CAESIUM IMMOBILIZATION USING ZEOLITE-CONTAINING ROCKS AND HIGH TEMPERATURE TREATMENT

摘要

A systematic study was undertaken or the chemical stability and structure or Cs-containing materials, prepared from Cs-saturated zeolite-containing rocks (ZCRs) by high temperature roasting, mainly in the absence of boron compounds.rnCs was introduced into the ZCRs via two procedures: (1) via an ion-exchange process between Cs in solution and the ZCRs; (2) by mixing ZCR powder with CsNO_3 solution. Cs-containing ZCRs prepared by the two methods were heated at various temperatures up to 1400 ℃. The chemical stability of the resulting glass-crystalline material was determined by measuring the amount of Cs passing into solution at various pH levels ranging from 1 to 12. The experiments lasted from 1 to 28 or more days.rnIt was found that heating ZCR at 1000 ℃ results in the strong immobilisation of Cs, Al, Na, K, Mg regardless of the way in which Cs was introduced into the ZCR. The lowest quantities of leached cations Cs, Al, Si, Na, K, Ca, Mg were detected in neutral solutions. The amount of Cs leaching into solution at pH=5 from products roasted at tempreatures >1000 ℃ was < 10~(-5) g cm~(-2) d-1 for the ZCR of all deposits studied. For the clay, the corresponding amount of Cs was about one order of magnitude higher. The crystal structure of the roasted material depends strongly on its Cs content: at a Cs content lower than 1 mg-equiv g~(-1), the material comprised a predominantly amorphous phase; at higher Cs content, it was a mixture of amorphous and crystalline phases: ortho-rhombic (CsAlSi_5O_(12)) and/or cubic - pollucite (CsAlSi_2O_6). The quantity of crystalline phases was greater when the second procedure for introducing Cs into the rocks was used.rnThus, from the results obtained here, ZCRs are shown to be a good candidate for radioactive waste conditioning and final disposal. Such a wasteform may be used not only in radionuclide decontamination processes, but also as a flux additive resulting in the formation of chemically- and mechanically-resistant Cs-containing glass-crystalline materials.