Uranium cubes reveal why the German nuclear project failed

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During World War II, physicists on the Manhattan Project worried that the Nazi German nuclear program might achieve the first nuclear bomb before the US and its allies. That’s not what transpired, but historians have debated just how close the Germans came to achieving criticality in their last attempt at building a reactor: the B8 pile.

第二次世界大战期间，曼哈顿计划的物理学家们曾担心纳粹德国的核计划可能会在美国及其盟国之前研制出第一枚核弹。事实并非如此，但历史学家们一直在争论德国人在最后一次建造反应堆的尝试——B8反应堆——中究竟距离实现临界状态有多近。

Werner Heisenberg, who led that effort, claimed that they were close and had only needed a little more uranium. But according to a paper published in the Proceedings of the National Academy of Sciences Nexus, that was not the case: the project simply didn’t have sufficient uranium and heavy water to build a working bomb, given the design they were using.

领导该项目的维尔纳·海森堡声称他们已经非常接近成功，只需要再多一点铀。但根据发表在《美国国家科学院院刊 Nexus》（Proceedings of the National Academy of Sciences Nexus）上的一篇论文，事实并非如此：鉴于他们所使用的设计，该项目根本没有足够的铀和重水来制造出一枚可用的炸弹。

This particular research started in 2013, when University of Maryland physicist Timothy Koeth—a co-author on the new paper—received the gift of a mysterious heavy metal cube from a friend. He instantly recognized it as one of the uranium cubes used by German scientists in their unsuccessful attempt to build a working nuclear reactor. Only 14 such cubes are known to exist today.

这项研究始于2013年，当时马里兰大学物理学家、这篇新论文的合著者蒂莫西·科思（Timothy Koeth）收到了一位朋友赠送的一个神秘重金属立方体。他立刻认出这是德国科学家在试图建造核反应堆失败时所使用的铀立方体之一。据了解，目前仅存14个这样的立方体。

In 2019, we reported on Koeth’s quest to track down the cube’s origins, as well as several other similar cubes that had somehow found their way across the Atlantic. At the time, Koeth and his partner in the quest, then-graduate student Miriam “Mimi” Hiebert, had tracked down ten such cubes around the US. For instance, the Smithsonian Institute had a German uranium cube in storage. They tracked a third cube to Harvard University, where it was regularly passed around to students in introductory physics classes as a curiosity.

2019年，我们报道了科思追踪该立方体来源的探索过程，以及其他几个不知何故流落到大西洋彼岸的类似立方体。当时，科思和他的研究伙伴、时任研究生米里亚姆·“咪咪”·希伯特（Miriam “Mimi” Hiebert）已经在全美追踪到了十个这样的立方体。例如，史密森尼学会就收藏了一个德国铀立方体。他们还追踪到了哈佛大学的第三个立方体，在那里，它经常被当作稀奇之物在物理入门课上传阅。

Underpinning the Manhattan Project in the US was the fear that German scientists under Adolf Hitler’s Nazi regime would beat the Allies to a nuclear bomb. The Germans had a two-year head start, but according to Koeth, “fierce competition over finite resources, bitter interpersonal rivalries, and ineffectual scientific management” resulted in significant delays in their progress toward achieving a sustained nuclear reaction.

美国曼哈顿计划的背后，是担心阿道夫·希特勒纳粹政权下的德国科学家会抢在盟军之前研制出核弹。德国人领先了两年，但据科思称，“对有限资源的激烈竞争、尖锐的人际矛盾以及低效的科学管理”导致他们在实现持续核反应的进程中严重滞后。

German nuclear scientists were separated into three isolated groups based in Berlin (B), Gottow (G), and Leipzig (L). Heisenberg headed the Berlin group, and as the Allied forces advanced in the winter of 1944, Heisenberg moved his team to a cave under a castle in a small town called Haigerloch—now the site of the Atomkeller Museum. That’s where the group built the B8 reactor.

德国核科学家被分为三个独立的组，分别位于柏林（B）、戈托（G）和莱比锡（L）。海森堡领导柏林小组。1944年冬天，随着盟军的推进，海森堡将他的团队转移到了海格洛赫（Haigerloch）一个小镇城堡下的洞穴中——那里现在是原子地窖博物馆（Atomkeller Museum）的所在地。该小组就是在那里建造了B8反应堆。

It resembled an “ominous chandelier,” per Koeth, because it was composed of 664 uranium cubes strung together with aircraft cable and then submerged in a tank of heavy water shielded by graphite to prevent radiation exposure.

