The Containment of Soviet Underground Nuclear Explosions

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Syndrome................................................................................................. .35 2.12Statistics on the Containment History of Soviet Tests..........................................................38 2.13A Few Examples............................................................................................................. .......43 Chapter 3. MONITORING ACCIDENTAL RADIATION RELEASES....................................................46 3.1Criteria for Conducting a Test.............................................................................................. .46 3.2Examples of Containment Failures.......................................................................................47 3.3Groundwater Contamination.................................................................................................47 3.4How Safe is Safe Enough?...................................................................................................5 0 Acknowledgements 50 Bibliography 51 On the Cover: Section of a cline of sight d pipe, extending from one of the three tunnels at Degelen mountain test site no. 169/2. This pipe, which is about 2 meters in diameter, was evacuated during the nuclear test at this site, on the 4 th of October, 1989. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 3 PREFACE This report was prepared in 2000-2001 under a contract between the U.S. Geological Survey and the Institute of Dynamics of Geospheres of the Russian Academy of Sciences (Contract no. OOHQSA0727 . This goal of the project was to produce, for Soviet underground nuclear testing, a report mirroring Dr. Gregory van der Vink 9s report, c Containment of Underground Nuclear Explosions d, published in 1989 by the U.S. Congress 9 Office of Technology Assessment. That report reviewed in detail the containment of U.S. nuclear tests at the Nevada Test site, with special attention devoted to safety and environmental aspects of underground nuclear testing. The current report uses as its outline the structure of the OTA report, modified to accommodate the differences between Soviet and U.S. nuclear testing practices and containment experiences. The basic contract reports that were provided by the Russian authors of this study have been supplemented by other published information and some unpublished works, improved citations of the literature, and a number of illustrations. Most of this report is, therefore, based on a translation from Russian to American English that was subsequently edited. The original Russian reports are available upon request to W. Leith ( email: wleith@usgs.gov ). Basic data on Soviet underground nuclear tests are published in the six volumes of c Nuclear Testing in the USSR. d Volume 1 of this series (published by RFYaTs-VNIIEF , 1997, as an update to MINATOM , 1996; a nearly-identical version was also published by IzdAT , 1997) contains summary data for all Soviet nuclear tests, excluding hydronuclear tests. Volume 2 presents information on the technology of nuclear testing; Volume 3, on military and political aspects of nuclear testing; Volume 4, summary information for 115 of the so-called cPeaceful Nuclear Explosions, d (PNEs); Volume 5, nuclear testing and the environment. Note that, the numbers of underground nuclear tests of various types reported in these publications are inconsistent and may, in places, differ from that presented herein: For example, for the Peaceful Nuclear Explosions (PNEs), Volume 2 ( op cit ) lists 115 PNEs; while Volume 1 lists 124, and both Sultanov et al (1993) and Laushkin et al (1995) list 122 PNEs. The inconsistencies are apparently due to how tests were categorized. The categorization of underground nuclear tests used herein is that of Adushkin and Laushkin ( Experience monitoring UNTs with the seismic network of the former USSR , FSSN, v.3, 1996); this is shown graphically on the next page, as well as in Tables 1 and 2. For the Semipalatinsk test site, a book reviewing nuclear testing there was published by MEDVIO- EKSTREM in 1997. Additional data are available in several publications, including, for under- ground tests: Bocharov et al (1989), Adushkin et al (1995), and Leith (1998). A summary of the record of containment of underground nuclear tests at the Semipalatinsk test site is available in Gorin et al (1993). Summary data for Soviet nuclear tests in the atmosphere is available in Dubasov et al (1993), and height-of-burst and other data for atmospheric nuclear explosions are available in Andryushin et al (1998). The latter reference also includes a listing of hydronuclear experiments conducted at the Semipalatinsk test site. A number of these hydronuclear tests (which resulted in plutonium contamination of the local environment) were conducted in tunnels at the Degelen Mountain test site, and may therefore be on interest for containment studies. For the Novaya Zemlya test site, books reviewing nuclear testing there were published by the Khlopin Radium Institute (1999) and IzdAT (2000). Additional information on nuclear test containment is available in Mikhaylov and Chernyshev (1991), Mikhaylov et al (1991), Chelyukanov and Savel 9ev (1992) and in Andrianov and Bazhenov (1992). The latter includes detailed descriptions for a number of containment failures at Novaya Zemlya that (summarized in this report). In general, the containment record at Novaya Zemlya has been poor (and is described some in detail, herein); this is of concern since it is now the only declared Russian nuclear test site. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 4 Number of Soviet Underground Nuclear Tests, categorized in terms of test emplacement mode (tunnel, borehole, shaft), and color coded by location (STS = Semipalatinsk Test Site, 363 tests; NZTS = Novaya Zemlya test sites, 42 tests; PNE = Peaceful Nuclear Explosions, 122 tests, including 5 PNEs on the STS). Hydronuclear tests are not included. NZTS borehole tests (6) NZTS, tunnel tests (36) STS borehole tests (133) STS tunnel tests (225) PNE, boreholes, STS (5) PNE, boreholes (114) PNE tunnel tests (2) Dnepr 1 & 2 PNE shaft test (1) " Clivazh " Distribution of 522 Soviet Underground Nuclear Tests USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 5 CHAPTER 1. The Nuclear Testing Program 1.1Introduction Conducting nuclear tests is one of the most important elements in the technology of creating and improving nuclear weapons. In 1963, due to the serious environmental consequences of nuclear testing in the atmosphere, hydrosphere and in near space, the USSR, USA and Great Britain signed the Treaty on Prohibiting Nuclear Weapons Testing in Three Media (in the atmosphere, in space and under water), also known as the Limited Test Ban Treaty . Currently, this Treaty is signed by 117 nations. The 1963 Treaty ushered in a new stage in nuclear testing, in which nuclear tests were conducted only underground. Nevertheless, this did not obviate the question of the environmental consequences of nuclear tests. The results from early underground nuclear tests indicated that releases of radioactive materials into the atmosphere were possible. As a result, during the entire period of underground testing, serious research was conducted to guarantee the ecological safety of underground nuclear explosions. One of the primary directions of this research was to guarantee the so-called ccamoufletic d (contained) nature of nuclear explosions, that is, to conduct underground explosions under conditions