A model for estimating the total absorbed dose to the thyroid in Swedish inhabitants following the Chernobyl Nuclear Power Plant accident: implications for existing international estimates and future model applications.

The time-integrated absorbed dose to the thyroid gland in the years after a fallout event can indicate the potential excess number of thyroid cancers among young individuals after a radionuclide release. Typical mean values of the absorbed dose to the thyroid have been calculated previously using reported data on radioiodine obtained from air sampling and dairy milk surveys in Sweden after the Chernobyl fallout, not including the contribution from 134Cs and 137Cs. We have developed a model for Swedish conditions taking these additional dose contributions into account. Our estimate of the average time-integrated absorbed dose to the thyroid, D th,tot, during the first 5 years after fallout ranged from 0.5-4.1 mGy for infants and from 0.3-3.3 mGy for adults. The contribution to D th,tot from 131I through inhalation and milk consumption varied considerably among different regions of Sweden, ranging from 9%-79% in infants, and from 4%-58% in adults. The external irradiation and exposure from the ingestion of 134,137Cs in foodstuffs accounted for the remaining contributions to D th,tot (i.e. up to 96% for adults). These large variations can be explained by the highly diverse conditions in the regions studied, such as different degrees of fractionation between wet and dry deposition, different grazing restrictions on dairy cattle, and differences in 134,137Cs transfers through food resulting from differences in the local fallout. It is our conclusion that the main contribution to D th,tot from nuclear power plant fallout in areas subjected to predominantly wet deposition will be from external exposure from ground deposition, followed by internal exposure from contaminated food containing the long-lived fission product 137Cs and the neutron-activated fission product 134Cs. The contribution from 134,137Cs to the thyroid absorbed dose should thus be taken into account in future epidemiological studies.

Detection of radioactive fragments in patients after radiological or nuclear emergencies using computed tomography and digital radiography.

A comparison has been carried out between standard-dose computed tomography, non-diagnostic computed tomography and digital radiography with respect to their suitability for detecting radioactive fragments associated with nuclear or radiological events such as debris from radiological dispersal devices. The purpose was to investigate if radiographic imaging is justified for the detection and localisation of radioactive fragments in affected patients. Fragments of uranium (U), copper (Cu), iron (Fe) and volcanic ash with effective diameters ranging from (approximately) 100 to 700 µm were selected. The fragments were positioned at two different locations on an anatomical torso phantom and images were produced with standard-dose CT, non-diagnostic CT and digital radiography. Capsules with radionuclides of (137)Cs, (60)Co and (99m)Tc were also positioned in the phantom and the effective doses were estimated for radionuclide exposures as well as for standard-dose CT, non-diagnostic CT and digital radiography. For standard-dose CT and digital radiography, U, Cu and Fe fragments were detected in sizes down to 100-180, 250-300 and 300-400 µm respectively. For the non-diagnostic CT the results were 180-250 µm (for U), 300-400 µm (for Cu) and 400-500 µm (for Fe). The effective dose from the standard-dose CT, non-diagnostic CT and digital radiography was 5.6, 1.9 and 0.76 mSv. Corresponding doses from (137)Co, (60)Co and (99m)Tc positioned at the site of fragments were in the range of 0.07-0.1, 0.32-0.45 and 0.08-0.09 mSv per MBq during 24 h. We conclude that, for a number of gamma emitters with activity levels on the order of magnitude of megabecquerel, imaging using ionising radiation can be justified since the effective dose from the radionuclides will exceed the dose from the radiological examination.

Tritium in urine from members of the general public and occupationally exposed workers in Lund, Sweden, prior to operation of the European Spallation Source.

A powerful neutron source, the European Spallation Source (ESS), is currently under construction in Lund, Sweden (~90 000 inhabitants). Levels of tritium (3H) in urine were estimated in members of the public in Lund and employees at the ESS using liquid scintillation counting, to obtain baseline levels before the start of operation of the ESS. These were compared with levels in other occupationally exposed radiation workers. Both the spallation reaction in the ESS tungsten target and the activation of various materials by the protons produced by the 5 MW linear accelerator will generate tritium, which will be released into the atmosphere mainly as tritiated water (HTO). Urinary HTO activity concentrations were determined in a total of 55 individuals belonging to four different categories: ESS employees, neighbours of the ESS, members of the general public in Lund and exposed workers from other facilities. The participants were asked to provide information on their beverage intake the day before urine sampling. The urine samples were filtered on activated charcoal and distilled before analysis. The effect of sample preparation on the isotope fractionation of urine samples was investigated by isotope ratio mass spectrometry (IRMS) of 2H/1H, which showed no influence. IRMS was also used to investigate if the ratio between the stable hydrogen isotopes (2H/1H) could provide useful data of the origin, and hence the tritium concentration, of various types of drinking water. Urinary HTO activity concentrations determined using liquid scintillation counting (LSC) were found to be below the minimum detectable activity (MDA) of 2.1 Bq⋅L-1 for most of the participants. Five of the workers actively handling organic tritiated material were found to have activity concentrations between 3.5 and 11 Bq⋅L-1, which were higher than the average value in local tap water of 1.5 ± 0.6 Bq⋅L-1. The results will be used to evaluate the radiological impact on the population from future releases of tritium resulting from the operation of the ESS.

