Measurements and modelling of 137Cs distribution on ground due to the Chernobyl accident: a 27-y follow-up study in Northern Greece.

Following the Chernobyl accident, an area of ∼1000 m(2) in the University farm of the Aristotle University of Thessaloniki was considered as a test ground for radioecological measurements. The radiocesium deposition in this area, due to the Chernobyl accident, was 20 kBq m(-2). The profile of (137)Cs in the soil of this area was measured systematically from 1987 to 2012. The form of the profile has changed over the years. During the 1987-2000 period the (137)Cs distribution was reproducible by a sum of two exponentials. However, at least since 2005 the (137)Cs distribution can be successfully fitted by a single exponential function. The long-time (∼27 y) evolution study of the (137)Cs distribution in soil permit one to extract with the use of a simple compartment model, the mean vertical migration velocity of (137)Cs. Vertical migration of (137)Cs in soil is a very slow process. The mean vertical migration velocity is estimated to be 0.14 cm y(-1).The relative good comparison between the time dependence of the (137)Cs distribution in soil and the model predictions indicate that the simple model used is realistic.

Experimental and theoretical study of radon distribution in soil.

Radon concentration as a function of the soil depth (0-2.6 m) was measured during the years 2002-2003 and 2003-2004 on the Aristotle University campus. Radium distribution in soil was found constant. On the contrary, as expected, radon concentration increased with soil depth. However, the radon concentration did not follow the well known monotonous increase, which levels off to a saturation value. In both radon distributions, radon concentration increased up to a soil depth of about 80 cm, seemed to remain constant at depths of 80-130 cm, and then increased again. The experimental distribution was reproduced by solving the general transport equation (diffusion and advection). The main finding of the numerical investigation is that the aforementioned, experimentally observed, profile of radon concentration can be explained theoretically by the existence of two soil layers with different diffusion-advection characteristics. Soil sample analysis verified the existence of two different soil layers. Different boundary conditions of the radon concentration at the soil surface were used for the solution of the diffusion-advection equation. It was found that the calculated radon concentration in the soil is, away from the soil surface, the same for the two boundary conditions used. However, from the (frequently used) boundary condition of zero radon concentration at the soil surface, the experimental profile of the radon concentration at the soil surface cannot be deduced. On the contrary, with more appropriate boundary conditions the radon concentration at the soil surface could be deduced from the experimental profile. The equivalent diffusion coefficient could be uncovered from the experimental profile, which can then be used to estimate the radon current, which is important, for example, for the estimation of radon entrance to dwellings.

Follow up study of radiocesium contamination in a Greek forest ecosystem.

Radiocesiun distribution in the different parts of a Quercus conferta Kit ecosystem in Northern Greece was measured in 2005-2006, twenty years after the Chernobyl accident. The comparison between the results of this study and those previously measured (1993-1995) in the same ecosystem gives information about the long-term behavior of 137Cs in forest ecosystems. The major part of the 137Cs inventory is still in the upper layers of the soil. The radiocesium distribution in soil is fixed and has been in equilibrium at least since 1993, when the first measurements were performed. The major contamination mechanism of leaves and wood is root uptake.

Twenty-year follow-up study of radiocesium migration in soil.

The profile of (137)Cs present in undisturbed soil due to the Chernobyl accident was measured repeatedly for approximately 20 y. The vertical migration of (137)Cs in soil is a very slow process. The mean vertical migration velocity is estimated at approximately 0.1-0.2 cm y(-1). A method based on in situ gamma spectrometry measurements and Monte Carlo computations, aimed at estimating the profile of (137)Cs without performing any soil sampling, is investigated.

Pilot study of indoor radon in Greek workplaces.

Radon and gamma dose rate measurements have been performed in 561 workplaces in 19 prefectures of Greece. The distribution of radon concentration can be well described by a log-normal distribution. Most of the radon concentrations are between 50 and 200 Bq m(-3) with an arithmetic mean of 123 Bq m(-3). The maximum measured value of radon gas concentration is 695 Bq m(-3). About 10% of the workplaces exceed 200 Bq m(-3). Only a small fraction ( approximately 1%) of workplaces exceed the European Commission action level (400 Bq m(-3)). Despite the relative small fraction of workplaces which exceed the value of 400 Bq m(-3), it is clear from the results of the present work that for certain prefectures, further and more extensive research is needed.

Simultaneous measurements of indoor radon, radon-thoron progeny and high-resolution gamma spectrometry in Greek dwellings.

