Patent Number: 053295714
Section: summary

The present invention relates to a reactor core for a nuclear reactor of boiling water type with a plurality of vertical fuel assemblies, each one containing a plurality of fuel rods, which are arranged between a bottom-tie plate and a top-tie plate in a surrounding vertical casing 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 spaced relationship in the lateral direction. Each 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, and out through the top-tie plate. Further, each fuel assembly is arranged with interspaces with respect to adjacent fuel assembly and possibly with a channel arranged internally in the fuel assembly for conducting water through the interspaces and through the internal channel (if any) in a vertical direction from below and upwards through the core. In operation of a nuclear reactor of boiling water type 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 that the water film which is present on the surface of each fuel rod disappears or is broken down in limited regions, 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. 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 by providing outlet holes for water at a certain level in some of the fuel assemblies. In comparison with a fuel assembly without outlet holes, a fuel assembly with outlet holes, inserted into a core with equal pressure drop, provides a greater flow up to the level of the hole and a lower flow above the level of the hole. The greater flow in the lower part of the fuel assembly gives a lower steam content up to the level of the hole, which together with the lower flow rate above the level of the hole gives improved dryout conditions in the fuel assembly. The lower flow rate above the level of the hole contributes to the improvement in that the water film on each fuel assembly grows in thickness as a consequence of reduced friction between the water film and the steam-water mixture flowing outside the fuel rods. What characterizes the present invention is, more particularly, that outlet holes for water for release of water from the space between the fuel rods to the interspaces between the fuel assemblies and/or to any internal channels in the fuel assemblies are arranged in the casing at a level positioned above the centre of the active length of the fuel rods and below at least the two uppermost spacers. By the active length of a fuel rods is meant in this application that axial part of the fuel rod which contains nuclear fuel material. The total area of the holes suitably constitutes 5-80% and preferably 20-40% of the flow area in the fuel assembly. By flow area in a fuel assembly is meant in this application that area in a horizontal section in the fuel assembly which is not occupied by fuel rods. The percentage of fuel assemblies which are arranged with outlet holes for water suitably constitutes 10-75% and preferably 25-55% of the total number of fuel assemblies in the core. When the burnup in a reactor of the above-mentioned kind has progressed so far that the smallest acceptable core reactivity margin has been attained, a partial recharge is carried out. By balancing in a suitable way how much fuel that is to be replaced as well as the enrichment of the replacement fuel, an excess reactivity is obtained which permits a certain energy output until the next refuelling. During the partial recharge, one-fifth of the fuel, for example, can be replaced every operating year (or any other suitable operating cycle) as from the end of the second operating year. This means that the fuel in the exemplified case remains in the core for five years during steady state, but that part of the fuel which is replaced during the initial stage is used for a shorter period of time. Casings with outlet holes for water according to the present invention are intended to be used in fuel assemblies with fresh fuel during the first or the first two operating cycles. Then the casing in each fuel assembly is replaced by a casing without outlet holes. The reason is that the risk of dryout is greatest when the fuel is fresh and has a high reactivity and hence high power.