Natural Thorium Research Articles

Combined analysis of KamLAND and Borexino neutrino signals from Th and U decays in the Earth’s interior

The KamLAND and Borexino experiments have detected electron antineutrinos produced in the decay chains of natural thorium and uranium (Th and U geoneutrinos). We analyze the energy spectra of current geoneutrino data in combination with solar and long-baseline reactor neutrino data, with marginalized three-neutrino oscillation parameters. We consider the case with unconstrained Th and U event rates in KamLAND and Borexino, as well as cases with fewer degrees of freedom, as obtained by successively assuming for both experiments a common Th/U ratio, a common scaling of Th+U event rates, and a chondritic Th/U value. In combination, KamLAND and Borexino can reject the null hypothesis (no geoneutrino signal) at 5 sigma. Interesting bounds or indications emerge on the Th+U geoneutrino rates and on the Th/U ratio, in broad agreement with typical Earth model expectations. Conversely, the results disfavor the hypothesis of a georeactor in the Earth's core, if its power exceeds a few TW. The interplay of KamLAND and Borexino geoneutrino data is highlighted.

Observations and modelling of thoron and its progeny in the soil–atmosphere–plant system

Samples of pasture vegetation, mainly Trifolium pratensis, were collected at the Botanic Garden of the University of Bologna during the period 1998–2000 and measured by gamma-spectrometry for determining thoron progeny. Concentrations of 212Pb were between 1.5 and 20 Bq m −2, with individual peaks up to 70 Bq m −2. Soil samples were collected at the same location and physically characterised. Their chemical composition (particularly Th and U) was determined by X-ray fluorescence spectroscopy. Lead-212 on plants mainly originates from dry and wet deposition of this isotope generated in the lower atmosphere by the decay of its short-lived precursor 220Rn, which is produced in the upper soil layers as a member of the natural thorium decay chain and exhales into the atmosphere. Concentrations of 220Rn in the atmosphere depend on (1) the amount of Th present in soil, (2) the radon fraction which escapes from the soil minerals into the soil pore space, (3) its transport into the atmosphere, and (4) its redistribution within the atmosphere. The mobility of radon in soil pore space can vary by orders of magnitude depending on the soil water content, thus being the main factor for varying concentrations of 220Rn and 212Pb in the atmosphere. We present a simple model to predict concentrations of thoron in air and its progeny deposited from the atmosphere, which takes into account varying soil moisture contents calculated by the OPUS code. Results of this model show close agreement with our observations.

Fast reactors in the energy security for the stable development of Russia

New aspects of the development of a future nuclear power system based on the advanced technologies of a closed nuclear fuel cycle with fast-neutron reactors are discussed on the basis of an analysis of systems problems pertaining to present-day nuclear power. The systems requirements ensuring adequate fuel for nuclear power with any installed capacity and maximum use of natural uranium and thorium brought into the fuel cycle are formulated. Sodium-cooled fast reactors, which thus far possess the highest level of technological readiness for commercialization, are given a special role in the formation of a new technological platform for large-scale nuclear power of the 21st century.

Occupational exposure to natural radionuclides due to mining activities in Ibadan, Southwestern Nigeria

The activity concentrations of potassium, uranium and thorium in minerals and soil samples from a mining site in Ibadan, Southwestern Nigeria were measured using gamma ray spectroscopy method. Effective dose per annum has been calculated from the activity concentrations of dominant gamma-emitting natural radionuclides, potassium, uranium and thorium. Samples collected include minerals (beryl, quartz and feldspar), soil samples from the mining pits, heaps and undisturbed land around the mining site. The activity concentrations of 40 K, 238 U and 232 Th, respectively in Bq kg -1 in the mineral samples were as follows: 1985 ± 16, 4.8 ± 0.9 and 11.8 ± 5.8 for beryl sample, 115.1 ± 27.9, 5.0 ± 1.3 and 6.3 ± 5.0 for feldspar samples and 1421 ± 122, <4.8 and 20.1 ± 3.5 for quartz samples. For the soil samples, the mean activity concentrations of 40 K, 238 U and 232 Th, respectively, were 314.2 ± 5.7, 27.7 ± 2.6 and 11.5 ± 5.9 Bq kg -1 for soil samples from the pits and 278.1 ± 5.4, 21.1 ± 2.0 and 15.3 ± 7.5 Bq kg -1 for soil samples from heaps. The mean activity concentrations of soil samples from the undisturbed land around the mining site were 194.3 ± 25.2, 14.5 ± 5.1 and 13.3 ± 5.9 Bq kg -1 for 40 K, 238 U and 232 Th, respectively. The effective dose to which the miners are exposed according to exposure scenarios were calculated as 89.9 µSv y -1 for digging and handling of soil and mineral samples in the pit and 63.6 µSv y -1 for handling of soil and mineral