which guarantee keeping the primary radioactive by-products that are most dangerous to man, within the area immediately surrounding the explosion location. The basis for providing containment of nuclear tests was the concept of creating a relatively impermeable barrier to the migration of the non-condensed radioactive gasses found in the explosion cavity under high temperatures and pressures. For this barrier, it was suggested to use the layer of rock above the underground explosion. As data were obtained on the level of rock damage in the close-in zone of the nuclear explosion, concepts improved about zones of irreversible behavior of the rock massif and of the rock composing it (the so-called czonal d approach to describing the mechanical effects of an underground nuclear explosion, see below). It was established that, in addition to buckling/crushing zones of the rock and massif damage directly abutting the explosion containment cavity, there exists an area of damaged medium near the free surface (the spall damage zone) and in the vicinity of large, tectonic faults, which significantly reduces the insulating properties of the rock massif. It is also asserted by Russian scientists that underground nuclear tests conducted in the USSR from 1985 to 1990 support the conclusion that the task of guaranteeing containment of underground nuclear explosions has been successfully solved at the former Soviet nuclear test sites. 1.2Definitions of Containment The Limited Test Ban Treaty (LTBT) established an international requirement for adequate containment of the radioactive products of underground nuclear explosions. Article 1.1(b) of this treaty prohibits an explosion that "...causes radioactive fallout [in Russian, c posadkha d] to be present outside of the territorial limits of the State under whose jurisdiction or control such explosion is conducted." Using this simple criterion, one might judge that the majority of U.S., Soviet, Chinese and French underground nuclear tests were "contained," but only to the extent that they were compliance with the LTBT. However, the U.S., the Soviets/Russians and the French have since developed more rigorous requirements for containment. The U.S. defines successful containment as "such that a test results in no radioactivity detectable off-site as measured by normal monitoring equipment, and no unanticipated release of radioactivity on site within a 24-hour period following execution" . The U.S. further characterizes prompt (seconds to hours), high- release containment failures as "venting", and late-time, small, slow radiation releases as "seeps" (associated with changes in atmospheric pressure; see OTA, 1989). Also defined are "controlled tunnel purgings," which are mostly small, intentional releases of gasses trapped in sealed tunnels, and "operational releases," which are also small releases upon post-test sampling (tunnel reentry or drill- back). Ventings from early U.S. underground nuclear tests (e.g., DesMoines , 1962, and Baneberry , 1970) USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 6 account for the major radioactivity release (more than 25 million curies). Following Baneberry, the U.S. halted testing for some months to review its testing procedures and record of containment. After implementing changes in U.S. testing procedures following Baneberry, there have been only two ventings that released more than 1000 curies ( Diagonal Line , 1971 and Riola , 1980), and only one seep releasing more than 100 curies ( Tierra , 1984). Also since Baneberry, all but one U.S. underground nuclear test (Cannikin) were emplaced in relatively porous volcanic rocks or alluvium at the Nevada Test Site; i.e., in geologic environments quite different from those of the main Soviet nuclear test sites. These differences in test environments are reflected in contrasting containment practices between the two countries. While the above definitions of containment are useful and can be used to review the available information on the containment of underground nuclear tests, under the Comprehensive Nuclear Test Ban Treaty (CTBT), the de facto criterion for "containment" will be undetectability by the radionuclide monitoring network of the International Monitoring System (IMS). Although there is no specific capability established for the IMS, it has been evaluated with respect to its ability to detect the venting of 10% of the radioactive gasses from a 1 kt underground nuclear explosion within 12 hours of the explosion (specifically, about 10 14 Bq of 133 Xe; see the Working Papers of the Conference on Disarmament CD/NTB/WP-224 and CD/NTB/WP-283, 1995). In terms of the historically 3used terms for describing nuclear test containment, this criterion would be categorized as a cprompt vent d. 1.3A Brief History of Nuclear Testing in the USSR Nuclear testing in the USSR began with the test of the RDS-1 nuclear bomb 1 , detonated on 29 August 1949. The 22 kt charge was an implosion design with plutonium as the working substance. It was placed on a metal tower, 37.5 m high, at Training Test Site No. 2 of the USSR Ministry of Defense, a specially equipped field located 170 km from Semipalatinsk, Kazakhstan (at that time the Kazakh Soviet Socialist Republic). This test site later became known as the Semipalatinsk Polygon. In addition to testing the nuclear charge itself, approaches were developed in this experiment for recording the primary effects of a nuclear explosion in the atmosphere. Specially developed equipment was used to record the optical and electromagnetic effects, the parameters of the atmospheric shock wave and the yield of the explosion. The organization responsible for instrumental observations of the physical and mechanical parameters of a nuclear explosion, including the development and creation of special recording devices, was headed by M.A. Sadovskiy, who later became an academician of the Russian Academy of Sciences and Director of the Institute of Physics of the Earth (IPE). The latter institution included the Special Sector ( Spetssektor ) with the independent rights of a structural subdivision. This Spetssektor was the leading scientific organization for studying the physics of nuclear explosions. The preparations for and the actual conduct of the first test determined the primary issues that had to be solved in the process of nuclear testing; it also determined the circle of organizations needed to participate in nuclear testing. The term cunderground nuclear test d can be defined as the near-simultaneous detonation of one or more nuclear charges inside one underground excavation (a tunnel, shaft or borehole). With this definition, of the 742 nuclear tests were conducted during the entire period of nuclear testing in the USSR, from 1949 to 1990, 522 tests (70%) were conducted underground. This includes 122 underground nuclear tests that were conducted cin the interests of the national economy d (the so-called Peaceful Nuclear Explosions , or PNEs). Note that, because many Soviet tests included multiple nuclear devices, detonated within a few milliseconds to seconds, the number of nuclear charges detonated (approximately 969) was significantly greater than the number of underground nuclear tests. The composite energy yield (TNT-equivalent) of all Soviet nuclear tests is estimated at 285.4 megatons (Mt). After the first atomic bomb test, it became clear that nuclear weapons would be constantly improved, and that it was necessary to organize a systematic nuclear testing program. Academician M.A. Sadovskiy led preparations for the technical task of designing the test site. In 1950, Training Test Site No. 2 (UP-2) became the Semipalatinsk Test Site. The test site territory had a specialized military unit assigned to it, as well as command and research services. The scientific director of the test site was M.A. Sadovskiy, who was previously the scientific director of UP-2. 