Simultaneous indoor radon, radon-thoron progeny and high-resolution in situ gamma spectrometry measurements, with portable high-purity Ge detector were performed in 26 dwellings of Thessaloniki, the second largest town of Greece, during March 2003-January 2005. The radon gas was measured with an AlphaGUARD ionisation chamber (in each of the 26 dwellings) every 10 min, for a time period between 7 and 10 d. Most of the values of radon gas concentration are between 20 and 30 Bq m(-3), with an arithmetic mean of 34 Bq m(-3). The maximum measured value of radon gas concentration is 516 Bq m(-3). The comparison between the radon gas measurements, performed with AlphaGUARD and short-term electret ionisation chamber, shows very good agreement, taking into account the relative short time period of the measurement and the relative low radon gas concentration. Radon and thoron progeny were measured with a SILENA (model 4s) instrument. From the radon and radon progeny measurements, the equilibrium factor F could be deduced. Most of the measurements of the equilibrium factor are within the range 0.4-0.5. The mean value of the equilibrium factor F is 0.49 +/- 0.10, i.e. close to the typical value of 0.4 adopted by UNSCEAR. The mean equilibrium equivalent thoron concentration measured in the 26 dwellings is EEC(thoron) = 1.38 +/- 0.79 Bq m(-3). The mean equilibrium equivalent thoron to radon ratio concentration, measured in the 26 dwellings, is 0.1 +/- 0.06. The mean total absorbed dose rate in air, owing to gamma radiation, is 58 +/- 12 nGy h(-1). The contribution of the different radionuclides to the total indoor gamma dose rate in air is 38% due to 40K, 36% due to thorium series and 26% due to uranium series. The annual effective dose, due to the different source terms (radon, thoron and external gamma radiation), is 1.05, 0.39 and 0.28 mSv, respectively.

Derived release limits for the greek research reactor site based on a diagnostic atmospheric modeling system for irregular terrain.

The upper limits for the rate of release of radionuclides into the atmosphere, i.e., the "derived release limits," are calculated for the Greek Research Reactor (GRR-1) in order to determine possible operational schemes compatible with the effective dose limits for the general population. GRR-1 is located at the northwestern foot of Hymettos Mountain and at the eastern border of the urbanized area of Athens basin. Due to the topographic complexity of the region, the meteorological and atmospheric dispersion calculations were based on a numerical modeling system that is especially designed to work over irregular terrains by using a prismatic unstructured grid. The calculation of derived release limits was made using guidelines and methods that conform to the system of dose limits prescribed by the European radiation protection regulations.

Radiological maps of outdoor and indoor gamma dose rates in Greek urban areas obtained by in situ gamma spectrometry.

The results obtained from 259 indoor and outdoor in situ gamma spectrometry measurements with a portable Ge detector and 707 total gamma dose rate measurements with an NaI detector in urban areas of 16 Greek islands are presented. From the in situ gamma spectra, the absorbed dose rate in air due to Uranium series, Thorium series, (40)K and (137)Cs are derived and discussed. The results obtained from the present work in conjunction with those reported previously were used for the realization of a complete indoor and outdoor gamma radiation map of Greek urban areas using in situ gamma spectrometry with portable Ge detector.

A combination study of indoor radon and in situ gamma spectrometry measurements in Greek dwellings.

The aim of the present study was to investigate of a possible correlation between indoor radon and indoor gamma dose rates deduced by in situ gamma spectrometry measurements by using a portable HPGe detector. Indoor radon and high resolution in situ gamma spectrometry measurements were performed in 60 apartments in Thessaloniki, the second largest city of Greece. Geometric mean radon concentration is 52 Bq m(-3). The mean total absorbed dose rate in air due to gamma radiation is 56 +/- 9 nGy h(-1). The contribution of the different radionuclides to the total indoor gamma dose rate in air is 41% due to 40K, 36% due to the thorium series and 23% due to the uranium series. No correlation was found between indoor gamma dose rate due to the uranium series and indoor radon for ground and first floor apartments. For upper floor apartments (above the second floor) a weak correlation is observed. The mean annual effective dose due to radon is 1.15 mSv, i.e., more than four times higher compared to the effective dose due to gamma radiation (0.27 mSv).

Long-term measurements of radon equilibrium factor in Greek dwellings.

