Migration and distribution of 237Np in saltmarsh sediments

Abstract

Neptunium is present in the environment from a number of nuclear related sources and due to its presence in High, Intermediate and Low Level wastes, long half-life (t½ =2.13 × 106 y), availability, high radiotoxicity and secondary source from ingrowth of 241Am (t½ =432.6 y, α) and 241Pu (t½ =14.33 y, β-) is a major component of the dose commitment to the public. It is highly mobile in the environment and readily absorbed by man therefore radiologically significant especially for longer term waste management and safety case assessments.

Due to difficulties with analysis there have been limited studies of the behaviour of neptunium in the environment required to understand the impact of its mobility and distribution; important when considering the burial and storage of nuclear waste. A review of cited methods showed most were based on the analysis of 237Np in a specific sample matrix. As the nuclear industry in the UK moves towards decommissioning phase, there is an increased demand for the analysis of more complex and varied samples types. In this study a method of the determination of 237Np was developed which was capable of analysing a wide variety of sample matrices. During the development phase, the lack of reference materials, especially for 237Np in solid matrices, became evident and to overcome a novel approach was developed using experimental design with the method scope based on elemental composition rather than sample type, increasing the flexibility of the method.

Since 1952 authorised releases of many fission products and transuranic radioisotopes, including 237Np, have been discharged into the Irish Sea from the Sellafield nuclear site. The radionuclides become attached, dependent on particle reactivity, to sedimentary particles and incorporated by sedimentary deposition and suspension processes into intertidal and estuarine environments resulting in the contamination of the surrounding environment. Parallel to the coastline is an area of fine-grain size dominated sediments commonly known as the ‘mud patch’ where sedimentary particles, with associated radionuclides, accumulate as a result of tidal movement and currents in the shallow Western Irish Sea basin. Particulates are redistributed and deposited onto the saltmarsh as a result of tidal processes and storm events.

Ravenglass saltmarsh is a highly dynamic ecosystem situated in the mouth of the River Esk, 5 km from Sellafield site. Like other saltmarsh environments, sediment both rapidly accumulates and degrades due to perturbations in sea level and storm events retaining a chronological profile of radionuclide discharge history. Research has shown that the Ravenglass saltmarsh is one of the most radioactively contaminated areas within the Irish Sea due to the significant levels of radionuclides, accumulated during tidal processes. The developed method was used to analyse surface scrape samples taken over a twenty five year period and an intertidal marsh sediment core from Ravenglass saltmarsh to gain a greater understanding of the behaviour, distribution and mobility of 237Np in the environment. Surface scrape samples provide a pragmatic method of measuring sediment contamination over large areas. The approach is used by most routine environmental monitoring programs, with the results used to calculate dose transfer factors and ascertain risk of exposure to the general public. The samples indicate recent contamination and are used in this study to identify temporal and spatial distribution across the saltmarsh. Samples from across the saltmarsh were analysed for a range of radionuclides (106Ru, 137Cs, Pu alpha, 237Np and 241Am) at four time points over the twenty five years. The greatest rate of change was reported for 106Ru, 137Cs and 237Np followed by Pu alpha and 241Am with a number of factors influencing the rate of reduction with time such as radiological half-lives, decreasing discharges and remobilisation. Considerable spatial variation for the different radionuclides across the saltmarsh was also observed, which with time became less defined. The highest activity concentrations of long-lived radionuclides were in low energy areas, typically where higher rates of sedimentation and vegetation occurred. The trend was reversed for the shorter-lived radionuclide, 106Ru, with higher activity concentrations observed in high energy areas where there was frequent tidal inundation. The variation shows the importance of sampling from the exact same location for long term studies to ensure any changes in data are not an artefact of spatial variation but reflect temporal trends.

The radionuclide profile of 237Np was determined in a sediment core taken from the saltmarsh which had a distinct geochemical redox profile. The core had been previously analysed for other radionuclides (137Cs, 241Am and Pu α) showed a chronological profile which had a strong correlation with published Sellafield discharge records indicating that the sediment was largely undisturbed. The highest activities of 237Np were in the upper sections of the profile with the presence of two minor peaks further down and activities decreasing to below the limit of detection at a depth of 21.5 cm. The profile was partially reconciled with the Sellafield effluent discharges however less defined than for other radionuclides. The lack of correlation was due to limited discharge data, increased 237Np activities in the upper sections and a differing profile distribution compared to the other transuranic nuclides possibly indicating redistribution of Np within the core. The distribution profile was consistent with other cores sampled at the Ravenglass saltmarsh which did not show redox zonation indicating 237Np is not redox sensitive in this environment.

Comparison of nuclide ratios in the surface scrape and core concluded 237Np exhibited conservative behaviour, similar to 137Cs, and activities closely reflecting Sellafield discharges due to the lower affinity for inorganic particles. This was in contrast to Pu alpha which exhibited a higher affinity for inorganic particles becoming strongly associated with the fine grained sediment and exhibiting non-conservative behaviour with input sources originating from discharge and the mud patch with results reflecting both current and historic contamination. Such information is essential in assessing the long-term behaviour of 237Np in the environment, importance in radiological dose assessment and mobility and distribution in long term storage options such as geological disposal.

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