Effect of uncertainty in transfer rates for the ICPR publication 67 biokinetic model on dose estimation for 239Pu from results of individual monitoring. International Commission on Radiological Protection

The radiation dose due to internal exposures from 239Pu is mainly estimated by measuring actual urinary or faecal excretion of activity and comparing those values with the standard excretion rates calculated from the models of the International Commission on Radiological Protection (ICRP). Recently, on the other hand, uncertainties in the ICRP's models and parameters are under consideration because of the paucity of human data. In addition, there is a possibility of variation between individuals. A code has been developed to reproduce the ICRP's dose coefficients and excretion rates for 239Pu, which is one of the most important elements for occupational exposure. By using this code, the respective transfer rates for the ICRP Publication 67 biokinetic model were modified, and the effect owing to these changes on present hazard assessment was investigated. As a result, it was shown that dose estimates for workers exposed to 239Pu were not very sensitive to changes in these transfer rates.     

Comparisons of 239Pu inhalation doses calculated with ICRP 67 and proposed systemic models

The International Commission on Radiological Protection (ICRP) has issued an age-specific systemic biokinetic model for plutonium (Pu), which was later modified to give better agreement with measured urinary excretion data. Recently, the current ICRP systemic Pu model was improved by Leggett et al. based on recently developed data. Incorporation of 239Pu in the human body may result in significant internal radiation exposure. In the present work, the retentions in organs and tissues, the equivalent dose and effective dose from 239Pu for workers and members of the public were estimated and compared under the current ICRP and the proposed models. 239Pu contents in liver and in other soft tissue calculated with the proposed model are higher than predicted by the ICRP model, whereas bone content is lower than predicted by the ICRP model. Based on the proposed model, the inhalation equivalent dose coefficient in some organs, e.g. liver and kidneys, is increased, but there is no significant change in the effective inhalation dose coefficients of 239Pu for workers and members of the public.     

Uncertainty analysis of doses from ingestion of plutonium and americium

Uncertainty analyses have been performed on the biokinetic model for americium currently used by the International Commission on Radiological Protection (ICRP), and the model for plutonium recently derived by Leggett, considering acute intakes by ingestion by adult members of the public. The analyses calculated distributions of doses per unit intake. Those parameters having the greatest impact on prospective doses were identified by sensitivity analysis; the most important were the fraction absorbed from the alimentary tract, f(1), and rates of uptake from blood to bone surfaces. Probability distributions were selected based on the observed distribution of plutonium and americium in human subjects where possible; the distributions for f(1) reflected uncertainty on the average value of this parameter for non-specified plutonium and americium compounds ingested by adult members of the public. The calculated distributions of effective doses for ingested (239)Pu and (241)Am were well described by log-normal distributions, with doses varying by around a factor of 3 above and below the central values; the distributions contain the current ICRP Publication 67 dose coefficients for ingestion of (239)Pu and (241)Am by adult members of the public. Uncertainty on f(1) values had the greatest impact on doses, particularly effective dose. It is concluded that: (1) more precise data on f(1) values would have a greater effect in reducing uncertainties on doses from ingested (239)Pu and (241)Am, than reducing uncertainty on other model parameter values and (2) the results support the dose coefficients (Sv Bq(-1) intake) derived by ICRP for ingestion of (239)Pu and (241)Am by adult members of the public.     

An analysis of a puncture wound case with medical intervention

A worker noted a small wound to his thumb when leaving a work site that was undergoing decontamination because of past operations with plutonium (Pu) and americium (Am). Direct surveys of the wound site confirmed the presence of contamination. The chelating agent Ca-DTPA was administered via a nebuliser within an hour after discovery of the wound. External measurements were made of the wound site and wound dressings; 24-h urinary excretion data were collected periodically and the Pu and Am urine content was determined. Zn-DTPA was administered on three occasions. The ICRP Pu systemic model was modified to consider the enhanced urinary excretion following administration of the chelating agents. The analysis indicated that the wound resulted in an initial deposition of 400 Bq 238Pu, 2240 Bq (239/240)Pu and 1060 Bq 241Am. About 70% of the initial wound activity was removed by surgical procedures and less than 1% of the wound activity was removed by chelation therapy. This paper compares the observed urinary excretion data with that indicated by a simulation of the kinetics of the transfer from the wound site and the kinetics of the chelating agent and Pu.     

