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Content archived on 2024-04-15

INDIVIDUAL AND SOCIAL RADIATION RISKS - RESULTING FROM THE OPERATION OF NUCLEAR FACILITIES AND ASSESSMENT OF RISKS DERIVED FROM ENHANCED NATURAL AND ARTIFICIAL RADIOACTIVITY

Objective

The objectives are:
to carry out surveys of radon concentrations in homes in those countries where insufficient data is available;
to develop and test techniques for identifying areas with a potential for high radon concentrations in homes, both on a large scale and for individual building sites;
to improve the understanding of, and develop mathematical models of, the movement of radon from the ground to subfloor spaces and into buildings;
to develop and test countermeasures against radon in homes using laboratory and field studies.
The main objective of the project is the estimation of the radiation risk resulting from the operation of nuclear power plants (NPP), in the framework of the extensive nonuniformity of the population and environment in Greece, with emphasis on adopting appropriate population distribution criteria. This risk will be set in perspective by comparing it to the corresponding risks of alternate energy sources and other technological activities in Greece.

The methodology developed on siting criteria is aimed to deal with the problems stemming from the demographic idiomorphy of Greece, where one third of the population of the country is concentrated in Athens city and suburbs, with the rest of the country exhibiting small population densities. Using this methodology, it becomes clear that by comparing the risk of a postulated severe nuclear accident to the risk of energy production, a decision aid tool for siting nuclear power plants near large population centres is formulated in respect to the social radiation risk. This approach has advantages: it can be used in conjunction with other approaches thus strengthening the final decision, it takes into account the social risk and it can be considered as a very desirable supplement to the more common site selection approaches used up to now, which satisfy mainly individual risk criteria, and finally it is very simple and can ease public fears to some extent by being conservative in certain aspects, (eg the postulated occurrence of a severe reactor accident, even the employment of a worst case wind rose in the calculations if desired.

The present methodology can be utilized as a sitingdecision aid tool in the presence of a large population centre, when deciding on the acceptability of sites, or when 2 or more sites, which are equivalent otherwise, are compared. It can be also used to determine the maximum allowable magnitude of a NPP at different sites, that will limit the risk from the reference severe accident to levels below those of energy production.

The developed methodology, for emergency response planning (ERP) optimisation satisfies the need for provision of 'adequate protection' by setting conflicting objectives and trying to optimise them all. It has been proposed several times that the uncertainty problem should be addressed by the so called 'worst case scenario'. This approach assumes that if an ERP has been planned to confront the worst possible conditions. it can also provide adequate protection in the case of less severe accidents, which in addition are more frequent. In most cases, planning against the worst accident will possibly result in deciding evacuation of large areas. However, evacuation is not always the best policy, since, for example, if the wind speed is such that the cloud is going to catch the evacuees, sheltering and leaving the cloud pass will certainly be a better policy. As this example indicates, the uncertainty in the parameters of the accident strongly affects the ERP and it should be taken into account when planning against emergencies.

It has also been proposed to use a single criterion, such as radiation dose, as a measure of effectiveness of an ERP. However, particular problems have been created from such use of radiation dose, problems deriving from the fact that the health effects are not linear functions of the dose. For example, we are not interested in the exact level of dose, if it has exceeded a certain limit, since above this limit any level of dose produces the same health effects. Therefore, it seems that explicit use of societal health effects as measures of effectiveness of a particular ERP provides a much more clear understanding o the consequences and hence a framework for evaluating alternative ERPs that facilitates communication of the results to both decision makers and interested parties.

The assessment of the consequences of Greek research reactor (GRR) under various operating conditions indicated that serious cons equences rather than the insignificant ones are to be expected in the event of the design base accident (DBA) for the continuous operation schedule with the exception of the thyroid dose and the nonfatal thyroid latent health effects, for the current operating schedule. In the event of the most severe credible accident the major part of the nonsignificant consequences would be due to the early exposure.

The policy of banning port entry to a nuclear powered ship in a large port, would not differ in essence from the siting policy followed in land based nuclear power stations, where proximity to large population centres is avoided.

When examining the different emergency plans, some points appear to need further improvement. Such points are: the standardization of nomenclature regarding the seriousness of emergencies together with comprehensive notification procedures which will be of a great help in establishing cooperation between neighboring countries;
the generalised use of dedicated communication lines between authorities and services involved in an emergency planning procedure;
the knowledge necessary to establish efficient decision making capabilities as far as sheltering and/or evacuation of a given population group is concerned (the natural shielding factor presented in buildings and houses around a given nuclear power plant is an important element to be taken into account in decision making procedures);
the mechanisms for controlling the contamination of foodstuffs taking into account that the recent Chernobyl accident showed the lack of adequate preparedness.

