Session: 05-04: Investigations of Cement Based Materials and Waste Matrices

Paper Number: 109898

109898 - Study of the Immobilization of Molten Salt in Cementitious and Alkali-Activated Matrices 

The Waste and Disposal group at the Belgian Nuclear Research Centre (SCK CEN) is studying the immobilization of several waste streams in alkali-activated and cementitious matrices. One of these wastes is the so-called molten salt. This salt is a residue of the molten salt oxidation (MSO) process, in which organic waste is treated prior to immobilization. Such organic wastes, as ion exchange resins, are oxidised in a reactor with a temperature of up to 1000 °C. The molten salt generated in the process must then be immobilized in a stable and durable matrix prior to disposal. At SCK CEN we studied a sodium carbonate molten salt enriched in radionuclides in carbonate forms. For the tests here mentioned, an inactive surrogate molten salt was prepared, meaning no radioactive material was handled.

In this study we focus on one blended cementitious system and one alkali-activated system to immobilize the molten salt. Previous trials to directly immobilize the salt, as it was produced, in these matrices have proven unsuccessful. The molten salt was too hygroscopic and sensitive to humidity and temperature changes. A new approach was developed by the SCK CEN group to pre-treat the molten salt prior to immobilisation. A series of trial tests show that the best route was a pre-treatment with calcium hydroxide, converting the sodium carbonate into sodium hydroxide and calcium carbonates. In this paper, we show the first results of ongoing tests to prove the long-term durability of the final waste form. The samples immobilised in blended cementitious systems have a waste loading of 10 and 14 %, hereunder named CH_10 and CH_14, respectively. The samples immobilised in alkali-activated system have a waste loading of 10 and 20%, hereunder named BFS_10 and BFS_20, respectively.

Early age properties of samples from both samples were satisfactory. The transformed molten salt was mixed with the cementitious or alkali-activated materials and cured in a sealed bag, at 20 °C, for 28 days prior the start of the tests. After the cure, the mechanical strengths of some samples were measured. The flexural strength of the alkali-activated samples was 1.4 and 2.0 MPa for BFS_10 and BFS_20, respectively and of the cemented samples was 3.1 and 3.8 MPa for CH_10 and CH_14, respectively. This is above the limit of 1 MPa stablished on the waste acceptance criteria (ACRIA) for immobilised waste-forms. The compressive strength was 37 and 42 MPa for BFS_10 and BFS_20, respectively, and 11.8 and 14.4 MPa for CH_10 and CH_14, respectively. The limit stablished on the ACRIA is 8 MPa. Prismatic samples of 4 x 4 x 16 cm³  were used for carbonation tests. The samples were pre-conditioned at 20 °C and a R.H. of 60%. After pre-conditioning, the samples were exposed to an atmosphere of 1% CO₂  at 20 °C. Results are only available for the BFS_10 and BFS_20 samples. They show a larger carbonation front in samples with 10 % waste loading. These samples also presented a higher mass gain during the tests. Further analysis on the samples are going to be carried out to explain this behaviour. TGA and XRD measurements are foresee to be done to show the changes in the mineralogy of the samples under tests. Leaching experiments, in which the samples are exposed to a large volume of leachant, are still ongoing.

Up to now, the results show promising recipes to immobilise the pre-treated molten salt in both blended-cement and alkali-activated matrices.

Presenting Author: Eduardo Ferreira SCK CEN

Presenting Author Biography: Obtained a Bachelor Degree in Environmental Chemistry at University of São Paulo and a PhD in Nuclear Technology at Nuclear and Energy Research Institute in São Paulo, Brazil. Working for more than 5 years in the Belgian Nuclear Research Institute, in the field of behavior of cementitious materials, immobilization of complex radioactive waste in cementitious and novel matrices, immobilization of liquid radioactive waste, among others.