Search for a superheavy nuclide with A=292 and neutron-deficient thorium isotopes in natural thorianite

In the search for long-lived superheavy elements (SHEs) in nature the claim for the detection of a SHE species with A = 292 in a commercially available thorium solution at an abundance of (1–10) × 10−12 relative to 232Th using ICP-SFMS has been raised very recently by Marinov et al. The same group also claimed the finding of four long-lived isomeric states in neutron-deficient thorium isotopes at an abundance of (1–10) × 10−11 relative to 232Th using the same experimental method. The aim of our study was to examine, whether the ratios mentioned above could also be found in a natural thorium mineral, using AMS as a much more sensitive technique assuring the complete suppression of molecular background. Chemically untreated thorianite (ThO2) was used for both measurements. In addition, commercial thorium(IV) oxide (thoria) powder was also analyzed in the SHE measurements. No events were observed in the A = 292 SHE search, resulting in an upper limit of 4 × 10−15 for the abundance of such a species relative to 232Th. For two neutron-deficient thorium isotopes, 211Th and 218Th, we find upper limits of 5 × 10−15. For 213Th and 217Th we observed a few events – albeit uncertain – which would put their abundance also into the 10−15 range.

Overview of Fission Safety for Laser ICF Fission Energy

This paper reviews fission safety design principles and approaches for coupling inertial confinement fusion (ICF) to a subcritical fission blanket, in Laser ICF Fission Energy (LIFE) power plants. LIFE power plants use fuel derived from either spent fuel from fission reactors, or from natural uranium or thorium. By using fusion neutrons, LIFE plants can fission a large fraction of the heavy metal in the fuel, and thus eliminate any need for enrichment of uranium or reprocessing of the fission fuel used in the LIFE plants. This generates security and economic benefits. This paper reviews fission design, safety and licensing issues that are relevant to LIFE power plants.

Spectral interference corrections for the measurement of 238U in materials rich in thorium by a high resolution γ-ray spectrometry

In this study, the spectral interferences are investigated for the analytical peaks at 63.3 keV of 234Th and 1001.0 keV of 234mPa, which are often used in the measurement of 238U activity by the γ-ray spectrometry. The correction methods are suggested to estimate the net peak areas of the γ-rays overlapping the analytical peaks, due to the contribution of 232Th that may not be negligible in materials rich in natural thorium. The activity results for the certified reference materials (CRMs) containing U and Th were measured with a well type Ge detector. The self-absorption and true coincidence-summing (TCS) effects were also taken into account in the measurements. It is found that ignoring the contributions of the interference γ-rays of 232Th and 235U to the mixed peak at 63.3 keV of 234Th ( 238U) leads to the remarkably large systematic influence of 0.8–122% in the measured 238U activity, but in case of ignoring the contribution of 232Th via the interference γ-ray at 1000.7 keV of 228Ac to the mixed peak at 1001 keV of 234mPa ( 238U) results in relatively smaller systematic influence of 0.05–3%, depending on thorium contents in the samples. The present results showed that the necessary correction for the spectral interferences besides self-absorption and TCS effects is also very important to obtain more accurate 238U activity results. Additionally, if one ignores the contribution of 232Th to both 238U and 40K activities in materials, the maximum systematic influence on the effective radiation dose is estimated to be ~6% and ~1% via the analytical peaks at 63.3 and 1001 keV for measurement of the 238U activity, respectively.

Symbiotic systems consisting of large-FBR and small water-cooled thorium reactors (WTR)

Small long life water-cooled thorium reactors (WTR; 30–300 MWth) have been investigated. For realizing thorium cycle of the reactors, a uranium–thorium mixture core is introduced to fast breeder reactors (FBR; 3000 MWth) to be a 233U producer. In the present study, two distinct metallic fuel pins, with natural uranium and thorium, are loaded into a large sodium-cooled FBR. The FBR itself is self-sustained by the plutonium produced in the uranium pins. Under the equilibrium burnup state, the FBR spent fuels are periodically discharged with a certain discharge rate and then separated. Some actinides are returned to the FBR core while 233U, which is discharged from the thorium pins, is utilized for the WTR fresh fuel. Fissile support capability is the main investigated parameter of the study. The system achieves higher support capability at higher burnup and lower power of the WTR, and shows that larger number of uranium pins is better for the FBR criticality while larger number of thorium pins and lower burnup give better support factor capability. For a symbiotic system consisting 3000 MWth FBR and 100 MWth WTRs, where discharged fuel burnup is 96 and 60 GWd/t for the FBR and WTRs, one FBR can support 5 WTRs.

