Estelle Davesne

Absorption of plutonium compounds in the respiratory tract

In order to optimise the monitoring of potentially exposed workers, it is desirable to determine specific values of absorption for the compounds handled. This study derives specific values of absorption rates for different chemical forms of plutonium from in vitro and animal (monkeys, dogs, mice, rats) experiments, and from human contamination cases. Different published experimental data have been reinterpreted here to derive values for the absorption parameters, f(r), s(r) and s(s), used in the human respiratory tract model currently adopted by the International Commission on Radiological Protection (ICRP). The consequences of the use of these values were investigated by calculating related committed effective doses per unit intake. Average and median estimates were calculated for f(r), s(r), and s(s) for each plutonium compound, that can be used as default values for specific chemical forms instead of the current reference types. Nevertheless, it was shown that the use of the current ICRP reference absorption types provides reasonable approximations. Moreover, this work provides estimates of the variability in pulmonary absorption and, therefore, facilitates analyses of the uncertainties associated with assessments, either from bioassay measurements or from prospective calculations, of intake and dose.

Concerted Uranium Research in Europe (CURE): toward a collaborative project integrating dosimetry, epidemiology and radiobiology to study the effects of occupational uranium exposure.

The potential health impacts of chronic exposures to uranium, as they occur in occupational settings, are not well characterized. Most epidemiological studies have been limited by small sample sizes, and a lack of harmonization of methods used to quantify radiation doses resulting from uranium exposure. Experimental studies have shown that uranium has biological effects, but their implications for human health are not clear. New studies that would combine the strengths of large, well-designed epidemiological datasets with those of state-of-the-art biological methods would help improve the characterization of the biological and health effects of occupational uranium exposure. The aim of the European Commission concerted action CURE (Concerted Uranium Research in Europe) was to develop protocols for such a future collaborative research project, in which dosimetry, epidemiology and biology would be integrated to better characterize the effects of occupational uranium exposure. These protocols were developed from existing European cohorts of workers exposed to uranium together with expertise in epidemiology, biology and dosimetry of CURE partner institutions. The preparatory work of CURE should allow a large scale collaborative project to be launched, in order to better characterize the effects of uranium exposure and more generally of alpha particles and low doses of ionizing radiation.

Absorption of americium compounds in the respiratory tract

Abstract Purpose: To improve the dosimetry of incorporated americium (Am) and to contribute to radiation protection by characterizing the absorption kinetics of inhaled Am compounds. Material and methods: In vitro dissolution tests, animal experiments and human contamination cases published in the literature were reviewed. The data were analyzed with biokinetic models consistent with the current publications of the International Commission on Radiological Protection. Results: Material-specific dissolution parameter values with consequent assignment to absorption Types are proposed as well as representative central values for the different chemical forms of Am. Conclusions: The absorption of Am oxide is consistent with the moderate absorption Type M while Am nitrate appears more soluble. Am associated with plutonium oxide usually follows its slow absorption Type S. However, the large variability observed stresses the value of investigating the specific absorption kinetics for Am compounds which represent a significant risk of internal exposure.

Optimisation of internal contamination monitoring programme by integration of uncertainties.

Potential internal contamination of workers is monitored by periodic bioassay measurements interpreted in terms of intake and committed effective dose by the use of biokinetic and dosimetric models. After a prospective evaluation of exposure at a workplace, a suitable monitoring programme can be defined by choosing adequate measurement techniques and frequency. In this study, the sensitivity of a programme is evaluated by the minimum intake and dose, which may be detected with a given level of confidence by taking into account uncertainties on exposure conditions and measurements. This is made for programme optimisation, which is performed by comparing the sensitivities of different alternative programmes. These methods were applied at the AREVA NC reprocessing plant and support the current monitoring programme as the best compromise between the cost of the measurements and the sensitivity of the programme.

Modeling the imprecision in prospective dosimetry of internal exposure to uranium.

The dosimetry of internal exposure to radionuclides is performed on the basis of biokinetic and dosimetric models. For prospective purpose, the organ or effective dose resulting from potential conditions of exposure can be calculated by applying these models with dedicated software. However, it is acknowledged that a significant uncertainty is associated with such calculation due to the variability of individual cases and to the possible lack of knowledge about some factors influencing the dosimetry. This uncertainty has been studied in a range of situations by modeling the uncertainty on the model parameters by probability distributions and propagating this uncertainty onto the dose result by Monte Carlo calculation. However, while probability distributions are well adapted to model the known variability of a parameter, they may lead to an unrealistically low estimate of the uncertainty due to a lack of knowledge about some input parameters. Here we present a mathematical method, based on the Dempster-Shafer theory, to deal with such imprecise knowledge. We apply this method to the prospective dosimetry of inhaled uranium dust in the nuclear fuel cycle when its physico-chemical properties are not precisely known. The results show an increased estimation of the range of uncertainty as compared to the application of a probabilistic method. This Dempster-Shafer method may valuably be applied in future prospective dosimetry of internal exposure in order to more realistically estimate the uncertainty resulting from an imprecise knowledge of the parameters of the dose calculation.

