GB900821A - Forced flow once through steam generator associated with a gas-cooled nuclear reactor - Google Patents

Abstract

900,821. Forced-flow steam boilers. VEREINIGTE KESSELWERKE A.G. April 24, 1959, No. 14117/59. Class 123 (2). [Also in Group XL (a)] A forced-flow, once-through steam boiler 3 is disposed in a double-walled pressure vessel 10, 11 above a nuclear reactor 1 and is heated by the gas coolant from the reactor, the gases being circulated by external fans 7 which force the gases downwardly through an annular passage 2 and upwardly through the reactor and the tube bank of the generator. Dampers 16 are provided in the gas passage 2 so that some or all of the gases can by-pass the reactor. The space between the double-walls of the pressure vessel is filled with rammed concrete 12 and, where the gas circulating tubes 6, 8 and the water and steam tubes 27, 33 pass through the upper part of the vessel walls, with gravel which allows of expansion movements of the tubes, the gas tubes being fitted with expansion joints 14 whilst the water and steam tubes 27, 33 have expansion bends 15 in this region. The tube system of the generator consists of an external annular feedwater header 26, Fig. 2, divided into a number, say four, sections from each of which a number of parallel connected tubes 27 pass into the pressure vessel and are therein serially connected in a number of superposed " pancake" platens forming an economizer 28, an evaporating section 29 and a superheater 32. Between the evaporator and superheater all the tubes are connected to an external annular mixing header 30 divided into four sections each provided with means for removing salts-contaminated water whilst the superheated steam flows through the tubes 33 into a similarly divided external annular header 34 provided with superheat temperature controlling water sprays. In accordance with the invention, the "pancake" " platens are, as shown in Fig. 3a, formed of tubes bent to form identical, equally spaced involutes 17 extending from a central space which is occupied by a tube 35, capped at its lower end 36 to prevent gas short-circuiting, to the periphery of the generator which thus has a circular section. The involute form of the tubes ensures that they are equally spaced throughout each platen and therefore receive the same amount of heat from the upflowing gases. The platens are supported by tubes 31 conveying steam from the mixing header 30 to the superheater. In a modification, Fig. 3b, not shown, each platen consists of U-tubes, the arms of which are bent into parallel extending involutes, the involutes in adjacent platens being oppositely directed. If a leak should occur in a tube of any one of the four independent sections of the system, isolating valves for that section are closed, the reactor is shut down and the gas by-pass dampers 16 changed over to prevent gases carrying reactor residual heat into the boiler. The three undamaged sections are drained and the outlet header of the damaged section opened into a screened vessel. Radioactivity can then be detected in the damaged tube where it enters the header and both ends of this tube are then sealed off. The generator can then be put into operation again with the reduced heating surface.