Patent Number: 051695928
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT In general terms, the present invention applies to a reactor comprising the following known components, as shown in FIG. 1: (1) a core 2 containing fuel rods in which a nuclear reaction takes place, giving off nuclear power which is spread between a top and a bottom of said core and which is transformed into heat. PA1 (2) a heat exchange circuit for causing a heat exchange fluid to penetrate into the core via an inlet duct 4 and to flow through the core under drive from a primary pump 8, leaving the core via an outlet duct 6 so as to remove said heat therefrom. This circuit delivers the heat to a heat receiver 10 having varying needs and conventionally constituted by a steam generator 10 which transfer the power to a secondary circuit. The pump 8 is provided with a speed sensor which provides a pump speed signal VP. PA1 (3) clusters of control rods 12 driven by mechanisms 13 for penetrating on command into the core from the top thereof in order to control the nuclear reaction therein. PA1 (4) a power measuring system 14 for delivering a total power signal DT representative of the nuclear power. This system takes the difference between the temperatures measured by two temperature detectors 20 and 22 placed on the inlet and outlet ducts 4 and 6, respectively. PA1 (5) a power difference measuring system 16 for providing a power difference signal DI representative of an axial difference in the nuclear power between the top and the bottom of the core. This system receives signals from two neutron detectors 15 and 17 respectively placed facing the top and the bottom of the core 2. PA1 (6) a temperature measurement system 18 for providing an operating temperature signal representative of an operating temperature which is a temperature of said heat exchange or cooling fluid. This system takes the average of the temperatures measured by the detectors 20 and 22. PA1 a power control system 24 acting on some of the control clusters 12 as a function, in particular, of variations in the needs of the heat receiver 10, so as to ensure that the heat receiver receives heat power that corresponds, at least approximately, to its needs; and PA1 an excess power protection system 30 suitable for providing an emergency stop signal causing at least some of said control clusters to penetrate into the core so as to stop said nuclear reaction quickly. To this end, this system receives at least the total power signal DT and the operating temperature signal ST. It provides an emergency stop signal AR to the mechanism 13 when the nuclear power represented by said total power signal DT exceeds an emergency stop limit LP which is defined on the basis at least of the temperature signal ST. PA1 a filter circuit 40 and a multiplier 42 for processing the pump speed signal VP; PA1 a circuit 44 for processing the axial power difference signal DI; PA1 a filter circuit 46 for processing the nuclear power signal DT; and PA1 a circuit 48 for providing an additional, constant signal to summing circuits 50 and 52 which provide the emergency stop signal AR. In conventional manner, the control system for the reactor comprises: The excess power protection system 30 and a high temperature protection system 32 both receive all of the signals mentioned above, with the high temperature protection system also receiving a primary pressure signal. According to the present invention, the emergency stop limit LP is lowered for values of the temperature signal ST which correspond to operating temperatures T that are less than a reference temperature TR. This emergency stop limit preferably obeys a positive slope law as a function of operating temperature ST, and this law is preferably linear. Also preferably, the reference temperature TR lies between 270.degree. C. and 320.degree. C., and preferably between 290.degree. C. and 300.degree. C. when the method is applied to a pressurized water reactor. Also preferably, when the operating temperature ST is 20.degree. C. less than the reference temperature TR, the value of the emergency stop limit LP is less than one-half the value that it has when said operating temperature is a little higher than said reference temperature. As shown in FIG. 2, a known protection system suitable for constituting the protection system 30 comprises: The temperature signal ST representative of the operating temperature of the reactor passes via a filter circuit 54. It is then applied to the summing circuit 50 via two paths. A first path includes a filter circuit 56 and a multiplier 58. A second path includes firstly a summing circuit 60 which also receives a signal PN representative of a nominal temperature. Thereafter it includes a multiplier 62. The emergency stop limit JP obtained in this way is shown in FIG. 3. The excess power protection system of the present invention for constituting the system 30 is identical to that shown in FIG. 2 except with respect to the second abovementioned path, as can be seen in FIG. 4. This second path is now constituted by a function generator 64 which causes the emergency stop limit LP to vary, as shown in FIG. 5. In FIG. 5, curve ET shows the evolution of a transient due to uncontrolled increase of the load in the secondary heat exchange circuit. This transient is initially the same as that shown by the curve ET in FIG. 3. However, in this case it is interrupted at a point 66 which corresponds to the system of FIG. 4 delivering an emergency stop signal. It may be observed that this modification makes it possible to trigger an emergency reactor stop before reaching safety criteria. It therefore makes it possible to guarantee that these criteria are satisfied in the event of an excessive increase in the load. In addition, it gives rise to more flexible dimensioning of the excess power system assembly, making it possible to use larger operating margins during certain transients in normal operation.