1 The abbreviation cRDS d stands approximately for Made in Russia [ c Russko Delano Sama d ] USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 7 Between 1949 and 1989, a total of 483 tests were conducted at the Semipalatinsk Test Site. This comprises nearly 65% of the overall number of nuclear explosions conducted by the USSR. For its entire existence, the Semipalatinsk Test Site was the primary location for conducting nuclear testing. The last nuclear test at the Semipalatinsk Test Site was conducted at the Balapan site on 19 October 1989. The data from this explosion are as follows: Time of detonation: 12:49:59.98 seconds Coordinates: 49 o 55 915 dN ; 78 o 54 924 dE # of nuclear charges: three Total charge yield:~85 kt Charge emplacement depths:628, 592 and 556 m Rock at the hypocenters:siliceous sandstone Gas content of rock:12 % (by weight at 1000 degrees Celsius) By the mid-1950s, it became necessary to establish a new test site for nuclear tests were being planned for water environments and for high-yield (megaton) nuclear charges. A test site was created on the territory of the Novaya Zemlya archipelago with the 31 July 1954 Decree of the Central Committee of the USSR Communist Party and the Council of Ministers of the USSR. Since its establishment, a total of 133 nuclear tests were conducted on the Novaya Zemlya Test Site, including 88 explosions in the atmosphere, 3 explosions under water, and 42 underground nuclear explosions, of which 36 were conducted in tunnels and 6 in deep boreholes. The first nuclear explosion at the Novaya Zemlya test site was conducted underwater at Chornaya Guba on 21 September 1955. It was at this test site that the USSR conducted on 30 October 1961 the highest yield nuclear test of any nation a ~50 Megaton charge exploded in the air at a height of 4000 m. The last test at Novaya Zemlya was conducted on 24 October, 1990, in tunnel A-13N. This was the last Soviet nuclear test (since then, no nuclear tests have been conducted by Russia). It is asserted by Russian scientists that, by the end of nuclear testing, the methodology for guaranteeing containment was so well developed that the last tests at Semipalatinsk and Novaya Zemlya, were conducted with practically total containment (i.e., with only an insignificant migration to the surface of light, non- condensible radioactive components in the form of inert gasses). The number of nuclear tests conducted per year in the USSR is not equally distributed (see Figure 1). 0 10 20 30 40 50 60 70 80 1949 1952 1955 1958 1961 1964 1967 1970 1973 1976 1979 1982 1985 1988 Year Number of tests Figure 1: Annual Number of nuclear tests in the USSR. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 8 Figure 1: Annual Number of nuclear tests in the USSR. The Soviets did not conduct nuclear tests In 1950 and 1952. This was related to the specifics of the initial stage of work in creating nuclear weapons. From 1959 until August 1961, the USSR did not conduct nuclear tests due to a moratorium on nuclear testing together with the USA and Great Britain. From 1963 until March 1964, nuclear tests were not conducted because of preparations for concluding the Limited Test Ban Treaty and the transition to a program of underground nuclear testing. From August 1985 to February 1987, and from November 1989 to October 1990, nuclear testing and nuclear explosions were not conducted due to USSR participation in testing moratoria. Table 1 presents a distribution of the quantity of nuclear tests and explosions according to the conditions in which they were conducted. It is clear that the greatest number of nuclear explosions were conducted in underground conditions (70%). Of these, 122 nuclear tests were conducted in the interests of the national economy, five of which were at the Semipalatinsk Test Site and three at Mangyshlak. Table 1. Distribution of the number of nuclear tests and explosions according to the environmental conditions in which they were conducted. Underground Emplacement Conditions SurfaceAtmos- pheric High-Altitude and Space Underwater + water surface Tunnel and shaft Borehole Total Number of Explosions 3217583+2263 + 1258742 The transition to conducting underground nuclear tests occurred later in the USSR than in the United States. The first underground nuclear test at the Nevada Test Site, code named Uncle , was a 1.2 kt cratering explosion conducted on 29 November 1951, within the framework of the Jangle program (this explosion was also known as Jangle -4). Four years later, on 23 March 1955, a second cratering nuclear explosion, Ess (1 kt, part of Operation Teapot) , was conducted at the Nevada test site. While the tests were not contained, since the charges were detonated at a shallow depth (5.2 and 20.4 m for Uncle Teapot explosions, respectively), these experiments were significant in terms of developing a methodology for conducting nuclear tests in underground conditions. The first U.S. contained nuclear explosion, Rainier, was conducted on 19 September 1957, using a proven device with a known yield of 1.7 kt. In the USSR, preparations for conducting underground nuclear explosions began in 1957 (by the end of 1957, the USA had already conducted 5 underground nuclear explosions: 3 in boreholes and 2 in tunnels, including Rainier ). A large series of underground explosions of chemical (TNT) explosives weighing 1, 10 and 1000 tons was conducted in the clays and sandy loams in the steppes close to the settlement of Kabulsai, in the Kazakh SSR. The main task of these explosions was to study the cscale d effect for buried explosions, through which an increase in charge energy (and, correspondingly, the depth of the explosion) the law of geometric similarity of the excavation effect is broken, as a result of the increase in the role of the gravity force. It is necessary to account for this effect in order to guarantee the conditions of containment when conducting underground nuclear tests (see Section 2.4). These preparations continued into 1959, when in the rock massif of Tuya-Muyun (Kirghiz SSR), two trotyl (TNT) charges weighing 190 and 600 tons were exploded underground, in order to determine the conditions for containing large-scale nuclear explosions. At the same time, in the rock massif of Degelen Mountain at the Semipalatinsk Test Site, Tunnel V-1 was being prepared for the first underground nuclear explosion in hard rock. In order to ascertain the technical tasking, an experimental detonation of a 600-ton trotyl (TNT) charge was conducted in a nearby tunnel, V-2, on 5 June 1961 4i.e., just before the nuclear explosion in tunnel V-1. The mechanical effects of this chemical explosion test, including the seismic signal, were studied in detail. Four months later, after analyzing the results of the chemical explosion in tunnel V-2, the first fully- contained, underground nuclear explosion in the USSR was conducted on 11 October 1961, in tunnel V-1 USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 9 (four years after Rainier ). In this experiment, a nuclear charge with a yield of 1.2 kt was detonated at a depth of 118 m. Because the mechanical effects of this explosion turned out to be somewhat weaker than expected, the requirements for containment were fully achieved. Note that, at about the same time, France (which had already conduced nuclear tests in the atmosphere) conducted its first underground nuclear explosion, code-named Agate , at the Reggane test site in the Sahara on 7 November 1961, with a yield of less than 20 kt. Thus, the USA, USSR and France had begun systematically conducting underground nuclear tests, in preparation for the LTBT. By 1962, the USA had conducted 58 underground nuclear explosions; the USSR, one test in underground conditions, and France, one underground explosion. 