Isotopes 238U Research Articles

Measurement of partial (n, n’γ) reaction cross-sections on highly radioactive nuclei of interest for energy production

In the context of the development of Gen. IV nuclear reactors, the GIF (Generation IV. International Forum) has selected six innovative technologies. Among them, one can highlight the concept of breeding for 232Th/233U and 238U/239Pu fuel cycles. But those nuclei, crucial for such cycles, suffer from a lack of precise knowledge (nuclear structure, reaction cross sections). In particular, it has been demonstrated that neutron inelastic scattering reaction cross sections are not known with sufficient precision for the isotopes 238U and 239Pu, and not known at all experimentally for 233U. In order to perform simulations of innovative reactor cores for the development of those technologies, the knowledge of the reaction cross section has to be improved which implies that new measurements have to be done. The GRAPhEME (GeRmanium array for Actinides PrEcise MEasurements) experimental setup, developed by the IPHC laboratory from CNRS and installed at the EC-JRC-Geel GELINA facility is a powerful tool to answer this need [1, 2]. Combining the prompt γ-ray spectroscopy and the time-of-flight methods, it measures partial (n, xnγ) reaction cross sections. This paper reports on the improvements made on the GRAPhEME setup and data analysis methodology to tackle the challenge of (n, xnγ) cross section measurements on high activity actinides. Results obtained so far on 233U are presented compared to TALYS calculations.

Natural Radioactivity and Chemical Evolution on the Early Earth: Prebiotic Chemistry and Oxygenation

It is generally recognized that the evolution of the early Earth was affected by an external energy source: radiation from the early Sun. The hypothesis about the important role of natural radioactivity, as a source of internal energy in the evolution of the early Earth, is considered and substantiated in this work. The decay of the long-lived isotopes 232Th, 238U, 235U, and 40K in the Global Ocean initiated the oxygenation of the hydro- and atmosphere, and the abiogenesis. The content of isotopes in the ocean and the kinetics of their decay, the values of the absorbed dose and dose rate, and the efficiency of sea water radiolysis, as a function of time, were calculated. The ocean served as both a "reservoir" that collected components of the early atmosphere and products of their transformations, and a "converter" in which further chemical reactions of these compounds took place. Radical mechanisms were proposed for the formation of simple amino acids, sugars, and nitrogen bases, i.e., the key structures of all living things, and also for the formation of oxygen. The calculation results confirm the possible important role of natural radioactivity in the evolution of terrestrial matter, and the emergence of life.

Long-term change in uranium migration processes in highly eroded granite, demonstrated by uranium series disequilibrium in fracture-filling materials

The mineralization and mobilization of uranium (U) are strongly associated with subsurface redox processes, which can affect the U inventory of deep geological repositories of nuclear waste and are to be considered in safety assessments. The Korea Atomic Energy Research Institute (KAERI) underground research tunnel (KURT) is located in an active groundwater recharge area in granitic bedrock, where oxidizing meteoric water infiltrates the subsurface through permeable zones of highly weathered granite, forming multiple redox fronts. Fracture-filling materials (FFMs) are U-enriched, with concentrations of up to ∼50 mg/kg. We examined long-lived radioactive isotopes of 230Th, 232Th, 234U, 235U, and 238U and other redox-sensitive elements and isotopes, e.g., Fe, δ56Fe, and Ce/Ce*, in the FFMs to understand fluctuations in redox conditions in the granitic fracture system. The primary focus was on U migration processes and to decipher the depths of meteoric water infiltration over geological timescales. Enrichment of uranium and other redox-sensitive elements occurred in FFMs at relatively shallow depths (∼180 m below ground level, bgl), implying that the U distribution was associated with redox interactions of the intruding groundwater that precipitated the secondary minerals. Interaction of FFMs with groundwater caused activity ratios (AR) of the FFMs to deviate moderately from secular equilibrium (AR = 1); 234U/238U AR of 0.87–1.25, and 230Th/238U AR of 0.76–1.22. A dynamic redox transition zone, witnessed by uranium series disequilibrium, shows both deposition and complex redistribution in the 120–205 Ka time frame, is confined to the upper ∼180 m bgl of the fractured rock. The results provide new insights into the long-term paleo-redox processes in the complex granitic fracture systems, which are analogs of potential deep-seated geological repositories for high-level radioactive waste.

