Patent Number: 040240177
Section: claims

1. A method of measuring burn-up of nuclear fuel in a nuclear reactor including a core, comprising the steps of introducing a neutron flux probe of constant composition of at least two different nuclides which deliver two separable activities produced by two different neutron energies to a measuring point of the reactor core the burn-up of which is to be measured during operation of the reactor, the probe producing two measuring signals, each measuring signal being a function of the local flux of a different neutron energy changing with the burn-up at the measuring point, comparing the two signals and computing the burn-up on the basis of the comparison. 2. The method of claim 1, wherein the probe is moved into into the reactor core, the probe being capable of delivering two separable activities with at least two different neutron energies, the probe is permitted to remain in the nuclear fuel for a predetermined period of time, the probe is then removed and subjected to a measurement of the activities, and the burn-up is computed by comparing at least two of the activities. 3. The method of claim 2, wherein the activities are separable by their decay characteristics, the number of pulses of each energy is determined in at least two measuring channels associated with the activities, and the burn-up is computed by comparing the determined numbers of pulses. 4. The method of claim 2, wherein the activities are separable in time, at least two activity measurements are made in at least two predetermined time intervals, and the burn-up is computed by comparing the activity measurements. 5. The method of claim 2, wherein the probe is constituted by a column of balls or a helical spring, and the column of balls is introduced into the nuclear fuel through guide tubes distributed through the core. 6. The method of claim 2, wherein the probe comprises manganese and a metal selected from the group consisting of nickel, vanadium and gold, or vanadium and nickel. 7. The method of claim 6, wherein the probe also comprises a metal selected from the group consisting of iron, chromium and titanium. 8. The method of claim 1, wherein fission chambers are located in the fuel, the fission chambers being charged with different fissile materials, and the measuring signals are produced in the fission chambers. 9. The method of claim 8, whrein the fission chambers are fixedly arranged in the nuclear fuel. 10. The method of claim 8, wherein the fission chambers are temporarily introduced into the nuclear fuel to produce the measurements. 11. The method of claim 8, wherein at least two of the fission chambers are formed into a structural unit. 12. The method of claim 8, wherein the reactor has fixedly arranged fission chambers and associated calibrating chambers arranged to be moved through guide tubes distributed through the nuclear fuel during the operation of the reactor, comprising the steps of introducing additional fission chambers charged with a different fissile material into a respective one of the guide tubes to produce the measuring signals for comparison, the measuring signals being computed when the movable additional fission chambers are at the level of the associated fixed fission chambers. 13. An apparatus for measuring burn-up of nuclear fuel in a nuclear reactor, comprising two fission chambers of different sensitivity constant in time with respect to the local neutron spectrum of the reactor and changing as a function of the local burn-up, the fission chambers being arranged in the reactor core at a measuring point and each fission chamber producing a measuring output signal, each measuring output signal being a different function of the local neutron energy distribution changing as a function of the burn-up, and means designed to compute a value associated with the local burn-up on the basis of comparison of the two output signals. 14. An apparatus for measuring burn-up of nuclear fuel in a nuclear reactor, including a core and guide tube means leading into the core, said guide tube means having an activation section within the core, comprising 1. a measuring system comprising elements of an alloy of at least two components having a constant mixing ratio, the components of the elements delivering two separable activities produced by two different neutron energies, and the elements being movable through the activation section of the guide tube means into the reactor core,  2. a detector unit outside the reactor and connected to the measuring system, the movable elements being received from the activation section in the reactor core through the guide tube means,  3. means arranged to receive and compare the output signals and designed to compute a value associated with the local burn-up on the basis of comparison of the output signals. 15. The apparatus of claim 14, wherein the elements are balls. 16. The apparatus of claim 14, wherein the elements consist of a helical spring.