Patent Number: 
Section: claims

1. A spent fuel pool flux measurement system for measuring and monitoring axial flux to determine subcriticality, in a nuclear power plant, which comprises:a spent fuel pool;a neutron source located in the spent fuel pool;an analytical tool to generate analytical axial flux curves for a plurality of different concentrations in soluble boron, generate an index of analytical axial flux curves based on the analytical axial flux curves, determine a slope for each analytical axial flux curve and determine an effective multiplication factor of neutrons based on the slope;one or more neutron detectors installed in the spent fuel pool and arranged to generate signals resulting from neutron interactions in the spent fuel pool;a counting device to receive and count said signals generated by the one or more neutron detectors;a connecting means to electrically connect the one or more neutron detectors to the counting device;a signal analyzer to receive counted signals from the counting device and to determine reactivity of fuel assemblies in the spent fuel pool based on counted signals;a power supply unit for the neutron detectors, the counting device and the signal analyzer; anda computer coupled to the signal analyzer and configured to store the index of analytical axial flux curves, generate a measured axial flux curve, determine a slope of the measured axial flux curve and correlate the slope of the measured axial flux curve with a slope of an axial flux curve in the index of analytical axial flux curves to obtain a value of a measured effective multiplication factor. 2. The system of claim 1, wherein the one or more neutron detectors comprises at least one silicon carbide semiconductor diode. 3. The system of claim 2, wherein the at least one silicon carbide semiconductor diode operates in neutron pulse mode when exposed to high gamma-ray fields. 4. The system of claim 1, wherein the value of the effective multiplication factor is less than or equal to 0.95 in normal conditions. 5. The system of claim 1, wherein regions of the spent fuel pool having the highest reactivity are determined by Monte Carlo simulations and the index of analytical axial flux curves. 6. The system of claim 1, wherein the nuclear power plant is a pressurized water reactor. 7. The system of claim 1, wherein the plurality of different concentrations in soluble boron is 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm and 2400 ppm. 8. The system of claim 1, wherein the analytical tool is a Monte Carlo analysis.