1.4Limits on Nuclear Testing The early nuclear testing in the atmosphere created globally-detectable quantities of radioactive by- products in the atmosphere and, as a consequently led to the radioactive contamination of the Earth 9s surface well beyond the boundaries of the test sites. In 1963, the Limited Nuclear Test Ban Treaty was concluded, requiring strict limitations on the release of radioactive by-products into the atmosphere during underground nuclear explosions (although no monitoring system was provided for in the Treaty). It was declared, that the concentration of radioactive by-products in air masses that pass beyond the territorial boundaries of a state conducting tests must not exceed the global background values of corresponding isotopes in the atmosphere. In addition to this, it was necessary to adhere to standards of radiation safety for the population. This, in turn, added practical limitations on the time of the onset of release of radioactive by-products from the explosion cavity into the atmosphere, even in those cases when this release was insignificant. These containment requirements could be achieved by: 1) the fundamental selection of the depth of emplacement of the nuclear charge, and 2) the development and implementation of special measures for sealing-off (stemming) the emplacement tunnels and boreholes. By the early 1970s, it was clear that the limitations on underground nuclear explosions imposed by the LTBT did not guarantee the full measure of integrated radiation safety of the country that was testing the nuclear weapons, or of neighboring States, and in 1974, the USA and USSR reached agreement on limiting the yield of underground tests to 150 kt (Threshold Test Ban Treaty). Later, in 1976, the USA and the USSR signed the Treaty on Threshold Limitations for Underground Nuclear Explosions Conducted in the Interests of the National Economy (a.k.a. the Peaceful Nuclear Explosions Treaty). In accordance with this Treaty, the yield of a nuclear charge used for conducting peaceful nuclear explosions was limited also to 150 kt (in the case of a multiple-device explosion, the total yield of the charges could not exceed 1.5 Mt). Although the last two treaties were not ratified until 1990, both countries governed their nuclear activities according to the treaty conditions. Thus, the agreements reached in 1974 dictated that nuclear tests should be conducted only on designated test sites (although peaceful nuclear explosions were permitted at other locations), and the yield of the nuclear charge must not exceed 150 kt. 1.5Types of Underground Nuclear Tests The Soviet underground nuclear explosions can be divided into two groups, based on the geometry of the underground emplacement: 1) explosions conducted in near-horizontal excavations (adits or tunnels), and 2) explosions conducted in deep near-vertical boreholes. In total (excluding hydronuclear explosions), 258 Soviet nuclear explosions were conducted in boreholes, and 263 in tunnels. Table 2 summarizes the numbers of nuclear explosions in tunnels and boreholes for the Semipalatinsk, Novaya Zemlya and the Peaceful Nuclear Explosion sites. From Table 2, it is clear that at Semipalatinsk, the portion of tunnel explosions comprised 62%, and at Novaya Zemlya, practically 85%. Underground Peaceful Nuclear Explosions, conducted in the interests of the national economy, were almost exclusively done in boreholes, with the exception of the Dnepr-1 and Dnepr-2 projects (tunnel-type emplacements) and the Klivazh project (a shaft-type emplacement). USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 10 Table 2. Comparison of the numbers of Soviet underground nuclear explosions conducted in deep boreholes, tunnels and mines (note that the numbers for Semipalatinsk include five PNEs conducted in boreholes). SemipalatinskNovaya ZemlyaPNEs (beyond test sites) TunnelBoreholeTunnelBoreholeTunnelBoreholeMineshaft 22513836621141 At the Semipalatinsk test site (STS), explosions in tunnels were conducted in Degelen Mountain. At the northern Novaya Zemlya test site (NZTS or NZ), explosions in tunnels were conducted in rock massifs along the Matochkin Shar Strait. The excavated tunnel works ranged from 200 m to 2 km long, usually made with a diameter of about 3 m. In accordance with the engineering requirements of a specific test, the system of underground works could be highly simple (one straight tunnel) or it could have a branching system of works. For example, in the test in Tunnel 704 at Degelen, one straight tunnel was used, with the charge placed at the end of the tunnel. In the complex test at Tunnel 169/2 (Degelen, 4 October 1989), two diverging tunnels were used, with the goal of observing the effects of a simultaneous release of radiation along two beams out to the surface. A third example is the complex system of underground works used for the multiple-device test in Tunnel A-37 (11 October, 1982) at the northern Novaya Zemlya test site (see Spungin et al , 1998). Figure 2 shows a plan view of the tunnel complex for this 80-kt test, in which four charges of differing yields were detonated in separate drifts. Figure 2. Plan of Tunnel A-37, northern Novaya Zemlya test site (Matochkin Shar). The sites marked ZR-1, ZR-2, etc. (for czero room d), are the locations of the nuclear charges within the tunnel complex . Elevation contours are in meters. 0 400 m ZR-4 ZR-3 ZR-2 ZR-1 600 550 500 450 400 350 300 250 200 150 100 USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 11 In several cases after a test in a single, linear tunnel, a system of access drifts was excavated for research into the properties of rock at various distances and azimuths from the explosion source. For example, Figure 3 presents the plan for the underground works excavated in the rock massif, which was destroyed by the explosion in the cTunnel V-1 d experiment in 1961 at the Degelen test site (STS). Figure 3. Plan view of the nuclear test-experiment, cV-1 d, of 11 October 1961 (Degelen test site). Note that the side access drifts were constructed after the test. The nuclear charge was placed, as a rule, at the end of the tunnel in a specially equipped room, known as the cend box d (in U.S. usage, czero room d). In some cases (for example, in tunnels 148 and Dnepr -1, when the underground works were used to direct the ejection of radioactive explosion products), the charge was placed at some distance from the end of the tunnel. Recording sensors were placed along the tunnel in specially made pits or recesses in the walls of the tunnel. All the recording equipment was placed either inside, near or close to the portal of the tunnel, or at distances of less than 1 km from it (i.e., close-in and remote measuring points). In the case of irradiation experiments (in which a radiation beam was directed to the surface), the tunnel was specially equipped so that, immediately after the radiation beam exited to the portal area where the test objects were placed, the tunnel was closed off, so as to protect the test objects from debris, as well as to contain the