Circulatory disease in French nuclear fuel cycle workers chronically exposed to uranium: a nested case–control study

Objectives There is growing evidence of an association between low-dose external γ-radiation and circulatory system diseases (CSDs), yet sparse data exist about an association with chronic internal uranium exposure and the role of non-radiation risk factors. We conducted a nested case–control study of French AREVA NC Pierrelatte nuclear workers employed between 1960 and 2005 to estimate CSD risks adjusting for major CSD risk factors (smoking, blood pressure, body mass index, total cholesterol and glycaemia) and external γ-radiation dose. Methods The study included 102 cases of death from CSD and 416 controls individually matched on age, gender, birth cohort and socio-professional status. Information on CSD risk factors was collected from occupational medical records. Organ-specific absorbed doses were estimated using biomonitoring data, taking into account exposure regime and uranium physicochemical properties. External γ-radiation was measured by individual dosimeter badges. Analysis was conducted with conditional logistic regression. Results Workers were exposed to very low radiation doses (mean γ-radiation dose 2 and lung uranium dose 1 mGy). A positive but imprecise association was observed (excess OR per mGy 0.2, 95% CI 0.004 to 0.5). Results obtained after adjustment suggest that uranium exposure might be an independent CSD risk factor. Conclusions Our results suggest that a positive association might exist between internal uranium exposure and CSD mortality, not confounded by CSD risk factors. Future work should focus on numerous uncertainties associated with internal uranium dose estimation and on understanding biological pathway of CSD after protracted low-dose internal radiation exposure.

Influence of DTPA Treatment on Internal Dose Estimates.

AbstractIn case of internal contamination with plutonium materials, a treatment with diethylene triamine pentaacetic acid (DTPA) can be administered in order to reduce plutonium body burden and consequently avoid some radiation dose. DTPA intravenous injections or inhalation can start almost immediately after intake, in parallel with urinary and fecal bioassay sampling for dosimetric follow-up. However, urine and feces excretion will be significantly enhanced by the DTPA treatment. As internal dose is calculated from bioassay results, the DTPA effect on excretion has to be taken into account. A common method to correct bioassay data is to divide it by a factor representing the excretion enhancement under DTPA treatment by intravenous injection. Its value may be based on a nominal reference or observed after a break in the treatment. The aim of this study was to estimate the influence of this factor on internal dose by comparing the dose estimated using default or upper and lower values of the enhancement factor for 11 contamination cases. The observed upper and lower values of the enhancement factor were 18.7 and 63.0 for plutonium and 24.9 and 28.8 for americium. For americium, a default factor of 25 is proposed. This work demonstrates that the use of a default DTPA enhancement factor allows the determination of the magnitude of the contamination because dose estimated could vary by a factor of 2 depending on the value of the individual DTPA enhancement factor. In case of significant intake, an individual enhancement factor should be determined to obtain a more reliable dose assessment.

Physico-chemical characteristics of uranium compounds: A review

Abstract Purpose: To collect values of parameters describing the physico-chemical properties of different uranium compounds in order to quantify their variability and to propose specific parameters for different workplaces and compounds. Material and methods: The published values of absorption parameters, gastrointestinal absorption fractions, activity median aerodynamic diameters and geometric standard deviations of the particle size activity distribution were collected. Results: Average and median values for each chemical form and workplace were determined for these parameters. These values can be used when no precise information is known for dose assessment following internal contamination by uranium. This review presents and discusses the variability of these parameters for the different uranium chemical forms and workplaces. Finally, sensitivity of the dose coefficients to these parameters was quantified. Conclusion: Specific parameter values for different workplaces and compounds were proposed and the variability quantified.

Collective dosimetry to distinguish occupational exposure to natural uranium from alimentary uranium background in bioassay measurements

Abstract Purpose: To assess occupational exposure from uranium bioassay results which are low and impacted by dietary intakes. Material and methods: First, the bioassay results of a group of workers exposed to UO2 were compiled along with results of a control group. A Bayesian approach was developed to account for dietary intakes in the calculation of the committed effective dose from occupational exposure of a group of workers. Results: Significant differences in uranium bioassay between the exposed and control groups were found establishing an occupational contamination of the exposed group of workers. Because uranium alimentary excretion estimated from the control group is very variable leading to unreliable individual dose assessment, a collective dosimetric approach was chosen. Applying the Bayesian method, all annual committed effective doses for the exposed group were estimated to be below 0.5 mSv with 95% confidence. Conclusions: The Bayesian method presented here is well designed to derive best estimate and dose distribution for a group of workers when a contamination is difficult to discriminate from a natural background or alimentary excretion.

INTEGRATION OF UNCERTAINTIES INTO INTERNAL CONTAMINATION MONITORING

Potential internal contaminations of workers are monitored by periodic bioassays interpreted in terms of intake and committed effective dose through biokinetic and dosimetric models. After a prospective evaluation of exposure at a workplace, a suitable monitoring program can be defined by the choice of measurement techniques and frequency of measurements. However, the actual conditions of exposure are usually not well defined and the measurements are subject to errors. In this study we took into consideration the uncertainties associated with a routine monitoring program in order to evaluate the minimum intake and dose detectable for a given level of confidence. Major sources of uncertainty are the contamination time, the size distribution and absorption into blood of the incorporated particles, and the measurement errors. Different assumptions may be applied to model uncertain knowledge, which lead to different statistical approaches. The available information is modeled here by classical or Bayesian probability distributions. These techniques are implemented in the OPSCI software under development. This methodology was applied to the monitoring program of workers in charge of plutonium purification at the AREVA NC reprocessing facility (La Hague, France). A sensitivity analysis was carried out to determine the important parameters for the minimum detectable dose. The methods presented here may be used for assessment of any other routine monitoring program through the comparison of the minimum detectable dose for a given confidence level with dose constraints.