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The International Atomic Energy Agency is developing technology for the continuous monitoring of inprocess UF 6 gas at centrifuge enrichment plants. These unattended detection nodes would be located on the high-pressure portion of the plant's header pipes, where the gas flow from multiple cascades is consolidated. The time-dependent relative enrichment data from the online monitor will be combined with subsequent mass measurements of the corresponding feed, product and tail cylinders to produce a mass measurement of 235U in each cylinder. The On-line Enrichment Monitor (OLEM) will utilize NaI(Tl) spectrometers, gas pressure gauges and possibly temperature sensors. This paper describes the conceptual design of the OLEM and the Monte Carlo modeling that has been performed to support the development process. Simulated OLEM responses are used to explore potential instrument design options (e.g. collimator configuration) and candidate data analysis methods, and to gain insight into the nature and magnitude of the various uncertainties that arise in online enrichment monitoring. Prominent among these uncertainties is the correction for the 235U signal coming from deposits on the walls of the piping; potential correction methods for this wall-deposit contribution are described. An OLEM case-study scenario is defined and the expected statistical uncertainties over a representative range of plant operating conditions (e.g. pressure, enrichment and wall-deposit thickness) are calculated. |
