Patent Number: 048715085
Section: claims

1. 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 form 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, said method comprising the steps of (a) operating said light water boiling reactor for an operating period until refuelling is needed, and then (b) exchanging in some control rod units control rods used during the operating period for new control rods having a reactivity worth which in a cold shut-down reactor is at least 6% higher than the original reactivity worth of the exchanged control rods, while maintaining in other control rod units control rods used during the operating period, the number of control rods with a higher reactivity worth after the exchange amounting to 40-60% of the total number of control rods of the reactor core. 2. Method according to claim 1, wherein in a cold shutdown reactor the reactivity worth of the new control rods with the higher reactivity worth is 10-20% 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. 3. Method according to claim 1, wherein the exchange of control rods for new control rods with a higher reactivity worth takes place in control rod units 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. 4. Method according to claim 3, in a number of control rod units distributed over the central zone of the reactor core there are used control rods which have been used in the reactor during the operating period and that at least the control rods in those control rods units which are located adjacent to each such control rod unit with a used control rod and in the same rows, perpendicular to each other, as said control rod unit with a used control rod, are exchanged for new control rods having a higher reactivity worth. 5. Method according to claim 3 or 4, wherein in regions within the central zone of the reactor core comprising 3.times.3 control rod units, control rods used during the operating period are exchanged for new control rods with a higher reactivity worth in that control rod unit which is located in the centre and in those four control rod units which are located in the same rows, perpendicular to each other, as said control rod unit located in the centre, whereas control rod which have been used during the operating period are used in the remaining four control rod units 6. Method according to claim 3 or 4, wherein when exchanging control rods new control rods with a higher reactivity worth are arranged in three control rod units at most, positioned adjacent to each other, in the same row of control rod units. 7. Method according to claim 3 or 4, wherein of the control rods used during the operating period, 50-50% of the total number of control rods in the central part of the reactor core are exchanged for new control rods with a higher reactivity worth.