Pyrochemical Decontamination Process Development for Volume Reduction of Intermediate Level Waste from PWR Decommissioning

Currently, 454 nuclear power plants are operating globally and account for about 11% of the total electric power generation. About 66% of these nuclear power plants have been in operation for more than 30 years, and the proportion of such old nuclear power plants is expected to continuously increase. In Korea, the Kori Unit #1 reactor has been decided to permanent shutdown, and 7 PWR reactors will reached the design life time before 2030. As a result, the need to prepare for the decommissioning of nuclear power plants is increasing, and the management of intermediate and low-level radioactive waste from decommissioning is also becoming important. Among them, the amount of the intermediate level metal waste from reactor internals periphery to reactor core is about 66 tons. The reactor internals are made of stainless steel and are exposed to high neutron flux for a long period. As the stainless steel is activated, long living activation products such as C¹⁴, Nb⁹⁴, Ni⁵⁹ and Ni⁶³ and short living activation products such as Co⁶⁰ are generated. However, according to the current regulations, intermediate level radioactive wastes containing a large amount of long living radionuclides can not be disposed of due to the total activity limits of Gyeongju repository. Therefore, most of the countries as well as Korea kept them in the interim storage on site and waiting for the construction of high-level radioactive waste or spent nuclear fuel repository. In order to solve this problem, it is necessary to develop decontamination technology for long living intermediate level wastes. Based on ORIGEN-2 modeling with some assumptions, pressurized water reactor internals were modeled. The radioactivity of C¹⁴, Nb⁹⁴, Ni⁵⁹, Ni⁶³ and Co⁶⁰ were 1.83E+06Bq/g, 2.88E+04Bq/g, 5.40E+06Bq/g, 1.05E+07Bq/g, 1.35E+09Bq/g, respectively. The decontamination factors required for disposal of all reactor internals from 20 units are 8.2, 259.1, 73.0, 94.7, and 36.6, respectively. In this paper, electrorefining process that takes good advantage of theoretically very low secondary waste generation was suggested for decontamination process. LiCl-KCl eutectic salt, which has lower operating temperature than fluoride salt (LiF-KF) and has less corrosion problem, was used as electrolyte. Approach for electrorefining is recovering Fe and Cr with high tendency to oxidation by leaving longliving nuclides using standard potential difference between them. In order to investigate the behavior of the major elements (Fe, Co, Ni, Cr) in the LiCl-KCl, cyclic voltammetry at 500°C was performed. All nuclides except for Cr showed a single pair of redox peak. Cr showed two pairs of redox peaks, but the oxidation peak of Fe was -0.2 ~ -0.1V [vs. 1 wt. % Ag/AgCl]. The oxidation behavior between Cr²⁺ and Cr³⁺ was negligible in that region. Apparent reduction potentials and diffusion coefficients in molten salt were obtained based on the results of cyclic voltammetry. In addition, a database for related studies was constructed by acquiring exchange current density and charge transfer coefficient of each nuclide through Linear Polarization Method. The achievability of the decontamination factors through electrorefining was evaluated by conducting REFIN modeling, 1-D time dependent simulation code. It showed that decontamination factors for Nb and Co can be achieved through electrorefining. In case of Ni, it is possible to achieve decontamination factor by 2 successive electrorefining. Electrorefining experiments were performed to verify the modeling results. LiCl-KCl-3 wt. % FeCl2 was used as electrolyte, and a type 304 stainless steel rod was used as anode. During the experiment, baskets were installed around the anode to recover the metals that could come off from the anode and the cathode. The applied potential on anode were -0.2V[vs. 1 wt. % Ag/AgCl], -0.1V[vs. 1 wt. % Ag / AgCl], 0V[vs. 1 wt. % Ag/AgCl]. After electrorefining, the cathode surface, inside the baskets and bulk salts were analyzed by ICP-MS and XRD analysis was performed to confirm the deposition on cathode. As a result, it was confirmed that Fe was electrodeposited on the surface of the cathode and the decontamination factors of Co and Ni was decreased as the applied potential on anode was increased. However, in all three experiments, it was confirmed that the required decontamination factor can be satisfied when electrorefining process is repeated twice. However, in the lab-scale electrorefining experiments, the problem of the formation of the limiting current was confirmed and the new cell design was designed using the CFX code to overcome the issues. The design of new baskets in the cell was modeled as a porous structure, and the particle tracking and the IR drop modeling were performed. The IR drop between anode and cathode was more than 2 times greater than that of the non-porous baskets. These findings suggest that if a pilot-scale electrorefiner is manufactured, it would result in reduction of 132.6 billion KRW economically