Factors Influencing Contaminant Transport in Vadose Zone of Near Surface Radioactive Waste Disposal Facility

Abstract

Radioactive waste is generally classified as low and intermediate level waste (LILW) and high-level waste (HLW), based on their level of radioactivity and the time taken for decay. The low and intermediate level solid/solidified wastes are emplaced in near surface shallow land repository and are termed as Near Surface Disposal Facility (NSDF). A near surface disposal facility (NSDF) is proposed to be built at Kalpakkam, Tamil Nadu. Kalpakkam is situated about 70 km south of Chennai (12o33’ N Lat and 80o11’ E Long) along the east coast of India. Several of India’s nuclear installations like Madras Atomic Power Station, Fast Breeder Test Reactor, Kalpakkam Reprocessing Plant and Indira Gandhi Centre for Atomic Research (IGCAR) are located in Kalpakkam. Typical reinforced concrete trench (RCT) for disposal of low and intermediate level nuclear waste have major portion of RCT being located below the ground surface and above the water table, i.e., in the vadose zone. Available studies on the mineralogical and physico-chemical characterization of clays at Kalpakkam nuclear plant site have not focused on the mineralogical, physico-chemical and hydraulic properties of soils in vadose zone. The physico-chemical properties of soils are important as they strongly affect the fate and mobility of radioactive contaminants in the sub-surface soil. The hydraulic properties of soils in the vadose zone differ from those in the saturated zone owing to dis-continuity in water filled voids that makes the flow path more tortuous; additionally presence of suction in soil voids renders gradient causing flow to vary with the soil moisture content. Strontium is the selected contaminant solute in this thesis as the waste inventory of proposed NSDF at Kalpakkam shall predominantly contain radioactive strontium and cesium ions. The unsaturated (vadose) zone housing the buried portion of RCT is important as hydraulic characteristics (void ratio, degree of saturation, volumetric water content, suction and unsaturated permeability coefficient) influence the transport of moisture and thereby of the solute dissolved in the moisture to underlying groundwater table, upon breach of the RCT. The physico-chemical properties (pore water chemistry, cation exchange capacity of soils) of the vadose zone soil play significant role in solute transport by way of adsorption/ion-exchange and desorption of the contaminant. Given the significance of hydraulic and physico-chemical properties of the vadose zone soil in influencing solute transport and the lack of studies in this direction in the Indian context, the present thesis examined strontium adsorption and transport for soils obtained from the vadose zone region of proposed NSDF at Kalpakkam, Tamil Nadu, India from 2 pits (termed Pit 1 and Pit 2). Strontium adsorption studies by the NSDF soils were accomplished from batch and miscible displacement column experiments.