39-years Performance of Cemented Radioactive Waste in a Mound Type Repository
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39-years Performance of Cemented Radioactive Waste in a Mound Type Repository Igor A. Sobolev, Sergey A. Dmitriev, Alexander S. Barinov, Galina A. Varlakova, Zoya I. Golubeva, Irene V. Startceva, and Michael I. Ojovan1 Scientific and Industrial Association “Radon”, Moscow, Russia; 1 Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, United Kingdom ABSTRACT Long-term field tests of cemented aqueous radioactive wastes in an experimental mound type surface repository were carried out at Moscow Scientific and Industrial Association “Radon” from 1965 to 2004. Aqueous radioactive wastes of different compositions containing short-lived radionuclides including 90Sr and 137Cs at concentrations from 0.34 to 1.8 MBq/L were immobilized using cementation technology. Water solution to cement ratio was 0.66, grout mixing time 10-15 minutes, and cement paste hardening time 7 days. 73 cement blocks with a volume of 0.027 m3 were disposed of for long-term tests in a simple mound type surface repository. The atmospheric precipitates, which contacted radioactive cement blocks, were collected and analyzed for the content of radionuclides. In August 2004 the experimental repository was opened, cemented blocks, underlying and covering materials were retrieved for analyses. XRD analyses showed that along with amorphous tobermorite gel the main crystal phases in cements are calcite and portlandite. Both visual inspection and radiometric analyses demonstrate that cemented blocks are in good condition and that the cement paste has retained radionuclides from the wastes. Thus after 39 years of storage in the mound type repository the cemented aqueous wastes are reliable immobilized. INTRODUCTION Cementation of radioactive waste has been practiced for many years to immobilize low and intermediate level wastes [1-4]. Cemented radioactive wastes are characterized by good compressive strength, good thermal, chemical and physical stability. Moreover the alkaline chemistry of hydraulic cements ensures low solubility for most radioactive waste radionuclides. The prominent advantages of cement immobilization are due to the inexpensiveness and readily availability of hydraulic cements, simple and low cost processing at ambient temperature. Cement matrix acts as diffusion barrier and provides sorption and reaction sites. Cementation is suitable for sludge, liquors, emulsified organic liquids and dry solids. Most of cementation technologies utilize hydraulic cements among which ordinary Portland cement (OPC) is of most use. Phases formed during cement hydration influence its structure and properties. The most important is tobermorite gel (Calcium Silicate Hydro-gel, CSH), which is the main cementing component of the paste. Setting and hardening behavior, strength and dimensional stability depend primarily on the tobermorite gel. Development of the microstructure of hydrated cement occurs after the paste has set and continues for months (and even years) after placement. OPC is typically 95-98% hydrated after 1
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