Uncertainty Propagation of Fission Product Yields from Uranium and Plutonium in Pebble-Bed HTGR Burnup Calculation

Quantifying fission product yield uncertainty contribution to reactor burnup calculation is an important aspect of pebble-bed High Temperature Gas-cooled Reactor (pebble-bed HTGR) uncertainty analysis. In this work, uncertainty propagation of fission product yield to pebble-bed HTGR burnup calculation is conducted. Uncertainty of fission product yields from four fissile isotopes, namely 233U, 235U, 239Pu and 241Pu, are considered. The stochastic sampling-based uncertainty analysis method is adopted and fission product yield covariance matrices are estimated from ENDF/B-VII.1. The covariance matrix for each fissile actinide is estimated based on the Bayesian method and fission product yields are assigned with log-normal distribution in the sampling process with the Latin Hypercube Sampling (LHS) method. Since the fission fraction from 239Pu plays an important role in fissions of fuels with high burnup value in pebble-bed HTGR, the fission product yield uncertainty contribution from 239Pu is highlighted in this work. The result shows that, in the burnup equilibrium state of pebble-bed HTGR, fission product yield uncertainty contributions from 235U and 239Pu to relative uncertainty of keff are 0.027% and 0.026%, respectively. The overall uncertainty contribution from four fissile isotopes (233U, 235U, 239Pu and 241Pu) to relative uncertainty of equilibrium core keff is 0.038%. Furthermore, fission product yield uncertainty has an important contribution to the nuclide density uncertainty of fission products. The most relative uncertainty, 10.82%, is observed in 109Ag contributed from the fission product yield uncertainty of 239Pu at the burnup equilibrium state. This indicates the uncertainty contribution from the fission product yield of 239Pu cannot be neglected in pebble-bed HTGR burnup uncertainty analysis.

DEVELOPMENT OF SILICON DETECTORS AND ELECTRONIC UNITS FOR RADIOMETER ALPHA-, BETAAND GAMMA-RADIATION

Spectrometry of nuclear radiation using semiconductor detectors (SPDs) originated in the early 1960s. Of all the tasks of nuclear radiation spectrometry, the most urgent is the creation of specialized instruments for scientific research and experiments. With the improvement of PPD manufacturing technology, new opportunities for improving the operational parameters of PPDs appear, and the range of tasks solved with their help is expanding. The article describes a special manufacturing technology for a silicon detector with a large sensitive area and working volume, as well as radiometers based on them. The developed technology for the manufacture of detectors ensures the minimum thickness of the “input” and “output” windows, which is important for measuring low-energy particles and allows measurements in 2π-geometry. In world practice, such detectors are rarely used. The paper presents the structure and block diagram of the developed radiometric device based on large detectors for rapid measurement of low-intensity radiation of radioactive elements. Detectors are developed taking into account the planned physical experiments and are manufactured for certain types of radiometers to perform a specific task. The developed device is designed to measure the volumetric activity of radon alpha radiation and the activity of beta and gamma radiation of natural isotopes (238U, 234U, 232Th, 226Ra, 222Rn, 218Po, 214Bi, etc.) in various media. The device is mobile, compact and safe, and can be used both in the field and stationary. The article presents the results of a study of the activities of alpha, beta and gamma radiation of soil air from a well 1.5 m deep. on the territory of one of the foothill regions of the republic. Measurements are taken in real time, online and the data is displayed on a computer monitor.

