Device for Operative Monitoring of Fissionable Materials

Abstract

A device providing high-accuracy, high-speed detection of fissile materials, the device comprising two coaxially arranged cylinders: an inner cylinder made of lead, which acts as a gamma shield and as a neutron multiplier; and an outer cylinder made of polyethylene, which acts as a neutron thermalizer. A pedestal capable of vertical axial movement is disposed in the lower part of the inner cylinder. Fifteen helium-3 counters with cadmium filters can be built into the wall of the outer cylinder in a circle, parallel to the generatrix. An isotropic deuterium-tritium 14 MeV neutron generator can be mounted in the wall of the outer cylinder, perpendicular to the generatrix. In one aspect, before installation, a vessel and a structural material contained therein are subjected to gamma scanning. Then, using the movable pedestal, the vessel is mounted in the inner cylinder so the center of mass of the structural material is opposite detectors.

Claims

1. A device for the operational non-destructive investigation of low concentrations of fissile materials in closed vessels based on helium-3 counters with cadmium filters and a pulsed neutron generator, installed in a polyethylene container, characterized in that the device has the form of two coaxially arranged cylinders, the inner one being from lead, which at the same time serves as gamma radiation protection and as a neutron breeder, the outer one being from polyethylene and serving as a neutron moderator, wherein in the lower part of the inner cylinder a pedestal is mounted with the possibility of its vertical movement along an axis, which positions the preliminarily gamma-scanned vessel with the SM in the inner cylinder in such a way that the center of mass of the SM is located opposite the detectors.

2. The device according to claim 1, characterized in that fifteen helium-3 counters with cadmium filters are equidistantly installed in the wall of the outer cylinder on a circle parallel to the generatrix.

Description

[0009] The inventors of the present invention were confronted with the task to develop an installation for determining small amounts (0.001 mass %) FM in SM of SHPA which is functioning under conditions of a high-power gamma-background and which is insensitive to the spatial arrangement of SM in a receiving vessel.

[0010] The technical result provided by the above set of features is a high (compared to the method of passive neutron control) accuracy and speed of determination of the FM due to the 12-fold increase of the signal level above the three-fold error of the background, the possibility of a direct unambiguous determination of the content of plutonium-239 and an indirect determination of the other actinides at a known ratio Pu:Am, as well as a non-sensitivity of the results of measurement towards the spatial arrangement of sample and gamma-irradiation. In addition, the installation can operate in a passive mode, i.e., determine the content of Cm. Consequently, knowing the content of Cm-242 and Cm-244, as well as Pu-239, it is possible to find the ratio of M.sub.Pu239/(M.sub.Cm242+M.sub.Cm244), which in its turn makes it possible to reliably estimate the masses of any other actinides included in the SM of SHPA and heat producing elements.

[0011] The technical result is achieved in that the basis of the design of the proposed invention is two coaxially arranged cylinders: an inner one of lead, serving at the same time as gamma radiation protection and a neutron breeder, and an outer one from polyethylene serving as a neutron thermolysator. Fifteen helium-3 counters with cadmium filters are equidistantly installed in the wall of the outer cylinder in a circle parallel to the generatrix. A pulsed isotropic deuterium-tritium 14 MeV neutron generator is mounted perpendicular to the generatrix in the wall of the outer cylinder.

[0012] In the lower part of the inner cylinder there is a pedestal, with the possibility of its vertical displacement along the axis. The scheme of the apparatus is shown in FIG. 3. Before being loaded into the installation, the vessel with SM is subjected to gamma scanning in order to establish a center of mass. Then, by means of the movable pedestal, the vessel is placed in the inner cylinder in such a manner that the center of mass of the SM is located opposite the detectors.

[0013] The technical solution of the invention is based on a method of differential attenuation of neutrons using a combination of Helium-3 counters with cadmium filters, a pulsed 14 MeV deuterium-tritium neutron generator, and a lead protection against gamma radiation serving as a neutron breeder at the same time. Preliminary gamma-scanning of a receiving vessel and a mobile pedestal make it possible to achieve a high efficiency of signal registration due to accurately positioning the mass center of CM between the neutron generator and the detectors. The effect of the position of the FM in the receiving vessel on the response of the helium-3 counters is presented in FIG. 2.