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  1. Polyethylene density uncertainty in the simulation of neutron detectors

    Simulations with the Monte Carlo N-Particle® (MCNP®) code are frequently used to study neutron coincidence counters for international safeguards. Reducing sources of uncertainty enables more accurate simulations, which reduces the need for expensive or impossible measurements. Accurately specifying the density of polyethylene is crucial for accurate simulations. The density of polyethylene ranges from 0.88 to 0.97 g/cm3 and high-density polyethylene (HDPE) ranges from about 0.944 to 0.965 g/cm3. The density of a specific detector should be measured exactly for the best simulations. Here, the density of the blue/white Active Well Coincidence Counter (AWCC) bulk polyethylene was measured to be 0.9594more » +- 0.0013 g/cm3 at 1-sigma confidence. The density was calculated from repeated measurements of the various polyethylene dimensions for volume, and weighing the polyethylene for the mass. The AWCC neutron detection efficiency was simulated for each polyethylene density. The relative range of efficiencies (3σ) was 3.4% for unspecified polyethylene, 0.9% for HDPE, and 0.3% for the measured density. The impact of modeling small features such as screws was also studied and was found to be a negligible 0.2%. Measuring polyethylene density can reduce its role in uncertainty from one of the largest to one of the smallest contributors.« less
  2. A new generation of uranium coincidence fast neutron collars for assay of LWR fresh fuel assemblies

    The active uranium neutron coincidence collar provides a means of non-destructively assaying the fissile linear density of Light Water Reactor fresh fuel assemblies containing low enriched uranium. These neutron collars can operate in two modes: a thermal and a fast mode. In fast mode, a neutron collar has an added cadmium (Cd) liner in the sample cavity of the detector to reduce the impact of the burnable poison (thermal neutron absorber) on the detector signal (doubles). The main advantage for operating in fast mode is a detected signal that is less dependent of the burnable neutron poison content and thusmore » less dependent on facility operator declarations. The drawback is that operating in fast mode requires a longer measurement time (~hour vs tens of minutes for thermal mode) to achieve the statistically needed precision in the measurements. The trend in the modern reactor fuel assemblies is moving to higher burnup by using higher initial enrichment and, consequently, a higher number of burnable poison rods to compensate the initial neutron reactivity. The increase of the burnable poison loading has motivated the development of a new generation of high efficiency fast neutron collars to allow practical measurements in-field by nuclear inspectors. This paper describes the development and performance evaluation of a new generation of neutron collars, for both boiling water reactor (BWR) and pressurized water reactor (PWR) fuels, jointly developed between the US Department of Energy, through Los Alamos National Laboratory, and the Euratom Safeguards Directorate of the European Commission. In this work, we present here calibrations with reference fuel assemblies at Los Alamos National Laboratory as well as the results of in-field measurement campaigns in fuel fabrication plants with modern commercial fuel assemblies. The experimental results show that a typical PWR verification can be made in a total time of 30 min with an uncertainty in the measured mass of 2% at one standard deviation (1σ). A BWR verification can be made in 47 min with an uncertainty in the measured mass of 1.9% at 1σ, or a total time of 20 min with 1σ uncertainty in the measured mass of 2.5%.« less
  3. Evaluation of Various Collimators for UF6 Gas Pipe Measurement

    On-line gamma measurements of the enrichment of uranium in UF6 gas in pipes is carried out as part of international safeguards. The 186 keV line from 235U is measured in NaI detectors. One complication in the analysis is that the 186 keV line also comes from the (time-dependent) amount of uranium-bearing material deposited on the interior walls of the pipe. This contribution has to be subtracted before the enrichment of the gas can be calculated. The relative contributions to the total 186 keV count rate from the gas and from the deposit depends on the amount of deposit, but alsomore » on the type of collimation/shielding used between the NaI detector and the gas pipe. Several different collimation/shielding designs have been simulated in order to determine which provides the best performance.« less
  4. Action Sheet 55 Measurement Campaign Summary

    Neutron collars have long been used by international inspectorates to determine the 235U loading of fresh light water reactor (LWR) fuel in nuclear fuel fabrication facilities. Burnable poisons are strong thermal neutron absorbers that are frequently used to increase the lifetime of LWR fuel in a reactor. These poisons have made the traditionally thermal neutron collar measurements challenging because they force inspectors to rely on operator declarations of the burnable poison content of the fuel in order to correct for the decreased count rate caused by the added thermal neutron absorption of the poison. Work has been conducted internationally tomore » try to eradicate the reliance on operator declarations. The Euratom Fast Collar for BWR Fuel (EFCB) is one promising technique to manage this issue.« less
  5. EURATOM Fast Collar for BWR (EFCB) Field Calibration Exercise

    A neutron collar for the measurement of fresh BWR fuel assemblies (EFCB) has been designed and built by Los Alamos National Laboratory (LANL) and the Euratom Safeguards Directorate (Euratom). The design was similar to a previous collar designed for the measurement of fresh PWR assemblies. A key feature of both of these collars is that they operate in fast mode (preventing the return of thermal neutrons into the assembly by means of a cadmium liner) to reduce the effect of burnable poisons on the measurement. The current report describes the results of the field calibration exercise that was carried outmore » at the ENUSA Fuel Fabrication Plant in Juzbado, Spain from May 28 to May 31, 2019.« less
  6. Nondestructive measurements of residual 235U mass of Israeli Research Reactor-1 fuel using the Advanced Experimental Fuel Counter

