9.3.2.1 Monitoring of contamination from nuclear accidents
Nuclear accidents often cause a large range of contamination, and bring great losses to people's lives and national economy, which attracts worldwide attention. Geophysical monitoring of nuclear accidents can be roughly divided into two major parts: one is the rapid monitoring of a large area that begins after a nuclear accident occurs, in order to understand the scope and direction of the day-to-day spread of contamination and to take the appropriate preventive countermeasures; and the second is the long-term monitoring of all nuclear facilities, so that in the event of an accident, it is possible to understand the original radioactive background as well as to track the gradual elimination of the process of post-accident contamination.
(1) Chernobyl nuclear accident monitoring
Early before the completion of the nuclear power plant, the Soviet Union's Ukrainian Academy of Sciences from the early 1960s through the monitoring station in Kiev on the area around Kiev (including the Chernobyl region) to carry out long-term monitoring of the radioactive environment. Parameters monitored included gamma radiation background values (measured with a radiometer), measurements of the radioactivity of fallout (collected with a flat-bottomed disk measuring 40 cm x 40 cm, with a piece of glycerine-soaked filter paper laid on the bottom of the disk, for a period of two weeks, with samples placed in porcelain crucibles and heated in an electric furnace to 500° C for ashing, followed by determination of the intensity of their β-radiation), and detections of radioactive contamination of the soil (at a depth of 10 mm, with a square sampler, 5 cm under the ground surface). depth of 5cm below the surface with a square sampler 10cm × 10cm samples, samples air-dried, ground, sieved, and then determine the intensity of its β-radiation).
Before the accident, the dose rate of γ radiation was 10 to 12 μR/h (background value), and after the accident on April 26, 1986, it rose to 5 mR/h on April 30, which was about 500 times higher than the background value. The gamma radiation values varied strongly in the following days and were related to the continued leakage of radioactive material and weather changes.There was also another peak on May 9 after the reactor exploded again.At the end of 1986, the gamma radiation was reduced to 50 μR/h, and then again in 1992 (before the monitoring was published) to 16-18 μR/h, which is close to the background value before the accident.
The beta activity in soil (by soil mass) was 550-740 Bq/kg before the accident, and rose to 29,600 Bq/kg after the accident.
The mass activity of radioactive 90Sr was 3.7-22.2 Bq/kg before the accident, and rose tenfold after the accident.
In order to understand the regional distribution of the contamination, the Swedish Geological Survey used two geophysical specialized aircraft to conduct aerial gamma energy spectrum measurements at an altitude of 150m, and the results of the measurements from May 1 to 6, 1986 are shown in Figure 9.12. Significantly high values were found near Gavle. The focus of the survey in the latter days was shifted to southern Sweden to see if cows could be allowed to graze on the spring-fresh pasture at that location.Coverage of the rest of Sweden with east-west survey lines at 100 km line spacing from May 5 to 8 revealed that the contaminated area was continuously expanding in the direction of the Swedish-Norwegian border. From May 9 to June 9 the whole of Sweden was covered with 50km line spacing aerial surveys, and in some anomalous areas the survey lines were encrypted to 2km.The Soviet Union has conducted aerial gamma energy spectrum surveys with scales of 1:100,000, 1:200,000, and 1:500,000 in an area of 527,400km in the country after April 28, 1986, in order to monitor the areas where radioactive contamination has dispersed.
Figure 9.12 Contour map of Swedish aerial γ-ray exposure rate (exposure rate in μR/h)
(2) Tracking of nuclear-powered satellites
Because satellites disintegrate into multiple fragments after entering the atmosphere, monitoring is carried out in a wide area around the landing orbit, relying mainly on aerial γ-energy-spectrum measurements, and then on ground inspections after detecting anomalies.