The time variation of the radon equilibrium factor F was measured every four hours in the Nuclear Technology Laboratory of the Aristotle University of Thessaloniki (in northern Greece) during October 1998-April 1999. The time dependence of the mean weekly value of radon equilibrium factor F is relatively small. During October 1999-May 2000 the radon equilibrium factor was measured in 25 apartments randomly distributed in Thessaloniki. The mean value of the equilibrium factor F is 0.47 +/- 0.09, close to the typical value of 0.4 adopted by UNSCEAR.

Measurements with a Ge detector and Monte Carlo computations of dose rate yields due to cosmic muons.

The present work shows how portable Ge detectors can be useful for measurements of the dose rate due to ionizing cosmic radiation. The methodology proposed converts the cosmic radiation induced background in a Ge crystal (energy range above 3 MeV) to the absorbed dose rate due to muons, which are responsible for 75% of the cosmic radiation dose rate at sea level. The key point is to observe in the high energy range (above 20 MeV) the broad muon peak resulting from the most probable energy loss of muons in the Ge detector. An energy shift of the muon peak was observed, as expected, for increasing dimensions of three Ge crystals (10%, 20%, and 70% efficiency). Taking into account the dimensions of the three detectors the location of the three muon peaks was reproduced by Monte Carlo computations using the GEANT code. The absorbed dose rate due to muons has been measured in 50 indoor and outdoor locations at Thessaloniki, the second largest town of Greece, with a portable Ge detector and converted to the absorbed dose rate due to muons in an ICRU sphere representing the human body by using a factor derived from Monte Carlo computations. The outdoor and indoor mean muon dose rate was 25 nGy h(-1) and 17.8 nGy h(-1), respectively. The shielding factor for the 40 indoor measurements ranges from 0.5 to 0.9 with a most probable value between 0.7-0.8.

Extended survey of indoor and outdoor terrestrial gamma radiation in Greek urban areas by in situ gamma spectrometry with a portable Ge detector.

The results obtained from more than 1000 indoor and outdoor in situ gamma spectrometry measurements in 41 towns (from all geographic subdivisions) of the Greek mainland (not islands) are presented. From the in situ gamma spectra the absorbed dose rate in air due to uranium series, thorium series, 40K and 137Cs are derived and discussed.

Derivation of indoor gamma dose rate from high resolution in situ gamma ray spectra.

The dose build up factor B related to the ratio of primary to scattered gamma radiation in indoor environment was calculated with Monte Carlo simulations using the MCNP code for different indoor geometries and gamma source distributions. The main conclusion is that the B factor does not depend strongly on parameters such as dimensions of the rooms, the thickness of walls, the density of the building materials, and the gamma source geometry. The calculated dose build up factors were used within the framework of the full absorption peak analysis method in order to deduce the dose rates in air from about 100 indoor in situ gamma spectrometry measurements performed at the town of Thessaloniki, Greece. A spectral "stripping method," which has been recently developed, was applied to the same 100 spectrometry measurements. The results of the dose rates obtained by the two methods are compared and discussed.

Monte Carlo calculation of dose rate conversion factors for external exposure to photon emitters in soil.

The dose rate conversion factors D(CF) (absorbed dose rate in air per unit activity per unit of soil mass, nGy h(-1) per Bq kg(-1)) are calculated 1 m above ground for photon emitters of natural radionuclides uniformly distributed in the soil. Three Monte Carlo codes are used: 1) The MCNP code of Los Alamos; 2) The GEANT code of CERN; and 3) a Monte Carlo code developed in the Nuclear Technology Laboratory of the Aristotle University of Thessaloniki. The accuracy of the Monte Carlo results is tested by the comparison of the unscattered flux obtained by the three Monte Carlo codes with an independent straightforward calculation. All codes and particularly the MCNP calculate accurately the absorbed dose rate in air due to the unscattered radiation. For the total radiation (unscattered plus scattered) the D(CF) values calculated from the three codes are in very good agreement between them. The comparison between these results and the results deduced previously by other authors indicates a good agreement (less than 15% of difference) for photon energies above 1,500 keV. Antithetically, the agreement is not as good (difference of 20-30%) for the low energy photons.

Contribution of 137Cs to the total absorbed gamma dose rate in air in a Greek forest ecosystem: measurements and Monte Carlo computations.