radioactive by-products of the explosion. After conducting a nuclear explosion, the tunnels were generally studied from the portal to the innermost stemming, which was not equipped with a hermetically sealed pass-through. Such studies made it possible to obtain information on the level of damage of the rock at various distances from the explosion, and of the underground works. In several experiments, the working tunnels were completely opened up in order to gain access into the containment cavity. damaged zone experimental boreholes access drifts Tunnel V-1 Main Drift crushed zone block- fractured zone cavity USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 12 To conduct nuclear explosions in flat terrain, vertical boreholes about 1 meter in diameter were drilled (the actual diameter of the hole varied with depth). Depths ranged from 200 m to 2 km for different tests. In several experiments, after the nuclear explosion, several investigation boreholes were excavated in order to study the properties of the damaged rock. For illustration, Figure 4 below presents the plan of the borehole in the cBorehole 102 d experiment at the Balapan test site (Semipalatinsk). The boreholes made after the explosion, including one, which had a very complex shape, intersected the rock located in the various zones of the underground nuclear explosion. By analyzing the core material of these boreholes, it was possible to ascertain the characteristics of the rock and of the massif in zones of rock buckling/crushing, the damaged rock zone, and also in fracture zones (for induced and block fracturing). 1.6Locations Where Nuclear Tests Were Conducted Nuclear tests were conducted in specially equipped areas of the Semipalatinsk and Novaya Zemlya Test Sites (Figures 5 and 6). In addition, 122 underground nuclear explosions were conducted outside of these main test sites (Peaceful Nuclear Explosions, or PNE 9s, conducted cin the interests of the national economy d). PNE test locations span a vast region of the former Soviet Union (in Siberia, Central Asia and in the European part of Russia, see Figure 7) and a wide range of geological environments. Figure 4. Structure of the central zone of the explosion in borehole 102. The vertical lines labeled cB d are the post-test exploratory boreholes. spall zone cavity zone of rock contortion damaged zone high-density fissure zone block fractured zone clay gravel sediments B B B B B 0 - 100 m - 200 m - 300 m - 400 m USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 13 Pavlodarsk Karaganda Semipalatinsk Experimental Field Degelen Balapan Shaft1003 Sargal Tchag Figure 5. Semipalatinsk nuclear testing cPolygon d, showing the location of the Degelen and Balapan underground test sites, and the atmospheric test site ( cExperimental Field d). Figure 6. Novaya Zemlya nuclear testing Polygon, including the underground test sites at Matochkin Shar and Krasino, and the atmospheric test site at Chernaya Guba. Matochkin Shar Krasino Atmospheric Test Site USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 14 Figure 7. Map of the former USSR, showing where cPeaceful Nuclear Explosions d were conducted outside of the main nuclear test sites (black dots). Five PNEs were also conducted on the Semipalatinsk Test Site. Distribution of underground nuclear explosions by location: LocationNumber of tests Semipalatinsk Test Sites363* Novaya Zemlya Test Sites 42 outside of declared test sites 117** * including 5 PNEs conducted on the STS ** at sites in the Russian Federation; European Russia; Asiatic Russia; Ukrainian SSR, Kazakh SSR; Uzbek SSR; and Turkmenskaya SSR 1.7Underground Testing at the Semipalatinsk Test Site The Semipalatinsk Test Site was the primary location for conducting nuclear tests in the USSR. Of a total of 742 Soviet nuclear tests, 483 were conducted at STS. The existence of a good infrastructure at STS, and sufficiently mild climatic conditions (compared to the Novaya Zemlya Test Site), permitted conducting nuclear tests there at any time of year. These factors also permitted cnon-standard d configurations for underground tests, including nuclear detonations with an intentional release of radiation to the surface. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 15 During the early period of nuclear testing (from 29 August 1949 to 25 December 1962), mainly aerial nuclear explosions were conducted at STS (this also includes surface nuclear explosions which, in many respects, can be considered catmospheric d explosions). Aerial explosions were conducted within the boundaries of the Experimental Field (Figure 8; see location on Figure 5). During this early testing period of activity at the STS, it should be noted that it was also here that the Soviets first dropped a nuclear bomb from an airplane (18 October 1951), and also first conducted a thermonuclear explosion, with a yield of 450 kt (12 August 1953). In this same period at STS, the first underground test of a nuclear charge (tunnel V-1) was conducted, as was the first underground test for the purposes of studying the effects of radiation on military equipment with the release of radiation to the surface (tunnel A-1). After the 1963-1964 moratorium on nuclear explosions, the nuclear testing program was conducted under the conditions stipulated by the Limited Test Ban Treaty. Since the 15 th of March, 1964, underground nuclear explosions were conducted only in tunnels or deep bore- holes. On 15 January 1965, on STS but away from the active sites, the first underground nuclear explosion in a borehole was conducted beneath the Shagan riverbed. This explosion was designed to create an ejection crater (Figure 9), and is considered to be the first nuclear explosion for peaceful purposes in the USSR. In this experiment, the technology was developed for using the excavation action of nuclear explosions in the interests of the national economy. In particular, as a result of the explosion (conducted in borehole 1004 at 178 m depth), a crater was formed, 415 m in diameter, 100 m deep, with a volume of 6.4 million cubic meters. After it filled with water, this was used as a water reservoir for the dry Kazakh steppe. (Actually, the volume of the reservoir was significantly greater than the volume of the crater, since the raised lip of the crater that was formed during the explosion served well as a dam, and a second reservoir was formed adjacent to the crater). The most powerful nuclear explosion conducted in a tunnel at the STS was the cTunnel E-1 d experiment (13 February 1966), in which the charge yield was 125 kt. The most powerful test in a borehole at the Balapan test field was the cBorehole 1061 d experiment (2 November 1972) in which a 165 kt charge was tested. It was also at the STS where the methodology for conducting a salvo (multiple-device) explosion in a single tunnel was tested (at Tunnel 14, on 3rd December 1966), and where the USSR first conducted a multiple (salvo) explosion in two tunnels (Tunnels Z-2 and 140, on 10 December 1972). Figure 9. The Soviet nuclear explo- sion, cShagan, d of 15 January 1965, designed to create a crater and dam a water reservoir. Figure 8. View of towers constructed for recor- ding the effects of atmospheric nuclear tests at the cExperimental Field d at the Semipala- tinsk Test Site. