for a more detailed understanding of the invention , attention is invited to fig1 . as illustrated , a spider 10 , formed from stainless steel , or the like , is provided with a centrally disposed , generally cylindrical hub 11 that has a longitudinal axis 12 . the spider 10 has a number of spider arms that extend radially from the hub 11 in a plane that is perpendicular to the axis 12 . for purposes of simplified description , only spider arms 13 , 14 , 15 , 16 , 17 , 20 and 21 which are viewed in the plane of of fig1 of the drawing are shown . a group of burnable poison rods 22 , 23 , 24 , 25 , 26 , 27 and 30 are attached to their respective spider arms . all of these burnable poison rods are long , slender tubes in which the longitudinal axis of each of these tubes is parallel with the longitudinal axis 12 of the hub 11 . as illustrated in more detail with respect to the burnable poison rod 30 , a hollow tube 31 forms the outer surface of the rod . on longitudinal end of the tube 31 is sealed by means of a solid plug 32 that is welded or otherwise secured in the open tube end . within the tube 31 and bearing against the plug 32 is a stack of short cylindrical burnable poison pellets 33 . the individual cylindrical pellets in the stack 33 are pressed together in a longitudinal direction by means of a spring 34 which is lodged between the last pellet in the stack 33 and a plug 35 that is secured in the other open end of the hollow tube 31 . turning now to fig3 it can be seen that the plug 35 is provided with a chamfered end 36 that provides a lead in for a cylindrical portion 37 . the inside diameter of the hollow tube 31 matches the outside diameter of the cylindrical portion to an extent that a snug fit is achieved . as shown in the drawing , the cylindrical portion , 37 terminates in a radially protruding shoulder 40 that engages the transverse end of the hollow tube 31 to which it is welded . in the illustrative embodiment of the invention , the diameter of the hollow tube 31 in order to establish a flush relationship between an exposed cylindrical portion 41 of the plug 35 that extends beyond the transverse end of the tube 31 and the outer surface of the tube . a transverse flange 42 that is spaced longitudinally from the shoulder 40 terminates the exposed cylindrical portion 41 of the plug 35 . a stem 43 protrudes in a longitudinal direction from the central portion of the transverse flange 42 . most of the shank that forms the stem 43 is formed from solid metal stock . a longitudinal bore 44 is formed , however , in the stem 43 for a depth that extends from the free end of the stem through a distance that is about half of the stem length . a portion of a stem 4 as manufactured , and before full installation on a spider arm 46 , is shown in fig2 . thus the stem 42 has a generally cylindrical exterior configuration . bore 47 , that extends from the free end of the stem 4 to about half its length , also is of a generally cylindrical shape with the exception of the bottom of the bore which is formed into the shape of a shallow cone . it will be recalled that the stem 45 is received in the spider arm 46 . to accomplish this end , the spider arm 46 is provided with a generally cylindrical passageway 50 that has a longitudinal axis 51 which is essentially coincident with the longitudinal axis of the stem 45 and the burnable poison rod ( not shown in fig2 ). in accordance with a characteristic feature of the invention , an enlarged recess 52 is formed within the passageway 50 . note in this respect , that the recess 52 overlaps only a portion of the bore 47 in the stem 45 . a truncated conical surface 53 affords a transition from the recess 52 to a bore 54 which forms the balance of the passageway 50 . as shown in fig2 the bore 54 has a slightly larger diameter than the outside diameter of the stem 45 that is lodged in the passageway 50 . in operation , the stem 45 is inserted into the appropriate passageway 50 . the stem is mechanically deformed through hydroforming , mechanical flaring or other suitable means , to match the contour of the truncated conical surface 53 and the enlarged recess 52 and to provide a designed &# 34 ; weak point &# 34 ; for poison rod removal , as illustrated in fig3 . in this manner , the burnable poison rod is secured to the associated spider arm without making use of costly and difficult to handle weld fittings , and the like . it will be recalled , moreover , that different metals often are used for the spider arm and for the burnable poison rod . the truncated conical surface 55 , however , provides a tapered surface that alleviates the effect of the different coefficients of thermal expansion . typically , as the temperature increases , the exemplary stainless steel spider arm expands in an axial direction more than the poison rod end plug 35 ( usually zircalloy ). in this manner , the axial stresses that otherwise would have been generated are alleviated because of the radical loosening of the parts . thus , because the spider arm also expands radially more than the plug 35 , the conical interface between the adjacent deformed portion of the stem 43 and the conical surface 53 shifts axially downward . the two parts remain in intimate contact radially and axially , but the strain in the stem 45 has been materially reduced relative to the stresses that otherwise would have been generated . thus , although a tight joint is maintained throughout the entire range of reactor operating temperatures , the structure nevertheless provides a predetermined design &# 34 ; weak point &# 34 ; to facilitate removal of the burnable poison rod as described subsequently in more complete detail . a further salient feature of the invention is shown in fig4 . as illustrated , a plug 56 seals an end of a burnable poison rod 57 . the plug 56 has a stem 60 , the longitudinal axis of which is coincident with longitudinal axis 61 of the burnable poison rod 57 . a bore 62 extends longitudinally through about half the length of the stem 60 . as shown in the drawing , this bore has been expanded or upset through about half of its length to produce an enlarged cavity that enables the outer diameter of the adjacent portion of the stem 60 to bear against an enlarged recess 64 in a spider arm 65 . the bore 62 has a truncated conical taper 63 that also provides a transition to a smaller diameter bore portion 66 . the smaller diameter bore portion 66 , moreover , terminates in a cone 67 . in accordance with a salient feature of the invention , a mandrel 70 is pressed into the bore 62 in the direction of arrow 71 . the mandrel 70 , preferably formed in the shape of a slender , cylindrical metal shaft , has a diameter that is slightly smaller than the diameter of the smaller diameter bore portion 66 . the longitudinal axis of the mandrel 70 is generally coincident with the longitudinal axis 61 of the burnable poison rod 57 . to remove the burnable poison rod 57 from the spider arm 65 in a manner that will avoid undesirable shearing or sawing procedures it is only necessary to press the mandrel 70 in the direction of the arrow 71 with force sufficient to extrude the stem 60 through small diameter recess 72 , thereby freeing the stem 60 from the spider arm 65 . naturally although not shown in the drawing , a group of mandrels can be ganged together on a plate . these mandrels , aligned with and inserted into corresponding bores on respective burnable poison rods mounted in a spider , can press all of the rods out of the spider in one operation . in this way , a potentially hazardous , expensive and tedious process of shearing or sawing each burnable poison rod from the spider is avoided .