The Soviet Union's nuclear reactor-powered Cosmos-954 satellite fell in northwestern Canada at the end of 1977 and the beginning of 1978, and in early 1978 the Canadian Department of National Defense and the U.S. Department of Energy collaborated to track the location of the satellite's fallen debris in Canada. First, based on the computer-predicted satellite fallout orbit, a fallout area 800km long and 50km wide was delineated from the eastern end of the Great Slave Lake to Becker Lake near Hudson Bay, and divided into 14 sections. Four C-130 Heracles aircraft were used to make aerial gamma energy spectroscopy measurements at a line distance of 1.853 km and an altitude of 500 m above the ground. The Geological Survey of Canada's spectral system first detected a radioactive source in Lot 1 on the ice at the eastern end of Lake Danu, and a census of the entire area was conducted on January 31, which revealed that all radioactive debris fell within a 10-km-wide band, within which a detailed survey was conducted at a line spacing of 500 m and an altitude of 250 m above the ground. In view of the impossibility of lowering the altitude of the Hercules aircraft any further, a helicopter detection system was also employed, and a number of weak sources of radioactivity were found on the ice in Lot 9, all of which could not be detected at Hercules altitude, and which were later analyzed in small fragments that were shown to be part of the reactor core. Thereafter, the helicopter system detected additional radioactive debris along the southern shore of Lake Danu (Figure 9.13), which drifted with the northerly winds to the south side of the booking track. By the end of March another systematic helicopter gamma spectrometry measurement was made on the ice of Lake Danu, and analysis of the data provided further evidence that the reactor core had completely disintegrated upon entry into the atmosphere. In the summer of the same year, the Atomic Energy Surveillance Authority of Canada (AEMAC) did further monitoring and cleanup to ensure removal of all hazardous materials,*** recovering about 3,500 pieces of debris as far south as 480km south of the satellite's orbit.
9.3.2.2 Monitoring of contamination from mine prospecting and metallurgical processing
In addition to uranium deposits, many non-ferrous, precious, and rare metals, rare earth elements and phosphorus deposits, etc. are also accompanied by a large number of radioactive elements, and the exploration, mining, beneficiation and smelting of these deposits will lead to radioactive pollution. In order to remove these contaminants and to understand the effectiveness of the removal, all need to be monitored.
(1) Pollution and Monitoring of Tailings Sites
At the stage of geological exploration, although the ore deposits are not handed over to the industry for mining, works such as horizontal tunnels, vertical shafts, and shallow wells are used during the exploration process, which contaminate the mining areas with natural radioactive elements. During the mining process of the deposit, the stacking and transportation of ores and waste rocks caused more extensive pollution, and the tailings and slag produced during the metallurgical process are also a non-negligible source of pollution.
Figure 9.13 Distribution of total gamma ray counts caused by radioactive debris from the Cosmos-954 satellite in the greater NuLake area
The U.S. Department of Energy made aerial radioactivity surveys in the Salt Lake Valley from 1979 to 1980 to delineate the extent of the tailings site and to guide ground surveys. The measurement system was mounted on a helicopter, and the detector consisted of 20 NaI crystals, each with a volume of 645.7 cm3, an aerial height of 46 m, and a line spacing of 76 m. Based on the measurement data, a contour map of the exposure rate was plotted as shown in Fig. 9.14 (a), and a map of the range of distribution of the 226Ra counts above the background value was plotted as shown in Fig. 9.14 (b). The background irradiation volume rate varied between 430 and 645 fA/kg (1 μR/h = 71.667 fA/kg). The highest exposure rate exceeded 1 x 105 fA/kg at the tailings pile, and there were two high exposure rate prominences to the north of the tailings pile, one to the west thought to be caused by wind-blown tailings, and one to the east along the railroad tracks, which could have been caused by radioactive material being transported at the time of the measurements, or by the transportation of loose ore or tailings along the railroad tracks. Other radiation anomalies along the railroad are also presumed to be caused by scattered material.
Using data from this aerial radioactivity survey, the Salt Lake City Health Department and the Utah Department of Health delineated 14 previously unknown areas of radiological anomalies, and ground inspections identified nine sites as belonging to tailings from a uranium processing plant, one to uranium ore, three to radioactive slag, and one to stored processing equipment. All of these contaminated lots identified in the early 1980s were cleaned up.
