ABSTRACT The International Atomic Energy Agency (IAEA) defines uranium with a 235U isotope concentration of 20 percent as the threshold between low-enriched uranium (LEU) and highly enriched uranium (HEU), and as a significant waypoint on the path towards weapon-grade uranium (typically above 90 percent 235U enrichment). The distinction between LEU and HEU is widely used in shaping nonproliferation policy, and it has featured prominently in commentary over Iran's nuclear program and the series of Nuclear Security Summits that since 2010 have sought to minimize civilian stockpiles and use of HEU. Yet the origin of this threshold is obscure, dating back 6 decades. This research note traces the political origin and the technical basis for this limit.

中文翻译：

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论从高浓铀中划定低浓铀极限的起源和意义

摘要 国际原子能机构 (IAEA) 将 235U 同位素浓度为 20% 的铀定义为低浓缩铀 (LEU) 和高浓缩铀 (HEU) 之间的阈值，并且是通往武器级的重要途径铀（通常超过 90% 的 235U 浓缩）。低浓铀和高浓铀之间的区别被广泛用于制定防扩散政策，并在对伊朗核计划的评论和自 2010 年以来一直寻求最大限度减少民用库存和高浓铀使用的一系列核安全峰会中占有突出地位。然而，这个门槛的起源是模糊的，可以追溯到 6 年前。本研究笔记追溯了这一限制的政治起源和技术基础。