Natural Reactor Research Articles

Hydrothermal clay mineral formation in the uraniferous Paleoproterozoic FA Formation, Francevillian basin, Gabon

The spatial distribution of neoformed clay minerals was investigated in the Paleoproterozoic FA Formation of the Francevillian basin, south-east Gabon, which hosts high-grade U ore deposits associated with the only known occurrence of natural nuclear reactors. Illite appears as the main clay phase in the lower fluvial unit. In the mineralized upper fluvio-deltaic-tidal unit, the clay assemblage is more diversified and commonly characterized by illite, chamosite, berthierine and chamosite/berthierine mixed layers, usually in association with alteration products of U-bearing minerals. The clay mineral assemblage thus potentially constitutes a mineralogical marker for the regional distribution of U ore deposits.Polytype species of the clay phases indicate mineral transformations in an environment characterized by a high fluid/rock ratio. According to crystalline structure, mineral chemistry, thermodynamic modeling and geochronology, clay phases seem to be mainly hygrometer, rather than exclusively a thermometer, and their formation, as well as associated dissolution-precipitation of U-bearing minerals took place between ca. 2040 and 2010Ma ago, at temperatures of about 240±30°C. Using previous burial estimates, this suggests the operation of hydrothermal processes controlled by an external heat source, likely associated with volcanism during deposition of the FD and FE formations. Hydrothermal activity, involving oxidized fluids, would have driven U remobilization and, ultimately, formation of high-grade U ore deposits. The thermal history proposed here allows for a better understanding of the conditions during burial of the sedimentary succession and the origin of its rich U endowment.

Percolation of diagenetic fluids in the Archaean basement of the Franceville basin

The Palaeoproterozoic Franceville basin, Gabon, is mainly known for its high-grade uranium deposits, which are the only ones known to act as natural nuclear fission reactors. Previous work in the Kiéné region investigated the nature of the fluids responsible for these natural nuclear reactors. The present work focuses on the top of the Archaean granitic basement, specifically, to identify and date the successive alteration events that affected this basement just below the unconformity separating it from the Palaeoproterozoic basin. Core from four drill holes crosscutting the basin–basement unconformity have been studied. Dating is based on U–Pb isotopic analyses performed on monazite. The origin of fluids is discussed from the study of fluid inclusion planes (FIP) in quartz from basement granitoids. From the deepest part of the drill holes to the unconformable boundary with the basin, propylitic alteration assemblages are progressively replaced by illite and locally by a phengite+Fe chlorite±Fe oxide assemblage. Illitic alteration is particularly strong along the sediment–granitoid contact and is associated with quartz dissolution. It was followed by calcite and anhydrite precipitation as fracture fillings. U–Pb isotopic dating outlines three successive events: a 3.0–2.9-Ga primary magmatic event, a 2.6-Ga propylitic alteration and a late 1.9-Ga diagenetic event. Fluid inclusion microthermometry suggests the circulation of three types of fluids: (1) a Na–Ca-rich diagenetic brine, (2) a moderately saline (diagenetic+meteoric) fluid, and (3) a low-salinity fluid of probable meteoric origin. These fluids are similar to those previously identified within the overlying sedimentary rocks of the Franceville basin. Overall, the data collected in this study show that the Proterozoic–Archaean unconformity has operated as a major flow corridor for fluids circulation, around 1.9Ga.

Development of a numerical thermal-hydraulic model to simulate forced and natural circulation reactor loop and non-linear analysis of parallel channel instabilities

The present work focuses on the development of a numerical model which simulates the hydrodynamics of forced and natural circulation boiling water nuclear reactor loops and demonstrates its capability to analyse density wave instability of the reactor undergoing in-phase and out-of-phase modes of oscillations. The numerical algorithm integrates a compressible and distributed fluid flow model used for simulating the two-phase flow phenomenon in the channels of the reactor core with an incompressible lumped parameter model which accounts for sub-cooled water flow in various ex-core and recirculation components of the reactor loop. The algorithm used in the present hybrid model explicitly addresses the uncertainties related to boundary conditions existing in the literature to simulate parallel channel instability analysis for forced and natural circulation boiling reactor loops. Extensive numerical experimentations are performed to search the stability thresholds for both the forced and natural circulation reactor loops undergoing in-phase and out-ofphase instability modes.