Modelling of bioassay data from a Pu wound treated by repeated DTPA perfusions: biokinetics and dosimetric approaches

The aim of this study is to model plutonium (Pu) excretion from the analysis of a well-documented Pu wound case involving repeated diethylene-triamine-penta-acetic acid (DTPA) perfusions up to 390 d and monitoring up to 3109 d. Three modelling approaches were simultaneously applied involving: (1) release of soluble Pu from the wound, estimated with the ICRP66 dissolution model, (2) systemic behaviour of Pu by using ICRP67 model, but also two new models recently reported and (3) additional 'Pu-DTPA' compartments which transfer Pu directly to urinary compartment from blood, interstitial fluids and liver. The best fit of simulations to biological data was obtained by using the new Leggett's systemic model and assuming the presence of three DTPA compartments. Calculations have shown that DTPA treatments have contributed to a 3-fold reduction of the effective dose. Thus, reduction of doses associated with the DTPA treatments can be estimated by modelling which is useful to improve the efficacy of a DTPA treatment schedule based on a diminution of risk.     

Anomalously high excretion of Pu in urine following inhalation of plutonium nitrate?

A study of the biokinetics of inhaled plutonium nitrate in two volunteers has been carried out. Low doses (approximately 80 microSv) were achievable because tracers of high isotopic purity were used: 237Pu (measurable by X ray spectrometry) and 244Pu (measurable by accelerator mass spectrometry). Lung retention, amount in blood, uptake to the liver and skeleton, and urinary and faecal excretion were measured. The measured urinary excretion rates are about a factor of three higher than those predicted from urine excretion data measured following intravenous injection of plutonium to the same volunteers. If similar biokinetic behaviour occurs in workers exposed to plutonium nitrate, intakes by inhalation and corresponding committed doses assessed by urine bioassay could be consistently overestimated by a similar factor.     

Uncertainties in doses calculated according to ICRP recommendations after inhalation of 239PUO2 and early chest monitoring

This study estimates uncertainties in Pu biokinetics and effective doses calculated after an acute inhalation exposure to (239)PuO(2) according to ICRP recommendations (default values for aerosols size and PuO(2) dissolution parameters). This was performed using the most recently reported variations in model parameters and simulations after a Monte Carlo approach. Without chest monitoring, uncertainties in thoracic retention and plutonium excretion was 8-10 (95% confidence interval as the ratio between 97.5 and 2.5 percentiles of the lognormal distributions) up to 900 d after exposure. Early chest monitoring reduces significantly the uncertainties in plutonium biokinetics and doses which remain within a 95% confidence interval of 2.3 as compared with 6.6, without monitoring. Analysis of bioassay data previously reported shows that the dose delivered to some individuals can be out of the confidence interval, which was mostly due to an inhibition of the late mechanical clearance of the alveolar interstitium.     

Modifying the ICRP 66 dosimetry model based on results obtained from Mayak plutonium workers

Results obtained in a study of the microscopic distribution of plutonium in the lungs of deceased Pu workers from the Mayak Production Association showed that the long-term retention of Pu was greater than predicted by the current ICRP 66 respiratory tract dosimetry model (HRTM). These data were therefore applied to the HRTM by modifying selected parameters, namely the transfer rate of Pu from the transformed state compartment and the fraction of Pu that transfers to the bound state compartment. Invoking the latter compartment into the modelling allowed a better representation of the long-term Pu retention as well as providing a convenient means of describing the workplace-specific characteristics of the different Pu aerosols found in the Mayak plant. In particular, the present model describes a significantly greater long-term retention of Pu nitrate aerosols in the lung compared with the Type M default.     