Status report on AMS measurements of plutonium isotopes using the 1MV Tandetron Accelerator at IFIN-HH

Accelerator Mass Spectrometry is the most sensitive measurement method that allows us to determine 239, 240, and 244Pu radioisotopes in environmental samples with extremely low isotopic concentrations. By measuring plutonium isotopes, we focus our research activity on two main directions, monitoring of environmental nuclear pollution and nuclear astrophysics.This report gives an overview of the progress that was made in the determination of plutonium isotopes at IFIN-HH, and it starts with the studies performed for the transport and measurement of actinides using the 1MV Tandetron Accelerator. The 238U and 232Th isotopes were used as pilot beams for the determination of ion transport parameters, after which the isotopic ratios 239Pu/242Pu, 240Pu/242Pu, and 244Pu/242Pu were measured using the new Plutonium Standard ColPuS. The results obtained for the isotopic ratios agree with the consensus values of the standard. In addition, we developed radiochemical procedures for thorium, plutonium, and uranium separation, the chemical recoveries of uranium and thorium using UTEVA and TEVA resins are presented, where we used an Agilent ICP-MS system (Inductively Coupled Plasma Mass Spectrometry). The radiochemical separation is important mainly for obtaining target materials that can provide microscopic pilot beams as U and/or Th. Finally, a short overview is provided of the molecular interference challenges that are encountered when measuring plutonium isotopes.

Abundances of Uranium and Thorium Elements in Earth Estimated by Geoneutrino Spectroscopy

AbstractThe decay of the primordial isotopes 238U, 235U, 232Th, and 40K has contributed to the terrestrial heat budget throughout the Earth's history. Hence, the individual abundance of those isotopes are key parameters in reconstructing contemporary Earth models. The geoneutrinos produced by the radioactive decays of uranium and thorium have been observed with the Kamioka Liquid‐Scintillator Antineutrino Detector (KamLAND). Those measurements have been improved with more than 18‐year observation time, and improvement in detector background levels mainly with an 8‐year nearly reactor‐free period, which now permit spectroscopy with geoneutrinos. Our results yield the first constraint on both uranium and thorium heat contributions. The KamLAND result is consistent with geochemical estimations based on elemental abundances of chondritic meteorites and mantle peridotites. The High‐Q model is disfavored at 99.76% C.L. and a fully radiogenic model is excluded at 5.2σ assuming a homogeneous heat producing element distribution in the mantle.

Evaluation of performance and precision of a radiochemical procedure for quantification of low concentrations of 234U and 238U in water samples

ABSTRACT Uranium isotopes 238U, 234U, and 235U are alpha-particle emitters and can be directly measured by alpha spectrometry. To evaluate the accuracy of the determination of low concentration of U isotopes in water samples, six spiked water samples were analysed by extraction chromatography using uranium selective resin. The separated uranium isotopes were electrodeplated on a disc of stainless steel and were measured by an alpha spectrometer for the determination of radiochemical yield using 232U isotopic tracer. Three certified IAEA reference water samples (IAEA-Cu-2010-03, IAEA-420 and IAEA-421) have been analysed for method validation. The measurement uncertainty sources were identified, and the combined standard uncertainty of the measurement has been determined. Although the activity of uranium isotopes in the certified reference samples was very low, the obtained results were reliable and were within the certified range with 95% confidence level. The characteristic limits (the decision threshold and detection limit) were calculated. The combined uncertainty of the results was mostly attributed to the count rates in the region of interest of the analyte and the tracer in the blank as well as in the sample spectra. Based on the results of the acceptance criteria including Z-score, u-test, trueness and precision, the used radiochemical procedure was found accurate and precise as well.

