Jørn Roed

STABLE TRACER AEROSOL DEPOSITION MEASUREMENTS IN A TEST CHAMBER

Abstract In order to provide data for evaluating the exposure of domestic building occupants to particulate pollutants from a variety of sources it is necessary to carry out aerosol deposition experiments in real house environments. To complement such studies in real houses, in which many influences on aerosol deposition exist simultaneously, it is desirable to carry out aerosol deposition experiments in test chambers under controlled conditions. An aerosol test chamber, with the dimensions of a small room, is described. The chamber is fitted with air sampling ports so that aerosol deposition velocities can be inferred by monitoring tracer aerosol concentration decay. The use of neutron-activatable aerosol facilitates simultaneous surface sampling. This is not possible with conventional light-scattering aerosol detection techniques and thus provides valuable information on mass transport limited deposition. Aerosol deposition experiments have been carried out, under turbulent conditions using monodisperse neutron-activatable particles of four sizes. Results are compared with the theory of Corner and Pendlebury (1951, Proc. Phys. Soc. B64 , 645–654). Surface sampling has been used to obtain additional information from average aerosol deposition velocities. The potential of the test chamber as a means of assessing the influence of internal building surface character upon aerosol deposition velocity is illustrated.

The behaviour of Chernobyl 137Cs, 134Cs and 106Ru in undisturbed soil: implications for external radiation

Following the Chernobyl accident, deposits of 137Cs, 134Cs and 106Ru on exposed surfaces in urban areas were found to be the major contributors to the external radiation dose. The fixation of these radionuclides in soils was investigated by the determination of their vertical distribution in and through sequential extractions. Four years after the Chernobyl accident it was found that most of the caesium remained firmly fixed in the topmost 2 cm. In the deeper layers of the soil it was less strongly bound. The ruthenium was found to be less strongly bound than the caesium and had penetrated a little deeper. The study indicates the need for countermeasures if the long-term deeper. The study indicates the need for countermeasures if the long-term radiation dose to the local populace is to be reduced significantly

Weathering of radiocaesium contamination on urban streets, walls and roofs

Recent investigations in Russia have emphasised the significance of dose contributions from contamination on urban streets and roof pavings, and, typically to a lesser extent, walls in the urban environment. The crucial factor determining the magnitude of these contributions is the retention of the contamination by the different types of urban surface. Since the Chernobyl accident, a series of long-term field studies has been carried out on urban streets, walls and roofs, to examine the weathering processes of 137Cs on the various surface types. The derived time-functions are applied to estimate resultant long-term doses to inhabitants of an urban centre. The paper highlights the effect on caesium retention of surface material characteristics.

Radiation dose implications of airborne contaminant deposition to humans

Abstract— In nuclear accident consequence assessment, dose contributions from radionuclide deposition on the human body have in the past generally been either ignored or estimated on the basis of rather simple models. Recent experimental work has improved the state of knowledge of relevant processes and parameter ranges. The results presented in this paper represent a first approach to a detailed assessment of doses from radiopollutant deposition on the human body, based on contaminant-specific data. Both the dose to skin from beta-emitters and the whole-body dose from gamma-emitters on body surfaces were found to give potentially significant contributions to dose. Further, skin penetration of some contaminants could lead to significant internal doses.

Decontamination in a Russian settlement

Decontamination was carried out in an area with three houses in Novo Bobovichi, Bryansk region, Russia, in the autumn of 1995. It was demonstrated that significant reductions in the dose rate both indoor (DRF = 0.34) and outdoor (DRF = 0.20) can be achieved when a controlled cleaning is undertaken. This paper describes the decontamination work carried out and the results obtained. The roofs of the houses were swept and cleaned by special roof cleaning equipment. The soil around the houses was removed by hand while carefully monitoring the ground for residual contamination. By monitoring the decline in the dose rate during the different stages of the work the dose reducing effect of each action has been measured.