Uncertainty analysis of the urinary excretion of plutonium

A quantitative estimate of the uncertainty of the urinary excretion of plutonium predicted by available biokinetic models is provided. Urinary excretion is primarily considered here because the monitoring of internal contamination of plutonium mainly relies on measurements of activity in urine samples. A previous paper has identified the most significant transfer rates for urinary plutonium excretion following an acute intake. That analysis is used here as a screening method to reduce the number of model parameters to be considered. A log-normal distribution was assumed for the probability distribution of the model parameters. The spread of the values, represented by the geometric standard deviation (GSD), is explicitly calculated, as few indications of the range of variation of systemic transfer rates are available. Different values for the GSD were considered. Assuming a certain GSD for all the systemic rate constants, random values of the rates were generated (by means of a Monte Carlo simulation with a Latin hypercube sampling scheme) and the resulting predictions of urine bioassay measurements were calculated. The comparison of the mean and variance of the predictions with the available data from several studies performed on different subjects provides information about the GSD of model parameters that represents the intersubject variation of transfer parameters.     

Early faecal excretion of inhaled plutonium

Special individual monitoring has been performed for suspected cases of inhalation of plutonium at the Tokai Works of the Japan Nuclear Cycle Development Institute (JNC). Some experimental data obtained from this special individual monitoring during the past 20 years are presented and discussed in this paper. Our experience suggests the following conclusions. The daily plutonium excretion rate, normalised to the total excretion for the first 5 days after inhalation, was approximately in agreement with the latest ICRP 78 dosimetric model. Maximum faccal excretion is observed on the second day after inhalation of plutonium compounds. On the other hand, the activity ratio for total alpha activity observed in early faeces to that detected in nasal swabs showed a wide distribution range, and it was proven that variations in this ratio followed a log-normal distribution. The logarithmic mean probability is about 2.1 for PuO2 and about 15.7 for Pu(NO3)4. In practice, a conservative dose assessment from nasal swabs can be performed on the basis of these experimental ratios.     

Surface-seeking radionuclides in the skeleton: current approach and recent developments in biokinetic modelling for humans and beagles

In the last decade, the biokinetics of surface-seeking radionuclides in the skeleton has been the object of several studies. Investigations were carried out to determine the kinetics of plutonium and americium in the skeleton of humans and beagles. As a result of these investigations, in recent years the models presented by ICRP in Publication 67 for humans were partially revised, particularly the skeletal part. The aim of the present work is to present recent developments in the biokinetic modelling of surface-seeking radionuclides (plutonium and americium) in beagles and humans. Various assumptions and physiological interpretations of the different approaches to the biokinetic modelling of the skeleton are discussed. Current ICRP concepts and skeleton modelling of plutonium and americium in humans are compared to the latest developments in biokinetic modelling in beagles.     

Prediction of monitoring data for 239Pu accidentally injected via wound site based on the proposed NCRP wound model

In response to the consultation from the National Council on Radiation Protection and Measurements (NCRP) to the International Commission on Radiological Protection (ICRP) committee 2, retention and excretion of 239Pu deposited at wound site were calculated by coupling together the proposed NCRP wound model and the current ICRP systemic model of Pu. The physicochemical forms considered were the soluble form categorized into 'Strong Retention', and the colloidal, particulate and fragmentary forms. The results are summarized as follows. If in soluble form, immediate medical intervention is needed to prevent uptake of radionuclides to body tissues, and prompt wound monitoring is essential for an accurate estimation of the initially deposited radioactivity. If in particulate form, a multi-component exponential equation leads to an overestimation of the absorption rate to blood because of significant lymph node drainage. The committed doses in the organs for direct transfer of 239Pu to the blood may be applied to every cases of accidental injection except for fragments.     

USTUR whole body case 0262: 33-y follow-up of PuO2 in a skin wound and associated axillary node