Double Cascades for Purification of Reprocessed Uranium Hexafluoride from 232, 234, 236U Isotopes

The paper offers a double-cascade scheme for reducing the concentration of 232, 234, 236U isotopes in reprocessed uranium hexafluoride. The greatest decrease of the ratio between the masses of 236U and 235U is provided in the product of the first ordinary cascade enriched by 235U at the concentration of less than 20%. For this purpose, a special mode of stages operation is determined. Enrichment by 232, 234U is performed in the second ordinary cascade, which is fed by the product of the first cascade. After being purified from 232, 234U, the waste flow is diluted till the concentration of 235U is less than 5%. This paper describes the methodology for calculating the parameters of cascades with the stage separation factors correlating with gas centrifuges. This methodology served as a basis for a computational experiment. It is demonstrated that the output gained after the dilution meets the requirements of the American Society for Testing and Materials C996-20 specification for the commercial grade of low-enriched uranium hexafluoride in terms of 232, 234U isotopes. The content of 236U in it is several times less than during the direct enrichment of reprocessed uranium hexafluoride.

Application of radio-analytical technique for determination of “Age” of nuclear materials for nuclear forensics

Determination of “Age” – the time elapsed since the last chemical purification – of a nuclear material is an important parameter in Nuclear Forensic analysis. Estimation of age may give crucial acumen of the possible production facility and the declared origin – if available – in case of a suspected nuclear material which is out of regulatory control. Mass spectrometry is the most common analytical technique used for chronological studies due to its superior precision (lower uncertainty). Though alpha spectrometry can be used for age determination, it better suits for irradiated fuel based radio-chronology because of higher specific activity of irradiated actinides (actinides formed by irradiation of actinides within a reactor) like plutonium. This paper describes the application of alpha spectrometry for the age determination in some natural U-metal samples with known history. The method utilises the radioactive non-equilibrium existing between 234U and 230Th isotopes in the sample for the elucidation of age of the material. Optimisation of a chemical procedure for the separation of ultra-trace amount of thorium isotopes from bulk uranium material using double column anion exchange chromatography is also presented. The chemical recoveries of uranium and thorium obtained with the present method were >95%. The uncertainty in age determination was <10% and ∼15% for older (∼51 years) and newer (∼11 years) samples respectively. The method is also used to elucidate the age of an unknown uranium material and found to be 39 ± 4 years old.

Combined laser ablation-tuneable diode laser absorption spectroscopy and laser-induced breakdown spectroscopy for rapid isotopic analysis of uranium

Laser-induced breakdown spectroscopy (LIBS) can provide immediate in situ elemental characterisation of a sample from a stand-off distance with no sample preparation for bulk, minor and trace species. These attributes suggest the technique could be applied to many processes in the nuclear industry. However, at present, LIBS cannot quickly and easily be used to determine the isotopic composition of a sample, which limits its potential. We report the development of a system comprising laser ablation-tuneable diode laser absorption spectroscopy (LA-TDLAS) and LIBS which can maintain the benefits of traditional LIBS and enable isotopic analysis of certain elements, including lithium and uranium. In this feasibility study, we used this approach to analyse a uranium metal wire sample under ambient conditions of pressure and temperature. Simultaneously, from the same laser produced plasma, the 235U isotope fraction was determined with LA-TDLAS while the plasma emission was used to obtain the electron density of the plasma using Stark broadening of uranium and sodium emission lines. The accuracy of the isotopic analysis performed with LA-TDLAS was limited by the noise of the absorption spectra and low concentration of the minor 235U isotope (0.7%). The limit of detection was estimated to be 1.5 wt%. Using the Doppler broadening of absorption lineshapes, the plasma temperature was calculated to drop from 25,000 K to 2000 K during the plasma lifetime. The electron density rapidly decreased from 6 × 1016 cm−3 immediately after laser ablation to around 3 × 1016 cm−3 after a few μs. The combination of LIBS and LA-TDLAS opens the possibility of rapid elemental and isotopic analysis which would be useful across the nuclear fuel cycle. Furthermore, the LA-TDLAS technique could be further developed for a portable measuring instrument for nuclear forensics applications.

