Paul Linsalata

Rare-earth elements in groundwaters from the Osamu Utsumi mine and Morro do Ferro analogue study sites, Poços de Caldas, Brazil

Abstract Data are presented on rare-earth elements (REE) in prefiltered ( 450 nm) from the Osamu Utsumi uranium mine and the Morro do Ferro thorium-REE-deposit. Groundwaters from both sites typically contain between 1–50 μg/l of total REE, but can reach values of up to 160 μg/l in the deepest borehole F4 (U-Mine: 150–415 m). Even higher REE concentrations of up to 29 mg/l were measured in acidic, sulfate-rich near-surface waters of the same site. The chondrite-normalized REE patterns in deeper, more reducing groundwaters and in their corresponding suspended particle fractions are similar to those observed in the bedrock (phonolites), indicating that bedrock leaching and secondary mineral sorption occurred without significant fractionation between these elements, in accordance with the only small variations in the stability constants of the expected REE-sulfate complexes in these waters. Groundwaters from the unsaturated zone of both sites show a very characteristic cerium depletion (less pronounced than that observed in the corresponding suspended particulate fractions), which is most probably related to the oxidation of Ce (III) under the prevailing E h -conditions of these waters (600 to 800 mV), and to sorption/precipitation reactions of the much less soluble Ce(IV) species. Coarse particulate matter (>450 nm ), composed mainly of amorphous ferric hydrous oxides, has a strong capacity for sorption of REE. This is shown by its very high REE concentrations, in some boreholes > 8,000 μg/g (total REE), and by the calculated association ratios R a (ml/g), which are in the order of 10 5 to 10 6 . The implications of these findings for the migration behavior of REE in both environments are discussed.

Natural analogues for the transuranic actinide elements: An investigation in Minas Gerais, Brazil

A highly weathered deposit of thorium and rare earth elements located near the summit of a hill in the state of Minas Gerais, Brazil, is being studied as an analogue for a radioactive waste repository that sometime in the distant future may be eroded to the surface or intruded by groundwater Thorium serves as an analogue for Pu4+, and La3+ as an analogue for Cm3+ and Am3+ The mobilization rate constants of the analogue elements by groundwater are so slow (of the order of 10−9 per year) as to suggest that essentially complete radioactive decay of the transuranic actinides would occur in place even under the relatively unfavorable conditions that exist at a site such as this

Natural decay series studies of the redox front system in the Poços de Caldas uranium mineralization

Abstract As part of a major natural analogue study of relevance to radioactive waste disposal, natural decay series disequilibrium techniques were used to provide information on the mechanisms and rates of processes occurring at redox fronts in an extensive uranium deposit near Pocos de Caldas, Brazil. This topic is of direct relevance to waste disposal safety assessment in view of the postulated development of radiolysis-induced oxidizing conditions in respositories. The study confirmed the downwards displacement of uranium by oxidizing groundwater interacting with the reduced rock, resulting in downwards movement of the redox front, with associated cyclical oxidation-induced dissolution and reduction-induced redeposition of uranium generating enhanced uranium concentrations at the redox fronts. Deposition of uranium occurs on both sides of the redox fronts, with reduction to insoluble U4+ in the reduced rock and scavenging by secondary iron oxides being identified as the respective mechanisms of deposition. Radioactive disequilibrium considerations indicate that, while the uranium deposited in the oxidized rock is retained for times of at least 106 y, some slow redissolution does occur, possibly involving mainly preferential loss of 234U. Sequential leaching indicates that the redeposited uranium in both the oxidized and the reduced rock is more readily dissolved than the uranium in the unaltered reduced rock. It is concluded that the observed distributions of uranium were produced by diffusion in both directions away from the redox fronts in conjunction with groundwater flow downward along the direction of the fronts. Limited redistribution of thorium has occurred, but at a level at least two orders of magnitude less intense than that of uranium and there is a systematic separation of uranium from thorium with increasing depth in the rock. At two locations where the oxidized rock overlies the reduced rock, natural decay series disequilibrium indicated a rate of movement of the front of 2–20 m in 106 y, suggesting that the rate of movement of the redox front is controlled by the rate of regional erosion (about 12 m in 106 y). At a location where the reduced rock overlies the oxidized rock, the front was estimated to have moved less than 1 cm in 106 y. This location was also the site of the most intense mineralization of uranium in the samples studied. 226Ra mobility over distances of the order of 10 m was observed, along with preferential transfer of 226Ra from the reduced to the oxidized rock at the deeper redox fronts. The postulated maximum rate of far-field movement of a repository-related redox front of about 50 m in 106 y is concluded to be a realistic value for use in models. Scavenging of radionuclides by secondary minerals forming in oxidized rocks is identified as a potentially important retardation process which might not be apparent from purely thermodynamic considerations. The effect of a repository-related redox front would be initially to retard radionuclide movement by at least partial deposition at the front. If such a redox front were to penetrate a sufficient distance through the far field, it could, however, result in the breakthrough of a pulse of radionuclides to the near-surface zone.

