Patent Number: 
Section: claims

1. A method for regulating operational parameters of the core of a pressurized water nuclear reactor, the nuclear reactor having:a core which is divided into an upper zone and a lower zone, and produces thermal power;a plurality of groups of rods for controlling the reactivity of the core, each of which occupies, in the core, a plurality of insertion positions which are stepped vertically starting from an upper position;means for inserting each group of rods in the core vertically;a primary circuit which assures the circulation of a primary cooling fluid through the core;means for adjusting the concentration of a neutron-absorbent component in the primary cooling fluid; andmeans for acquiring values which are representative of the conditions of operation of the core of the reactor,wherein the regulation method comprises:a step of acquiring values representative of (1) thermal power produced in the upper and lower zones of the core, (2) temperatures of hot and cold branches, and (3)flow of primary cooling fluid, which are representative of the conditions of operation of the core of the reactor;a step of evaluation of actual values of (i) mean temperature of the primary cooling fluid in the core, (ii) axial distribution of thermal power between the upper and lower zones of the core, and (iii) a parameter representative of capacity to increase power of the reactor corresponding to the thermal power which is produced by the core when the groups of rods are raised to the vicinity of the upper position, which values (i)-(iii) are collectively the operational parameters, this evaluation being performed at least as a function of the values acquired;a step of selection of either a first control law or a second control law to control (a) the concentration of the absorbent component, and (b) positions of insertion of the groups of rods, the first control law being different from the second control law, wherein the first control law is selected as soon as at least one group of rods is in an insertion position lower than a predetermined position, and the second control law is selected as soon as all the groups of rods are in respective insertion positions above said predetermined position;a step of calculating set points of the operational parameters at least as a function of control set points; anda step of regulation of the operational parameters as a function of set points relating to said parameters and of the actual values evaluated, by means of the selected control law,wherein the first control law comprises the following steps, in any order:modifying the positions of insertion of the groups of rods as a function of the deviation of the mean temperature of the primary cooling fluid in the core with regard to its set point, in order to regulate the mean temperature of the primary cooling fluid in the core to its set point;modifying the positions of insertion of the groups of rods as a function of the deviation of the axial distribution of power with regard to its set point, in order to regulate the axial distribution of power to its set point; andadjusting the concentration of the neutron-absorbent component in the primary cooling fluid as a function of the deviation from its set point of the parameter which is representative of the capacity to increase power of the reactor corresponding to the thermal power produced by the core when the groups of rods are raised to the vicinity of the upper position, which is representative of the capacity to increase the power of the reactor to its set point, in order to regulate the said increase power capacity to its set point; andwherein the second control law comprises the following steps, in any order:modifying the positions of insertion of the groups of rods as a function of the deviation of the mean temperature of the primary cooling fluid in the core with regard to its set point, in order to regulate the mean temperature of the primary cooling fluid in the core to its set point;adjusting the concentration of the neutron-absorbent component in the primary cooling fluid as a function of the deviation of the axial distribution of power with regard to its set point, in order to regulate the axial distribution of power to its set point; andmodifying the positions of insertion of the groups of rods as a function of the deviation from its set point of the parameter which is representative of the capacity to increase power of the reactor corresponding to the thermal power produced by the core when the groups of rods are raised to the vicinity of the upper position, which is representative of the capacity to increase the power of the reactor to its set point, in order to regulate the said increase power capacity to its set point. 2. The method according to claim 1, wherein the step of calculating set points of the operational parameters further comprises a sub-step of calculation of the mean temperature set point of the primary cooling fluid in the core, on the basis of a value which is representative of the power provided to an electricity network which is supplied by the reactor. 3. The method according to claim 2, wherein the step of calculating set points of the operational parameters further comprises a sub-step of distribution of the groups of rods into a sub-set for control of the mean temperature of the primary cooling fluid in the core, and a heavy sub-set which assures substantially the control of the axial distribution of power, the groups of rods of the heavy sub-set being inserted less far than those of the other sub-set. 4. The method according to claim 3, wherein the heavy sub-set is always positioned in the upper half of the core. 5. The method according to claim 3, wherein the parameter which is representative of the capacity to increase the power of the reactor is determined at least on the basis of the positions of insertion of the group(s) of rods of the control sub-set, the step of calculating set points of the operational parameters comprising a sub-step of calculation of a position set point for the group(s) of rods of the control sub-set as a function of a set point for the capacity to increase the power and to values acquired. 