A tank for a liquid-filled shell transformer or shell reactor, comprising: - a lower tank portion comprising a bottom plate and lower sidewalls that are substantially vertical and end in a first horizontal flange located at or near an upper end of the lower sidewalls, - an upper tank portion, wherein the upper tank portion comprises an inner tank portion with inner sidewalls that are substantially vertical and end in a second horizontal flange at a lower end of the inner sidewalls, and an outer tank portion with outer sidewalls that are substantially vertical and end in a third horizontal flange located at a lower end of the outer sidewalls, wherein the outer sidewalls are arranged radially outwardly with respect to the inner sidewalls such that the outer tank portion surrounds the inner tank portion, - a short circuit beam mounted inside an upper inner portion of the inner sidewalls of the inner tank portion and joined to the inner portion of the inner sidewalls to form an integral structure, such that the inner sidewalls of the inner tank portion substantially maintain their shape or suffer only a small deformation in case of a sudden rise in the internal pressure generated by an internal arc fault, and - a tank cover configured to be joined to an upper end of the outer sidewalls of the outer tank portion, wherein - the lower tank portion and the inner tank portion are joined together along a substantially horizontal first perimetric joining line formed by the first horizontal flange and the second horizontal flange, - the lower tank portion and the inner tank portion define an internal space for housing an active part of the shell transformer or shell reactor and an insulating liquid, - a space is formed between the inner sidewalls and the outer sidewalls, wherein the space is in fluid communication with the internal space for housing the active part of the shell transformer or shell reactor and insulating liquid, and - the outer tank portion and the inner tank portion are joined together along a substantially horizontal second perimetric joining line formed by the second horizontal flange and the third horizontal flange, such that the outer sidewalls of the outer tank portion and of the attached cover can bulge outwards in case of a sudden rise in the internal pressure generated by an internal arc fault. - a lower tank portion comprising a bottom plate and lower sidewalls that are substantially vertical and end in a first horizontal flange located at or near an upper end of the lower sidewalls, - an upper tank portion, wherein the upper tank portion comprises an inner tank portion with inner sidewalls that are substantially vertical and end in a second horizontal flange at a lower end of the inner sidewalls, and an outer tank portion with outer sidewalls that are substantially vertical and end in a third horizontal flange located at a lower end of the outer sidewalls, wherein the outer sidewalls are arranged radially outwardly with respect to the inner sidewalls such that the outer tank portion surrounds the inner tank portion, - a short circuit beam mounted inside an upper inner portion of the inner sidewalls of the inner tank portion and joined to the inner portion of the inner sidewalls to form an integral structure, such that the inner sidewalls of the inner tank portion substantially maintain their shape or suffer only a small deformation in case of a sudden rise in the internal pressure generated by an internal arc fault, and - a tank cover configured to be joined to an upper end of the outer sidewalls of the outer tank portion, wherein - the lower tank portion and the inner tank portion are joined together along a substantially horizontal first perimetric joining line formed by the first horizontal flange and the second horizontal flange, - the lower tank portion and the inner tank portion define an internal space for housing an active part of the shell transformer or shell reactor and an insulating liquid, - a space is formed between the inner sidewalls and the outer sidewalls, wherein the space is in fluid communication with the internal space for housing the active part of the shell transformer or shell reactor and insulating liquid, and - the outer tank portion and the inner tank portion are joined together along a substantially horizontal second perimetric joining line formed by the second horizontal flange and the third horizontal flange, such that the outer sidewalls of the outer tank portion and of the attached cover can bulge outwards in case of a sudden rise in the internal pressure generated by an internal arc fault.