Patent Number: 048428051
Section: description

FIGS. 1, 2, and 3 are curves of values measured when a control cluster falls, in which it can be seen that the steam pressure at the inlet to the turbine and the speed of the pumps do not vary, whereas the total nuclear power falls very rapidly and returns to its prior value after several tens of seconds. The fall of a cluster is immediately observed by the measurement chambers in the form of a limited, but very rapid drop in the total power. A fallen cluster gives rise to a power drop which is restricted to a few fuel assemblies. This drop affects the total power for a period of a few seconds. Thereafter, the nuclear power regulation loop acts on the control clusters which have not fallen in order to return the total power to the reference value. FIGS. 4, 5, and 6 show the variations in the same magnitudes as FIGS. 1, 2, and 3 but for a configuration of the power station which is referred to as "isolation after a grid fault" because this configuration is established automatically in the event of a fault appearing on the power distribution grid. The power demand on the boiler constituted by the nuclear reactor is then reduced to a low, non-zero level. This level is selected to be high enough, firstly to satisfy the power station's own requirements during the isolation period, and secondly to subsequently enable the station to return rapidly to the power level requested by the grid once the faults thereon have been cleared. FIG. 4 shows a very rapid drop in steam pressure at the inlet to the turbine. This pressure drop causes the speed of the turbo-alternator set to vary and thus gives rise to small variations in the frequency of the electricity supplied. These frequency variations give rise to identical variations in the speed of the pumps in the primary cooling fluid circuits. FIG. 5 shows that the systems for regulating the nuclear power of the reactor cause the positions of the control rods to vary in order to reduce said power. The variations in various parameters, and in particular in the speed of the pumps, prevent the nuclear power from dropping smoothly, and may even cause it to oscillate for several seconds about the original power value before dropping as shown in FIG. 6. FIGS. 7, 8, and 9 show the variations in the same magnitudes when the station is isolated by its operators. The curves representing the speed of the pumps and the nuclear power are similar to those of FIGS. 4 and 5, except at the origin where the measured magnitudes do not decrease prior to oscillating. FIGS. 10, 11, and 12 are the corresponding curves in the event of the turbine being tripped out of service without it being necessary to perform an emergency stop. The inlet pressure to the turbine begins by dropping rapidly, and then falls off more slowly (FIG. 10). The speed of the primary pumps remains constant and the nuclear power decreases slowly (FIG. 12). FIGS. 13, 14, and 15 are the corresponding curves in the event of a transient grid fault which is cleared very quickly and which does not justify isolating the power station. The inlet pressure to the turbine drops very quickly (FIG. 13) as for power station isolation, but then returns very quickly to its original value. The pump speed drops slightly before returning to its initial value (FIG. 14). The nuclear power begins by dropping smoothly like the pump speed, and then oscillates prior to returning to its initial value (FIG. 15). FIGS. 1, 2, and 3 show that in the event of a control cluster falling, only the nuclear power varies, whereas in the other events (FIGS. 4 to 15) at least one of the other two external parameters shown varies in addition to the nuclear power varying. The variations in nuclear power may be so large as to give rise to an emergency stop of the reactor. In order to avoid the drawbacks of such an unwanted stop, the present invention teaches that the value of the derivative at each point on the curves should be calculated and that the calculated values should be compared with predetermined threshold values. An emergency stop will be initiated if, and only if the nuclear power derivative signal reaches the corresponding threshold. FIG. 16 is a block diagram of a circuit suitable for implementing the method in accordance with the invention. Measurement signals are available representative of the speed of the primary pumps 1, of the steam pressure at the inlet to the turbine 2, and of the nuclear power 3. The signals 1 and 2 represent two parameters which are said to be "external" to the reactor. These three signals are applied to the inputs of three respective differentiator circuits 4a, 4b, and 4c which constitute the above-differentiating means. The derivative signals 9 and 10 at the outlets from the circuits 4a and 4b are applied firstly to respective delay circuits 5a and 5b and secondly to respective pairs of threshold relays 6e & 6f and 6g & 6h. It will be understood that such relays constitute the threshold means mentioned above. The delayed derivative signals 7 and 8 at the outputs from the circuits 5a and 5b are likewise applied to pairs of threshold relays 6a & 6b and 6c & 6d. The derivative signal 11 at the output from the differentiator circuit 4c is applied to a threshold relay 6j. The threshold relays deliver respective signals having values 0 or 1 . Each of the signals 7, 8, 9, and 10 is applied to a pair of threshold relays so as to detect excursions beyond a positive threshold (+) and a negative threshold (-), between which thresholds the signals are at zero. Each of the signals 12a, 12b, 12c, 12d, 12e, 12f, 12g, and 12h from the threshold relays is applied to an input to a logic NOR circuit 13. This circuit delivers a logical "1" output if, and only if none of its input signals is at 1. The output signal 12j from the threshold relay 6j is set to 1 when the signal 11 is below a negative threshold. The output signal from the circuit 13 and the output signal from the threshold relay 6j are applied to respective inputs of an AND gate 14. When both input signals to the AND gate are in the "1" state, the output signal 15 is likewise in the "1" state and this signal is used as an emergency stop signal for the reactor. Given the durations of the various signals, it turns out that regardless of the instants at which the pump speed signal or the turbine inlet steam pressure signal vary within predetermined time limits, the variations in the derivations of these signals are taken into account by the threshold relays at the same instants that the nuclear power varies. For example, and with reference to FIG. 6, the signals 9, 10, and 11 are taken into account at instant t1, and the delayed signals 7 and 8 together with the signal 11 are taken into account at instant t2.