This whole body donation case (USTUR Registrant) involved two suspected PuO2 inhalation intakes, each indicated by a measurable Pu alpha activity in a single urine sample, followed about 1(1/2) y later by a puncture wound to the thumb while working in a Pu glovebox. The study is concerned with modelling simultaneously the biokinetics of deposition and retention in the respiratory tract and at the wound site; and the biokinetics of Pu subsequently transferred to other body organs, until the donor's death. Urine samples taken after the wound incident had readily measurable Pu alpha activity over the next 14 y, before dropping below the minimum detectable excretion rate (<0.4 mBq d(-1)). The Registrant died about 33 y after the wound intake, at the age of 71, from hepatocellular carcinoma with extensive metastases. At autopsy, all major soft tissue organs were harvested for analysis of their 238Pu, 239+240Pu and 241Am content. The amount of 239+240Pu retained at the wound site was 68 +/- 7 Bq (1 SD), measured by low-energy planar Ge spectrometry. A further 56.0 +/- 1.2 Bq was retained in an associated axillary lymph node, measured by radiochemistry. Simultaneous mathematical analysis (modelling) of all in vivo urinary excretion data, together with the measured lung, thoracic lymph node, wound, axillary lymph node and systemic tissue contents at death, yielded estimated intake amounts of 757 and 1504 Bq, respectively, for the first and second inhalation incidents, and 204 Bq for the total wound intake. The inhaled Pu material was highly insoluble, with an estimated long-term absorption rate from the lungs of 2 x 10(-5) d(-1). The Pu material deposited at the wound site was mixed: approximately 14% was rapidly absorbed, approximately 49% was absorbed at the rate of about 6 x 10(-5) d(-1), and the remainder ( approximately 37%) was absorbed extremely slowly (at the rate of about 5 x 10(-6) d(-1)). Thus, it was estimated that only approximately 40% of the Pu initially deposited in the wound had been absorbed systemically over the 33-y period until the donor's death. The biokinetic modelling also indicated that, in this individual case, some of the parameter values (rate constants) incorporated in the ICRP Publication 67 Pu model were up to a factor of 2 different from ICRP's recommended values (for reference man).     

Twenty-year follow-up of a Pu/Am inhalation case

In 1983 a technician inhaled a mixture of Pu/Am aerosols in an accidental situation in the hotlab of Paul Scherrer Institute (PSI). This case is of interest for long-term follow-up since the technician was relatively young (26 y) at the time of intake, no chelating agent was used to alter retention and excretion and the inhaled activity was rather high ( approximately 20 kBq of alpha emitters). The results obtained from periodic lung counts, urinary and faecal excretions as well as from some bone and liver measurements up to the year 2003 are presented. The measurements were mainly made at PSI but also at FZK Karlsruhe, Germany, and PNNL Hanford, USA. The evaluation and dose estimation of this case was done by several institutions, such as FZK, PNNL and NRPB in addition to PSI. Elements of the case were used in international biokinetic model validation programs by EURADOS/EULEP and IAEA and the (241)Am data are given as example in Annex E of the ICRP 'Guide for the Practical Application of the ICRP Human Respiratory Tract Model'. An overview is given on the various results obtained by the different institutions using their models and methods for interpretation of the measured data. While estimation of intake varies by more than an order of magnitude, final estimation of effective committed dose varies only in the range of 0.5-1.5 Sv.     

Sensitivity analysis of the urinary excretion of plutonium

The transfer parameters of a new age-related model for plutonium metabolism, which have the largest influence on urinary excretion, are determined by applying a sensitivity analysis and assuming a direct uptake into blood. Realistic cases of contamination via ingestion and inhalation were considered as well and sensitivity coefficients for non-systemic parameters were calculated. The most important parameters in relation to the path of intake are identified and the effects of the modifying factors proposed by the ICRP in the case of alteration of respiratory tract physiology are briefly examined.     

Can default ICRP f1 values be applied to determine radiation dose from the intake of diet-incorporated thorium?

Data on the daily urinary excretion of thorium (Th) was obtained from 15 non-exposed adult German subjects. A radiochemical neutron activation analysis method was developed and standardised especially for this purpose. The daily urinary excretion of 232Th was found to be in the range 1.9-14.9 microBq d(-1) with a mean (+/-SD) value of 6.5 (+/-4.3) microBq d(-1). Using this excretion value and reported data on dietary intake of Th for a similar German population, the gastrointestinal absorption factor (f1 value) proposed by the International Commission on Radiological Protection (ICRP) was tested. Although the daily excretion of 232Th observed in the present study was comparable to some of the currently reported values in certain other countries, it was higher than the excretion value calculated by applying the biokinetic model of Th proposed by ICRP for the dietary intake values. The study showed that the default ICRP values of the f1 factor for diet-incorporated Th may not be applicable.     

