Patent Number: 048715085
Section: abstract

A core of a light water boiling reactor comprises a plurality of vertical fuel assemblies (10) and a plurality of control rods, each control rod comprising four vertical blades arranged in a cruciform. The control rods are arranged with each one of their blades between two fuel assemblies located in the same row, such that each control rod together with four fuel assemblies arranged around the blades of the control rod form a unit, the control rod unit (30, 30-o), having an at least substantially square cross-section. The control rod units are arranged in a symmetrical lattice with each control rod unit included in two rows of control rod units perpendicular to each other. After a period of operation of the reactor, when exchanging fuel rods which are present in the reactor at the time of exchange and have been used during the operating period, for new control rods, there are arranged in some control rod units (dark squares) control rods with a reactivity worth which is higher than the original reactivity worth of the control rods which have been used, whereas in other control rod units (light squares) there are used control rods which have been used in the reactor during the operating period (FIG. 2).