Reactor safeguards regimes are intended to detect illicit or suspicious uses of reactor facilities. Depending on the regime, examples of illicit use could include unauthorized changes in the rate of plutonium production within a reactor, a reduction in the level of irradiation of fuel to facilitate later removal of fissile material, or the actual diversion of fissile material from the reactor. Safeguards monitoring systems are currently in place at about half of the world's power reactors, and at hundreds of research reactors worldwide. These are largely safeguarded by indirect means that do not involve the direct measurement of the fissile isotopic content of the reactor, but instead consist primarily of semi-annual or annual inspections of coded tags and seals placed on fuel assemblies, and measures such as video surveillance of spent fuel cooling ponds. Direct measurements are typically made off-line, before or after fuel are introduced into the reactor. Real-time, online quantitative measurements of reactor core power and isotopic composition have been demonstrated in more recent times with large, expensive anti-neutrino detectors [1]. This technique is based upon variations in detectable antineutnno yield from differing isotopes, as exhibited in Table I. Count rates of anti-neutrino detectors are low, with a very high background.
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