    In 2018, a measurement campaign took place with participants from Los Alamos National Laboratory (LANL), the Nuclear Research Centre-Negev (NRCN) and Soreq Nuclear Research Center (SNRC) at the Israeli Research Reactor-1 (IRR-1) in which 14 of the reactor’s used fuel assemblies (FAs) with varied amount of depletion were measured with the nondestructive assay instrument Advanced Experimental Fuel Counter (AEFC). Designed for safeguards purposes, the AEFC measures both neutrons emitted from the FA (passive neutrons) and fission neutrons induced by an external neutron source (in this experiment, 252Cf). Signals recorded with the AEFC include total neutron count rates (Singles), time-correlated neutronmore » count rates (Doubles), and total gamma-ray count rates. The 235U content of the FAs was previously assessed by two independent methods: (1) measurement of the transparency of the FA to low-energy gamma rays from an activated rhenium source (rhenium gamma transmission, or the RGT method) and (2) calculation of the 30-year burnup history of the core using detailed three-dimensional Monte-Carlo core depletion calculations. The results from the FAs that had been measured via the RGT method were used to construct the calibration curves, which translate the AEFC count rates to 235U mass. Then, the calibration was evaluated using AEFC measurements of six additional FAs that were not measured via the RGT method. From the results, it was determined the Doubles calibration curve was more reliable than that of the Singles and follows a simple second-order polynomial fit for the whole range of residual 235U mass content, albeit with larger statistical uncertainty. Detailed uncertainties quantification was conducted for both the AEFC Singles and Doubles. This includes the analysis of statistical uncertainties, calibration uncertainty, and random uncertainties due to the sensitivity of the AEFC to several sources of uncertainty, namely the FA position, FA orientation, interrogation source position, and ambient pool temperature. Finally, an overall total uncertainty of 6 g of 235U is estimated for the Singles and Doubles, which is mainly due to calibration uncertainty (for the Singles) and statistical uncertainty (for the Doubles), and which constitutes 3%–6% of the 235U total mass in the FAs, depending on their level of depletion.« less
  7. Time behavior of the emission of 1274 keV gamma-rays from UF6

    22Na ingrowth originating from fluorine-alpha interactions has been proposed as a chronometer for UF6 cylinders. Furthermore, these proposals have ignored the prompt 1274 keV gamma-ray contribution from 19F(α, p). We have shown that this contribution is significant and cannot be ignored when using this as a chronometer for UF6.
  8. Annual IPCA2 Performance Report for JFY18

    This report summarizes the results of monthly control measurements of IPCA2 performed over the period of October 2018 through March 2019 and represents an annual performance overview for JFY18. Note that the start of the measurements in October 2018 was defined by the start of the contract agreement. Monthly measurements of Plutonium neutron detection efficiency, AmLi stability, Curium stability and HPGe gamma spectra of Plutonium standards were performed and analyzed. All the results are shown with respect to original control bounds established from 2013- 2017 data in. Updated control bounds established based on this JFY18 data for use during JFY19more » measurements are summarized in Appendix A. Based on Pu efficiency measurements, the performance of the IPCA2 during this reporting period was stable within 0.6% at 1 σ level. Measurements were compared to room temperature and humidity and no dependence was found for any detector performance. The end of this report contains summary of all IPCA2 measurements performed since October 2018 and includes discussion of troubleshooting and repair activities performed over the reporting period.« less
  9. Characterizing laser-plasma ion accelerators driving an intense neutron beam via nuclear signatures

    Compact, bright neutron sources are opening up several emerging applications including detection of nuclear materials for national security applications. At Los Alamos National Laboratory, we have used a short-pulse laser to accelerate deuterons in the relativistic transparency regime. These deuterons impinge on a beryllium converter to generate neutrons. During the initial experiments where these neutrons were used for active interrogation of uranium and plutonium, we observed β-delayed neutron production from decay of 9Li, formed by the high-energy deuteron bombardment of the beryllium converter. Analysis of the delayed neutrons provides novel evidence of the divergence of the highest energy portion ofmore » the deuterons (i.e., above 10 MeV/nucleon) from the laser axis, a documented feature of the breakout afterburner laser-plasma ion acceleration mechanism. Furthermore, these delayed neutrons form the basis of non-intrusive diagnostics for determining the features of deuteron acceleration as well as monitoring neutron production for the next generation of laser-driven neutron sources.« less
  10. A comparison of Monte Carlo fission models for safeguards neutron coincidence counters

    Monte Carlo simulation is a powerful tool used to model neutron coincidence detectors for international safeguards. The simulation has typically sampled properties such as the fission neutron multiplicity, energy, and direction, from independent probability density functions. However, multiplicity counters detect event-based neutron correlations and thus more accurate fission event modeling is needed. To respond to this need, the Fission Reaction Event Yield Algorithm (FREYA) and the Cascading Gamma-ray Multiplicity with Fission (CGMF) models were added in the newest version of MCNP, MCNP6.2. The models simulate individual fission events conserving momentum, energy, and angular momentum such that correlated particles are emitted.more » The effects of the new models on simulations of safeguards neutron coincidence counters were studied and compared to standard MCNP simulations. The MCNPX-PoliMi model was also included in the comparison. The properties of fission neutrons from safeguards relevant isotopes were compared to literature references. Then a hypothetical simplified detector was modeled to isolate the effects of specific differences between models. Experimental measurements from previous work were modeled and agreements were compared. Lastly, the probabilities of correlated events occurring in the experimental measurements were calculated with the different models. For example, the probability was calculated of detecting neutrons from both induced fission in uranium and spontaneous fission of Cf-252 in the same fission chain.« less
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