Steam drum level control studies of a natural circulation multi loop reactor

Abstract The proposed heavy water moderated and light water cooled pressure tube type boiling water reactor works on natural circulation at all power levels. It has parallel inter-connected loops with 452 boiling channels in the main heat transport system configuration. These multiple (four) interconnected loops influence the steam drum level control adversely through the common reactor inlet header. Alternate design studies made earlier for efficient control of SD levels have shown favorable results. This has lead to explore further the present scheme with the compartmentalization of CRIH into four compartments catering to four loops separately. The conventional 3-element level control has been found to be working satisfactorily. The interconnections between ECCS header and inlet header compartments have also increased the safety margin for various LOCA and design basis events. The paper deals with the SD level control aspects for this novel MHT configuration which has been analyzed for various PIEs (Postulated Initiating Events) and found to be satisfactory.

Parametric Core Analysis and Economic Feasibility of a Thorium Heavy Water Breeder Reactor

A parametric study was performed on the thorium-to-233U breeder fuel cycle for pressurized heavy water reactors (PHWRs) similar to the existing CANDU type. The objective was to estimate the sensitivity of the thorium breeder PHWR energy cost to fuel reprocessing costs, reactor capital costs, fuel specific power, fuel-to-moderator ratio, and reactor size and to find optimal parameters to minimize the energy cost for reasonable economic assumptions. A baseline model thorium heavy water breeder reactor (THWBR) was developed from these parameters to show how an existing PHWR would perform economically if fueled only with thorium and the 233U bred and reprocessed from that thorium. This study found that the baseline model THWBR is not cost competitive with the current PHWR fuel cycle using natural uranium but may be significantly closer in cost to the natural uranium fuel cycle than models discussed in previous publications. Because the proposed thorium reactor can, with the assistance of some thorium fuel reprocessing, achieve a higher average fuel burnup than the once-through natural uranium cycle, the waste management costs will be lower while the reprocessing costs will be higher than the natural uranium fuel system. When the strategic and proliferation-resistance values are included, the thorium breeder PHWR may be competitive with natural uranium PHWRs and light water reactors in some markets. The next phase of our study is expected to show how to use novel combinations of unconventional PHWR core geometries to increase the breeding ratio and fuel burnup, decrease the reprocessing requirements, and make a thermal breeder reactor more economical.

Theoretical and experimental study on single-phase natural circulation under inclined conditions

The natural circulation reactor is widely used in marine environments where thermo-hydraulic performance is heavily affected by the heaving, pitching, and inclining of a ship. This paper theoretically and experimentally investigated steady-state single-phase natural circulation under inclined conditions. Results showed that energy transported by natural circulation was proportional to 1.5 times the power of the temperature difference between the hot leg and the cold leg. Furthermore, a parameter, k, was presented that revealed the comprehensive influence of working fluid properties, resistance characteristics, gravity fields, and loop configurations. k was treated as the criterion for the circulation ability of a loop and it also acted as the basis for evaluating and optimizing different designs. Analysis under the guidance of k was confirmed by a series of experiments performed on a symmetrical two-circuit loop. Both theoretical and experimental results showed that the inclination restrained overall circulation due to the decrease in average altitude difference between the steam generators and the electric heater. The disparity in branch circulations increased with the increase in the inclined angle. A loop design consisting of a large altitude difference and a small width was preferable to confine the influence of inclination. However, if the loop width was too small, it caused a severe reduction in the circulation ability for large angle inclinations.

Criticality of Oklo Natural Reactors: Realistic Model of Reaction Zone 9

Explanation of the so-called Oklo phenomenon can only be found by associating neutron physics and geological observations. The framework of Reactor Zone #9 (RZ9) from the Oklo uranium deposit in Gabon is discussed. A realistic model of RZ9 is proposed and studied with present-day numerical tools. Anchor points for this model are set by means of a selection of relevant parameters acting on geological conditions and neutron-transport physics, including ore homogeneity, initial reactor thickness, reflector effect and initial neutron poison content. Based on Monte Carlo simulations, the present modeling enables discussion of start-up conditions of the RZ9 reactor at optimal overlap of two opposing long-term processes: geochemical enrichment of the uranium content of the ore and its progressive impoverishment in 235U. On the basis of the present work, the inception and operation of Oklo natural reactors can be discussed from the largest to smallest initial core size, whereas before, only the largest ones had found some explanation.

