[en] Automated monitors for the determination of airborne radioactivity are weidely in use as 'early warning' systems for nuclear emergencies, but seldom applied in quantitative studies to the appearance of natural radioactivity (especially radon and short-lived decay-products). This report describes how a specific type of airborne gross-α/β monitor (FAG FHT59S), can be applied for this purpose. The main focus is on the performance of the applied algorithm which recalculates actual air activity concentrations from primary measurements, especially for circumstances contradicting assumptions made by the manufacturer. Furthermore, a complete uncertainty analysis has been carried out. As a result, a formula has been derived which converts, within a reasonable degree of uncertainty, the monitor recordings to the equilibrium-equivalent decay-product concentration (EEDC) of Rn-222. NRM-data of Rn-222 (progeny) obtained in this way agree with results of previous radon surveys carried out in the Netherlands. (orig.)

[en] The accident at the Chernobyl nuclear power station in April 1986 and its consequences were reason for the Dutch government to evaluate and improve the facilities and the preparedness for nuclear emergency management in the Netherlands. The results of the evaluation have been elaborated in operational terms in the National Plan for Nuclear Emergency Planning and Response (EPR). During an accident with radioactive material the Technical Information Group (TIG) coordinates the measuring activities of the so-called Support Centres. According to the EPR, measuring activities of Support Centre RIVM are focussed on the collection and processing of data on emissions, concentrations, depositions and radiation doses from soil and air. This report describes the measuring strategy of RIVM for nuclear emergencies. The measuring strategy and the measuring plan, the latter deduced from the measuring strategy, concentrate on explicit answers to the following central questions: what has to be measured, by whom, where, when and how, and why? The demands of the TIG and the specification of tasks and operational facilities of Support Centre RIVM are considered as starting-points, limiting conditions and constraints for the measuring strategy. These items are converted to explicit choices for the measuring strategy and the default measuring plan. This report further includes a list of contacts of Support Centre RIVM with other (research) institutes, inside and outside the Netherlands, which may be relevant during a nuclear emergency. 3 figs., 2 tabs., 22 refs

[en] After the first three introductory chapters (background information in chapter 1, applied quantities and units in chapter 2, and a general overview of source-dependent radiation doses in the Netherlands and dose distributions among the Dutch population in chapter 3), chapter 4 starts with a description of all relevant sources and processes contributing to the (natural) background and causing its variations in space and time. Chapter 5 discusses the National Radioactivity Monitoring network, evaluates the possibilities (and limits) of the NRM for the present purpose and highlights some applications of the results of this study. An analysis of ambient dose-rate data, evaluated in Chapter 6, aims at a complete and coherent evaluation of all relevant sources and processes contributing to (variations in) outdoor external radiation levels. The main goals were to provide simple (phenomenological) expressions for the assessment of the temporal and spatial variations in the natural background, including best estimates and uncertainty ranges for the applied parameters, and for the probability that certain radiation levels occur. Some applications of the results are demonstrated. Chapter 7 presents the results of an analysis of natural airborne radioactivity data. The aim was to examine the dynamic behaviour of ²²²Rn and short-lived decay products in the Dutch outdoor environment and to reveal the influence of various meteorological parameters. The activity concentration of ²²²Rn progeny in the outdoor environment is determined by a complex process of exhalation, horizontal and vertical dispersion, radioactive decay, attachment to aerosols and deposition on the ground surface of attached and unattached decay products. Many parameters are involved in the assessment of the concentration of ²²²Rn (progeny) on a certain spot and its variations in time. This process is modelled in Chapter 8. The final conclusions of this study are summarised in Chapter 9, which includes recommendations for further research. 85 figs., 16 tabs., 164 refs., 2 appendices

[en] In the Netherlands and many other countries research in the field of natural radioactivity is focused on the prevention of radon in the indoor environment. However, also the occurrence of natural radioactivity in the outdoor environment is of interest. Therefore, data of the Dutch National Radioactivity Monitoring (NRM) are analyzed. Applied quantities and units selected from the perspective of radiation protection are explained in Chapter 2. Chapter 3 presents a general overview of source-dependent radiation doses in the Netherlands and dose distributions among the Dutch population. Chapter 4 starts with a description of all relevant sources and processes contributing to the (natural) background and causing its variations in space and time. Chapter 5 discusses the NRM network, evaluates the possibilities (and limits) of the NRM for the present purpose and highlights some applications of the results of this study. An analysis of ambient dose-rate data, evaluated in Chapter 6, aims at a complete and coherent evaluation of all relevant sources and processes contributing to (variations in) outdoor external radiation levels. Some applications of the results are demonstrated. Chapter 7 presents the results of an analysis of natural airborne radioactivity data. The aim was to examine the dynamic behaviour of ²²²Rn and short-lived decay products in the Dutch outdoor environment and to reveal the influence of various meteorological parameters. The activity concentration of ²²²Rn progeny in the outdoor environment is determined by a complex process of exhalation, horizontal and vertical dispersion, radioactive decay, attachment to aerosols and deposition on the ground surface of attached and unattached decay products. Many parameters are involved in the assessment of the concentration of ²²²Rn (progeny) on a certain spot and its variations in time. This process is modelled in Chapter 8. figs., tabs., appendices, refs

