Abstract
Identification of the position of a localized neutron source, or that of local inhomogeneities in a multiplying or scattering medium (such as the presence of small, strong absorbers) is possible by measurement of the neutron flux in several spatial points, and applying an unfolding procedure. It was suggested earlier, and it was confirmed by both simulations and pilot measurements, that if, in addition to the usually measured scalar (angularly integrated) flux, the neutron current vector or its diffusion approximation (the flux gradient vector) is also considered, the efficiency and accuracy of the unfolding procedure is significantly enhanced. Therefore, in support of a recently started project, whose goal is to detect missing (replaced) fuel pins in a spent fuel assembly by non-intrusive methods, this idea is followed up. The development and use of a dedicated neutron detector for within-assembly measurements of the neutron scalar flux and its gradient are planned. The detector design is based on four small, fiber-mounted scintillation detector tips, arranged in a rectangular pattern. Such a detector is capable of measuring the two Cartesian components of the flux gradient vector in the horizontal plane. This paper presents an initial evaluation of the detector design, through Monte Carlo simulations in a hypothetical scenario.
Highlights
In nuclear engineering, many applications are related to the task of locating the position of a neutron source, or some strong inhomogeneity, from the measurement of the neutron flux in a multiplying or neutron scattering medium
Later the possibility of locating the position of a neutron source in a water tank from the measurement of the scalar flux and its gradient in one single point was demonstrated experimentally, with the unfolding procedure being supported by Monte Carlo simulations [5]
The detector is based on four thin LiF/ZnS(Ag) optical fiber-mounted neutron scintillators arranged in an aluminum matrix according to a rectangular pattern
Summary
Many applications are related to the task of locating the position of a neutron source, or some strong inhomogeneity (e.g. the presence of a strong absorber), from the measurement of the neutron flux in a multiplying or neutron scattering medium. The measurement of the flux gradient or the neutron current was made possible by the use of very thin detectors (about 1 mm), developed in Japan [8,9], which allows to obtain the scalar neutron flux with a high spatial resolution In these detectors a small volume of a mixture of neutron converter and scintillation material is mounted on the tip of a light guiding fiber. The idea is to perform measurements concurrently in several radial points of a fuel assembly, and comparing the measured flux shape with the one calculated from the declared data of the (intact) fuel assembly (to discover the absence of fuel pins), as well as with calculations with defect fuel (to identify the position of the missing pins) This is clearly an inverse problem, similar to the identification of a neutron source or a strong absorber in a homogeneous medium, substantially more complicated. The results of the numerical investigation of the performance of the detector are discussed and conclusions are drawn
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