Patent Number: 
Section: claims

1. A method for operating a nuclear light water reactor during an operation cycle including an initial control rod cycle and a plurality of subsequent control rod cycles following successively after the initial control rod cycle,said reactor comprising a core, said core comprisinga plurality of cells each including four elongated fuel units and a control rod position, said elongated fuel units being arranged in parallel with each other and each including a plurality of fuel rods containing a nuclear fuel in the form of fissile material,a plurality of control rods, each of said cells including a control rod position, each of said control rods being introduceable in a respective one of said control rod positions, and at least some of said fuel units including a control rod dependent addition of a burnable absorber,said method comprising:introducing substantially all of said control rods in the core before the reactor is started and an operation cycle is initiated,operating the reactor during said operating cycle without replacement or relocation of said fuel units, wherein the operation cycle is at least 18 months,operating the reactor during the initial control rod cycle of said operation cycle with a first control rod configuration with a first group of control rods at least partly introduced and the control rods other than those of the first group extracted,maintaining the control rods in position with substantially no movements thereof during the initial control rod cycle,operating the reactor during a first of the subsequent control rod cycles with a second control rod configuration with the first group of control rods extracted and a second group of control rods at least partly introduced; andoperating the reactor during a second of the subsequent control rod cycles with a third control rod configuration with the second group of control rods extracted and a third group of control rods at least partly introduced;wherein the initial control rod cycle is significantly longer than each of the subsequent control rod cycles. 2. A method according to claim 1, wherein the subsequent control rod cycles also include:operating the reactor during a third of the subsequent control rod cycles with a fourth control rod configuration with the third group of control rods extracted and a fourth group of control rods at least partly introduced. 3. A method according to claim 2, wherein the subsequent control rod cycles also include:operating the reactor during a fourth of the subsequent control rod cycles with a fifth control rod configuration with the fourth group of control rods extracted and a fifth group of control rods at least partly introduced.