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Thorium From Natural SourcesNew Perspectives For Incorporation Monitoring?

J.Zingler, R. HilleDepartment for Safety and Radiation Protection

Research Centre Juelich, D-52425 Juelich, Germany

AbstractThe natural content of the isotopes 228Th, 230Th and 232Th in food samples is investigated by

means of α-spectroscopy. The results are presented and evaluated. In particular, attention is paidto aspects of gender and regional fluctuations. The results are compared with earlier investigationsof food and faeces samples.

1 Introduction

Occuring in the ecosystem under normal conditions, thorium displays a high degree of radiotoxicityafter incorporation. In the case of occupational exposure to thorium, reliable knowledge of the activity intakefrom natural sources, of the metabolic behaviour and occurrence of its isotopes in excrement is indispensable forincorporation monitoring. Workers are exposed to thorium in many professional fields such as welding withthoriated electrodes, production and use of gas mantles or in nuclear power technology (particularly in high-temperature reactors).

Knowledge of the activity of thorium from natural environmental sources in human bodies and itsexcretion is too scanty to ensure the radiation protection of occupationally exposed persons, particularly if theadditional thorium concentrations are low. A long-term project including experimental work on balancing thenatural uptake and excretion of thorium will provide new data and gives information and insights in this field.

The results for the daily intake of thorium by food which is the main source of non-occupationalexposure are evaluated, paying attention to aspects of gender and regional fluctuations. The dietary intake isrelated to values in excretion samples (cf. [1]) which were collected in a time-correlated manner by the samesubjects.

2 Sample Processing

The food samples were collected as duplicate portion samples of 24 h. Food sampling wasaccompanied by the preparation of a nutrition record for the respective day. The faeces samples were collectedone day after the food sampling. The volunteers were healthy adults, living in one of the German regions ofJuelich, Dresden, Erlangen or Kiel (cf. fig. 1). These regions were chosen in order to obtain findings from abroad variation of areas of Germany. None of the volunteers was occupationally exposed to thorium. Sixvolunteers were prepared to participate in a long-term project and sampled bimonthly over a period of two years.

The activity of the isotopes 228Th, 230Th, 232Th in samples of food duplicates was determined by α-spectroscopy after radiochemical processing.

The food samples were wet-mineralized after drying and dry ashing (500℃ ). An oxalatecoprecipitation was chosen as the first enrichment step. For the final separation of thorium, an anion exchangeresin (Dowex 1x8) was applied. The samples were prepared for measurement by electrodeposition and weremeasured for about 22 hours. 229Th was used as the internal standard.

A small aliquote of the ash was incinerated at a temperature of 700 ℃ in order to calculate themineral weight of the samples.

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3 Results and Discussion

The results of the analysis of 24 h-food duplicates are reported in the following subsections. 123samples from 54 volunteers were considered. The average age of the subjects was 40 years and their averagedweight 72 kg.

3.1 The total collective

An averaged weight of 2.9 ± 0.81 kg of food was consumed per day, of which 1.9 ± 0.8 kg equallingroughly two thirds of the total consumptions were beverages. The mineral weight of the food samples was 13.4± 5.2 g.

The activity of thorium in duplicate portions of food of 24 h is shown in tab. 1. It is calculated fromthe distribution over all 123 measured samples. The sample volume permits a satisfactory statistical approach.

Table 1: Daily intake of thorium from food (mBq/d)

Isotope Median Confidence interval Minimum Maximum V (%)228Th 37.9 34.6…40.8 7.0 557 19230Th 5.5 4.6… 6.3 1.2 67 14232Th 4.1 2.8…4.9 0.5 49 17

The median of the activity was determined as 37.9 mBq/d for 228Th, 5.5 mBq/d for 230Th and 4.1mBq/d for 232Th. The confidence intervals in the table are calculated on a 90 percent basis. The range greatlyvaries over two orders of magnitude. The variation coefficient V denotes the quotient of standard deviation andmean and gives an idea about the shape of the distribution, i.e. small values of V express a nearly Gaussianshaped distribution.

Table 2 and 3 show the specific activities in food. In the first case, the activity has been related tothe wet weight of the samples, in the latter it has been related to the mineral weight.

