Deposition of radionuclides, their subsequent relocation in the environment and resulting implications. Report

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European Commission
radiation protection
Deposition of radionuclides, their subsequent
relocation in the environment and
resulting implications European Commission
radiation protection
Deposition of radionuclides, their subsequent
relocation in the environment and
resulting implications
J. Tschiersch, G. Frank, U. Hillmann, P. Jacob, R. Meckbach, H.G. Paretzke and F. Trautner
GSF­Forschungszentrum für Umwelt und Gesundheit
85758 Oberschleißheim
Germany
J. Roed, K.G. Andersson and C. Lange
Risø National Laboratory, MIL­114
4000 Roskilde
Denmark
Α. J. Η. Goddard and M. A. Byrne
Imperial College of Science, Technology and Medicine
London SW7 2BX
United Kingdom
J. Brown and J.A Jones
National Radiological Protection Board
Chilton, Didcot, Oxon ΟΧ11 ORQ
United Kingdom
K. Rybácek, J. Palágyi, S. Palágyi and M. Tomásek
Institute of Nuclear Physics
18086 Prague 8
Czech Republic
I. Navarcik, A. Mitro, V. Jansta, I. Datelinka and A. Cipáková
Institute of Radioecology, j.s.c
04061 Kosice
Slovak Republic
P. Zombori and I. Fehér
KFKI Atomic Energy Research Institute
1525 Budapest
Hungary
Final Report
Directorate­General
Science, Research and Development
1995 EUR 16604 EN Published by the
EUROPEAN COMMISSION
Directorate-General XIII
Telecommunications, Information Market and Exploitation of Research
L-2920 Luxembourg
LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of
the Commission is responsible for the use which might be made of the
following information
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1995
ISBN 92-827-4903-7
© ECSC-EC-EAEC, Brussels · Luxembourg, 1995
Printed in Germany Summary
The research programme "Deposition of artificial radionuclides, their subsequent relocation in the
environment and implications for radiation exposure" was initiated by the Commission of the
European Communities under the Contract FI3P-CT92-0038. The main objective of the
programme was to improve, where necessary, the models and their parameterizations used in
estimating the intensity and spatial distribution of deposited radionuclides and the total impact of
such deposits in assessments of the accidental releases of radioactivity. For that purpose several
experimental studies on specific relevant problems were performed and models were developed.
The results of the experimental programmes were reviewed and considered for incorporation into
nuclear accident consequences assessment codes. The direct link between experimental studies,
model development and the improvement of accident consequences assessment codes was one of
the main features of the project.
Field studies have been undertaken at GSF with the aim of improving the characterisation of the
wet deposition process of radionuclides by appropriate parameters. In addition to the
measurement of the particle size distribution, special emphasis was given to the simultaneous
determination of the size distribution of the rain drops. A newly developed rain drop spectrometer
(pluvio-spectrometer) was used for this purpose. It was found that the scavenging process is
strongly influenced by the rain drop size distribution. The variation of scavenging rate with particle
size is small when many of the rain drops are large. However, the g rate varies much
more with particle size when small rain drops are predominating. A comparison with values
calculated in accident consequence codes showed that, in general, at lower precipitation intensities
the scavenging efficiency is underestimated, but at higher intensities it tends to be overestimated. It
is proposed to develop a parameterization based on particle and rain drop size distribution rather
than on rain intensity only. Deposition by fog proved to be a very efficient path for deposition.
High deposition velocities have been measured particularly for large and hygroscopic aerosol
particles.
The process of aerosol deposition on indoor surfaces has implications for human exposure to
particulate contaminants of both outdoor and indoor origin. Indoor deposition rates have been
determined by monitoring the decay of tracer aerosol in three Danish and one British house by
Risø and Imperial College. Monodisperse aerosols from 0.5 to 6.5 μπι in size were labelled with
dysprosium and released in the houses. Sequential air samples were analysed by neutron
activation. The results were consistent with increasing deposition velocities for increasing particle
size and increasing degree of furnishing. An empirical formula was found by a power regression of
the deposition velocity, vd, as a function of the aerodynamic diameter of the particles, dp :
Vd = 1.23 · 10"4 · (dp in μπι)065 ms'1. Using this formula dose reduction factors were calculated as a
function of particle size.
It has long been recognised that a relationship exists between the occurrence of some adverse
health effects and the deposition of airborne contaminants onto skin. In order to extend the
database for aerosol n velocities to skin, and also to hair and clothing, comprehensive
measurements were carried out by Imperial College and Risø working in close collaboration. It
was found that deposition velocities to skin are at least one order of magnitude higher than to
internal building surfaces. Considerable variability exists in the values measured, partially
III attributable to differences in the volunteers. It is suggested that the deposition velocity of aerosol
particles of micrometer dimensions to skin are in the order of 10"2 m/s. Tests with heated and
unheated aluminium cylinders indicate that the measured deposition velocity was higher to the
heated surface. This finding demonstrates that body heat may be responsible for a significant
proportion of the differences between the deposition velocity values observed to human and inert
subjects. The volunteers differ in their arm-hair growth. More aerosol particles were deposited on
a hairy arm than on a non-hairy arm, indicating the strong influence of the surface roughness on
aerosol deposition velocities. Hair deposition has not been seen to differ significantly from skin
deposition; however, a satisfactory protocol for determining hair surface is still under
development. A subject which must also be considered when assessing radiological risk due to
aerosol deposition on the body is the efficiency of removal of the deposited material through
active decontamination procedures. The removal efficiency for silica particles was found to be
higher by washing than by wiping. The mean decontamination efficiency was about 52%.
Following caesium deposition from the atmosphere to the ground, fields with undisturbed caesium
depth distributions are a major source for the external exposure of the population. Two methods
for the determination of the gamma dose rate in air due to caesium distributions in soil have been
applied by GSF, Risø, INP Prague and IR Kosice. The first method is soil sampling and laboratory
measurements, the second method is the peak-to-valley evaluation of in situ gamma-ray spectra, a
method that has been developed in the framework of this project. From the obtained results,
gamma dose rates in air due to caesium in the soil have been calculated. These data have been
analytically approximated to allow an easy use of the results in external dose assessment models.
The new analytical approximations were compared to former approximations utilising
measurement results from Southern Bavaria after the Chernobyl accident and from the New York
area after the atmospheric weapons test fallout. The new approach gives a better approximation to
the measured data at medium and long times after deposition.
Weathering of radioactive substances in urban areas was studied, and the calculation of location
factors in urban dry deposition scenarios was performed by Risø and GSF. The time series of
measurements of weathering on urban surfaces in the town of Gävle (Sweden) now spans over
almost a decade and has provided a reference against which existing models can be validated.
Weathering measurements were also made in other parts of Europe (Bavaria and Russia) and the
results were found to agree very well. The weathering effect on radiocaesium on paved surfaces in
Bavaria since the Chernobyl accident has been followed continually from the early phase.
Location factors were calculated for various dry deposition scenarios in houses offering different
shielding, partly based on experimental data, and it was established that indoor deposition may
have great influence on the location factors. Other calculations based on in situ gamma
spectrometry in Bavarian environments (mainly affected by wet deposition) gave location factors
which were comparable to those found by the first approach for a medium shielded house.
The behaviour of long-lived radionuclides in the atmosphere, (their concentration in ground level
air, wet and dry deposition and resuspension) was studied. A co