A nuclear power plant comprising: a nuclear steam supply system; a containment vessel for hermetically housing the nuclear steam supply system; a reservoir comprising water, housed within or within proximity of the containment vessel; spent nuclear fuel submerged within the reservoir and cooled by the water; and a thermosiphon extending from within the reservoir to an exterior of the containment vessel for transferring heat in the reservoir to air outside of the containment vessel, characterized in that said thermosiphon comprises a first and second heat exchanger (52, 54) with the second heat exchanger at a higher elevation than the first heat exchanger, wherein the first heat exchanger is at least partially immersed within the reservoir which in use places the water in the reservoir on the first side of the first heat exchanger in thermal communication with a working fluid on the second side of the first heat exchanger, the second side of the first heat exchanger is in fluid communication with the first side of the second heat exchanger through a closed loop in which the working fluid circulates, which in use places the working fluid on the first side of the second heat exchanger in thermal communication with ambient air outside the containment vessel on the second side of the second heat exchanger, and in that the containment vessel comprises a steel shell extending substantially around a containment and has substantially vertical walls that extend from a lower portion of the containment to an upper portion of the containment, and in that the nuclear power plant further includes an air baffle which extends substantially around and is spaced from the vertical walls of the containment vessel with an air inlet at a lower elevation of the air baffle and an air outlet at an upper elevation of the air baffle, the second heat exchanger being supported within an annulus between the vertical wall of the containment vessel and the air baffle. a nuclear steam supply system; a containment vessel for hermetically housing the nuclear steam supply system; a reservoir comprising water, housed within or within proximity of the containment vessel; spent nuclear fuel submerged within the reservoir and cooled by the water; and a thermosiphon extending from within the reservoir to an exterior of the containment vessel for transferring heat in the reservoir to air outside of the containment vessel, characterized in that said thermosiphon comprises a first and second heat exchanger (52, 54) with the second heat exchanger at a higher elevation than the first heat exchanger, wherein the first heat exchanger is at least partially immersed within the reservoir which in use places the water in the reservoir on the first side of the first heat exchanger in thermal communication with a working fluid on the second side of the first heat exchanger, the second side of the first heat exchanger is in fluid communication with the first side of the second heat exchanger through a closed loop in which the working fluid circulates, which in use places the working fluid on the first side of the second heat exchanger in thermal communication with ambient air outside the containment vessel on the second side of the second heat exchanger, and in that the containment vessel comprises a steel shell extending substantially around a containment and has substantially vertical walls that extend from a lower portion of the containment to an upper portion of the containment, and in that the nuclear power plant further includes an air baffle which extends substantially around and is spaced from the vertical walls of the containment vessel with an air inlet at a lower elevation of the air baffle and an air outlet at an upper elevation of the air baffle, the second heat exchanger being supported within an annulus between the vertical wall of the containment vessel and the air baffle.