(19) As a result of the six model test exercises and the field sampling programme undertaken by the TWG during the four-year international collaborative programme, a number of recommendations that derive from the conclusions above can be made concerning: a) modelling tritium behaviour due to chronic atmospheric and sub-surface sources; b) field sampling and data acquisition methods; and c) future studies.
MODELLING
(i) Dry deposition is an important process that contributes to long term average soil moisture concentrations of HTO and should be included in models.
(ii) Secondary sources of HTO due to re-emission from soil can be ignored for ground level atmospheric sources and at distances >500 m from elevated sources but must be considered if the source is sub-surface.
(iii) To calculate near-surface soil moisture concentrations due to an atmospheric source, a more accurate estimate is made if soil moisture concentrations are related to air rather than rain concentrations.
(iv) In the absence of a suitable model, the assumption that the soil moisture concentration (Bq l-1) is 0.3 times the air concentration (Bq l-1) can be used. In screening assessments, a conservative value of 0.5 can be adopted.
(v) For HT releases, air and near-surface soil concentrations of HTO can be obtained from empirical data obtained in the CRL chronic HT release experiment.
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(vi) For tritium transport down through deeper soil layers, and up from contaminated aquifers, the division of the model soil compartment into a large number of thin layers is likely to give more accurate results than if a small number of thick layers is used.
DATA ACQUISITION METHODS
(i) It is essential to have good source data and accurate measurements for model inputs (e.g.
well-defined meteorological data statistics, soil properties) in order to achieve good assessment predictions and for developing, testing and applying models.
(ii) One of the reasons for the differences between predictions and observations in the atmospheric scenarios may lie in a mismatch between the assumptions of the models and the realities of the data. Most of the models assume equilibrium conditions when in fact the air concentrations over a given sampling site fluctuate continually in response to changing atmospheric conditions. Moreover, the observed soil and grass concentrations are normally determined from samples taken irregularly throughout the growing season.
Averages of these samples may not reflect true mean values, which are what the models are designed to predict. Sampling programmes should be planned to yield mean concentrations, which are the quantities of primary interest in dose assessments.
(iii) Models can be used to highlight problems with observational data and it should not be automatically assumed that model predictions are inaccurate if they do not agree with the observations.
FUTURE STUDIES
(i) We have adequate models for assessing air moisture, soil moisture and plant TFWT concentrations of HTO resulting from both short term and chronic releases of HTO.
Work is now required to develop and test models for OBT formation and translocation in food crops, animals and aquatic systems. More work is also needed to understand and model tritium deposition to snow and the fate of the tritium when the snowpack melts.
(ii) Further experimental work is needed to confirm the concentration ratios determined in the CRL HT chronic release study and to extend the ratios obtained to sites with climate, soil and plant properties different from those at CRL.
(iii) Data sets are required to test models for sub-surface processes and pathways involved in tritium transport in saturated and unsaturated media following chronic atmospheric or sub-surface sources of tritium.
(iv) Data are required to improve modelling of wet and dry deposition to soil.
(v) Further work is needed to determine if tritium can be retained in the environment to a significant extent if a previously constant source of tritium is reduced or cut off.
(vi) Tracer experiments are required to provide information on tritium dispersivity in the saturated and unsaturated zones.
REFERENCES
[1] BARRY, P.J., WATKINS, B.M., BELOT, Y., DAVIS, P.A., EDLUND, O., GALERIU, D., RASKOB, W., RUSSELL, S., TOGAWA, O., Intercomparison of model predictions of tritium concentrations in soil and foods following acute airborne HTO exposure, Journal of Environmental Radioactivity 42 (1999) 191–207.
[2] BIOMOVS II, Tritium in the Food Chain: Intercomparison of model predictions of contamination in soil, crops, milk and beef, after a short exposure to tritiated water vapour in air, Published on behalf of the BIOMOVS II Steering Committee by the Swedish Radiation Protection Institute, Sweden, ISBN 91-972134-7-0 (1996a).
[3] BIOMOVS II, Tritium in the Food Chain: Comparison of Predicted and Observed Behaviour, A: Re-emission from soil and vegetation, B: Formation of organically bound tritium in grain of Spring Wheat, Published on behalf of the BIOMOVS II Steering Committee by the Swedish Radiation Protection Institute, Sweden ISBN 91-972958-2-5 (1996b).
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ANNEX I
MODEL DESCRIPTIONS
I–A. MODELS FOR ATMOSPHERIC RELEASES I–A.1. Model used by JAERI, Japan
H. Amano, M. Andoh, T. Takahashi
ETDOSE (Environmental Tritium model for DOSE estimation), was developed by M.A. Andoh, T. Takahashi and H. Amano for dose assessment of tritium emissions in the surface environment. ETDOSE calculates tritium concentrations in air; soil, plant free water and OBT, and estimates dose impact by inhalation of air and ingestion of food for an acute and a chronic release of HT and HTO [1].