6. The method according to claim 5, wherein the step of regulation by means of the first control law further comprises:a sub-step of calculation of a displacement to be carried out for the group(s) of rods of the control sub-set as a function of the set point and to the actual value of the mean temperature of the primary cooling fluid in the core; anda sub-step of modification of the position(s) of insertion of the group(s) of rods of the control sub-set as a function of the displacements calculated for the purpose of regulating the mean temperature of the primary cooling fluid in the core to the set point. 7. The method according to claim 5, wherein the step of regulation by means of the first control law further comprises:a sub-step of calculation of the displacements to be carried out for the set(s) of rods of the control sub-set and of a displacement to be carried out for the heavy sub-set as a function of at least the set point and the actual value of the axial distribution of thermal power; anda sub-step of modification of the position(s) of insertion of the group(s) of rods of the control sub-set and/or of the heavy sub-set as a function of the displacements calculated for the purpose of regulating the axial distribution of thermal power to the set point. 8. The method according to claim 7, wherein, when the mean temperature of the cooling fluid is in a dead band around its set point, the control sub-set and the heavy sub-set are displaced in inverse directions in order to regulate the axial distribution of thermal power to its set point. 9. The method according to claim 5, wherein the step of regulation by means of the first control law further comprises:a sub-step of calculation of the concentration of the neutron-absorbent component as a function of the set point and to the actual value of the parameter representative of the capacity to increase the power of the reactor; anda sub-step of adjustment of the concentration of the neutron-absorbent component in the primary cooling fluid to the concentration calculated in order to regulate the parameter representative of the capacity to increase the power of the reactor to its set point. 10. The method according to claim 5, wherein the step of regulation by means of the second control law further comprises:a sub-step of calculation of the displacement(s) to be carried out for the group(s) of rods of the control sub-set and of the displacement to be carried out for the heavy sub-set as a function of the set point and to the actual value of the mean temperature of the primary cooling fluid in the core, and as a function of the set point and the actual position of the group; anda sub-step of modification of the position(s) of insertion of the group(s) of the control sub-set and/or of the heavy sub-set as a function of displacements calculated, for the purpose of regulating the mean temperature of the primary cooling fluid in the core to the set point. 11. The method according to claim 10, wherein the control sub-set is displaced first in order to regulate the mean temperature of the primary cooling fluid in the core, the heavy sub-set being displaced when the control sub-set has reached the limits of a dead band centered on its position set point. 12. The method according to claim 5, wherein the step of regulation by means of the second control law further comprises:a sub-step of calculation of the concentration of the neutron-absorbent component as a function of the set point and to the actual value of the axial distribution of thermal power; anda sub-step of adjustment of the concentration of the neutron-absorbent component in the primary cooling fluid at the concentration calculated in order to regulate the axial distribution of thermal power to the set point. 13. The method according to claim 5, wherein the step of regulation by means of the second control law further comprises:a sub-step of calculation of the displacement(s) to be carried out for the group(s of rods of the control sub-set and of the displacement to be carried out for the heavy sub-set as a function of at least the set point and to an actual value of the positions of insertion of the group(s) of rods of the control sub-set; anda sub-step of modification of the positions of insertion of the group(s) of the control sub-set and/or of the heavy sub-set as a function of the displacements calculated, in order to maintain the group(s) of the control sub-set in a dead band around the insertion position set point. 14. The method according to claim 13, wherein, when the mean temperature of the cooling fluid is in a dead band around its set point, the control sub-set and the heavy sub-set are displaced in inverse directions in order to keep the group(s) of the control sub-set in the said dead band around its insertion position set point. 15. The method according to claim 3, wherein the groups of rods of the control sub-set are inserted or extracted sequentially when the thermal power produced by the core varies, two groups which are inserted or extracted in succession, each group having respective insertion positions which are separated from one another by a difference which is constantly less than a predetermined limit. 16. The method according to claim 1, wherein the regulation method is automatic. 17. The method according to claim 1, wherein the set point of the parameter representative of the capacity to increase the power of the reactor is a set point for the positions of insertion of several groups of rods, calculated based on a set point of capacity to increase the power of the reactor and on an actual value of a primary thermal power of the core.