Thermal behaviour of Bituminized Waste Products
Résumé
In France, bitumen was chosen during the 60's as an efficient embedding material for low and medium activity nuclear waste (sludges), owing to its high chemical inertness, impermeability, and confining ability. In the scope of the nuclear safety process, the thermal behavior of these bituminized sludges should be investigated during their whole life: it is thus crucial to study the physicochemical and thermochemical phenomena involved when they are exposed to heat. Thermogravimetric analysis coupled to differential thermal analysis was used to assess kinetic parameters related to reactions that occur when these materials are exposed to non-isothermal heat conditions, in various atmospheres (nitrogen, air). Firstly, a simplified system, pure bitumen, was studied. The degree of complexity of the studied system was then increased, by taking into account salts present in significant quantities in the nuclear sludges, in particular sodium nitrate.
Under nitrogen atmosphere, pure bitumen shows a single thermal event, which is attributed to its pyrolysis. Under air, it undergoes three exothermal and one endothermal events that result from combustion and cracking reactions, respectively. Under nitrogen, bitumen embedding sodium nitrate shows an exothermic peak due to the combustion reaction of the bitumen by the nitrates. Activation energy for each event has been determined using several kinetic methods from non-isothermal heat conditions.