Ecological Half-time of Radiocaesium From the Chernobyl Accident and from Nuclear Weapons Fallout as Measured in a South Swedish Population

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1 Ecological Half-time of Radiocaesium From the Chernobyl Accident and from Nuclear Weapons Fallout as Measured in a South Swedish Population C.L. Rääf, B. Hemdal, S. Mattsson Dept. of Radiation Physics, Malmö university hospital, SE Malmö Sweden. INTRODUCTION The aim of this study was to investigate the time variation of the whole-body burden levels of 137 Cs in a south Swedish population after the Chernobyl fallout in 1986 (1, 2) and to compare the results with data obtained from the same reference group during the 1960s and 70s (3, 4, 5), as well as with contemporary studies on other Swedish populations. Radiocaesium from the nuclear weapons fallout consisted almost solely of 137 Cs (6) and it was estimated that the intake of dairy and beef products accounted for a large part of the 137 Cs intake in the Lund reference group (4), which is in accordance with international observations (7). Between 1960 and 1980 the cumulated deposition of fallout 137 Cs from atmospheric bomb tests was about 2 kbq m -2 in the Lund region (8, 9), which is in reasonable agreement with results from similar latitudes in the northern hemisphere except for areas with very high precipitation (10, 11). The ground deposition levels from the Chernobyl fallout in the province of Skåne was typically 1 kbq m -2 (9), which is low in comparison with most other regions in Sweden (11). Furthermore the aggregate transfer of caesium from soil deposition to man through the ingestion of contaminated foodstuffs was calculated based on detailed deposition data in the region (9) in combination with the results of in vivo concentration in the control group. METHODS Background The Lund reference group was formed in 1960 (3, 4), and originally consisted of a small group of some 3 to 17 individuals who were subject to in vivo determinations of the body burden of 137 Cs and 40 K from 1960 to At the beginning of 1964, the group was enlarged to 34 urban inhabitants of Lund and its surroundings, of which a majority worked in one specific factory in Lund (5). Many of these 34 subjects remained in the group and were investigated at least once a year between 1964 and 1980 (except for 1969 and 1972) with a whole-body counter at the Department of Radiation Physics in Lund, and between 1987 and 1994 at the Department of Radiation Physics in Malmö. The duration of the period of investigation is thus longer than 30 years and consists of 34 different measuring occasions. The composition of the group by age and gender is given in Table 1. The experimental design of the study between 1987 and 1994 has been described in Rääf et al., 2000 (2). In this evaluation of data, the effective ecological half-time, T ½,eff, in the Lund reference group, was determined from the time dependence of the whole-body content values based on 137 Cs determination in all the subjects in the control group. Table 1 Composition of the Lund Reference Group by age [years] and gender at the beginning and end of the two study periods. Period No. of participants Age (Mean ± 1 S.D.) Females Males Both Females Males Both Pre-Chernobyl Beginning: ± ± ±12.4 End: ± ± ±13.9 Post-Chernobyl Beginning: ± ± ±13.5 End: ± ± ±12.2 Ecological half-time of whole-body content of caesium in man Earlier studies (5, 12, 13) have shown that the internal contamination of radiocaesium following a 1

