Investigation of selective separation of radioactive cesium from NPP evaporator bottom residue

Abstract

In the Hungarian PWR-type (4 blocks of VVER-440/213) nuclear power plant of Paks the radioactive waste waters are collected in common tanks. Up to the present, the low salinity solutions were evaporated (by adding sodium-hydroxide) till 400 g/dm3 salt content (pH~12-13) and stored for burial. Currently there is about 6500 m3 concentrated evaporator bottom residue in the tanks of the NPP. A Liquid Wastewater Treatment Technology (LWT) was developed to treat this wastewater before solidification and burial. The aim of this technology is selectíve separation of cesium and cobalt isotopes into small volume, then the remaining inactive chemicals could be treated as chemical waste. The radioactive liquids contain these radioactive isotopes with 104-106 Bq/L activity concentrations. The long-life radionuclides are present in very low concentration (10-9-10-12 mol/dm3) as ions, suspended, colloid particles and in complex (EDTA, oxalate, citrate) forms. In this treatment technology, two cesiumselective ion exchanger applied, the CsTreat and the CsFix.changer for the selective separation of radiocesium isotopes (134Cs, 137Cs). A third Russian selective sorbent Termoxid-35 was also tested.
CsTreat and Csfix have near the same Cs ion exchange capacity, sorbent Termoxid-35 was not effective for these residues. Capacity of CsTreat and CsFix depends on the sorbent charging technique into column and for CsTreat on the potassium content. Ion exchange properties of sorbent CsFix were independent of potassium concentration. Columns were more effective in serial coupling (2-3 columns). The screen size for inlet and outlet should be increased from 0.032mm to0.18mm to avoid plugging. Further research is required to increase the slope of breakthrough curves.