Study on radiogenic lead recovery from residues in thorium facilities using ion exchange and electrochemical process

Brazil has one of the biggest mineral thorium reserves, enabling the use of this material in its nuclear reactors. Consequently, this cycle of the fuel would need an initial purification stage of the natural thorium, generating residues from chemical treatment. This work provides operational parameters for the recovery of existing values in these residues, especially the radiogenic lead, that is a radioisotope of thorium decay chain, using ionic exchange technique associated to the electrochemical one. The treatment by ionic exchange in anionic resin and hydrochloric acid medium, provides about 33.4% of radiogenic lead. At the electrochemical process, lead was reduced to a metal in nitric acid medium, presenting a recovery of 98%. The electrochemical process presents an increase in the cost, nevertheless the technological importance of the radiogenic lead in the production of new elements, besides being a strategic material, justifies its use.

Determination of Polonium-210 in various foodstuffs after microwave digestion

Natural radioactivity is associated with the vast mineral resources in South Africa in such concentrations that the radionuclides from the natural uranium and thorium decay series are found to pose concern for public exposure to communities living around these areas. The radiological impact of all operations is monitored as part of the license obligations, imposed by the South African National Nuclear Regulator Act [1]. Two main pathways giving rise to significant exposures are of interest: (a) direct ingestion resulting from regular and continuous use of contaminated water for drinking purposes, and (b) regular consumption of fish and other food products harvested from and/or grown in contaminated areas. To measure the individual nuclides in foodstuffs at the required sensitivity level in order to evaluate the yearly dose due to an individual source at a screening level of 25 μSv/a, one is faced with a required lower limit of determination (LLD) of 0.1 to 0.5 Bq/kg for certain foodstuffs. For some of the nuclides this LLD can only be obtained with radiochemical separation through acid destruction of dried foodstuffs followed by individual element separations. In this study the digestion of foodstuffs in an open-vessel microwave system followed by the determination of 210Po through radiochemical separation by spontaneous deposition onto silver discs and subsequent measurement by α-spectrometry, has been evaluated. The levels of 210Po in a variety of foodstuffs were determined and the estimated dose for the adult age group, resulting from consumption was evaluated. The dose from 210Po varied from 4 to 250 μSv/a for the various foodstuffs, with the largest intake from fish (about 0.5 Bq/day). The assessment of natural radionuclides in foods allowed us to evaluate the items that present the highest risk to the population, and compare this to the limits established by the National Nuclear Regulator (NNR). Not much data of this kind is available in South Africa. However, one would need more accurate consumption/intake values to calculate the actual yearly dose and the potential radiological impact on the public.

A novel concept of QUADRISO particles – Part III: Applications to the plutonium–thorium fuel cycle

In the present study, a plutonium–thorium fuel cycle is investigated including the 233U production and utilization. A prismatic thermal High Temperature Gas Reactor (HTGR) and the novel concept of quadruple isotropic (QUADRISO) coated particles, designed at the Argonne National Laboratory, have been used for the study. In absorbing QUADRISO particles, a burnable poison layer surrounds the central fuel kernel to flatten the reactivity curve as a function of time. At the beginning of life, the fuel in the QUADRISO particles is hidden from neutrons, since they get absorbed in the burnable poison before they reach the fuel kernel. Only when the burnable poison depletes, neutrons start streaming into the fuel kernel inducing fission reactions and compensating the fuel depletion of ordinary TRISO particles. In fertile QUADRISO particles, the absorber layer is replaced by natural thorium with the purpose of flattening the excess of reactivity by the thorium resonances and producing 233U. The above configuration has been compared with a configuration where fissile (neptunium–plutonium oxide from Light Water Reactors irradiated fuel) and fertile (natural thorium oxide) fuels are homogeneously mixed in the kernel of ordinary TRISO particles. For the 233U utilization, the core has been equipped with europium oxide absorbing QUADRISO particles.

