Solidification of medium-level wastes at the burial site

Abstract

Solidification of radioactive wastes directly in storage tanks is one way to make use of defective burial sites and burial sites which have exceeded their service life and contain low-and medium-level sludge. This approach can be used to solidify the bottom sludge residues, which, as a rule, remain in the storage tanks after the tanks have been emptied.The process of solidification of medium-level sludge directly in the storage tank has been investigated and tested commercially in application to sludge storage at one of the radiochemical plants in this country. The tank, 30 m high and 12 mm in diameter, is made of ferroconcrete and it is lined on the inside with 4 mm thick stainless steel sheets (Fig. 1).The sludge residues in a defective storage tank of the type indicated above have the following chemical composition (g/liter): sodium nitrate 325, sodium acetate 6, potassium (sodium) ferrocyanide 125, Fe 6, AI, Cr, Mn 0.5 each, pH 6-7, solid:liquid = 1:2, the computed sludge volume 80 m 3, and radioactivity 3.7-101~ Bq/liter (-100% 137Cs).The process of solidification of the wastes requires mixing with the binder components. Therefore, one of the main requirements which the technology of solidification inside the storage tank must meet is that this process had to be eliminated.A large group of binders was tested for solidification of ferrocyanide sludge in the absence of mixing and heating: Portland cement, furnace and ferrochrome slags, perlite mixed with an alkali, and the systems magnesium oxide + orthophosphoric acid, zinc oxide + orthophosphoric acid, and copper oxide + orthophosphoric acid.On the basis of investigations performed on a sludge simulator and real ferrocyanide sludge, the system orthophosphoric acid + magnesium oxide was chosen as the binder. Caustic magnesite, which is a waste product of largecapacity production of refractory materials, was used as the source of magnesium oxide. Caustic magnesite is a highly dispersed powder with a density of 3.3-3.4 g/cm 3 and the following chemical composition (mass %): magnesium oxide 78.6, calcium oxide 3.5, silicon oxide 1.86, total sesquioxides 2.1, carbonic acid 9.25, and the rest is water.A sequence in which the binder components are loaded into the sludge was tested on a laboratory scale and a prototype storage tank (Fig. 2) using orthophosphoric acid and caustic magnesite. The mass ratio of the sludge, orthophosphoric acid, and caustic magnesite necessary for coalescing the components and obtaining concrete block based on magnesium phosphate binder was chosen. The magnesium phosphate binder is a complex binder, whose components, in the absence of mixing, perform different functions which determine the sequence in which the components are loaded into the storage tank. This loading sequence was determined using a transparent and a stainless steel prototype setups with the mass ratio of the model sludge:orthophosphoric acid:magnesite ---1:0.3:0.5:Orthophosphoric acid, which is denser than the sludge, is poured in to stir up the sediment; in the transpa...