Andrei V. Gouskov, Lev B.Prozorov, Sergei L. Speshilov
Moscow Scientific and Industrial Association "Radon"

Investigations of actual contaminated water interaction with geological barriers are very important for radioactive waste (RAW) storage facilities safety assessment. Use of mathematical models for radionuclide migration prediction needs many data to describe real situation. Use of another object data (or from literature) for predictive modeling often causes some errors. Moreover some data obtained earlier for another purposes are often inaccurate. The complex of researches for geological-and-ecological safety assessment of RAW isolation in concrete geological-and-hydro-geological conditions is offered to your attention and is illustrated by the real example.

The Moscow SIA "Radon" accomplishes processing and deposition of RAW in central region of Russia and is leading organization in the system of 16 similar facilities in Russia. It renders scientific-and-methodical and technical support to these enterprises.

The disposition of the "Radon" specialized enterprises on territory of Russian Federation is shown on Fig.1. The "Radon" system was created in the beginning of the 60-th years for gathering, processing, and temporary storage of low and intermediate level radioactive waste (LLW/ILW). It is accumulated now about 100000 cubic meters of LLW/ILW with total activity more than 1 million Ci and many ionization radiation sources with total activity exceeding 1,5 million Ku in Russian "Radon" waste storage facilities.

Almost all facilities constructed in 60-s years are repositories of a near surface type. Ecological requests to such type of structures were changed essentially for last 40 years. Generally adopted principle of insurance of safety during RAW isolation is now the principle of multibarrier cover.

The essential role is played by artificial engineering barriers preventing radionuclides to get in the sphere of human's activity. However, the period of RAW potential danger is about 300-500 years and it is very probable, that the engineering barriers will be destroyed (infringed) for this time. Therefore a natural geological barrier just plays the most important role from our point of view. And so investigation and prognostication of radionuclides migration is important for an evaluation of geological barriers efficiency and safety of repositories.

Some laboratory and field researches of containing rocks protective properties on one of the waste storage facility in Volga-river region were carried out by the authors in 1996-97. The main objective of researches was waste storage safety assessment on inspected facility site from the geological and hydro-geological point of view.

The complex of researches for an evaluation of geological-and-ecological safety of LLW/ILW isolation in concrete geological-and-hydro-geological conditions has been developed and tested by the authors.

This complex includes:

• prospecting drilling for clarification of rock variability in a top of a geological section;
• determination (obtaining) of filtration properties of the rocks being a bottom of RAW facility in natural conditions (field experiments);
• determination of contaminated water structure (full chemical and radionuclidal structure) and it's hydrochemical properties;
• laboratory study of real radioactive solutions interaction with containing rocks for determination of approximate (rough) significances of migration parameters in static conditions and in dynamics (on filtration or diffusion models);
• field experiments for selected mathematical model verifying and it's parameters clarification;
• prediction of possible migration of radionuclides from a repository under an engineering barrier destruction conditions for long term.

Now consider the geological conditions of specific wastes storage facility in Volga river region. The RAW storage facility site is placed in a hallow cutting an ancient hill. The preliminary information showed that upper 20-30 meters of a geological section are clays. Predictive calculation for cesium-137 transport in case of complete engineering barrier destruction was carried out in 1994 on this basis. Migration and filtration parameters for clays and cesium-137 were taken for this prediction from literature.

On the first stage the geologic information was investigated and updated by the authors. The researches conducted in 1996 have shown that the bottom of RAW facility is on the level of a powerful diluvial loamy sand layer with high filtration properties. The obtained data showed, that the predictive modeling having been executed earlier was not correct.

Drilling of an additional directional well (45° ) under RAW facility No1 confirmed that a diluvial loamy sands layer is a foot and containing rock for this facility.

Three more exploratory wells drilled in 1997 allowed us to get a longitudinal and lateral sections of the site (Fig. 2).

Geological investigations also showed that in spite of the presence of rather nontight rock layers there was no temporary ground water horizon and more over there was no real possibility to format it. This is stipulated by availability of dense loam layer (0.4-1.4 m) on a top of aeration zone section and by large slope of land surface causing practically all precipitation to spent for a surface drain, transpiration and evaporation.

On the second stage filtration properties of the rock were investigated in laboratory and field conditions. Rock examples were taken for laboratory investigations on the first stage. Determination was carried out according to methodology and standards being used in Russia. The field experiments also were carried out in a distance of 5 m from the facility to determine filtration properties of the loamy sand. It was dug a drug hole of 1.5 x 1.4 m in plan and 1.5 m depth. Three filtration experiments were fulfilled in it according to standard methodology. It allowed to obtain such data as: transport porosity - 0.53, filtration coefficient - 0.69 m/day, factor of hydrodispersion - 0.00206 sq.m/day, water unsaturation - 0.33.

Therefore geological features of the site can be characterized as follows:

• large thickness of aeration zone (the upper aquifer is on depth of more than 60 meters);
• the diluvial loamy sand layer with 11 meter thickness (permeability 0.5 - 1 meter per day) in a top of geologic section is spread by tight rocks (aquiclude), which reliably protects the aquifer from possible contamination from a land surface;
• small amount of atmospheric precipitation and rather large slope of a surface (up to 20 degrees) cause an absence of sporadic near-surface underground water and aquifers.
• The negative characteristics of inspected object are:
• accumulated water presence in a repository with solid wastes;
• layout of facility bottom in rather nontight rocks.

