Conceptual Design And Initial Evaluation Of A Neutron Flux Gradient Detector For Partial Defect Testing Of Spent Nuclear Fuel

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 has been suggested earlier, and it was confirmed by both simulations and pilot measurements, that if in addition to the usually measured scalar (angularly integrated) flux, also the neutron current vector, or its diffusion theory approximation, the flux gradient vector is also measured, this can enhance the efficiency and accuracy of the unfolding procedure significantly. Therefore, in support of a recently started project in collaboration between Chalmers and SCK CEN, whose goal is to detect missing (replaced) fuel pins in a spent fuel assembly by non-intrusive methods for safeguard purposes, this idea is followed up. For the within-assembly neutron measurements, the use of a dedicated neutron gradient detector is 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 a full feasibility and sensitivity study of the detector design, through Monte-Carlo simulations of the complete design and a measurement scenario.