Conceptual Ideas for New Nondestructive UF6 Cylinder Assay Techniques

Miller, Karen A.¹ 

[1] Los Alamos National Laboratory

关键词: BREMSSTRAHLUNG;    DE-EXCITATION;    FEASIBILITY STUDIES;    GAMMA RADIATION;    ISOTOPE SEPARATION;    ISOTOPE SEPARATION PLANTS;    LIQUID SCINTILLATORS;    NEUTRONS;    NONDESTRUCTIVE ANALYSIS;    ON-SITE INSPECTION;    PHOSPHORS;    PHOTON BEAMS;    PHOTONS;    RESONANCE FLUORESCENCE;    SAFEGUARDS;    URANIUM;    VERIFICATION;    CYLINDERS;    URANIUM HEXAFLUORIDE;   

DOI  :  10.2172/1039686
RP-ID  :  LA-UR-12-21067
PID  :  OSTI ID: 1039686
Others  :  TRN: US1202275

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来源: SciTech Connect

【 摘 要 】

Nondestructive assay (NDA) measurements of uranium cylinders play an important role in helping the International Atomic Energy Agency (IAEA) safeguard uranium enrichment plants. Traditionally, these measurements have consisted of a scale or load cell to determine the mass of UF{sub 6} in the cylinder combined with a gamma-ray measurement of the 186 keV peak from {sup 235}U to determine enrichment. More recently, Los Alamos National Laboratory (LANL) and Pacific Northwest National Laboratory (PNNL) have developed systems that exploit the passive neutron signal from UF{sub 6} to determine uranium mass and/or enrichment. These include the Uranium Cylinder Assay System (UCAS), the Passive Neutron Enrichment Meter (PNEM), and the Hybrid Enrichment Verification Array (HEVA). The purpose of this report is to provide the IAEA with new ideas on technologies that may or may not be under active development but could be useful for UF{sub 6} cylinder assay. To begin, we have included two feasibility studies of active interrogation techniques. There is a long history of active interrogation in the field of nuclear safeguards, especially for uranium assay. Both of the active techniques provide a direct measure of {sup 235}U content. The first is an active neutron method based on the existing PNEM design that uses a correlated {sup 252}Cf interrogation source. This technique shows great promise for UF{sub 6} cylinder assay and is based on advanced technology that could be implemented in the field in the near term. The second active technique is nuclear resonance fluorescence (NRF). In the NRF technique, a bremsstrahlung photon beam could be used to illuminate the cylinder, and high-resolution gamma-ray detectors would detect the characteristic de-excitation photons. The results of the feasibility study show that under certain measurement geometries, NRF is impractical for UF6 cylinder assay, but the 'grazing transmission' and 'secant transmission' geometries have more potential for this application and should be assessed quantitatively. The next set of techniques leverage scintillator detectors that are sensitive to both neutron and gamma radiation. The first is the BC-523A capture-gated organic liquid scintillator. The detector response from several different neutron energies has been characterized and is included in the study. The BC-523A has not yet been tested with UF{sub 6} cylinders, but the application appears to be well suited for this technology. The second detector type is a relatively new inorganic scintillator called CLYC. CLYC provides a complementary detection approach to the HEVA and PNEM systems that could be used to determine uranium enrichment in UF{sub 6} cylinders. In this section, the conceptual idea for an integrated CLYC-HEVA/PNEM system is explored that could yield more precision and robustness against systemic uncertainties than any one of the systems by itself. This is followed by a feasibility study on using alpha-particle-induced reaction gamma-rays as a way to estimate {sup 234}U abundance in UF{sub 6}. Until now, there has been no readily available estimate of the strength of these reaction gamma-rays. Thick target yields of the chief reaction gammas are computed and show that they are too weak for practical safeguards applications. In special circumstances where long count times are permissible, the 1,275 keV F({alpha},x{gamma}) is observable. Its strength could help verify an operator declaration provided other knowledge is available (especially the age). The other F({alpha},x{gamma}) lines are concealed by the dominant uranium line spectrum and associated continuum. Finally, the last section provides several ideas for electromagnetic and acoustic nondestructive evaluation (NDE) techniques. These can be used to measure cylinder wall thickness, which is a source of systematic uncertainty for gamma-ray-based NDA techniques; characterize the UF{sub 6} filling profile inside the cylinder, which is a source of systematic uncertainty for neutron-based NDA techniques; locate hidden objects inside the cylinder; and provide a unique identification of cylinders. Acoustic and electromagnetic NDE techniques are complementary to NDA measurements, and may improve the accuracy and continuity of knowledge of UF{sub 6} measurements of interest to the IAEA. As concepts and approaches for enrichment plant safeguards continue to evolve to meet modern challenges, the conceptual ideas explored in this report, along with more traditional techniques, help define the toolkit of technologies available for UF{sub 6} cylinder assay. Whether the application is an unattended cylinder verification station or an on-site inspection, the basic building blocks can be tailored to provide the best solution given competing constraints such as size and weight limitations, required precision, mechanical complexity, cost, stability, robustness, etc.