Concerning the shielding activities, the ALGOL-60 code, DEPSHIELD of the Danish Riso National Laboratory for the calculation of shielding factors of buildings and houses has been significantly modified and rewritten in Fortran-77. The modified version, named SHIELD-F, has been used to estimate an average shielding factor for the population of the Attica basin.

A computer program ADREA-I, has been developed which is particularly suitable for atmospheric transport and dispersion calculations over realistic topographies, of any degree of complexity. ADREA-I is intended to be a useful tool in analysing the radiation risk in complex environments, where simple codes reach their limits.

The model verification studies performed showed satisfactory agreement with experimental results increasing the confidence as an appropriate transport model in complex terrain environments. However, further verification studies of the code, based on experimental data, are needed.

The first verification and demonstration studies (ie triangular ridge problem, building block problem and Nebraska boundary layer) showed the capability of the code to treat the complex domain satisfactorily.

The sea breeze verification study, based on the Alaskan Beaufort Sea Coast data, showed that the results obtained were in relatively good agreement with those obtained by Kozo (1982). This verifies the capability of the code to be applied successfully to problems including land/ water interfaces.

The studies of the effects of a large natural barrier on sea breeze and contamination patterns gave reasonable results for the diurnal evolution of the phenomena. All these applications demonstrate the code ability to treat complicated dispersion problems, where terrain disturbances, inhomogeneities and complex atmospheric conditions, such as stagnant prevailing atomosphere, are combined. In all the above cases, the main conclusion was that the mountain presence seems to exert a drastic effect on the pollutants distribution, keeping the lower altitudes of the opposite mountain side relatively clear.

The first test of the ADREA-I code, against real field data, (Wangara experiment), provided satisfactory results for the mean boundary layer structure.

The air/ground interaction modelling review shows that the majority of the mesoscale theoretical studies eith er utilise a periodic heating function to prescribe the ground surface temperaturevariation, or use the surface heat energy budget equation. The latter is a more advanced approach being the most appropriate in complex terrain simulations and permitting feedback between the ground temperature and the atmosphere. In ADREA-I code, both cases are included. The first test of the surface heat budget model, reproduced the experimental Wangara data satisfactorily.

A seasonal analysis was performed on the flow field data over the Greek territory with representative months of December, March, June and September.

The flow field data in the 850 mbs over the Greek territory show that the prevailing wind directions are either of the west, southwest sector (December, March) or north sector (June, September). For the latter period the southeast sector is highly dominated by northerly winds due to the existence of the Cyprus thermal low. The flow field on the surface is strongly influenced by the area topography (high mountains, frequent land/sea interchanges).

Greece appears to be one of the European countries with the highest concentration of radon-222 in thermal waters which are used in balneotherapy. The violation of the justification principle and as low as reasonably achievable (ALARA) principle during this use of ionising radiation has been repeatedly pointed out, although the benefits from this practice (if related to radioactivity at all) seem to be confirmed by centuries of application.

The relatively low content of radon-226 in soil and the warm climate (high average ventilation rates) of the Athens region (with a population of 3.5 million) are the reasons for the observed low concentrations of radon-222 decay products in the indoor air. Higher values have been observed in Thessaloniki, and there are similar indications from other regions of Northern Greece. The use of track etched detectors for a wider radon survey is planned, but it seems that the Greek population will appear to be exposed to indoor radon to a lesser extent than the population of most of the European Community (EC) countries.

The Greek building materials investigated so far do not create any concern from a radiological point of view. This is confirmed not only by the observed indoor radon levels, but also by the values of the exposition rate measured in parallel. There are nevertheless certain cases of mine areas where further investigation is necessary.

The results of the survey of natural radioactivity in Greek soils indicate that the country is characterized, by conventional levels, as compared to the EC area. The studies of natural and artificial radioactivity in the Greek marine environment have created a good reference base. It was used after May 1986 to evaluate the impact of the Chernobyl accident and can be used for any similar comparisons in the future. In addition, the studies brought about the selection of some marine organisms (especially algae) as suitable bioindicators for certain natural and artificial radionuclides.% L
The basic comments on the findings related to the impact of the Chernobyl accident are as follows:
The properties of the Greek soils favour the fixation of caesium and reduce its availability to the growing plants (the 1987 dose to the average Greek adult due to the root uptake pathway (including projections for meat and milk through the animal feedingstuffs) does not exceed 10 uSv);
the most important short term pathways include the consumption of animal products (grain and pastry during the winter of 1987 to 1987, reflects the contamination of the 1986 harvest);
the major late pathway is the external irradiation due to the radioactive caesium deposited in rural areas (the 50 years critical group dose from external irradiation approaches 3000 uSv, while for the average Greek its value does not exceed 200 uSv);
the special concern about the hot particles (HP) is related to the fact that they lead to a type of internal exposure not comparable with any natural one (the collective dose due to inhalation of HPs seems to be relatively low even if a "quality factor" of 10 is used with respect to the conventional form of radioactive materials, but the peak values of HP activities observed call for further research and special epidemiological attention).

