Uranium Decay Chain Research Articles

The potential of lichens as long-term biomonitors of natural and artificial radionuclides

Lichens were used as biomonitors of Chernobyl fallout 137Cs, of cosmogenic 7Be and of radioactive members of the natural uranium and thorium decay chains. Samples were taken from two locations in France, including lichens sampled at different distances of a coal fired power plant and close to a uranium ore processing waste disposal site. All samples were analyzed gamma-spectrometrically after equilibrium concentrations of short-lived isotopes had been attained. Activity concentrations of the members of the uranium and thorium decay chains in Parmelia sulcata sampled 1994 decrease with distance from the plant, whereas in lichens taken at the waste disposal site a decrease with time was observed. Comparison of 7Be activity concentrations measured in lichens with atmospheric deposition rates confirms that P. sulcata can be used as a quantitative biomonitor of radioactive trace substances. Retention half-lives calculated with a simple one-compartment model are 2.6±1.2 years for cesium, which was detected in all samples even more than a decade after the Chernobyl accident, and of 0.7 (±0.1) to 3.3 (±0.7) years for lead. Consequences of our results for model identifiability and parameter estimation of a two-compartment model are discussed.

Radionuclides in the Loire river estuary (France) : Sources, transport and fate

Anthropogenic radionuclide inputs in the Loire estuary (French Atlantic coast) consist of radioactive releases from 14 nuclear reactors located along the Loire river basin, and of fallout from nuclear weapon tests and from the Tchernobyl accident. To estimate to what extent radionuclides associated with sediment accumulate in the estuary, three complementary approaches were used: field surveys, laboratory experiments and numerical modelling. Sampling of bottom sediments, water and suspended solids was carried out at 8 different dates over a 15 month-period. Analysis covered 14C, 90Sr, 3H, the naturally occurring gamma-emitters (uranium and thorium decay chains, 7Be and 40K), and the artificial gamma emitters (mainly cobalt and cesium isotopes). Processes of radionuclide transport and exchange between dissolved and particulate phases were included in a previously developed estuary specific 2D-hydrodynamic model. Equations of sorption and desorption kinetics were derived from laboratory experiments conducted at different salinities. Simulations carried out for two river discharge conditions (low summer flow, high winter flow) allowed to follow radionuclide desorption in the estuary. For long term simulations, a simplified model was developed. It provided estimates of the amount of radionuclides expelled out of the estuary under dissolved and particulate forms, of the transit time for both forms and of the variations in radionuclides concentrations in the fluid mud. Based on computed results and observations, contributions from different origins (natural, military, industrial) to the inventory of radioactivity in the estuary are presented.

Radionuclides in the Loire river estuary (France): Sources, transport and fate

Anthropogenic radionuclide inputs in the Loire estuary (French Atlantic coast) consist of radioactive releases from 14 nuclear reactors located along the Loire river basin, and of fallout from nuclear weapon tests and from the Tchernobyl accident. To estimate to what extent radionuclides associated with sediment accumulate in the estuary, three complementary approaches were used: field surveys, laboratory experiments and numerical modelling. Sampling of bottom sediments, water and suspended solids was carried out at 8 different dates over a 15 month-period . Analysis covered I4 C, 90 Sr ,3H, the naturally occurring gamma-emitters (uranium and thorium decay chains, 7 Be and 40 K), and the artificial gamma emitters (mainly cobalt and cesium isotopes). To gain information on the contamination history of the estuary, sediment cores were also collected at different locations inside and outside the estuarine zone. Processes of radionuclide transport and exchange between dissolved and particulate phases were included in a previously developed estuary specific 2D-hydrodynamic model. Equations of sorption and desorption kinetics were derived from laboratory experiments conducted at different salinities. Simulations carried out for two river discharge conditions (low summer flow, high winter flow) allowed to follow radionuclide desorption in the estuary. For long term simulations, a simplified model was developed. It provided estimates of the amount of radionuclides expelled out of the estuary under dissolved and particulate forms, of the transit time for both forms and of the variations in radionuclides concentrations in the fluid mud. Based on computed results and observations, contributions from different origins (natural, military, industrial, marine, continental) to the inventory of radioactivity in the estuary are presented.

