Method for treating a surface with a treatment gel, and treatment gel

A gel for treating a radioactive surface consisting of a colloidal aqueous solution consisting:

• 5 to 25% by weight, based on the weight of a gel, of a mixture of pyrogenated silica and precipitated silica, the pyrogenated silica representing 8% by weight of the gel, the precipitated silica representing 0.5% to 1% by weight of the gel,
• 0.5 to 4 mol/l of an active treatment agent, wherein the active treatment agent is selected from a group consisting of acids, bases, and mixtures thereof,
• water, and
• optionally 0.05 to 1 mol/l of an oxidizing agent with a normal oxidation-reduction potential E0 larger than 1.4 V in a strong acid medium or of the reduced form of this oxidizing agent,
• wherein said gel forms a solid, dry residue having a particle size in the range from 0.1 to 3 mm in size.