(2) Pollution from Coal Mining and Coal Combustion and Monitoring
Many important coal-mining areas have formed large areas of radioactive contamination during coal mining. For example, the Ruhr mines in Germany have found activity concentrations of up to 13 kBq/m3 due to the 226Ra content of the water pumped to the surface from the mines, and up to 63 kBq/m3 into the underground pits.The total activity due to the 226Ra content of the water pumped out of all the mines in the Ruhr district each year***is 37 GBq. The distribution of radioactive contamination at the surface is related to a large extent to the chemical composition of the water, and *** there are two types of radium-containing water. **There are two classes of radium-containing water, Class A containing little or no sulfate, but containing Ba2+ ions; Class B water contains a large amount of sulfate, but no Ba2+ ions. Radium does not precipitate in class B water, while radium in class A water, when mixed with sulfate, radium and barium precipitate at the same time, forming radioactive deposits. Many coal mines have been mining coal for more than a hundred years, forming a very thick sediment layer where mine wastewater flows through, with a mass activity of 150kBq/kg, and leading to the contamination of soils and plants, with a soil mass activity of 0.2-31kBq/kg, and fresh plants on both sides of the watercourse containing 226Ra, which is 1kBq/kg.
At present many developing countries of the world are using Coal as the main source of energy, so fly ash has become a source of radioactive contamination over a large area. According to the statistics of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), a cogeneration plant that burns 10t of coal per day releases to the atmosphere 238U with a radioactivity of up to 1,850kBq, and a 1,000MW cogeneration plant discharges 5×105t of fly ash per year, of which 1.4×105t is discharged into the atmosphere. Surveys have shown that the cancer mortality rate due to fly ash radioactivity is 30 times higher around thermal power plants than around nuclear power plants.
Figure 9.14 Aerial Radioactivity Measurements in the Salt Lake Valley
(3) Radioactive Pollution and Monitoring in Oil Exploitation and Transportation
Radioactive contamination during oil development comes mainly from radioactive logging. The radioactive substances used in logging mainly include neutron sources and isotopes, such as americium-beryllium (241Am-Be) neutron source, 137Cs, 226Ra, 131Ba, 131I, 113Sn, 113In gamma source, etc. The radioactive contamination in the logging process is mainly caused by improper operation, such as: due to careless operation, the configuration of the activation fluid splashed into the external environment; in the process of opening the bottle and dispensing, dilution and mixing, there is a 131I aerosol escaped, resulting in air contamination; in the injection of 131I activation fluid into the injection wells, due to improper operation, resulting in the surface contamination around the wellsite; logging process of staining the well tubing and well tools, and so on.
In petrochemical production, pressure equipment (such as boiler furnace tube, liquefied petroleum gas ball tanks, liquefied petroleum gas tanker, pressurized containers, pipelines, etc.) flaw detection, level control, level measurement, density determination, material dosage, chemical composition analysis, and medical treatment of fluoroscopy, radiography, treatment of diseases, etc., the widespread use of radiological technology. In the level, liquid level, density, material dose, chemical composition analysis of radioactive isotope sources of dose, activity is generally a few millicurie (mCi), rarely more than 1000 mCi. However, under normal working conditions, whether engaged in industrial flaw detection or isotope instrumentation operators, the health of the body will not be subjected to radioactive damage.
Oil field on the radioactive contamination of a large area, and can even be reflected in the 1:500,000 aviation γ energy spectrum measurements, the pollutants to radium and its decay products are mainly uranium, thorium content does not exceed the background value of the soil. The enterprise used the route automobile energy spectrum measurement in the Stavropol border area measured 40 oil and gas fields, the surface of which is all contaminated by radioactive waste, found more than 300 contaminated sections, γ-ray irradiation rate of 60 to 3000 μR / h, most of which is in the range of 100 to 1000 μR / h.
(4)Radioactive pollution and monitoring of phosphate fertilizer
In the natural environment there is a stable ****-biological relationship between phosphorus and uranium, the raw material of phosphate fertilizer --- phosphorus ore contains biased uranium, phosphorus fertilizer by-products contain a high number of uranium decay products, which will give the Phosphate fertilizer plant around the environment caused by radioactive contamination.