Uranium and uranium decay series radionuclide dynamics in bone of rats following chronic uranium ore dust inhalation.

The accumulation and release of uranium and some uranium decay chain radionuclides were measured in the bones of rats that had been chronically exposed to inhaled uranium ore dust during the first half (approximately) of their natural adult lifespan. Endochondral bone (femur, tibia, humerus, radius, and ulna), membrane bone (skull roofing bones) and muscle of Sprague-Dawley rats (n = 55) that died at various times up to 65 weeks after the end of chronic inhalation of uranium ore dust aerosol (4.2 h d(-1) for 65 wk) and from age matched controls (n = 10), were analyzed for uranium, 230Th, 226Ra, 210Pb, and 210Po. Overall, during the period of dust inhalation, the nuclides accumulated in the above order of decreasing concentration in dry bone. However, the results demonstrate that there was some differential accumulation of uranium and uranium decay series radionuclides in muscle and two bone types of rats during the chronic inhalation period. The data also show that the bone levels of some, but not all, radionuclides decreased significantly with time after inhalation ceased. Lung uranium concentration at the time of death was a highly significant covariant for temporal changes in the levels of some radionuclides in both endochondral bone and membrane bone, indicating that lung remained a major source of these isotopes for accumulation in these bone types after ore dust inhalation had ceased. For some isotopes, the two bone types behaved differently during the dust inhalation period, and differently again after the dust inhalation ceased. The relative behavior of one bone type compared to the other for a particular isotope during the dust inhalation period did not predict the relative behavior after dust inhalation ceased. However, a faster accumulation of one bone type compared to the other for a particular isotope during the dust inhalation period predicted a faster decrease after dust inhalation ended.

Radiological Evaluation of Gold Mining Activities in Ariab (Eastern Sudan)

Calculations of individual effective dose per annum have been made from the activity concentrations of dominant gamma-emitting radionuclides from uranium and thorium decay chains in ore, solid waste (spoil heap) and soil samples collected from Adassedakh and Hadal Auatib gold mines in Ariab (Eastern Sudan). Measurements were performed using a high-resolution gamma spectrometer. Independent of location, the data show that the activity concentration for 226Ra and 232Th increases in the order: soil>spoil heap>ore. This order basically confirms that the activity levels in the spoil heap are low relative to natural background (i.e. soil samples), and thus the expected contribution in enhancement of natural background radiation is correspondingly low. The concept of concentration ratio (CR) has been applied to quantify the degree of accumulation of radionuclides in the solid waste (heap). The calculated CR values revealed that 232Th is most accumulated while there is no accumulation of 40K. The absorbed dose rates in air at a height of 1 m as estimated using DRCF from activities of 241Pb, 214Bi, 212Pb, 228Ac, 208Tl and 40K ranges from 6.6 to 32.3 nGy.h-1 (Adassedakh), and 2.2 to 26.1 nGy.h-1 (Hadal Auatib). The principal contributors to the total dose rate are 40K, 214Bi and 228Ac which generate almost 90%. The corresponding annual effective doses fall within the range: 0.04-0.20 mSv.y-1 for Adassedakh and 0.01-0.16 mSv.y-1 for Hadal Auatib. Although, these estimates do not include the internal radiation burden due to inhalation of radon and its decay products, the values obtained are insignificant relative to the dose limit specified for occupational exposure of any worker.

