Abstract

Over the past 50 years, the explosions of nuclear weapons and releases from the nuclear industry have introduced anthropogenic plutonium (Pu) into the environment. Of the artificial radionuclides released to the environment from nuclear activities, the transuranium elements, including plutonium, are a major concern. This concern arises from their very long half-lives, their possible accumulation in bone (in particular plutonium) as well as a relatively high radiotoxicity due to a-radiation.

The environmental behaviour of plutonium is complex. The main purpose of this research work was to advance understanding of the behaviour of plutonium in the environment, in both highly contaminated as well as low level areas, in order to address the following Key Questions:

1. Can the plutonium isotopic signatures be used effectively in complex environmental systems to define the relative contributions from potential sources?
2. Can the extent of spread of environmental contamination from a particular source be determined from the isotopic signatures of plutonium?
3. Does the chemical speciation of plutonium affect its mobility in the environment?  Conversely, does the nature of the environment (environmental conditions) affect the speciation of plutonium, and hence its mobility?

Research objectives therefore also include source identification, characterisation of the speciation of plutonium in the environment, and investigation of the effect of these factors on the environmental mobility of plutonium.

Plutonium chemistry and its behaviour in the environment are reviewed in Chapter 1. The chemistry of Pu at macro-concentrations has been extensively studied, and thermodynamic constants derived for their chemical reactions. However, the environmental chemistry of plutonium is made more complex as the element is generally present in micro-concentrations, which leads to additional properties, such as colloid formation and adsorption, becoming more dominant.  In aqueous environments, plutonium compounds exists in four oxidation states, all of which are capable of forming relatively stable complexes under various environmental and biological conditions.

In the Arctic environment, the main source of plutonium is from atmospheric weapons testing, but previous studies of plutonium in the Kara Sea have shown that, at certain sites, other releases have given rise to enhanced local concentrations. Since different plutonium sources are characterised by distinctive plutonium-isotope ratios, evidence of a localised influence can be supported by clear perturbations in the plutonium-isotope ratio fingerprints as compared to the known ratio in global fallout. A short presentation of past, present and potential Pu sources in the Arctic is given in Chapter 2. 

The generally low levels of plutonium in environmental samples, often combined with limited sample sizes, necessitate reliable low-level techniques for determination of Pu isotopes. The analytical methodology used in this work is presented in Chapter 3, which includes alpha-spectrometry, inductively coupled plasma-mass spectrometry (ICP-MS) and accelerator mass spectrometry (AMS). ICP-MS and AMS have proved to be powerful methods for measuring low-level Pu activity concentrations, and both techniques offer the advantage of giving information on Pu isotope ratios. Based on procedural blanks, detection limits for AMS were below 1 fg Pu (equivalent to ca. 2 mBq ²³⁹Pu). AMS also showed less interferences and memory and matrix effects as compared to routine ICP-MS techniques.

Chapter 4 summarises the main results of the seven research papers that form the basis of this thesis. This has been divided into studies on source term identification, field-studies on plutonium speciation and mobility, and laboratory studies on the kinetics of Pu interaction and distribution coefficients (K_d) with soil and sediment. Source identification work includes determination of plutonium concentrations and isotope ratios in sediment samples collected during various expeditions to the Kara Sea, the Ob and Yenisey estuaries and their river systems. Measurements of sediments collected from dumping sites at Novaya Zemlya showed deviation from global fallout ²⁴⁰Pu/²³⁹Pu ratios, indicating releases from reactors and waste containers. Both the Ob and Yenisey have weapons-grade plutonium sources in their catchment areas, including the three Russian plutonium production and reprocessing plants at Mayak PA, the Siberian Chemical Combine (SCC) and the Krasnoyarsk Mining and Krasnoyarsk Mining and Chemical Industrial Complex (KMCIC). Analysis of sediment samples showed a clear influence from a low ²⁴⁰Pu/²³⁹Pu source (weapon Pu) in the Yenisey Estuary, whereas plutonium in Ob Estuary sediments was dominated by global fallout. The results also show an increase in plutonium concentration and a decrease in isotope ratio going upstream from the Yenisey estuary (Yenisey River).

