[en] The design of fuel elements for the pressurised heavy water reactors being built in India is based on thin zircaloy cladding of collapsible type dictated by the use of natural uranium dioxide fuel material. In order to establish reliable design criteria for satisfactory performance during the life time, a thorough understanding of the behaviour of such fuel elements is necessary. In this paper, the following problems related to structural stability associated with the use of thin-walled cladding are examined. (i) Longitudinal Ridge Formation: The dominant collapsed mode for the cladding tube is found to be an oval with a single lobe in the transverse section, the shape depending on the diametral clearance between the pellet and the sheath. (ii) Circumferential Ridge Formation: Circumferential ridging at pellet interfaces commonly observed in irradiated fuel, have been analysed on the basis of hourglass shape by Veeder, AECL-2660 (1967). In the present model, however, it is predicted that circumferential ridging could form even in the absence of a temperature gradient, as is observed during out-of-pile autoclave tests. (iii) Collapse into Axial Gaps: Collapse into axial gaps between adjacent pellets or between pellets and end-plugs is also analysed. Calculations by classical formulae indicate that the critical pressure is overestimated by a factor of about 2.5. (Auth.)