Constraints on r-process nucleosynthesis from 129I and 247Cm in the early Solar system

ABSTRACT GW170817 has confirmed binary neutron star mergers as one of the sites for rapid neutron capture (r) process. However, there are large theoretical and experimental uncertainties associated with the resulting nucleosynthesis calculations and additional sites may be needed to explain all the existing observations. In this regard, abundances of short-lived radioactive isotopes (SLRIs) in the early Solar system (ESS), which are synthesized exclusively by r-process, can provide independent clues regarding the nature of r-process events. In this work, we study the evolution of r-process SLRIs 129I and 247Cm as well as the corresponding reference isotopes 127I and 235U at the solar location. We consider up to three different sources that have distinct 129I/247Cm production ratios corresponding to the varied r-process conditions in different astrophysical scenarios. In contrast to results reported in a recent study, we find that 129I and 247Cm in the ESS do not come entirely from a single major event but get contributions from at least two more minor contributors. This has a dramatic effect on the evolution of the 129I/247Cm ratio, such that the measured ESS value in meteorites may not correspond to that of the ‘last’ major r-process event. Interestingly, however, we find that the 129I/247Cm ratio, in combination with the observed 129I/127I and 247Cm/235U ratio in the ESS, can still provide important constraints on the properties of proposed r-process sources operating in the Milky Way.

THE MODEL OF DESCRIPTION OF EXTRACTION OF URANIUM-235 AND URANIUM-238 ISOTOPES COMPLEXES IN A SUPERCRITICAL CARBON DIOXIDE LAYER HEATED FROM BELLOW

The models and analytical solutions for description of the extraction of 235U and 238U isotopes complexes in case of availability of water microdrops located both in a uniform temperature field and in a non-uniform temperature field of the heated from bellow horizontal layer of the supercritical carbon dioxide are suggested. It has been indicated that in a uniform temperature field the efficiency of supercritical fluid extraction with carbon dioxide complexes of uranium isotopes reaches a maximum at the limit of water solubility in supercritical carbon dioxide. Theoretical calculations have demonstrated that the maximum concentration of 235U isotope complexes heated from below in a supercritical carbon dioxide layer reaches 1.2% and is observed near the lower layer boundary. The procedure for supercritical fluid extraction with carbon dioxide is described, in which the concentration of 235U isotope complexes in the supercritical fluid will be higher than the natural one.

РАДОНОВЫЕ ВОДЫ БОЛЬШОЙ БЕЛОКУРИХИ: СОВРЕМЕННОЕ ИСПОЛЬЗОВАНИЕ И ПЕРСПЕКТИВНЫЕ УЧАСТКИ РАЗВИТИЯ КУРОРТА

At present, a tourist and recreational cluster is being formed around the federal resort of Belokurikha, which includes the adjacent territories of the Smolensk, Altai and Soloneshensky municipal districts of the Altai Territory. To determine the directions of spatial development in the cluster, information is needed on the prospects for discovering new sources of radon waters. The purpose of the study is to assess the current level of use of radon waters by the resort of Belokurikha, to identify geological and geochemical prospecting indicators of radon waters and to assess the prospects for their detection within the tourist cluster "Bolshaya Belokurikha". A field study of the territory and sampling of granitoids from the Belokurikhinsky pluton were carried out. The analysis of isotopes of uranium and thorium from granitoids was carried out according to the classical method at the Center for Isotope Research of the Russian Geological Research Institute (St. Petersburg). The analysis of the current water use was carried out based on the data of the Hydroservice of the resort of Belokurikha on water consumption for 2019. The volume of radon water consumption by the sanatoriums of the Belokurikha resort in 2019 varied throughout the year from 435.1 m3 / day in August to 282.1 m3 / day in December. This does not exceed the operational reserves of the Belokurikhinskoye deposit, which are 600 m3 / day. Potential areas for the detection of radon waters are confined to the deep sub-latitudinal Belokurikha fault, which runs along the border of the Pre-Altai plain and the Altai Mountains, and especially to the zones of intersection of faults of different directions. This is the main geological prospecting indicator. The geochemical prospecting indicator is the increased content of uranium in rocks. The highest concentrations of 238U and 232Th isotopes in the granitoids of the Belokurikhinsky pluton were found for aplites and pegmatites. Thus, the entire strip of the transition zone from the Biysk-Barnaul depression to the mountain-folded structure of Altai has prospects for radon waters, due to a complex of features for prospecting and factors of water formation with the participation of the vadose component and of deep waters leaching minerals with increased radioactivity from aplites and pegmatites. Prospective sites for carrying out detailed prospecting for radon waters are located in Belokurikha-2 in the area of the village Ulyanovka and near the village of Danilovka. Here, in addition to indirect prospecting indicators, there are also direct ones (radon content in waters).

