Patent Number: 060350105
Section: description

Referring now to FIGS. 1 and 2, the monitor according to the invention comprises a lead block 1 in which a gamma detector 2 for gamma spectroscopy as well as two neutron detectors 3 are located. The block has a front face 8 which is the measurement face of the monitor and intended to be as close as possible to the fuel pin or assembly to be measured. A collimator 4 extends from the front face 8 through the block to the gamma detector 2 in the rear part of the block. The bore hole of the collimator has a diameter of about 2 or 3 mm and the block height is about 270 mm, the dimensions of the front face 8 being for example 200 mm.times.200 mm. In accordance with the state of the art, the gamma detector 2 contains a crystal which operates at room temperature, for example a CdTe crystal or a CdZnTe crystal. A preamplifier is integrated into the detector and outlet conductors (not shown) are connected to nuclear instrumentation for treating and analysing the measurement results. In such a detector, an energy resolution (FWHM) of 7.2 keV at 662 keV is achieved which permits gamma spectroscopy of fission products in the spent fuel. The collimator 4 may be equipped with a removable tube made of tungsten or the like in order to enhance the shielding of the detector. This tube may be removable in order to adapt the bore hole diameter or the material of this tube to the required collimator function. The two cylindrical neutron detectors 3 are disposed perpendicularly to the scanning direction (in FIG. 2 along the pin axis) as well as parallel to each other and parallel to the front face 8 in the block close to this front face on either side of the collimator 4. The distance between the axis of each detector 3 and the axis of the collimator 4 must respond to contradictory requirements: On the one hand, this distance should be large enough in order not to weaken the shielding effect of the lead block around the collimator hole by the presence of the neutron detectors and moderating material, and on the other hand this distance should be as small as possible, so that the neutron and gamma detectors are as close as possible to the point under investigation. Due to the fact that the neutron emission is measured by two detectors disposed on either side of the ideal location which is occupied by the collimator 4, the measurement results of these two neutron detectors can be combined and a mean value can be established which corresponds with high precision to the neutron emission activity along the axis of the collimator 4. The neutron detectors can be .sup.235 U fission chambers which are sensitive to thermal neutrons but very insensitive to the intense gamma radiation emitted by the spent fuel. The incident fast neutrons are thermalized by a moderator material 5 which surrounds the detectors. In a first embodiment as shown in FIGS. 1 and 2, this material completely surrounds the detectors and has a cylindrical tubular shape. According to a second embodiment of the invention shown in FIG. 3, the moderator material 5' occupies only a sector of about 90.degree. out of the entire periphery around the detector axis. This sector begins in a plane parallel to the collimator axis and extends towards the collimator. Such a configuration enhances the contribution of neutrons coming from a reduced length range of the pins 7 on both sides of the collimator axis and reduces the contribution of neutrons from outside that range. Thus, a higher precision is obtained. According to an additional feature, at least one of the neutron detectors together with its moderator is associated to mechanical means 9 for insertion of a cadmium sheet 6 around the moderator material or for withdrawing it therefrom. The detector with the cadmium sheet in place responds mainly to fast neutrons which pass through the cadmium in the detector while thermal and low energy neutrons are absorbed. Such a measure gives an additional information which facilitates the characterization of the fuel, under wet storage. The cadmium sheet 6 is inserted during a separate scanning cycle performed after a main scanning cycle in which both neutron detectors are used without shielding by a cadmium sheet. The monitor as shown above can be used for either passive or active neutron measurements. In this latter case, a neutron source 10 such as an isotopic neutron source or a neutron generator must be placed in alignment with the collimator axis beyond the fuel pin or assembly. An arm must then be provided (not shown) which holds the source 10 and which is rigidly connected to the lead block 1. A moderating material can then be placed between the source 10 and the fuel pin or assembly for thermalising the incident neutrons. The invention is not restricted to the embodiments as described and shown in the drawings. The lead block may have rectangular or circular cross-section. The collimator may be constituted purely by a bore hole in the lead block, or an insert tube may be used, made of tungsten or another appropriate material to shield the gamma radiation. The neutron detectors may be of any other appropriate type such as .sup.3 He, BF.sub.3. The monitor can be used in a dry storage environment as well as in a storage pool, if an appropriate water-tight casing is used. Finally, the monitor according to the invention can be used not only for scanning fuel pins parallel to their axis, but also for monitoring the burn-up profile of a fuel assembly. In this latter case, a relative scanning movement is performed by which the lead block turns around the assembly. The monitor could incorporate additional features for optimising the measurements: for example, to achieve repeatable counting geometries and to define the respective measuring position.