Patent Number: 039649660
Section: description

DESCRIPTION OF THE INVENTION The accompanying drawing shows only so much of a liquid-metalcooled fast breeder reactor as is necessary to illustrate the setting of the present invention, the complete reactor being disclosed in U.S. Patent application Ser. No. 503,149, filed Sept. 4, 1974. The reactor includes a generally cylindrical pressure vessel 10 closed at the bottom by a bell 11 which bounds an inlet plenum 12. The vessel 10 has a plurality of inlet nozzles 13 above the bell 11 through which a heat-exchange fluid such as liquid sodium is supplied to the plenum 12 under pressure. The reactor core (not shown) is surrounded by a core barrel 14 and rests on support plate 15 which is supported from vessel 10 by conical skirt 16. A core catcher 17 according to the present invention of diameter greater than the reactor core is disposed in the pressure vessel 10 within inlet plenum 12 below the reactor core. Core catcher 17 includes a horizontal, circular baffle plate 18 having a plurality of openings 19 distributed in concentric circles about a central opening. Vertical heat transfer tubes 20 (4-inch, Type 304 stainless steel, schedule 5 pipe) are mounted in each of said openings by roll forming and welding. It will be noted that the flared end 21 of the tubes extends a short distance below the bottom of the baffle plate and that roll 21A is above the top of the baffle plate. The double attachment gives added assurance that a tube will never break loose during operation. Heat transfer tubes 20 are each drilled with three 2-inch diameter flow holes 22 near the end thereof and butt welded to an end cap 23 to close off the end. Baffle plate 18 is welded at its periphery to an imperforate, cylindrical baffle 24 to provide edge support for the plate. Core catcher 17 is supported from core support plate 15 by six uniformly spaced hanger rods 25, the lower end of each of which is welded to one of six radial beams 26 joined at the center by full-penetration welding to a hub 27. Radial beams 26 are intermittently welded to the bottom surface of baffle plate 18, channels 28 in the top of the beams accommodating the flared ends 21 of the vertical tubes. The end of radial beams 26 and the hub 27 are also depressed sufficiently to accommodate the flared end 21 of a central heat transfer tube 20. Radial beams 26 provide support for baffle plate 18 in bending. Hanger rods 25 are attached to core support plate 15 by means of retainers 29 such that up and down loads can be transmitted. Hanger rods 25 also provide lateral support for the core catcher by transmitting lateral loads by shear into the core support plate. Radial motions that may develop in the core catcher during transient phases of operation are allowed to occur by bearings 30 having slots 31 therein which are welded to outwardly extending lip 32 on cylindrical baffle 24 within notches 33 therein. By allowing the relative motion to occur between the core catcher and the hanger rod, thermal stresses are minimized. Important features of this invention follow: a. Containment volume can handle a complete core, even if the debris falls by a single stream through the core support plate. PA1 b. Adequate cooling of the heat-generating debris provided by natural insulation with no forced flow required. PA1 c. No device external to the reactor vessel is required. PA1 d. No attachments or modifications to the reactor vessel are required. PA1 c. Fabrication can be accomplished using conventional techniques. PA1 f. The core catcher provides for the accommodation of thermal expansion during transients. PA1 g. Criticality of debris is prevented by spatial separation. PA1 h. A screening effect is provided by the baffle assembly to eliminate possible flow blockage near the core inlet. In the extremely remote chance that an accident occurs which causes all or a part of the fuel to become molten, the fuel will cascade down through the core support plate and onto the core catcher between the heat-exchange tubes of the present invention wherein sodium continuing to flow through heat-exchange tubes by natural convection will cool the molten fuel and the heat-exchange tubes interspersed in the mass of molten fuel will eliminate any possibility that the fuel can attain a critical mass.