Transport pathways of Th, U, Ra and La from soil to cattle tissues

Abstract A field study was conducted in an area of background radioactivity (Orange County, New York) to assess tissue concentrations, soil-to-tissue concentration ratios (CR) and the comparative bioavailability of isotopic Th, U, Ra and light rare earth elements (REE) (i.e. La, Ce, Nd and Sm) in adult steers. The study was undertaken because few field or laboratory data exist on the dietary transfer of these elements to steer tissues and because the elements chosen for study bear many physical, chemical and biological properties which are similar to those of the transuranic actinide elements (i.e. Pu, Am and Cm) contained in high-level nuclear waste. It was shown by analysis of the bovine diet that soil ingestion, occurring incident to grazing, purposefully and/or as surficial contamination on feeds, would under most conditions account for the majority of intake when all possible intake pathways are considered. Exceptions are for the uranium isotopes and 230 Th, which were ingested primarily with a phosphate mineral feed supplement. Because the intakes of most of these elements are dominated by soil ingestion, we have calculated CR values as μg kg −1 (fresh tissue weight) per μg kg −1 (dry soil), as opposed to the more routinely used F ƒ ratio ( F ƒ = μ g kg −1 (fresh muscle weight) per μg day −1 (intake rate from feed)).

Distribution and mobilization of cerium, lanthanum and neodymium in the Morro do Ferro basin, Brazil

Abstract The distribution of the light rare-earth elements (LREEs) Ce, La and Nd in the Morro do Ferro ore body and drainage basin is presented. The ore body, which contains an estimated 3·10 7 kg Th and more than 1·10 8 kg REE, is located near the summit of the hill. The REEs are chemically similar to the trivalent actinides and therefore can be studied as analogues for certain of these elements. Ce is concentrated in the surface (i.e. 0–50 cm) layer of both the ore body and the drainage basin. There appears to be an association between Ce and Th and also between Nd and La in samples collected from the surface and at depths extending to 60 m. Enrichment of Ce is especially evident in soils over the ore body while La and Nd appear to be depleted in this zone. The mobilization rate of La by groundwater solubilization is tentatively estimated to be 1.8·10 −9 yr. −1 , which is very close to the rate previously estimated for Th. The mobilization rate of La due to stormflow erosion is on the order of 10 −6 yr. −1 and is similar to that reported earlier for Th.

Plutonium-239, 240 and plutonium-238 in sediments of the Hudson River estuary.

Plutonium-239,240 and plutonium-238 were determined in 59 Hudson River sediment dredge samples collected during 1973-77 in the vicinity of the Indian Point Nuclear Power Station. Acid leaching followed by solvent extraction, electrodeposition, and alpha-spectrometry were used to extract, purify, and quantitate plutonium isotopes present in these samples. Annual median plutonium-238/plutonium-239,240 isotopic activity ratios in surficial sediments were 0.032 (1973-74), 0.035 (1975), 0.042 (1976), and 0.040 (1977). The source of these nuclides in the estuary was identified by analysis of the sample isotopic activity ratios. On the basis of the sampling regimen and the methods used, it is concluded that no input, other than that of fallout, has contributed significantly to the plutonium burden in Hudson sediments. (1 map, 9 references, 9 tables)

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

Abstract 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.

