Iodine transport in a severe accident at the high-flux isotope reactor

One of the accident sequences postulated for the high-flux isotope reactor (HFIR) at Oak Ridge National Laboratory (ORNL) involves a large-break loss-of-coolant accident (LOCA) with drainage outside the containment building and into the process waste system. This scenario presents an iodine release pathway to the environment that bypasses the building filtration system. A companion paper describes, in more detail, the HFIR reactor and the modeling of this accident sequence using the MELCOR code. Initial calculations based on the simple iodine transport models in MELCOR indicated significant releases, which posed serious violations of safety standards. Due to known deficiencies in these models, more detailed iodine transport calculations were necessary. Even though few studies have been performed for research reactors, models of iodine transport in light-water-reactor (LWR) containments have recently been developed for the US Nuclear Regulatory Commission. Because the conditions encountered at HFIR are similar, these models have been used to obtain iodine source terms for the postulated LOCA. The abundance of cooling water in the various pipes and tanks suggests that most iodine transport involves aqueous behavior. Initial release from melting fuel most likely occurs in steam, but continuous flow quickly moves released fission products and fragments of melted fuel into liquid. Transport through the system occurs by liquid-phase convection. Speciation is due predominantly to aqueous hydrolysis and radiolysis reactions that depend highly oil pH and somewhat on temperature, radiation dose, and concentrations. Environmental release is driven by evaporation rates from water in those volumes that have both as and liquid regions