In southwestern Spain near the confluence of the Odier and Tinto rivers into the sea there is a large phosphoric acid plant for the manufacture of phosphate fertilizers, the raw material for the phosphorite, which contains a large number of uranium-based radionuclides. Phosphoric acid is produced in Spain by treating the raw rock with sulphuric acid, during which a calcium sulphate precipitate (CaSO4-2H2O), known as phosphogypsum, is formed, and this by-product is either discharged directly into the Odiel River or piled up around the plant. Therefore, it was necessary to estimate the amount of nuclides discharged annually from the plant into the surrounding environment. In addition, the radioelement content of several commercial fertilizers in southwestern Spain was determined to estimate their radioecological impact on agricultural land.
All the investigations were based on the determination of U isotopes, 226Ra and 210Po and 40K in solid and liquid samples. Knowing the amount of phosphogypsum produced per year and the average value of mass activity of U, 226Ra and 210Po in it, it was concluded that the total activity of U isotope discharged per year in the vicinity of the plant is about 0.6 TBq, the total activity of 210Po is 1.8 TBq, the total activity of 226Ra is 1.8 TBq, and that 80% of the total amount of all kinds of radionuclides are deposited in a phosphogypsum pile, and the rest is discharged directly into the Odier River, and the stored Phosphogypsum was also gradually dissolved by water into the river. The water reaching the Tinto River had a238U activity concentration of 40 Bq/L,226Ra of 0.9 Bq/L, and210Po of 9 Bq/L. To study the contamination of the river, samples of the water system's sediment were also taken; the samples weighed a few kilograms wet, were dried, ground, and mixed, and then were measured on a high-purity germanium detector covered with a 10-cm-thick lead screen lined with a 2-mm copper liner to allow for the measurement of the lower mass activities .
Phosphate fertilizer plant environmental radioactive contamination has also been found in China. The General Corporation of the Nuclear Industry in the suburbs of Shanghai, aviation γ energy spectrum measurements, had found 10 × 10-6 uranium anomalies, 45 times the background value, it was found to be caused by phosphate fertilizer factory phosphate powder.
9.3.2.3 Radioactive Pollution and Monitoring of Building Materials
Besides radon escaping from the rock and soil of the foundation of a house, the building materials may also contain certain radioactive elements, and thus may also be a source of radioactive pollution. When the mass activity of radium in building materials is higher than 37Bq/kg, it will become an important source of radon in indoor air. In some places, industrial wastes are used as raw materials for manufacturing building materials, which may bring radioactive pollutants from industrial wastes indoors. For example, the use of fly ash or coal slag to manufacture building materials was once considered a good way to utilize waste, but when the radioactive element content of coal is high, it can lead to serious consequences. China's Nuclear Industry Corporation once made a survey on the indoor absorbed dose rate of houses built of stone cinder, and found that the absorbed dose rate of γ radiation of stone cinder brick houses was 3 to 9 times higher than that of the control houses. China uses Baiyun Ebo tailings and slag as raw materials to manufacture cement in factories, and the indoor radon concentration in houses constructed with cement produced by them is 4 to 6 times higher than that in the control group. And the survey results of radioactivity of commonly used building materials in the United States show that wood radiates the least radon and concrete the most.
China's residential houses are mostly made of bricks as building materials, which have the highest radioactive 40K mass activity of 148Bq/kg, Ra of 37-185Bq/kg, and thorium of 37-185Bq/kg.
For the natural building materials, the building materials industry standard (JC518-93) classifies them into three categories, which are shown in Table 9.4.
Table 9.4 China's natural building materials nuclear radiation classification standards
Russia's Institute of Exploration Geophysics proposes to use the following parameters of radiation indoor residential radiation dose monitoring of building materials.
