Patent Number: 059563800
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a diagrammatic, longitudinal-sectional view, which is not to scale, of a containment 8 of a boiling water nuclear power facility having an apparatus 7 for determining neutron flux density n. The containment 8 encloses a reactor pressure vessel 9, in which a reactor core 3 is disposed. The containment 8 as well as the reactor pressure vessel 9 are directed along a major axis 17. A plurality of fuel assemblies or elements 4 are disposed in the reactor core 3 and are likewise directed along the major axis 17. The fuel assemblies 4 form a source 1 which emits neutrons as a consequence of nuclear fission. Measurement lances 13, only two of which are illustrated for reasons of clarity, are provided between a number of mutually adjacent fuel assemblies 4 and are directed parallel to the major axis 17. Each measurement lance 13 has a drive 10 outside the reactor pressure vessel 9 for moving a push rod 12 in the measurement lance 13. An ionization chamber 2, in particular a so-called fission chamber, which forms a neutron detector, is disposed within an end of each push rod 12. The drive 10 allows the ionization chamber 2 to be moved into the reactor core 3, between the fuel assemblies 4 and, as is particularly important during power operation of the nuclear power facility, the drive 10 allows the ionization chamber 2 to be withdrawn from the reactor core 3 again. Each ionization chamber 2 is connected through a connecting cable 11, possibly through a plurality of connecting cables 11, to a preamplifier 14 which is disposed outside the containment 8. The ionization chamber 2 as well as the preamplifier 14 are part of a measurement device 5 for producing and transmitting a first measurement signal S.sub.1 and a second measurement signal S.sub.2, which are used to form a wide-range signal W that is associated uniquely with the neutron flux density n. Each preamplifier 14 is connected to an evaluation device 6 in which the wide-range signal W is determined. The evaluation device 6 is connected to an output unit 15, in particular for displaying the wide-range signal W, for example on a screen or on a printer. The evaluation device 6 is furthermore connected to a reactor safety system 16 of the nuclear power facility. Neutron flux densities as well as, in particular, major changes in the neutron flux densities which lead to the conclusion that a rapid change has occurred in the reactor power level, can thus be detected through the apparatus 7 and can be input into the reactor safety system 16. The neutron flux density n determined from the wide-range signal W can thus be used as a reliable signal for triggering reactor scramming. FIG. 2 graphically shows a profile of a wide-range signal W (drawn as a solid line) as a function of the neutron flux density n of a nuclear power facility. The wide-range signal W is a function which is defined region-by-region and has three directly mutually adjacent regions. A first region extends from a neutron flux density n=0 to a neutron flux density n=n.sub.1. A second region extends from the neutron flux density n=n.sub.1 to a higher neutron flux density n=n.sub.2. The second region is followed by a third region. The second region is referred to as an overlapping region .DELTA.. In the overlapping region .DELTA., the wide-range signal W is a monotonally rising function which is formed from the first measurement signal S.sub.1 and the second measurement signal S.sub.2. The first measurement signal S.sub.1 is a monotonal function of the neutron flux density n from the neutron flux density n=0 to a neutron flux density n=n.sub.10. The first measurement signal S.sub.1 is preferably a pulsed signal from the ionization chamber 2. The second measurement signal S.sub.2 is a monotonally rising function of the neutron flux density n in a region of the neutron flux density n which starts at a second limit flux density n.sub.20. In the first region, that is to say up to the neutron flux density n=n.sub.1, the wide-range signal W is equated to the first measurement signal S.sub.1. Since, in practice, the neutron flux density n=n.sub.1 is unknown, the opposite procedure is used, namely a first signal value N.sub.1, which is uniquely associated with the neutron flux density n=n.sub.1, of the first measurement signal S.sub.1 is assumed to be the lower limit of the overlapping region .DELTA.. If the first measurement signal S.sub.1 is less than or equal to this first signal value N.sub.1, then the wide-range signal W is set to be identical to the first measurement signal S.sub.1. The wide-range signal W is formed analogously by the second measurement signal S.sub.2 for values of the neutron flux density n greater than n.sub.2. Since the second