Behaviour of Short-Lived Iodines in Operating UO2 Fuel Elements

Abstract

Sweep Gas experiments have been done to determine the behaviour of short-lived fission products within operating UO₂, fuel elements at linear powers of 45, 54, and 60 kW/m, and to burnups of 70, 80, and 50 MWh/kgU respectively. Although radioiodine transport was not observed directly during normal operation, equilibrium gap inventories for I-131 were deduced from the shutdown decay behaviour of the fission gases. These inventories were a strong function of fuel power and ranged from 126 GBq (3.4 Ci) to 2520 GBq (68 Ci) over the range tested. We conclude that the iodine inventory was adsorbed onto the fuel and/or sheath surfaces with a volatile fraction of less than 10^-2 and a charcoal-filter-penetrating fraction of less than 2x10^-5.

Two of the tests defected and allowed high temperature pressurized-water coolant into the sweep gas system. Differences between the conditions of these failures allowed a qualitative comparison between radioiodine transport in the liquid and vapour phases of the coolant. Comparison with results for dry helium showed that transport was 10² times greater in the vapour phase and 10⁴ times greater in the liquid phase.