Estimation of lung absorption parameters for oxides of 238Pu.

Following inhalation of an aerosol of relatively insoluble particles, it is usually found that the fractional dissolution rate of material retained in the lungs decreases with time, and the amount remaining undissolved can be represented simply by a decreasing exponential function with two or more components. A few exceptions are known, in which the dissolution rate increases with time. The most important in the context of radiological protection is probably that of 238Pu dioxide. Several published comprehensive data sets, from animal studies and accidental human exposures, have been analysed using the Human Respiratory Tract Model (HRTM) of the International Commission on Radiological Protection. The HRTM contains a simplified representation of particle dissolution in the respiratory tract, suitable for routine radiological protection purposes. Still, it was found to have sufficient flexibility to represent the measurement data in most of these cases. Although the 238Pu dioxide showed a wide range of behaviour in the different studies, there was good agreement between the absorption behaviour modelled for two studies involving 'ceramic' 238Pu dioxide as used in spacecraft radioisotope thermoelectric generators: a long-term experimental study in dogs and an accidental exposure involving a group of workers.

Modelling particle retention in the alveolar-interstitial region of the human lungs.

Better information is available now on long-term particle retention in the human lungs than there was in 1994, when the human respiratory tract model (HRTM) was adopted by the International Commission on Radiological Protection (ICRP). Three recent studies are especially useful because they provide such information for groups of people who inhaled very similar aerosols. For all three the HRTM significantly underestimates lung retention of insoluble material. The purpose of this work was to improve the modelling of long-term retention in the deep lung. A simple physiologically based model developed to predict lung and lymph node particle retention in coal miners was found to represent lung retention in these studies adequately. Instead of the three alveolar-interstitial (AI) compartments in the HRTM, it has an alveolar compartment which clears to the bronchial tree and to a second compartment, representing the interstitium, which clears only to lymph nodes. The main difference from the HRTM AI model is that a significant fraction of the AI deposit is sequestered in the interstitium. To obtain default parameter values for general use, the model was fitted to data from the three recent studies, and also the experimental data used in development of the HRTM to define particle transport from the AI region for the first year after intake. The result of the analysis is that about 40% of the AI deposit of insoluble particles is sequestered in the interstitium and the remaining fraction is cleared to the ciliated airways with a half-time of about 300 days. For some long-lived radionuclides in relatively insoluble form (type S), this increased retention increases the lung dose per unit intake by 50-100% compared to the HRTM value.

ICRP Publication 137: Occupational Intakes of Radionuclides: Part 3.

Abstract ­: The 2007 Recommendations of the International Commission on Radiological Protection (ICRP, 2007) introduced changes that affect the calculation of effective dose, and implied a revision of the dose coefficients for internal exposure, published previously in the Publication 30 series (ICRP, 1979, 1980, 1981, 1988) and Publication 68 (ICRP, 1994). In addition, new data are now available that support an update of the radionuclide-specific information given in Publications 54 and 78 (ICRP, 1988a, 1997b) for the design of monitoring programmes and retrospective assessment of occupational internal doses. Provision of new biokinetic models, dose coefficients, monitoring methods, and bioassay data was performed by Committee 2, Task Group 21 on Internal Dosimetry, and Task Group 4 on Dose Calculations. A new series, the Occupational Intakes of Radionuclides (OIR) series, will replace the Publication 30 series and Publications 54, 68, and 78. OIR Part 1 has been issued (ICRP, 2015), and describes the assessment of internal occupational exposure to radionuclides, biokinetic and dosimetric models, methods of individual and workplace monitoring, and general aspects of retrospective dose assessment. OIR Part 2 (ICRP, 2016), this current publication and upcoming publications in the OIR series (Parts 4 and 5) provide data on individual elements and their radioisotopes, including information on chemical forms encountered in the workplace; a list of principal radioisotopes and their physical half-lives and decay modes; the parameter values of the reference biokinetic model; and data on monitoring techniques for the radioisotopes encountered most commonly in workplaces. Reviews of data on inhalation, ingestion, and systemic biokinetics are also provided for most of the elements. Dosimetric data provided in the printed publications of the OIR series include tables of committed effective dose per intake (Sv Bq−1 intake) for inhalation and ingestion, tables of committed effective dose per content (Sv Bq−1 measurement) for inhalation, and graphs of retention and excretion data per Bq intake for inhalation. These data are provided for all absorption types and for the most common isotope(s) of each element. The electronic annex that accompanies the OIR series of publications contains a comprehensive set of committed effective and equivalent dose coefficients, committed effective dose per content functions, and reference bioassay functions. Data are provided for inhalation, ingestion, and direct input to blood. This third publication in the series provides the above data for the following elements: ruthenium (Ru), antimony (Sb), tellurium (Te), iodine (I), caesium (Cs), barium (Ba), iridium (Ir), lead (Pb), bismuth (Bi), polonium (Po), radon (Rn), radium (Ra), thorium (Th), and uranium (U).

Note on the autocorrelation coefficient as a test statistic for assessment of the goodness-of-fit of biokinetic models to multiple bioassay data sets.

The European Commission project IDEAS has produced guidelines for internal dose assessments from monitoring data. A key stage in the guidelines requires assessment of the goodness-of-fit of biokinetic models to bioassay data. The present note extends the use of an autocorrelation coefficient to assess the fits of multiple types of bioassay quantity simultaneously.

Dosimetric calculations for uranium miners for epidemiological studies.

Epidemiological studies on uranium miners are being carried out to quantify the risk of cancer based on organ dose calculations. Mathematical models have been applied to calculate the annual absorbed doses to regions of the lung, red bone marrow, liver, kidney and stomach for each individual miner arising from exposure to radon gas, radon progeny and long-lived radionuclides (LLR) present in the uranium ore dust and to external gamma radiation. The methodology and dosimetric models used to calculate these organ doses are described and the resulting doses for unit exposure to each source (radon gas, radon progeny and LLR) are presented. The results of dosimetric calculations for a typical German miner are also given. For this miner, the absorbed dose to the central regions of the lung is dominated by the dose arising from exposure to radon progeny, whereas the absorbed dose to the red bone marrow is dominated by the external gamma dose. The uncertainties in the absorbed dose to regions of the lung arising from unit exposure to radon progeny are also discussed. These dose estimates are being used in epidemiological studies of cancer in uranium miners.

Particle clearance in the alveolar-interstitial region of the human lungs: model validation.

New information on particle retention of inhaled insoluble material indicates that the ICRP Human Respiratory Tract Model (HRTM) significantly underestimates long-term retention in the lungs. In a previous paper, the information from three studies was reviewed, and a model developed to predict particle retention in the lungs of coal miners was adapted in order to obtain parameter values for general use to predict particle retention in the alveolar-interstitial (AI) region. The model is physiologically based and simpler than the HRTM, requiring two instead of three compartments to model the AI region. The main difference from the HRTM AI model is that a significant fraction, about 35 %, of the AI deposit of insoluble material remains sequestered in the interstitium. The new model is here applied to the analysis of two well-known contamination cases with several years of follow-up data.

Feedback stabilization of multiple resistive wall modes.

Active feedback stabilization of multiple independent resistive wall modes is experimentally demonstrated in a reversed-field pinch plasma. A reproducible simultaneous suppression of several nonresonant resistive wall modes is achieved. Coupling of different modes due to the limited number of the feedback coils is observed in agreement with theory. The feedback stabilization of nonresonant RWMs also has an effect on tearing modes that are resonant in the central plasma, leading to a significant prolongation of the discharge pulse.