The invention relates to a device for sealing a pump of a nuclear power station. Said includes: a mechanical packing and a fluid header. The header includes: a first plurality of surfaces that cooperate with said mechanical packing; a second plurality of surfaces that are designed to cooperate with a pump housing; and a plurality of ducts. The plurality of ducts form, in an operating state, a first fluid circuit that constitutes a thermal barrier between the first plurality of surfaces and the second plurality of surfaces, and a second fluid circuit that supplies fluid to said mechanical packing in order to cool it.