A Bayesian method to localize lost gamma sources.

A feasibility study of a Bayesian based algorithm for orphan source localization by means of mobile gamma spectrometry is presented. The method was tested on three types of gamma sources (137Cs, 133Ba and 131I) using a HPGe detector mounted on a vehicle. Estimates on source activity and source locations were within 51% and 29% of actual values, respectively. Further studies are required to validate and develop this method for additional source-detector configurations and gamma radiation background conditions.

RETROSPECTIVE DOSIMETRY USING SALTED SNACKS AND NUTS: A FEASIBILITY STUDY.

The possibility of using ordinary household table salt for dosimetry is suggested by its high sensitivity to ionising radiation, which generates a readout of optically stimulated luminescence (OSL). However, to exploit this finding for retrospective human dosimetry, it would be needed to find salt in close proximity to the exposed individual. Finding salty snacks frequently tucked into handbags, backpacks or pockets seemed to be a possibility; these items therefore became the test materials of the present study. The aluminium or cardboard packages used to exclude the moisture that makes crisps and nuts go soft and stale also helps to retain the induced OSL signal. Therefore, different snacks, either their salt component alone or mixed with the snack, are exposed to ionising radiation and then were assessed for their dosimetric properties. The results indicate the feasibility of using some salty snacks for dosimetry, with a minimum detectable dose as low as 0.2 mGy.

Transfer of 137Cs from Chernobyl debris and nuclear weapons fallout to different Swedish population groups.

Data from measurements on the body burden of (134)Cs, (137)Cs and (40)K in various Swedish populations between 1959 and 2001 has been compiled into a national database. The compilation is a co-operation between the Departments of Radiation Physics in Malmö and Göteborg, the National Radiation Protection Authority (SSI) and the Swedish Defense Research Agency (FOI). In a previous study the effective ecological half time and the associated effective dose to various Swedish populations due to internal contamination of (134)Cs and (137)Cs have been assessed using the database. In this study values of human body burden have been combined with data on the local and regional ground deposition of fallout from nuclear weapons tests (only (137)Cs) and Chernobyl debris (both (134)Cs and (137)Cs), which have enabled estimates of the radioecological transfer in the studied populations. The assessment of the database shows that the transfer of radiocesium from Chernobyl fallout to humans varies considerably between various populations in Sweden. In terms of committed effective dose over a 70 y period from internal contamination per unit activity deposition, the general (predominantly urban) Swedish population obtains 20-30 microSv/kBq m(-2). Four categories of populations exhibit higher radioecological transfer than the general population; i.) reindeer herders ( approximately 700 microSv/kBq m(-2)), ii.) hunters in the counties dominated by forest vegetation ( approximately 100 microSv/kBq m(-2)), iii.) rural non-farming populations living in sub-arctic areas (40-150 microSv/kBq m(-2)), and iv.) farmers ( approximately 50 microSv/kBq m(-2)). Two important factors determine the aggregate transfer from ground deposition to man; i.) dietary habits (intakes of foodstuff originating from natural and semi-natural ecosystems), and ii.) inclination to follow the recommended food restriction by the authorities. The transfer to the general population is considerably lower ( approximately a factor of 3) for the Chernobyl fallout than during the 1960s and 70s, which is partly explained by a higher awareness of the pathways of radiocaesium to man both by the public and by the regulating authorities, and by the time-pattern of the nuclear weapons fallout during the growth season in Sweden.

Ecological half-time and effective dose from chernobyl debris and from nuclear weapons fallout of 137Cs as measured in different Swedish populations.

The fallout in Sweden of radiocesium from nuclear weapons tests during the 1960's (137Cs) and from the Chernobyl accident in 1986 (134Cs and 137Cs) has transferred to humans through different ecological pathways. Data from whole-body burden measurements of 134Cs, 137Cs, and 40K in various Swedish populations between 1964 and 2002 have been compiled. This database enables an evaluation of the temporal and geographical dependence of the transfer of radiocesium from ground deposition to humans and the associated absorbed dose. The body burdens of 137Cs gradually decrease after the peak values reached in 1965 from nuclear weapons fallout and in 1987 from the Chernobyl fallout, but at a varying rate depending on the population. Assuming a dual exponential decrease, a short-term component of typically 1-2 y and a long-term component of 5-10 y are found in urban populations in Sweden. Among reindeer herders and hunters the effective ecological half-time is mono-exponential with a half-time of 5-7 y. The estimated time-integrated effective dose to an individual during a period of 50 y from the Chernobyl fallout is, on average, approximately 10 mSv for reindeer herders, which is 10-100 times higher than the estimated dose received by urban populations in the three major Swedish urban areas (Malmö, Göteborg, and Stockholm).