2 deposition is characterised by an initial build-up phase, the length of which depends on the season during which the fallout takes place and on the dietary habits of the population. After the Chernobyl fallout in Sweden, the build-up phase was typically 1 to 1.5 years (12, 14, 13). The maximum of the physical-decay-corrected contamination level is then followed by an exponential decrease, provided that the population is not exposed to additional deposition or that no general change in dietary habits takes place. The atmospheric nuclear weapons tests at the end of the 1950s and beginning of the 1960s resulted in a continuous deposition of radiocaesium in the Northern Hemisphere, lasting many years. Due to the increased mixing of the stratosphere with lower atmospheric layers during springtime in the Northern Hemisphere, the so-called spring injection, a certain increase in the fallout rate was observed each spring and summer (15). This type of deposition extended the time taken for the internal contamination levels in humans to reach their maxima and delayed the reduction in radiocaesium body burdens. In the case of the Lund reference group, the maximum values were not obtained until , at least one year after the peak annual deposition and the SALT II agreement in 1963, which virtually ended the era of large-scale atmospheric nuclear weapons testing. From 1965, the average contamination levels declined in a more or less exponential fashion, although significant additional continuous deposition from the atmosphere occurred more than a decade after the SALT II agreement due to the retention of the released fission products in the atmospheric layer of several years (16, 10, 15). The population in Lund was thus continuously exposed to contaminated foodstuffs all through the 1970s. An ecological half-time, T ½,eco (Eqs 1 2), can be defined for the Lund reference group in terms of the gradual decrease in in-vivo caesium concentration levels, and was estimated to be about 1.5 years (14) until monitoring was discontinued in At that time, the contamination levels had, on average, fallen to at least one tenth of the peak values in the middle of the 1960s. ln 2 T ½,eco = Slope of logarithm of whole-body concentration (corrected for physical decay) vs. time (1) Expressed in mathematical terms, the effective and ecological half-times are related according to: = + (2) T½, eff T½, eco T½, phys where T ½,phys is the physical half-life of the radionuclide. In the present work, T ½,eff denotes the elimination rate of the whole-body concentration in man in accordance with the definition of transfer factors and kinetic models in the literature (17, 10). An exponential retention function was fitted to the mean whole-body concentration values, a pop (t), from the post-chernobyl in-vivo measurements between 1987 and 1994, by the aid of the statistical software, STATISTICA 6.0 (18). The values of T ½,eco could then be obtained with a statistical confidence interval, enabling the prediction of the future activity concentration levels in the Lund reference group. The presence of pre- Chernobyl 137 Cs during the post-chernobyl study period was corrected for, since the Chernobyl fallout occurred at a time when pre-chernobyl radiocaesium remained in the environment and was available to humans via similar food chains as Chernobyl caesium. Using data from the study conducted between 1964 and 1980, a fitted exponential expression of a(t) pop was used as a baseline, on which the Chernobyl 137 Cs was superposed to yield the total average body burden of 137 Cs per kg body weight. Thus (Eq. 3): a pop,chernobyl (t) = a pop,post-chernobyl (t) a pop,pre-chernobyl (t) (3) Extrapolating data from the study between 1964 and 1980, it could be assumed that, on average, up to 0.4 Bq/kg of the activity concentration of 137 C in the individuals of the Lund reference group in spring 1987 could be attributed to pre-chernobyl radiocaesium. This value is, however, uncertain, mainly due to a two-component exponential decrease in the mean body burden concentration levels between 1965 and 1980, of which the longterm component was difficult to determine by non-linear estimation methods. Aggregate transfer factor The long-term transfer of caesium into man can be described by using the ratio between the timeintegrated activity concentration, based on the mathematical fit to the measured in vivo 137 Cs concentration in the control group, a pop (t), divided by the time-integrated number of disintegrations per m 2 of 137 Cs deposited on ground, A dep (t), from a specific fallout event. That is (Eq. 4): 2

3 T ag,int = t0 t0 a A pop tot () t dt () t dt (4) or (Eq. 5) No. of disintegrations per kg body weight of a given radionuclide that will occur in individuals due to a single or continuous deposition on a certain geographical area T ag,int = No. of disintegrations of deposited activity per m 2 during a given time period from a single or continuous deposition in that geographical area (5) where λ phys is the physical disintegration constant (=ln2/t ½,phys ) for a given radionuclide and A tot (t) [Bq m -2 ] is the total activity per m 2 ground at time t. A similar procedure as Eq. 4 has previously been suggested by UNSCEAR, 1977 (10). The principal difference between the UNSCEAR procedure and that used in this work, is that the successive physical decay of 137 Cs from each annual deposition cumulated during the time interval of interest is taken into account, in order to obtain the cumulated number of disintegrations per m 2 of soil during a given period of time. The fraction of the substance that is available to root uptake by crops and thus to the long-term transfer through the ecosystem into man, is partly related to the cumulated number of disintegrations in the upper layers of soil. The dimension of the time-aggregated transfer factor then becomes [(Bq kg -1 )/kbq m -2 ]. The advantage of this somewhat more cumbersome procedure is that the time pattern of the fallout rate is taken into account, which is useful when comparing two such different fallout patterns as the fallout from the nuclear weapons tests and Chernobyl. Calculation of committed effective dose, E(t 0 <t<t 0 +50) The effective dose from radioactive caesium, which is assumed to be distributed homogeneously in the human body (19), can be calculated using conversion factors, S (µsv/h / kbq), taken from the literature (20), where a total body activity of 1 kbq 137 Cs corresponds to a mean absorbed dose rate of 35 µsv/year, for an adult male weighting 70 kg (21). Using the mathematical fit to the measured data as a prognostic tool for the future levels of radiocaesium body burden, the time integral of the average whole-body activity of caesium, A pop (t), can be related to the committed effective dose to an average adult individual in the population, <D pop >. t = year < D ( t < t < t ) >= S A ( t) dt pop 0 1 t= t0 pop (6) 22); The average dose rate to the population can also be estimated through a more complex relationship (20, 134 Et ( < t< t; Cs) = = w A ( t) dt = w a ( t) dt t 0 pop, Cs Et ( < t< t; Cs) = t 0 pop, Cs = w A ( t) dt = w a ( t) dt t 0 pop, Cs t 0 pop, Cs 137 (7) (8) where E(t 0 <t<t 1 ) is given in msv if the unit of t is years, w is the individual body-weight and a pop (t) (=A pop (t)/w), is the fitted function of the mean whole-body concentration of 137 Cs or 134 Cs. The most straightforward method of obtaining an average committed dose to the individuals in the Lund reference group was by applying the average weight of all the individuals during the two study periods and multiply the weight by the time-integrated average whole-body concentration of caesium. Provided that a single exponential decline can be fitted to the observed a(t), Eq. 7 can be integrated over a time interval extending up to 3