Workplaces with natural radionuclides – investigation results from Bavaria

Abstract Since the amendment of the German Radiation Protection Ordinance in the year 2001 special workplaces with increased exposure due to natural terrestrial radiation have been subject to legal regulations. After the Radiation Protection Ordinance came into force the Bavarian Environment Agency informed all water supply facilities, breweries, dairies, mineral water manufactures, operators of underground workplaces and radon spas that the annual radon exposure of their staff has to be estimated. In Bavaria approximately 3330 companies with workplaces with exposure to radon were investigated. Herein the water supply facilities represented the largest group with 2550 companies. In about 96 % of all investigated companies the radon exposure of the staff is below the action level of 6 mSv per year. In 75 water supply facilities and two mines the radon exposure of the staff exceeded the action level (6 mSv/a) or the annual limit (20 mSv/a). Workplaces with increased exposure due to uranium and thorium are of secondary importance in Bavaria. Within the scope of a research project 13 companies were found, that still use thoriated welding electrodes. Only a few facilities use thoriated mantles, thorium alloys or natural thorium and uranium. The effective dose of all examined workers was below the action level of 6 mSv per year.

Transuranium elements and fission products in technological channels of unit No. 2 of Chernobyl Nuclear Power Plant

A gamma-spectrometric and radiochemical analysis was carried out of a material (Zr-2.5% Nb alloy) of technological channels (TC) of Chernobyl Nuclear Power Plant (NPP) power unit No. 2 RBMK-1000 reactor, being under the beginning stage of decommissioning. Activities of 90Sr, 137Cs, 238Pu, 239+240Pu, 241Am and 244Cm were determined. It was established that the main source of the revealed actinides and fission products was an impurity of natural thorium and uranium in TC source material on the level of several tenths of ppm. Impurity analysis of TC source material was performed by neutron activation analysis (NAA) and inductively coupled plasma mass spectrometry (ICP-MS). Fission product and transuranium element activities measured were compared with the results of prognostic calculations performed with the help of the NAAPRO code.

Reactor physics ideas to design novel reactors with faster fissile growth

There are several types of fission reactors operating in the world adopting generally the open fuel cycle which considers the naturally available fissile nuclide, viz., 235U. The accumulated discharged fuel is considered as waste in some countries. However the discharged fuel contains the precious man-made fissile plutonium which would provide the sole means of harnessing the nuclear energy from either depleted uranium or the natural thorium in future. It must be emphasized that the present day power reactors use just about 0.5% of the mined uranium and it would be imprudent to discard the rest of the mass as waste. It is therefore necessary to explore ways and means of exploiting the fertile mass which has the potential of providing the energy without the green house effects for millennia to come. This has to be done by innovating means of large scale fertile to fissile conversion and then using the man-made fissile material for sustenance as well as growth of fission nuclear power. This paper attempts to give a broad picture of the available options and the challenges in realizing the theoretical possibilities.

Measurement of natural radioactivity and radon exhalation rate from rock samples of Jaduguda uranium mines and its radiological implications

The Singhbhum shear zone is a 200km long arcuate belt in Jharkhand state situated in eastern India. The central part between Jaduguda–Bhatin–Nimdih, Narwapahr–Garadih–Turamdih is rich in uranium. Presence of uranium in the host rocks and the prevalence of a confined atmosphere within mines could result in enhanced concentration of radon (222Rn) gas and its progeny. Inhalation of radon daughter products is a major contributor to the radiation dose to exposed subjects. By using high resolution γ-ray spectroscopic system various radionuclides in the rock samples, collected from different places of Jaduguda uranium mines have been identified quantitatively based on the characteristic spectral peaks. The activity concentrations of the natural radionuclides, uranium (238U), thorium (232Th) and potassium (40K) were measured in the rock samples and radiological parameters were calculated. Uranium concentration was found to vary from 123±7Bqkg−1 to 40,858±174Bqkg−1. Activity of thorium was not significant in the samples, whereas, few samples have shown potassium activity from 162±11Bqkg−1 to 9024±189Bqkg−1. Radon exhalation rates from these samples were also measured using “Sealed Can technique” and found to vary from 4.2±0.05 to 13.7±0.08 Bq m−2h−1. A positive correlation was found between the radon exhalation rate and the uranium activity. The absorbed dose rates vary from 63.6 to 18876.4nGyh−1, with an average value of 7054.2nGyh−1. The annual external effective dose rates vary from 0.7 to 23.2mSvy−1. Radium equivalent activities (Raeq) varied from 134.3 to 40858.0Bqkg−1. Value of external hazard index (Hex) varied from 0.4 to 110.4 with an average value of 41.2.