I–A.1.1. Structure of the model
ETDOSE includes two systems that user can choose for calculation of atmospheric distribution patterns of tritium. One is “rectangular grid system”. This system is based on TRIMOD, which was developed by Dr. Belot [2]. In this system, the downwind area from the release point to the sampling point is divided into rectangular elements. Tritium dispersed into a given sector deposits on each rectangular element and re-emits to the atmosphere soon from the area source.
Another is “numerical integral system”. In this system, not only the downwind area to the sampling point, but also the upwind area and the downwind area from the sampling point are included for calculations of secondary tritium dispersion. In a circle area, with the center of the source point and the radius of twice the length of the distance from the source to the sampling point, tritium is dispersed under given condition of the wind direction. Tritium re-emitted from the deposition point is dispersed to the sampling point according to the wind direction.
We calculated scenario 1 (atmospheric transport) by using “rectangular grid system” for HTO release and “numerical integral system” for HT release.
I–A.1.2. Primary dispersion
The basic equation is the sector averaged Gaussian equation. Deposited concentration of tritium is postulated to be equal in the same downwind distance in the given sector. It needs the frequency of wind blows into the chosen direction, wind speed at effective height of release and stability as input data. The tritium concentration is calculated by superimposing of all results for each weather stability class multiplied by that frequency.
I–A.1.3. Secondary dispersion
Re-emission from soil surface is considered to occur one time. Flux of re-emission is assumed to be equal to flux of deposition that given in scenario 1; 3.0E-4m/s for HT release and 3.0E-3 for HTO release. In both HT and HTO release, the chemical form of re-emission is HTO. User can give wind velocity for re-emission plume as input data, and in the calculation of scenario 1, we used wind velocity at 60m height.
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I–A.1.4. Wet deposition
Wet deposition is assumed to occur only for HTO (for primary release of HTO and re-emission) and affect the soil HTO concentration and the re-emission rate. The wet deposition rate is calculated by using the washout coefficient. The tritium concentration in rainwater is obtained by dividing the wet deposition rate by the average precipitation rate.
I–A.1.5. Buildup in soil and vegetation
Soil: The tritium concentration in soil water is assumed to be equilibrium with that in the air moisture on the soil surface. The tritium concentration in soil water is obtained by multiplying the tritium concentration in the air moisture by the isotopic ratio HTO/H2O in liquid and vapor (=1.1), and adding the tritium concentration of rainwater.
Plant HTO: A plant takes water from both the soil and the atmosphere. The relative humidity coordinates which amount of tritium comes from atmosphere or from the soil. The concentration in plant free water (Cpw) is calculated by the next equation,
SW p
aw
pw aEC E C
C = +(1− )
wherea is the isotopic ratios HTO/H2O in liquid and vapor (=1.1),Cawp is the concentration of HTO in air humidity at the plant leaf surface, Csw is the concentration in soil surface water, E is the relative humidity. In the calculation for scenario 1, HTO concentration in air humidity at the plant leaf surface was same value as the specific activity in soil surface air because the plant was assumed to be short grass.
Plant OBT: The tritium concentration in plant organic matter (OBT) is calculated by multiplying the HTO concentration in the plant free water by the ratio of OBT/HTO in an equilibrium state. The ratio of OBT/HTO was determined as 0.8 for calculation of Scenario 1.
References
[1] ANDOH, M., TAKAHASHI, T., AMANO, H., Development of an environmental tritium model; ETDOSE, Proceedings of the Cross-Over Symposium “New Approach for Studies on Environmental Radioactivity “, JAERI-Conf 99-001, 161–169 (1999) (in Japanese).
[2] BELOT, Y., CAMUS, H., A code to predict the atmospheric concentration of HTO following an accidental release of HT, in Tritium in the Environment and Work Place, Final report on Fusion Safety and Environment for the period 1989–1991, Institut de Protection et de Sureté Nucléaire, France (1991).
I–A.2. MODEL USED BY COMMISSARIAT À L’ÉNERGIE ATOMIQUE, FRANCE Y. Belot
TRIMASS1, is a code written in Fortran 77, that was developed at CEA by Y. Belot and G. Guinois for assessment of tritium atmospheric dispersion during routine operations. This is a multi-source, sector–averaged model, that requires the user to define as input data the source(s), and terrain characteristics and the weather annual-averaged conditions. The model is applicable to flat terrains and to a certain extent to rolling terrains showing small elevational differences.