据科思描述，它看起来像一个“不祥的吊灯”，因为它由664个用航空电缆串联起来的铀立方体组成，然后浸没在一个由石墨屏蔽以防止辐射的重水箱中。

As the German scientists were racing against time, Manhattan Project lead Lieutenant General Leslie Groves kicked off a covert mission dubbed “Alsos,” with the express purpose of gathering information and materials related to Germany’s scientific research. When Allied forces closed in, Heisenberg took apart the B8 experiment and buried the uranium cubes in a field, ferreting away key documentation in a latrine. (Pity Samuel Goudsmit, the poor physicist who had to dig those out.) Heisenberg escaped by bicycle, carrying a few cubes in a backpack.

正当德国科学家与时间赛跑时，曼哈顿计划负责人莱斯利·格罗夫斯（Leslie Groves）中将发起了一项代号为“阿尔索斯”（Alsos）的秘密任务，其明确目的是收集与德国科学研究相关的信息和材料。当盟军逼近时，海森堡拆除了B8实验装置，将铀立方体埋在田野里，并将关键文件藏在厕所里。（可怜的物理学家塞缪尔·古德斯米特不得不把它们挖出来。）海森堡骑自行车逃跑，背包里还带着几个立方体。

A critical reassessment: As Heisenberg reported, the German scientists’ final experiment failed because the amount of uranium in the cubes was insufficient to trigger a sustained nuclear reaction. But Heisenberg was confident that “a slight increase in its size would have been sufficient to start off the process of energy production.” A model described in a 2009 paper supported that, showing the group would have needed only 50 percent more uranium cubes to make the design work.

关键的重新评估：正如海森堡所报告的那样，德国科学家的最后一次实验失败了，因为立方体中的铀含量不足以引发持续的核反应。但海森堡坚信，“只要稍微增加一点尺寸，就足以启动能量生产过程。”2009年的一篇论文中描述的模型支持了这一观点，显示该小组只需要多出50%的铀立方体就能使该设计成功。

Koeth and Hiebert uncovered a box of declassified documents about German uranium in the National Archives and found about 400 other uranium cubes from a separate reactor experiment by the Gottow group. They concluded that the combined inventory would have been more than enough to achieve criticality in the B8 reactor, suggesting that Germany’s secretive, isolationist approach hampered their nuclear program because the two groups weren’t sharing information or resources.

科思和希伯特在国家档案馆发现了一箱关于德国铀的解密文件，并发现了来自戈托小组另一项反应堆实验的约400个铀立方体。他们得出结论，将这些库存加在一起，足以使B8反应堆达到临界状态，这表明德国秘密的孤立主义做法阻碍了他们的核计划，因为这两个小组之间没有共享信息或资源。

This latest paper builds on that earlier work but reaches a different conclusion. Koeth, Hiebert, and their co-authors analyzed two of the surviving uranium cubes, along with combing through historical records, technical reports, correspondence, and similar archival sources to learn more about the uranium, heavy water, and graphite blocks used for the B8 reactor experiment.

这篇最新的论文建立在早期工作的基础上，但得出了不同的结论。科思、希伯特及其合著者分析了两个现存的铀立方体，并梳理了历史记录、技术报告、信件和类似的档案资料，以进一步了解用于B8反应堆实验的铀、重水和石墨块。

Among their findings: The B8 reactor would have needed between 1,292 and 1,775 uranium cubes to achieve criticality, and twice the heavy water produced by the Norsk Hydro plant in Vemork, Norway—the only source at the time—over the course of the entire war. In short, “Our calculations suggest Heisenberg significantly underestimated the materials required,” the authors wrote.

他们的发现包括：B8反应堆需要1,292到1,775个铀立方体才能达到临界状态，并且在整个战争期间，其所需的重水是挪威维莫克（Vemork）的挪威水电公司（Norsk Hydro）工厂——当时唯一的来源——所生产总量的两倍。简而言之，作者写道：“我们的计算表明，海森堡严重低估了所需的材料。”

The authors also cited prior work noting key differences in American and German graphite production. The former used petroleum coke. While some of this supply reached Germany, the British naval blockade in January 1940 deprived the Germans of that vital resource. The German nuclear physicists had to use metallurgical coke from the Ruhr, which had twice as many impurities as American coke. (The US scientists had similar issues until they located an unusually low-boron coke in Pennsylvania.) Thus, “no natural uranium, graphite-moderated pile could have gone critical” with the materials available to the German scientists at the time.

作者还引用了之前的研究，指出了美国和德国在石墨生产上的关键差异。前者使用石油焦。虽然部分供应到达了德国，但1940年1月的英国海军封锁使德国人失去了这一重要资源。德国核物理学家不得不使用鲁尔区的冶金焦，其杂质含量是美国焦炭的两倍。（美国科学家也曾遇到类似问题，直到他们在宾夕法尼亚州找到了硼含量极低的焦炭。）因此，以当时德国科学家所能获得的材料，“任何天然铀、石墨慢化反应堆都不可能达到临界状态”。