Chapter 3—Containing Underground Nuclear Explosions ¢ 51 radioactive material escaped downthe pipe and then seeped from the HLOSpipe tunnel into the bypass tunnel. Subsequently, the tunnel was intentionally vented so that experimental equipment could be recovered. 7. Mighty Oak (April 10, 1986, horizontal tunnel test, less than 20 kilotons, no unintentional release of radioactive material.) During the Mighty Oak test, the closure system near the working point was over-pressured and failed. The escaped pressure and temperature caused both the MAC and the GSACto fail. The toss of the stemming plug near the working pointleft the tunnel an open pathway from the cavity. Temperatures and pressures on the closed TAPS door reached 2,000 °F and 1,400 poundsper square inch. After 50 seconds, the center part (approximately 6 feet in diameter) of the TAPS door broke through. With the closures removed, the stemming column squeezed out through the tunnel. Radioactive material leaked from vesselI, into vessel IT, and into vessel III, where it was successfully contained. Approximately 85 percentofthe data from the primetest objectives was recovered, although about $32 million of normally recoverable and reusable equipment was lost.** Controlled purging of the tunnel began 12 days after the test and continued intermittently from April 22 to May 19, when weather conditions were favorable. A total of 36,000 Ci were released to the atmosphere during this period. IS THERE A REAL ESTATE PROBLEM AT NTS? There have been over 600 underground and 100 aboveground nuclear test explosions at the Nevada Test Site. With testing continuing at a rate of about a dozen tests a year, the question of whetherthere will eventually be no more room to test has been raised. While such a concern maybe justified for the most convenient areas under the simplest arrangements, it is not justified for the test area in general. Using the drill-hole spacing of approximately onehalf the depth of burial, high-yield tests can be spaced about 1,000 feet apart, and low-yield tests can be spaced at distances of a few hundred feet. Consequently, a suitable square mile oftest site may provide space for up to 25 high-yield tests or over 300 low-yield tests. Even with testing occurring at a rate of 12 tests a year, the 1,350 square miles oftest site provide considerable space suitable for testing. In recent years, attempts have been made to use space more economically, so that the most convenient locations will remain available. Tests have traditionally been spaced in only 2-dimensions. It maybe possible to space tests 3-dimensionally, that is, with testing located below or aboveearliertests. Additionally, the test spacing has been mostly for convenience. If available testing areas become scarce, it may become possible to test at closer spacing, Or even to test at the same location as a previoustest. Area for horizontal tunnel tests will also be available for the future. The N-tunnel area has been extended and has a sizable area for future testing. P-tunnel, which is used for low-yield effects tests. has only beenstarted. (See figure 2-4 in ch. 2 of this report.) Within Rainier and Aqueduct Mesa alone, there is enough area to continue tunnel tests at a rate of two a year for at least the next 30 years. ‘Consequently, lack of adequate real estate will not be a problem for nuclear testing for at least several more decades. TIRED MOUNTAIN SYNDROME? The ‘‘Tired Mountain Syndrome"’ hypothesis postulates that repeated testing in Rainier Mesa has created a ‘‘tired’’ mountain that no longer has the strength to contain future tests. Support for this concer has come from the observation of cracks in the ground on top of the Mesa and from seismological measurements, indicating that large volumes of rock lose strength during an underground test. Debate exists, however, over both the inference that the weakened rock is a danger to containment, and the premise that large volumes of rock are being weakened by nuclear testing. Basic to the concern over tired mountain syndrome is the assumption that weakened rock will adversely affect containment. As discussed previously, only in an extremesituation, such as detonating an explosion in water-saturated clay, would rock strength be a factor in contributing to a leak of radioactive material.*5 For example, manytests have 34Containment and Safety Reviewfor the Mighty Oak Nuclear Weapon Effects Test, U.S. Department of Energy, Nevada Operations Office, NVO-311, May 1, 1987. 35See earlier section ‘‘Why do nuclear tests remain contained?’’