Projected-length distributions of fission-fragment tracks from U and Th thin film sources in muscovite

Thin films of natural uranium and thorium deposited on muscovite were used as sources of neutron-induced fission fragments. Fragment energy loss in thin-source geometry is negligible. In this way, the observed fragment range results from the interaction of the fragment with the detector material. This characteristic enables the investigation of asymmetric fission and etching, through measurements of projected track length distributions in muscovite micas coupled with thin films. The means and standard deviations of the etchable length distributions of the heavy and light fission-fragment tracks were estimated by fitting a theoretical equation to the experimental data.The light fission-fragment accounts for ∼54% and the heavy fission-fragment for ∼46% of the etchable length of a full fission track. This average partition is the same for tracks from thermal-neutron-induced fission of 235U and fast-neutron-induced fission of 232Th. The mean etchable length of uranium fission tracks is ∼2.5% longer than that of thorium fission tracks. This difference is at the resolution limit of these measurements but correlates with the difference in the mean combined initial kinetic energies of the fission fragments. The mean etchable length of uranium fission tracks in muscovite is ∼5% shorter than their calculated latent track length, supporting earlier estimates of a length deficit of this magnitude. The length deficit and the standard deviation of the etchable length distribution of the light fission-fragment tracks are twice the equivalent values for the heavy fission-fragment tracks. This is interpreted in terms of a vt-profile (track etch rate) that depends on the mass of the track-forming particle.

Performance of natural uranium- and thorium-fueled fast breeder reactors (FBRs) for 233U fissile production

The performance of natural uranium and thorium-fueled fast breeder reactors (FBRs) for producing 233U fissile material, which does not exist in nature, is investigated. It is recognized that excess neutrons from FBRs with good neutron economic characteristics can be efficiently used for producing 233U. Two distinct metallic fuel pins, one with natural uranium and another with natural thorium, are loaded into a large sodium-cooled FBR. 233U and the associated-U isotopes are extracted from the thorium fuel pins. The FBR itself is self-sustained by plutonium produced in the uranium fuel pins. Under the equilibrium state, both uranium and thorium spent fuels are periodically discharged with a certain discharge rate and then separated. All discharged fission products are removed and all discharged actinides are returned to the FBRs except the discharged uranium utilized for fresh fuel of the other thorium-cycled reactors. 233U-production rate of the FBRs as a function of both the uranium–thorium fuel pins fraction in the core and the discharge fuel burnup is estimated. The result shows that larger fraction of uranium pins is better for the FBR criticality while larger fraction of thorium fuel pins and lower fuel burnup give higher 233U production rate.

A comparison of alpha and gamma spectrometry for environmental natural radioactivity surveys

An α-spectrometry, using automated borate fusion and sequential extraction and exchange chromatography, was used to determine the uranium and thorium based on environmental radioactivity of 20 soil samples. The same set of the samples was analysed using γ-spectrometry with an HPGe detector. The two data sets were checked for coherence using Z-score and χ 2 statistical tests. We show that γ-spectrometry is a valid alternative to time-consuming α-spectrometry for the determination of natural uranium and thorium activity in soil (activity range: 12.5–58.2 Bq/kg). The measured activities were compared with the theoretical activities to ensure secular equilibrium in the 238U and 232Th series. For 226Ra, a special study was made on deconvolution of the 186 keV multiplet with the Levenberg–Marquardt algorithm. Finally, the combined use of Z-score and χ 2-tests was found to be a powerful tool for comparing the results obtained with two different methods.

Biokinetic modelling of natural thorium in humans by ingestion

The retention of naturally occurring thorium (228Th, 230Th, 232Th) in model compartments and its daily urinary and faecal excretion after acute and chronic injections and ingestions were calculated for male and female subjects of six age groups based on the current age-dependent biokinetic model for thorium (Th) recommended by the International Commission on Radiological Protection (ICRP). The results are tabulated in a database. The calculated contents of 228,230,232Th in organs or tissues using their reference concentrations in foodstuffs for the European population are compared with autopsy data. The model prediction of 232Th in whole body for a 50-year-old unexposed person is 22 mBq, 86% of that in skeleton, 9.7% in other soft tissues, 3.4% in liver, 0.7% in kidneys and 0.01% in blood. The modelling predicts lower contents of the natural Th isotopes in whole body, especially in blood compared with measured data for the unexposed public. Modelled 232Th daily urinary excretions are 5 to 10 times less than bio-assay data from the authors' own laboratory.

Search for β-delayed fission ofAc228

Radium was radiochemically separated from natural thorium. Thin $^{228}\mathrm{Ra}$$\stackrel{{\ensuremath{\beta}}^{\ensuremath{-}}}{\ensuremath{\rightarrow}}$$^{228}\mathrm{Ac}$ sources were prepared and exposed to mica fission track detectors, and measured by an HPGe $\ensuremath{\gamma}$-ray detector. The $\ensuremath{\beta}$-delayed fission events of $^{228}\mathrm{Ac}$ were observed and its \ensuremath{\beta}-delayed fission probability was found to be ($5\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$.