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 16 The geological structure of the two main underground test areas at STS differ from one another, in terms of the amount of tectonic (natural) fracturing present in the rock massifs. The Degelen rock massif, where nuclear tests were conducted in tunnels, is highly fractured and characterized by the presence of a noticeable layer of weathered rock, whose physical and mechanical properties differ significantly from the properties of the bedrock. The Degelen area is also characterized by the presence of a large number of tectonic faults and extensive fractures. Each of the excavated tunnels in the Degelen rock massif intersects, as a rule, several level IV-VI tectonic faults. This makes the task of containing the by-products from an underground nuclear explosion in a rock massif of special current interest. It should be noted that the primary containment conditions for underground nuclear explosions were obtained from the results of the Semipalatinsk experiments. It was here that the primary correlations were established; these determine the effect of tectonic damage on the laws governing the release into the atmosphere of by-products from an underground nuclear explosion. 1.8Announcement of Nuclear Tests The organizational system for nuclear testing in the USSR stipulated informing the administrative organizations and the nation 9s population of the conduct of underground nuclear explosions. Informing the administrative organizations was dictated by the necessity of appropriating the corresponding financial funding, as well as the means for engineering the tests (communications, utilities, ensuring safety, messages, and so forth). Since the system for setting up and organizing nuclear explosions was located in two USSR ministries (Ministry of Defense (MOD) of the USSR and USSR Minatom), the Office of the Council of Ministers of the USSR developed the measures for ensuring nuclear testing. The Council of Ministers developed a list of necessary documentation of oral and written reports on completing all work related to the nuclear test. A special Decree of the Council of Ministers regulated the sequence of announcing the nuclear explosions within USSR territory. Thus, in the first quarter of each year, a Joint Decree of the USSR MINATOM and the USSR MOD determined the plan for nuclear testing in the current year. This same Decree had an appendix, which laid out the plan for specific measures to ensure safe testing. All changes in the operational plan of measurements or the schedule of testing were reported without delay to the USSR Council of Ministers by MINATOM and MOD. After completing all preliminary measures and completing preparations for each test, a Representative of the State Commission on Testing prepared a special report to the USSR Council of Ministers and to the CPSU Central Committee on the readiness to conduct the test. This report contained a description of the nature and purpose of the test, as well as a request for approval to conduct the test. After the decision is made to conduct the test, the Council of Ministers and the USSR Defense Council inform the leadership of the appropriate test site. At the same time, no later than two days before the planned test, the administrative leadership, and before that, the party leadership of the region abutting the test site were also informed of the test. The fact that information on conducting the nuclear test was transferred and accepted was documented for all stages in the announcement process. A planned test was announced to the population living in population centers close to the test site only one day prior to the test. Information on conducting nuclear tests and peaceful nuclear explosions was not accessible to the general population. After the nuclear tests were conducted, the USSR population was periodically informed by way of informational messages in official periodicals (e.g., the c Pravda d and c Izvestiya d newspapers). Regular publications on conducted tests began to appear only in 1987. During the time of atmospheric testing, while there were generally no messages published, there were several exceptions. For example, on 8 August 1953, TASS announced in the c Izvestiya d newspaper the planned, 12 August 1953 test of the first hydrogen bomb at STS. In October, 1961, the General Secretary of the CPSU, N.S. Khrushchev, made an announcement at the 22 nd Congress of the CPSU regarding the planned, 30 October 1961 test of the most powerful hydrogen bomb in the world (the number 100 megatons was stated). Indeed, such a test was conducted at Novaya Zemlya, but with an actual yield of 58 Mt (apparently, at the last moment, it was decided to conduct the test with a less-than- full yield). When the nation 9s population was informed of the test, the actual charge yield was not noted. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 17 Often, a yield range was stated (for example, ccharge yield of up to 20 kt d). However, as a rule, the purposes of the test were announced. 1.9Detonation Authority and Procedures The task of carrying out nuclear tests was entrusted to specially-created subdivisions of the Ministry of Atomic Energy (specifically, the Ministry of Medium Machine Building, MMMB, of the USSR, later MINATOM ), and the Ministry of Defense (MOD). A special- ized Directorate of the MOD was created in 1949. The Chief Directorate of the MMMB, made up of military specialists, was created in 1957. The tasking of this Directorate included the special acceptance of nuclear char-ges, populating military units with specialists on using nuclear charges, and conducting nuclear tests. In addition to MINATOM and MOD, a wide range of organizations belonging to various agencies also participated in the fielding and evaluation of nuclear tests (Figure 11). These organizations, first and foremost, the Academy of Sciences of the USSR, researched the physical effects of the nuclear explosion, as well as its damaging effects. The State Commission on Nuclear Testing and the Interagency Commission on Seismic and Radiation Safety of Nuclear Explosions played an important role in organizing and conducting the tests (these commissions had different names at different times). The main task of the State Commission was to provide quality and timely nuclear tests and to obtain the physical characteristics of the tested nuclear charge, as well as other information stipulated by the test program. The State Commission was also responsible for maintaining secrecy of the testing. The Commission was staffed by representatives from MINATOM and MOD, and included a scientific director who developed the charge, and representatives of associated institutes from MINATOM and other agencies. It also included representatives of the organizations who produced the charge and of the customer. The Interagency Commission was responsible for ensuring nuclear test safety. At special meetings of this commission, the conditions for conducting the explosion were examined (when conducting underground explosions, special attention was given to the issue of containment of radioactive explosion by-products under the ground). According to the results of their examination of a specific test design, either the selected explosion parameters were deemed permissible, or corrections were added to the construction of the underground works and stemming components. At the same time, these Commission meetings also developed the requirements for the meteorological situation at the time of the nuclear test. The Interagency Commission included specialists from various agencies: MOD, MINATOM, USSR Academy of Sciences and the Hydrometeorological Service of the USSR. After preparing the appropriate conclusions, the possibility of conducting the test was examined at a high government level in the USSR Council of Ministers and the CPSU Central