Patent Number: 048715085
Section: summary

BACKGROUND OF THE INVENTION A core in a boiling reactor normally comprises several hundred fuel assemblies. These are arranged vertically in the core and have an at least substantially square cross-section. Each fuel assembly consists of a bundle of fuel rods which are surrounded by a fuel channel. The core also comprises a large number of control rods, which may each comprise four vertical blades arranged in a cruciform configuration and provided with a neutron absorber material. The fuel assemblies are normally arranged in a symmetrical lattice, with each fuel assembly included in two rows of fuel assemblies, located perpendicular to each other, and the control rods normally being arranged with each one of their absorber blades between two fuel assemblies located in the same row, so that each control rod together with four fuel assemblies, arranged around its blades, forms a unit, in the following called control rod unit, having an at least substantially square cross-section and so that the control rod units are arranged in a symmetrical lattice, with each control rod unit included in two rows of control rod units located perpendicular to each other. The present invention relates to the operation of a nuclear reactor of the type described above. When the burnup in a reactor has progressed so far that the smallest acceptable core reactivity margin has been attained, a partial recharging of fuel is carried out. By suitably balancing the amount of fuel to be replaced as well as the fissile enrichment of the replacement fuel, an excess reactivity is effected which permits a certain energy output until the next refuelling occasion. In connection with refuelling, when the reactor is shut down, an exchange of control rods is also carried out, if necessary, in which control rods the reactivity worth (by consumption of neutron absorber) has dropped to a predetermined value, or which control rods exhibit defects. Normally, the exchange of control rods is carried out if the reactivity worth in one-fourth of the length of the control rod has dropped to 90% of the original reactivity worth. By reactivity worth of a control rod is meant that change in reactivity which follows the complete insertion of a fully withdrawn control rod in a critical reactor under given conditions. When exchanging control rods, control rods existing in the reactor at the time of the exchange are replaced, according to the state of the art, by control rods having the same reactivity worth as that of the existing ones in their original state. In this way, the behaviour of the reactor is maintained unchanged. The control rods in a boiling reactor bind a certain amount of reactivity in the reactor. Together with burnable neutron absorbers, for example digadolinium trioxide, in the fuel, a smallest permissible shutdown margin is obtained for a given core. If it is desired to achieve longer fuel cycles, i.e., a longer operating time between two refuellings, or a larger energy output in the reactor, the shutdown margin in a core with existing control rods has to be dimensioned using an increased quantity of burnable neutron absorber. Such use of an increased quantity of burnable absorber results in reactivity losses because of increased reactivity penalty of residual burnable absorber at the end of a fuel cycle (such as because of an increased content of isotopes of gadolinium with a low absorption cross-section for neutrons). Thus, an increased use of a burnable absorber enables a greater flexibility regarding the design of the core with reactor fuel but at the same time results in increased losses because of the reactivity penalty of residual neutron absorber. SUMMARY OF THE INVENTION The present invention is based on the realization that the increased shutdown margin, which is required for longer fuel cycles or for a greater energy output, can be brought about without the above-described disadvantages--consisting of the increased reactivity penalty caused by the use of an increased amount of neutron absorbers--occurring. According to the invention, this is achieved by the fact that some of the control rods located in the reactor, when exchanging control rods, are replaced by control rods having a higher reactivity worth than the original reactivity worth of the control rods present in the reactor at the time of exchange, whereas other control rods used during the earlier operation of the reactor are maintained in the reactor. The invention makes it possible, with a limited number of control rods with a higher reactivity worth, to achieve the same operational advantages as if all the existing control rods were to be exchanged for others with a higher reactivity worth. The advantages thus gained are very considerable. The present invention relates more particularly to a method for the operation of a light water boiling reactor with a core comprising a plurality of vertical fuel assemblies having an at least substantially square cross-section, each fuel assembly consisting of a bundle of fuel rods surrounded by a fuel channel, and a plurality of control rods, each control rod comprising four vertical blades arranged in a cruciform and provided with a neutron absorber material, the fuel assemblies being arranged in a symmetrical lattice with each fuel assembly included in two rows of fuel assemblies which are perpendicular to each other and the control rods being arranged with each one of their blades between two fuel assemblies located in the same row, so that each control rod together with four fuel assemblies, arranged around the blades of the control rod, forms one unit, a control rod unit, having an at least substantially square cross-section, the control rod units being arranged in a symmetrical lattice with each control rod unit included in two rows of control rod units perpendicular to each other, characterized in that, after a period of operation of the reactor, when exchanging control rods, which are present in the reactor at the time of exchange and which have been used during the operating period, for new control rods, there are arranged in some control rod units control rods having a reactivity worth which is higher than the original reactivity worth of the control rods which are present in the reactor at the time of exchange and have been used during the operating period, whereas in other control rod units there are used control rods which have been used in the reactor during the operating period. The reactivity worth, in a cold shutdown reactor, of the newly inserted control rods with the higher reactivity worth is at least 6% higher, preferably 10-20% higher, than the original reactivity worth of the control rods present in the reactor at the time of the exchange. By original reactivity worth of a control rod is meant in this patent application the reactivity worth of a control rod in a non-used state. To utilize the invention in full, the exchange of existing control rods for new control rods with a higher reactivity worth only takes place in control rod units located in a central zone of the reactor core, which is located inside an edge zone extending around the reactor core and comprising those control rod units which are located furthest out in the reactor core in each row of control rod units. According to one embodiment of the invention, control rods which have been used during a preceding operating period are maintained in a number of control rod units distributed over the central zone of the reactor core, whereas at least the control rods in those control rod units which are located adjacent to each such control rod unit with maintained control rod and in the same rows perpendicular to each other as this control rod unit with maintained control rod are replaced by new control rods with a higher reactivity worth. Control rods used during the earlier operating period in regions within the central part of the reactor core, comprising 3.times.3 control rod units, can then be exchanged for new control rods with a higher reactivity worth in that control rod unit which is located in the centre of the region and in those four control rod units 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 maintained in the remaining four control rod units. The exchange of fuel rods is preferably performed such that new control rods with a higher reactivity worth are arranged in three control rod units at most, located adjacent to each other, in the same row of control rod units. The number of control rods used during the earlier operating period, which are replaced by new control rods, preferably amounts to 40-60% of the number of control rods in the entire reactor core and to 50-80% of the number of control rods in the central zone of the reactor core.