The influence of some rare earth elements as neutron absorbers on the inception of Oklo natural nuclear reactors.

A totally reflected core model was built to estimate the infinite multiplication factor ${{k}}_{{\infty}}$ as a parametric function using MCNP code. Thus, it was possible to evaluate the influence of a specific physical parameter on the criticality occurrence of the Oklo phenomenon, namely initial Poisons (IP: Gd, Sm and Nd). In fact, these rare earth elements, prior to criticality occurrence in Oklo reaction zones (RZs), are considered as a key parameter in the present study. Thus, it was possible to construct isocritical lines, ${{k}}_{{\infty}}\left({{V}}_{{UO}{2}},{{\varPhi}}_{{C}}\ \right)\cong{1}$, over a significant range of Uraninite volume fraction: ${{V}}_{{UO}{2}}\left[\%\right]{\in}\left[{0};{40}\right]$. The corresponding critical porosity ${{\varPhi}}_{{C}}$ is obtained for a given value of ${{V}}_{{UO}{2}}$ by Python-driven MCNP5 calculations. By including realistic measurements of IP for different RZs, it was possible to distinguish the corresponding inception circumstances for the natural RZs considered here.

Evidence of fast neutron operating in Oklo.

Since the discovery of the first reactor zone (RZ) at the Oklo uranium deposit in 1972, many isotopic studies have been performed to understand the mechanism of the operation as fission reactors and to trace the migration behaviors of fissiogenic isotopes produced in the Oklo RZs. As the representative parameters to characterize the operating conditions of RZs, neutron fluence generated in RZ, duration of RZ operation, restitution factor of 235U from α decay of 239Pu produced by neutron capture of 238U and the proportion of fission events due to 235U, 238U and 239Pu are compiled and compared with individual RZs. In particular, one of the Oklo RZs, RZ 13, shows several specific features in the view point of isotopic and nuclear characteristics. By comparison of the data between RZ13 and other RZs, fission contribution of 238U for RZ13 is found to be significantly higher than those of other RZs.

Parallels between natural Oklo reaction zones and industrial reactors dynamics.

The first man-made nuclear reactor was developed by Fermi and collaborators at the University of Chicago and reached criticality in December 1942. This was the confirmation that men were able to use sustained fission reactions in order to produce energy. Following this success, nuclear reactors studies gave rise to several families of reactors corresponding to different orientations and technical choices. They are linked mainly to the choice of fuel (natural uranium, enriched uranium, plutonium, thorium), coolant (water, carbon dioxide, helium, sodium, ...) and moderator for slow neutron reactors (graphite, light water, heavy water). Out of all these choices, the pressurized water reactor (PWR) family is the closest to the Oklo natural reactors. Many intriguing similarities are observed and discussed in the present article. Our present-day understanding of the PWR operating conditions has been a great help for understanding the Oklo reactors.

The Oklo phenomenon, the discovery, first questions and first answers.

The discovery in Nature of 16 reactions zones where sustained fission chain reaction occurred 1.95 b.y ago raised numerous questions in many scientific fields. Fifty years later, some of them are still unanswered. This article recalls the history of this discovery and the scientific facts before discussing the present-day understanding of this complex phenomenon.

Electrochemical, spectroscopic, and structural evidence for the mild hydrolysis of tetracyanoethylene, TCNE, to form the 2,3,3-tricyanoacrylamidate ligand: isolation of an unexpected quadruply-bonded polymeric material [Mo2(O2CCMe3)3((NC)2CC(CN)CONH)]infinity.

Under strictly anhydrous conditions, no reaction occurs between Mo(2)(O(2)CCMe(3))(4) and tetracyanoethylene, TCNE, at room temperature, but after addition of 1 equiv of water, a reaction proceeds to form [Mo(2)(O(2)CCMe(3))(3)((NC)(2)CC(CN)CONH)], 1. The compound contains a quadruple-bonded Mo(2) unit and the 2,3,3-tricyanoacrylamidate anion as a ligand (TC3A), a very unusual hydrolyzed form of TCNE. Two different solid-state structures were obtained after crystallization of 1. Crystals obtained from CH(2)Cl(2) consist of a two-dimensional network, and crystals grown from a C(6)H(6) solution form a 1-D chain motif. In both cases, the TC3A ligand acts as a polydentate ligand involving a bidentate OCN bridging unit and two CN groups. The electrochemical and spectroscopic (IR, UV/vis/near-IR, NMR, EPR) properties of 1 support the formulation in solution as a discrete 1:1 complex of the TC3A donor ligand and a Mo(2) unit with no charge transfer. The coordinated TC3A ligand exhibits redox properties similar to those of free TCNE.