Radon-based assessment of stability effects on potential radiological releases

It is a requirement of nuclear energy and research facilities to conduct continuous and comprehensive atmospheric monitoring in order to better forecast public or environmental exposure to routine or accidental releases of radioactive substances to the atmosphere. A key aspect of such monitoring programs is the assessment of the atmospheric mixing state (or “stability”). Whether these facilities are in dense urban areas, or surrounded by heavily vegetated exclusion zones, local roughness heterogeneity can hamper attempts to accurately categorise stability by conventional meteorological techniques. Based on an analysis of 8 months of hourly climatology and atmospheric radon observations from a 60 m tower at the IFIN-HH nuclear research facility (Bucharest, Romania), we develop and apply a continuous (i.e. not categorical) radon-based scheme for the classification of the nocturnal atmospheric stability state. We demonstrate the superior performance of the radon-based technique to Pasquill-Gifford or bulk Richardson number stability typing at this site where heterogeneous roughness elements reach to 15 m a.g.l. Under stable nocturnal conditions the Pasquill-Gifford scheme overestimates the atmosphere’s capacity to dilute pollutants with near-surface sources by 20% compared to the radon-based scheme. Under these conditions, near-surface wind speeds drop well below 1 m s-1 and nocturnal mixing depths vary from ~25 m to less than 10 m a.g.l. Climatological parameters are characterised by season and 4 arbitrarily-defined nocturnal stability categories. Benchmarks (based on 10/50/90th percentile distributions) of 30-60 m wind and temperature gradients are devised for each stability category for evaluation of model performance. Lastly, nocturnal radon-derived effective mixing depth estimates constrained by tower observations are used to better-constrain the seasonal variability in the Bucharest regional radon flux: 13 mBq m-2 s-1 (winter), 18 mBq m-2 s-1 (summer)

Modelling H-3 and C-14 transfer to farm animals and their products

The radionuclides {sup 14}C and {sup 3}H may both be released from nuclear facilities. These radionuclides differ from most others in that they are isotopes of macro-elements which form the basis of animal tissues, feed and, in the case of {sup 3}H, water. There are few published values describing the transfer of {sup 3}H and {sup 14}C from feed to animal derived food products. Approaches are described which enable the prediction of {sup 14}C and {sup 3}H transfer parameter values from readily available information on the stable H or C concentration of animal feeds, tissues and milk, water turnover rates, and feed intakes and digestibilities. It is recommended that the concentration ratio between feed and animal product activity concentrations be used as it is less variable than the transfer coefficient (ratio between radionuclide activity concentration in animal milk or tissue to the daily intake of a radionuclide)

A model approach for tritium dynamics in wild mammals

Tritium (3H) transfer into environment must be modelled differently than the transfer of other radionuclides released from nuclear reactors because hydrogen represents the building blocks of life. A solid understanding of 3H behaviour is essential because 3H may be released in large quantities from CANDU (CANada Deuterium Uranium) reactors and from future thermonuclear reactors. Recently, the authors published a complex dynamic metabolic model for 3H and 14C transfer in farm and wild animals, but the model applications for wild biota were restricted to too few examples and mostly for 14C transfer. In this study, the model is applied to few selected wild animals for 3H uptake. Despite the lack of any experimental data for wild animals, the results presented in this study are less uncertain than for many other radionuclides and can provide a useful estimation for biota radioprotection

A versatile model for tritium transfer from atmosphere to plant and soil

The need to increase the predictive power of risk assessment for large tritium releases implies a process level aproach for model development. Tritium transfer for atmosphere to plant and the conversion in organically bound tritium depend strongly on plant characteristics, season, and meteorological conditions. In order to cope with this large variability and to avoid also, expensive calibration experiments, we developped a model using knowledge of plant physiology, agrometeorology, soil sciences, hydrology, and climatology. The transfer of tritiated water to plant is modelled with resistance approach including sparce canopy. The canopy resistance is modelled using Jarvis-Calvet approach modified in order to directly use the canopy photosynthesis rate. The crop growth model WOFOST is used for photosynthesis rate both for canopy resistence and formation of organically bound tritium, also. Using this formalism, the tritium transfer parameters are directely linked to known processes and parameters from agricultural sciences. The model predictions for tritium in wheat are closed to a factor two to experimental data without any calibration. The model also is tested for rice and soybean and can be applied for various plants and environmental conditions. For sparce canopy the model uses coupled equations between soil and plants

Modelling the transfer of

Future development of nuclear energy in the frame of climate change and sustainable development needs an increased safety and consequently, robust models of environmental transfer of radionuclides. Tritium and Carbon are life elements and must be treated separately from trace elements. The IAEA promoted EMRAS (Environmental Modelling for Radiation Safety) project in order to decrease uncertainties in the predictive capability of environmental models, including the cases of aquatic and biota. To understand the processes and models reliability, nine scenarios have been developed. The Working Group contributed to the Revision of “Handbook of parameter values for the prediction of radionuclide transfer in temperate environments” TRS 364, as well. The main task of this paper is to propose ways for models' predictive power improvements, based on lessons learnt from EMRAS' exercises

Using WOFOST crop model for data base derivation of tritium and terrestrial food chain modules in RODOS

The European Commission Project RODOS is developing a coherent methodology for a Real-time On-line DecisiOn Support System for Nuclear Emergency across Europe. Within this system there is a special module to model the transfer of triated water from releases to terrestrial foods. In order to model the transfer of tritiated water from air to various plants, the conversion to organically bound tritium, and the partition to edible parts of the plant, both the mean dynamics of leaf area index and a physiological description of canopy photosynthesis are required. The WOFOST crop growth model has been selected as a basis for deriving tritium transfer dynamics to plants. Its ability to reproduce site-specific biomass growth of various plants (not only from Europe) is demonstrated in this paper, as well as its compatibility to other photosynthesis models. We have tested that this model can simulate limited fertilisation situations via the adaptation of two important parameters. After adaptation of model parameters to site-specific plant growth data, multi-annual mean dynamics can be obtained using meteorological data for subsequent years

Tritium dynamics in large fish – a model test

Tritium can represent a key radionuclide in the aquatic environment, in some cases, contributing significantly to the doses received by aquatic non-human biota and by humans due to aquatic releases. Recently, the necessity to have a robust assessment of tritium routine and accidental risk emissions for large nuclear installations increased the interest in the topic. In the present study, the recent experiments concerning tritium transfer in adult rainbow trout are described. The updated model concerning the dynamics of tritium transfer in aquatic food chain (AQUATRIT model) developed by the authors is applied and tested for these experimental data. The model predicts the experimental data with a factor of 2 to 3 and the potential improvements of the model are discussed. The present model results emphasize that in the field conditions, the major factors influencing the OBT biological loss rate are the temperature and the prey availability while, the OBT uptake is mainly influenced by the fish growth rates. The main goals of this study are to enhance the robustness of aquatic models for tritium risk assessment and to fulfil a gap for aquatic pathways in environment

Towards a model for the dynamic transfer of tritium and carbon in mammals

Available data have been analysed to test the hypothesis that both 3H and 14C transfer in mammals can be accounted for by an understanding of metabolism. Data obtained from various 14C and 3H experiments with rats and sheep have been analysed to assess the multi-component retention function of various organs and identify any relationship between half-times and component contribution. Our hypothesis was that component half-times for 14C and 3H are similar after intakes of organic compounds. Similarities in the tritium and carbon dynamics between rat and sheep were observed supporting the hypothesis. For fast and slow components of muscle half-time, allometric relationships have been derived. The results obtained could be used in the development of a human biokinetic model