Reassessment of Reactor Coolant and Iodine Chemistry under Accident Conditions

Under severe accident conditions, the chemistry of reactor coolant could be significantly different from that under normal operating conditions. Injection of standby liquid control (SBLC) solution, release of fission product cesium (Cs) from damaged fuel rods, and addition of NaOH and Na2S2O3 in spray water will certainly enhance the alkalinity of water. In addition, oxidation of fuel cladding (zircaloy) by high-temperature water and steam would produce a significant amount of hydrogen, and hydrogen in water could effectively suppress water radiolysis and make the water more reducing. It is believed that the chemistry and transport behavior of iodine released from damaged fuel rods are strongly dependent on the chemistry of reactor water under accident conditions, and it is expected that the released iodine will most likely remain in nonvolatile forms in water. The chemistry of reactor coolant and the activities of iodine released from a damaged core are reviewed in detail in this paper.