Soil sample moisture content as a function of time during oven drying for gamma-ray spectroscopic measurements

In routine gamma-ray spectroscopic analysis of collected soil samples, procedure often calls to remove soil moisture by oven drying overnight at a temperature of 100°C [1]. Oven drying not only minimizes the gamma-ray self-attenuation of soil samples due to the absence of water during the gamma-ray spectroscopic analysis, but also allows for a straightforward calculation of the specific activity of radionuclides in soil, historically based on the sample dry weight. Because radon exhalation is strongly dependent on moisture [2, 3], knowledge of the oven-drying time dependence of the soil moisture content, combined with radon exhalation measurements during oven drying and at room temperature for varying soil moisture contents, would allow conclusions to be made on how the oven-drying radon exhalation rate depends on soil moisture content. Determinations of the oven-drying radon exhalation from soil samples allow corrections to be made for the immediate laboratory gamma-ray spectroscopy of radionuclides in the natural uranium decay chain. This paper presents the results of soil moisture content measurements during oven drying and suggests useful empirical fits to the moisture data.

Development of maps of radon-prone areas using radon measurements in houses

Radon is a radioactive gas arising from the uranium decay chain, and can enter houses from the ground. High radon exposures have been shown to cause lung cancer. Many governments and international bodies have therefore recommended that radon exposures in houses should be limited. Radon levels in houses vary widely from area to area depending on local geology. However, the relationship with geology is complex and varies between rock types, within single rock types and is affected by superficial cover. Even on a single geology, radon levels vary widely depending on house characteristics and the living habits of the occupants. In order to identify houses with high radon levels, it is necessary to map the problem. In the UK, this has been carried out using lognormal modelling of results of measurements of radon levels in houses around the country. Such maps have been the basis of advice from NRPB to the government on limiting radon exposures. Methods of deriving maps at 5 km and 1 km grid square resolution are discussed with examples.

Beta doses in tooth enamel by “one-group” theory and the ROSY ESR dating software

A new method of calculating beta doses for one-dimensional geometries, based on “one-group” theory, is presented. In typical situations it is found that this approach leads to average beta doses which are significantly lower (by up to 50%) than those from Grün's DATA software. For various natural sources, our beta dose rate estimates are slightly greater than expected from calculations based on Monte Carlo data, while the DATA results agree well with the experimental results of Aitken et al. (1985) for the uranium and thorium decay chains. Further experimental evidence is desirable to resolve this issue. We compare age estimates calculated using new ESR dating software called ROSY, which incorporates this approach, with those from DATA. Because of the difference in beta dose estimates, ROSY ages are generally older than the corresponding DATA ages, there being little difference in gamma and alpha dose estimates. We do however observe significant discrepancies in cosmic dose estimates. Examination of results for El Castillo Cave, Spain, by Rink et al. (1996) suggests that the ROSY age estimates are the more accurate.

Peculiar radon spot in Hungary

For the public, indoor radon is the main source of exposure from ionizing radiation. Radon gas originates from the radioactive decay chain of uranium deposited in rocks or in building materials. In the reviews mostly a rather steady radon exhalation has been assumed. In a village of North-East Hungary, however, high radon concentrations have been measured, differing strongly in neighbouring houses and varying in time, due to the interplay of several geochemical phenomena.

The effects of disequilibria in the uranium and thorium decay chains on burial dose rates in fluvial sediments

Buried sediments receive about 53% of their annual dose of ionising radiation from radionuclides in the uranium and thorium decay chains. In luminescence dating of sediment samples, it is usually assumed that the dose rate does not change over the period of burial, implying that the uranium and thorium decay series are in secular equilibrium. For the 232Th decay chain there is little literature available on the equilibrium conditions in sediments, but given the short half-lives of the longer-lived daughters in the series, 228Ra (5.75 years) and 228Th (1.91 years), the decay chain is expected to be in secular equilibrium in most natural materials. However, for the 238U decay chain, disequilibrium is commonplace in the surficial environment and the half-lives of several members of this decay chain ( 234U, 230Th, 226Ra) are sufficiently long that any disequilibrium, once established, may persist for millennia. In these circumstances, the dose rate will vary with time unless the decay rate is matched by the transport and deposition of the relevant (unsupported) nuclide. We present data from a variety of fluvial and lacustrine depositional environments, and demonstrate that disequilibria is common in these Australian surficial sediments. The origins of the disequilibria and their likely evolution in time are discussed. The effect on the dose rate is assessed and, in the majority of cases, is found to be comparable with other luminescence dating uncertainties of typically 5–10%.

