Patent Number: 049869523
Section: description

As shown in FIG. 1, a prior art pressurized water nuclear reactor comprises a core 2 having a vertical axis A. The plan of this core is in the form of a square grid whose edges constitute, in part, the above-mentioned circumscribing square. The corners of the square are omitted since they do not form parts of the core. That is why they are not shown. Only the middle portions such as 4 of the sides of the square are shown since they constitute parts of the edge of the core. The core has two median axes parallel to pairs of its sides, and two diagonal axes parallel to its diagonals. Four neutron flux detectors D1, D2, D3, and D4 are disposed on the diagonal axes of the core, outside and close to the core. I.e., they are disposed around the vertical axis A at the following angular positions 45.degree., 135.degree., 225.degree., and 315.degree. measured from an angular origin extending upwardly in the figure. Each of the four identical protection chains includes one of the detectors, for example the detector D1, together with a primary treatment circuit operating as a differentiator R1, followed by a threshold member S1. This circuit provides the above-mentioned primary fall signal, in the event that an accidental fall of a cluster of control rods has occurred in the vicinity of the detector, or even at a distance from the detector if the previously established neutron flux distribution makes that possible. A secondary treatment circuit 6 receives the fall signals provided at the outputs from the four protection chains. These signals are said primary fall signals. The secondary circuit provides the above-mentioned secondary fall signal if it receives two signals at the outputs from said chains. This secondary fall signal controls measures for protecting the reactor by means not shown which cause all of the control rods to fall almost immediately. Starting from this prior system, a characteristic of the invention lies in using four additional detectors and in coupling them with the four conventional detectors within the protection chains. These additional detectors are not necessarily identical to the four conventional detectors. They are situated on the median axes, and outside the core. It is preferable for the detectors to be uniformly distributed, although an angular pitch of 45.degree. is not absolutely necessary. The two systems shown in FIGS. 2 and 3 ensure that in the event of an accidental cluster fall, a minimum of three detectors belonging to three different protection chains will be in the proximity of the cluster(s), thus giving rise to a large fall signal. The system thus ensures 2/4 logic protection even in the event of one of the chains being faulty. The two systems differ as follows: The first system shown in FIG. 2 has the advantage of its chains being symmetrical: each chain has one diagonal detector and one median detector. The second system shown in FIG. 3 is optimized with respect to the probability of detecting an accidental fall: four adjacent detectors always belong to four different chains. However, the chains are not symmetrical with respect to the detectors: two of the chains are fitted solely with diagonal position detectors, while the other two chains use median position detectors only. One or other of these two solutions is selected depending on the characteristics of the detectors, which characteristics may depend on their median or diagonal position, and taking account of the fact that the treatment circuits used in accordance with the invention are typically incorporated in treatment units which provide protection functions other than those described above. More specifically, and as shown in FIG. 2, the first system in accordance with the invention comprises eight detectors: D1A, D4B, D3A, D1B, D2A, D4A, and D2B at angular positions of 45.degree., 90.degree., 135.degree., 180.degree., 225.degree., 270.degree., 315.degree., and 360.degree. respectively, with a single chain, e.g. the first, including two detectors D1A and D1B whose references include the same digit. In addition, each chain includes two primary treatment circuits RIA, SIA and RIB, S1B associated with respective ones of its two detectors, together with an intermediate treatment circuit P1 constituted by an OR gate. In the second system in accordance with the invention as shown in FIG. 3, the detectors are in the same positions as in the first system but in the following order: D1C, D2C, D3C, D4C, DID, D2D, D3D, and D4D, with the digit likewise specifying the number of the chain to which the detector belongs. For example, the first chain comprises primary treatment circuits R1C, SIC and R1D, S1D associated with the detectors D1C and D1D respectively, together with the intermediate treatment circuit P1. In other respects, this system is identical to the first.