Study the radioactive concentration for soil samples contaminated with depleted uranium in Al-Nahrawan site at Baghdad governorate using high purity germanium detector

This study is investigating the radioactivity in soil samples at the Al-Nahrawan site. The radiation survey appears there are 3000 square meters area are contaminated with DU in AL-Nahrawan site identified using Geiger–Müller (GM) for radiation survey and gamma spectrometry for 52 soil samples analysis taken from AL-Nahrawan site at different depths (0-70) cm and different locations. The results of gamma analysis using high-purity germanium show that the ratio between 235U/238U is less than 0.00720 (neutral ratio), and it has different values from 0.002-0.00588, and the average value of radioactive nuclides concentration for (238U, 235U and 40K) are (76019.61, 259.55 and 147.5) Bq/kg respectively, these values are higher than the (BG) radioactive concentration levels in Iraq for (238U and 235U) isotopes, and the analysis of 40K concentration appeared in the acceptable limits. The health effects of depleted uranium in the human body it is exposed to many health troubles through the entry of uranium oxide particles.

Estimating the Concentrations of Natural Isotopes of 238 U and 232Th and Radiation Dose Rates for Wasit Province-Iraq by Gr-460 system

This paper includes the measurement of the concentrations of natural radioactive isotopes of 238U, 232Th, and radiation dose rates for selected areas of Wasit province.GR-460 system has been used for the radiological survey operations, it is a portable system, installed inside a minibus and inside the system, there is a Sodium Iodide Detector (NaI), that has the ability to measure the concentrations of natural radioactive isotopes in (ppm) unit and measure the radiation dose rates in µR/h unit. The measurement results showed the absence of any significant increase in the 238U and 232Th concentration where the concentration of isotopes 238U and 232Th, were 3.35 and 5.46 ppm respectively, it is authorized and acceptable. The values of radiation dose rates ranged between 45-65 ոSv/հ, and all these values are within the natural radiation background. Eleven radioactive sources type 226Ra have been detected beside destroyed industrial facility because of the war 2003 they had been used in the lightning arresters technology in the last regime. These sources treated and placed in armored containers and taken to the Iraqi main store (Iraqi Organization of Atomic Energy Site), the radiation dose measurements ranged between 72- 48 µR/h, which is higher than the natural radiation background.

New reference materials for trace-levels of actinide elements in plutonium

Abstract Two plutonium oxides were prepared as unique reference materials for measurement of actinide elements present as trace constituents. Each reference material unit is approximately 200 mg of PuO2 powder in a quartz glass bottle. Characterized attributes of the oxides included mass fractions of plutonium, americium, neptunium, and uranium. Isotope-amount ratios were also determined for plutonium and uranium, but neptunium and americium were observed to be monoisotopic 237Np and 241Am. Measurements for characterization and verification of the attributes show that plutonium and trace actinides are homogeneous with the exception of limited heterogeneity for uranium, primarily observed for the 238U isotope. Model purification ages calculated from measured americium and uranium attribute values are consistent with material histories and indicate that these impurities are predominantly due to the decay of plutonium isotopes.