Removal of radioactive fallout from surface of soil and grassed surfaces using peelable coatings

Abstract In the wake of the Chernobyl accident it was found that the greatest contribution to the dose rate in the urban environment came from isotopes of radiocaesium deposited in open areas such as gardens and parks. Without intervention, the contamination would remain on or close to the soil surface for many decades. The efficiency and cost-effectiveness of peelable coatings as a means for removing the contamination was investigated. From small-scale tests it was found that peelable coatings based on polyvinyl alcohol and lignin were effective and inexpensive, both removing more than 90% of the contamination from dry soil.

A Nordic preparedness guide for early clean-up in radioactively contaminated residential areas

Abstract Experience gained from environmental radiological measurements, experimentation and modelling over the past decade has been applied in the development of a Nordic emergency preparedness catalogue which can be used to overview the cost–benefit aspects of an early implementation of measures to reduce long-term external radiation doses to people living in a contaminated residential complex. The catalogue contains information on the dose-reductive effect of a number of actions which are thought to be particularly well suited for the task. The requirements, restrictions, implications and cost elements of each method are listed in a format which is easy to use in a comparative analysis. The data show that if action is taken soon after an accident, it is often possible at relatively little cost to reduce the lifetime external dose to inhabitants of a contaminated area by a factor of 10. The use of the catalogue is discussed in relation to internationally applied intervention levels and concepts as well as to non-radiological aspects.

The skim and burial plough : A new implement for reclamation of radioactively contaminated land

Abstract Radionuclides accidentally released to the atmosphere in severe nuclear accidents can give rise to widespread radioactive contamination of land. If no remedial action is taken, the contamination may constitute a long-term external radiation hazard. Similarly, agricultural and dairy produce from the contaminated land may be a source of internal radiation. A newly developed agricultural implement named the ‘kim and burial’ plough has been built and tested and shown to be an effective countermeasure against both of these potential hazards but without the disadvantages of some other land reclamation methods. The new plough skims off a shallow layer of the contaminated top soil and buries it at a depth of about 45 cm without inverting the 5–45 cm horizon. The results are that radiation levels at the soil surface are greatly reduced, the contamination becomes much less available for plant uptake and in most cases there is little or no effect on soil quality. The efficacy of the new plough has been demonstrated under particularly severe field conditions.

Triple digging: a simple method for restoration of radioactively contaminated urban soil areas

External radiation doses to urban or suburban populations after the Chernobyl accident in 1986 are often dominated by contributions from soil areas near dwellings. Previously reported procedures for reclamation of contaminated soil are generally inadequate or expensive to carry out in this type of area. Therefore, a new technique has been examined in a heavily contaminated area of Russia. The triple digging method requires only the use of a shovel and can be carried out by the local populace. The field test in 1995 showed that although the contamination had migrated rather deep into the soil, triple digging can still reduce the external dose rate by as much as a factor of 10, if a large part of the open area is treated. The method has little impact on the soil fertility and has the beneficial side-effect that it reduces the uptake of contamination to plants grown in the area.

Indoor aerosol deposition studies using neutron-activatable tracers

Abstract The occupants of buildings may be exposed to a variety of particulate pollutants, which constitute a health hazard when airborne or when deposited on the internal building surfaces. In order to provide generic data for exposure evaluation, Imperial College, in collaboration with the Danish National Laboratory at Riso, have developed a method for measuring indoor aerosol deposition. Monodisperse particles are labelled with a neutron-activatable tracer, so that, when the particles are dispersed in indoor air, air filter samples can be collected for neutron activation analysis (NAA). The aerosol time decay constant can then be determined and, from that, the aerosol deposition velocity can be inferred. Full-scale experiments have been carried out in two houses: a single storey typical Danish dwelling and a centre-terraced Building Research Establishment British test house. Aerosol deposition velocities have been measured, under a variety of conditions, for two particulate sizes: 2 and 4 μm aerodynamic diameter. The experimental techniques used and the results obtained are presented. In addition, for the case of an unfurnished room, a comparison of the results obtained with the theory presented by Nazaroff and Cass (1989) is made. The results indicate that the technique is suitable for studying indoor particulate pollution cycling.