Patent Number: 
Section: claims

1. A process for monitoring at least one operating parameter of a nuclear reactor core of a nuclear power station power unit, the core having a number of juxtaposed fuel assemblies arranged over the height of the core, the process comprising the steps: introducing a plurality of detectors, each having a string of spaced stacked detector units, into at least some of the fuel assemblies of the core for measuring neutron flux, the detectors being fixed and distributed over the height of the core;  during operation of the nuclear reactor, at specified time intervals:  a) measuring the neutron bulk flux distribution using a maximum number of detectors being equal to 15% of the number of fuel assemblies;  b) employing a neutron flux calculation code in conjunction with measurements provided by the detectors to obtain an instantaneous neutron bulk flux distribution throughout the entire core in the form of a set of neutron flux values at points distributed throughout the core,  c) calculating at least one core operating parameter from the instantaneous neutron bulk flux distribution; and  d) raising an alarm if at least one operating parameter is outside a preselected range. 2. The monitoring process according to  claim 1 , together with the following steps to obtain the instantaneous neutron flux distribution throughout the entire core: claim 1 instantaneously calculating, on the reactor site, the bulk flux distribution inside the core, in the form of a set of neutron flux values at the various points distributed throughout the core, comprising a first sub-set of instrumented positions where the neutron flux measuring detectors are located and a second sub-set of non-instrumented positions, the instantaneously calculating being based on parameters originating from the power unit plant, using the neutron flux calculation code;  calculating, for each instrumented position, the difference between the flux values obtained by measurement and the corresponding values calculated from the parameters originating from the power unit plant;  extrapolating the corresponding differences for every non-instrumented position, from the differences relating to the instrumented positions;  algebraically adding the values of the instrumented and non-instrumented differences to the bulk flux distribution values obtained from the parameters originating from the power unit plant, to obtain the measured value of the bulk flux distribution for every point distributed throughout the core. 3. The monitoring process according to  claim 1  together with the following steps to obtain the instantaneous neutron flux distribution throughout the core: claim 1 instantaneously calculating, on the reactor site, the neutron bulk flux distribution inside the core, in the form of a set of neutron flux values at the various points distributed throughout the core, comprising a set of instrumented positions where the neutron flux measurement detectors are located, the instantaneous calculating being based on parameters originating from the power unit plant, using the neutron flux calculation code;  calculating, for each instrumented position, the differences between the neutron flux values obtained by measurement and the corresponding values calculated from the parameters originating from the power unit plant;  the calculated differences being used to correct defining parameters of the neutron flux calculation code; and  performing a second instantaneous calculation on the nuclear reactor site of the instantaneous neutron flux distribution inside the core based on the parameters originating from the power unit plant, using the neutron flux calculation code which includes corrected defining parameters. 4. The monitoring process according to  claim 1  wherein the instantaneous values of the parameters coming from the power unit plant and current neutron flux values determined for every point in the core are used to calculate new instantaneous neutron flux values simply, using the calculation code. claim 1 5. The monitoring process according to  claim 1  wherein, for a core containing a number of fuel assemblies approximating 200, fewer than 30 detectors are used. claim 1 6. The monitoring process according to  claim 5  wherein, for a core containing 193 fuel assemblies, 16 detectors are used. claim 5 7. The monitoring process according to  claim 1  wherein, the at least one operating parameter of the nuclear reactor core is chosen from the group of the following parameters: linear power density P lin , critical heating ratio (CHR, or DNB ratio), axial power imbalance PI ax , azimuthal power imbalance PI az , and negative reactivity margin NRM. claim 1 8. The monitoring process according to  claim 1 , wherein conditioning processing of the measured signals from the detectors includes the steps of isolating, for each signal, an electric signal due to a phenomenon of fast electron production; and by applying an inverse transfer function, determining a neutron flux value from electric current isolated from the current signal. claim 1 9. The monitor process according to  claim 1 , wherein neutron flux measurements are obtained by reading outputs of the flux detectors at time intervals of less than one minute. claim 1 10. An apparatus for monitoring at least one operating parameter of a nuclear reactor core of a nuclear power station power unit, the core having a number of juxtaposed fuel assemblies arranged over the height of the core, the apparatus comprising: a plurality of detectors, each having a string of spaced stacked detector units, located in at least some of the fuel assemblies of the core for measuring neutron flux, the detectors being fixed and distributed over the height of the core;  the maximum number of detectors being equal to 15% of the number of fuel assemblies;  means for conditioning signals from the detectors;  processing means for determining the power distribution in the core and the at least one core operating parameter;  means for comparing the at least one operating parameter with at least one predetermined limit and for generating an alarm signal if the limit is exceeded; and  means for displaying the alarm signal. 11. The apparatus according to  claim 10 , wherein the neutron flux measurement detectors are self-powered neutron detectors. claim 10 12. The apparatus according to  claim 11 , wherein the self-powered neutron detectors comprise a transmitter made of a rhodium-based material. claim 11