1 Standard deviation

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Table 2: Specific activity of thorium in food (mBq/kg wet weight)

Isotope Median Confidence interval Minimum Maximum V (%)228Th 14.1 12.8…14.9 2.46 408 6230Th 2.0 1.8… 2.2 0.38 16 5232Th 1.5 1.1…1.9 0.17 18 7

Table 3: Specific activity of thorium in food (mBq/g mineral weight)

Isotope Median Confidence interval Minimum Maximum V (%)228Th 3.08 2.60…3.46 6.97e-1 33.3 10230Th 0.44 0.40…0.53 6.69e-2 2.3 8232Th 0.33 0.28…0.44 3.01e-2 3.1 12

The variation coefficient of the activity concentrations is obviously smaller than that of the activityrates (diurnal values). In particular, scattering is the lowest if related to wet weight.

The ratio of the activity of 228Th and 232Th is 9.2 and varies greatly (range: 1.8 – 50, confidenceinterval: 8.0 – 11.2). Its value is far from one as in environmental samples due to the higher mobility of 228Ra,the parent nuclide of 228Th.

3.2 Gender and regional fluctuationsThe discussion of gender dependencies and regional differences will be confined here to the 232Th

isotope.Figures 2 and 3 present the averaged values of the daily activity intake. The straight line drawn and

marked At refers to the level of the weighted mean over the total collective. The error bars represent thecharacteristic 90%–range of the sample distribution for the person or the group of persons. The values for thegoups of persons M(ale), F(emale) as well as JU (Juelich), DD (Dresden), ER (Erlangen) and KI (Kiel) havebeen calculated without the values of the long-term subjects so that there is no a priori dependence of theparameters.

Fig. 2 shows on the left the value for all male subjects (M) as well as for the male long-term subjectsA – D and on the right above F the result for all the female subjects and for the female long-term subjects A andB. The values of the male and female collectives do not differ from the result for all values. The valuestypical of the long-term subjects are covered by the characteristic range of the groups' results. It is remarkable,

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however, that the median of the values for male long-term subjects is always above the total value, whereas it isclearly below for the female long-term subjects.

Fig. 3 shows the results for the respective region groups JU, DD, ER and KI and for the associatedindividual long-term subjects A and B. In this case, too, the values typical of the long-term subjects as well asthe total value of the activity are covered by the characteristic range of the groups' results.

3.3 Comparison with excretion data and dose calculationDuring the project, the volunteers also collected 24-h faeces samples one day after food sampling.

From the measurement of 108 faeces samples, the following results were obtained: the median of the activitywas determined as 43.9 mBq/d for 228Th, 6.7 mBq/d for 230 Th and 5.4 mBq/d for 232Th (cf. [1]).

The activity excreted exceeds that taken in by 16% for 228Th, by 22% for 230 Th and even by 32% for232Th. The difference is probably of a statistical nature. Due to earlier investigations, for example, by Fisenne[2] in New York or Kolb [3] in Berlin, the amount of daily inhaled thorium is only about 30 µBq/d of eachisotope at a respiratory rate of 1.2 m3/h. This value cannot explain the elevated excretion. However, theactivity concentration of thorium in air should be examined in the four regions of this study, since fluctuationsmight be observed like in other environmental samples (food, water, soil).

A calculation of the annual effective dose according to ICRP 72 [4] from the measured ingestionvalues provides a dose of 1.4 µSv/a from ingestion of thorium from natural sources. For comparison:UNSCEAR [5] specifies a dose of 0.8 µSv/a for the same case.

3.4 Comparison of the present results with other duplicate portion studiesTab. 4 shows a comparison of the values according to tab. 1 of this study with values reported by other

authors. In this comparison, only those publications have been referred to which also addressed duplicateportion studies, in contrast to market basket studies. An exception are the citations from ICRP 23 andUNSCEAR.

Moreover, only 232Th is considered since the procedure with ICP-MS or NAA dose not allow anyother comparison.

The values scatter over one order of magnitude, those from Germany being fairly homogeneous.

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Table 4: Comparison of daily intakes of 232Th (mBq/d) between present results and published data

Activity Sampling location Sampling time2 Method Literature4.1 ± 0.3 Germany (4 places) 1997-1999 α Present result2.8 ± 0.33.5 ± 0.39.00.9 ± 0.41.1 ± 1.51.7 ± 0.82.0

GermanyGermany (Munich)India (general)U.S. (Los Alamos, NM)Japan (Mito)Japan (31 places)Ukraine

1981198619871987-19881984-19871981-19821994

α

αNAAICP-MSICP-MSICP-MSICP-MS

Frindik[6]Schieferdecker & Frindik[7]Sunta et al.[8]Shiraishi et al.[9]Shiraishi et al.[9]Shiraishi et al.[10]Shiraishi et al.[11]

