Discovery of fissionogenic Cs and Ba capture five years after Oklo reactor shutdown

Abstract

Understanding the release and sequestration of specific radioactive signatures into the environment is of extreme importance for long-term nuclear waste storage and reactor accident mitigation. Recent accidents at the Fukushima and Chernobyl nuclear reactors released radioactive Cs andCs into the environment, the former of which is still live today. We have studied the migration of fission products in the Oklo natural nuclear reactor using an isotope imaging capability, the NAval Ultra-Trace Isotope Laboratory's Universal Spectrometer (NAUTILUS) at the US Naval Research Laboratory. In Oklo reactor zone (RZ) 13, we have identified the most depleted natural U of any known material with a U/U ratio of 0.3655 ± 0.0007% (2σ). This sample contains the most extreme natural burnup in Sm,Eu, Gd, andGd, which demonstrates that it was sourced from the most active Oklo reactor region. We have discovered that fissionogenic Cs and Ba were captured by Ru metal/sulfide aggregates shortly following reactor shutdown. Isochrons from the Ru aggregates place their closure time at 4.98 ± 0.56 y after the end of criticality. Most fissionogenic Ba andBa in the Ru migrated and was incorporated as Cs over this period. Excesses in Ba in the Ru point to the burnup ofCs. Cesium and Ba were retained in the Ru despite local volcanic activity since the reactor shutdown and the high level of activity during reactor operation.

An overview of NRL's NAUTILUS: a combination SIMS-AMS for spatially resolved trace isotope analysis

“…A demonstration of U-Pb chronometry from uraninite minerals from the reactor zone 13 of the Oklo natural nuclear reactor is shown in Figure 5. Lead isotopes in this sample from reactor zone 13 5,[28][29][30] are discordant due to nearby volcanism roughly 671 Ma ago, in good agreement with earlier work 2 . A sample of titanite from a Grenville skarn (OLT-1) was used as an age reference 1 .…”

“…As discussed previously, Figure 14 shows a comparison of direct, molecule-free imaging on the SIMS-SSAMS versus imaging on the SIMS alone, which was full of interfering species from the heterogeneous matrix. Figure 15 shows an additional example of where direct isotope ratio imaging of Ba and Cs isotopes allowed us to locate an anomalous hotspot containing fissionogenic 135 Ba, 137 Ba, and 138 Ba that was otherwise indistinguishable via SIMS 5 . This hotspot was later correlated to the presence of Ru metal and sulfides, which apparently captured live fissionogenic 135 Cs and 137 Cs within 5 years of the Oklo natural nuclear reactor ceasing criticality.…”

“…We have successfully designed and built the NAval Ultra-Trace Isotope Laboratory's Universal Spectrometer (NAUTILUS) at the U.S. Naval Research Laboratory (NRL) [4][5][6][7][8][9] , which combines the modified hardware from two commercial instruments, an Ametek Cameca ims 4f SIMS 10 and a National Electrostatics Corporation (NEC) single-stage AMS (SSAMS) [11][12][13] , together with custom control hardware and software. The motivation for this novel combination MS-MS instrument is to utilize the aforementioned advantages of each technique in a manner which simultaneously mitigates each technique's drawbacks.…”

“…Ordinarily these do not increase the overall signal-to-noise (SNR) ratio of a measurement since they often enhance the magnitude and complexity of the molecular background in addition to enhancing the signal of interest. The capabilities of the NAUTILUS have been demonstrated through the discovery of fissionogenic Cs and Ba captured in Ru metal/sulfide aggregates within Oklo reactor materials 5 ; direct measurement and identification of 236 U in U3O8 particles covered with monazite "dirt" 4 ; and through direct measurement of trace rare earth elements (REEs) in hibonite and other minerals where the molecular background was (10 4 -10 5 )× higher than the signals of interest 6 . What follows is a description of the NAUTILUS instrument and its performance.…”

Spatially Resolved Characterisation of Low Mass Fraction Uranium Glass Working Reference Materials

“…A more complete description of the NAUTILUS instrument and demonstration of its capabilities can be found here (Groopman et al 2018(Groopman et al , 2020(Groopman et al , 2021. Briefly, the NAUTILUS is a combination of two well-known mass spectrometers, a modified CAMECA ims-4f SIMS and a AE 300 keV National Electrostatics Corporation SSAMS.…”