On 29 July 1996, at 01:49 GMT, the People's Republic of China detonated an underground explosion at their Lop Nor test facility in a remote part of XinJiang autonomous region. Within hours of the test, the Chinese government announced that it had concluded its nuclear testing program and was ready to join a moratorium on the testing of nuclear weapons. The subsequent approval of the Comprehensive Test Ban Treaty (CTBT) by the United Nations in September, which permanently outlaws nuclear weapons testing, makes it quite possible that the 29 July test will be the last nuclear explosion for some time - maybe forever! Figure 1 shows the P waves from the explosion at several IRIS stations. The magnitude of the event was mb = 5.2, which translates to an explosive yield of approximately 10 kilotons.
|Figure 1. Recording to the P-waves from the "Last Nuclear Test" at several IRIS stations.|
Much less has been written about the Chinese nuclear weapons program than for either the United States or Soviet programs. There is one excellent reference, Lewis and Litai (1988, "China builds the Bomb"), and this article draws heavily from this work. The Chinese quest for nuclear weapons began in 1955 after the "Taiwan Strait Crisis," in which the US made threats of using nuclear weapons to halt Chinese invasions of islands held by the Taiwan government. Previous to this time, Mao Zedong had labeled the atomic bomb "a paper tiger," which only had modest strategic value in the struggle between communism and imperialism. Mao was fond of stating that nuclear attack could kill hundreds of millions of Chinese, but given time the population would breed and grow, eventually overwhelming the enemy.
Unlike the Manhattan project, the Chinese knew a priori what it would take to build the atomic bomb. Thus, they were able to develop various aspects of nuclear technology, such as mining uranium, design of the bomb, and production of a nuclear initiator, in parallel. The Chinese plan called for a heavy reliance on Soviet advisors and followed the "Soviet Blue Print" to the bomb. In fact, the Soviets originally agreed to supply the Chinese an actual prototype bomb before 1960. However, almost immediately, Sino-Soviet relations began to deteriorate, and by 1959 the Soviets were seeking to hinder the Chinese program. The Soviets officially suspended their nuclear assistance in a letter dated 20 June 1959. In sarcastic recognition of this event, the Chinese chose the code name "596" for their first atomic bomb.
The parallel development of the Chinese nuclear program had some unusual consequences. One of the most unusual of these was the decision to use U235 as the fissile material in the first weapon. There are two principle fissile materials used in nuclear weapons: U235 and Pu239. Plutonium has a relatively short half-life (22,000 years) and is not naturally occurring. On the other hand, U235 is found in nature, although is it is much less abundant than U238 (the natural ratio is 1:147). There are several methods for separating U235 from U238, although the most common is through a process called gaseous diffusion. In this process, uranium hexafloride gas is forced through literally tens of thousands of nickel barriers. The slight mass difference between U235 and U238 means that the U235 gas is preferentially passed through the barrier. It is a tremendously expensive and time consuming process to produce large amounts of U235; for weapons-grade fissile material, U235 needs to be "enriched" to the 90% level. On the other hand, only modest enrichment of U235 is required to fuel a reactor that can be tuned to produce plutonium. In the early 1960's the Chinese uranium isotope separation facilities were far ahead of their programs for building plutonium production reactors. Economic pressures in 1960 caused the Chinese to halt work on their reactors and concentrate on the uranium enrichment facilities - hence the decision to use U235 in the first weapon.
The first Chinese test of a nuclear device took place at the Lop Nor test site on 16 October 1964. The weapon was an implosion-type device suspended from a 120m tall tower. The detonation produced an explosive yield about 15 kt, which is typical for a first-generation weapon test. The news of the explosion was met with wild celebration in Beijing.
The Chinese had an accelerated program to transition their nuclear devices into weapons. The second Chinese test (14 May 1965) was a test of a bomber-delivered weapon based on the 596 design. The fourth test was of a missile warhead. Unlike the US or Soviet programs, the Chinese chose to test their entire missile delivery system in their first test. On October 27, 1966, a missile was launched some 800 km west of Lop Nor; this missile flew over several major cities before detonating in a 20-kt explosion at the test site.
Even before the detonation of 596, the Chinese had begun work on the problem of developing a hydrogen bomb. Again, the Chinese development program benefited from knowledge of the Soviet and American weapons programs. The third and fifth Chinese nuclear explosions were experiments to test the properties of thermonuclear materials, principally lithium-6 deuteride. These tests were "boosted" fission weapons; fusion fuel was placed in the core of an implosion fission weapon. The yields of these weapons were on the order of several hundred kilotons.
It took the Chinese only six tests to arrive at a true thermonuclear weapon. On June 17, 1967, a medium-range bomber dropped a bomb over Lop Nor which detonated as a 3-Mt explosion. The rapid development of the Chinese H-bomb is even more remarkable when one considers the political turmoil in China at the time. In 1966, Mao Zedong declared the Great Proletarian Cultural Revolution, and scientists were considered elitist and therefore, suspect in the great battle to empower the proletariat. Nuclear scientists were forced into the countryside; technicians questioned scientific authority, and many of the facilities necessary for the production of nuclear materials were shut down. In one of the more bizarre episodes, one of Mao's nephews led a band of "revolutionaries" to Lop Nor to take over and operate the facility for "the people."
Despite the setbacks of the Cultural Revolution, the Chinese nuclear program survived. In the summer of 1965 Mao wrote a poem called "Two Birds: A Dialogue." In this poem, a giant bird, called a peng, is questioned on what is the purpose of being so big and strong. The bird's response is that "because I can land wherever I want" (Salisbury, 1992). Many have interpreted this poem as a commentary on the Chinese challenge to the nuclear programs of the Soviet Union and the US. The 29 July 1996 explosion marked at least the 44th Chinese test (P. Richards, unpublished list). Today the Chinese are thought to possess a sophisticated nuclear arsenal, third in size only to the those of the US and Russia.
