The Russian concept for deep geological disposal of solidified high-level radioactive waste (HLW) is to create several large regional repositories, especially near two large radiochemical enterprises: PA Mayak in the Ural Mountains; and the Mining-Chemical Combine (MCC) in Siberia.

The research on substatiation and selection of a site was carried out over several years by the Khlopin Radium Institute, Minatom, the Ministry of Sciences, and Krasnoyarsk Land. Site selection is based on criteria developed by the IAEA, on concepts of safe isolation of radioactive wastes into geological formations, on social-economical and other aspects.

The works are carried out stage by stage, with successive enlargement of scale, detailing the search, and justifying safety of proposed sites.

Site selection

To determine the most uniform and least dislocated subsurface areas, complex analysis of geological-geophysical and geodetic-topographical information was carried out. Special attention was paid to intraplatform granitoid massifs and the Archean metamorphic complexes framing these massifs.

Originally, 20 areas were selected among the most ancient gneiss bedding and massifs of granitoid rocks covering about 22,000km2. Each was evaluated by several criteria. The granitoids of the Nizhnekanskiy massif were identified as the most promising geological formation for solidified HLW disposal. This massif is one of the largest granitoid massifs of Middle Siberia, with an outcrop area of over 1500km2. It consists of granitoids of three different time phases, as follows:

• Dioritoid phase, consisting of diorites, quartz-diorites, tonalites, and granodiorites.

• The Main Granite phase, consisting of biotite granites, leucocratic granites and alaskite.

• The vein phase.

The age of the massif’s rocks is 910+/- 60 million years. According to the data of gravitation surveying, the massif has a thickness of 4-6km.

Due to combined investigations on scales 1:100,0000 – 1:50,000, carried out in the northern part of the Nizhnekanskiy massif, including electrical surveying, lineament analysis, geomorphological analysis, geological modeling using gravimetric survey data, there were selected the two most promising sites: the “Itatskiy” and the “Kamennyi” ones. Each of the sites has an area about 15-20km2, and are 25-30km from the MCC.

Site study

It is necessary to find granitoid blocks with minimal fracturing and water permeability. Most attention was given to geophysical works, first of all, to electrical surveying using methods of audiomagnetotelluric sounding, radiomagnetotelluric profiling, and vertical electrical sounding. These methods measure the value of apparent specific electrical resistance of rocks, which is mostly defined by water saturation of the rock that, in most cases, interplays with water permeability of rocks.

Electrical surveying showed sufficiently large monolithic granitoid blocks at both sites. These blocks have low specific electrical resistivity (from 7000 up to 50,000 Vm), while zones of jointy (near-surface and near-fracture) granitoids have lower values of specific resistance. Monolithic granitoids are traced up to 3-5km in depth. The borders of monolithic rock blocks are tectonic.

In 1998-99, a number of shallow (10-50m) and four deep (300, 500, 500 and 700m) boreholes were drilled at the “Itatskiy” and “Kamennyi” sites. Study of borehole cores and logging data confirmed the presence of monolithic granitoids having low fracturing and water saturation.

Thus the data on deep structure of the massif, obtained by surface (indirect) geophysical methods and is confirmed by direct data of drilling only up to a depth of 700m. The foremost task of further studying the massif is to drill deep boreholes up to 2-3km.

Examination of rocks forming the massif showed they are different varieties of granitoids, predominantly leucocratic and biotite granites, granodiorites, tonalites, and quartz-diorites. The presence of such accessory minerals as zircon, monazite, different iron oxides and sulphides in granitoids is an important factor. On the one hand, these minerals intensify the barrier role of host-rocks containing the repository. On the other hand, they allow matrices to develop for the disposal of transuranium elements, which are geochemically compatible with the host-rock. In particular, to dispose of the actinide fraction of HLW in granitoids, the Radium Institute developed the optimal compositions of crystalline ceramics on the basis of Zircon (ZrAn)SiO4, zirconium dioxide (ZrGdAn)O2, and others.

The rocks are the main barrier against radionuclides for disposal into the biosphere. Therefore, the empirical data on water permeability, radiation and heat stability, and also retention properties of rocks relating to the main long-lived radionuclides of HLW are important. Special radiochemical studies show that the retention of cesium, plutonium and americium by sorption for granitoid rocks of the Nizhnekanskiy massif is high enough, indicated by high values of distribution coefficients. It was also determined that plutonium, americium and curium can be sorbed on accessory minerals of granitoids and the degree of their transfer into solid phase increases with phase contact time. This increasing and low values of desorption suggest an isomorphous replacement of chemical analogues by radionuclides in solid phase, such as in zircon and monazite. Incorporation of radionuclides via diffusion and their retention by minerals limit the migration range of long-lived radionuclides in monolithic rocks under the influence of migrating water.

The results of geological-geophysical investigations serve as a basis for designing an underground research laboratory. Two variants are being considered: using the system of boreholes drilled from the surface, and the combinations of drift and boreholes. Realisation of one variant will depend on a series of different research, organisational and economical aspects.