Water examples were taken from the same disposal facility for laboratory tests. On the third stage chemical structure of contaminated water was determined: HCO₃^- - 96.1 mg/l, NO₃^- - 3.7 mg/l, SO₄^2-- 7.5 mg/l, K⁺- 100.0 mg/l, Mg²⁺- 16.8 mg/l, Ca²⁺- 16.0 mg/l, pH - 7.65. Underground water chemical analysis is rather different: HCO₃^- - 488 mg/l, SO₄^2-- 720 mg/l, Na⁺ & K⁺- 37.5 mg/l, Mg²⁺- 42.9 mg/l, Ca²⁺- 326.6 mg/l.As a whole, chemical structure of investigated solutions testifies that they were derivated owing to interaction of the atmospheric precipitation with solid radioactive wastes disposed in facility. It may be supposed that the water has been accumulated in RAW facility before it was closed.

Radionuclidal structure: ²²⁶Ra - 13.1 Bq/l, ⁶⁰Co - 41 Bq/l, ¹³⁷Cs - 8407.5 Bq/l, ²³²Th - not detected. Cesium - 137 can be considered as the main contaminant and the most important indicator.

Laboratory study of real radioactive solutions interaction with containing rocks was studied on the next stage.

It is known, that the interaction of actual contaminating solutions with containing rocks is rather complex. The migration of contaminants in layers is determined by convective transport (filtering), by longitudinal and transversal hydrodispersion, by difference between density of displacing and displaced liquids, by delay in deadlock (isolated) pores, by chemical interaction, by physical sorption, by diffusion, by natural decay etc. A quantitative evaluation of migration is rather difficult because of contaminated water, as a rule, is mixed and contains a few migrants. The separate contaminants besides can be in the several forms. The forms of migrants- contaminants can not always be determined rather precisely. Such situation makes interpretation of experiment outcomes much more difficult.

The indicated features complicate studying of water contaminants migration very much. It is necessary to carry out tests for each object: for concrete mixed water and for concrete containing rocks.

Some properties of solution and rock were studied earlier and just some properties of interaction between solution and rock were studied on the fourth stage.

At first we supposed that the interaction can be described by the constant factor of distribution for cesium-137 in liquid and in solid phase. A number of physical tests were been carried out in static to get this coefficient. It was about 11000 mL/g but the chemical equilibrium time exceeded 250 hours. The solution was diluted and strengthened to obtain a number of different concentrations and as a result the sorption isotherm (Fig. 3).

It is obvious that obtained data are only approximate significances of migration parameters.

Another more data were obtained in dynamics. Filtration column of 0.5 m length was filled by the real loamy sand and real solution of 1.4 m length (contaminated atmospheric precipitation) was filtrated through the column. Cesium-137 distribution in loamy sand after filtration showed that the mathematical solution for "unlimited" adsorption capability of rock can be used to describe the real solution and rock interaction process. In this case the length in which cesium-137 relative concentration (in liquid) changes from 0.99 down to 0.01, taking into consideration the dispersion, can be evaluated as follows [1]:

Where - factor of a dispersion;

The factor of sorption velocity has been defined as a =19.303 1/day (a =0.804 1/hour) based on this laboratory test. Predictive calculation of cesium-137 linear transport has been carried out. The outcomes show that the length (depth) of contamination will not exceed 0.25 m for such a solution and for such a loamy sand. The use of "unlimited" adsorption capability model is probably suitable for this specific conditions because of rather low cesium-137 concentration in this solution and fairly high sorption capacity of this loamy sand.

The field experiment was carried out on the fifth stage to verify this model. Three tests were fulfilled in the same drug hole as on the second stage. Cesium-137 concentration in rock after 1, 2 and 3 m filtration accordingly is shown on the Figure 4.

It can be seen that contaminated zone does not exceed 0.15-0.17 m independently of solution volume and total activity. This confirms that the used mathematical model is correct to describe the linear (one-dimensional) process of actual contaminated water interaction with geological barriers for conditions being under consideration.

Field experiment outcomes are processing this time and the final results will be obtained later.

Long-term experience of the authors in the field of radioactive isotopes migration allows us to make a conclusion, that the most effective models for practical purposes are such mathematical models, which are based on empirical outcomes of researches of interaction of actual contaminated waters with concrete geological rocks. It is necessary to execute complex laboratory and field experiments for such researches. It is not possible to make the authentic prognosis of migration and reliable conclusion about natural barrier efficiency only on the grounds of some theoretical researches.

In some cases radionuclide transport predictive model does not claim a numerical solution of the migration equations, and can base on a known analytical solution for a linear stream.

The researches carried out on one waste storage facility have shown, that protective properties of geological barrier allow not only to store wastes on this site reliably, but also to bury solid wastes in near-surface capacities. It will require to provide the site by the monitoring system and reliable protection during all period of potential danger of wastes.

1. F.M.Bochever, N.N.Lapshin, A.E.Oradovskaja. "Underground water protection from contamination," NEDRA, Moscow (1979), in Russian.

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