The methodology work related to the introduction of the track etch detector technology included:
the development of a user friendly method for manual determination of the track density;
the study of personal variations in counting efficiency and the introduction of personal operator efficiency coefficients to correct for these variations;
the construction of a small calibration chamber, suitable for the simultaneous exposure of 8 measuring pots or several unprotected films;
the determination of optimal etching conditions (duration, etching solution density and temperature) in order to minimise the low limit of detection and to achieve maximum repeatability of the results;
the selection of suitable, light tight metal pots for housing the detector films.

The first phase of the survey of the natural radioactivity of Greek soils has been completed. About 600 samples from the whole country have been analysed by the use of high resolution gamma spectrometry. The specific activities of the major gamma emitting radionuclides of the uranium and thorium series as well as of potassium-40 have been determined. Paired measurements of the external exposition rate and the soil radioactivity have been made in several areas. A statistical analysis of the results is in progress in order to determine the regional averages and variations.

A new version of the laboratory device for the determination of the exhalation rate of radon has been developed and calibrated. Determination of exhalation rates down to 1E-5 Bq/s is possible from samples of volume up to 4 l. The device can also be used as an alternative calibration chamber for track etch detectors. The sensitivity of the system allows the testing of various materials for radon leakages.
LNETI (P), University of Cantabria (E), University of Athens (GR) and NSCR Demokritos (GR) will carry out surveys of the exposure of the population to radon in Portugal, Spain and Greece. These will be carried out using passive etched track detectors and active measurement techniques. Additional data will be collected on the radon decay product equilibrium factors and on the origins and characteristics of the radon sources. The average radon concentrations in homes will be calculated, and the variations in concentrations mapped.

BGS (UK) and NRPB (UK) will identify appropriate geological and radiological parameters for radon potential mapping. Existing data will be evaluated, and programmes of collection of relevant data including radionuclide contents of rocks and soils will be started in defined areas. This information will be used to construct maps of radon potential, which will be compared with data on radon concentrations in homes, both from earlier surveys and from new surveys designed to test the validity of the maps.

KVI (NL), CSTC (BE), TNO (NL) Risoe (DK) and SSI (S) have under development mathematical models of radon movement and availability in the soil, movement into buildings through subfloor spaces where present, and subsequent dilution and dispersion. Different models emphasise different parts of this process. These laboratories will meet to exchange information on their models and to draw up a programme of model comparison. The results from the models will also be compared as closely as possible with measurements and will be used to identify the most important parameters for measurement in assessing radon problems in homes.

KVI (NL), the Technical University of Denmark and Risoe (DK) will study soil factors influencing radon availability to buildings using laboratory and field studies. These will include the radon exhalation rate of materials, the influence of porosity, permeability and groundwater on radon movement, and the development of improved instrumentation for characterizing soils on site. The results of this work will be used as input for the mathematical models described above.

WTCB (BE) and TNO (NL) will carry out laboratory and site studies of constructional factors influencing the entry of radon into buildings from the ground. The insight gained will be used in the development and testing of remedial and preventive measures to avoid high levels. This will include the testing of the effectiveness and durability of barrier and diversion techniques for preventing the entry of soil gas.

In the UK, some existing buildings have had remedial measures installed and thousands of new buildings have been constructed using antiradon designs. NRPB will survey the radon levels in a representative sample of these buildings to determine the effectiveness and durability of different countermeasures in practice.

In view of the wide scope of these contracts, it has been found necessary to set up small working groups to coordinate the work on particular topics where there might otherwise be duplication of effort. These groups will exchange information and will meet as required. NRPB will attend the meetings of the groups. The Technical University of Denmark, KVI and the University of Cantabria will collaborate on the subject of soil permeability measurements. KVI, Risoe, CSTC and the Technical University of Denmark will collaborate on models of radon movement in soils and into homes. SSI, CSTC and TNO will collaborate on compartmental models of airborne radon movements within homes, also using data from earlier KVI studies. Within each group the first named laboratory will take the lead in arranging the collaboration, with assistance from NRPB as required.

Apart from cooperation on the specific topics mentioned, all the laboratories will maintain communication with each other and with laboratories in Europe and North America on topics of mutual interest.

Topic(s)

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Call for proposal

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Coordinator

GREEK ATOMIC ENERGY COMMISSION
EU contribution
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Address
NRC "DEMOCRITOS" 153 10 AGHIA PARASKEVI
15310 ATHINAI
Greece

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