Method for immobilising radioactive waste including at least alkaline salts in order to obtain an immobilisation matrix, the method including at least: - a step (201, 401) of pretreatment of initial radioactive waste (200, 400) in order to obtain pretreated radioactive waste; - a step (206, 406) of mixing pretreated radioactive waste with the addition of compounds capable of vitrifying or ceramifying the pretreated radioactive waste; such compounds include at least: i. an alkaline borosilicate base with at least 0.7 to 1.5 mol of silica per mole of alkaline oxide contained in the pretreated radioactive waste, ii. an oxide stabilising and protecting the matrix from corrosion in a quantity from 0.05 mol to 0.1 mol per mole of alkaline oxide contained in the pretreated radioactive waste, iii. an additional mixture of oxides in a quantity from 0.8 mol to 2 mol per mole of alkaline oxide contained in the pretreated radioactive waste, the mixture including at least one metal oxide and participating in formation of the matrix and making it possible to obtain both easier melting and better water resistance, - a step for calcining and then melting (202, 402) of the mixture at a temperature below 1050°C to obtain a molten mass; - a step for cooling the molten mass to obtain (203, 403) a matrix for immobilisation of radioactive waste, the initial radioactive waste being an evaporator concentrate, the pretreatment step including a step for extraction of 70% to 90% of borax (Na 2 B 4 O 7) from the initial radioactive waste, the pretreated radioactive waste obtained having a molar ratio of boric anhydride (B 2 O 3) to alkaline oxides of between 0.15 and 1. - a step (201, 401) of pretreatment of initial radioactive waste (200, 400) in order to obtain pretreated radioactive waste; - a step (206, 406) of mixing pretreated radioactive waste with the addition of compounds capable of vitrifying or ceramifying the pretreated radioactive waste; such compounds include at least: i. an alkaline borosilicate base with at least 0.7 to 1.5 mol of silica per mole of alkaline oxide contained in the pretreated radioactive waste, ii. an oxide stabilising and protecting the matrix from corrosion in a quantity from 0.05 mol to 0.1 mol per mole of alkaline oxide contained in the pretreated radioactive waste, iii. an additional mixture of oxides in a quantity from 0.8 mol to 2 mol per mole of alkaline oxide contained in the pretreated radioactive waste, the mixture including at least one metal oxide and participating in formation of the matrix and making it possible to obtain both easier melting and better water resistance, i. an alkaline borosilicate base with at least 0.7 to 1.5 mol of silica per mole of alkaline oxide contained in the pretreated radioactive waste, ii. an oxide stabilising and protecting the matrix from corrosion in a quantity from 0.05 mol to 0.1 mol per mole of alkaline oxide contained in the pretreated radioactive waste, iii. an additional mixture of oxides in a quantity from 0.8 mol to 2 mol per mole of alkaline oxide contained in the pretreated radioactive waste, the mixture including at least one metal oxide and participating in formation of the matrix and making it possible to obtain both easier melting and better water resistance, - a step for calcining and then melting (202, 402) of the mixture at a temperature below 1050°C to obtain a molten mass; - a step for cooling the molten mass to obtain (203, 403) a matrix for immobilisation of radioactive waste,