Abstract
At the decommissioning of the nuclear power plant A1 (NPP-A1) at Jaslovske Bohunice, the radioactive (RA) waste of different physical and chemical characteristics has to be treated. A part of this waste cannot be stabilized directly by standard – running technologies for RA waste treatment installed in Jaslovske Bohunice. Among the most specific was, an extremely reactive, concentrated chromate-sulphuric acid (CSA) that had been used as a strong oxidizing agent for cleaning technological heavy-water tanks more than 15 years ago.
Prior to solve the problem of radioactive elements stabilization for long-term disposal, it was necessary to solve the problem of CSA chemical stabilization. With respect to radiation safety regulations, the direct neutralization of CSA with strong bases was excluded from the very beginning because of the extremely strong exothermic character of the reaction and the possibility of thermal explosion. After laboratory experiments, with both the inactive simulants and real CSA, the neutralization of undiluted CSA with a hydrate of secondary salt of ortho-phosphoric acid was found to be the best solution. The reaction of 96 w% sulphuric acid/CSA with a powder form of the phosphate salt is calm, fast enough, slightly exothermic, and yields the reaction product in the powder form. More over, the main part of the radioactive elements that are contaminants of the CSA undergo during this process chemical transformation into very slightly soluble phosphate structures.
The powder form of the reaction product is, in the next step, immobilized into the solid matrix by cementation technology. Besides the Portland cement (PC), the powder of calcium hydroxide is also introduced. This reacts in the cement slurry with primary phosphates and converts them into less soluble secondary phosphates, and also enables to form apatite structures at the process of cement slurry hardening. As a result, the contaminating radioactive ions, are immobilized not only physically within the solid matrix, but they are also chemically bound into stable and very slightly soluble chemical structures.
Based on the described method, the technology was build-up in the area of the nuclear power plant. The core of the technology is the chemical reactor with the coat-cooler, stirring device, and input jets for liquid media, input device for solid/powder media and output device for emptying the reactor vessel. The technological process is managed from the central control board. Processed CSA is injected/spaterred into the reactor vessel with stirred phosphate salt. After finishing neutralization reaction, indicated by the time-dependent temperature profile, the powders of PC and calcium hydroxide are introduced and homogenized with the reaction product. The last step is an injection of water, formation of cement slurry that is permanently stirred, and finally emptied-out into 200 l barrel where slurry is left to harden.
By this, cyclic batch-based technological regime, the total amount of stored-contaminated CSA was processed, and 20 barrels, each of 200 l, of immobilized/stabilized – hardened radioactive waste have been prepared for long-term disposal. The amount of embedded salts into the cement matrix was chosen as to fulfill the acceptance criteria for the Slovak radioactive waste repository at Mochovce.
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