IODINE-STEEL REACTIONS UNDER SEVERE ACCIDENT CONDITIONS IN LIGHT-WATER REACTORS

Owing to large surface areas, the reaction of volatile molecular iodin e (I-2) with steel surfaces in the containment may play an important r ole in predicting the source term to the environment. Both wall retent ion of iodine and conversion of volatile into non-volatile iodine comp ounds at steel surfaces have to be considered. Two types of laboratory experiment were carried out at Siemens (KWU) in order to investigate the reaction of I-2 at steel surfaces representative for German power plants. (1) For steel coupons submerged in an I-2 solution at T = 50, 90 or 140 degrees C the reaction rate of the I-2-I- conversion was det ermined. No iodine loading was observed on the steel in the aqueous ph ase tests. I-2 reacts with the steel components (Fe, Cr or Ni) to form metal iodides on the surface which are all immediately dissolved in w ater under dissociation into the metal and the iodide ions. From these experiments, the I-2-I- conversion rate constants over the temperatur e range 50-140 degrees C as well as the activation energy were determi ned. The measured data are suitable to be included in severe accident iodine codes such as IMPAIR. (2) Steel tubes were exposed to a steam-I -2 flow under dry air at T = 120 degrees C and steam-condensing condit ions at T = 120 and 160 degrees C. In dry air, I-2 was retained on the steel surface and a deposition rare constant was measured. Under stea m-condensing conditions there is an effective conversion of volatile I -2 to non-volatile I- which is subsequently washed off from the steel surface. The I-2-I- conversion rate constants suitable for modelling t his process were determined. No temperature dependence was found in th e range 120-160 degrees C.