Adaptation of neutron safeguards measurements for the front-end of the modern nuclear fuel cycle




Broughton, David Paul

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The application of international safeguards to the commercial nuclear fuel cycle prompts the need to assay ²³⁵U content in un-irradiated uranium in the front end of the fuel cycle. The highlypenetrating nature of neutrons makes them a common observable used for indication of bulk ²³⁵U content, however correlations are complicated by the fact that neutrons emissions from a sample depend on factors including multiplication, detection efficiency, and ²³⁵U/²³⁴U ratio for passive UF₆ measurements as ²³⁴U-driven ¹⁹F(α,n) reactions are the primary neutron source in low enriched UF₆. The neutron-based systems studied in this work are rooted in theory and detector development of the 1970s and 1980s. Changes in the commercial nuclear industry have led to increases in systematic uncertainty. This work focuses on addressing systematic uncertainties for two independent neutron-based verification systems: a set of passive ³He detectors for verification of UF₆ in large 30B storage cylinders, and the active neutron collar (UNCL) for verification of fresh fuel assemblies. Previous passive neutron UF₆ measurements found calibrations directly correlating total counts (Singles) to ²³⁵U mass and coincidence counts (Doubles) to enrichment were sensitive to UF₆ distribution and ²³⁵U/²³⁴U ratio. The proposed enricher-specific calibrations to reduce uncertainty from these factors rely on consistency of an enrichers practices. The goal here is to produce a general technique for measurement of any and all cylinders with a single calibration. Experimental and simulated data were used to demonstrate relative UF₆ distribution corresponds to the count rate ratio between two neutron detectors positioned to primarily sample different regions of the cylinder. Experimental evaluation was used to evaluate proposed new analysis algorithms to reduce analysis assumptions. This method correlates the distribution corrected Singles to ²³⁴U mass, which may then be converted to ²³⁵U mass using the correlation of the distribution corrected Doubles-to-(Singles squared) to the enrichment weighted ²³⁵U/²³⁴U ratio. This independently addresses both sources of systematic uncertainty, and provides strong evidence supporting the use of this system for safeguarding large gas centrifuge enrichment facilities in the future. The UNCL instrument has been a cornerstone of fresh fuel safeguards for decades using the empirical universal calibration and correction factors generated in the 1980s. Since this period PWR fuel designs have seen dramatic increases in Gadolinia burnable poison content to reduce the excess initial reactivity for the higher enrichment fuel used to enable longer periods between refueling. This requires extrapolation from the range over which the traditional poison rod correction was originally evaluated and introduces considerable systematic uncertainty. Due to the limited variability in commercial fuel assemblies and the prohibitive cost of constructing a sufficient range of reference calibration assemblies a high fidelity MCNP model was constructed here using engineering drawings of the reference fuel assembly jig, EC Curto, corresponding to the 16×16 PWR array used in Brazilian Angra type II and III fuel. A range of simulations and measurements were used in a sensitivity analysis to gauge total simulation uncertainty relative to total measurement uncertainty. This analysis supported the use of simulated relative response to update the poison rod correction. Evaluated factors include: simulated AmLi neutron launch spectrum, AmLi neutron emission rate, AmLi anisotropicity, high density polyethylene density, the position of the fuel assembly within the detector, and the experimental statistical uncertainty. Following this, the poison rod correction was updated using a set of 287 simulated fuel assemblies. Simulated relative responses were used to evaluate the functional form of the poison correction and determine optimal parameters. This update used between 4-24 poison rods ranging 2-11 wt.% Gd₂O₃ within assemblies having mean enrichments ranging 2.5-5%. Experimental validation of the updated coefficients using seven measurements shows bias is reduced by an order of magnitude and σ [subscript R] reduced from 10% to <2%. Two updated poison corrections are provided, the most accurate of which is directly usable in the existing analysis code (INCC) use by inspectorate at fuel fabrication facilities worldwide. As a whole, this work addressed systematic uncertainties affecting neutron-based verification measurements resulting largely from the impact of increased SWU availability on enrichment practices, and the benefits of increased fuel burnup motivating increases in enrichment and corresponding poison content in PWR fuel designs. These revisions to existing safeguards verification techniques conducted here were a direct response to changes in the commercial nuclear industry.


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