Patent Number: 040452824
Section: description

It should be noted in addition that, in this description, the expression "fuel assemblies" designates both the assemblies which contain fissile material and the assemblies which contain only fertile material. This figure shows diagrammatically and to a complete extent only two of the N measuring channels, namely those designated by the references A and B. Each of these N measuring channels is connected to one of the N monitored reactor core assemblies; in consequence, the complete association of the two computing circuits C and D together with all the N measuring channels will be explained in general principle without being completely illustrated. There are shown at 1, 2, 3 and 4 in the FIGURE four of the N temperature detectors constituted by thermocouples which each supply one of the N measuring channels such as A and B. Either of said measuring channels is identical in design with the channel A which comprises the following devices in series: an element for amplifying the measurement previously performed by the thermocouple 1, said element being constituted for example by two cascade-connected amplifiers 5 and 6 which have a suitable gain in order to detect small temperature variations; a summing amplifier 7 having three inputs to which are applied respectively in accordance with the essential feature of the invention, the signal which has been previously amplified for measuring the outlet temperature of the fuel assembly corresponding to the measuring channel A, the signal produced by the circuit C being such as to correspond to the opposite of the mean reactor core outlet temperature and the signal produced by the circuit D being such as to correspond to the fraction of the difference between the mean "hot" (T.sub.1) and "cold" (T.sub.2) core outlet temperatures so that the signal emitted by said summing amplifier 7 of channel A is initially of zero value; devices 8, 9 and 10 comprise respectively a threshold for pre-alarm, emergency shutdown and control rod drop constituted for example by open-loop amplifiers which can be actuated by positive or negative signals by means of a diode 7a and an amplifier 7b having a gain of 2 for rectifying the negative polarities, said diode and amplifier being connected in shunt on the output of said summing amplifier 7; regrouping OR-circuits 11, 12, 13 which trigger the standard safety actions such as pre-alarm, emergency shutdown or a control rod drop when the signals which may be delivered by each of said devices 8, 9 and 10 of each measuring channel which is similar to the channel A are applied respectively to said OR-circuits. The composition of the computing circuits C and D is directly dependent on the operations to be performed on the basis of two sets of temperatures measured on the one hand on a predetermined number of "hotter" core assemblies and on the other hand by a predetermined number of "colder" core assemblies. Thus the circuits C and D first have in common two operational amplifiers 14 and 15: the amplifier 14 for generating a signal corresponding to the mean "hot" (T.sub.1) core outlet temperature being placed at the output of a certain number of measuring amplifiers 6 selected from those which form part of measuring channels such as the channel A which are connected to "hot" core assemblies; the amplifier 15 for generating a signal corresponding to the mean "cold" (T.sub.2) core outlet temperature being placed at the output of a certain number of measuring amplifiers 6' selected from those which form part of measuring channels connected to "cold" core assemblies. The computing circuit C, the output of which feeds one of the three inputs of the summing amplifier 7 by means of a signal corresponding to the opposite value of the mean core outlet temperature is provided at the output of said amplifiers 14 and 15 with a summing amplifier 16 having a gain of 2:1 followed by an inverting amplifier 17. The circuit D, the output of which also feeds one of the three inputs of the summing amplifier 7 by means of a signal corresponding to the fraction of the difference between the mean "hot" and "cold" core outlet temperatures comprises a summing amplifier 19 followed by an assembly 20 placed at the output of the amplifier 14 for generating the mean "hot" temperature and at the output of an inverting amplifier 18 which is in turn placed at the output of the amplifier 15 for generating the mean "cold" temperature. By way of example, the assembly 20 aforesaid is constituted by a series of amplifiers having symmetrical outputs between which are connected N potentiometers for permitting initial adjustment of the fraction of the difference between the mean "hot" and "cold" core outlet temperatures in order to ensure that the signal initially emitted by the summing amplifier 7 is zero within each of the N measuring channels. In this example, the measuring amplifiers 5 and 6 have a gain in the vicinity of 235, the amplifiers having outputs which are symmetrical with the assembly 20 have a gain in the vicinity of 1.5, thus making it possible to obtain a satisfactory signal at the output of the summing amplifier 7. The core of a fast reactor contains about 85 fuel assemblies and it is sufficient to supply each of the operational amplifiers 14 and 15 with groups of 10 signals. It should finally be pointed out that the arrows such as the arrow 21 shown in the accompanying FIGURE represent diagrammatically a number of different signalling relays which make it possible, for example in the event of overstepping of emergency shutdown thresholds or of a control rod drop, to switch the output of the summing amplifier 7 to an external incident recorder. It is readily apparent that, at the time of monitoring of the nuclear reactor core, the different means shown diagrammatically in the FIGURE are conveniently arranged within suitable drawers fixed on a synoptic unit. This unit permits initial adjustment of the potentiometers of the assembly 20 as well as the adjustment on the one hand of the pre-alarm and emergency shutdown threshold values combined as a rule by means of a switch having preset voltages delivered by amplifiers in which the output values can be controlled and, on the other hand, the adjustment of the control-rod drop threshold values of the devices 10 which are smaller in number than the core assemblies but can be connected to either of the measuring channels. The synoptic unit aforesaid also serves to carry out the control operation proper by means of indicator lamps for signals delivered after the regrouping OR-circuits 11, 12, 13 and each pre-alarm device 8 when these latter are supplied. The aforesaid drawers which contain the means shown diagrammatically in the FIGURE are clearly connected to an assembly for collecting all the mesurements of the reactor core thermocouples, to the emergency bays for the transfer of orders for control rod drop and emergency shutdown, and also to the control-room console.