Patent Number: 049833502
Section: summary

The present invention relates to controlling a nuclear power station and more specifically to detecting the fall of a control cluster into the core of the reactor of such a station. Such detection must be reliable even if a component or a set of components in a protective system happens to be faulty. It is important for the fall of a cluster to be detected with all the required safetY in spite of the possibility of such a faulty system. Clusters are constituted by neutron-absorbing rods and nuclear power, i.e. the power evolved by the reaction, is regulated and controlled by moving the clusters. They are distributed over the horizontal section of the core. They are vertically movable in both directions. For this purpose they are hooked onto an appropriate mechanism which is situated above the core and which is controlled to cause the absorbing rods of a cluster to penetrate into the core to a greater or lesser extent. A failure in this mechanism or in the hooking member may result in a cluster falling into the core. Such a fall gives rise to a local reduction in nuclear power, and thus to a reduction in the overall power of the core. Since there is a power-regulating servo loop, such a decrease is rapidly compensated by raising other control clusters. However, this gives rise to various drawbacks including neutron flux distortion, thereby slowing down combustion of the fuel elements close to the fallen cluster and accelerating combustion elsewhere. In addition, the range of action possible on the reaction is reduced. That is why it is desirable to detect such accidental falls as reliably and as quickly as possible in order to perform an emergency reactor stop and reestablish normal operation thereof. It is also necessary to avoid performing such an emergency stop if that is unnecessary, since such a stop is expensive, in particular because it reduces the availability of the nuclear power station for providing electricity. In a known method for detecting falls of control clusters, use is made of detection means which are also applicable to other types of accident. More precisely, these means detect the excessive temperatures caused by pockets of steam appearing at various points in the core along the rods constituting the fuel elements. When a cluster falls accidentally, the first rise in nuclear power to a certain level after the fall has taken place causes such an excess temperature or pocket of steam to appear along a fuel element whose combustion is accelerated by the flux distortion. Different automatic protection means then cause an emergencY reactor stop. Thereafter the cause of the stop is determined and the consequences of the accident are repaired prior to putting the reactor back into service. This known method suffers from the drawback that a fairly long period of time may elapse before the nuclear power is raised high enough to give rise to the above-described stop process, particularly if the anti-reactivity of the fallen cluster is relatively low. The combustion rate of some fuel elements can thus be increased, thereby initiating nucleated boiling. A particular object of the present invention is to provide reliable and rapid detection of the fall of a control cluster, even when the position and/or the low degree of anti-reactivity of the fallen cluster gives rise to only a small variation in nuclear power, while simultaneously avoiding as far as possible the danger of a false detection giving rise to an unnecessary emergency reactor stop. Another object of the invention, by virtue of such detection, is to provide better protection for the reactor of a power station against the damaging consequences which would arise from continuing the nuclear reaction in normal service after a cluster has fallen, while preserving the availability of the power station. The invention seeks to achieve these objects in a manner which is both simple and cheap. That is why the present invention provides a method of detecting the fall of a control cluster in a nuclear reactor, wherein "sensitive" parameters sensitive to displacements of the control clusters of the reactor are measured on a long-term basis, and a cluster fall signal is generated when one of said parameters varies at a speed greater than a predetermined alarm threshold corresponding to said parameter, the method being characterized by the fact that said sensitive parameters which are measured include parameters of at least a first type and a second type, said first type being constituted by position parameters representative of the positions of said control clusters themselves, and said second type being constituted by parameters which are sensitive to displacements of control clusters even when said clusters are situated at a distance from points where said parameters are measured, that the parameters of each of these two types are measured in at least two zones of the core of said reactor, and that said cluster fall signal is generated when at least two of said sensitive parameters varies at a rate greater than said corresponding alarm thresholds, with one of said two parameters being one of said position parameters. In a preferred disposition, said second type of sensitive parameter and optionally a third type of sensitive parameter are constituted by neutron flux parameters representative of the nuclear power evolved in each of said zones of the core or/and by heat flux parameters representative of the heat flux removed by cooling fluid flowing through each of said zones. The present invention also provides an apparatus for detecting the fall of a control cluster into the core of a nuclear reactor, said core comprising a plurality of zones each provided with: heat measurement means for providing a heat flux signal corresponding to said zone and representative of heat flux removed by the flow of a cooling fluid through said zone; PA1 neutron measuring means for providing a neutron flux signal corresponding to said zone and representative of nuclear power in said zone; and PA1 a plurality of control clusters, with each of said control clusters being provided with measurement means for providing a cluster position signal representative of the position of said cluster; PA1 said apparatus comprising differentiation and comparison means for receiving "sensitive" signals sensitive to the displacements of the control clusters and for providing corresponding alarm signals whenever said sensitive signals vary at a rate greater than corresponding predetermined alarm thresholds; and PA1 logical processor means for providing a cluster fall signal in the presence of a plurality of said alarm signals; PA1 said apparatus being characterized by the fact that said alarm signals include at least signals of a first type and of a second type, said first type being constituted by position alarm signals corresponding to said cluster position signals and a second type being constituted by heat alarm signals corresponding to said heat flux signals and/or by neutron alarm signals corresponding to said neutron flux signals, said logic processing means providing said cluster fall signal on receiving at least two of said alarm signals including at least one of said position alarm signals. PA1 said logic processing means comprise: PA1 said differentiation and comparison means are associated with said primary logic units in such a manner that, together with a corresponding portion of said means, each of said logic units constitutes an acquisition unit; PA1 said acquisition units constitute a succession corresponding to the succession of said zones in the core of the nuclear reactor; and PA1 each of said acquisition units receives a group of said cluster position signals corresponding to one of said core zones, said neutron flux signal corresponding to another core zone, and said heat flux signal corresponding to yet another core zone, such that said neutron flux signals, said heat flux signals, and said groups of cluster position signals are each received by one and one only of the acquisition units. The following preferred dispositions in accordance with the invention may also be adopted: primary logic units each of which receives said position alarm signals corresponding to at least one of said core zones and at least one of said second type alarm signals corresponding to another of said zones, and for providing a primary detection signal when at least one of said alarm signals is present; and PA2 a combination circuit receiving said primary detection signals and said position alarm signals and providing said cluster fall signal when at least two of said primary detection signals and at least one of said position alarm signals are present; An implementation of the invention is described by way of non-limiting example and with reference to the accompanying diagrammatic figures. In the description, those dispositions which are mentioned below as being preferred in accordance with the present invention should be considered as being used. It should also be understood that the elements described and shown may be replaced by other elements providing the same technical functions without going beyond the scope of the invention.