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Long-term retention and chemical fractionation of fissionogenic Cs and Tc in Oklo natural nuclear reactor fuel
Applied Geochemistry ( IF 3.4 ) Pub Date : 2021-07-03 , DOI: 10.1016/j.apgeochem.2021.105047
Evan E. Groopman 1 , Larry R. Nittler 2 , David G. Willingham 1 , Alex P. Meshik 3 , Olga V. Pravdivtseva 3
Affiliation  

We present the results of a coordinated NAUTILUS and NanoSIMS isotopic study of epsilon (ε) phase metallic aggregates from the Oklo natural nuclear reactor zone (RZ) 13. We observed that fissionogenic Tc and Cs were heterogeneously sequestered within the aggregates. Isotopes of these elements are relevant for improving the safety of spent nuclear fuel storage and reactor operation on generational timescales. Like the noble metals, nearly all of the Tc was retained within the reactor, though its abundance relative to Ru in the metallic aggregates varied by a factor of 10. The neutron fluence estimated from the production of 100Ru from neutron capture on 99Tc was estimated to be up to 1.2 × 1021 n·cm-2. In contrast to Tc, nearly all of the fissionogenic Cs in the reactors was lost from the reactor fuel. The metallic aggregates contain the only phases yet identified to have sequestered radiocesium. Fissionogenic Cs isotopes decay over vastly different timescales, but were incorporated and retained within the ε-phase in proportions similar to stable 133Cs. This indicates that retention began during criticality and sequestration lasted billions of years, despite local geologic activity and the presence of nearby magmatic dikes. Using fissionogenic Ba isotopes, we estimated that the metallic aggregates continually incorporated their radioactive Cs parents during criticality, though the majority of Cs was flushed out of the reactor on a characteristic timescale of 2.7 ± 0.6 years. We found that the abundance of Bi was correlated to Rh and Pd, and speculate that this may have been due to primary Np–Rh and Np–Pd alloys forming during or shortly after criticality. Using Pb–Pb data from uraninite and galena grains surrounding the metallic aggregates, we also inferred a final Pb mobility age of 298 Ma for RZ13, which is more recent than most estimates from other RZs.



中文翻译:

Oklo 天然核反应堆燃料中裂变源 Cs 和 Tc 的长期保留和化学分馏

我们展示了来自 Oklo 天然核反应堆区 (RZ) 13 的 epsilon (ε) 相金属聚集体的协调 NAUTILUS 和 NanoSIMS 同位素研究结果。我们观察到裂变 Tc 和 Cs 异质隔离在聚集体中。这些元素的同位素与在代际时间尺度上提高乏核燃料储存和反应堆运行的安全性有关。与贵金属一样,几乎所有的 Tc 都保留在反应堆中,尽管它在金属聚集体中相对于 Ru 的丰度变化了 10 倍。根据99 Tc中子捕获产生的100 Ru估计的中子注量是估计可达 1.2 × 10 21 n·cm - 2. 与 Tc 相比,反应堆中几乎所有的裂变 Cs 都从反应堆燃料中流失了。金属聚集体包含唯一的相,但已被确定为螯合放射性铯。裂变 Cs 同位素在截然不同的时间尺度上衰变,但以与稳定133相似的比例结合并保留在 ε 相中CS。这表明,尽管当地的地质活动和附近存在岩浆岩脉,但在临界期开始保留,并且封存持续了数十亿年。使用裂变 Ba 同位素,我们估计金属聚集体在临界期间不断结合其放射性 Cs 母体,尽管大多数 Cs 在 2.7 ± 0.6 年的特征时间尺度上被冲出反应堆。我们发现 Bi 的丰度与 Rh 和 Pd 相关,并推测这可能是由于在临界期间或临界后不久形成了初级 Np-Rh 和 Np-Pd 合金。使用来自金属聚集体周围的铀矿和方铅矿颗粒的 Pb-Pb 数据,我们还推断出 RZ13 的最终 Pb 迁移年龄为 298 Ma,这比其他 RZ 的大多数估计值更新。

更新日期:2021-07-08
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