Accurate determination of uranium and thorium in environmental samples using alpha track-etch technique

A theory of the methods of uranium determination in environmental samples based on the measurement of alpha activity of radon and its daughters and on the total alpha activity of all the members of the uranium decay chain using the track-etch technique is given in this paper. It is shown that for samples in which there is radioactive disequilibrium, these methods cannot be used to obtain meaningful and accurate results. It also follows from the theory that an accurate estimate of uranium and thorium-232 in environmental samples, such as soils, foodstuffs, flora and fauna, cannot be obtained from the measurement of the radioactivity of radon daughter nuclides in view of the radioactive disequilibrium in these open systems.

Etched track detectors in solar neutrino experiments

Within the framework of LOREX experiment the mineral Lorandite (TlAsS2) of the old (5 × 106 years) arsenic mine Allchar (Kavadarci, Macedonia) will be used to estimate average solar neutrino flux from the amounts of 205Pb isotope (T12=1.5 × 107 years) induced by 205Tl (ve, e-205Pb reaction. In order to evaluate the amount of the 205Pb isotope induced by 204Pb (n, γ) 205Pb reaction the thermal neutron flux inside and above the mine was measured by our high sensitive etched track neutron dosemeters (CR-39/BE, CR-39/BN-1). The thermal neutron flux was found to be (2.7 ± 0.4) × 10-3 cm-2 s-1. In addition to cosmic muon interactions and spontaneous fissions the neutrons can be produced through various (α, n) reactions due to the high alpha particle flux from the decay chains of uranium and thorium. Therefore radon concentration in the mine was measured by our diffusion chamber type dosemeter utilizing CR-39 detector and was found to be 1.5 × 104 Bq m-3. The possibilities to extend the application of etched track detectors to another solar neutrino experiments are discussed.

The behavior of the uranium decay chain nuclides and thorium during the flank eruptions of Kilauea (Hawaii) between 1983 and 1985

The concentrations of members of the 238U decay chain and 232Th have been determined for the lavas that erupted on the East Rift Zone of Kilauea Volcano, Hawaii (Puu Oo) between January 1983 and January 1985. There was a decrease during the first 180 days in the abundances of all nuclides, following the behavior of the incompatible elements. (230Th/238U) varies with (232Th/238U) yielding a batch process age for the source magma of 127,800 ±28,500 (2σ) y, similar to East Pacific Rise basalts. No (226Ra/230Th) disequilibrium was evident at Puu Oo although Haleakala and Loihi show significant excesses of (226Ra) over (230Th). The initial (210Pb) excess relative to (226Ra) implies strong incompatibility of 210Pb probably with the help of chloride complexing, and the deficiency in later episodes indicates volatilization from the melt mediated by the formation of volatile chloride compounds.

Developing measuring technique in radioprotection for tracking radon 222 “in situ”

Abstract Radon 222 a natural radioactive gas belonging to the decay chain of uranium 238, is the main constituent of atmospheric radioactivity. Tracking its concentration is thus fundamental for radioprotection ( whether in mining developments or housing) uranium and geologic prospecting and crustal deformations observation (particularly those preceding seismic or volcanic occurences). An experimental setup using solid state nuclear track detectors (SSNTD) and a proportional counter is used for taking additional measurements in order to track this gas “in situ”. The low cost of SSNTDs and their easy use enables the entire survey area to be displayed cartographically. Each detector reveals the integrated radioactive activity over a two month period for each experimental area observed on the site. The proportional counter, immediately indicates all variations in the radon emanation level for a given experimental area on the site through its continuous measuring function. These two techniques used jointly reveal the spatial and temporal distribution of radon emanation. An application for radon level estimation in buildings is presented.

