Gamma Radiation Effects on Time-Dependent Iodine Partitioning.
A need for characterization of the iodine source -term used in safety calculations for hypothesized fission reactor core disruptive accidents has motivated this study in iodine volatility. Previous experimental studies have been directed at evaluating volatility of iodine at a single time shortly (1-12 h) after introduction into the aqueous -phase. This work has explored the very important variables of time in solution and gamma radiation dose rate for a range of iodine concentrations (10('-8) - 10('-5) gI/ml) and pH's (5, 9 and 11). All experiments were performed at (TURN)25(DEGREES)C, first in the absence of a significant radiation field and later with a gamma radiation dose rate ranging from .003 MRad/hr to .06 MRad/hr. Iodine was introduced either as molecular I(,2) or NaI with I-131 (8.04 day half -life) as a tracer. Conditions present during the course of the experiment governed iodine solution chemistry thus determining whether volatile forms (I(,2) and HOI) would predominate. Results of experiments with nonirradiated systems indicated very little volatility with NaI initiated studies. I(,2) initiated systems at pH 5 were the most volatile whereas pH 9 and 11 I(,2) systems showed decreasing volatility with time. This has been explained by analysis of iodine aqueous chemistry. Total iodine concentration dependence has also been addressed. A pronounced radiation-induced reduction in iodine volatility in pH 5 iodide solutions has been demonstrated as well as a dose rate dependence in the transient phase. This effect is more pronounced in higher concentration solutions. As with nonirradiated systems, irradiated alkaline solutions exhibit low volatility. Explanations for the behavior of irradiated systems based on iodine radiolytic chemistry are provided. A computer based model incorporating water radiolysis and iodine radiolytic chemical reactions has been formulated and tested. The model successfully predicts radiation induced volatility changes in pH 5 iodide systems. The experimentally observed dose rate dependence is also verified; however, insufficient knowledge of alkaline system iodine radiolytic interactions prevented success under those conditions.
- Pub Date:
- Physics: Nuclear