Donald M. Nelson

Concentration and separation of actinides from urine using a supported bifunctional organophosphorus extractant

Abstract A method has been developed to isolate quantitatively the actinide elements (as a group) from urine samples and to produce near massless electrodeposits suitable for high-resolution α-spectrometry. The actinides are concentrated from the bulk urine sample by coprecipitation with calcium phosphate. The precipitate is then ashed, dissolved in dilute nitric acid and passed through a column of octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) dissolved in tributyl phosphate (TBP) supported on an inert substrate (Amberlite XAD-7) which absorbs the actinides. The actinides are stripped from the column with dilute ammonium hydrogenoxalate, electrodeposited on a stainless-steel disk and counted on an α-spectrometer. Recoveries of added tracers are uniformly high (⩾ 90%) so that samples can be routinely run without yield monitors, permitting an α peak anywhere in the spectrum to be detected with few interferences. Detection limits are influenced only by the available sample size, counting time and counter backgrounds.

Pu(V) as the stable form of oxidized plutonium in natural waters

Abstract This work presents analytical evidence supporting the proposition that Pu(V) is the sole or predominant form of oxidized plutonium in natural waters. Two independent methods, the selective adsorption of Pu(VI) by silica gel, and the somewhat less selective coprecipitation of Pu(V) with calcium carbonate, were developed to separate Pu(V) from Pu(VI). Measurements of ambient plutonium in several natural waters by these methods found only Pu(V). In laboratory tracer studies, Pu(VI) was shown to be highly unstable in dilute bicarbonate solution and in Lake Michigan water, reducing in first-order fashion to Pu(V).

Time-dependent modeling of fallout radionuclide transport in a drainage basin: Significance of “slow” erosional and “fast” hydrological components

Abstract The sediment-depth distributions of 238Pu, 239,240Pu, 241Am and 137Cs have been measured in a sediment core from the Saguenay Fjord, Quebec, Canada, which has been dated to better than 1-yr. resolution over the past 100 yr. using 210Pb. The historical record of fallout radionuclide fluxes to the sediments has been estimated using a 210Pb constant flux model and variations in the flux distributions have been directly related to changes in the atmospheric input function. Rates of radionuclide transport through the drainage basin have been predicted using a two-component time-dependent model composed of terrestrial (soil) and water phases, the latter including catchment basins upstream of the fjord. The residence times of 137Cs and 239,240Pu in the soil component are 1000 and 3000 yr., respectively, while both radionuclides have equivalent residence times of 1 yr. in the water column component. The atmospheric burn-up of a nuclear-powered SNAP-9A satellite in 1964 has resulted in a well-defined 238 Pu 239,240 Pu horizon appearing in the sediments at the 1967–1968 level. The magnitude and timing of the SNAP-9A-derived 238Pu distribution are in agreement with the drainage basin model predictions and confirm the absence of post-depositional remobilization of Pu in these anoxic sediment regimes. Theoretical estimates of the change in the 241 Am 239,240 Pu activity ratio in sediments deposited since the 1950s, determined using the drainage basin model, are in agreement with the experimental results and indicate that the initial 241 Pu 239,240 Pu activity ratio for the 1950s weapons tests was 15 or more while the later weapons tests were characterized by an activity ratio closer to 12.

Plutonium oxidation state distributions in the Pacific Ocean during 1980–1981

Abstract The concentration of plutonium and its oxidation state distribution have been measured as a function of depth at three locations in the North Pacific Ocean. Concentration profiles were similar to those observed in the same area during 1974 with primary maxima at depths of a few hundred meters below the surface and secondary maxima near the bottom. Oxidation state distribution profiles were similar at the three locations with plutonium about equally divided between the reduced and oxidized forms except near the bottom. There the oxidized form was more abundant and comprised ∼ 90% of the total plutonium. No major change in oxidation state occurred at the depths of the shallow concentration maxima suggesting that their formation and persistence were not dependent upon a redox change. The concentration maxima near the bottom coincided with the enhanced abundance of the poorly sorbed oxidized form suggesting that they resulted from the loss of oxidized plutonium from the sediments.

Oxidation states of plutonium in carbonate-rich natural waters

Abstract Adsorption of actinide elements to calcium carbonate in waters of moderate to high carbonate concentration has been shown to be dependent on the oxidation state. This property has been exploited for distinguishing the (III) and (IV) oxidation states of plutonium. Ambient plutonium in Mono Lake, California, a natural high-carbonate water, appeared to be entirely in the (IV) oxidation state. Tracer experiments in carbonate media and in Mono Lake water, in the presence of atmospheric oxygen, confirmed that Pu(III) is rapidly oxidized under these conditions.

The annual cycle of plutonium in the water column of a warm, monomictic reservoir

Abstract An annual cycle occurs in the 239,240 Pu inventories of the water column of Pond B, an 87-ha warm monomictic reservoir on the US Department of Energys Savannah River Site in Barnwell Co., South Carolina. The pond has elevated concentrations of 238 Pu and 239,240 Pu in sediments due to releases from former reactor operations and continues to receive additional Pu input from atmospheric deposition. The 239,240 Pu inventories in the water column display annual cycles that differ between surface waters (i.e. 0–6 m deep) and deeper waters ( i . e . > 6 m deep ) that are anoxic in summer months. For surface waters, the 239,240 Pu inventory increases following turnover in November to a maximum in March followed by a decline until later summer when minimum inventories occur. For deeper waters, the 239,240 Pu inventories increase rapidly following turnover and reach maximum values in March. The inventories in deeper waters remain large from March until turnover. Maximum inventories for the entire water column occur in March with minimum inventories at turnover in October and November. Turnover results in a redistribution of Pu across water depth but no measurable Pu loss from the water column. Ratios of 238 Pu: 239,240 Pu indicate that the cycle involves primarily Pu from sediment sources with little influence from atmospheric sources. Thus, the cycle represents net remobilization of 239,240 Pu from the sediments to the water column during the oxic, holomictic portion of the year followed by a net loss of Pu from the water column once stratification occurs.

Comparison of distribution coefficients for americium and curium: Effects of pH and naturally occurring colloids

Abstract Distribution coefficients (K D ) were measured by a batch technique for sorption of americium-243 and curium-244 to Lake Michigan sediment from four different natural surface waters. The dependences of K D values on pH and on the concentrations of naturally occurring colloidal organic carbon (COC) were studied. The comparison of K D values showed that, while changes in pH and in COC concentrations cause significant changes in sorption for both americium and curium, neither variable causes a significant difference in sorption between these two elements.

239,240Pu, 241Am and 232Th in lakes: The effects of seasonal anoxia

A pond which undergoes seasonal anoxia as a result of thermal stratification was studied to assess the possible release of actinide elements from anoxic sediments to the pond waters. Concentration profiles of 239,240Pu, 241Am, 232Th, iron and manganese in the water column show significant increases in concentration for all elements in the anoxic deep waters. However, calculated total inventories of the elements in the pond water indicate no apparent input of actinide elements to the system during summer stratification. Similar inventory calculations for 239,240Pu in a previously studied system support the results of this study. The lack of a significant increase in pond inventories of actinide elements coupled with a lack of correspondence of concentration profiles of the actinide elements to the seasonal Fe and Mn concentrations profiles argues that the increases in concentrations observed for these elements in anoxic bottom waters result from the downward transport of material already in the water column rather than release from the sediments.