Behavior of Actinides and Other Difficult to Measure Radionuclides in the Melting of Contaminated Steel

Metal waste from nuclear power plants is normally contaminated with beta and gamma emitters mainly due to corrosion product radionuclides. Metal waste originating from reprocessing and fuel fabrication plants is contaminated only with alpha emitters (uranium). So far, only radionuclides that can be measured by gamma spectrometry can be quantified. The behavior of alpha emitters is investigated using an artificially added radionuclide in melt experiments.During its 1984–1988 program on decommissioning of nuclear installations, the Commission of the European Communities concluded a 2-yr research contract with Siemens AG, UB Kraftwerk Union on the behavior of radionuclides that are difficult to measure in the melting of steel.Investigation of the radionuclides 55Fe, 63Ni, and 90Sr began with melt experiments on 55Fe (considered an epsilon emitter) at laboratory scale, which showed that this nuclide is probably as homogeneously distributed in the melt as 60Co; thus, 60Co can be used as an isotopic indicator for 55Fe. In another melt experiment, 241Am was artificially added to metal waste and melted, showing a decontamination factor of ∼100 even with a very small quantity added (4 × 10−7 g 241Am).As of mid-1988, four melt experiments, each with different melt parameters, have been carried out. The last experiment relates to the melting of carbon steel with metallic uranium additions; although this experiment is not yet completely evaluated, problems related to the direct alpha measuring technique may arise from the disturbance of the radiochemical equilibrium of the uranium decay chain in the melt process.

Radiogenic production of 10Be and 26Al in uranium and thorium ores: Implications for studying terrestrial samples containing low levels of 10Be and 26Al

10Be and 26Al are produced by cosmic-ray-induced spallation of atmospheric constituents, and also as has recently been shown, by spallation in the rocks exposed to cosmic rays. We now present experimental data showing that these two isotopes can also be produced by radiation from uranium and thorium decay chain radionuclides contained within the host rocks. We have measured the 10Be and 26Al concentrations in a number of shielded uranium (spontaneous fission half-life ~10 26 years) and thorium (no spontaneous-fission-decay mode) ores using accelerator mass spectrometry. The concentrations of 10Be and 26Al range from 2.8·10 6 to 29.6·10 6 and 1.1 · 10 7 to 1.43· 10 atoms g −, respectively. The high concentrations of 10Be and 26Al imply unacceptably-high branching ratios (~ 10 −4 to 10 −1) for their production from spontaneous fission and coupled with the presence of 10Be and 26A1 in the Th ores, eliminate spontaneous fission as the major source of either isotope. The large 26A1 to 10Be ratio found in all the samples eliminates atmospheric production. The most likely sources are the alpha reactions, 7 Li( α, p) 10 Be and 23 Na( α, n) 26 Al, and neutron reactions, 9 Be( n, γ) 10 Be, 10 B( n, p) 10 Be and 13 C( n, α) 10 Be. Our work provides an upper limit on the contribution of in situ radioactivity to samples containing low levels of 10Be and 26Al. We have calculated the equilibrium concentrations of 10Be and 26Al in a wide variety of geologic materials including granites, ultramafic rocks, basalts, shales, pelagic sediments, ferromanganese nodules, tektites and Libyan Desert Glass. The calculated concentrations of 10Be and 26Al range from <1 to 4100 and 9 to 10.3 · 10 6 atoms g −1, respectively. Implications of these calculations are discussed. In general, the calculated amounts of radiogenically produced 10Be in geological materials are quite small and frequently negligible compared to cosmogenic production, while that of 26Al are significant and sometimes the dominant source.

