GB1184188A - Improvements in or relating to Nuclear Reactors - Google Patents

Abstract

1,184,188. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. 10 Oct., 1967 [21 Oct., 1966], No. 47423/66. Heading G6C. The levitation of a fuel stringer within a fuel channel of a nuclear reactor is resisted by the use of a coolant pressure difference. In the embodiment illustrated, a component 1 of a fuel stringer, e.g. a neutron scattering plug, has an extension 3 of reduced external size, a further component 4 sealing between the extension 3 and the wall 2 of the fuel channel. The component 4 has a pair of sliding seals 5 contacting the wall 2 and a pair of sliding seals 6 contacting the extension 3, and is held stationary with respect to the channel. As described in Specification 882,714, part of the fuel coolant flow, i.e. pressurized gas, is introduced at a pressure in excess of that prevailing elsewhere in the channel by means of passages 7 into the annulus between the component 1 and the channel wall 2 and flows downwards through the core at the bottom of which it joins other coolant flow and passes up through the interiors of the fuel elements of the stringer and the component 1. The pressurized gas from the passages 7 enters the space between the components 1 and 4, the component 1 acting as a plunger exerting a downward pressure on the sleeves of the fuel elements on which it rests. Levitation of the fuel elements is thus resisted. The component 4 carries a spider 8 through which extends a tie-rod 9 having an upper termination 10 which, after the spider is raised a short distance, abuts on the spider and lifts the tie-rod 9, this in turn raising the fuel elements. A connection 12 releasably connects a component 11 to the component 4. The component 11 is normally a spacing piece connecting the fuel and neutron plug parts of the stringer with parts having a gag valve and a shield and seal plug serving to close the respective refuelling access aperture in the reactor pressure vessel. The component 4 has a lost motion interconnection 18 with the component 1 consisting of abutments 19 and 20 adapted to cooperate after a certain degree of raising of component 4 relative to component 1 in excess of the abovementioned short distance. On raising the stringer, an upward pull on component 11 is transmitted to component 4 which when raised beyond the short distance lifts the fuel elements. The pressure resisting levitation continues to hold down component 1 until the seals 5 are clear of the channel. The abutments 19 and 20 are not brought into contact until after removal of the stringer from the reactor when they support the component 1 after the tie-rod 9 has been severed. The force resisting levitation may, if desired, be utilized only during refuelling by arranging components 1 and 4 so that they completely close together when the stringer is fully inserted with the weight of component 4 holding down component 1.