[en] In 1994, the Sector Substances and Risks of RIVM decided to strengthen strategically its research into risk assessment methodology. In this report the research area of human exposure assessment at the RIVM is outlined. A representative selection of human exposure assessment models for both chemical substances and radiation is analysed with regard to aim, principle, degree of model analyses and values of default parameter. For comparison, a model to assess human exposure to micro-organisms is included as well. All models are operational or nearly so in the production of risk assessments in the Sector Substances and Risks and also in the Sectors Public Health Research and Environmental Research. The models discussed all have a defined area of application and support risk management. The research areas of exposure assessment for substances and radiation are compared and many methodological analogies are apparent. However, at the level of models and parameters an in-depth analysis of analogies and explained or unexplained differences is lacking. A detailed examination of organisation aspects and RIVM-models for human exposure assessment learns that all relevant areas of interest are covered. For all routes of exposure the reach of the actual risk and exposure assessment methodology is large. A more uniform coverage is attained for radiation than for chemical substances. For both areas the estimation and registration of emissions can be improved. The development of risk assessment systems and related harmonisation proJects have already attention for many years (e.g. CSOIL, USES, RIBRON). It is concluded that the RIVM requires a broad, up-to-date range of instruments for exposure assessment and active involvement in all kinds of national and international relevant networks. The RIVM should also remain involved in the development and evaluation of methodology and in projects aiming at harmonisation. 2 figs., 9 tabs., 64 refs

[en] The title LMR network, which started March 1990, was succeeded by the NMR, early 1996. In this report the performance of the LMR over the years 1993-1995 is evaluated and a data review is presented. Apart from the relocation of two of the 58 measuring sites, the general specifications of the LMR have not been changed in the period 1993-1995. However, the nuclide-specific air activity monitor for the detection of aerosol-bound γ-emitters broke down in 1993. A potential successor for this complex monitor was rejected due to frequent failure during a test period. Another monitor type is currently being examined. The technical properties of the two field monitors used, the Bitt Technology RS02/RM10E proportional counter device and the FAG FHT59S airborne gross-α/β-activity monitor, are described. Measurements of external radiation, expressed in ambient dose-equivalent rate, H^*(10), contain a systematic error for the correction of which a conversion formula is presented. The natural α-activity concentration as determined by the aerosol monitor is (during non-emergency situations) proportional to the equilibrium-equivalent decay product concentration of ²²²Rn progeny in air. Nuclear accidents with any impact for the Netherlands did not occur. However, automatic warning messages indicating that radiological data were recorded above one of the warning levels were received many times. Some of these warnings were 'real' due to natural causes or industrial activities. Most of the warnings, however, were false, for instance, due to malfunction of equipment. In 1994, but especially in 1995, the number of 'false' messages was relatively large, partly due to the ageing of the instruments and partly due to the extreme weather conditions of 1995. Nevertheless, the availability of LMR data in the period 1993-1995 was higher than in previous years: dose-rate on average some 96%, air activity approximately 91%. The monitoring results of the LMR have increased the knowledge of the variations in the natural background radiation and the parameters determining it. 14 figs., 4 tabs., 3 appendices, 48 refs

[en] Several sources and processes contribute to the natural radiation background level, causing significant fluctuations in time. Quantified knowledge on the probability of these variations is desirable for many reasons, e.g. to discriminate between natural and human-induced factors or to support the management of nuclear emergency networks. Frequency distributions of ambient dose rate, as observed by the Dutch National Radioactivity Monitoring Network over the period 1990-1994, have been explained through the joint contribution of five time-varying sources, including counting statistics. Normalised probability density functions (with a normal shape for noise, terrestrial and cosmogenic radiation, and an exponential shape for airborne and deposited radioactivity from radon progeny) add up to one joint probability density function, which agrees with long-term data distributions over four orders of magnitude. This comparison yields parameter values describing the probable impact of rainout and washout of radon progeny and the typical fluctuation band of terrestrial radiation as observed in the Netherlands. (Author)

