Abstract: A waste incinerator system, particularly suitable for bulk waste contaminated with radioactivity, is provided with a primary, upright combustion chamber with a removable, frusto-conical bottom section. The chamber is of the type having a double shell with cooling air passage therethrough. The cooling air is introduced at two different, vertically spaced levels, whereby the temperature control of the double shell can be effected by selectively controlling the volume of air incoming at the two levels. Only the bottom, frusto-conical removable portion of the combustion chamber is provided with refractory lining, the remainder of the chamber having a heat resistant steel inside wall, whereby the build-up of radioactivity levels within the chamber is reduced. The refractory of the frusto-conical bottom portion of the chamber is replaceable.