity of operation, and waste management, silver sorbents are superior to caustic scrubbing, and, or these sorbents, we prefer the silver zeolites. No method has been fully demonstrated, from laboratory-scale through pilot-plant testing, to be an effective iodine trap at the low iodine concentration (2 x 10-11 mol I/L) expected in the MOG of the HWVP in the presence of the other gaseous off gas components. In terms of compatibility of the trapping technology with the components in the MOG, there is some question about the resistance of the silver zeolite's aluminosilicate matrix to the fluoride component in the off gas. The caustic scrubber has no compatibility problems with the MOG off gas; however, the acidic components such as CO2 will increase the volume of waste produced and could affect the efficiency of the iodine trapping. To apply these gaseous iodine trapping technologies to the HWVP, further development work would be required. Neither method has been demonstrated at the very low iodine concentrations that exist in the off gas, which are 0.01% to 1% of the found in nuclear fuel dissolver off gases for which these technologies were developed. Furthermore, the large excess of other reactive and trappable gases in the HWVP off gas imposes a heavy load on the trapping medium, could impede iodine trapping, and could have deleterious effects on the trapping medium itself. For silver zeolites, other trappable gases such as chlorine, which are in gross excess of the iodine in the off gas, will compete for the active sites in the silver zeolite. In applying a silver zeolite to the HWVP, 99-9% of the silver would be used to trap chlorine with less than 0.1% of the silver employed in the zeolite bed used for iodine trapping. It is also difficult to predict what will happen when the aluminosilicate framework of the zeolite is exposed to the reactive gas, HF, which is also present in the off gas and is known to attack silicates. In the case of caustic scrubbing, because of the low iodine concentration in the off gas, essentially all of the caustic will be used for CO2 removal, a small fraction for chlorine and fluorine removal, and a trace amount for iodine removal. NO2, which should exist largely as NO, will not be removed.« less