9.3.2.4 Selection and investigation of sites for disposal of nuclear waste
Each country selects geological bodies suitable for storing nuclear waste according to its own conditions, but two types have been studied the most so far: salt bodies and bodies of y crystallized rocks. Salt is considered to be the best geological medium for storing nuclear waste, with the advantages of dryness of the undamaged salt layer, easy healing of fissures created in the salt, easier absorption of heat released by nuclear waste by salt than by other rocks, strong shielding of rays by salt, high compressive strength of salt, and generally located in areas with low seismic activity. Some other countries, because of their respective geological conditions, have mainly studied the use of deep crystalline rocks to store nuclear waste. Countries such as Canada and Sweden, where most of the territory belongs to Precambrian geology, have studied gneisses, granites, gabbros, and so on. The ability of these rocks to store nuclear waste depends mainly on the activity of the groundwater in them. Since the only channel for groundwater in crystalline rocks is the fissure, circling the fissure zone and studying its water content is an important task. The following conditions are taken into account in the specific selection of a storage site: the terrain is flat and therefore the hydraulic gradient is small, the major fissure zones should not pass through the site, the number of small fissure zones should be as small as possible, and locations where mines are likely to be found should be avoided.
Other geologic bodies studied are clays, basalts, tuffs, shales, sandstones, gypsum, and carbonates are also targets to consider. Generally, carbonates are unsuitable, but carbonate lenses surrounded by impermeable rocks are worth studying. In addition to geologic bodies on land, studies of seafloor rocks have begun.
(1) Geophysical work in the site selection survey of salt bodies
A. Census of salt bodies
In order to store nuclear wastes, it is necessary first to know the depth, thickness, and configuration of the salt layer, and to circle out the salt bodies suitable for storage, and the general tendency is to store the nuclear wastes in salt mounds.
Gravimetry. Gravity method can be effective for surveying salt mounds. The density of salt is stable at 2.1×103kg/m3, which tends to be lower than that of the surrounding rock (2.2×103 to 2.4×103kg/m3), and gravity lows of n×10 to n×100 g.u. can be measured on salt mounds. When there is a thick layer of gypsum in the upper part of the salt mound, due to the high density of gypsum, the result is the formation of gravity high on the background of weak gravity low. When the salt dome is surrounded by dense igneous rocks (igneous rocks were intruded during the formation of the salt dome), a circular gravity high appears at the edge of the gravity low. The surface relief of salt dunes can be studied by a combination of high-precision gravity and seismic measurements. When the gravity field in the salt dune area is very complex (the gravity field is a combined reflection of the supra- and infra-salt stratigraphy, the salt layer and the basement), the minimization method is used for the interpretation: firstly, a model is proposed on the basis of the geologic-geophysical data, and then the model curve that is the best match to the observed gravity anomalies is automatically selected, so that the sum of squares of the two deviations is equal to the minimum value.
Electrical measurements. Salt than the surrounding rock resistivity is high, is the electrical reference layer, the salt layer structure in the past with direct current bathymetry research, in recent years is more and more use of geodetic current method and magnetic geodetic current method. When determining the buried depth of the salt body by the geocurrent method, the statistical relationship between the average field strength of the geocurrent and the depth of the salt layer is utilized, so a small amount of drilling and seismic data should be mastered. Areas of high mean field strength correspond to salt mounds and salt walls, and many of the local formations circled in this way have been confirmed by seismicity or drilling.
Seismic surveys. Seismic reflection and refraction methods are effective in detecting salt undulations in areas of sedimentary rocks with relatively simple tectonics. For example, the location and morphology of the Morse Salt Dome, chosen in Denmark for the storage of nuclear waste, is based on the distribution of reflective surfaces. In some cases the ground seismic method can only locate the top gentle part of the salt dunes. The morphology and location of the sidewalls are difficult to determine, and this can be done using well seismic.
In short, in the site selection, in order to study the salt layer structure, generally first use gravity and electric method, the combination of the two can be more detailed to determine the size and shape of the salt layer structure in the plane. The seismic network is arranged according to the results of the gravity and electrical methods, and the depth of the salt body can be accurately determined by the seismic method, while the location and morphology of the sidewalls of the salt body can be accurately determined by utilizing the seismic in the well.
B. Study of the internal structure of the salt body
In order to determine whether the salt body is suitable for the storage of nuclear waste, it is necessary to study the internal structure of the salt body, i.e., the amount of impurities (intercalation) it contains, its water content, and the degree of development of fissures.
