The reduction of radioactive waste volume is an important issue for the management of the nuclear fuel cycle. The purpose of this study is to create a technique to drastically reduce low-level liquid waste generated by a spent fuel reprocessing plant employing PUREX technology.
In the PUREX plant, NOx gas is used as an oxidizing reagent for adjustment of the Pu valence in the Pu purification stage. The spent NOx gas is recovered as nitric acid and a certain amount of recovered nitric acid becomes low-level waste (LLW). As NOx gas is produced by the chemical reaction of nitric acid and sodium nitrite, a considerable quantity of non-radioactive sodium nitrate solution is discharged. From the standpoint of the impact on the environment, this discharge must be reduced.
The objective of this research is to develop an electrolysis process by which NOx gas is directly produced from recovered nitric acid. Using this technology, the low-level sodium nitrate waste can be reduced and the nonradioactive sodium nitrate waste from the present NOx production process can be entirely eliminated.
The study was performed in the following two steps:
Phase 1: Fundamental research;
Phase 2: Engineering study.
Fundamental research: A study on the conditions of nitric acid decomposition by electrolysis was performed. The composition of NO and NO2 and its quantity are determined by electrolyte concentration, cathode material and current density. Catholyte reduced electrochemically is in a state of chemical equilibrium expressed by NO, NO2 (N2O4), HNO2, HNO3 and H2O. Nitric acid concentration, temperature and partial pressure of NOx are important process parameters since NOx generation current efficiency and NOx gas composition are dependent on a chemical equilibrium.
Engineering study: The pilot plant was designed and constructed. The process configuration of the pilot plant is the same as that of the anticipated actual plant and its NOx generation capacity is one eighth of the actual plant. The plant mainly consists of electrolysis unit, NOx compression and storage unit, sodium nitrite oxidation unit, nitric acid distillation unit, and NOx recovery and off-gas treatment unit. NOx gas and HNO2 are generated as a result of nitric acid reduction in the electrolysis unit. NOx gas is transported under negative pressure to the compression/storage unit by a compressor and is stored in tanks under high pressure. After adjustment of gas composition by O2 supply, NOx gas is fed to the user process in the case of an actual plant. In the pilot test plant, NOx gas is transported to the nitric acid recovery step, in which NOx gas is recovered as nitric acid and reused in the pilot plant.
As a result of operation of the pilot plant, the process performance, equipment performance, and long-term operation stability were proven. It was confirmed through the pilot plant operation that this technology was immediately applicable to a PUREX reprocessing plant.