One of the main consequences of underground nuclear explosions (UNE) is destruction and radioactive contamination of mountain rocks where UNE was performed. Radioactive contamination of mountain rocks may be subdivided into primary and secondary contamination.
Physical and mechanical destruction of mountain rocks is studied in more detail. As an example, the figure presents destruction of rocks at the testing ground Balapan. Our investigations made it possible to develop methodology for detecting zones of rock destruction due to both single nuclear explosions and due to numerous explosions in places with high number of explosions per unit area. Analysis of integral porosity spatial distribution shows that the strongest rock destruction occurs in the middle part of Balapan testing ground in vicinity of wells 1314, 1304 and 1344; smaller destruction is registered to the north-east in vicinity of well 1203. In this case, integral porosity consists of porosity of mountain rocks and values of “voids” in geological medium formed as a result of rock deformation by UNE. In the areas with high UNE density, integral porosity, as a whole, increases with depth to a depth of 400 m, and starts to decrease at higher depths. Maximal porosity in anomalous zones of destruction equals to 10% (depths 300-400 m), minimal porosity is registered beyond the explosion areas.
As a result of UNE, several zones with different degree of geological medium disruption are formed. For example, for a <150 kilotons explosion performed 500 m below surface, several spherical zones with different rates of rock destruction are formed: I – cavity (area of rock burn-out of a radius up to 5 m), II – 70m thick crushed zone, it is a destroyed rock turned into crushed stone, III – a 150 m zone of intensive fractures, IV – zone of block fractures stretching up to 100m. For the given example zone of local irreversible changes can stretch to a distance of 500m from nuclear cavity, i.e. reach the day surface.
It is characterized by displacements of mountain rocks in the form of opening-up of tectonic fractures and faults existing before the explosion.
Primary radioactive contamination of the geological medium during UNE occurs as a result of spreading of high-temperature gaseous radioactive fluids from central cavities up to the day surface along the complex systems of earlier existing and activated under the UNE impact zones of fractured disruptions and high fracturing. In the central zone a radioactive melt containing up to 40% of radioactive products is formed, the other part of radioactive products is spread in the areas of irreversible rock deformation.
Secondary radioactive contamination of mountain rocks at Semipalatinsk Nuclear Test Site is caused by presence of ground waters on all technical grounds where UNE were made. When ground water reaches zones of irreversible deformations, it dissolves artificial radionuclides and moves them out in the geological medium transporting them to considerable distances. In this case, the main contamination spreads through radionuclide redistribution with water in the mountain range by sorption on the walls of channels along which water moves. An indirect proof of existence of such process is the presence of tritium in drainage waters of the fluorite mining pit located in the northern part of Degelen range. At the same time, absence of other nuclides in drainage waters shows that front of their secondary contamination has not reached the boundaries of the Degelen Mountains. It should be noted that this problem is very actual and important for the SNTS. It is caused by the fact that, by chance, SNTS territory is reach in mineral resources. Uncontrolled mining of mineral resources may result in their irreversible loss.
Therefore, radioactive products formed in the places of UNE explosions at SNTS territory are distributed in the geological medium in three conventional zones. Their main part is concentrated in the block of mountain rocks with radius up to 100 m. This zone is characterized by radioactive contamination with high concentration of artificial radionuclides, which creates real radiation hazard. The volume of the second zone is much greater, it can reach the day surface as it can have radius of 500 m and more, but the level of radioactive contamination in this zone is much lower. The size and shape of the third zone vary widely as it depends on hydrogeological conditions of the area, degree of rocks deformation and peculiarities of radioactivity distribution during primary contamination.
A unique feature of UNE explosions at the Site is that most UNE on the Balapan testing ground were made in rocks containing coaly materials. The presence of ground waters and high temperatures typical of UNE gave rise to the process of underground gasification of mountain rocks. Such process under the ground is fraught with a real danger of formation of “charged” wells with outbursts of underground gas accompanied by its inflammation. The most powerful catastrophic manifestation of gas presence on the territory happened in April, 1992, on the testing ground Balapan, in the near-mouth part of “charged” well “Glubokaya” (UNE was made in 1977). In April, 1992, in the vicinity of the mouth of “charged” well the ground collapsed and formed a hole. Formation of a hole was accompanied by an explosion heard at a distance of 10 km and fire seen at the same distance for two days. The crater formed in this accident had a diameter (in the upper part) more than 50 m and depth up to 20m.
As an indirect proof of still undergoing underground burning, one can present the results of near-surface thermometric survey in the epicenter zone of “charged” well 1207. Registered thermal anomaly of the size of about 100×200 m is displaced at about 200 m away from the well mouth (UNE epicenter).
Investigations showed that composition of released gases corresponds to the composition of gaseous products of thermal oxygen-free destruction of coal-bearing rocks, which is witnessed by high concentrations of methane, carbon oxide, sulfurous anhydride and hydrogen sulphide.
Therefore, in addition to hazards from artificial radiation exposure, the Test Site hides some other hazards caused by effects of residual manifestations of long-tem geothermal activity in mountain rocks formerly embedding UNE. Such processes at SNTS lands may be accompanied by catastrophic phenomena and are to be carefully monitored.