Anthony Lavietes

Real-Time, Fast Neutron Coincidence Assay of Plutonium With a 4-Channel Multiplexed Analyzer and Organic Scintillators

The design, principle of operation and the results of measurements made with a four-channel organic scintillator system are described. The system comprises four detectors and a multiplexed analyzer for the real-time parallel processing of fast neutron events. The function of the real-time, digital multiple-channel pulse-shape discrimination analyzer is described together with the results of laboratory-based measurements with 252Cf, 241Am-Li and plutonium. The analyzer is based on a single-board solution with integrated high-voltage supplies and graphical user interface. It has been developed to meet the requirements of nuclear materials assay of relevance to safeguards and security. Data are presented for the real-time coincidence assay of plutonium in terms of doubles count rate versus mass. This includes an assessment of the limiting mass uncertainty for coincidence assay based on a 100 s measurement period and samples in the range 0-50 g. Measurements of count rate versus order of multiplicity for 252Cf and 241Am-Li and combinations of both are also presented.

Liquid scintillator-based neutron detector development

The IAEA, in collaboration with the Joint Research Center (Ispra, IT) and Hybrid Instruments (UK), is developing a liquid scintillator-based neutron coincidence counting system to address a number of safeguards applications. Interest in this technology is increasing with the advent of high-flash point, nonhazardous scintillating fluids coupled with significant advances in signal processing electronics. Together, these developments have provided the enabling technologies to allow liquid scintillators to be implemented outside of a laboratory environment. Another important aspect of this detector technology is that it can be used with the current installed infrastructure of safeguards assay instruments and data acquisition electronics. It is also an excellent candidate for the replacement of 3He-based systems in many applications. As such, a comparison to an existing 3He-based system will be presented to contrast the differences and benefits for several applications. This paper will describe the experiments and associated modeling activities engaged to carefully characterize the detection system and refine the models. The latest version of MCNPX-PoliMi Monte Carlo modeling code was used to address the specific requirements of liquid scintillators. Additionally, this development activity has driven the collaborative development with Hybrid Instruments of a high-performance pulse shape discriminator (PSD) unit. Specific applications will be described with particular emphasis on those in which liquid scintillators provide immediate benefit over traditional detection methods.

A 4-channel multiplex analyzer for real-time, parallel processing of fast scintillators

The design, principle of operation and the results of the first measurements made with a 4-channel multiplexed analyzer for real-time parallel processing of fast scintillation detectors is described. Recent advancements in the performance of organic scintillation media for the detection of fast neutrons, not least the reduction in hazard and the recent advent of plastic scintillation materials exhibiting Pulse Shape Discrimination (PSD), has highlighted the possibility of using multiple detectors in systems based on detectors exploiting these media. In this paper a real-time, digital multiple-channel PSD analyzer is described based on a single-board solution with an integrated high-voltage supply and graphical user interface. It has been developed to meet the requirements of nuclear materials assay of relevance to safeguards and security, and tested in a variety of related laboratory-based environments.

Development of a liquid scintillator-based active interrogation system for LEU fuel assemblies

The IAEA, in collaboration with the Joint Research Center (Ispra, IT) and Hybrid Instruments (Lancaster, UK), has developed a full scale, liquid scintillator-based active interrogation system to determine uranium (U) mass in fresh fuel assemblies. The system implements an array of moderate volume (~1000ml) liquid scintillator detectors, a multichannel pulse shape discrimination (PSD) system, and a high-speed data acquisition and signal processing system to assess the U content of fresh fuel assemblies. Extensive MCNPX-PoliMi modelling has been carried out to refine the system design and optimize the detector performance. These measurements, traditionally performed with 3He-based assay systems (e.g., Uranium Neutron Coincidence Collar [UNCL], Active Well Coincidence Collar [AWCC]), can now be performed with higher precision in a fraction of the acquisition time. The system uses a high-flashpoint, non-hazardous scintillating fluid (EJ309) enabling their use in commercial nuclear facilities and achieves significantly enhanced performance and capabilities through the combination of extremely short gate times, adjustable energy detection threshold, real-time PSD electronics, and high-speed, FPGA-based data acquisition. Given the possible applications, this technology is also an excellent candidate for the replacement of select 3He-based systems. Comparisons to existing 3He-based active interrogation systems are presented where possible to provide a baseline performance reference. This paper will describe the laboratory experiments and associated modelling activities undertaken to develop and initially test the prototype detection system.

Real-Time Capabilities of a Digital Analyzer for Mixed-Field Assay Using Scintillation Detectors

Scintillation detectors offer a single-step detection method for fast neutrons and necessitate real-time acquisition, whereas this is redundant in two-stage thermal detection systems using helium-3 and lithium-6, where the fast neutrons need to be thermalized prior to detection. The relative affordability of scintillation detectors and the associated fast digital acquisition systems have enabled entirely new measurement setups that can consist of sizeable detector arrays. These detectors in most cases rely on photomultiplier tubes, which have significant tolerances and result in variations in detector response functions. The detector tolerances and other environmental instabilities must be accounted for in measurements that depend on matched detector performance. This paper presents recent advances made to a high-speed FPGA-based digitizer. The technology described offers a complete solution for fast-neutron scintillation detectors by integrating multichannel high-speed data acquisition technology with dedicated detector high-voltage supplies. This configuration has significant advantages for large detector arrays that require uniform detector responses. We report on bespoke control software and firmware techniques that exploit real-time functionality to reduce setup and acquisition time, increase repeatability, and reduce statistical uncertainties.

Gamma and neutron detector performance in a MOX fuel fabrication plant environment

Effectively implementing safeguards in fuel reprocessing and fuel fabrication plants requires the extensive use of state-of-the-art instrumentation for accurate material characterization. The ability to rapidly and continuously determine the material balance in a large facility is essential to control and reduce the risk of diversion. Multiple detection technologies are implemented that provide the necessary Continuity of Knowledge (CoK) and inventory control required for effective safeguards. In a Pu/U-oxide (MOX) fuel processing facility, much of the detection technology is based on moderated 3He neutron detectors. Recent concerns with respect to the possible lack of sufficient quantities of 3He has resulted in the interest in developing alternative detection technologies for current and future needs and applications. To address this situation, we have begun a series of experiments at a MOX fuel fabrication facility to determine the performance and suitability of liquid scintillator neutron detectors. MOX fuel in the form of reactor fuel assembly fuel rods and large volume MOX powder containers typically found in such production facilities were measured. In addition, high-purity Germanium (HPGe) and cadmium zinc telluride (CZT) detectors will be included to determine their respective performance characteristics in a large and complex fuel production environment. The results of these experiments will be used for consideration in the development and deployment of new safeguards systems in large, complex facilities such as the Japan Mixed-Oxide Fuel Fabrication Facility (J-MOX) or the Rokkasho Reprocessing Plant (RRP). This paper will present and discuss the results of the first series of experiments, conducted at PPFF (Plutonium Fuel Development Center Plutonium Fuel Facility) of JAEA, as well as describe the next steps in this process.