Chapter 3 Containing Underground Nuclear Explosions Underground nuclear tests are designed and reviewedfor containment, with redundancy and conservatism in eachstep. INTRODUCTION The United States’ first underground nuclear test, codenamed‘‘Pascal-A,’’ was detonated at the bot- tom of a 499-foot open drill-hole on July 26, 1957.! Although Pascal-A marked the beginning of undergroundtesting, above ground testing continued for another 6 years. With testing simultaneously occurring aboveground,the release of radioactive material from underground explosions was atfirst not a major concern. Consequently, Pascal-A, like many of the early underground tests that were to follow, was conducted ‘‘roman candle’’ style in an open shaft atmospheric testing was conducted in the Christmas Island and Johnston Island area of the Pacific. From 1961 through 1963, many of the undergroundtests vented radioactive material. The amounts were small, however, in comparison to releases from aboveground testing also occurring at that time. With the success of the Rainier test, efforts were made to understand the basic phenomenology of contained underground explosions. Field efforts included tunneling into the radioactive zone, laboratory measurements, and theoretical work to model the containment process. Through additionaltests, experience was gained in tunnel-stemming processes and the effects of changing yields. The early attempts to explain the physical reason why under- that allowed venting. As public sensitivity to fallout increased, guidelines for testing in Nevada became morestringent. In 1956, the weapons laboratories pursued efforts to reduce fallout by using the lowest possible test yields, by applying reduced fission yield or clean technology, and by containing explosions underground. Of these approaches, only underground testing offered hope for eliminating fallout. The objective was to contain the radioactive material, yet still collect all required information. The first experiment designed to contain an explosion completely underground was the *‘Rainier’’ test, which was detonated on September 19, 1957. A nuclear device with a known yield of 1.7 kilotons was selected for the test. The test was designed with two objectives: 1) to prevent the release of radioactivity to the atmosphere, and 2) to determine whether diagnostic information could be obtained from an underground test. The test was successful in both objectives. Five more tests were conducted the following year to confirm the adequacy of such testing for nuclear weapons development. ground nuclear explosions do not always fracture rock to the surface did little more than postulate the hypothetical existence of a ‘‘mystical magical membrane.’’ In fact, it took more than a decade of underground testing before theories for the physical basis for containment were developed. In 1963, U.S. atmospheric testing ended when the United States signed the Limited Test Ban Treaty prohibiting nuclear test explosions in any environment other than underground. The treaty also prohibits any explosion that: ... Causes radioactive debris to be present outside the territorial limits of the State under whose jurisdiction or control such explosion is conducted.’ With the venting of radioactive debris from underground explosions restricted by treaty, containment techniques improved. Although many U.S. tests continued to produce accidental releases of radioactive material, most releases were only detectable within the boundaries of the Nevada TestSite. In 1970, however, a test codenamed ~*Baneberry”’ resulted in a prompt, massive venting. Radioactive material from Baneberry was tracked as far as the Canadian border and focused concern about both the environmental safety and the weaty compliance of In November 1958, public concern over radioactive fallout brought about a nuclear testing moratorium that lasted nearly 3 years. After the United States resumed testing in September, 1961, almost all testing in Nevada was done underground, while 'The first underground test was the United States’ 100th nuclear explosion. 2it is interesting to note that even with an open shaft, 90% ofthe fission products created by Pascal-A were contained underground. 3Arucle 1,1(b). 1963 Limited Test Ban Treaty -31-