Use of faecal excretion function for quick estimation of initial and existing lung burden for occupational exposure control due to 239Pu for S-class of intake

Two functions namely initial lung deposition and lung retention per unit faecal excretion rate are constructed primarily based on the excretion pattern of four subjects exposed to (239)Pu. In the absence of initial clear knowledge about the class of compound inhaled, faecal to urine excretion ratio was used to infer the type of inhaled class. Trends in the urine and faecal data had suggested that the intake was due to mixed class of plutonium compound for each case. With the assumption of 1:1 mixture of plutonium M&S class inhaled compound, faecal excretion rates for only S-class intake were worked out. Uptake inferred based on their urinary data had suggested it to be of similar level within 40% of uncertainty for 5 microm particle size distribution. Data, all the four cases, were pooled for analysis citing the similar level of intake, particle size distribution, nature and pattern of work. The function obtained was tested for S-class lung retention and faecal excretion rate value. These functions are handy tool for estimating initial lung burden and lung retention value for low level of S-class plutonium intake based on subject faecal analysis data.     

Plutonium isotopes in marine sediments and some biota from the Sudanese coast of the Red Sea

Measurements of (239+240)Pu and (238)Pu were carried out on marine biota as well as on sediments from the fringing reefs area extending towards north and south (Flamingo Bay) of PortSudan harbour. The analyses were performed using radiochemical separation and alpha spectrometry. The range of the activity concentrations in marine sediments, in mBq kg(-1) dry weight, was found to be from 5.10 to 82.00 for (239+240)Pu and from 0.89 to 8.63 for (238)Pu. Corresponding activity concentrations of (239+240)Pu and (238)Pu in sediments from the harbours at PortSudan and Sawakin were 53-301 and 8.29-28.6 (PortSudan) and 163-343 and 4.7 (Sawakin), respectively. The higher values for plutonium in marine algae suggest their suitability as an indicator species for monitoring purposes. The results obtained are generally lower than those found by other studies and show that the Red Sea environment is mildly affected by plutonium contamination. Activity ratios of plutonium isotopes confirm that the existence of plutonium in the Red Sea is mainly due to atmospheric global fallout.     

A model for the transfer of alkaline earth elements to the fetus.

A biokinetic model has been developed for the transfer of calcium, strontium, barium and radium to the human fetus. For the mother, ICRP models were adapted for pregnancy to include increases in gastrointestinal absorption, urinary excretion and bone turnover rates. The fetus was modelled with blood, soft tissue and bone compartments. Fetal requirements for Ca were determined by skeletal calcification, and recyling between fetal and maternal blood was inlcluded. Daily transfer of Sr, Ba and Ra to the fetus was taken to be lower than for Ca by factors of 0.6 for Sr and 0.4 for Ba and Ra. For acute intakes in late pregnancy at 35 weeks after conception, when maximum transfer occurs, the model predicts whole-body fetus:mother concentration ratios (C(F):C(M)) of 18 for Ca, 8 for Sr and 2 for Ba and Ra, respectively. Estimates of committed equivalent doses to the red bone marrow of offspring, including in utero and postnatal dose, after maternal ingestion in late pregnancy, were greater than corresponding doses in adults by factors of 20-31 for 45Ca, 2-3 for 90Sr and 3-4 for 226Ra but slightly lower (0.8-1.9) for 133Ba.     

A generic biokinetic model for Carbon-14

The generic biokinetic model currently recommended by the International Commission on Radiological Protection (ICRP) for the treatment of systemic radiocarbon assumes uniform distribution of activity in tissues and a biological half-time of 40 d. This model is intended to generate cautiously high estimates of dose per unit intake of C-14 and, in fact, generally predicts a much higher effective dose than systemic models that have been developed on the basis of biokinetic studies of specific carbon compounds. The simplistic model formulation precludes its application as a bioassay model or adjustment to fit case-specific bioassay data. This paper proposes a new generic biokinetic model for systemic radiocarbon that is less conservative than the current ICRP model but maintains sufficient conservatism to overestimate the effective dose coefficients generated by most radiocarbon-compound-specific models. The proposed model includes two systemic pools with different biological half-times representing an initial systemic form of absorbed radiocarbon, a submodel describing the behaviour of labelled carbon dioxide produced in vivo, and three excretion pathways: breath, urine and faeces. Generic excretion rates along each path are based on multi-phase excretion curves observed in experimental studies of radiocarbons. The generic model structure is designed so that the user may adjust the level of dosimetric conservatism to fit the information at hand and may adjust parameter values for consistency with subject-specific or site-specific bioassay data.