Hydrothermal Hot-pressing Research Articles

Immobilization of Low-Level Radioactive Waste Containing Sodium Sulfate by Hydro-thermal Hot-Pressing

Click to increase image sizeClick to decrease image sizeKEYWORDS: hydrothermal hot-pressingradioactive waste disposalimmobilizationsolidificationsodium sulfateslow-level radioactive wasteswaste forms

Immobilization of low-level radioactive waste containing sodium sulfate by hydrothermal hot-pressing.

Immobilization of low-level radioactive waste containing sodium sulfate by hydrothermal hot-pressing.

水熱ホットプレス法によるほう酸ナトリウム含有廃棄物の固化

Powder of Na2B4O7, a simulated low-level radioactive waste concentrate discharged from PWR power plants, was successfully immobilized into a borosilicate glass matrix by hydrothermal hot-pressing. Waste forms with high waste contents up to 55_??_ were produced. Addition of Ca(OH)2 to a starting powder reduced leachability of waste forms. Solid powder of Na3PO4·12 H2O gave water which was essential to the solidification by hydrothermal hot-pressing. The optimum conditions to produce a waste form with high mechanical strength and low leachability, were determined as follows; reaction temperature: 350°C, reaction pressure: 21 MPa, reaction time: 10 min, heating and cooling rate: 1°C/min. The waste form with 35_??_ waste content produced under the optimum conditions, had high density (2.44 g/cm3), high volumetric reduction (product/waste=1/2), high compressive strength (240 MPa), low leachability (11 g/m2·d determined by leach tests at 40°C for 28 d in deionized water) and high thermal stability (not melted below 600°C). It was shown that a large waste form with 30_??_ waste content, 30 cm in diameter, was successfully produced by hydrothermal hot-pressing.

Immobilization of cesium into pollucite structure by hydrothermal hot-pressing.

In order to immobilize Cs into pollucite structure, hydrothermal reactions of Cs with amorphous aluminosilicate (siliceous sinter) and with mixtures of low-quartz (silicastone) and Al(OH)3, were performed. In both cases, pollucite was formed in 5-N NaOH solution above 200°C for 10 min. Synthesized pollucite was solid solution between pollucite and analcite. Waste forms in which Cs was immobilized in pollucite structure were successfully produced by the hydrothermal hot-pressing method using the silica matrix (mixture of low-quartz and amorphous aluminosilicate) containing Al(OH)3 and NaAlO2. Effect of hot-pressing conditions on leachability of the waste forms was investigated. The waste form containing 10w/o CsOH produced at 300°C and 49 MPa for 24 h, had Cs leach rate of 3.15 g/m2·d by Soxhlet leach tests for 7 d. The waste form was porous, but had high compressive strength and thermal stability.

Immobilization of Cesium into Pollucite Structure by Hydrothermal Hot-Pressing

In order to immobilize Cs into pollucite structure, hydrothermal reactions of Cs with amorphous aluminosilicate (siliceous sinter) and with mixtures of low-quartz (silicastone) and Al(OH)3, were performed. In both cases, pollucite was formed in 5-N NaOH solution above 200°C for 10 min. Synthesized pollucite was solid solution between pollucite and analcite. Waste forms in which Cs was immobilized in pollucite structure were successfully produced by the hydrothermal hot-pressing method using the silica matrix (mixture of low-quartz and amorphous aluminosilicate) containing Al(OH)3 and NaAlO2. Effect of hot-pressing conditions on leachability of the waste forms was investigated. The waste form containing 10w/o CsOH produced at 300°C and 49 MPa for 24 h, had Cs leach rate of 3.15 g/m2·d by Soxhlet leach tests for 7 d. The waste form was porous, but had high compressive strength and thermal stability.

Solidification of Powders in SiO<sub>2</sub>-Fe<sub>2</sub>O<sub>3</sub> and SiO<sub>2</sub>-ZrO<sub>2</sub> System by Hydrothermal Hot-Pressing

A silicastone powder (low-quartz) containing Fe2O3 and ZrO2 up to 30wt% was kneaded with 10N NaOH solution, and then hot-pressed at 300°C and 28MPa for 3h to produce solidified bodies. During hydrothermal hot-pressing, shrinkage of starting materials was measured. Solidified bodies containing Fe2O3 consisted of low-quartz, hematite and acmite (NaFeSi2O6). The formation of acmite suppressed linkage of silica stone grains and reduced compressive strength of the solidified bodies. It also reduced the shrinkage of starting materials. Solidified bodies containing ZrO2 contained low-quartz and monoclinic ZrO2. No reaction products were detected by X-ray powder diffraction. Addition of ZrO2 up to 20wt% increased the shrinkage of starting materials and compressive strength of the solidified bodies. Mobility of silicastone grains at the early stage of hydrothermal hot-pressing was increased by addition of ZrO2. Similar phenomenon was observed by addition of graphite, a solid lubricant.