Determining the amount of impurities (intercalation). The relative purity of the salt is an important factor in its ability to store nuclear waste, and the presence of impurities reduces the compressive strength of the salt layer and its ability to shield against radiation. The impurities contained in the salt body include mud components, gypsum, etc. The mud components either form separate interlayers or are mixed with the salt to form mud salts. The purity of Middle and Upper Permian salt formations was evaluated in the Palo Duro Basin, Texas, USA, using natural gamma logging and density gamma-γ logging. gamma-ray intensity was correlated with the mud content because of the higher amount of thorium in the muddy fractions. gamma-γ logging-derived densities, on the other hand, showed a linear correlation with the percent gypsum content. Mean values of γ-intensity were calculated for each salt layer in each borehole. The γ-intensity of the interlayer less than 30 ft was averaged with the salt layer, and when the interlayer was thicker than 30 ft, the salt layer was treated as two separate layers, from which contour plots of the γ-ray intensities of the different rotations were compiled, which is essentially a map of the distribution of the mud content, and from which the area with the lowest mud content can be selected as the site for storing the nuclear waste.
In the Salt Valley region of the United States has also been utilized vertical seismic profiling, based on the different wave speeds to divide the salt in the sandwich. And at the Morse Salt Mounds in Denmark the entrapment within the salt has been studied using gravity in wells.
Studying water content. The water content of the salt body is a potential hazard to the creation of nuclear waste, dissolving part of the salt into brine, reducing the mechanical strength of the salt and corroding the waste container. Measuring the water content of the salt body can be done using neutron logging with 255Cf as the neutron source. Tests have shown that the peak of hydrogen itself in the released γ-ray spectrum is weak and cannot be used as a scale for evaluating water content, but the interaction of fast neutrons with the nuclei of Na and Cl atoms can be utilized to measure water content with the following parameters:The ratio of the inelastic scattering peak of the Na neutron to the capture peak of the Cl neutron. Inelastic scattering refers to the Na nucleus absorbs a neutron and release a neutron and γ-rays, γ-rays peak position in 138keV; neutron capture refers to the Cl nucleus captures a neutron and release γ-rays, the position of its peak in 789keV. the ratio of the above and the water content is proportional. The United States has used the transient electromagnetic method to determine the location of the brine, in the actual detection found that the location of the brine and the transient electromagnetic method of one-dimensional inversion of the low-resistance layer of the location of a fairly good match.
Understanding the degree of fissure development. In order to ensure the safety of the nuclear waste repository, it is necessary to understand the degree of fracture development of the salt layer. The main methods are in-well electrical methods (especially radio wave methods) and sonic logging. The high resistivity of salt, the small loss of electromagnetic wave propagation, the large detection distance of radio wave method, the resistivity or dielectric constant of the interlayer or fissure is different from that of salt, all these are favorable conditions for the application of radio wave method. The radio wave method includes the transmission and reflection methods, the transmission method measures the attenuation of the signal between the holes, while the reflection method, with the transmitting and receiving antennas located in the same hole, measures the travel time of the electromagnetic pulse and the characteristics of the reflective layer. Homogeneous salt does not produce significant reflections, and reflections increase as the number of fissures increases. Salt without clefts has high resistivity and low attenuation, while salt with many clefts has low resistivity and high attenuation. Therefore, a salt body with low attenuation and fewer reflections is more suitable for storing nuclear waste.
Different parameters such as reflected wave amplitude, sonic velocity and interval time can be utilized in determining the location of the fissure zone using sonic logging.
(2) Geophysical work in siting and investigation of deep crystalline rock bodies
Nuclear waste is proposed to be stored in mine-like processing caves at a depth of 500-1000m in deep crystalline rock bodies of granite. The geophysical work is divided into three phases during the siting and investigation of the deep crystalline rock body, i.e., site screening, site evaluation, and investigation during the excavation of the caves.
