Reassessment of Reactor Coolant and Iodine Chemistry under Accident Conditions

Abstract

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 Na 2 S 2 O 3 in spray water will certainly enhance the alkalinity of water. In addition, oxidation of fuel cladding (zircaloy) by hightemperature 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.

Adsorption of Iodine Species (I3–, I–, and IO3–) at the Nuclear Paint Monolayer–Water Interface and Its Relevance to a Nuclear Accident Scenario

“…The present investigation provides molecular-level insight into the interfacial adsorption/interaction of the inorganic iodine species (IO 3 – , I – , and I 3 – ) on the paints used in nuclear installations. The radiolytic condition of the containment building along with the undertaken safety measures such as spray may greatly vary the pH of the humid environment inside the containment building. , The HD-VSFG measurement reveals that the paint–water interface becomes positively charged at acidic conditions and net neutral or uncharged at the alkaline condition due to protonation/deprotonation of the amine groups present in the paint. Variation of the charge characteristics greatly affects its interaction with the iodine species and the associated water at the interface.…”

“…Under an accidental condition, the iodine released from the reactor core is mainly inorganic iodide (e.g., I − ) 9−11 which is subsequently converted into other inorganic iodine species such as molecular iodine, triiodide, and oxyanions in the oxidizing radiolytic environment of the containment. 12,13 However, the formation of organic iodide inside the containment is not well understood. Gamma irradiation of organic compound in iodine-containing solution leads to the formation of organic iodides including methyl, ethyl, and isopropyl iodides.…”

“…Under an accidental condition, the iodine released from the reactor core is mainly inorganic iodide (e.g., I – ) − which is subsequently converted into other inorganic iodine species such as molecular iodine, triiodide, and oxyanions in the oxidizing radiolytic environment of the containment. , However, the formation of organic iodide inside the containment is not well understood. Gamma irradiation of organic compound in iodine-containing solution leads to the formation of organic iodides including methyl, ethyl, and isopropyl iodides. , The first step toward the formation of organic iodide is the proximity of inorganic iodine to an organic matter inside the reactor containment. , The major source of organic matter is the painted surfaces of the walls and accessories inside the containment building, which cover a large area.…”

“…We measured the self-assembled paint monolayer–water interfaces in the “presence” and “absence” of the iodine-species (I – , I 3 – , and IO 3 – ) in the aqueous phase by using broadband HD-VSFG as well as narrowband classical-VSFG techniques. To mimic the varying acidic/alkaline condition inside the containment, we varied the pH of the bulk aqueous phase (pH = 2.9, 5.8, and 9.5). , As observed by the preferred orientation of interfacial water, the paint–water interface is net neutral at alkaline (pH 9.5) condition and positively charged at acidic (pH 2.9) and even weakly acidic (pH 5.8) conditions. The inorganic iodides, especially I 3 – and I – , significantly adsorb at the paint–water interface at acidic pHs, while I – and IO 3 – (I 3 – is unstable at alkaline pH) interact weakly with the paint at the alkaline condition.…”

Heterodyne-Detected Sum Frequency Generation Study of Adsorption of I– at Model Paint–Water Interface and Its Relevance to Post-Nuclear Accident Scenario

“…The prominent species are the CsI aerosol particles, which are likely to settle in the sump and instantaneously dissociate into Cs + and I – . Further, under such accident conditions, the pH of sump water, calculated to be alkaline (∼10–11), can also become acidic due to dissolution of nitric acid, formed from nitrate/nitrite generated during the radiolysis of humid air inside the containment, as well as absorption of HCl formed from Cl – ions generated during radiolytic decomposition of cable insulation and other polyvinyl chlorides. , Regarding the paints used in containment buildings, they are mostly of epoxy type based on diglycidyl ether of bisphenol A (DGEBA) and cured with hardeners such as amides, amines, or alcohols . Since the paint is a complex mixture of a variety of organic compounds, it represents a complicated system to study using the interface-selective and phase-sensitive HD-VSFG technique (vide infra).…”