Suitability of Local Clays for Radioactive Waste Disposal

Suitability of Local Clays for Radioactive Waste Disposal

Abstract

Six local clays namely Bomo, Bukuru, Kankara, Manjahota, Onibode and Upkor were studied for use as waste-form for fixation of strontium ion in radioactive wastes. The clays were characterized with respect to cation exchange capacity, organic carbon content and crystal structure. Then the most relevant properties of clay waste-forms: shrinkage, density and porosity were studied for Bomo clay and finally, the prepared wasteforms were tested for leaching characteristics. Bomo clay had the highest cation exchange capacity (CEC) of 44.45 and 50.50 meq/l00g for raw and beneficiated respectively. It was followed by Bukuru clay. The other clays had rather low CEC values of between 26.1 and 15.32 meq/l00g lor both raw and beneficiated. Bomo clay contained quartz, illite and montmorillonite with some muscovite in the raw sample. While Bukuru clay contained predominantly quartz and disordered kaolinite. The other clays contained predominantly kaolinite, illite and quartz minerals. Bomo clay had the best characteristics for fixation and disposal of radionuclides. The shrinkages of the waste-forms were generally higher for the beneficiated than for the raw samples. The bulk density ranged between 1.80 x 103and 1.94 x 103 kg/m3 and apparent porosity ranged between 17.2 – 24.2%. For low strontium concentration of 1587.5 ug/g the mass fixed in clay mineral varied between 63.0 – 70.9 percent of the initial mass used for equilibration while 30.0-41.3 was obtained vii for high concentration of 7875 ug/g. The highest percentage cation loading obtained were 50 and 73 of the total CEC of Bomo clay for low and high concentrations. Bomo clay gave promising result of over 99% retention of fixed strontium after 25 days of leaching. The cumulative mass leached for 25 days ranged between 9.0 ug/g for BL1 and 3.5 ug/g for sample BL3 for the low concentration and 20.0 ug/g for sample BH1 and 2.0 ug/g for sample BH3 for high concentration. For beneficiated sample, the mass leached decreased sharply as temperature increased from 800-1000 C. The most fixation was obtained by beneficiating the clay and firing to 1000 C for both low and high concentrations. However, considerable fixation was obtained by using a raw clay even at 800 C. The leach rate approximately obeyed the natural decay law: R = R e The distribution coefficient, a generally increased with increase in temperature. The values ranged between 1632 for sample BL1 and 4516 for sample BL3 for low concentration and 2566 for sample BH1 and 20478 for sample BH3 for high concentration.