Patent GB-1103248-A (GB)
Title: Nuclear reactor with fuel element cooled by boiling water and steam
Inventor: MARGEN PETER HEINRICH ERWIN
IPC: G21C15/06
Filed: 1965-07-22
Abstract: 1,103,248. Fuel elements. ATOMENERGI A.B. 22 July, 1965 [24 July, 1964; 7 Dec., 1964], No. 31314/65. Heading G6C. A fuel element comprises a body of fissile fuel having at least two opposed heat-exchanging surfaces, one of the surfaces being cooled by water which is heated to boiling by the fuel to produce steam and the other of the surfaces being cooled by the steam so produced, and a heat-insulating layer on at least part of one of the surfaces for directing the major part of the heat produced behind that part of the surface towards a corresponding part of the opposed heat-exchanging surface where the efficiency of cooling is greater. In a first embodiment (Fig. 1), three tubular fuel bodies la, 1b, 1c are enclosed between an outer protective coating 2 and an inner protective coating 3, the coatings being of zirconium, for example, and the lower portion 15 of the fuel element is supported on a seat 16 in the bottom 4 of the reactor vessel. The top fuel body la is attached to a hollow head 7, 17 containing canals 6 opening on its cylindrical outer wall. The top portion 17 of the head has a flange-like portion 33 which can be seized by a lifting tool when the fuel element is to be removed from the reactor. The pipe 25 leading down from the seat 16 is then closed by a valve 26 and pressurized water supplied by a pipe 24 through a duct 23 to a groove 22 within the seat 16, resulting in the fuel element being lifted from the seat 16. The head 7, 17 is slidable in a tube 31 having a series of lower openings 21, and a series of upper openings 20 registering with the openings of the canals 6 in the head. The tube 31 is mounted between two plates 29, 32 defining a space for steam and water. The bottom of the tube 31 is fastened to the top of a tube 8 extending between the plate 29 and a lower plate 30, water acting as moderator filling the space 27 between these two plates. Feed water is supplied below the plate 30, flowing upward through the space 28 between the fuel element and the tube 8 while boiling. A mixture of steam and water flows out through the openings 21, the steam flowing through the openings 20 and canals 6 down through the fuel element while being superheated. The outer surface of the fuel body la and the inner surface of the fuel body 1c have vertical gas-filled grooves between the fuel and the coatings 2, 3, respectively. As a result, a comparatively large part of the heat produced in the fuel body 1c is directed outwardly to heat the water in the boiler space 28 while a comparatively large part of the heat produced in the fuel body la is directed inwardly to superheat the steam. This is consistent with the natural cooling capacity, as the water has its highest cooling capacity at the bottom of the fuel element whereas the steam has its highest cooling capacity at the top of the fuel element. In a second embodiment (Fig. 5, not shown), a fuel element comprises four coaxial tubes of fissile material, boiling water flowing upward between the outer tube and a coaxial cylinder defining the fuel channel, between the two intermediate tubes, and inside the inner tube. The steam produced flows down between the two outermost tubes and between the two innermost tubes while being superheated. This fuel element is designed to operate under such conditions that the boiling water has a higher cooling capacity than the steam being superheated. Consequently, the steam-cooled surfaces of the fuel tubes are each provided with a heat-insulating layer whereas the water-cooled surfaces merely have a protective coating. In a third embodiment (Figs. 6 and 7, not shown), the fuel element comprises a cylindrical body of fissile material through which seven holes extend lengthwise. All surfaces of the body are covered with a protective coating. The upper portion of the outer cylindrical surface of the body has a heat-insulating layer formed by a plurality, of grooves between the fuel and the coating, similar layers being provided on the lower portions of the walls of the holes. In operation, boiling water flows up between the fuel element and a surrounding flow tube, the steam produced flowing down through the holes while being superheated.