The invention relates to a fluidized bed reactor chamber made up of peripheral waterwalls (1, 1') constituted by tubes interconnected by fins and having a bottom portion (1A) lined with a layer of refractory material (3A), a top portion (1B), and a zone (1C) referred to as an intermediate zone situated between said bottom and top portions, at least two waterwalls being connected to each other along a substantially vertical edge, and said peripheral waterwalls in said intermediate zone (1C) being bent outwards angularly relative to the vertical plane defined by the top portion (1B) of each peripheral waterwall. According to the invention a horizontal fraction only of each of said peripheral walls is bent in this way, said fractions being disposed on either side of said edge.