I–A.2.1. Structure of the model
The average concentrations of tritium in air humidity, rain water, soil moisture, plant water and plant organics are obtained from the code, as output data, at given points, and/or at the points of an uniform user–specified rectangular grid. The resulting data matrix can be optionally visualised as contour plots.
The calculation grid is defined by contiguous rectangular terrain elements of given size and number in two rectangular directions. The size and the number of the terrain elements can be chosen, but an increase in the number of elements is at the expense of an increase in the calculation time (about half-an-hour for 201 × 201 terrain elements).
The main primary source is placed at the centre of the grid. The position of each of the other primary sources, and also of all receptors, is defined with respect to the main primary source, taking the Ox axis from West to East direction, and the Oy axis from South to North direction.
I–A.2.2. Primary dispersion
The basic equation is the sector-averaged Gaussian equation for a 'continuously' emitting point-source. This equation is based on the assumption that that all wind directions within a given sector occur with equal probability. It uses the probability distributions of wind direction, wind speed and stability given as input to the model. No provision is made for plume broadening at the source nor for plume depletion.
The vertical dispersion parameter is expressed as a function of both distance and mesoscale roughness length by using the Hosker's formulae (1974) derived from Smith's results (1972), and assuming that the airflow displacement height is negligible in the sampling zones of interest. As an alternative, the user can choose the Brigg's results (Briggs 1973) which have been collated for unspecified small roughness lengths (between 0.03 and 0.3 m).
The wind speed is extrapolated to the height of each source by using the vertical profiles proposed by van Ulden and Holtslag (1985). In case of several sources, the contribution of each source is calculated in the same manner with only changes in the relative coordinates of each new source and receptor(s). The total primary concentration is determined by superimposing the contributions of all sources.
I–A.2.3. Secondary dispersion
The secondary concentration of tritium as HTO, due to reemission of tritium deposited on the soil surface, is determined by estimating as indicated below, in the section on soil and re-emission, the upward flux of tritium at each terrain element (or grid node), and summing up
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over space the contributions of each of the terrain elements to the secondary concentration at each point of the grid.
It is assumed that the secondary and primary dispersion proceed under identical weather conditions, except for the wind speed which is assumed to be substantially reduced close to the ground where the re-emitted tritium comes from. As suggested by Smith and Singer (1966), the wind speed is extrapolated to the height 0.6 σ, where σ is the vertical dispersion parameter. This extrapolation is made by using the vertical profiles of wind speed referenced in the section above.
I–A.2.4. Wet deposition
The wet deposition of HTO is determined from a formula based on the use of an apparent washout coefficient. The long-term time averaged wet deposition flux at the receptor is estimated by the sector-averaged formula:
) /(uρθ Q
Fw = ΛΦ
where Q is the average emission rate; Λ is the washout rate of tritiated water for the average precipitation rate during the period of interest; Φ is the joint frequency of rain and wind into the sector containing the receptor j; u is the average wind speed; ρ the distance between source and receptor and θ the angle of the wind direction sector.
The washout rate of tritiated water depends on source height, distance from the source and rain intensity. In the case of elevated sources, the washout rate coefficient is nearly constant over a wide area (Belot 1998). The average concentration of tritium in rainwater is simply obtained by dividing the wet deposition flux so obtained by the corresponding average precipitation rate.
I–A.2.5. Buildup in soil and vegetation
The estimation of the soil water concentration is obtained by writing the balance of tritium average inputs and outputs to and from the root layer. The inputs are attributable to dry and wet deposition, the outputs to infiltration, plant uptake and re-emission from soil surface, the losses due to radioactive decay being negligible. The steady-state concentration of tritium in the soil water so obtained is independent of the thickness of the root layer and depends only on the relative magnitude of inputs and outputs. This yields the following formula:
) /(
)
( t w e s r
s v F v I
C = χ+ ρ +
where Cs is the concentration of tritium in soil water; vt is the transfer velocity of HT or HTO from atmosphere to soil; χ is the average concentration of tritium in the atmosphere; Fw is the average flux density of tritium wet deposition; ve is the exchange velocity of HTO between soil and the atmosphere; ρs is the water concentration in air saturated at soil surface temperature; Ir is the infiltration rate of water through the root layer. The density of the re-emission flux is assumed to be equal to veρsCs, that is to be proportional to the concentration of tritium in soil water.
The main difficulty resides in the estimation of the transfer / exchange velocities. If we consider only HTO, the transfer velocity vt is assumed to be equal to the exchange velocity ve, and is set equal to a value comprised between 0.001 and 0.003 m s-1 depending of average soil moisture and mean wind speed close to ground level. For HT, the transfer velocity is assumed to be ten times lower than the exchange velocity.