The absorbed gamma dose rate in air 1 m above soil due to natural gamma emitters and 137Cs from the Chernobyl accident was determined inside a Quercus conferta Kit ecosystem in Northern Greece by combination of Monte Carlo simulations with the MCNP code and in-situ gamma spectrometry measurements. The total absorbed gamma dose rate in air is about 64 nGy h(-1), where 40% of this value is due to 137Cs and 60% to natural gamma emitters. The Monte Carlo simulations indicated that the gamma absorbed dose rate in air due to 137Cs is mainly due (70%) to unscattered radiation and to a lesser extent (30%) to the scattered radiation. The results obtained with the Monte Carlo simulations for the unscattered radiation were in very good agreement with the experimental values deduced by in-situ gamma spectrometry measurements. From the combination of the Monte Carlo simulations and in-situ gamma spectrometry measurements a conversion factor C = 1 nGy h(-1)/kBq m(-2) was deduced for 137Cs. This factor must be used with caution and only for forest sites similar to the one used for this work.

Diffusion model of radon exhalation rates.

Radon migration through walls is considered using an effective diffusion model. One and two dimensional models as well as a spherical geometry are compared. It is concluded that either the 1-dimensional or spherical model, both having analytical solution, may be used for practical purposes. Application is made for typical cases in Athens, Greece.

Monte Carlo based method for conversion of in-situ gamma ray spectra obtained with a portable Ge detector to an incident photon flux energy distribution.

A Monte Carlo based method for the conversion of an in-situ gamma-ray spectrum obtained with a portable Ge detector to photon flux energy distribution is proposed. The spectrum is first stripped of the partial absorption and cosmic-ray events leaving only the events corresponding to the full absorption of a gamma ray. Applying to the resulting spectrum the full absorption efficiency curve of the detector determined by calibrated point sources and Monte Carlo simulations, the photon flux energy distribution is deduced. The events corresponding to partial absorption in the detector are determined by Monte Carlo simulations for different incident photon energies and angles using the CERN's GEANT library. Using the detector's characteristics given by the manufacturer as input it is impossible to reproduce experimental spectra obtained with point sources. A transition zone of increasing charge collection efficiency has to be introduced in the simulation geometry, after the inactive Ge layer, in order to obtain good agreement between the simulated and experimental spectra. The functional form of the charge collection efficiency is deduced from a diffusion model.

Radiocesium contamination in a submediterranean semi-natural ecosystem following the Chernobyl accident: measurements and models.

Radiocesium dynamics in a Quercus conferta Kit ecosystem in Northern Greece have been extensively studied over the years 1993-1995. Radiocesium distribution in the different parts of the ecosystem was measured. A total 137Cs inventory of 243+/-66 MBq ha(-1) due to the Chernobyl accident was measured in all parts of the ecosystem. Almost 90% of this inventory is still in the upper layers of the soil and the forest floor. In particular 13.4% is in the forest floor, 52.6% in the Ah horizon, and 23.4% in the upper 5 cm of the soil. Only 2.2% of this inventory is in the above ground biomass. The mean total 137Cs deposited on the forest floor from the above ground biomass is 0.18 MBq ha(-1) y(-1). Cesium leaching from the forest floor is negligible. The radiocesium distribution in soil is fixed and in equilibrium, at least since 1993. Most of radiocesium is not available for migration. Cesium migration in soil was modeled by a) an "equivalent diffusion" model with different initial conditions and b) a "compartment" model derived from a diffusion-advection model. A compartment model for the contamination of living biomass is proposed. The total absorbed dose rate in air as well as the contribution due to 137Cs from the Chernobyl accident was determined inside the forest, by in-situ gamma spectrometry.

Long term radiocesium contamination of fruit trees following the Chernobyl accident.

Radiocesium contamination from the Chernobyl accident of fruits and leaves from various fruit trees was systematically studied from 1990 to 1995 on two agricultural experimentation farms in Northern Greece. The results are discussed in the framework of a previously published model describing the long-term radiocesium contamination mechanism of deciduous fruit trees after a nuclear accident. The results of the present work qualitatively verify the model predictions.

Radiocesium distribution in undisturbed soil: measurements and diffusion-advection model.

The profile of 137Cs, due to Chernobyl accident, in undisturbed soil was measured experimentally over the years 1987 to 1994 and was found to remain practically fixed in the upper 30 cm of soil since 1987. Total deposition of 137Cs at the site was 20 kBq m-2, and approximately 80% of that is in the upper 10 cm. The profile has two slopes in semilogarythmic scale, i.e., it appears as the sum of two exponentials. The contribution of weapons fallout is found to be negligible. Pure diffusion and diffusion-advection models of cesium migration are investigated. It is found that the pure diffusion model cannot reproduce the double slope, while the diffusion advection model can.