KamLAND-Experiment and Soliton-Like Nuclear Georeactor. Part 1. Comparison of Theory with Experiment

We give an alternative description of the data produced in the KamLAND experiment, assuming the existence of a natural nuclear reactor on the boundary of the liquid and solid phases of the Earth core. Analyzing the uncertainty of antineutrino spectrum of georeactor origin, we show that the theoretical (which takes into account the soliton-like nuclear georeactor) total reactor antineutrino spectra describe with good accuracy the experimental KamLAND-data over the years of 2002-2007 and 2002-2009, respectively. At the same time, the parameters of mixing (△m221=2.5×10-5 eV2, tan2θ12=0.437) calculated within the framework of georeactor hypothesis substantially differ from the parameters of mixing ((△m221=7.49×10-5 eV2, tan2θ12=0.436) obtained in KamLAND-experiment for total exposure over the period of 2002-2009. By triangulation of KamLAND and Borexino data we have constructed the coordinate location of soliton-like nuclear georeactors on the boundary of the liquid and solid phases of the Earth core.

Sun, Earth, radioactive ore: Common periodicity

The study of natural radioactivity of objects which are able to change their temporal timing feature is presented. It is of interest to compare the latest data on the activity of the Sun and the periodicity of solar neutrinos and the temporal characteristics of the radioactive source. That is, to conduct a search for the possible influence of external sources for radioactivity. There are cycles 5 min, 18 min and 53 min found in solar physics. The cycle of 27 days corresponds to the activity of the Sun. During of the solar activeity these temporal pulsations are lost in a strong variation of solar wind (Neugebauer, NASA). The Stanford University scientists (P. Starrek, G. Valter and M. Vitlend) have found the cycle of 28.4 days as pulsations of the solar neutrinos. Neutrinos come from the depths of the Sun and they tell about the frequency of oscillations of solar bowels. It is also seen online: Kostyantynivska L. V. Solar activity. Search experiment is better to have a known but modified experiment. Experiments on monitoring natural radioactivity and the possible influence from the Sun were previously carried out by measuring the variations of the gamma-ray sample of ore from the TransBaikal uranium deposit; the characteristics of the sample are known. The spectrum of temporal variations in the activity of the sample Zabaikalskaya radioactive ore contains peaks which coincide with the period of natural oscillations of the Sun. The capture cross section of the radioactive heavy deformed nucleus in time decay increases in many orders and is able to interact with the stream of solar neutrinos which are modulated by own oscillations of the Sun. The picks of spectrum of long-period oscillations of the Earth exceed its own contain peaks that match the value with an accuracy of 1% 3% with peaks of its own oscillations of the Sun. The mechanism of excitation of these oscillations is similar to the nature of variations in the activity of a radioactive sample of ore. These effects are included in the mechanisms of interaction of the Earth—the Sun systems and impact on seismicity; search problem of existing natural nuclear reactor inside Earth core.

Comparison of a post-closure transient criticality model with the Oklo natural reactors

AbstractIn support of the disposal system safety case for a geological disposal facility (GDF) there is a requirement to consider 'what-if' hypothetical scenarios for post-closure nuclear criticality. Although all such scenarios are considered very unlikely, one 'what-if' scenario is the mobilization of fissile material from a number of waste packages and its slow accumulation within the GDF or the immediate surroundings. Should sufficient fissile material accumulate a quasi-steady-state (QSS) transient criticality event could result. A computer model has been developed to understand the evolution and consequences of such an event.Since a postulated QSS criticality could persist for many millennia, building confidence in the modelling approach is difficult. However, the Oklo natural reactors in Africa operated for similar durations around two billion years ago, providing a natural analogue for comparison. This paper describes the modelling approach, its application to hypothetical criticality events for a GDF, and how the model can be compared to Oklo. The model results are found to be in agreement with the observational evidence from Oklo, building confidence in the use of the QSS model to simulate postulated post-closure criticality events in GDFs.

γ-ray fluxes in Oklo natural reactors

Uncertainty in the operating temperatures of Oklo reactor zones impacts the precision of bounds derived for time variation of the fine structure constant $\alpha$. Improved $^{176}$Lu/$^{175}$Lu thermometry has been discussed but its usefulness may be complicated by photo excitation of the isomeric state $^{176m}$Lu by $^{176}$Lu($\gamma,\gamma^\prime $) fluorescence. We calculate prompt, delayed and equilibrium $\gamma$-ray fluxes due to fission of $^{235}$U in pulsed mode operation of Oklo zone RZ10. We use Monte Carlo modeling to calculate the prompt flux. We use improved data libraries to estimate delayed and equilibrium spectra and fluxes. We find $\gamma$-ray fluxes as a function of energy and derive values for the coefficients $\lambda_{\gamma,\gamma^\prime}$ that describe burn-up of $^{176}$Lu through the isomeric $^{176m}$Lu state. The contribution of the ($\gamma,\gamma^\prime $) channel to the $^{176}$Lu/$^{175}$Lu isotopic ratio is negligible in comparison to the neutron burn-up channels. Lutetium thermometry is fully applicable to analyses of Oklo reactor data.

