Patent Number: 049833502
Section: description

As shown in FIG. 1, the core 2 of the reactor in a pressurized water nuclear power station is constituted by juxtaposing housings such as 4 in a horizontal square mesh grid, which housings extend vertically over the entire height of the core and contain fuel elements (not shown). Some of these housings are occupied by control clusters (not shown) each of which is provided, above the core, firstly with a mechanism (not shown) for controlling vertical displacement thereof, and secondly with measuring means such as CZ2A and CZ2B for permanently measuring the height of the cluster, i.e. the position within its housing. These measuring means provide position signals such as Z2A and Z2B. The core 2 has a vertical axis 6 and is divided about two planes of symmetry 5 and 6 into four quadrants Q1, Q2, Q3, and Q4 which follow one another angularly around said axis and which constitute the said core zones. Elements specific to any one such quadrant are designated by one or more letters followed by one of the numbers 1 to 4 which is the number of the quandrant. An additional letter following the number is used in alphabetical order to distinguish between different ones of a plurality of specific elements of the same nature, for example said cluster positioning measuring means CZ2A, CZ2B, etc. ..., which belong to the same quadrant, e.g. the second quadrant Q2. In each quadrant, the number of control clusters is 18, for example, and likewise there are 18 distinct measuring means used for indicating the positions of the clusters, and 18 distinct position signals such as Z2A and Z2B which are provided by these measuring means. The core 2 is contained in a pressure vessel 8 capable of withstanding the water pressure of a primary cooling circuit and containing an internal skirt 10 surrounding the core. The water arrives at the top of the pressure vessel via inlet ducts, it descends around said skirt to the bottom of the vessel, it passes radially to the inside of the skirt at the bottom of the vessel, and it rises while cooling the fuel elements prior to leaving from the top of the vessel via outlet ducts which are connected for that purpose to the skirt. More precisely, the fuel elements in the quadrants Q1, Q2, Q3, and Q4 are cooled by water which enters via four inlet ducts RA1, RA2, RA3, and RA4, and which leaves via four outlet ducts RB1, RB2, RB3, and RB4 each constituting a part of a respective one of four primary cooling circuits. These ducts are provided with temperature sensors TA1, TA2, TA3, TA4, TB1, TB2, TB3, and TB4 respectively. The two temperature sensors such as TA3 and TB3 associated with the same quadrant such as Q3 are connected to means such as CP3 for measuring the heat flux and generating a heat flux signal such as P3 which is representative of the heat flux removed by the flow of water in the quadrant. In order to obtain this signal, the measuring means such as CP3 multiplies the inlet to outlet temperature difference by the water flow rate, said flow rate being given by means (not shown) which measure, for example, the speed of the pumps in the primary cooling circuit which includes the inlet and outlet ducts in question. The measuring means CP3 also performs various correction operations that are not related to the present invention, in particular measuring the pressure in the cooling circuit pressurizer, so as to ensure that the resulting heat flux signal is as accurate a representation as possible of variations in heat flux over time. The measuring means CP1, CP2, and CP4 operate like the means CP3 for the purpose of providing respective signals P1, P2, and P4. Each of the four quadrants Q1, Q2, Q3, and Q4 is provided with measuring means CF1, CF2, CF3, and CF4 providing a neutron flux signal F1, F2, F3, and F4 representative of the mean neutron flux within each of the quadrants respectively. The signals representative of cluster position, heat flux, and neutron flux obtained in this way constitute the above-mentioned "sensitive" signals, with the "sensitive" parameters being those represented by said signals. As shown in FIG. 2, apparatus in accordance with the invention comprises four acquisition units U1, U2, U3, and U4 which receive said sensitive signals. Each of these units, e.g. U1, receives the neutron flux signal, e.g. F1, corresponding to one of the quadrants, e.g. Q1, the heat flux signal, e.g. P4, corresponding to another quadrant, e.g. Q4, and the group of cluster position signals, e.g. Z2A, Z2B, etc. . . . , corresponding to yet another quadrant, e.g. Q2. For each of these signals, each of said units includes differentiation and comparison means such as 12 which receive a corresponding sensitive signal and which provide a corresponding alarm signal whenever its sensitive signal varies at a rate greater than a corresponding predetermined alarm threshold. The processing to which each sensitive signal is subjected is in fact more complex than that in order to take account, in particular, of the various time offsets in these signals relative to the real physical parameters that they represent and due to measurement conditions. Each alarm signal, e.g. F'1 or Z'2A is designated by the same letters and numerals as the corresponding sensitive signal, e.g. F1 or Z2A, together with a prime symbol. Within each of the acquisition units U1, U2, U3, and U4, a respective primarY logic unit L1, L2, L3, or L4 receives all of the alarm signals provided by the differentiation and comparison means such as 12 and provides a respective primary detection signal D1, D2, D3, or D4 whenever at least one of the arm signals is present. A circuit 14 receives these primary detection signals and provides a signal 16 whenever at least two of the primary detection signals are present. Further, an OR gate 18 receives all of the position alarm signals Z'1A, Z'1B, . . . , Z'2A, Z'2B, . . . , Z'3A, . . . , Z'4A, . . . obtained from said cluster position signals and provides a signal 20 whenever at least one of these position alarm signals is present. An AND gate 22 provides a cluster fall signal 24 whenever the signal 16 and the signal 20 are present simultaneously. The circuit 14 and the gates 18 and 22 constitute the above-mentioned combination circuit. It can be seen in FIG. 2 that an emergency reactor stop can be caused by the cluster fall detection signal 24 only if there is a rapid drop in neutron flux or in heat flux accompanied by at least one rapid drop in a cluster position signal. In particular, there is no danger of an untimely emergency stop when the power station is isolated, i.e. when it is temporarily disconnected from the electricity grid to which it normally supplies power. During such isolation, the reactor power decreases progressively in order to adjust the power to the desired value. The control clusters are inserted so as to travel along their full stroke over a period of several minutes, whereas a falling cluster takes less than one minute to fall.