4 50 years, which is a time span recommended by the ICRP (23) for the calculation of the committed effective dose for adults, E(0<t<50 years). The following calculations were carried out in order to obtain the committed dose (0<t<50 years) contribution from 137 Cs in msv to the Lund reference group (t in unit of years) as well as the time-integrated aggregate transfer factor; for pre-chernobyl 137 Cs the integration time was set from 1945 to 1995, thus starting with the year of the first nuclear detonations. Pre-Chernobyl: E(t 0 = 1945<t<1995; Cs) Lund = w a, 137 () t dt Lund pop Cs (9) For Chernobyl 137 Cs and 134 Cs, the average individual committed dose is given by: Post-Chernobyl: E(t 0 =1986<t<2036; Cs) Lund = w a, 137 () t dt Lund 2036 pop Cs E(t 0 =1986<t<2036; Cs) Lund = w a, 134 () t dt Lund pop Cs 1986 (10) The effect of pre-chernobyl 137 Cs in the second study period was estimated by applying the mathematical fit to the background corrected values of a pop from Eq. 3. RESULTS Activity concentration of 137 Cs and 134 Cs Whole-body concentrations of pre-chernobyl 137 Cs in the Lund reference group reached their peak values of about 12 Bq kg -1 at the beginning of 1965, after which an exponential decline was observed with a half-time of 1.3±0.2 (1 SE) y. A slower decline was observed at the end of the 1960s and the 1970s (Table 2). Using annual deposition data from Ljungbyhed, 43 km north of the city of Lund (9), it was observed that the in vivo levels of 137 Cs in the Lund reference group exhibited a one year delay in the time-pattern relative to that of the deposition in the Lund region (Fig 1). The mean concentration of Chernobyl 137 Cs in vivo in the Lund reference group reached peak values in spring 1987 of (± 1 SE) 3.7±0.7 Bq kg -1 for 137 Cs and 1.5±0.3 Bq kg -1 for 134 Cs as average values for both sexes. Ecological half-time of whole-body concentration of caesium in man The corresponding ecological half-time for 137 Cs during the pre- and post-chernobyl study periods is given in Table 2. By fitting the parameters of an exponentially decreasing function to the whole-body concentration data from the Lund reference group between 1987 and 1994, the ecological half-time, T ½,eco (±1 SE), was found to be 2.6±0.2 years and the effective ecological half-time was found to be 2.4±0.2 years. When these data were corrected for the presence of pre-chernobyl 137 Cs, the value for T ½,eco (±1 SE) was found to be 1.8±0.1 y (Table 2). The ecological half-time of 134 Cs between 1987 and 1994 was found to be 2.1±0.2 years, when an exponential fit was used. The effective ecological half-time calculated from the exponential fit became 1.04±0.08 years. The corresponding value of the effective ecological half-time for 137 Cs found after the peak in vivo content values in late 1964 was 1.3±0.2 years (Table 2). From the non-linear fit of the data a second smaller component was found with a longer ecological half-time (18±50 years for both sexes; Table2). Results concerning the ecological half-time for 137 Cs from other Swedish populations groups exhibit values between 3 and 12 years (11). 4

5 Table 2 T ½,eff,conc [years] for 137 Cs in the Lund reference group (MV±1 SE) for the two study periods. Study period Pre-Chernobyl Post-Chernobyl Short-term component Long-term component Females 1.2 ± 0.3 (13 ± 20) 1.8 ± 0.2 Males 1.3 ± 0.3 (27 ± 130) 1.8 ± 0.1 Both 1.3 ± 0.2 (18 ± 50) 1.8 ± 0.2 Fig 1 Mean whole-body concentration of 137 Cs in the Lund reference group between 1960 and 1994 and mean annual deposition of 137 Cs between 1962 and 1980 at Ljungbyhed (43 km north of Lund; solid line) and Denmark (dotted line). This figure illustrates the 1-year delay between peak deposition rate and peak levels of 137 Cs in man. Annual 137 Cs deposition / Bq m -2 y Cs conc. in the LRG (Right hand scale) Cs in vivo conc. / Bq kg -1 Time-integrated aggregate transfer factor of 137 Cs in the Lund reference group The time-integrated aggregate transfer factor for post-chernobyl 137 Cs between 1986 and 2007 was estimated to be 0.6 Bq kg -1 / Bq m -2 (both sexes), with no background correction, and 0.4 Bq kg -1 /Bq m -2 when adjusted for pre-chernobyl 137 Cs. Using the second-order exponential fit to the average activity concentration in the Lund reference group between 1965 and 1980, the time-integrated transfer factor became a factor of ten times higher than the uncorrected value for post-chernobyl 137 Cs (Table 3). 5

