Patent Number: 
Section: claims

1. An absorber rod comprising: a stainless steel cladding;  a top plug and a bottom plug of stainless steel closing said cladding;  a column of neutron absorber pellets in said cladding; and  an end bar of hafnium fixed to the bottom plug by a purely mechanical connection. 2. A rod according to  claim 1 , wherein said neutron absorber pellets are boron carbide pellets. claim 1 3. A rod according to  claim 1 , wherein the mechanical connection is a connection by cold isostatic compression crimping. claim 1 4. A rod according to  claim 3 , wherein the bottom plug has a top portion engaged in the cladding and a flange bearing against the bottom edge of the cladding and welded to the cladding, and an extension in which circumferential grooves are machined, and the bar is terminated by a thin-walled tubular zone constituting a skirt deformed into the grooves of the plug extension. claim 3 5. A rod according to  claim 1 , wherein the bar has an initial protective oxide layer at least 5 xcexcm thick and has oxygen diffusion over a depth of at least 25 xcexcm. claim 1 6. A rod according to  claim 5 , wherein the protective layer is obtained by controlled oxidation of the hafnium bar, for 3 to 12 hours, at a temperature in the range 820xc2x0 C. to 950xc2x0 C., in oxygen diluted to a concentration of 1% to 50% in an inert gas, under low pressure. claim 5 7. A rod according to  claim 5 , wherein the protective layer is obtained by an oxidation and diffusion process in which the duration of oxidation lies in the range 0.1% to 10% of the total treatment time. claim 5 8. A rod according to  claim 1 , wherein the cladding and the plugs are made of austenitic stainless steel. claim 1 9. A rod according to  claim 1 , wherein an outside surface of the cladding, of the top plug and of the bottom plug is nitrided. claim 1 10. A rod according to  claim 2 , wherein the bar has a length in the range of 25% to 35% of a length of the column of boron carbide pellets. claim 2 11. An absorber rod according to any one of claims  1 ,  2 ,  3 ,  10 , in a nuclear reactor. 12. A control cluster for a nuclear reactor, said control cluster comprising a spider for connection to a control mechanism and a plurality of absorber rods, said absorber rods being connected by top ends to the spider, each absorber rod comprising: a stainless steel cladding;  a top plug and a bottom plug of stainless steel closing said cladding;  a column of neutron absorber pellets in said cladding; and  an end bar of hafnium fixed to the bottom plug by a purely mechanical connection, the end bars of hafnium being separate one from the other. 13. The control cluster of  claim 12 , wherein the neutron absorber pellets are boron carbide pellets. claim 12 14. The control cluster of  claim 12 , wherein the mechanical connection is a screwed connection. claim 12 15. The control cluster of  claim 12 , wherein the mechanical connection is a connection by cold isostatic compression crimping. claim 12 16. The control cluster of  claim 15 , wherein the bottom plug has a top portion engaged in the cladding and a flange bearing against a bottom edge of the cladding and welded to the cladding, and an extension in which circumferential grooves are machined, and the bar is terminated by a thin-walled tubular zone constituting a skirt deformed into the grooves of the plug extension. claim 15 17. The control cluster of  claim 12 , wherein the bar has an initial protective oxide layer at least 5 xcexcm thick and has oxygen diffusion over a depth of at least 25 xcexcm. claim 12 18. The control cluster of  claim 17 , wherein the protective layer is obtained by controlled oxidation of the hafnium bar, for 3 to 12 hours, at a temperature in the range 820xc2x0 C. to 950xc2x0 C., in oxygen diluted to a concentration of 1% to 50% in an inert gas, under low pressure. claim 17 19. The control cluster of  claim 17 , wherein the protective layer is obtained by an oxidation and diffusion process in which the duration of oxidation lies in the range 0.1% to 10% of the total treatment time. claim 17 20. The control cluster of  claim 12 , wherein the cladding and the plugs are made of austenitic stainless steel. claim 12 21. The control cluster of  claim 12 , wherein an outside surface of the cladding, of the top plug and of the bottom plug is nitrided. claim 12 22. The control cluster of  claim 13 , wherein the bar has a length in the range 25% to 35% of a length of the column of boron carbide pellets. claim 13