Committee. Both commissions began their work two or three months before the specific nuclear test. A special role befell the State Commission on the eve of the test. Several days before the nuclear test, this commission examined the preparedness of all participating services and the state of completion of the special measures for ensuring the test. Figure 10. The ~118 kiloton nuclear device that was deto- nated by the Soviets on 14 September, 1988, as part of the cJoint US-USSR Verification Experiment, in bore- hole 1350 at Balapan (Semipalatinsk test site). USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 18 The primary members of the Interagency Commission on Ensuring Test Safety participated in all meetings of the State Commission, which occurred in the initial test preparatory period. The last meeting of the commission occurred on the eve of the nuclear test. This was after the cDry Run d test, in which the preparedness of all services and the operation of all equipment was verified in a mode that was as close as possible to conducting the actual test (e.g., the operation of all services and equipment was verified minute by minute). At this final meeting of the State Commission, information concerning the meteorological situation was heard, and the final decision was taken on the possibility of conducting the nuclear test in the given specific situation. Immediately after a test was conducted, the State Commission collected the operational data, which characterized the test results. A commission meeting was convened to look over issues related to the achieved charge yield as compared to the planned yield; the results of studying the action of the explosion, and the radiation situation in the test location and in the region. At this meeting, a short report was made to the government on the results of the test (a more complete report was prepared after detailed processing of the obtained data). Figure 11. Diagram summarizing the authority and interactions of the various Soviet organizations participating in the conduct of nuclear tests, as described in the text. Note that the Chair of the State Commission on nuclear testing would change, depending on the purpose of the test. For example, if the experiment was a radiation effects test, the Chair would come from the military, while if it was a device performance test, the Chair would be from one of the weapons laboratories. Also, occasional participating organizations include (depending on the test purpose): Ministry of Electronics; Ministry of General Machine Building; Ministry of Heavy Machine Building; or Ministry of Aviation. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 19 Chapter 2CONTAINMENT OF UNDERGROUND NUCLEAR TESTS 2.1What Happens During an Underground Nuclear Explosion From the point of view of ensuring underground nuclear test containment, it is interesting to examine the two main processes which accompany a nuclear explosion in a rock massif: 1) the formation of a zone of irreversible deformation of the solid medium (the containment cavity, the zones of buckling/crushing, crushing and fracture formation); and 2) the formation of highly compressed radioactive gasses in the containment cavity. It should be noted that both these processes occur essentially simultaneously. Formation of highly compressed gasses in the containment cav ity The process of forming an explosive source during the detonation of a nuclear device is comprised of the formation of a cavity, which fills with gaseous by-products at high pressure. Estimates show that at the moment when the cavity reaches its maximum size (after roughly 0.1 sec/kt 1/3 ), the pressure within is equal to roughly 150-200 atm, and the temperature is 4000-5000 degrees Celsius. The processes of condensation of a portion of the gasses and the mass transfer of substances lead to a situation where 1- 10 sec/kt 1/3 after the explosion, a dynamic equilibrium of the gaseous mixture is established. At this time, the temperature of the gaseous by-products is 1500-2100 degrees Celsius, and the pressure settles to the pressure level of the non-condensable gasses. The overall quantity of non-condensable gasses, M , is determined by the relation: M = G q where is a coefficient characterizing the gas-forming properties of the rock, G is the specific mass of the rock participating in the gas formation, and q is the yield of the explosion. The formation of gas is determined by the physical-chemical composition of the rock and is related to the vaporization of rock on the wall of the cavity, the melting of the rock with subsequent vaporization of the water contained in it, and the thermal decomposition of the minerals. It also results from the strong compression of the rock (beyond the cavity) by the shock wave. Of the gaseous by-products that arise in the process of cavity formation, only the non-condensable portion filter through the permeable space of the rock massif. The formation of non-condensable gas during the explosion occurs in the zones of vaporization, melting and thermal decomposition of the rock. Table 3 presents the numbers characterizing the quantity of vaporized and melted material during a nuclear explosion in several rock types. The experiment results indicate the fact that the zone of gas release in dense, hard rock has a mass of close to (2-4) x 10 5 kg/kt, and in alluvium, 7 x 10 5 kg/kt. Table 3 Rock TypeSpecific mass of vaporized material (in tons per kt yield) Specific mass of the melted material (in tons per kt yield) Dry granite69300 +100 Moist tuff (18-20% H 2 0) 72500 +150 Dry tuff73200 -300 Alluvium107650 +50 Rock salt150800 The nature of the gas release depends on the type of rock. In silicate rocks (quartzites, granites, tuffs and alluvium), that contain water in the pore spaces, non-condensable gas is represented by water vapor. In carbonate rocks (limestones, dolomites, and some types of quartzites), the principal mass of the non- condensable gas is comprised of carbon dioxide, which is released as a result of the thermal decomposition of the rock. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 20 The gas temperature in the cavity should be considered to be equal to the melting temperature of the rock in silicate type rock (Table 4) and equal to the temperature of the thermal decomposition of the rock in a carbonate type rock. (Actually in the case of carbonate rock, a competition occurs between the processes of vaporization, melting, and thermal decomposition; nevertheless, the non-condensable gasses are represented, mainly by carbon dioxide). Table 4 Rock TypeShale, PorphyriteGraniteQuartzite, SandstoneAleurolite, Argillite Melting Temp., o C1220-14501340-13801400-17501120-1170 The thermal decomposition of dolomites and limestones occurs at temperatures of 825 to 900 degrees Celsius. Calculations confirm the fact that the mass of dolomite, which undergoes thermal decomposition during an underground nuclear explosion, is close to 180 tons per kiloton of nuclear charge. The temperature of the gasses in the cavity reaches close to 2200 degrees Celsius. As we see, the temperature of the non-condensable gasses in the cavity in the case of a carbonate type rock is higher in comparison with silicate type rock. This can be explained by the difference between the molecular weights of carbon dioxide ( µ =44) and of water ( µ =18). In practice, gas formation may occur simultaneously as a result of water vaporization, and as a result of the thermal decomposition of the minerals. It is namely by this method that it occurs at the Novaya Zemlya and Semipalatinsk test