Patent Number: 045267456
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS In FIGS. 1, 2, 3 and 4, the numeral 1 designates a fuel box which is attached to a sleeve-formed base 2. In use of the fuel assembly in a boiling water reactor, the exterior surfaces of the fuel box and the base face the space within the reactor but outside the fuel assembly. The fuel box 1, which is composed of four mutually equal sheet-metal elements 1', interconnected by means of four vertical strips 1", surrounds sixty-four fuel rods 3, twelve smaller water tubes 4 and one larger, central water tube 5. Each water tube forms a vertical channel extending along the fuel rods and conducting a water flow along but being separated from the fuel rods. The water tubes are mechanically connected to each other by means of a plurality of elongated linking members 6, which are attached with their ends to the strips 1". The fuel rods 3 rest with their lower ends on a bottom lattice device or element 7, which rests on two vertical supporting plates 8, which are welded to a hollow, cruciform water distributing member 9, which is provided with connection openings for the water tubes 4 and 5. The connection openings are constructed with annular supporting surfaces, against which the water tubes 4 and 5 rest at their lower end surfaces. The water distributing member 9 is provided with at least one (e.g. four) substantially radial tube 10, which opens out at the side surface of the fuel assembly and through which both the interior cavity of the member 9 and the water tubes 4 and 5 are hydraulically connected to space located radially outside the base 2. The base 2 of the fuel assembly has a circular, downwardly-facing inlet opening positioned below tubes 10, as illustrated. The inlet opening is surrounded by a substantially annular end surface 13. A guide member 14, which consists of a ring 14" and a plurality of rods 14' attached thereto, is arranged below the sleeve-formed base 2. The fuel assembly shown has a vertical center line and is intended to be supported, together with three similar fuel assemblies, by a common supporting plate intended for four fuel assemblies, said supporting plate being constructed with a conical supporting surface and a circular throttling opening for each fuel assembly. Below the radial tubes 10 and below the water distributing member 9, the wall of the base 2 is provided with at least one through-hole 12 which opens out at the side surface of the fuel assembly and opens inwardly directly into the interior of base 2, so that the inlet opening is hydraulically connected to the reactor space located radially outside the base. When the flow paths of the described fuel assembly are dimensioned in such a way that an optimum by-pass flow through the water tubes 4 and 5 is obtained at full reactor power, it cannot be avoided that a certain amount of boiling takes place in the water tubes 4 and 5 when the reactor power is reduced to a minimum by reducing the speed of the circulating pumps. To a certain extent such boiling gives rise to void formation at the upper part of the water tubes, which results in the pressure being considerably reduced in the water distributor 9, water thus flowing in through the channels 10 from the gaps located between the fuel assemblies. At a fully acceptable value of the void formation, the corresponding increase of the water flow through the radial channels 10 will be so great that a state of equilibrium will be created.