On the symmetry of photon detection arrays: A directionally sensitive 3D model.

A detector's ability to obtain the direction of a radioactive source is an invaluable operational asset. A 2D/3D model was developed based on directionally sensitive arrays. The average location of photon interactions within a symmetrical array yields the direction of the source. The model is validated with simulations and laboratory measurements, maximum systematic error being 5-10° at energies >200 keV and approaching zero at lower energies. The symmetry model yields the direction of a shielded source even when no full energy photons could be detected.

Efficiency calibrations of radiation detectors for source localization and characterization at long source-detector distances for gamma and neutron sources.

Deployment of radiation detectors under field conditions for the purposes of security, safety or response has increased in recent years. Effective use of such instruments in the field necessitates careful consideration of the efficiency of the detector - both peak and total - at distances which may extend beyond 100 m. Difficulties in addressing the determination of both peak and total efficiencies across the energy range of interest and at long distances reduces the utility of such systems in effectively characterising radiation sources in the field. Empirical approaches to such calibrations are difficult. Approaches such as Monte Carlo simulations can become challenging with respect to time and computational requirements as source-detector distances become greater and in consideration of total efficiency. This paper presents a computationally efficient method of calculating peak efficiency at distances more than 300 m using efficiency transfer from a parallel beam geometry to point sources at extended distances. The relationship between total and peak efficiency at extended distances is explored and means of estimating the total efficiency from the peak efficiency are discussed. The ratio of the total efficiency to the peak efficiency increases as a function of the source-detector distance. The relationship is linear at distances longer than 50 m and is independent of photon energy. Usefulness of the efficiency calibration as a function of the source-detector distance was demonstrated in a field experiment. Total efficiency calibration measurements were performed for a neutron counter. An AmBe source was then successfully localized and characterised using four measurements at arbitrary locations far away from the unknown source. This kind of capability is useful for the authorities responding to nuclear accidents or security events. It has important operational implications, including the safety of the personnel involved.

Angular insensitivity of a gamma spectrometer for in-field applications.

In-field measurements have particular challenges as compared with those conducted under laboratory conditions. Besides unknown source shielding, the source-detector distance varies and the detector orientation relative to the incident radiation is not necessarily constant. The incoming flux facing a detector is a parallel beam at long source-detector distances (>1 m). The counting efficiency depends on the tilting angle relative to the beam facing the detector. In principle, a cylindrical detector with a height-diameter ratio of π/4 (H/D = 0.785) exhibits the lowest angular dependency (41% at low energies for a tilting angle of 45° as compared with the orientation of the detector end cap relative to the beam). However, Monte Carlo simulations of a germanium detector showed that this variability can be greatly improved by slightly increasing H/D (0.84) and introducing a copper cladding around the detector (1.1 mm). The counting efficiency of such a detector is almost independent of the direction of photons arriving to the detector. The maximum deviation of 10% takes place at 200 keV.

Rapid in situ assessment of radiocesium wood contamination using field gamma-ray spectroscopy to optimise felling.

The Chernobyl nuclear power meltdown that took place in 1986 has left a radioactive contamination legacy that currently severely limits the economic potential of impacted regions including the Polessie State Radioecology Reserve in Southern Belarus. Extensive areas of forested land could potentially become economically viable for firewood and building materials if radioactive contamination, notably 137Cs, could be characterised faster, whilst closely adhering to regulatory limits. Currently, laboursome tree coring and unreliable transfer factors derived from limited soil sampling data are routinely employed in felling decision making, which has financial repercussions owed to the large amounts of waste produced and unnecessary transportation costs. In this study, it is demonstrated that a combination of targeted mobile gamma-ray spectrometry and a newly developed, lead shielded, in situ gamma-ray spectrometry method can significantly speed up the process of characterisation of 137Cs wood activity in the field. For the in situ method, Monte Carlo calibration routines were developed alongside spectral processing procedures to unfold spectra collected in the field allowing for separation of ground and tree spectral components. Isolated contributions from the tree could then be converted to activity. The method was validated at a test facility and then demonstrated at three separate sites with differing contamination levels. This technique showed that single trees could be measured within approximately 20% of the activity compared to conventional tree core data. However, some discrepancies were found which were attributed to under sampling using the tree corer and low count rates at the lowest activity site, prompting the need for further data collection to optimise the method. It was concluded that this real-time approach could be a valuable tool for management of contaminated forested areas, releasing valuable timber and ultimately reducing the risk associated with living and working in these areas.

GPS-coupled contaminant monitors on free-ranging Chernobyl wolves challenge a fundamental assumption in exposure assessments.

