FEATURES OF RADIONUCLIDES MIGRATION FROM THE
NEAR SURFACE TYPE RAW STORAGES
E.I.Veselov, V.V.Martianov, L.B.Prozorov
Mos SIA "Radon"
ABSTRACT
The near surface type depositories (storages) are used for low and midle level radioactivity wastes placement. The advantages of this depository type consist of construction simplicity, filling and operation. This type depository is used in Russia from the beginning of 60-th. During this time (more than 30 years) a significant knowleges containing information about processes which happen within the borders of RAW storages and geological environment are accumulated. The analysis of this information has allowed us to note some features concerning radionuclide migrations from RAW depositories.
INVESTIGATION
The IAEA concept (multi-barier) is used in designing and constructing RAW storages. Both geological (clay or loam layers) and engineers (RAW containers, concret blocks and isolation materials) bariers are important factors protecting environment from radionuclide migration.
The mathematical simulation is applied for radionuclide migration from RAW storages forecasting. The used (existing) radionuclide migration models assume the contaminated solutions transition in vertical direction (down wards) to the first aquifer from the surface. Then the process of radionuclides migration in this aquifer is simulated.
Fig. 1. Radionuclides migration prognostication (the traditional approach)
Practical experience of near surface RAW depository operation and the monitoring data have shown some difference from theoretical accounts.
There are some reasons forming these differences. The technology of RAW depository construction provides platform preparation on the surface or foundation ditch shaping, in which the design of depository is placed. After RAW depository filling and its cementation stayed free space is filled with earlier withdrawed ground. Under effect of building mechanisms on the territory of RAW placement the violation of superficial stratum ground structure up to depth 1.0 - 1.5 m is happend.
In this case clay being a basis of geological barriers, acquire increased filtration properties. The development of vegetation also results to superficial stratum structure violation.
The climatic conditions of an average band of Russia includes raining and snowing season periods , freezing and thawing of the ground, also promote to ground properties modification. The thermal conduction of concrete and clay is various. It is resulted to deformation processes on such contact as clay - concrete. The "body" of RAW storage is gradually deformed under the operation of temperature, dynamic pressure and leaching.
Besides, placement of the constructions on the significant territory results to diminution of evaporation square.
Thus, the human influence into environment results in perched water formation.
Fig. 2. Atmospheric precipitation
The previous hydro-geological researches were conducted for acknowledgement of perched water formation. The drilling data and ground tests have shown violation of clay structure, filtration properties magnification and local aquifer availability generated within the borders of RAW storage. It was determined, that the unbroken ground is characterised with filtration coefficient 0,001 m/day. The filtration coefficient of infringed clay are changed from 0,05 up to 0, 5m/day. The porosity of ground has increased from 0,30 up to 0,48-0,5. The preliminary analysis of underground waters structure has shown availability of leaching processes from the "body" of storage (Ca2+, Mg2+ and HCO3- magnification). The aquifer saturation happens as a result of atmospheric precipitation, and level of ground water is changed depending on the amount of precipitation. Ground water were not revealed in the unbroken grounds abroad RAW storage.
PROTECTIVE BARIERS AND RADIONUCLIDES MIGRATION MECHANISM
The local aquifer formation increases a possibility of radionuclides migration in an environment. For an evaluation of this possibility the script of radionuclides intrusion from protective barriers and their migrations in an external medium is developed.
The model of radionuclides intrusion assumes, that under influence of the natural factors (cycle a freezing - the thawing of atmospheric precipitation etc.) happens destruction of the protective natural barrier structure. This process results in formation of cracks and channels in the ground, and movement of natural water (especially in the places of ground and concrete contact). A bit later natural barrier destruction increases and natural water interacts with RAW storage engineering designs. The ground water penetrates inside a depository destroying it. Further these waters advance to leaching of radionuclides from RAW storage. The further contaminated water filtration or diffusion results to radionuclides accumulation in infringed grounds of RAW storage.
It is necessary to note, that the infringed grounds within the limits of foundation ditch are filled with water as well at the reason of formed aquifer. The perched groundwater, formed as resultof human activity, mix up with the contaminated waters of foundation ditch and bear radionuclides in environment.
Main radionuclides part (up to 95 %) migrates with surface ground waters. The smaller part (up to 5 %) will hit into underlying grounds by infiltration. Therefore main role in the radionuclides migration transport is played by manmade perched groundwaters, which may connect with hydrographic drainage net.
NEAR SURFACE TYPE RAW STORAGE INSPECTION
The pilot survey of RAW storages has shown their tightness and high reliability. The elimination has made for one depository, which represents double-storey design from a series of RAW blocks separately constructed at the miscellany time.
Inflow of atmospheric waters into the storage contour and radionuclides migration out of limits of the storage was revealed.
The data of clay and loam, forming a protective barrier around RAW storage , has shown significant increase of filtration properties of these rocks. The unbroken grounds have filtration coefficient not above 0,001m/day and porosity at a level 0.3. The filtration coefficient of researched rocks (clay and loam ) was in intervals from 0,01 up to 0,5m/day. Thus the filtration coefficient of the upper stratum (up to 0,1 m) reached the first meters per day.
It was found, that the level of ground waters is changed depending on atmospheric precipitation amount. Thus the lower part of storage is constantly flooded and is exposed to destroying effect of ground waters stream. The atmospheric precipitation influences the upper part of storage, penetrating through an infringed protective clay and loam barrier.
After removal of the upper protective ground barrier, on the ferro-concrete surface of storage a system of cracks and microcracks were found out. The storage constructional material researches have shown, that the water rate can reach 0,01 m3 per day through 1m2 of the surface.
The grounds radiation condition study has allowed to reveal a series of the radionuclide output centers. Data has shown the centers have a local character and are insignificant on scale.
The radiochemical analysis of ground has shown, that the anomalous sites are submitted by 137Cs and 90Sr. Thus the anomalous radionuclides concentration are found out up to depth 1,0-1,5m from a surface. Below than indicated boundary the increased radionuclides contents are not revealed.
Fig. 3. Depository plan.
The conducted researches have confirmed the main concept of violation of RAW storage structure model and radionuclides migration with the stream of ground waters.
CONCLUSION
Migration scenario includes next steps:
3. Penetration of contaminated ground water into present drainage network plays the main role in radionuclide spreading
REFERENCES