Evaluation of the performance of a neutron gradient detector for partial defect testing in spent nuclear fuel assemblies

In recent research efforts, neutron detectors consisting of bundles of optical fibers whose tips are covered in scintillation material have been investigated. These neutron detectors can be of a relatively small size since optical fibers have diameters of a few millimeters, so that they can be accommodated into narrow spaces, e.g., within nuclear fuel assemblies. Having an array of scintillators also enables the possibility to measure the flux gradient vector, in addition to the usually measured scalar flux. As a collaboration between Chalmers University of Technology and The Belgian Nuclear Research Center SCK CEN, a neutron detector based on four optical fiber-mounted scintillators, arranged in a rectangular pattern, is under development for the safeguard application of detecting possible missing fuel pins (which might or might not be replaced with non-nuclear material) in a spent fuel assembly. In principle, such a detector can measure the two cartesian components of the flux gradient vector in the horizontal plane, therefore enhancing the efficiency and accuracy in identifying possible inhomogeneities in a multiplying or scattering medium (such as the case of missing pins in a spent fuel assembly). The concept of the detector was already evaluated through a series of Monte-Carlo simulations, but for the simple scenario of a neutron source in a water tank. In this paper, a further step of the analysis is presented, i.e., the Monte-Carlo simulations of the performance of the detector in estimating the gradient of the neutron flux in a full PWR spent fuel assembly. Both the intact configuration and examples of partial defects at different locations and with different numbers of removed fuel pins are considered.