sites. At NZ, the rock massifs are represented by shales, sandstones and quartziitic sandstones with some limestone content. The shales, which are most widespread, have the following mineral composition: KAL 3 Si 3 O 10 (OH) 2 15-35% Fe 3 Al 2 Si 3 O 10 (OH) 2 15-25% SiO 2 10-30% Mg 3 Si 4 O 10 (OH) 2 3-30% FeS 2 0.1-3% Dolomite2-25% free water 0.05-0.8%. At the Balapan test area on STS, the rocks are represented by siltstones, sandy tuffs, argillites, porphyries and carboniferous shales. As a whole, these rocks contain the following minerals: Quartzites 20-50% Dolomites5-15% Mg 3 Si 2 O 5 (OH) 4 1-45% CaAl 2 Si 2 O 8 10-30% Mg 3 Si 4 O 10 (OH) 2 1-20% free water 0.8-1.2% The variation in mineralogical compositions is the main feature of the rock at STS and NZ. Practically all minerals contain hydroxyl and carbonate groups. As a result of the thermal decomposition of these minerals, gasses form: Co 2 , H 2 0, S 2 , H 2 S and others (along with infusible oxides: SiO 2 , MgO, Al 2 O 3 and others). As a result of comparative evaluations, it was found that the gas release at the NZ sites as a result of thermal decomposition of the shales occurs in a volume of rock massif, which is characterized by a mass of roughly 300 tons per kt of nuclear explosion. The equilibrium state of the gas and the cavity is upheld in this case due to the water vapor formed as a result of the decomposition of sericite, which appears during the moment of highest pressure of the saturated vapors of all the minerals comprising the shale. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 21 To simplify the calculations, it is permissible to consider only carbon dioxide and water as the principal components. The molecular weight of the non-condensable gas will be equal, in this case, to: µ = · /( · CO2/ µ CO2 + · H2O / µ H2O ) . In general, it is necessary to consider also the presence of the free water in the rock. In this case: · = · W + · H2O + · CO2 where · W is the specific content (weight) of free water. Formation of the zone of irreversible behavior of the rock The migration of gaseous by-products from an underground nuclear explosion, which, in the final count, determines the containment of the underground explosion, occurs along a permeable channel, which either existed initially in the rock massif, or was formed as a result of the explosion. As a result of the vaporization and melting of the rock at the focus of the explosion, and the subsequent displacement of the medium at great distances as a result of the shock and compressional waves, a containment cavity forms at the focus of the explosion. It is free from the rock volume, and filled with gaseous materials. In the area located beyond the cavity, the medium remains solid. However, as a result of the intense compression in the explosive wave, the rocks undergo intense changes. Their physical-mechanical properties and structure change noticeably. At greater distances, the rock is damaged due to the formation of rather fine pieces (crushing zone). Then, with increased distance from the focus of the underground explosion, the size of the pieces are such that one can speak of the formation of new fractures in the rock massif. Thus, the main zones of irreversible behavior of the medium, from the point of view of containment of the underground explosion are: the containment cavity, and the zones of rock buckling/crushing, crushing and fracture formation (Figures 3 and 4). It is specifically these zones that directly affect the migration of gaseous explosion by-products. The possibility of ensuring containment of an underground nuclear explosion is determined by the initial parameters of the gaseous explosion by-products in the containment cavity and the permeability of the rock massif in the area directly abutting the containment cavity. Similar investigations into the mechanical state of the rock and rock massif in the vicinity of the under- ground explosion, which were conducted using special excavations in the rock after the explosion (adits and deep boreholes) showed that the parameters of the zone of irreversible behavior of the medium is defined by the initial physical-mechanical properties of the rock and of the massif; these are as follows: Degelen Site ZoneCavityRock buckling/crushingRock crushingFracture formation relative size, m/kt 1/3 7-1010-1620-2535-40 Balapan Site ZoneCavityRock buckling/crushingRock crushingFracture formation relative size, m/kt 1/3 10-1414-1825-3050 Increase in Massif Permeability 1 3-8 x1.8-4 x1.3-3.8 x1.27-3 x Note here that ensuring containment of underground nuclear explosions is based on a prediction of the dimensions of the main zones of irreversible deformation of the rock massif during the explosion. 1 For the cavity, the estimated permeability increase is for that of the rock fill resulting from cavity collapse. USGS Open File Report 01-312: Containment of Soviet Underground Nuclear Explosions 22 2.3How Nuclear Explosions Remain Contained Radioactive materials that are produced in a nuclear explosion consist of a rock melt with materials of the nuclear device and gaseous products. As the containment ( camouflet ) cavity expands, a considerable fraction of the nuclear material is converted to the solid state as a result of cooling of the melt and condensation of some of the gaseous components. A certain amount of the radioactive products, mostly inert gases, as well as some products of dissociation of rocks and water vapor, remains in gaseous form. The nature of gas release depends on the type of rock: in rocks of silicate type (quartzites, granites, tuffs, alluvia) that contain water in the free state, the uncondensed gas is represented by water vapor, in rocks of carbonate type (limestones, dolomite, some types of quartzites), most of the uncondensed gas is carbon dioxide that is released as a result of thermal dissociation of rock. Uncondensed gaseous radioactive products of an underground nuclear explosion that have accumulated in the underground cavity following the first instant of the explosion begin to filter through the geological medium. There are several factors that contribute to this: 1 3 elevated permeability of rocks in the near zone of the explosion; 2 3 the presence of tectonic disturbances (faults and fractures) whose permeability is also increased by explosive action; 3 3 inadequate isolation of the underground excavation (tunnel, borehole). At the same time, it should be noted that a certain fraction of the uncondensed radioactive gas remains underground due to the following factors: 1 3 relatively low pressure of explosion products in the containment cavity (while the initial pressure is a few hundred MPa, the pressure of uncondensed gases at the instant that they begin filtering out of the cavity is on the order of 1-10 atm depending on the gas-forming properties of the rocks). 2 3 rather high porosity of the rock mass broken by the explosion (volume of voids formed is comparable to the volume to which uncondensed radioactive gases expand when pressure falls from the initial level to atmospheric); 3 3 with appropriate selection of the depth of burial of the device, the layer of ground situated above the charge chamber (especially the layer of unbroken rock) is good insulating material that prevents propagation of gaseous explosion products toward the free surface. 4 3 well executed stemming systems (and other measures to isolate underground excavations) likewise serve as a good insulator to stop propagation of uncondensed gases toward the surface of

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