As part of the studies on highly contaminated areas, plutonium activities and isotope ratios have been measured in soil and sediment samples collected from waste storage reservoirs at the Mayak PA area and downstream in the Techa River (a tributary of the Ob River). The “Mayak” Production Association (Mayak PA) was established in the late 1940s to produce plutonium for the Soviet Nuclear weapons programme. Authorized and accidental releases of radioactive waste have caused severe contamination to the surrounding areas.  The objective of the study was to determine the total inventory of Pu in the reservoirs and to identify the different sources contributing to the Pu contamination. Results show the presence of different sources and confirmed recent reports of civil reprocessing at Mayak. Determination of activity levels and isotope ratios in soil and sediment samples from the Techa River, support the hypothesis that most of the plutonium, like other radionuclides in the Techa River, originated from the very early waste discharges to the Techa River between 1949 and 1951. Analysis of reservoir sediment samples suggested that about 75% of the Pu isotopes could have been released to reservoir 10 during the early, weapons production operation of the plant, and that the majority of Pu in reservoir 10 today originates from later civil discharges. Enhanced ²⁴⁰Pu/²³⁹Pu ratios in river sediment upper layers (0-2 cm) between 50 and 250 km downstream from the plant, indicate a contribution from other, non-fallout sources.

 Knowledge of plutonium speciation in the Mayak area (Techa River) and in the Ob and Yenisey rivers and estuaries, and the processes controlling its behaviour in estuarine systems, can improve predictions of its transfer and subsequent environmental impact to Arctic Seas. As part of the present work, sequential extraction techniques were used to investigate the binding and mobility of plutonium in sediments from the rivers and estuaries of the Ob and Yenisey. Sequential extractions showed that between 47 and 80 % of the Pu in Yenisey River sediments and 35 to 53 % of the Pu in soils around the Techa River catchments area are mobilized with weak oxidizing agents, which can indicate that Pu is bound to organic material. In contrast, Pu in Ob and Yenisey estuarine sediments was more strongly bound, with 60 to 100 % being found in the HNO₃-extractable fraction. This change in speciation probably reflects that organic-bound Pu is mobilized and re-bound to the mineral phase (essentially Fe/Mn-hydroxides) when the sediments encounter the more alkaline and more saline water of the Ob and Yenisey estuaries.

Plutonium binding interactions determine the degree to which soils and sediments can act as a sink or a potential diffuse source of contaminants. Sensitivity analysis of assessment models demonstrates that the distribution coefficient (K_d – Bq/kg solid per Bq/L water) is one of the key parameters contributing to large uncertainties in the assessment. Furthermore, the interaction of Pu-species with solids (soils and sediments) is a time and temperature dependant process, with K_d usually increasing with time until pseudo-equilibrium/equilibrium is reached. Therefore, the contact time between contaminated sea water and sediments should be taken into account in dispersion and dose assessment models. In the present work, dynamic model experiments have been performed where different Pu-species are added as tracers to a "soil - rainwater system" and to a “sediment – seawater system”. After a defined contact time, the samples have been sequentially extracted and results are implemented in a dynamic box model to estimate interaction and fixation rates.

The interaction of all Pu-species with soils and sediments seems to be rapid and follows a two-step reaction. Up to contact times of a few weeks, the K_d for Pu^III,IV is about one order of magnitude higher than for Pu^V,VI and Pu^III,IV-organic. After only 1 hour of contact, between 33% and 40% of the plutonium was strongly bound to the soil and sediment components, i.e., only extractable with strong HNO₃. The extraction of bound Pu followed a similar pattern for all the original species, suggesting that the Pu interaction mechanism with soil and sediment was rather independent of the original species.

The results indicate that the distribution coefficient, K_d, for Pu depends on the Pu-species in question. Thus, soils and sediments act as a sink for Pu^III,IV (high K_d), while Pu^III,IV-organic and Pu^V,VI should be considered more mobile (low K_d). Furthermore, the interaction with inert fractions obtained from sequential extraction depends less on Pu-species than on the effect of increase with time. For short time interactions, species-specific time functions rather than K_d constants should be applied in assessment models. The interaction and fixation model demonstrate that the mobile Pu decreases rapidly with time. After about 1 hour contact time the mobile fraction is reduced to about 30% for Pu^V,VI and Pu^III,IV-organic and to less than 10% for Pu^III,IV. Extrapolation of the model gave similar patterns and "fixation" rates for the three Pu-species suggesting, once again, that the fixation process is not dependent on the original speciation of added Pu-tracer. However, the rate of initial association (short half-lives) is faster for Pu^III,IV than for Pu^V,VI and Pu^III,IV-organic, as expected, and therefore shows a dependence on the nature of the original Pu-species.