Patent Number: 048715085
Section: description

DETAILED DESCRIPTION FIG. 1 shows a small part of a horizontal section of a reactor core for a boiling reactor with vertical fuel assemblies. The section comprises nine full fuel assemblies 10. The total number of fuel assemblies in a complete cross-section amounts to several hundred. Each fuel assembly, for example 10a, is built up of a bundle of 64 fuel rods 11 in a square lattice and of a fuel channel 12 of zircaloy-4 of square cross-section surrounding the fuel rod bundle. The rods are held in their positions by so-called spacers (not shown) which are evenly distributed between the top tie plate and the bottom tie plate (also not shown) on the fuel assembly. Each fuel rod consists of a number of circular-cylindrical pellets of uranium dioxide as fuel, which are stacked on top of each other and canned in a cladding tube 13 of zircaloy-2. The spaces 14 between the fuel rods within the fuel channel are traversed by a coolant, in the exemplified case light water. The gaps 15a and 15b between the fuel assemblies are also traversed by a coolant of the same kind. The gaps 15b into which control rods 16 can be inserted are, in the illustrated case, wider than the gaps 15a in which there are no control rods. The cross-section also comprises neutron sources 17 as well as neutron detectors 18. One or more of the fuel rods may be exchanged for a non-energy producing rod. Thus, for example, rod 19 could be exchanged for a solid or water-filled rod of zircaloy-2. The fuel rods 20, 21, 22 and 23 are tie rods and are anchored to top tie plate and bottom tie plate in the fuel assembly. The control rods 16 have absorber blades 24, 25, 26 and 27 arranged in a cruciform. The centre of the control rod cross is designated 28. As will be clear from Figure 1, the fuel assemblies 10 are arranged in a symmetrical lattice with each fuel assembly included in two rows of fuel assemblies which are perpendicular to each other. The control rods 16 are arranged with each one of their blades between two fuel assemblies located in the same row. In this way, each control rod, for example 16b, together with four fuel assemblies 10b, 10c, 10d, 10e, arranged around its blades can be said to form a unit 30, previously called control rod unit, which in FIG. 1 is surrounded by a dashed line. A control rod unit has an at least substantially square cross-section, and, as will also be clear from FIG. 1, the control rod units are arranged in a symmetrical lattice with each control rod unit included in two rows of control rod units which are perpendicular to each other. FIG. 2 shows the control rod units in the reactor core and a number of separate fuel assemblies 10 arranged in outer regions of the core. The control rod units 30 are illustrated as light squares in those positions where, at the time of the fuel rod exchange, control rods used during the earlier operating period are arranged, and as dark squares where control rods used during the earlier operating period are replaced by new control rods with a higher reactivity worth, in the exemplified case with a 15% higher reactivity worth in a cold shutdown reactor, than the original reactivity worth of the control rods used during the earlier operating period. In the exemplified case, the exchange of control rods for new control rods has only been carried out in control rod units in a central zone 31 in the reactor core, which is located inside an edge zone 32 extending around the reactor core and comprising those control rod units 30a which are located furthest out in the reactor core in each row of control rod units. As will be clear from FIG. 2, after the control rod exchange a number of control rod units (light squares), which have been used during the earlier operating period, are distributed over the central zone. It is also clear that the control rods in those control rod units, for example 30c, 30d, 30e and 30f, which are located adjacent to each such control rod unit, for example in unit 30b, and which are located in the same rows perpendicular to each other as this control rod unit, consist of control rod units having a higher reactivity worth. From the case exemplified in FIG. 2 it is also clear that the reactor core comprises regions within the central zone comprising 3.times.3 control rod units, for example units 30g, 30h, 30i, 30j, 30k, 30l, 30m, 30n and 30o, where control rods used during the earlier operating period are exchanged for new control rods having a higher reactivity worth in that control rod unit which is located in the centre, i.e. in unit 30g, and in those four control rod units, i.e. in units 30h, 30i, 30j and 30k, which are located in the same rows perpendicular to each other as the control rod unit located in the centre, whereas control rods used during the earlier operating period are used in the remaining four control rod units, i.e. in units 30l, 30m, 30n and 30o. New control rods having a higher reactivity worth are arranged in three control rod units at the most, located adjacent to each other, for example in units 30g, 30h and 30i, in the same row of control rod units in the central zone 31. In the case illustrated in FIG. 2, 47% of the total number of control rods included in the reactor core and 63% of the total number of the control rods included in the central zone of the reactor are replaced by new control rods having a higher reactivity worth.