Measurements of external contaminant exposures on individual wildlife are rare because of difficulties in using contaminant monitors on free-ranging animals. Most wildlife contaminant exposure data are therefore simulated with computer models. Rarely are empirical exposure data available to verify model simulations, or to test fundamental assumptions inherent in exposure assessments. We used GPS-coupled contaminant monitors to quantify external exposures to individual wolves (Canis lupus) living within the Belarus portion of Chernobyl's 30-km exclusion zone. The study provided data on animal location and contaminant exposure every 35 min for 6 months, resulting in ~6600 individual locations and 137Cs external exposure readings per wolf, representing the most robust external exposure data published to date on free ranging animals. The data provided information on variation in external exposure for each animal over time, as well as variation in external exposure among the eight wolves across the landscape of Chernobyl. The exposure data were then used to test a fundamental assumption in screening-level risk assessments, espoused in guidance documents of the U.S. Environmental Protection Agency and U.S. Department of Energy, - Mean contaminant concentrations conservatively estimate individual external exposures. We tested this assumption by comparing our empirical data to a series of simulations using the ERICA modeling tool. We found that modeled simulations of mean external exposure (10.5 mGy y-1), based on various measures of central tendency, under-predicted mean exposures measured on five of the eight wolves wearing GPS-contaminant monitors (i.e., 12.3, 26.3, 28.0, 28.8 and 35.7 mGy y-1). If under-prediction of exposure occurs for some animals, then arguably the use of averaged contaminant concentrations to predict external exposure is not as conservative as proposed by current risk assessment guidance. Thus, a risk assessor's interpretation of simulated exposures in a screening-level risk analysis might be misguided if contaminant concentrations are based on measures of central tendency. We offer three suggestions for risk assessors to consider in order to reduce the probability of underestimating exposure in a screening-level risk assessment.

Reconstructing the deposition environment and long-term fate of Chernobyl 137Cs at the floodplain scale through mobile gamma spectrometry.

Cs-137 is considered to be the most significant anthropogenic contributor to human dose and presents a particularly difficult remediation challenge after a dispersal following nuclear incident. The Chernobyl Nuclear Power Plant meltdown in April 1986 represents the largest nuclear accident in history and released over 80 PBq of 137Cs into the environment. As a result, much of the land in close proximity to Chernobyl, which includes the Polessie State Radioecology Reserve in Belarus, remains highly contaminated with 137Cs to such an extent they remain uninhabitable. Whilst there is a broad scale understanding of the depositional patterns within and beyond the exclusion zone, detailed mapping of the distribution is often limited. New developments in mobile gamma spectrometry provide the opportunity to map the fallout of 137Cs and begin to reconstruct the depositional environment and the long-term behaviour of 137Cs in the environment. Here, full gamma spectrum analysis using algorithms based on the peak-valley ratio derived from Monte Carlo simulations are used to estimate the total 137Cs deposition and its depth distribution in the soil. The results revealed a pattern of 137Cs distribution consistent with the deposition occurring at a time of flooding, which is validated by review of satellite imagery acquired at similar times of the year. The results were also consistent with systematic burial of the fallout 137Cs by annual flooding events. These results were validated by sediment cores collected along a transect across the flood plain. The true merit of the approach was confirmed by exposing new insights into the spatial distribution and long term fate of 137Cs across the floodplain. Such systematic patterns of behaviour are likely to be fundamental to the understanding of the radioecological behaviour of 137Cs whilst also providing a tracer for quantifying the ecological controls on sediment movement and deposition at a landscape scale.

An in situ method for the high resolution mapping of 137Cs and estimation of vertical depth penetration in a highly contaminated environment.

The Chernobyl nuclear power plant meltdown has to date been the single largest release of radioactivity into the environment. As a result, radioactive contamination that poses a significant threat to human health still persists across much of Europe with the highest concentrations associated with Belarus, Ukraine, and western Russia. Of the radionuclides still prevalent with these territories 137Cs presents one of the most problematic remediation challenges. Principally, this is due to the localised spatial and vertical heterogeneity of contamination within the soil (~10's of meters), thus making it difficult to accurately characterise through conventional measurement techniques such as static in situ gamma-ray spectrometry or soil cores. Here, a practical solution has been explored, which utilises a large number of short-count time spectral measurements made using relatively inexpensive, lightweight, scintillators (sodium iodide and lanthanum bromide). This approach offers the added advantage of being able to estimate activity and burial depth of 137Cs contamination in much higher spatial resolution compared to traditional approaches. During the course of this work, detectors were calibrated using the Monte Carlo Simulations and depth distribution was estimated using the peak-to-valley ratio. Activity and depth estimates were then compared to five reference sites characterised using soil cores. Estimates were in good agreement with the reference sites, differences of ~25% and ~50% in total inventory were found for the three higher and two lower activity sites, respectively. It was concluded that slightly longer count times would be required for the lower activity (<1MBqm-2) sites. Modelling and reference site results suggest little advantage would be gained through the use of the substantially more expensive lanthanum bromide detector over the sodium iodide detector. Finally, the potential of the approach was demonstrated by mapping one of the sites and its surrounding area in high spatial resolution.