La-138/139 isotopic data and neutron fluences for Oklo RZ10 reactor

Background: Recent years have seen a renewed interest in the Oklo phenomenon, particularly in relation to the study of time variation of the fine structure constant $\ensuremath{\alpha}$. The neutron fluence is one of the crucial parameters for Oklo reactors. Several approaches to its determination were elaborated in the past.Purpose: We consider whether it is possible to use the present isotopic ${}^{138}$La$\ensuremath{-}{}^{139}$La data for RZ10 as an additional indicator of neutron fluences in the active cores of the reactors.Results: We calculate the dependence of the Oklo ${}^{138}$La abundance on neutron fluence and elemental lanthanum concentration.Conclusion: The neutron fluence in RZ10 can be deduced from lanthanum isotopic data, but requires reliable data on the primordial elemental abundance. Conversely, if the fluence is known, the isotope ratio provides information on the primordial lanthanum abundance that is not otherwise easily determined.

The distribution of iodine in the Croatian marine lake, Mir – The missing iodate

The marine chemistry of iodine has been studied in the marine lake, Mir, regarded as a natural reactor situated in the karstificated carbonate rocks of the Croatian Adriatic coast. The investigation covered the major variables: salinity, temperature, dissolved oxygen, pH and alkalinity, some nutrients, organic carbon, and iodide, iodate and organic-I. Lake Mir was found to be meso-trophic, with dynamic nutrient cycling of a magnitude usually associated with the temperate zone but within a Mediterranean clime. An essentially U-shaped pattern exists in the plot of nutrient concentration versus time for the June–November period studied. Together, the major variables and the nutrient chemistry confirm that Lake Mir is essentially isolated from the nearby (90 m) Adriatic seawater, and this also may explain the meso-trophic nature of the lake, with dry and wet deposition as the source of the extra nutrient. It is of note that iodate was essentially absent from Mir during the sampling period. This appears to be consistent with iodine's behaviour in the oceans in general, where iodate is reduced generally as a result of the presence of the biota. The chemistry of iodine in Lake Mir is consequently dominated by changes in iodide and organic-I concentrations, with the latter at higher concentrations than those found in seawater. Even so, the total iodine concentration in Mir is only about one-quarter of that in the adjacent Adriatic seawater, and again it is argued that this is probably a function of Lake Mir's isolation.

Fission yields of molybdenum in the Oklo natural reactor

The isotopic compositions of molybdenum in six uranium-rich samples from the Oklo Zone 9 natural reactor were accurately measured by thermal ionization mass spectrometry. The samples were subjected to an ion exchange separation process that removed the isobaric elements zirconium and ruthenium, with high efficiency and a low blank. Molybdenum possesses seven isotopes of which 92,94,96Mo are unaffected by the fission process, enabling the raw data to be corrected for isotope fractionation by normalising to 92Mo/96Mo, and to use 94Mo to correct for the primordial component in each of the fission-produced isotopes. This enables the relative fission yields of Mo to be calculated from the isotopic composition measurements, to give cumulative fission yields of 1:0.941:0.936:1.025 for 95,97,98,100Mo, respectively. These data demonstrate that the most important nuclear process involved in reactor Zone 9 was the thermal neutron fission of 235U. The consistency of the relative cumulative fission yields of all six samples from different locations in the reactor, implies that Mo is a mobile element in the uraninite comprising Zone 9, and that a significant fraction of molybdenum was mobilized within the reactor zone and probably escaped from Zone 9, a conclusion in agreement with earlier published work.

Did natural fission of 235U in the earth lead to formation of the supercontinent Columbia?

Steady decline in the percentage of 235U in terrestrial uranium made natural fission impossible after about 1.8Ga. Fission before 1.8Ga disturbed the lead isotope system at various places worldwide, such as Oklo, Gabon, and may have caused the first lead isotope paradox. Fission in areas of high uranium concentration may also have generated enough heat to localize sparse Archean and Paleoproterozoic UHT belts. The oldest widespread orogenic systems formed at approximately 2.0–1.8Ga after fission stopped contributing to the earth’s heat flow. These early orogenic systems partly created the supercontinent Columbia.