Radon escape from New Zealand speleothems

When calculating the environment radiation absorbed dose rate in ESR or TL dating, it is usual to allow for the possibility of dissequilibrium in the uranium-series decay chain, due to the escape of the radioactive gas, radon, from the sample before its decay to non-gaseous daughter products. Partial or complete radon escape may reduce the internal radiation dose to less than half that which would be received if no radon escaped. In this study a number of speleothems from a temperate climate, varying in age from 6 ka to more than 730 ka (ages determined by 234U/ 230Th and 235U/ 231Pa dating and by paleomagnetic analysis) have been analysed for thorium and polonium isotopes. As radon lies between thorium and polonium in the uranium decay chain, a comparison of thorium and polonium activities with the expected values, derived from the age of the sample and the initial concentrations, gives an estimate of the radon escape factor and its impact on environmental dose rates. The activity rates of thorium, uranium and polonium were measured separately by counting their α particle emissions after electroplanting the elements onto rotating stainless steel discs. Polonium was plated directly after dissolution of the sample and before loading the solution onto a heated cation column to separate the uranium and thorium fractions. The uranium fraction was further purified on an anion before plating. The recovery rates of uranium and thorium were monitored by spiking but the recovery rate of polonium must be regarded as a minimum; estimates of radon escape are therefore likely to be overestimates rather than underestimates. Of 19 speleothems studied, only two gave results inconsistent with values predicted on the basis of no radon loss (Chauvenet's test): one, a relatively young sample of 6 ka, showed evidence of radium coprecipitation with uranium, and one, a dog-tooth spar with an open crystal structure and a high porosity showed evidence for radon escape. A third sample, also a flowstone/spar with a higher porosity than normal, was marginally significant. As the degree of radon escape will vary from sample to sample, the environment dose rate must be regarded as having a high level of uncertainty for samples of this type, unless radon escape is determined experimentally. The remaining samples (12 stalagmites, 3 stalactites and 1 flowstone), of a more massive structure, showed no significant radon escape (Student's t-test), and therefore the environmental dose rate for such samples should be calculated on the basis of radon retention.

Exposure to long-lived members of the uranium and thorium decay chains

Human exposure to isotopes of uranium, thorium, radium, lead, polonium and some of their shorter-lived progeny in typical natural radiation background environments is reviewed. The sources and pathways of exposure considered include external irradiation of the whole body from typical levels of the primordial radionuclides in soils, and internal irradiation of various tissues following dietary or inhalation intake. Because the scientific literature in this area is replete with extensive reviews by national, e.g. NCRP and international, e.g. UNSCEAR committees, reliance is placed on their findings.

Neutron capture studies: I. Multiple capture reactions and implications for calculated beta-delayed fission rates; and II. The nuclear level structure of238Np

Astrophysical r-process calculations of transbismuth elements are of interest because certain actinide pairs can be treated as chronometers in determining the duration of nucleosynthesis. For one such calculation where a particularly long galactic age was derived, 21+2-4 Gyr, the author presents evidence that the effect of beta-delayed fission appears to be seriously overestimated in uranium decay chains with A=252 to 257. With this conclusion, it follows that this estimate of the galactic age must be considered more uncertain than if the calculated rates of beta-delayed fission were found to be acceptable. The nuclear level structure of 238Np has been investigated using the 237Np(n, gamma )238Np reaction and the alpha decay of 242mAm as experimental probes. Having established a level scheme for 238Np that includes 47 excited levels and 93 secondary transitions, the author finds a high degree of correspondence between the experimental band structure and that of a semi-empirical model developed to predict excitations in odd-odd deformed nuclei.

The use of alpha counting for determining the heat production due to the uranium and thorium decay chains

The count rate of alpha particles emitted form the surface of a sample can be readily used to determine the heat production rate due to U and Th in the sample. the conversion factor lies in the range 22–25 mJ/kg per year for an alpha particle count rate of 1.00 count/(ks cm 2), depending on the relative contributions of U and Th, and the extent of Rn escape. Use of a value of23 ± 1mJ/kg per year is recommended for most samples. Glassing the sample first overcomes errors due to inhomogeneity and Rn escape; in this case the conversion factor is about 10% higher. The apparatus is simple to use and is inexpensive.