The concentrations in plant water are predicted using the equation of Raney and Vaadia (1965) that relates the concentration plant water to the concentrations in air humidity and soil moisture. The concentrations of non-exchangeable tritium in the combustion water of organic matter in leaves is predicted by multiplying the concentrations in leaf water by an isotopic discrimination factor taken to be 0.6 (Kim and Baümgartner, 1994).
I–A.2.6. Application of the model to BIOMASS scenarios
Scenario 1: The concentrations in air are determined from both primary and secondary dispersion for the weather conditions given in the scenario. The vertical dispersion parameters are taken from Briggs. The average exchange velocity at soil surface is 0.003 m s-1 and the washout coefficient is 6 × 10-4 s-1 for a precipitation intensity of 1 mm hr-1.
Scenario 3: The concentrations in air are determined from the primary dispersion only, using the weather conditions given in the scenario. The vertical dispersion parameters are taken from Briggs. The average exchange velocity at soil surface is 0.003 m s-1 and the washout coefficient is 6 × 10-5 s-1 for a precipitation intensity of 1 mm hr-1.
Scenario 5: The model, theoretically devoted to nearly flat terrains, is nevertheless applied to the Valduc terrain, in spite of the fact that the terrain shows elevational differences of about 100 m over several km in certain directions. This is supported by the observation made elsewhere that elevational differences in excess of 50 m over several km do not show significant topographical effects (Hinds 1970). The concentrations in air are determined by summing the contributions of primary and secondary dispersion. The vertical dispersion parameters are taken from Hosker (1974), and correspond to a mesoscale roughness length of 1m, which is assumed to be representative of the patchy forested area that surrounds the Valdux center. The average exchange velocity at soil surface is 0.002 m s-1 and the washout coefficient is 6 × 10–5 s-1 for a precipitation intensity of 1 mm hr-1.
References
[1] BELOT, Y., Predicting the washout of tritiated water from atmospheric plumes, presented at the workshop of the IEA Task group on tritium Safety and Environmental Effects, held at AECL, Chalk River, Canada, on May 11–12 (1998).
[2] BRIGGS, G.A., Diffusion estimation for small emissions (draft), ATDL contribution file No. 79, Atmospheric Turbulence and Diffusion Laboratory, Oak Ridge, Tennessee (1973).
[3] HINDS, W.T., Diffusion over coastal mountains of Southern California, Atmospheric Environment4 (1970) 107–124.
[4] HOSKER, R.P., in Proc. Symp. on Physical Behaviour of Radioactive Contaminants in the Atmosphere, November 1973, Vienna IAEA, p 291 (1974).
[5] KIM, M.A., BAUMGÄRTNER, F., Equilibrium and non-equilibrium partition of tritium between organics and tissue water of different biological systems, Appl. Radiat.
Isot. 45 (1994) 353–360.
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[6] RANEY, R., VAADIA, Y., Movement and distribution of THO in tissue water and vapor transpired by shoots of Helianthus and Nicotiana, Plant Physiology 40 (1965) 383–388.
[7] SMITH, F.B., A scheme for estimating the vertical dispersion of a plume from a source near ground level, In Proc. Third Mtng of the Expert Panel on Air Pollution Modeling, Paris, France, NATO-CCHS report 14, Brussels (1972).
[8] SMITH, M.E., SINGER, I.A., An improved method of estimating concentrations and related phenomena from a point source emission, J. Appl. Meteor. 5, (1966) 631–639.
[9] VAN ULDEN, A.P., HOLSTLAG, A.M., Estimation of atmospheric boundary layer parameters for diffusion applications, J. Climate Appl. Meteor. 24 (1985) 1196–1207.
I–A.3. MODEL USED BY AECL, CANADA
P. Davis
I–A.3.1. Introduction
The AECL model was developed by P.A. Davis of Chalk River Laboratories, Canada, in 1997. It is a simple analytical model implemented in FORTRAN 77. It is meant to calculate realistic concentrations of tritium in air, rain, soil and plants due to chronic releases of tritiated hydrogen (HT) and tritiated water vapour (HTO) to the atmosphere. The model was developed specifically for use in BIOMASS as an aid in evaluating and revising the Canadian derived release limit model for tritium. The model is described by Davis (1998) but the code is not
The AECL model was developed by P.A. Davis of Chalk River Laboratories, Canada, in 1997. It is a simple analytical model implemented in FORTRAN 77. It is meant to calculate realistic concentrations of tritium in air, rain, soil and plants due to chronic releases of tritiated hydrogen (HT) and tritiated water vapour (HTO) to the atmosphere. The model was developed specifically for use in BIOMASS as an aid in evaluating and revising the Canadian derived release limit model for tritium. The model is described by Davis (1998) but the code is not