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BackgroundNeutron localization technology can be used for monitoring spent fuel reprocessing to prevent nuclear criticality accidents.PurposeThis study aims to investigate the effects of different neutron emission modes, 3He pressure, tube diameter, and moderators on the position resolution and detection efficiency of a position-sensitive 3He proportional counter.MethodsFirstly, a 3He proportional counter with a tube diameter, tube length, and 3He pressure of 2.54 cm, 80 cm, and 405 300 Pa, respectively, was taken as object of study, and the Monte Carlo (MC) algorithm was employed to simulate the effects of neutrons with different energies, neutron emission modes, 3He pressures, tube diameters, and moderating bodies on the position resolution and detection efficiency of a position-sensitive 3He proportional counter. Then, the effects of the time difference of arrival (TDOA) on the location of a source in an active zone, source to tube distances, thickness of moderating body, and position resolution of the 3He proportional counter were investigated. Finally, a linear fitting analysis of the active region was performed, and the locations of inactive regions were estimated to determine the positioning ability of the 3He tube.ResultsSimulation results show that the position resolution of this 3He proportional counter is equal in all axial sections, and the position-resolution limit of thermal neutron source is 0.17 cm. When the thickness of the moderating body is 5 cm, and the 244Cm source is close to the external wall of the moderating body, the neutron detection efficiency is 3.0%, and the theoretical position resolution is 9.25 cm whilst experimental result of position resolution is 18.50 cm, half of the theoretical value, and the location resolution limit is 2.64 cm after processed by the linear fitting curve of the active location and a time-difference crest.ConclusionsMC simulation and experimental tests show that the position-sensitive 3He proportional counter exhibits better position resolution and detection efficiency for the 244Cm source when the thickness of the moderating body is 2~3 cm, hence suitable for practical applications.

Abstract: The possibility of localization of an unknown neutron source in a bulky homogeneous medium (box) was studied. Previous study showed that for the planar case, a neutron source could be localized with accuracy better than 7%, by using the count rates ratio of two 3He detectors, positioned on the opposite faces of the box. The error is larger close to the edge of the box. Positioning of polypropylene slabs of different thicknesses between the detectors and the box, in order to cancel the effect of the edge of the box, was found to improve the accuracy of the localization of the source in points close to the faces of the box. The influence of the deviation of the source from the plane connecting the two 3He detectors was studied. It was found that for detectors placed on the centers of the sample faces, and neutron sources positioned at a large distance from the central plane, the calculation of the vertical distance between the source and the box plane by the usual R(x)/R(0) ratio with R(0) measured at the center of the box plane leads to quite large error, up to 15%. However, it was found that the error can be reduced by measuring R(0) of external source placed on the box face coplanar with the unknown source. In this case the accuracy does not depend on the distance from the central plane. Different ways to determine the plane of the unknown source were studied. It was found that it is impossible to determine it by looking for maximum count rate of one detector or the geometric mean of pair of detectors. However, it is possible to get better localization results of the source plane by using several pairs of detectors and finding a pair with maximal geometric mean. The experimental results were confirmed with MCNP simulations. It was found that using six detectors, three on each parallel faces of the box (460×420×200mm3), the location of the source can be found with an average error of 2.3% and a maximal error of 11%. [Copyright &y& Elsevier]

Fuel debris have a distinct neutron signature that can be detected to locate the said debris in a damaged nuclear power plant. Neutron measurement in a damaged PCV environment is however submitted to severe deployments constraints, including a high-dose-rate gamma background and limited available space. The study was therefore oriented towards small fission chambers (FC), with U-235-enriched active substrates. To investigate the expected performance of the FC in various irradiation conditions, a numerical model of the detector head was built. We describe the elaboration and experimental calibration of the numerical model and the Monte Carlo study of the fission rate inside U-235 coatings per generated neutron. The evaluation of a representative calibration coefficient then allowed us to carry out a multi-parameter performance study of a FC underwater, aiming at computing an explicit response function linking, on the one hand, the activity and spatial distribution of neutron emitters in a water container, with, one the other hand, the expected count rates measured by a fission chamber as a function of its radial and axial position inside the water volume. The FC underwater behavior was subsequently corroborated by a measurement campaign on a FC response, set at different positions inside a water drum, as a function of its axial and radial distance to a Cf-252 neutron source attached near the center of the container. We finally present an approach in which fuel debris localization is defined as an Inverse Problem, solvable with a Maximum-Likelihood Expectation Maximization (ML-EM) iterative algorithm. The projector matrix is built by capitalization on the results of the previously consolidated numerical studies. The ML-EM was tested on simulated data sets with a varying number of active voxels. Our first results indicate that, for a thermal neutron flux in the order of 10 n.cm−2.s−1 at the detector, originating voxels are identified with a spatial resolution in the radial plane in the order of 10 to 100 cm2. [ABSTRACT FROM AUTHOR]