Patent Number: 048266531
Section: summary

TECHNICAL FIELD The present invention relates to a fuel assembly with a lower lattice arrangement and a plurality of fuel rods supported by said arrangement, at least one fuel box which surrounds a corresponding group of fuel rods, a base with an inlet opening for reactor coolant and a circular downwardly-facing, substantially annular end surface, the fuel assembly having at least one vertical water channel for a water flow running along the fuel rods but being separated from said fuel rods. DISCUSSION OF PRIOR ART More particularly, the invention relates to a fuel assembly which is constructed in such a way that it is capable of being used, with advantage, in a boiling water reactor which is originally intended for fuel assemblies having no water channel of the kind mentioned above, without it being necessary to introduce any considerable change of the other components of the reactor. A problem, which will then be encountered, is to achieve the necessary by-pass flow through the central water channel(s) when the reactor power is reduced by reducing the circulating cooling flow. The problem cannot be solved by simply constructing the by-pass channel in question with greater hydraulic flow capacity, since this channel also must have a flow capacity which gives optimum water distribution at full reactor power. A fuel assembly according to the invention has the advantage of giving a sufficient by-pass flow at a greatly reduced reactor power and circulating cooling flow, without giving too great a by-pass flow at full power. The total water flow supplied to a fuel assembly is always subjected to a throttling at the inlet of the fuel assembly. In accordance with known theory, the energy losses appearing in connection with the throttling do not start to manifest themselves until at some distance downstream of the throttling opening, more particularly where the stream of water starts to widen out again. DISCLOSURE OF THE INVENTION The present invention is based on calculations showing that a by-pass flow through a vertical water channel in a fuel assembly, is reduced to a relatively small extent upon a substantial reduction of the circulating cooling flow, if the by-pass flow is supplied by the aid of a vertical supply tube, the lower end of which is below the level at which the throttled stream of water supplied to the fuel assembly has acquired an increased cross-section with only vertical stream lines. According to the invention, there is provided a fuel assembly having a vertical center line and comprising a lower lattice device and a plurality of fuel rods supported by said lattice device, and at least one fuel box which surrounds a corresponding group of fuel rods, a sleeve-like base with an inlet opening for reactor coolant and a circular, downwardly-facing, substantially annular end surface, the fuel assembly having at least one vertical water channel for a water flow running along the fuel rods and being separated therefrom. A supply tube with a downwardly-facing inlet opening is arranged in said base for the supply of water to the at least one water channel, the inlet opening of the supply tube being located below a horizontal plane, the height of which above the lowermost point of any other fuel assembly portions is equal to the inner diameter of said annular end surface. The fuel assembly is intended to be arranged in a reactor core in a conventional manner, with four fuel assemblies in each core module, water gaps between adjacent assemblies, and a control rod of cruciform cross-section in each module. In addition to the water flow flowing along the fuel rods and in contact with these, the reactor core is traversed by a first by-pass flow, which is located at the above-mentioned gaps formed between the fuel assemblies, and by a second by-pass flow, which is located at a plurality of vertical water channels of the above-mentioned kind.