Chapter 3 Containing Underground Nuclear Explosions . CONTENTS Page INTRODUCTION . 31 WHAT HAPPENS DURING AN UNDERGROUND NUCLEAR EXPLOSION 32 Microseconds . 32 Milliseconds . +. 32 Tenths of a Second . 32 A Few Seconds . 32 Minutes to Days . .. .. .. .. .. .. .. ... ........ 32 WHY NUCLEAR EXPLOSIONS REMAIN CONTAINED ... ...... SELECTING LOCATION, DEPTH, AND SPACING: . 35 REVIEWING A TEST SITE LOCATION . 37 CONTAINMENT EVALUATION PANEL . .38 CONTAINING VERTICAL SHAFT TESTS . 40 CONTAINING HORIZONTAL TUNNEL TESTS . .. .. .. .. .. .. ... ... ...... 41 TYPES OF RADIATION RELEASES . 46 Containment Failure: . 46 Late-Time Seep . 46 Controlled Tunnel Purging . 47 Operational Release . 47 RECORD OF CONTAINMENT . 47 Containment Evaluation Panel . 47 Vertical Drill Hole ’lasts . 48 Horizontal Tunnel Tests . 48 From the Perspective of Human Health Risk . 49 A FEW EXAMPLES: . 49 IS THERE A REAL ESTATE PROBLEM AT NTS? . 51 TIRED MOUNTAIN SYNDROME? . 51 HOW SAFE IS SAFE ENOUGH? . 54 Box Page 3-A. Baneberry . 33 Figures Figure Page 3-1. Formation of Stress “Containment Cage” . 35 3-2. Minimum Shot Separation for Drill Hole Tests . 38 3-3. Minimum Shot Separation for Tunnel Tests . 39 3-4. “Typical’’ Stemming Plan . 41 3-5. Three Redundant Containment Vessels . 42 3-6. Vessel I . 43 3-7. Vessel 1 Closures . 44 3-8. Tunnel Closure Sequence . 45 3-9. Typical Post-Shot Configuration . .46 3-10.4Radius of Decrease in Rock Strength . .. .. ... ... ....... 53 Table Page 3-1. Release From Underground Tests . .. .. .. .. .. .. .. .......8 48 Chapter 3 Containing Underground Nuclear Explosions Underground nuclear tests are designed and reviewed for containment, with redundancy and conservatism in each step. INTRODUCTION atmospheric testing was conducted in the Christmas Island and Johnston Island area of the Pacific. From The United States’ first underground nuclear test, 1961 through 1963, many of the underground tests codenamed ‘‘ Pascal-A,’ was detonated at the bot- vented radioactive material. The amounts were tom of a 499-foot open drill-hole on July 26, 1957.1 small, however, in comparison to releases from Although Pascal-A marked the beginning of under- aboveground testing also occurring at that time. ground testing, above ground testing continued for another 6 years. With testing simultaneously occur- With the success of the Rainier test, efforts were ring aboveground, the release of radioactive material made to understand the basic phenomenology of from underground explosions was at first not a major contained underground explosions. Field efforts concern. Consequently, Pascal-A, like many of the included tunneling into the radioactive zone, labora- early underground tests that were to follow, was tory measurements, and theoretical work to model conducted ‘‘reman candle’ style in an open shaft the containment process. Through additional tests, that allowed venting.2 experience was gained in tunnel-stemming proc- esses and the effects of changing yields. The early As public sensitivity to fallout increased, guide- attempts to explain the physical reason why under- lines for testing in Nevada became more stringent. In ground nuclear explosions do not always fracture 1956, the weapons laboratories pursued efforts to rock to the surface did little more than postulate the reduce fallout by using the lowest possible test hypothetical existence of a “mystical magical mem- yields, by applying reduced fission yield or clean brane.” In fact, it took more than a decade of technology, and by containing explosions under- underground testing before theories for the physical ground. Of these approaches, only underground basis for containment were developed. testing offered hope for eliminating fallout. The objective was to contain the radioactive material, yet In 1963, U.S. atmospheric testing ended when the still collect all required information. The first United States signed the Limited Test Ban Treaty experiment designed to contain an explosion com- prohibiting nuclear test explosions in any environ- pletely underground was the “Rainier” test, which ment other than underground. The treaty also was detonated on September 19, 1957. A nuclear prohibits any explosion that: device with a known yield of 1.7 kilotons was . causes radioactive debris to be present outside selected for the test. The test was designed with two the territorial limits of the State under whose objectives: 1) to prevent the release of radioactivity jurisdiction or control such explosion is conducted.3 to the atmosphere, and 2) to determine whether With the venting of radioactive debris from diagnostic information could be obtained from an underground explosions restricted by treaty, con- underground test. The test was successful in both tainment techniques improved. Although