Human metabolism of orally administered radioactive cobalt chloride.

This study investigated the human gastrointestinal uptake (f1) and subsequent whole-body retention of orally administered inorganic radioactive cobalt. Of eight adult volunteers aged between 24 and 68 years, seven were given solutions of (57)Co (T1/2 = 272 d) containing a stable cobalt carrier, and six were given carrier-free (58)Co (T1/2 = 71 d). The administered activities ranged between 25 and 103 kBq. The observed mean f1, based on 6 days accumulated urinary excretion sampling and whole-body counting, was 0.028 ± 0.0048 for carrier-free (58)Co, and 0.016 ± 0.0021 for carrier-associated (57)Co. These values were in reasonable agreement with values reported from previous studies involving a single intake of inorganic cobalt. The time pattern of the total retention (including residual cobalt in the GI tract) included a short-term component with a biological half-time of 0.71 ± 0.03 d (average ± 1 standard error of the mean for the two nuclides), an intermediate component with a mean half-time of 32 ± 8.5 d, and a long-term component (observed in two volunteers) with half-times ranging from 80 to 720 d for the two isotopes. From the present data we conclude that for the short-lived (57)Co and (58)Co, more than 95% of the internal absorbed dose was delivered within 7 days following oral intake, with a high individual variation influenced by the transit time of the unabsorbed cobalt through the gastro-intestinal tract.

Spatial variability of the dose rate from (137)Cs fallout in settlements in Russia and Belarus more than two decades after the Chernobyl accident.

Radionuclides from the 1986 Chernobyl accident were released and dispersed during a limited period of time, but under widely varying weather conditions. As a result, there was a high geographical variation in the deposited radioactive fallout per unit area over Europe, depending on the released composition of fission products and the weather during the 10 days of releases. If the plume from Chernobyl coincided with rain, then the radionuclides were unevenly distributed on the ground. However, large variations in the initial fallout also occurred locally or even on a meter scale. Over the ensuing years the initial deposition may have been altered further by different weathering processes or human activities such as agriculture, gardening, and decontamination measures. Using measurements taken more than two decades after the accident, we report on the inhomogeneous distribution of the ground deposition of the fission product (137)Cs and its influence on the dose rate 1 m above ground, on both large and small scales (10ths of km(2) - 1 m(2)), in the Gomel-Bryansk area close to the border between Belarus and Russia. The dose rate from the deposition was observed to vary by one order of magnitude depending on the size of the area considered, whether human processes were applied to the surface or not, and on location specific properties (e.g. radionuclide migration in soil).

Hair as an indicator of the body content of polonium in humans: preliminary results from study of five male volunteers.

The radionuclide (210)Po is of importance from a radiation protection view and has properties that cause special problems when attempting to determine the body content in humans. Estimates have traditionally been made from either urine and/or fecal samples, which require a time-consuming radiochemical preparation before alpha spectrometric determination. In order to find a more simple and less labor intensive method hair has been used as a bioindicator and investigated in this study. The relationship between intake and excretion in hair has been estimated in five volunteers who ingested radioactive polonium ((209)Po as a bio-tracer for (210)Po) in well determined quantities. Four of the volunteers were given 5-10 Bq (209)Po in a single intake (acute intake) and one volunteer has ingested a daily intake of 58.7 mBq (209)Po for a period of 180 d. Human hair was found to reflect the daily clearance of ingested polonium peaking at 0.001-0.01% d(-1) of the ingested amount, thereafter decreasing mono-exponentially, corresponding to a biological half-time of 10-20 days. For the case of protracted intake a mono-exponential build-up was observed with a half-time of 40 ± 5 d. In addition, after cessation of intake, a short-term component (74%) with a biological half-time of 16 ± 4 d, and a long-term component (26%) with a half-time of 93 ± 53 d were observed. It is concluded that hair can be used to detect not only the amount of ingested polonium but also whether the intake was protracted or acute.

Ecological half-time of radiocesium from Chernobyl debris and from nuclear weapons fallout as measured in a group of subjects from the south of Sweden.

