Patent Number: 049869523
Section: summary

The present invention relates to nuclear reactors. In general, it applies under the following circumstances: A reactor has a vertical axis. The nuclear reaction which takes place in the core of the reactor is accompanied by a neutron flux which is distributed around said axis (and vertically). Said core has vertical hollows distributed around said axis and it is provided with controllable reaction-inhibiting elements (control rods) capable of descending in said hollows in order to absorb said neutron flux, thereby controlling said nuclear reaction. As a result, an accidental fall of one of said reaction-inhibiting elements into one of said hollows locally absorbs said neutron flux and disturbs the neutron flux distribution around said axis. Continuing said nuclear reaction may then damage said core. That is why a protection system is provided. Such a system conventionally includes not less than three separate protection chains (for safety reasons). Each of these chains comprises: a neutron flux detector disposed at a distance from said axis for measuring a local neutron flux which, in the event of one of said reaction-inhibiting elements falling, is subjected to a reduction, with the reduction being larger the nearer said detector is to the hollow in which said fall has taken place; and a primary treatment circuit associated with said detector for providing a primary fall signal solely when the neutron flux measured by said detector is subjected to a reduction whose rate of decrease exceeds a predetermined speed threshold. Said detectors are angularly distributed around said axis. Said system further includes a secondary treatment circuit receiving the fall signals output by said protection chains and providing a secondary fall signal solely on receiving not less than two of said fall signals from two respective chains, thereby reducing the risk of such a secondary fall signal being provided when one of said reaction-inhibiting elements has not fallen; and means for limiting said nuclear reaction in the event that said secondary fall signal is provided. More specifically, consider the case of one or more clusters of control rods, i.e. reaction-inhibiting elements, falling into the core of a nuclear reactor of the pressurized water (PWR) type. The consequence of such a fall is to disturb the distribution of neutron flux in the core. This disturbance may be very large and could give rise to the crisis of the water boiling, thereby damaging the fuel rods, unless protective measures are undertaken. One such protective measure consists, in particular, in implementing said means for limiting the nuclear reaction. With respect to this, and other risks, the electrical protection and safety members of a nuclear reactor are divided into four distinct groups referred to as protection chains. Each of these groups is independent from the other groups. They are powered from distinct power supplies and they are located in separate premises. The cables interconnecting the various components of a given group follow paths which are different from and separate from the paths used by the other groups. The power of the reactor is measured by short response time neutron flux detectors installed outside the reactor, and this measurement is used by the safety members. For reasons of bulk and positioning, there is only one detector per measurement point rather than four probes as in the case of other measurements. Each of these detectors is attributed to one of the protection chains. In conventional manner, the core of the reactor is in the form of a square grid located within a circumscribing square. Relative to said circumscribing square, the four neutron flux detectors are placed on its diagonal axes (see FIG. 1). However, it should be understood that the system of the invention is equally applicable to a hexagonal grid reactor. Reactor-protecting measures are undertaken when two of the four chains (2/4 logic) observe a rapid flux drop which is more negative than a threshold speed which is typically 2% to 3% of nominal flux per second. This speed is the derivative of the measured flux with respect to time, with said derivative being frequency filtered. The threshold is passed almost immediately after the accident while the nuclear power is being transiently reduced by the falling cluster and before the feedback and regulation systems have had any substantial effect. Such feedback systems tend to re-establish equilibrium between the nuclear power and the power requested by the steam turbine which is conventionally powered by the nuclear reactor. The core must be protected even if one of the four protection chains is faulty (single fault criterion). The normal presence of one or more neutron-absorbing clusters for limiting nuclear power in the core sets up a radial disturbance of the neutron flux which is acceptable, per se. However, it has been found that when one or more clusters fall accidentally, such a disturbance can greatly attenuate the fall-detecting signal from one or even two of the more distant detectors from the cluster or clusters which have fallen accidentally. Under such circumstances, only the closest two detectors deliver a meaningful signal. It is necessary to assume that one of these detectors is faulty. That is why, within the above-mentioned single fault criterion, it must be assumed that the protective measures required by some cluster falls will not in fact be performed because said falls will not be detected by the protection system. Such falls which may potentially not be reliably detected must be taken into account when dimensioning the boiler, and therefore lead to boiler performance being limited. The particular object of the present invention is to improve the performance of a nuclear boiler by reducing the constraints imposed on the constructor by the risk of the protection system failing to detect the fall of a cluster. The present invention provides a system which comprises, in conventional manner: said protection chains; PA1 said secondary treatment circuits; and PA1 said means for limiting said nuclear reaction. PA1 Compared with the above-described prior art system, the system of the invention is characterized by the fact that each of said protection chains includes: PA1 at least two of said neutron flux detectors angularly separated by more than 90.degree. about said axis; PA1 two of said primary treatment circuits associated respectively with said two detectors for providing said primary fall signals; and PA1 an intermediate treatment circuit for receiving said primary fall signals and for providing an intermediate fall signal on receiving at least one primary fall signal from at least one of said two primary treatment circuits; PA1 said fall signals provided at the outputs from said protection chains and received by said secondary treatment circuit being constituted by said intermediate fall signals, such that said secondary fall signal is provided in the event of one said reaction-inhibiting elements falling even if the resulting reductions in neutron flux received by a plurality of said detectors which are relatively far away from said hollow receiving said falling reaction-inhibiting element do not exceed said predetermined variation speed threshold, and even in the event of one of said chains being faulty so that it prevents one of said primary fall signals from a detector relatively close to said hollow being generated. Preferably, when said reactor core has square symmetry in plan view, with the number of said protection chains being four, said detectors are angularly disposed around said axis at an angular pitch of 45.degree.. Each of said protection chains then comprises two of said detectors which are angularly spaced by 135.degree. or by 180.degree. about said axis. An implementation of the present invention in the context explained above is described in greater detail by way of non-limiting example with reference to the accompanying diagrammatic figures. When the same item appears in several figures, it is designated in all of them by the same reference symbol. It should be understood that the items mentioned may be replaced by other items performing the same technical functions.