A. Site Screening
A regional census is first carried out to screen several areas, each of which can cover thousands of square kilometers, as candidate treatment sites. During the screening process, it is important to understand the morphology and depth of the plutonic bodies, the surrounding geology, the location and orientation of major discontinuities, the characteristics of the cover, and the integrity of the rocks. Since the site screening is a regional survey involving a large area, rapid census geophysical methods, especially airborne geophysical methods, are selected. Aeromagnetic surveys have been used to determine the boundaries of y formed rock bodies and the rock-tectonic interfaces in the bodies. aeromagnetic surveys, generally carried out at the same time as aeromagnetic surveys, and aerial gamma spectrometry can also be used to delineate the boundaries of granite bodies, which can contain up to 8 × 10-6 of uranium, whereas the enclosing rocks are often less than 2 × 10-6. Aeromagnetic surveys are used to fill in the fissure projections of the fissure zones on the near-surface and to characterize the overburden. Fissure zones in the lake area can be delineated using shipborne sonar equipment. The integrity of the rock can be evaluated by measuring the overall resistivity of the rock, using methods such as the geomagnetic method (MT), the audio geomagnetic method (AMT), the transient electromagnetic method (TEM) and the DC resistivity method.
The ground gravity method has been used to determine the morphology and depth of y formed rock bodies and their geologic setting. Figure 9.15 shows a 39-km-long gravity profile spanning the bedrock in a north-south direction, which includes measured and modeled gravity profiles as well as interpreted profiles based on common local rock units. A gravity low of 100 g.u. associated with the rock base is very apparent, and the local gravity high superimposed on the gravity low is most likely caused by a high density of inclusions.
B. Site Evaluation
Site evaluation is a more detailed investigation in smaller, screened areas of up to 100km2 each, with the general objectives of circling the major rift zones, determining their geometry, performing lithologic infill mapping and characterizing the overburden.
High-resolution seismic reflection was applied to understand the depth of the rift zones as well as to discover y buried rift zones. Targets wider than 1/8 of the main wavelength of the seismic wave can be detected. For example, in granite with a P-wave velocity of about 5500m/s, a 5m-wide fissure zone can be detected if an operating frequency of about 150Hz is used. But the requirement to detect reflectors up to 1000m from the surface implies that useful reflections are contained within the first s of the seismic record, however, for the gun detector spacing commonly used for high-resolution seismic, there are also ground-roll waves arriving in this time period, and in order to minimize the effect of the ground-roll waves it is necessary to use frequency filtering, f-k filtering, and reduction of explosives to preserve the high-frequency component of the signal, and Select the appropriate detector distance to minimize the ground-rolling wave in the superposition.
Three other ways of applying geophysical methods to estimate the hydraulic permeability of the fissure have been proposed: one is to use the conductivity of the fissure space; the other is to use the loss of acoustic energy within the fissure; and the third is to use the response of the borehole to the fissure compression when the seismic wave passes through.
The laminar method is more useful for sites being prepared for excavation, where the number of boreholes is kept to a minimum to prevent the formation of new groundwater channels in the rock mass.
C. Investigations at the Excavation Stage
After the excavation of the cave for storing nuclear waste has begun, it is necessary to understand the hydrogeological and geomechanical conditions of the rock mass surrounding the cave. Because of the reduced objectives of this phase of the study, high-resolution and therefore high-frequency geophysical methods are to be used. Radar, ultrasonic and acoustic radiation methods have been used effectively.
Figure 9.15 A north-south gravity profile and two-dimensional gravity model (north on the right)
Using ultrasound, the thickness of the excavation damage zone can be determined. The safety of the excavation can be monitored using acoustic radiation measurements, and variations in acoustic radiation parameters can be used to predict possible rockbursts and locate them. In addition, acoustic radiation measurements are used to track the progress of grouting into the fracture zone, when accelerometers are placed in a series of boreholes adjacent to the fracture zone, and the intensity of acoustic radiation recorded during grouting is correlated with the progress of the grouting.
In summary, geophysical methods for siting and investigating nuclear waste disposal sites in areas of deep crystalline rocks can be used quickly and economically to achieve both a comprehensive understanding of the geology of a large area and a detailed evaluation and investigation of candidate sites. The various stages of geophysical work are summarized in Table 9.5. However, in addition to geophysical methods, a combination of other methods, particularly hydrogeological, geochemical, geological and rock mechanics methods, should be applied in all phases of the work. The geophysical methods should also be verified by drilling due to their multiplicity of interpretations.
Table 9.5 Geophysical work in geological disposal of nuclear waste in deep crystalline rock areas