M. Antonopoulos-Domis

Large-eddy simulation of a passive scalar in isotropic turbulence

TEMTY, a code for large-eddy simulation of a passive scalar in isotropic turbulence, is developed and proved by successful simulation of experiment. The role of each term in the scalar equation and the concept of prefiltering the scalar equation is examined. The ratio of the exponents in the decay of velocity and temperature intensities is found to parametrize with the ratio Λ u /Λ 0 , where Λ u , Λ 0 , are the velocity and temperature Taylor microscales respectively.

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.

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 CERNs GEANT library. Using the detectors 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.

Compartment model for long-term contamination prediction in deciduous fruit trees after a nuclear accident.

Radiocesium contamination from the Chernobyl accident of different parts (fruits, leaves, and shoots) of selected apricot trees in North Greece was systematically measured in 1987 and 1988. The results are presented and discussed in the framework of a simple compartment model describing the long-term contamination uptake mechanism of deciduous fruit trees after a nuclear accident.

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.

Radiocesium dynamics in fruit trees following the Chernobyl accident.

Contamination of fruits and leaves from various trees with 137Cs from the Chernobyl accident was systematically studied from 1987 to 1990 on two farms in Northern Greece. Measured biological half-lives for 137Cs are in good agreement with a recently presented model. Contamination of leaves and fruits of trees planted before the accident decays exponentially with time. Contamination of trees planted after the Chernobyl accident was also studied.

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.

Frequency dependence of acoustic resonances on blockage Position in a fast reactor subassembly wrapper

Abstract Monitoring changes of the acoustic resonance frequencies in the subassemblies of sodium cooled fast reactors is a potential method for blockage detection. The dependence of the resonance frequencies on blockage position along the subassembly has been investigated experimentally and the results compared with model predictions.

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.

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.