[en] Airborne radioactivity monitors put up in a network as a warning system in case of nuclear accidents were used to gain insight in the temporal variations of ²²²Rn and its decay products. Average equilibrium equivalent decay product concentrations (EEDCs) over a period of five years for stations across the country range from 1 to 3 Bq.m^-3. Hourly averaged concentrations show a diurnal variation at all stations. The amplitude of this variation is maximal in summer and minimal or non-existent in winter. Superimposed is a seasonal variation with a maximum in late autumn and a minimum in spring. Wind velocity appears to be the major variable controlling the variation in EEDC in the outdoor environment of the Netherlands. Humidity seems to be correlated with the EEDC of ²²²Rn, probably because, due to plateout, a better equilibrium between ²²²Rn and its decay products is installed. Reduced EEDC during and shortly after rainfall seems to indicate a dilution of ²²²Rn in ground surface air due to enhanced vertical mixing below the rain cloud. (Author)

[en] The significant variations in time exhibited by background radiation hinders a sensitive recognition of human-induced factors. A comprehensive study in the Netherlands has examined the influence of the various natural processes on the natural background using six years data from the Dutch nuclear emergency network. Results presented concentrate on temporal variations in ambient dose-equivalent rate, H*(10), and have led to simple expressions to model the ambient dose rate using a limited set of readily available parameters, i.e. air pressure, deposition rate and equilibrium equivalent decay product concentration of ²²²Rn, EEDC. Best values and uncertainty ranges of the applied parameters are reported. Remaining variations, e.g. due to variations in the cosmic radiation intensity and the radon soil profile, are shown to be small in the Netherlands, with one exception when the cosmogenic dose rate at sea level was decreased for a period of months due to a global deflection of the earth's magnetic field in the summer of 1991. The resulting compensation method for the natural ambient dose rate enables sensitive detection of anomalies, supporting the surveillance of nuclear installations and the management of nuclear emergency networks. (Author)

[en] A new Euratom directive demands that Member States establish a national action plan for indoor radon. Important requirements are a national reference level for the radon concentration in dwellings, actions to identify dwellings with radon concentrations that might exceed this reference level and the encouragement of appropriate measures to reduce the radon concentrations in dwellings where these are high. This paper provides ingredients and recommendations for a national action plan for radon in dwellings, applicable to the Netherlands. The approach presented here, which may serve as a model for other countries or regions with a comparatively favourable indoor radon situation, is based on the analysis of radon data from a national survey in more than 2500 Dutch dwellings, built since 1930. The annual average activity concentration of radon in dwellings in the Netherlands equals 15.6 ± 0.3 Bq m⁻³. The 50th and 95th percentiles were found to be 12.2 and 38.0 Bq m⁻³, respectively. In 0.4 per cent of the dwellings we found values above 100 Bq m⁻³. Radon concentrations showed correlations with type of dwelling, year of construction, ventilation system, soil type and smoking behaviour of inhabitants. The survey data suggest that it is feasible for the Netherlands to adopt a national reference level for radon in dwellings of 100 Bq m⁻³, in line with recommendations by WHO and ICRP. We were able to predict dwellings with a moderate probability for radon concentrations above 100 Bq m⁻³ by applying a combination of three selection criteria: location, type of dwelling and manner of ventilation. Of the existing 6.2 million dwellings in the Netherlands (built since 1930), approximately 23–24 thousand are suspected to exceed this level. Some 80% of these are found in the group of naturally ventilated single-family dwellings in either the southern part of Limburg (approx. 13 thousand) or the Meuse-Rhine-Waal river delta (approx. six thousand). This selected group of dwellings represents 7% of the housing stock. In contrast to many other countries in Europe and elsewhere, radon concentrations in dwellings above 200 Bq m⁻³ are very rare in the Netherlands. As a result, relatively simple and inexpensive measures in existing Dutch single-family dwellings will be sufficient to reduce indoor radon concentrations above the proposed national reference level of 100 Bq m⁻³ to values well below. - Highlights: • House, ventilation and soil type are the main predictors of radon in Dutch dwellings. • We propose a Dutch national reference level for radon in dwellings of 100 Bq m⁻³. • Three criteria predict which dwellings have a higher probability of elevated radon. • 7% of the housing stock encompasses 80% of the dwellings with elevated radon levels.