The addition ratio of water necessary to dissolve the nitrate salts of a lagoon sludge and the stabilization characteristics of these nitrate solutions have been investigated at KAERI for the treatment of two lagoon sludges. In a practical process, a water addition ratio of 1.5 is suitable to dissolve the nitrate salts to over 96 wt% without precipitation. A highly concentrated nitrate solution was thermally denitrated in two steps in a muffle furnace. The first denitration was performed at 250 ℃ for the denitration of ammonium nitrate and the second denitration was performed at 900 ℃ for the other nitrates. After the first denitration, we found that ammonium nitrate had decomposed completely. Sodium oxide, which is created from the decomposition of sodium nitrate, reacts readily with water, and it is known that alumina stabilizes sodium oxide to form Na2OㆍAl2O3. The second step of the thermal denitration was conducted after adding alumina to the residue of the first denitration. All the residues obtained after the thermal denitration with alumina consisted of calcium oxide and Na2OㆍAl2O3, which are stable compounds for storage.