Aqueous solution of iodide under gamma ray irradiation - fixation of iodide by adding soluble silver salt

An experiment on the fixation of iodide by adding soluble silver salt in the aqueous solution under the gamma ray irradiation was performed to establish a method for confinement of methyl iodide formation during the severe accident of a LWR plant. The formation of methyl iodide in the water phase of sump tank in the reactor pressure vessel containment building was presumed in this study by the reason of being of much obscure parts on the formation mechanism and transfer behavior of the methyl iodide under the severe accident conditions. And the separate effects tests on the chemical behavior of silver iodide and methyl iodide were performed under the gamma ray irradiation. Though the high dependence on the accident events sequence or the plant construction for the formation and transfer behavior of the fission products iodine was inferred, it was shown from the experimental results as above that the method to confine the formation of the methyl iodide by adding the soluble silver salt into the sump tank water at the accident is desirable. In conclusion: The cobalt 60 gamma ray irradiation (1.3 x 106 R/h x 2.75 h) to the silver iodide just after the precipitation in the aqueous solution did not give the effect to the stability of the precipitate. The aqueous solutions shut up in the stainless steel bombs as below were irradiated by the gamma ray from the spent fuel assembly(6.6 or 6.9 x 105 R/h, 1 or 2 h), respectively; CsI (7.89 x 10-4 M), CsI (7.69) x 10-5∼7.89 x 10-4 M) containing (COOH)2(5.56 x 10-5 M ∼1.11 x 10-3 M), H3BO3(3.24 x 10-2 M) and CH3COOH (1.67 x 10-5 M∼1.67 x 10-3 M), Fe3+ (1.79 x 10-3 M) and (COOH)2(2.2 x 10-4 M), or CsOH (3.2 x 10-6 M) and CH3COOH (1.67 x 10-5∼1.67 x 10-3 M), respectively. The methyl iodide occurred in each bomb just, after the irradiation(max. 2.7 x 10-1 %), but the formation mechanism of methyl iodide in each solution above were not resolved. The methyl iodide dissolved in the aqueous solutions(6.44 x 10-2 M) were irradiated by the cobalt 60 gamma ray (1.4 x 106 R/h x 4 h), respectively. Thirty two % of the methyl iodide above(in average of 4 data) were decomposed to the iodine (I2) and the iodide (I-). The decomposition rate of methyl iodide dissolved in the aqueous solution was greater than that in the gaseous state, apparently. In the aqueous solution shut up in the glass container, the methyl iodide was formed from the reaction between the methyl radical donor(dimethyl sulfate, 1 x 10-3 M or 2 x 10-3 M) and the iodine-131 tracer(about 7 micro Ci) at the conversion rate of about 3.5 %. In the aqueous solution above, the formation of methyl iodide decreased distinctly by the cobalt 60 gamma ray irradiation (7 x 105 R/h x 1 h) or the addition of the silver nitrate (1 x 10-4 M). Under the severe accident conditions of LWR, when the main source of methyl iodide formation is due to the reaction between the methyl radical donor and the soluble fission products iodide(containing the iodine formed from the reaction between the oxidation products by water radiolysis and the iodide ion), the addition of adequate amounts of silver nitrate to the water phase just before the dissolution of soluble fission products iodine can decrease distinctly the formation rate of methyl iodide by producing the silver iodide precipitate. (author)