Patent Number: 053234340
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fuel assembly for a nuclear reactor of boiling water type containing a plurality of vertical fuel rods, which are arranged between a bottom-tie tie plate and a top-tie plate in a surrounding vertical casing part and which extend through a number of spacers which are arranged in spaced relationship in the vertical direction and which together with the bottom-tie plate and the top-tie plate retain the fuel rods in a spaced relationship in a lateral direction. Each fuel rod in the fuel assembly comprises a cladding tube containing a nuclear fuel material and the fuel assembly is formed with an inlet for water for conducting water in through the bottom-tie plate, through the space between the fuel rods in the vertical casing part, and out through the top-tie plate. In the operation of a nuclear reactor with a core containing several fuel assemblies of the above-mentioned kind, there may be a risk of so-called dryout occurring, that is to say the disappearance or breaking down in limited regions of the water film which is present on the surface of each fuel rod, which locally leads to a considerably deteriorated heat transfer between the fuel rod and the water conducted through the fuel assembly, resulting in a greatly increased wall temperature of the fuel rod. The increased wall temperature may lead to damage with serious consequences arising on the fuel rod. SUMMARY OF THE INVENTION The present invention is based on the realization that the risk of the occurrence of the unfavourable events described above, caused by the occurrence of dryout, can be eliminated or considerably counteracted if measures are taken to maintain the power emission of the fuel rod, in defined regions along the extent of the fuel rod, sufficiently low to prevent the occurrence of dryout there, or to prevent damage from arising on the fuel rod, even if dryout should occur there. The regions on the fuel rod where the power emission needs to be maintained low are, according to the invention, the regions immediately below the spacers in the upper part of the fuel assembly. In these regions a redistribution of the flow takes place because of the blocking of the flow by the spacers, which may result in an increased risk of breakdown of the water film existing in these very positions. What characterizes the present invention is that in at least the main part (i.e., a majority) of the fuel rods, each fuel rod is adapted to give off a considerably lower power in those parts of the fuel rods, which are located immediately below the spacers in at least the uppermost one-third of the active length of the fuel rod, as compared to the power that the fuel rod is adapted to give off in the remainder of the active length of the fuel rod. By the active length of the fuel rod is meant in this patent application that axial part of the fuel rod which contains nuclear fuel material. Each one of those parts of a fuel rod, which is adapted to give off a considerably lower power, suitably has a length of 2-10 cm and is suitably adapted to give off a power which is at most one-third of the power which the fuel rod is adapted to give off, on average, in the remaining parts of its active length. Preferably, this part has a length of 3-8 cm and is adapted to give off a power of at most one-fourth of the power which the fuel rod is adapted to give off, on average, in the remaining parts of its active length. The lower power in the above-mentioned part of a fuel rod can be achieved by the use of a nuclear fuel material in this part, for example of the same kind as in the rest of the fuel rod but provided with a burnable absorber, for example Gd.sub.2 O.sub.3. It may also be achieve, inter alia, by the use of a non-fissile material with a low neutron absorption, for example zirconium or a zirconium alloy such as Zircaloy. The invention will be explained in greater detail by describing an embodiment with reference to the accompanying drawings.