The characteristic of dissolution reaction behavior of chromium oxide in nuclear power plant decontamination

Kim Junehyun

School of Architectural, Civil, Environmental and Energy Engineering,
Major in Environmental and Energy Engineering
The Graduate School, Kyungpook National University
Daegu, KOREA
(Supervised by Professor Choi Sang-June)


(Abstract)
Cr-rich Cr2O3 is present in the inner layer of the corrosion oxide film in the primary pressurized light water reactor system. Unlike Fe and Ni oxides, Cr2O3 does not dissolve by reduction or acidic dissolution. Cr2O3 is dissolved in solution using oxidizing agent. KMnO4 is used as oxidizing agent and it is divided into nitric permanganate (NP) and alkaline permangnate (AP) depending on pH condition. And HMnO4 (HP), which is prepared by passing KmnO4 solution through cation exchange resin column. HP is more efficient than NP and AP in the dissolution of Cr2O3, and it is also advantageous in reducing waste volume resulting from decontamination.
In this study, the dissolution behavior characteristic of chromium oxide and the effect of MnO2 in chemical decontamination were investigated with Cr2O3 pellet and simulated CRUD.

As a result of experiment the amount of chromium oxide dissolved was lower in solution having a higher concentration of nitric acid. This is because the higher the concentration of nitric acid the larger the surface charge difference between MnO2 and Cr2O3 pellet, which means that the amount of MnO2 accumulated on the pellet surface increases. Similarly an experiment to dissolve chromium oxide depending on concentration of HMnO4 showed that the higher the concentration of HMnO4, the higher the dissolution rate of chromium but saturated at ~ 3 mM. And the dissolution rate in 4, 5, 6 mM HmnO4 solution was reduced. An experiment using simulated CRUD showed that the amount of chromium dissolved was high as in the order of 1 cycle < 2 cycle < 3 cycle.

Based on these experimental results, the following conclusions were obtained: 1. At the same concentration of HMnO4, the higher the amount of H+ ions, the faster the dissolution rate of chromium oxide increased. This is because as the amount of H+ ions increases, the amount of MnO2 increases, and the difference in zeta potential between the surface of the pellet and MnO2 becomes greater, thereby increasing the amount of MnO2 accumulated on the surface of the pellet. Thus, it can be seen that further injection of an acid at the same concentration of HMnO4 is not efficient in dissolving chromium oxide.
2. There is no significant difference in chromium oxide dissolution of KMnO4 and HMnO4 under the same pH conditions. The difference between the two oxidizers is the presence or absence of K+ and NO3-. The effect of K+ and NO3- on the dissolution of chromium oxide is considered to be small when the concentration of H+ ion and MnO4- are the same.
3. The higher the acidity, the lower the dissolution rate of chromium oxide. As the concentration of HMnO4 decreases, the dissolution rate of chromium oxide is proportional to the concentration of HMnO4 and 1 mmol / L, And the amount of MnO2 in the Cr2O3 pellet is increased.
4. MnO2 accumulation in chromium oxide dissolution by Cr2O3 pellet was also found in chromium oxide dissolution by using the corroded specimens and it was found that the dissolution rate of chromium increased as the cycle was repeated. Therefore, in order to decontaminate the corrosion oxide film, it is helpful to increase the decontamination performance by performing decontamination several times.