123.6

W. Europe & N. AmericaWorldwide

19751993

ICRP 23 [12]UNSCEAR [5]

4 Conclusion

Activity concentrations of thorium from natural sources in food duplicates according to tab. 2 and 3scatter less than the activity rates according to tab. 1. For 232Th it was only possible to observe genderdependencies for individuals, regional fluctuations were insignificant. The analysis of excretion samples fromthe same collective of persons as the food samples in this study showed that the intake by ingestion seems to belower than the excretion. Other reasons than statistical, i. e. the amount of thorium in air, should beinvestigated. In the past, the effective dose by the incorporation of thorium by ingestion from natural sourceshas been underestimated. However, it only influences incorporation monitoring insignificantly since its valueof 1.4 µSv/a only accounts for about 7·10–5 of the dose limit for occupational exposure (20 mSv/a) proposed byICRP in Publication 60 [13].

References

[1] ZINGLER, J.: Excretion of Thorium due to Non-occupational Uptake for Adults from Different Regions ofGermany. In: Proceedings of the IRPA Regional Congress on Radiation Protection in Central Europe, heldin Budapest, Hungary, August 22-27, 1999, to be published

[2] FISENNE, I.M., PERRY, P.M.,DECKER, K.M. & KELLER, H.M.: The daily intake of 238U, 235U, 234U, 232Th,230Th, 228Th, 228Ra and 226Ra by New York City residents. Health Phys. 53(4), 1987,357

[3] KOLB, W.: Jahreszeitliche Schwankungen des Uran- und Thoriumgehalts in Aerosolen der bodennahen Luft.Fachverband für Strahlenschuts, FS-85-37-T, 1985, 11

[4] INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION: Age-dependent Doses to Members of thePublic from Intake of Radionuclides. ICRP Publication 72, Part 5 – Compilation of Ingestion andInhalation Dose Coefficients, Pergamon Press, Oxford, 1996

[5] UNITED NATIONS SCIENTIFIC COMMITTEE ON THE EFFECTS OF ATOMIC RADIATION (UNSCEAR): Sourcesand effects of ionizing radiation. New York: United Nations, 1993,67

[6] FRINDIK, O. in SCHELENZ, R.(ed.): Essentielle und toxische Inhaltsstoffe in der täglichen Gesamtnahrung.BfE Karlsruhe (BEF-R-83-02), 313ff

[7] SCHIEFERDECKER, H. & FINDIK, O.: Abschätzung der täglichen Plutoniuminkorporation vor und nachTschernobyl. In: Der Reaktorunfall in Tschernobyl —— Ergebnisse, Erfahrungen, Folgerungen. 7.Fachgespräch zur Überwachung der Umgebungsaktivität, Neuherberg (BfS—ISH), München, 16.-17.11.1987,173

[8] SUNTA, C.M., DANG, H.S. & JAISWAL, D. D.: Thorium in man and environment uptake and clearance. J.Radioanal. Nucl. Chem. 115, 1987, 149-158

[9] SHIRAISHI, K., MCINROY, J.F. & IGARASHI, Y.: Simultaneous multi-element analysis of diet samples byinductively coupled plasma mass spectrometry and inductively coupled plasma atomic emissionspectrometry. J. Nutr. Sci. Vitaminol. 36, 1990, 81-86

[10] SHIRAISHI, K., IGARASHI, Y., TAKAKU, Y., MASUDA, K., YOSHIMIZU, K., NISHIMURA, Y., HONGO, S. &YAMAGUCHI, H.: Daily intakes of 232Th and 238 U in Japanese males. Health Phys. 63, 1992, 187-191

2 Sampling time or reported time

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[11] SHIRAISHI, K., TAGAMI, K., BAN-NAI, T., YAMAMOTO, M., MURAMATSU, Y., Los, I.P., PHEDOSENKO, G.V.,KORZUN, V.N., TSIGNADOV, N.Y. & SEGEDA, I.I.: Daily intakes of 134Cs, 137Cs, 40K, 232Th and 238U inUkrainian adult males. Health Phys. 73, 1997, 814-819

[12] INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION: Report of the task group on reference man.ICRP Publication 23, Oxford, Pergamon Press, 1975

[13] INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION: 1990 Recommendations of theInternational Commission on Radiological Protection. ICRP Publication 60, Oxford, 1991