6 Table 3 Time-integrated aggregate transfer factor in [Bq kg -1 / kbq m -2 ] for 137 Cs based on prognosis from fitted data of the mean in vivo concentration in the Lund reference group and deposition data from Isaksson, 1997(9). Period Females Males Both Pre-Chernobyl ( ) Post-Chernobyl ( ) Calculation of committed effective dose, E(t 0 <t<t 0 +50) The average weight (± 1 SD) of the individuals in the Lund reference group during the post-chernobyl study (males and females together) was 71.0±10.6 kg. In the pre-chernobyl study the average weight for both sexes was found to be 69.3 kg. The estimated individual average committed effective dose from 137 Cs between 1945 and 1995 was 0.20 msv. The fraction of pre-1963 integrated activity required to calculate E(1945<t<1995) was found to be approximately 0.5. The resulting average committed effective dose from internal contamination to the individuals in the Lund reference group after Chernobyl, E(1986<t<2036) was thus found to be 0.033±0.008 (1 SE) msv from 137 Cs and 0.007±0.005 (1 SE) msv from 134 Cs. Correcting for pre-chernobyl 137 Cs present after 1986 gave a committed effective dose of msv from Chernobyl 137 Cs. CONCLUSION The large difference in the ecological behaviour of nuclear weapon s fallout and Chernobyl caesium found in this study is due to several reasons. The continuous nature of the pre-chernobyl fall-out resulted in significant direct contamination on growing crops, whereas the Chernobyl deposition took place just prior to the Swedish growing season. The main pathway of caesium from feed to man is by direct contamination of growing crops, rather than then root uptake of caesium in soil into the edible parts (24). Other factors, such as the difference between the chemical properties of the pre-chernobyl and Chernobyl 137 Cs, could have importance in the fixation of the radionuclide in the soil and the root uptake rate by growing crops, although one Nordic study suggested that the chemical specifiation of the fallout radiocaesium was of less importance for the aggregate transfer than the chemical and physical composition of the soil (25). Furthermore, human behaviour, in terms of countermeasures taken after the Chernobyl accident, might have led to greater consumer awareness of contaminated foodstuffs, which in turn could have reduced the total transfer rate of 137 Cs to normal consumers. To summarise the conclusions; No significant difference was found between the activity concentration of 137 Cs in males and females between 1987 and 1994, whereas males in the Lund reference group had a significantly higher 137 Cs concentration during the 1960s and 70s. This, together with data on the equivalent biological half-time of caesium (2), indicate that the difference observed between the sexes in the uptake of radiocaesium decreases with age. The effective ecological half-time, T ½,eff, for 137 Cs during the post-chernobyl study period, corrected for the presence of pre-chernobyl 137 Cs, was still found to be significantly longer (p<0.05) than in the years after No significant difference at the 95% confidence level was found between the effective ecological half-time, T ½,eff, for 137 Cs for males and females between 1987 and The use of potassium normalisation of the in vivo 137 Cs concentration levels yielded significantly smaller differences in terms of sex for normalised caesium body content in the Lund reference group over both study periods. Using the Cs/K ratio instead of 137 Cs concentration values during the pre-chernobyl study period on the time-integrated transfer factor appeared to decrease the difference between males and females. This trend is not consistent with the post-chernobyl study, where the use of the Cs/K ratio yielded greater differences between the sexes. Post-Chernobyl values for ecological half-time and aggregate transfer factor agree within a factor of two with those found in other Swedish studies (excluding the Sami populations). The estimated effects of pre-chernobyl 137 Cs in the post-chernobyl period were apparent both in the calculation of the average committed dose contribution to the Lund reference group, E(t 0 <t<50y), as well as in the calculations of the aggregate transfer factor, T ag. Calculation of the time-integrated accumulated transfer factor of fallout 137 Cs, T ag,int, shows that the nuclear weapons fallout 137 Cs was transferred at least 10 times more efficiently to humans than the Chernobyl 137 Cs. 6

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