measurement signal S.sub.2 in this region is likewise a monotonal function of the neutron flux density n, a signal value N.sub.2 of the second measurement signal S.sub.2 is uniquely allocated to the neutron flux density n=n.sub.2. The second signal value N.sub.2 is greater than the first signal value N.sub.1. The condition that the second measurement signal S.sub.2 is greater than the second signal value N.sub.2 is accordingly selected as the criterion for equating the wide-range signal W to the second measurement signal S.sub.2. The overlapping region .DELTA. defined by the neutron flux densities n.sub.1 and n.sub.2 is thus uniquely defined by the first signal value N.sub.1 and the second signal value N.sub.2. The first measurement signal S.sub.1 as well as the second measurement signal S.sub.2 are both monotonal functions of the neutron flux density n, at least in part of the overlapping region .DELTA.. In this overlapping region .DELTA., the wide-range signal W is determined both from the first measurement signal S.sub.1 and from the second measurement signal S.sub.2. More specifically, this is done in such a way that the wide-range signal W is the sum of the product of the first measurement signal S.sub.1 and a coefficient factor .alpha. dependent on the second measurement signal S.sub.2, and the product of the second measurement signal S.sub.2 and a coefficient factor .beta. dependent on the first measurement signal S.sub.1. The coefficient factors .alpha.,.beta. are chosen in each case in such a way that the wide-range signal W corresponds with the first signal value N.sub.1, that is to say with the first measurement signal S.sub.1, at the neutron flux density n.sub.1, and corresponds with the second signal value N.sub.2, that is to say with the second measurement signal S.sub.2, at the neutron flux density n.sub.2. Furthermore, the coefficient factors .alpha.,.beta. are chosen in such a way that the wide-range signal W is a monotonal function of the neutron flux density n in the overlapping region .DELTA.. This is achieved, for example, by the coefficient factor .beta., which is dependent on the first measurement signal S.sub.1, being proportional to the difference between the first signal value N.sub.1 and the first measurement signal S.sub.1. Analogously, the other coefficient factor .alpha. is proportional to the difference between the second measurement signal S.sub.2 and the second signal value N.sub.2. This choice means that, if the neutron flux density n=n.sub.1, then the wide-range signal W merges continuously into the overlapping region .DELTA., irrespective of the value of the second measurement signal S.sub.2. This also applies to the region boundary defined by the neutron flux density n=n.sub.2. Even if there is a shift in the start of the proportional dependency of the first measurement signal S.sub.1 and of the second measurement signal S.sub.2 of the neutron flux density n, for example as a result of the physical conditions in the ionization chamber 2 changing with time, the method ensures that the wide-range signal W is always a unique function of the neutron flux density n. The invention is distinguished by a method for determining the neutron flux density, particularly in a reactor core of a boiling water or pressurized water nuclear power facility, in which a transition of a wide-range signal in an overlapping region is ensured while maintaining a unique association with the neutron flux density. At low neutron flux densities, the wide-range signal is preferably identical to a pulsed signal from an ionization chamber, and at high neutron flux densities of up to about 10.sup.14 neutrons/(cm.sup.2 .multidot.s) the wide-range signal is identical to an alternating-current signal from the same ionization chamber. In the overlapping region, the wide-range signal is determined from a combination of the pulsed signal and the alternating-current signal. The combination ensures that the wide-range signal in each case merges continuously into the corresponding measurement signal, the pulsed signal or the alternating-current signal, at the boundaries of the overlapping region. The wide-range signal in the overlapping region is preferably formed by the sum of the product of the pulsed signal and a factor dependent on the alternating-current signal, and of the product of the alternating-current signal and a factor which is dependent on the pulsed signal. The wide-range signal which is formed in this way makes it possible to detect the neutron flux densities in a nuclear power facility from shut-down operation up to power operation at 100% of the rated power of the nuclear power facility. The method is particularly suitable for determining the neutron flux density in the reactor core of a boiling water nuclear power facility.