Conditioning by cementation of a nuclear waste comprising magnesium in metal form

The invention relates to a method of conditioning, by embedding in a cementitious matrix, a nuclear waste comprising magnesium metal and at least one material selected from graphite and steels, which method comprises the following steps: embedding the waste using a cementitious paste obtained by mixing a composition comprising a powder of a binder selected from hydraulic cements, vitrified blast-furnace slags and mixtures thereof, and an aqueous mixing solution; then - leaving the cementitious paste to harden in order to form the cementitious matrix; and is characterized in that the aqueous mixing solution comprises at least 1 mol/L of a strong base. The invention also relates to a composition suitable for being used in this conditioning method and also to a conditioning package obtained by said method. Applications: conditioning of waste from UNGG (for "Uranium Naturel - Graphite - Gaz carbonique [natural uranium - grapite - carbon dioxide]) or MAGNOX (for Magnesium Non OXidizing) nuclear reactors with a view to the storage thereof.