176Lu/175Lu thermometry for the Oklo natural reactors: A new examination of old data

Background: Lutetium thermometry has been used to analyze Oklo natural nuclear reactor zones but leads to widely varying and puzzling predictions for the temperatures ${T}_{O}$ which in turn impacts bounds on time variation of the fine structure constant $\ensuremath{\alpha}$.Purpose: We revisit results for reactor zone RZ10 in light of new measurements of the isomer branching ratio ${B}^{g}$ in ${}^{175}$Lu neutron capture at 5 and 25 keV.Method: We recalculate predictions for ${T}_{O}$ as a function of ${B}^{g}$ using realistic models of the Oklo neutron flux.Results: We find ${T}_{O}=100\ifmmode\pm\else\textpm\fi{}30$ ${}^{\ensuremath{\circ}}$C using a new value of ${B}^{g}$, in contrast to $350<{T}_{O}<500$ ${}^{\ensuremath{\circ}}$C using the evaluated value at thermal energy.Conclusions: Lutetium thermometry can be applicable to analyses of Oklo reactor data, but a better measurement of ${B}^{g}$ with thermal neutrons is needed to confirm the reliability of temperature predictions.

A Study on the Steady-State and Transient Behavior of Natural Circulation in REX-10

Research regarding small- and medium-sized nuclear reactors (SMRs) has increased because of multipurpose applications and increased safety. According to this tendency, a new conceptual nuclear reactor, the Regional Energy rX-10MWt (REX-10), is being designed. REX-10 adopts a way to remove heat by natural circulation and integrates the primary systems within a reactor pressure vessel. To evaluate the steady-state and transient behavior of natural circulation in REX-10, a NAtural Circulation TEst Reactor (NACTER) is designed using the scaling law. The ratio of the height and core power are 1/3 and 1/500, respectively.This research can be divided into three parts - a steady-state experiment, a transient experiment, and MARS (Multidimensional Analysis for Research Safety) code analysis. To investigate the natural circulation characteristics under the steady-state conditions, two parameters were chosen and various experiments were conducted. As a result of the steady-state experiment, we show that the most important parameter that affects the natural circulation behavior is the heater power. In addition, we carried out a transient experiment. The results of the transient experiment are that the NACTER facility is well controlled and guarantees safety in abrupt changes in experimental conditions. Finally, MARS code simulations were conducted. The MARS code results show good agreement with the experimental results.

Did natural fission of 235U in the earth lead to formation of the supercontinent Columbia?

Did natural fission of 235U in the earth lead to formation of the supercontinent Columbia?

Main aspects of the theory of ore-bearing weathering mantles in the 21st century

The theory of ore-bearing weathering mantles (WM) was developed at the Institute of Geological Sciences, USSR Academy of Sciences (currently the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences) in the 1930s. I.I. Ginzburg was an originator of this theory. The extensive new data on the bauxite-, Ni-, Au-, REE-bearing and other rare-metal WM has led to the development of a general theory of exogenic ore deposits, which has been confirmed by forecasting, prospecting, and exploration of solid mineral resources. On considering the current status of the WM theory based on the results of precision research methods (XRD, IRS, EPR spectroscopy, DTA, electron microscopy, etc.), the main aspects determining its advance can be pointed out: (1) physicochemical simulation of WM formation; (2) identification and study of nanominerals in WM; (3) study of the contribution of organic matter to weathering; and (4) use of WM as indicators of climatic change in the Earth’s history. As is presently established, WM are natural chemical reactors of nanoparticles of various minerals. The progress in nanomineralogy of WM will facilitate the choice of rational methods of mining and recovery of valuable components. As has been proved by electron microscopy, fossilized microbial communities, biofilms, and cyanobacterial mats occur in almost all WM. The importance of studying mutual relations of inorganic and organic substances of WM in more detail is evident. Development of the idea of using WM as indicators of paleoclimate will provide insights into global changes in the environment and complement the results obtained by traditional paleoclimatic methods. This aspect is especially topical in connection with widely discussed problem of global climatic warming.

Geophysics with Hawaiian Anti-neutrino Observatory (Hanohano)

The design studies are under way for the deep ocean anti-neutrino observatory located in the vicinity of the Big Island (Hawaii) with the main goal of measuring geo-neutrino flux from the mantle and core which can exclusively be done in a location far from the continental plates such is Hawaiian Islands chain. Hanohano will also accomplish the definitive measurement of the electron anti-neutrino signal from the core to observe or eliminate a hypothetical natural reactor in the Earthʼs core.