Abstract
Nuclear energy has become an interest due to its high reliability, safety and productivity making it the only immediately deployable replacement to carbon based energy production. The majority of radionuclides produced by fission are either burnable as fuels in nuclear reactors, have a short half-life, or are non-volatile. Of the few remaining volatile isotopes, iodine-129 is a concern due to its long half-life and bioaccumulative property. This have proven difficult to capture in a cost effective manner.A simpler method of iodine-129 capture involving sorption onto reduced silver mordenite has been investigated in the past. This has been effective in capturing both diatomic and organic iodides but the reactions and associated kinetics driving iodine retention have not yet been fully understood.
In the experiments presented, dynamic sorption tests of reduced silver mordenite were performed in air at 170 oC with 120 ppb methyl iodide. The sorbent was constructed by coating inert cordierite honeycomb with reduced silver mordenite allowing for tests of low masses of silver mordenite. Methyl iodide content was monitored by gas chromatograph with electron capture detection (GC-ECD) and flow rates were measured with a mass flow meter. The data indicated that the outlet concentration of methyl iodide had plateaued but did not return to the inlet concentration. The utilization of silver was calculated and found to be in excess of the available silver indicating a catalytic reaction was occurring obscuring the practical working capacity. Attempting to quantify reaction kinetics, flows through the system were varied and concentration measurements were taken.
Since GC-ECD could not detect hydrocarbons, another set of experiments were performed at a much higher methyl iodide content at 40 ppm. This involved the use of a gas chromatograph with flame ionization detection (GC-FID) for hydrocarbon content measurements. This was also performed with different water contents. The results indicated surface saturation as water content was increased and a change to less methylated hydrocarbons was observed.
When reviewing these results a the studies reported in literature a mechanism for iodine accumulation is difficult to establish. This is because these mechanisms are based on methyl cation formation which cannot participate in termination reactions. When considering a methyl radical reaction mechanism, accumulation of iodine is easy to incorporate indicating that a free radical reaction mechanism more likely.
This investigation has identified reaction rates, and conditions that affect reaction products. Effects upon iodine accumulation were also made that indicates that water had little influence upon the final quantity of iodine retained. The implications for the use of reduced silver mordenite includes acceptable exposure to high humidity environments and catalytic decomposition of all organic iodide into hydrogen iodide making detection of organic iodide an ineffective method of determining breakthrough. Further work is needed to determine the rate limiting step and the practical working capacity of the sorbent.