referring now to the figures of the drawing in detail and first , particularly , to fig1 thereof , an absorber rod 2 has an essentially cylindrical shape . at its lower and upper ends 4 and 6 it is conically shaped , i . e ., provided with a beveled edge 7 and 8 , respectively . the absorber rod 2 , that can be composed axially of a multiplicity of partial rods , is placed in a sheathing tube 3 that is indicated in the figure by a dash - and - dot line , by which it is surrounded in gas - sealed fashion . by “ lower end ” what is meant is the end of absorber rod 2 with it , in its installed state and in operation , is inserted , together with the sheathing tube 3 into a control rode guide tube of a burner . in a lower section 9 that adjoins this conical section 7 , the absorber rod 2 is formed with a multiplicity of recesses in the form of annular flutes or grooves 10 a . in the area of these recesses , the absorber rod 2 thus has a cross sectional surface perpendicular to its long axis that is markedly smaller than the cross sectional surface perpendicular to the long axis of the cylindrical sheathing tube 3 shown with dots and dashes in the figure . in one area 13 between the grooves 10 a or adjoining the grooves 10 a , the absorber rod 2 has a cylindrical shape and there has a diameter only slightly less than the diameter of the sheathing tube 3 , so that in this area 13 , only a small gap s , barely visible in the figure , to the sheathing tube 3 exists , on the order of magnitude of about 100 μm . in other words , only in a partial section of the circumferential surface of lower section 9 is the absorber rod 2 provided with recesses . in the exemplary embodiment , a depth d of the grooves is about 1 mm , and their width b is about 2 mm , so that with the seven grooves that are each at a distance of about 1 cm from each other , a free space results with a volume on the order of magnitude of about 270 mm 3 . into this free space , the swelling absorber rod 2 can extend without leading to a stretching of the sheathing tube 3 . in addition , it can be gleaned from fig1 a that the surface on which the absorber rod 2 in section 9 is in contact with the sheathing tube when installed , is only slightly reduced , only by about 10 - 20 % in the exemplary embodiment . instead of annular grooves 10 a , screwlike grooves 10 b can also be provided , as is shown in the figure by dashes . fig2 now shows a section from a control rod , in the sheathed tube 3 of which absorber rod 2 according to fig1 a , 1 b is shown . it can clearly be seen that between the absorber rod 2 and the inner surface of sheathing tube 3 , in its areas 13 adjoining the grooves , only a small gap exists , so that heat transport there is good . in the embodiment according to fig3 a and 3b , the recess is formed by a multiplicity of longitudinal grooves 10 c running in the axial direction , that extend in the depicted exemplary embodiment over the entire length of the absorber rod 2 , so that the absorber rod 2 is symmetrical , and the lower and upper ends 4 , 6 can be exchanged . it has been shown in practice that it suffices if the longitudinal grooves 10 c extend from the lower end 4 over a length / of about 100 - 300 mm , since it is only in this area that the neutron loading is very great and results in a pronounced swelling . between the longitudinal grooves 10 c are the areas 13 in which only a small gap exists between the absorber rod 2 and the sheathing tube 3 . in the embodiment form according to fig4 a and 4b , the conical areas or the beveled edges 7 , 8 are modified by a reduction in the bevel angle α to values between 2 ° and 30 ° as well as by an increase in the length h of beveled edges 7 , 8 , i . e ., the height of the conical area is altered to values greater than 1 mm . in this way , an increased free space is created into which the absorber material can swell . the bevel angle α is reduced and at the same time the height h of the conical area is increased . therefore , the free space produced by these beveled edges 7 , 8 can be increased while the front part a of absorber rod 2 remains the same . in this case also , it is sufficient to only modify one of the bevels 7 , 8 . if only one bevel is modified , for example bevel 7 , the lower end 4 of the absorber rod 2 is determined . in other words : the absorber rod can then be placed only in one direction in the sheathing tube of the control rod . additionally in the figure , at the lower end , a recess is made in the form of a central axial borehole 10 d , which likewise serves as a free space and can be implemented in addition to , or alternatively to , the measures explained above and below . typically such a borehole 10 d has a diameter d of about 3 mm and a depth t of about 50 - 100 mm . if such a borehole 10 d is made , care must be taken that the front part a , which simultaneously is the contact surface for absorber rod 2 on an interior surface of the sheathing tube , is not reduced by an appropriate configuration of the bevel 7 . the measure depicted in fig4 a , 4 b can be used also in combination with the grooves 10 a , 10 b running around as depicted in fig1 a , 1 b , or the longitudinal grooves 10 d depicted in fig3 a , 3 b . instead of the grooves , flutings or axial borehole shown , recesses can also be provided with other geometric forms , such as trough - shaped indentations or holes . the only thing that is primarily important is that additional free space results , into which the absorber rod can swell , and that they occupy at most a part of the circumferential surface of the lower section .