On the application of an environmental radiological assessment system to an anthropomorphic surrogate.

Recent developments have seen the expansion of the system of radiological protection for humans to one including protection of the environment against detrimental effects of radiation exposure, although a fully developed framework for integration of human and ecological risk assessment for radionuclides is only at an early stage. In the context of integration, significant differences exist between assessment methodologies for humans and the environment in terms of transfer, exposure, and dosimetry. The aim of this elaboration was to explore possible implications of the simplifications made within the system of environmental radiological protection in terms of the efficacy and robustness of dose-rate predictions. A comparison was conducted between human radiological assessment and environmental radiological assessment for an anthropomorphic surrogate, the results for which, produced by both the environmental and human-oriented risk assessment systems, were critically compared and contrasted. The adopted approach split the calculations into several parts, these being 1) physical transfer in an ecosystem, 2) transfer to humans, 3) internal doses to humans, and 4) external doses to humans. The calculations were carried out using both a human radiological assessment and ecological risk assessment system for the same surrogate. The results of this comparison provided indications as to where the 2 systems are amenable to possible integration and where such integration may prove difficult. Initial stage transport models seem to be an obvious component amenable for integration, although complete integration is arguably unattainable as the differences between endpoints mean that the relevant outputs from the models will not be the same. For the transfer and dosimetry components of 2 typical methodologies, it seems that the efficacy of the environmental system is radionuclide-dependent, the predictions given by the environmental system for (90) Sr and (60) Co being unsatisfactory and those for (239) Pu and (210) Po being evidently poor. Integration in this context might take the form of exploring the biokinetic models developed for humans with regard to selected animals and radionuclides. External dose assessment for environmental and human systems provide results for the surrogate that correspond quite closely providing an indication that integration in this regard is perhaps unnecessary.

Review of research on impacts to biota of discharges of naturally occurring radionuclides in produced water to the marine environment.

Produced water has been described as the largest volume waste stream in the exploration and production process of oil and gas. It is accompanied by discharges of naturally occurring radionuclides raising concerns over the potential radiological impacts of produced water on marine biota. In the Northern European marine environment, radioactivity in produced water has received substantial attention owing to the OSPAR Radioactive Substances Strategy which aims at achieving 'concentrations in the environment near background values for naturally occurring radioactive substances'. This review provides an overview of published research on the impacts to biota from naturally occurring radionuclides discharged in produced water by the offshore oil and gas industry. In addition to summarising studies and data that deal directly with the issue of dose and effect, the review also considers studies related to the impact of added chemicals on the fate of discharged radionuclides. The review clearly illustrates that only a limited number of studies have investigated possible impacts on biota from naturally occurring radionuclides present in produced water. Hence, although these studies indicate that the risk to the environment from naturally occurring radionuclides discharged in produced water is negligible, the substantial uncertainties involved in the assessments of impact make it difficult to be conclusive. With regard to the complexity involved in the problem under consideration there is a pressing need to supplement existing data and acquire new knowledge. Finally, the present work identifies some knowledge gaps to indicate future research requirements.

Application of an environmental impact assessment methodology to a site discharging low levels of radioactivity to a freshwater environment in Norway.

Significant shifts in opinion regarding environmental protection from ionising radiation have resulted in the development and availability of bespoke approaches for the assessment of impacts on wildlife from radioactive contaminants. The application of such assessment methodologies to actual situations, however, remains relatively limited. This paper describes the implementation of the ERICA Integrated Approach and associated tools within the context of routine discharges of radioactive materials to a freshwater environment. The article follows the implementation through its relevant stages and discusses strengths and weaknesses of the approach in relation to the case study. For current discharge levels, 137Cs and 60Co constitute the main dose contributors to the majority of reference organisms studied, although 241Am and 3H are the main contributors for the phyto- and zooplankton categories. Patterns are observed depending on whether the reference organism is sediment-associated or not. At current discharge levels, none of the reference organisms exceeded or approached the selected screening level, and impacts on biota could be regarded as negligible.