PURPOSE: To facilitate insertion into a jacket tube by fitting the base end parts of a couple of grip members to the side wall upper end parts of a U-shaped leaf spring.

CONSTITUTION: The base end parts 2a and 2a of the grip members 2 and 2 are fitted rotatably to the upper end parts of the side walls 1b and 1b of the leaf spring 1. A metal ring 3 is fitted onto the side wall recessed parts 1c and 1c of the leaf spring to energize the side walls 1b and 1b in spreading directions and closes the side walls 1b and 1b so as to increase the interval of 1d. At high temperature in nuclear reactor operation, the metal ring 3 which has large thermal expansion expands thermally to release the leaf spring side walls 1b and 1b to open, and consequently friction is eliminated between the grip members 2 and 2 and the internal surface of the jacket tube to allow fuel pellets to move.