A sealing device for a power station pump, said device comprising: a pump housing comprising first and second ducts (15, 16) for the passage of a fluid; a shaft; a first passage for the fluid defined between the rotary shaft and the pump housing; a mechanical packing mounted between the shaft and the pump housing and comprising friction elements for the rubbing-together of a rotating part and of a stationary part, said device being configured for having two states, a shut down first state in which no fluid circulates through said device and an operating second state in which the fluid flows in a flow circuit passing via: the first passage to supply the circuit with fluid, a second passage comprised between the pump housing and the packing and communicating with said friction elements (71, 72), the second passage then forming a means of cooling the friction elements (71, 72) and a thermal barrier, the first duct to supply the second passage with cooled fluid, the second duct to remove the hot fluid from the second passage, wherein the second passage comprises a first chamber, adjacent to the friction elements formed by the rotating part and the stationary part, extending radially between the packing, and the pump housing and axially between a cover and a fourth passage, the first chamber defining in the operating state a first thermal barrier; a second chamber comprised between the pump housing on the one hand and the packing on the other defining, in the operating state, a second thermal barrier, and a gap that runs longitudinally between the shaft and the pump housing, said gap running circumferentially and defining, in the operating state, a third thermal barrier, wherein the second chamber is positioned between the first chamber and the gap, characterized in that the second chamber is bounded: radially, by the shaft and a bore formed in the pump housing, axially, by the packing and the pump housing, and in that the sealing device is configured so that in operation the cooling fluid flows from the first duct to the first chamber, then through the second chamber and then through the gap. a pump housing comprising first and second ducts (15, 16) for the passage of a fluid; a shaft; a first passage for the fluid defined between the rotary shaft and the pump housing; a mechanical packing mounted between the shaft and the pump housing and comprising friction elements for the rubbing-together of a rotating part and of a stationary part, said device being configured for having two states, a shut down first state in which no fluid circulates through said device and an operating second state in which the fluid flows in a flow circuit passing via: the first passage to supply the circuit with fluid, a second passage comprised between the pump housing and the packing and communicating with said friction elements (71, 72), the second passage then forming a means of cooling the friction elements (71, 72) and a thermal barrier, the first duct to supply the second passage with cooled fluid, the second duct to remove the hot fluid from the second passage, wherein the second passage comprises a first chamber, adjacent to the friction elements formed by the rotating part and the stationary part, extending radially between the packing, and the pump housing and axially between a cover and a fourth passage, the first chamber defining in the operating state a first thermal barrier; a second chamber comprised between the pump housing on the one hand and the packing on the other defining, in the operating state, a second thermal barrier, and a gap that runs longitudinally between the shaft and the pump housing, said gap running circumferentially and defining, in the operating state, a third thermal barrier, wherein the second chamber is positioned between the first chamber and the gap, characterized in that the second chamber is bounded: radially, by the shaft and a bore formed in the pump housing, axially, by the packing and the pump housing, and in that the sealing device is configured so that in operation the cooling fluid flows from the first duct to the first chamber, then through the second chamber and then through the gap. the first passage to supply the circuit with fluid, a second passage comprised between the pump housing and the packing and communicating with said friction elements (71, 72), the second passage then forming a means of cooling the friction elements (71, 72) and a thermal barrier, the first duct to supply the second passage with cooled fluid, the second duct to remove the hot fluid from the second passage, wherein the second passage comprises a first chamber, adjacent to the friction elements formed by the rotating part and the stationary part, extending radially between the packing, and the pump housing and axially between a cover and a fourth passage, the first chamber defining in the operating state a first thermal barrier; a second chamber comprised between the pump housing on the one hand and the packing on the other defining, in the operating state, a second thermal barrier, and a gap that runs longitudinally between the shaft and the pump housing, said gap running circumferentially and defining, in the operating state, a third thermal barrier, wherein the second chamber is positioned between the first chamber and the gap, characterized in that the second chamber is bounded: radially, by the shaft and a bore formed in the pump housing, axially, by the packing and the pump housing, and in that the sealing device is configured so that in operation the cooling fluid flows from the first duct to the first chamber, then through the second chamber and then through the gap.