ES344973A1 - Nuclear Reactor Fuel Bundle - Google Patents

Abstract

A fuel bundle comprises a plurality of fuel rods arranged in a square array within a container of square section with the ends of the fuel rods supported by tie-plates. The lower tie-plate, as shown in Fig. 4, comprises a flat plate section 82 provided with receptacles 72, 721, 7211 for supporting the fuel rods and openings 74 to permit the passage of coolant, a first wall section 84 of square cross-section connected to the periphery of the plate section 82, and a second wall section 86 connected to the first wall section 84 and tapering from a square cross-section to a circular cross-section, the maximum angle of taper of the second wall section 86 being in the range 40 to 50 degrees. The tapered wall section 86 includes a conical seat 78 which, when the fuel bundle is mounted in a reactor core, rests on a support socket, and the wall section 86 merges into a cylindrical support section 90 from which extends a nose member 80 consisting of three curved sections 102 spaced 120 degrees apart and extending from the lower edge of the support section 90 to a common connection at their lowermost ends. The centre receptacle 721 has a square cross-section and receives a square support-pin located at the bottom of the centre fuel rod. The latter is provided with locking devices that are secured to spacers of the fuel bundle at appropriate axially-spaced positions. The centre fuel rod is thus prevented from rotating and the spacers remain fixed in position. A number of receptacles 7211 are provided with threads to receive threaded fuel rods so that the upper and lower tie plates may be mechanically interconnected. The container 62 housing the fuel bundle slides over the exterior surface of the square wall section 84 which is provided with shoulders 98 as an emergency stop for the container 62 in the event of tabs positioned across the upper corners of the container failing and the container sliding downwards. The length A represents the minimum overlapping length between the bottom of the container 62 and the wall section 84. The length B is the increase in overlap due to the fuel bundle being brought to the operating or hot condition. The degree of taper of the wall section 86 is critical and above a taper of 50 degrees there is fluid flow separation and void formation near the wall section 86. With a reduced taper, the overall height of the tie-plate becomes excessive. The use of the fuel bundle in a boiling water reactor is described, which reactor (Fig. 1, not shown) comprises a number of vertically extending control rod guide tubes, the upper ends of which are each provided with four sockets for supporting fuel bundles and a cruciform-shaped opening for a control rod. The water flowing from a supply chamber in the lower portion of the reactor pressure vessel is in two parallel streams: the first, consisting of about 90% of the total flow, passes through openings at the top of the control rod guide tubes and through the flow channels of the fuel bundles the second, consisting of the remaining 10% of the total flow, passes through annular openings between the upper ends of the control rod guide tubes and associated openings in a bottom grid plate and flows outside the flow channels of the fuel bundles and over the control rods. A shroud is mounted coaxially within the pressure vessel surrounding the reactor core to provide a downcomer annulus between the shroud and the vessel wall.