Determination of plutonium in sediments by solvent extraction and α-spectrometry

Abstract A simple technique for the determination of environmental levels of plutonium in a highly complex matrix (sediments containing very high amounts of iron and other metals) is reported. The sediments, collected from the Hudson River Estuary with an Emory dredge, were hand-homogenized before a sample aliquot was taken. Samples were airdried, weighed, spiked with 242Pu tracer, and heated at 400°C for 24 h. Plutonium was leached from the sediment with an acid mixture. The leachate was filtered, and plutonium coprecipitated with iron by adding ammonia solution. After dissolution, plutonium was extracted with 20% trilaurylamine in xylene, the extracts were thoroughly acid-washed to remove uranium and thorium traces, and plutonium was then back-extracted with 2 M sulfuric acid prior to electrodeposition onto a platinum planchet. The isotopic composition of plutonium was determined by α-spectrometry. Tracer yield and plutonium concentrations determined on aliquots of the same samples by this method and by an ion-exchange technique were not significantly different.

Light rare earth elements and natural series radionuclides within the Morro do Ferro

As part of the Pocos de Caldas natural analogue project, data are presented on natural series radionuclides (232Th, 230Th, 228Th, 234U, 238U and 231Pa) and the light rare earth elements (LREEs) La, Ce, Pr, Nd and Sm from two boreholes within the near surface thorium-rare earth element ore body known as Morro do Ferro (MF). Under defined conditions, a study of these elements can provide useful information on the long term behavior of chemical analogues present in high level waste (HLW) and on the processes affecting radionuclide migration. Within the deeply weathered mineralized zone, Th decreases exponentially with a half thickness of 10 meters from 5.7% in the surface meter to 0.1% at 36 meters. Since no fractionation of 228Ra is evident (based on 228Th232Th equilibrium), 228Ra is essentially decaying in-situ where it is radiogenically produced. Uranium isotopes (mean = 35 ppm as 238U) are found in near equilibrium (234U/238U range of 0.9 to 1.1) both within the mineralization and down along the hydraulic gradient. No evidence for any significant uranium accumulation exists. Leaching of uranium by infiltrating, oxidizing rain water is postulated as the dominant removal mechanism although preferential loss of 234U also occurs as evidenced by 234U/238U ratios of < 1.0 in rock and between 1.1 and 1.5 in ground water samples. Positive and negative Ce anomalies are reported and have been interpreted as occurring from relic redox fronts which are no longer visible.

Plutonium and radiocesium in the water column of the Hudson River estuary

Isotopes of plutonium (Pu), cesium (Cs), and cobalt (Co) introduced into the Hudson River Estuary from fallout deposition, the erosion of fallout-contaminated surface soils, and nuclear reactor effluent (isotopes of Cs and Co only) have been measured in water column samples collected from 1975 to 1980 Isotopic measurements conducted independently by two research groups utilizing different sampling and analytical techniques have been summarized. The major conclusions drawn from the work are that for water samples collected by the two laboratories over similar time periods, the mean concentrations of nonfilterable239,240Pu (<0.45 μm) were identical at 0.13 fCi/l, mean concentrations of both137Cs and239,240Pu in suspended particulates were more divergent at 2,270±920 pCi/kg (±1 SD) and 1,430±430 pCi/kg for137Cs, and 19±8 pCi/kg and 12±4 pCi/kg for239,240PuThe behavior of239,240Pu and137Cs within the water column is shown to diverge within brackish waters Specifically, the magnitude of the137Cs distribution coefficient (Kd) can be expressed as an inverse power function of the chloride ion concentrations for chlorinities between 0.1 and 4 g Cl−/l No difference in the239,240PuKd has been observed between fresh and brackish watersBased on the expected inventories of239,240Pu and137Cs within watershed soils, the current downstream transport of these radionuclides represents fractional mobilization rates on the order of 1–4 (×10−4) per year