In the summer of 1958, the Chinese began to search for a site to test their future nuclear weapons. The search carried them to the western deserts of XinJiang Province (now an autonomous region). The Soviet advisors to the Chinese recommended a site 150 km from a large city; their rationale was that it was perfect for testing devices with a maximum yield of 20 kilotons. The Chinese rejected the Soviet attempts to limit their nuclear program and chose a desert valley which lies on the ancient "silk road." In fact, Marco Polo visited the area in the thirteenth century and stayed at the city of Lop (now called Ruoqiang). Marco Polo wrote that it is a "well known fact that this desert is the abode of many evil spirits, which amuse travelers to their destruction with the most extraordinary illusions." The remoteness of the region was reaffirmed shortly before the first Chinese test. Part of the final test preparations included an aerial inspection, and much to the surprise of the Chinese, the aerial photos revealed an encampment of over 200 inhabitants living close to ground zero! Search parties eventually rounded up the group, which turned out to be a group of Kuomintang (Chinese Nationalists), fugitives that had avoided surrender in 1949.
Lop Nor is located in the northeastern edge of the Tarim basin, an unusual geologic province. The Tarim basin is a rigid block of Precambrian and Paleozoic rocks which have survived relatively undeformed during the ongoing IndiaŠEurasian collision. To the north of the Tarim basin is the Tien Shan; to the south is the Tibetan Plateau and the Hindu Kush. Most of the Tarim basin is covered with a Quaternary sedimentary sequence, although at the Lop Nor test site the sedimentary cover is thin (Matzko, 1994). The basement at the test site consists mainly of metaconglomerates, sandstones, and some Carboniferous-age granite. The basement outcrops as small knobs and ridges.
|Figure 2. Location of the underground nuclear test site at the Lop Nor test site. The topography is shaded and contoured. The location of the tests are shown with the dots, which are color coded according to size. The July 29, 1996 event was located in the cluster of tests on the western portion on the test site.|
The first underground test at Lop Nor took place on 22 September 1969. The last atmospheric test took place on 16 October 1980, all tests since have been underground. Table 1 lists the known underground tests at Lop Nor and their body wave magnitudes. Figure 2 shows the epicenters determined by ISC or the PDE for the events in Table 1.
The events are clustered in two groups. The events in the western portion of the test site are thought to have been detonated in horizontal shafts. The events in the basin are vertical emplacements. The use of horizontal and vertical shafts for testing is similar to the US testing program.|
The magnitude of the Chinese underground tests range from mb = 4.5 to mb = 6.6. In general, it is assumed that the magnitudeŠyield relationship for Lop Nor is approximately the same as that observed for the former Soviet test sites in Kazakhstan. Thus, an approximate yield relationship of the form mb = 4.45 + 0.75log Y can be used to assign yields to the Chinese tests. The largest test (21 May 1992) probably had a yield of about 700 kt. Note that most of the presummed horizontal-shaft tests are very small yields.
Most of the Chinese nuclear explosions are accompanied by tectonic release. The 21 May 1992 event produced seismograms with very large tangential motions (Figure 3 shows the waveforms at the IRIS/IDA station ARU). The tectonic release from this event can be modeled as a strike-slip earthquake (Gao et al., 1995) with a moment equivalent to a magnitude MS 5.1. The tectonic release is strongest for the vertical-shaft events; these are most likely detonated in granite, a material well suited for storing tectonic strain (Toksoz and Kehrer, 1972; Wallace, 1991). As the yield of Lop Nor explosions decreases, the orientation of the tectonic release changes to thrusting-type motion. This is exactly what is observed at the Nevada test site (Patton, 1991).
|Figure 3a. Seismic records for the May 21, 1992 Lop Nor test. The records are rotated such that the top trace is the vertical component and the bottom is the tangential component. Not the large size of the S and surface waves on the tangential component.||Figure 3b. Travel path for the seismograms shown in 3a. Although the path contains some extremes in topography, the best explanation for the tangential components is tectonic release.|
The 29 July 1996 nuclear explosion at Lop Nor was most likely emplaced in a horizontal shaft in the western side of the test site. The test marked the end of testing program which was in many ways different than the US nuclear weapons program. The Chinese had far fewer tests, yet obviously conducted tests for weapons design as well as to study the effects of nuclear weapons.
Gao, L., T. Wallace, and P. Richards (1995). Seismotectonics of the Lop Nor test site, to be submitted.
Patton, H. (1991). Seismic Moment estimation and scaling of the long-period explosion source spectrum, Taylor, Patton, Richards, eds., AGU Geophys. Mono. 65, 171-183.
Toksoz, M.N., and H.H. Kehrer (1972). Tectonic strain release by underground nuclear explosions and its effect on seismic discrimination, Geophys. J., 69, 141-161
Wallace, T. (1992). Body wave observations of tectonic release, in Explosion Seismology, Taylor, Patton, Richards, eds., AGU Geophys. Mono. 65, 161-170.
Matzko, R. (1994). Geology of the Chinese test site, near Lop Nor, Xinjing Uygun Autonomous Region, China, Engineering Geology, 36,173-181.
Lewis, J. W., and X. Litai (1988). China Builds the Bomb, Stanford University Press, Stanford, Calif.
Salisbury, H. E. (1992). The New Emperors, Avon Books, New York.
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