Work description

The rocks investigated were sampled from borehole cores. Investigations showed that in the most promising sites, the rocks are granitoids of all phases of Nizhnekanskiy complex. The most widespread granitoid varieties in the sites are leuvocratic granites, biotite granites, grandiorites, tonalites and quartz-diorites. The last three varieties (the ‘dioritoids’) belong to the First Phase. Between the three ‘dioritoids’, there are no distinctive borders, and chemical and mineral compositions are transient.

Besides main rock forming elements (O, Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K and P), there are some minor elements, with concentrations of less than 0.01% by weight. The minor elements include Ba, Sr, Zr. Concentration of uranium in the granitoids is low, and the concentration of thorium is greater than that of uranium.

The mineral compositions consist of different ratios of feldspars, quartz and micas. A specific feature of the granitoids of the Nizhnekanskiy massif is a high concentration of magnetite, up to 1.3%. Due to intensive chloritisation, chlorite concentration reaches 1-3%. Chlorite is often the main component of the material, filling old cracks and fractures. The main accessory minerals are: sphene, apatite, ilmenite, and zircon, and monazite and sulphides (pyrite and galena) found in the diorioids. Besides chlorite, secondary mineralisation is by epidote, zoisite and muscovite. Secondary iron oxides (hematite, limonite, martite), having high sorption characteristocs, occur in the dioritoids.

To characterise a value for matrix permeability of the rocks, experiments using samples of the granitoids were carried out. Round plates 4-9mm thick were sawed from rock samples and fixed in special cells. Air or water, with a fixed temperature, was forced under different pressures through the plates. The rate of air (or water) leakage through the plates was measured. The values of matrix permeability were calculated. They were 10-19 – 10-20m2 for the most monolithic specimins prepared from weathered surface samples. These values correspond, within an order of magnitude, to a hydraulic conductivity of about 10-7m/d. The last value is close to that required for construction of the repository (10-8 – 10-7m/d). One should expect that at depth, the matrix permeability of granitoids is much lower.

Some plates were measured both before and after single-pass heating at 150°C in water media during 100 hours. The increase in temperature increased the permeability up to 1.5-4 times.

Differential Thermal Analysis (DTA) showed that granodiorites have only two observed effects within the temperature range 20-900°C. These effects are dehydration of micas and the structural transition between low- and high-temperature modifications of quartz. The temperature range of mica dehydration in radiation-free samples lies within the range 110-570°C. However, gamma-irradiation of granitoids leads to an increase in the temmperature of this process up to 550-650°C after exposure for 107Gy. It should be noted that the dehydration process is observed only for granodiorites, where the portion of water-containing layer lattice silicates (biotite, chlorite) can reach 20%. In this case, total water loss through the rock is not more than 0.2-0.3% by weight. The second temperature effect is the a-b transformation of quartz at 573°C. This does not lead to mass losses, but it can cause intensive disturbance of rock solidity due to change of volume of the quartz grains. This transformation was observed for both groups of granitoids.

Examinations of retention properties of granitoids relative to long-lived radionuclides have been conducted at the Radium Institute for many years. Studies show that sorption retention of plutonium, americium and curium can be sorbed onto accessory minerals (zircon, monazite) in granitoids, and a degree of transfer into the solid phase grows with time of contact between phases. Such growth and low desorption values imply that isomorphic replacement of chemical analogues by radionuclides occurs in the solid phase.

results of the investigations

The main results of the HLW disposal site investigations carried out to date are:

• Selection of granitoids of the Nizhnekanskiy massif as a suitable geological formation for disposal of solidified HLW.

• Selection and stage-by-stage substantiation of two promising sites within the massif for construction of the HLW repository.

• Accumulation of data to construct a research laboratory and an ecologically safe HLW repository.

• Determination of the objectives for subsequent stages of the work (detailed geological exploration of a chosen site).

Rocks in the most promising sites at the Nizhnekanskiy granitoid massif are represented mainly by widespread stable rock-forming minerals. The presence of iron oxides and sulphides is a positive factor due to their high sorption ability. Some specific accessory minerals (zircon and monazite) are also sorbents for long-lived actinides.

The role of layer lattice silicates (micas, chlorite) is ambiguous. On one hand, these minerals are good sorbents. On the other hand, when overheated, these minerals can be a source of excess groundwater. However, due to low matrix permeability of the rocks, a contribution of this water will be insignificant relative to the total volume of mobile groundwater.

Low values of matrix permeability show that a portion of water passing through the granitoid monolith will be less than the groundwater passing along fractures. Most cracks and fractures are filled by secondary minerals having high sorption ability.

On the whole, rock specimins from the Nizhnekanskiy massif promote consideration of these rocks as a suitable host medium for HLW disposal. The final conclusion about the suitability of the rocks requires detailed geological exploration in the most promising site and experiments in situ, which will give more detailed data for long-term forecasts.
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