New insights on the immobilization of radioactive acidic liquid waste and cesium in amorphous aluminophosphate waste form

Radioactive acidic liquid waste (RALW), generated during the Chemical Oxidation Reduction Decontamination (CORD) and High Permanganate CORD Ultraviolet (HP-CORD UV) process of primary systems in nuclear power plants, contains various heavy metals and various radionuclides. These are typically removed by organic ion exchange resins, producing large amounts of secondary spent ion exchange resins (spent IERs) waste. This study aimed to reduce spent IER generation by directly using unpurified RALW as mixing solution for amorphous aluminophosphate (AAP) waste forms which use acidic solution as a raw material. In addition, Cs that may be present in RALW was added at 10 wt% to investigate its effect on the AAP structure. Neither the use of RALW as mixing solution nor the addition of Cs significantly altered the mechanical strength of AAP waste forms. The presence of heavy metal in RALW did not substantially change the local structure of Al and P in the AAP matrix. Multiscale characterization showed that Cs was mainly immobilized by forming amorphous cesium aluminophosphate and through physical incorporation within the AAP matrix. This research demonstrates that RALW can be effectively used in AAP waste forms and provides direct evidence of Cs immobilization mechanisms in AAP systems.