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An approach to developing a facility for detecting and nondestructive monitoring of fissile materials is examined. The approach is based on the use of a pulsed neutron source and different moderators in which the time dependences of the characteristics of the neutron and photon fields are distinguished with the help of a digital technology for processing scintillation signals. The computational and experimental studies which were performed make it possible to substantiate the main parameters of a real customs facility, which can be used at an airport, for detecting fissile materials. It is impossible to conceal fissile materials by means of radiation-absorbing screens, for example, lead or cadmium screens. The use of a pulsed source of neutrons with a deuterium-deuterium synthesis reaction precludes any possible contamination of the territory of the airport even if the neutron source is deliberately destroyed.A method based on pulsed neutron sources and different moderators, in which the time dependences of the characteristics of the neutron and photon fields are to be distinguished by digital technology for processing scintillation signals, is used in the present work to develop a facility to detect fissile materials. In such facilities, which provide different neutron moderation times, it is possible in practice to create conditions under which different energy groups of moderated neutrons exist some time after a neutron pulse. However, this possibility can be fully exploited only in combination with a detection system which makes it possible to separate neutrons and photons in the form of a pulse in the presence of a high load in the electronic channel. Replacing the currently widely used electronic units and proportional counters with analog-to-digital conversion boards with the scintillation responses of the neutrons and photons subsequently recorded in computer memory makes it possible to implement such a system. In this case, the responses are detected with zero dead time and high efficiency, characteristic of scintillation systems.The base of digitized pulses which has been developed makes it possible to use mathematical processing of the time and amplitude sequences of the pulses obtained, which could consist of separate independent electronic channels, to separate the responses of neutrons and photons in a single experiment, and to obtain information about the possible functionals of the neutron and photon responses of fissile materials as well as to investigate the time coincidences of fission events. Thus, there exists a practical possibility of developing a new generation of facilities, possessing substantial advantages over existing facilities, for active monitoring of fissile materials.The facilities for detecting and monitoring fissile materials with pulsed neutron sources have a complicated material composition with different neutron and photon field detectors incorporated in it. The interaction of the elements of such a facility can be so strong that its overall design scheme cannot meet the con...

Different approaches to detecting fissile materials which are being transported illegally across state borders or removed illegally from enterprises of the nuclear industry are examined. The content of fissile materials is determined using a pulsed neutron source, which is used in combination with two neutron moderators, separated by a screen with a large absorption cross section for thermal neutrons. The results of an experimental validation, the sensitivity limits, and the possibility of further development of these systems are presented.The determination of the content of fissile materials in the manufactured items of the nuclear power industry at the stages of fabriction and handling of these materials is asssociated with the safety of the operation of nuclear-power systems, the accounting system for nuclear materials, monitoring the wastes from nuclear production and the nonproliferation of nuclear materials and technology. At this time, the nonproliferation of nuclear materials and the related detection and monitoring of fissile materials during unauthorized shipment, specifically, during examination of passenger baggage in customs, is urgent. The existing methods of nondestructive monitoring are geared toward the detection and determination of the amount of fissile materials in closed volumes which cannot be opened.The setups used for detecting and monitoring fissile materials can be conditionally divided, according to their applications, into two large classes:• setups for nondestructive monitoring of fissile materials in the fuel cycle of nuclear reactors; • setups for detecting fissile materials in customs terminals. When the same physical method is used, the requirements for the parameters and the conditions of operation of the setups depend on the goals of their application and can be completely different. The class I setups are used mainly under laboratory conditions and at enterprises in the nuclear industry. A special team services them; they can have large shielding, so that the use of neutron sources with a high yield in them is permitted, and the frequency of inspections is determined by the production need. The basic requirements for setups of this class can be taken as follows:• the error in determining the composition of the object being inspected must satisfy the conditions of its operation in the nuclear-physical apparatus, gurantees of nonproliferation, or utilization of fissile materials; • the possibility of detecting highly enriched fissile materials when the self-screening effect is small; • the possibility of detecting samples with a complex heterogeneous structure, for example, a fuel assembly.