EPIC is a tool to study the transport of radionuclides in soil and aquifer.
The project and the partners
In collaboration wih the Ukrainian Institute of Agricultural Radiology (UIAR) and the Institute of Geological Sciences (IGS), the Institue for Radioprotection and Nuclear Safety (IRSN) has equipped and maintained since 1999 an experimental site in the exclusion zone of the Chernobyl Nuclear Power Plant, 2.5 kilometres west of power unit number 4 wich exploded in 1986.
The aim of the research carried out in this platform is to develop models to predict how radionuclides can migrate from a source of contamination to man. The involved Ukrainian Institutes use the experimental site and the subsequent numerical modelling for applied risk assessment analysis of the Chernobyl waste dump area, and more generally for the assessment of sites contaminated by nuclear fuel material.
The experimental site (EPIC site) provides a rare opportunity to study the transport of radionuclides from a waste trench in a reallife complex soil-aquifer system. This allows IRSN to acquire important knowhow in the handling of polluted sites (nuclear or other) and to compare mostly laboratory and modelling efforts with a field situation.
The field site: layout and instrumentation
The EPIC site is a rectangular area of 100 m x 80 m centred around one shallow trench filled where radioactive debris contaminated by fuel particles were burried after the accident.
There are three levels of interest on the site:
The unsaturated zone (UZ), at the surface, consists of eolian sand, almost pure quartz. In the unsaturated zone, there are three clusters each composed with captors for monitoring suction pressure, moisture content and temperature at different depths. One cluster records the variations of these parameters in the trench and the two others measure the background natural values of the same parameters. Ceramic samplers are used to determine the chemical and radiological composition of water in the UZ. A weather station measures rainfall, wind direction and speed, hours of sunshine, temperature, humidity, and temperature of the upper part of the soil.
The waste trench is present from soil surface to a depth of 2.5 m in the unsaturated zone and has also been equipped with instrumentation. It contains mainly sand contaminated with nuclear fuel particles spread on the ground after the explosion.and some decaying radioactive organic material.
The phreatic aquifer comprises both eolian sand and alluvial deposits. The alluvial deposits are more clayey and hence less permeable. In the aquifer, the piezometers allow observation of pressure variations and regular groundwater quality sampling.
Problem statement and methodology
Current investigations focus on the transport of strontium-90, cesium-137 and plutonium in the UZ and in the aquifer. The different levels, trench, unsaturated zone and aquifer, require a different scientific approach and an adapted methodology in order to formulate the corresponding conceptual models.
The main factors that play a role in transport of radionuclides are:
The dissolution rate of the nuclear fuel particles in the trench,
The diffusion and dispersion of radionuclides in both unsaturated and saturated zone,
The interaction of the radionuclides with the sediment.
Investigations are carried out on the field and completed in the laboratory in order to qualify the models.
The models qualified for strontium-90 migration form a basis for various modeling scenarios, in which radionuclide migration can be studied on large time scale (1 000 years) in the case of changing boundary conditions.
The case of the transport of strontium-90
The source term
The source term consists of trench filled with waste. Strontium dissolves and transfers from the trench to the unsaturated zone with percolating rainwater.
The unsaturated zone (UZ)
The transport of strontium in the UZ is governed by its water content or suction pressure and by its geological heterogeneity, as shows the heterogeneous distribution of strontium in the alluvial sand. To study these characteristics of the UZ, transport of water or a tracer is followed in time in several column tests filled with eolian sand installed in laboratory.
Such column tests allow to derive the relationship between suction pressure and water content and between hydraulic conductivity and water content, which are column hydrodynamic parameters. It is also possible to follow water flow and pollutants migration, in order to establish the transport models in unsaturated zone. The interaction of an element with the sand/silt matrix, another important factor; is quantified by its distribution coefficient or Kd (ml/g).
Transport through the aquifer
The grain size distribution of soil shows that alluvial sand is more silty and clayey than eolian, and confirms the difference in hydraulic behaviour between alluvial and eolian sand. The cation exchange capacity (CEC) for strontium-90 will be higher in the alluvial because of a higher clay content. This leads to a higher distribution coefficient for strontium-90.
Continuous pressure recording in different piezometers on site and in the exclusion zone give the direction and magnitude of the water flow through the aquifer. The change in pressure is mainly related to rainfall or snowmelt events.
Regular sampling campains follow changes in groundwater quality and the movement of radionuclides, strontium-90 in particular.
All of this information allowed the numerical simulation of the Sr90-plume from the trench through the UZ and in the aquifer as represented here in a cross-section along the cross-section AB.
Research to come in the next years will aim to :
Investigating the influence of transient flow variations, hydraulic and chemical,
Understanding the details of transport through the unsaturated zone,
Looking at the influence of the trench composition on the aquifer chemistry,
Identifying the possible transport of some transuranium elements with complex chemical behaviour.