Abstract: A decomposition vessel comprises a rigid bucket of a plastic material resistant to chemical attack, totally enclosed in a supporting metal vessel which consists of a cylindrical supporting member and of a flat cap adapted to be screwed onto the supporting member so as to close the vessel tightly. Sealing means are provided between the opposed flat faces on the cap and the supporting member in the form of a sealing disc adjacent the flat cap and an outwardly exending flange integral with the rigid bucket in contact with the disc and with an upper planar face of the supporting member.