Patent Number: 046997571
Section: claims

1. A fuel rod for use in a nuclear reactor, having a gas tight sheath, a stack of fuel pellets in said sheath, an end plug for said sheath and means for retaining said stack in abutment against said end plug during handling of said nuclear reactor fuel rod, wherein said means comprise a radially expandable element having a cross-sectional area in rest condition such as it frictionally engages an internal surface of said sheath; and temperature responsive means of a shape memory alloy operatively associated with said radially expandable element and having a transformation temperature above atmospheric temperature, said radially expandable element and temperature responsive element being constructed and arranged for said temperature responsive element to contract said radially expandable element out of frictional contact with said sheath said transformation temperature. 2. A fuel rod as claimed in claim 1, further comprising resilient means having a predetermined length at rest, located between said pellet stack and said radially expandable element. 3. A fuel rod as claimed in claim 2, wherein said resilient means comprises: a washer in contact with said stack; a first helical compression spring connected to said radially expandable element and said washer; and a second helical spring of shape memory alloy having a transformation temperature higher than the atmospheric temperature and lower than the transformation temperature of said temperature responsive means, said second spring being connected to said washer and to said radially expansable element whereby it exerts a drawing force on said washer above the transformation temperature of said second spring, said drawing force being higher than the compression force of the first spring. 4. A fuel rod as claimed in claim 1, wherein said radially expandable element consists of an annular element having an axis and at least one set of resilient contact fingers distributed about said axis for exerting said frictional force and wherein said temperature responsive means consists of a ring encircling said fingers. 5. A fuel rod as claimed in claim 1, wherein said radially expandable element consists of a split thimble having outwardly directed radial flanges and said temperature responsive means consist of an axially split ring located in a circular groove of said ring. 6. A nuclear fuel rod for use in upstanding condition, having an elongated sheath with an axis, a stack of fuel pellets in said sheath having a length lower than that of said sheath, an end plug for said sheath and means for retaining said stack in abutment against said end plug during handling of said nuclear reactor fuel rod, wherein said means comprise: a radially expandable thimble having a C-shaped cross-section and formed with a circumferential groove in the outer surface thereof, said groove being located between end portions and said thimble having a crosssectional area in rest condition such that said end portions frictionally engage an internal surface of said sheath when in said sheath;  and a ring of shape memory alloy having a transformation temperature above atmospheric temperature, located in said groove encircling said thimble and arranged for exerting on said thimble, when above said transformation temperature, a shirnking force sufficient for contracting said thimble out of frictional contact with said sheath. 7. A fuel rod as claimed in claim 1, wherein said radially expandable element consists of bellows having a rotational symetry about an axis of said rod and having outer folds dimensioned for frictional contact with said sheath. 8. A fuel rod as claimed in claim 7, wherein said temperature responsive means comprise helical spring means coaxial with said bellows and compressed between end surfaces of said bellows for exerting a force tending to axially expand said bellows. 9. A fuel rod as claimed in claim 1, wherein said radially expandable element consists of an axially split thimble and said temperature responsive element comprises coiled spring means of shape memory alloy provided with end portions anchored to said thimble in close proximity to said slit.