many U.S. Five more tests were conducted the objectives. tests continued to produce accidental releases of following year to confirm the adequacy of such radioactive material, most releases were only detect- testing for nuclear weapons development. able within the boundaries of the Nevada Test Site. In November 1958, public concern over radioac- In 1970, however, a test codenamed ‘‘Baneberry’ tive fallout brought about a nuclear testing morato- resulted in a prompt, massive venting. Radioactive rium that lasted nearly 3 years. After the United material from Baneberry was tracked as far as the States resumed testing in September, 1961, almost Canadian border and focused concern about both the all testing in Nevada was done underground, while environmental safety and the treaty compliance of IThc first underground icst wm the Uni[cd Slates’ 1 Wth nIJdeM explosion. 211 is intere5t1ng [. no(c tha[ even with ~ open shaft, 90% of the fission products created by Pascal-A were contained Underground 3A~iClc I, I (b). 1963 Limited Test Ban Trcaly -3 l– — 32 ● The Containment of Underground Nuclear Explosions the testing program. 4 Testing was suspended for 7 Tenths of a Second months while a detailed examination of testing practices was conducted by the Atomic Energy As the cavity continues to expand, the internal Commission. The examination resulted in new pressure decreases. Within a few tenths of a second, testing procedures and specific recommendations the pressure has dropped to a level roughly compara- for review of test containment. The procedures ble to the weight of the overlying rock. At this point, initiated as a consequence of Baneberry are the basis the cavity has reached its largest size and can no of present-day testing practices. longer grow.6 Meanwhile, the shockwave created by the explosion has traveled outward from the cavity, Today, safety is an overriding concern throughout crushing and fracturing rock. Eventually, the shock every step in the planning and execution of an wave weakens to the point where the rock is no underground nuclear test. Underground nuclear test longer crushed, but is merely compressed and then explosions are designed to be contained, reviewed returns to its original state. This compression and for containment, and conducted to minimize even relaxation phase becomes seismic waves that travel the most remote chance of an accidental release of through the Earth in the same manner as seismic radioactive material. Each step of the testing author- waves formed by an earthquake. ization procedure is concerned with safety; and conservatism and redundancy are built into the 5 system. A Few Seconds AN After a few seconds, the molten rock begins to WHAT HAPPENS DURING collect and solidify in a puddle at the bottom of the UNDERGROUND NUCLEAR cavity.7 Eventually, cooling causes the gas pressure EXPLOSION within the cavity to decrease. The detonation of a nuclear explosion under- ground creates phenomena that occur within the Minutes to Days following time flames: When the gas pressure in the cavity declines to the Microseconds point where it is no longer able to support the overlying rock, the cavity may collapse. The col- Within a microsecond (one-millionth of a sec- lapse occurs as overlying rock breaks into rubble and ond), the billions of atoms involved in a nuclear falls into the cavity void. As the process continues, explosion release their energy. Pressures within the the void region moves upward as rubble falls exploding nuclear weapon reach several million downward. The “chimneying” continues until: pounds per square inch; and temperatures are as high as 100 million degrees Centigrade. A strong shock . the void volume within the chimney completely wave is created by the explosion and moves outward fills with loose rubble, from the point of detonation. the chimney reaches a level where the shape of the void region and the strength of the rock can Milliseconds support the overburden material. or . the chimney reaches the surface. Within tens of milliseconds (thousandths of a second), the metal canister and surrounding rock are If the chimney reaches the surface, the ground sinks vaporized, creating a bubble of high pressure steam forming a saucer-like subsidence crater. Cavity and gas. A cavity is then formed both by the pressure collapse and chimney formation typically occur of the gas bubble and by the explosive momentum within a few hours of the detonation but sometimes imparted to the surrounding rock, take days or months. 4!kc for ex~p]e, Bruce A. Bolt, Nuclear Explosions and Eart@akes San Francisco, CA. (W.H. Freeman k CO., 1976). ~S= ‘ ~~tonatim &~ority and Procedures’ (ch. 2). %x the next section, “How explosions remain contained, ” for a detailed explanation of cavity