Geochemical exploration for buried sandstone-hosted uranium mineralization using mobile U and Pb isotopes: case study of the REB deposit, Great Divide Basin, Wyoming

Mobile radiogenic lead isotopes ( 206 Pb, 207 Pb, 208 Pb and 210 Pb) represent products of radioactive decay of their parental uranium and thorium isotopes ( 238 U, 235 U, 232 Th), and are considered potential geochemical pathfinders of buried sandstone-type uranium deposits. Soil samples collected along a geochemical traverse intersecting buried uranium roll front mineralization at the REB deposit in the Great Divide Basin, Wyoming, USA were studied. Mineralization of this deposit is hosted in weakly lithified arkosic sands, at a depth of 120–200 m, without a strong surficial expression of its presence at depth, which makes discovery of this deposit type difficult, slow and expensive. All soil samples have been analysed for ratios of the mobile long-lived Pb isotopes and their parental U and Th isotopes, determined from partial leach products obtained using a weak acid leaching technique. The samples were also analysed for trace elements, assayed both in the partial leach products and using conventional whole-soil sample assays. Ratios of the mobile radiogenic Pb isotopes to their parental U and Th isotopes ( 206 Pb/ 238 U, 207 Pb/ 235 U and 208 Pb/ 232 Th) determined in the partial leach products exhibit anomalous contents in the soil samples collected above the uranium rolls. The anomalous values are several times greater than background values, to lateral distances of 350–400 m outside of the roll fronts. Notably, conventional whole-soil assays have failed to detect the anomalies that were detected using mobile Pb isotopes. Supplementary material : Minor and trace elements assays of the geochemical samples are available at https://doi.org/10.6084/m9.figshare.c.5610980

SPATIAL NONUNIFORM DISTRIBUTION OF 235U ISOTOPE AT SUPERCRITICAL FLUID EXTRACTION WITH CARBON DIOXIDE IN A GRADIENT TEMPERATURE FIELD

The spatial redistribution of the 235U isotope of natural uranium in a gradient temperature field along the height of the reactor in supercritical carbon dioxide has been experimentally investigated. The scheme of the reactor is given and the principle of operation of the reactor is described. The method of preparation of initial samples from granite samples containing natural uranium and the procedure of extraction are described. The conclusion about the spatial redistribution of 235U isotopes in supercritical carbon dioxide is based on the analysis of gamma spectra of extracts. It is shown that the concentration of the 235U isotope in a supercritical fluid is maximal near the lower heated flange of the reactor, and decreases with approaching the upper, cooled flange. It was concluded that the separation factor of the 235U isotope in supercritical carbon dioxide can be about 1.2 ± 0.12.

Radiation-induced oxidation of iron in the ocean of the early Earth

Radiation-chemical mechanism of bivalent iron oxidation in the World Ocean is suggested and substantiated. The decay of primordial radioactive isotopes 40K, 235U, 238U, and 232Th, present on the Earth since the moment of its formation (4.5 Ga), initiated the water radiolysis with the generation of ionic and radical species (eaq–, •H, •OH) and formation of molecular products (H2O2 and H2). The mechanism of the radiation-chemical oxidation of Fe2+ in the World Ocean depending on pH and the presence of oxygen is considered. The Fe(III) hydrolysis is responsible for the iron transfer from the freely dissolved state in the ocean into the solid phase. Low solubility of the hydroxide favors the formation of insoluble oxides, magnetite and hematite. The calculations reveal the decisive role of 40K in the radiation-induced oxidation. The process occurred throughout the volume of the World Ocean with the efficiency corresponding to the solubility of potassium salts in it. Quantitative estimation shows that, in the period from 4.3 to 2.5 Ga, i.e., before the occurrence of the Great Oxidation Event, about ~1021 g of iron could be oxidized by the radiation-chemical mechanism.