From 1960 to 1980 and between 1987 and 1994 the whole-body content of 17Cs, and when possible also 134Cs, was measured in a group of subjects living in the city of Lund, Sweden (55.7 degrees N, 13.2 degrees E). The results have been analyzed to estimate the effective ecological half-time of fallout radiocesium in humans living in the area. The Lund area (The Province of Skåne) was subjected to a deposition of about 2 kBq m(-2) of pre-Chernobyl 137Cs from nuclear weapons testing and 1 kBq m(-2) of 137Cs from Chernobyl fallout in May 1986. The radiocesium from the nuclear weapons tests in the 1950's and 1960's still gave a significant contribution to the total 137Cs levels in humans in the post-Chernobyl study period (1987-1994) of about 0.4 Bq per kg body weight, which was about 10% of the peak post-Chernobyl concentration level of 137Cs (3.5-4 Bq kg(-1)) in 1987. The effective ecological half-time for 137Cs from Chernobyl was found to be 1.8 +/- 0.2 y. The aggregate transfer factor from deposition to mean activity concentration in man was estimated to be 3.6 Bq kg(-1)/kBq m(-2). These values may be compared with an effective ecological half-time of 1.3 y found in the reference group in the 1960's, and an aggregate transfer factor of 10 Bq kg(-1)/kBq m(-2). This difference is largely explained by the continuous nature of the global fallout leading to contamination on growing crops whereas the Chernobyl fallout occurred just prior to the South Swedish growing season, leading to less efficient transfer to crops and to human diet. The average committed individual effective dose (50 y) from ingested 137Cs from the Chernobyl fallout was estimated to be 0.02 mSv and from the nuclear weapons fallout (1945-1995) to be 0.20 mSv.

Urinary excretion measurements for the assessment of body burden of radiocaesium in man: differences between potassium and creatinine normalisation.

Relationships between 24-h urinary excretion and body burden of 137Cs obtained in a south-Swedish population, together with data from the literature, were applied to urine samples collected in 1994-95 from adult subjects living in Russia, in order to estimate their average body burden of 137Cs. Estimates obtained through creatinine normalisation of the 137Cs content in the Russian urine were a factor of 2 higher than estimates obtained through potassium normalisation. The accuracy of both these normalisation methods is discussed, as well as the influence of the difference in dietary habits between the two populations.

Human metabolism of radiocaesium revisited.

Three adult volunteers (2 males and 1 female) have ingested radioactive caesium in two edible forms in order to compare the gastrointestinal uptake and the subsequent short-term and long-term biokinetics. Foodstuffs made of fresh-water fish or mushrooms from regions in Sweden of high 137Cs deposition was ingested together with a 134CsCl-solution mixed with soft drinks. Whole-body countings of 134Cs and 137Cs were performed in the Malmo whole-body counter during a period of approximately 50 d prior to, and 200-300 d after the ingestion. Urine and faeces were collected the day before and up to 7-10 d after the test meals, and analysed for 134Cs and 137Cs. No significant difference in the gastrointestinal uptake fraction, f0=0.98, between caesium incorporated into a foodstuff matrix and caesium in ionic form was detected. Cumulated faecal excretion during 7 d after ingestion accounted for 2-3% of the intake. No significant difference in the long-term biological half-time was found between caesium present in a food matrix and in an aqueous solution in any of the volunteers.

A biokinetic study of 209Po in man.

Five adult volunteers participated in a biokinetic study of radioactive polonium. Portions of about 10 Bq of (209)Po were orally administrated to four of the volunteers in a single ingestion. The fifth volunteer ingested a daily amount of 53 mBq of 209Po for 243 d to study the time to achieve equilibrium between intake and excretion for protracted intakes. For the subjects ingesting single intakes of (209)Po complete sampling of urine and feces was subsequently collected the first few days upon the ingestion. The samples were processed with radiochemical extraction and analyzed with alpha spectrometry. In the study, the maximum daily excretion rates in feces were 18-50% of the ingested activity, observed within 3 d after intake. Regarding the urine excretion, the daily excretion peaked, on average, at 0.15-1% of the ingested activity within two days upon intake. These results indicate an average gastro-intestinal uptake fraction of 0.46±0.08, which agrees well with earlier biokinetic studies of polonium in man.

Using an optimised OSL single-aliquot regenerative-dose protocol for low-dose retrospective dosimetry on household salt.

The single-aliquot regenerative-dose (SAR) protocols used in retrospective dosimetry for optically stimulated luminescence measurements have been mainly developed for archaeological and geological dating of quartz and feldspar. The aim in this study is to find a read-out protocol that can generate the most reproducible signal for household salt (NaCl) at absorbed doses below 100 mGy. The relation between the reproducibility of the signal, in terms of the ratio between given absorbed dose and SAR-calculated dose, and parameters such as test-dose pre-heat temperature has been studied. It was found that a temperature of 200 °C yielded the best reproducibility in the SAR-calculated dose, which is a somewhat higher pre-heat temperature than what is used for dating of quartz.