When the US Nuked Mississippi

In 1964, the Pentagon carried out its only nuclear tests in the eastern US, detonating two underground nuclear devices in a rural area of Mississippi.

salmon device project dribble
The “Salmon” nuclear device                                              photo from WikiCommons

In October 1963, the United States and the Soviet Union, after nine years of negotiations, signed a Limited Test Ban Treaty which outlawed the testing of nuclear weapons in the air, under the sea, or in outer space. It was the first significant attempt to control the Cold War nuclear arms race.

The Treaty, however, did allow continued nuclear weapons testing underground (provided that the weapon was smaller than 150 kilotons–about ten times the size of the Hiroshima bomb). Partly, this was because it was presumed that the radiation and fallout would be contained by the earth’s surface. But also it was partly because nobody was really sure that an underground test could be detected by another nation anyway. It was agreed in the Treaty that both nations would be able to use “national technical means” to monitor each other for signs of illegal tests, but in the early 1960s, the only equipment potentially capable of detecting an underground explosion consisted largely of sensitive seismographs which could, at least in theory, detect the shock waves that would result from a nuclear explosion. But nobody was certain it would always work. And so, unsure that a ban on underground tests would actually be verifiable, it was decided to allow them to continue, and to put off any further action until it was demonstrated that a ban on such tests could in fact be enforced.

Further, even before the Treaty was signed the US had became concerned that there might be a way for the Soviets to cheat, and to carry out secret atomic tests that would be undetectable to the Americans. When a nuclear weapon was exploded underground, it blew out a large cavity that was sealed by a thick layer of molten material. If the Russians then lowered a second bomb inside this cavity and exploded it, the Americans thought, the underground air chamber might be able to muffle the shock waves from the blast, making them difficult or impossible to detect by seismographs on the earth’s surface. As a later Pentagon report put it: “If a large enough cavity were created underground, and a nuclear device of the right yield were detonated at the center of it, the air would cushion the shock and relatively little of the energy of the detonation would be transmitted to the cavity wall. Therefore, the earth shock or seismic signal transmitted to distant seismographs would be extremely small when compared to the energy transmitted from a tamped explosion of the same yield and at the same position.”

The Johnson Administration decided that the only way to know for sure would be to try it out for themselves and see if their instruments could detect it. The test would be known as “Project Dribble”, and it would be carried out under a Pentagon program called “Vela Uniform”, which had responsibility for monitoring all Soviet nuclear tests. The plan was laid out in detail, and in public, with the US describing exactly what it was going to do and why. To give the best possible test, it was decided that the initial underground explosion would be done inside a “salt dome”–an underground salt deposit that had been left behind by an ancient sea. These domes were found in many countries around the world and were often commercially mined, but the US considered that they could be turned to clandestine testing by using a jet of water to dissolve a large chamber inside the salt deposit, perhaps camouflaged by a civilian salt-mining operation. So, the original plan for Project Dribble called for a total of three underground nuclear explosions. One would be detonated inside a salt dome and excavate an underground chamber. After that, two more bombs would be set off inside the void to test the effectiveness of various possible methods of detecting the explosions and tracing their origins. (In addition to providing the best test conditions, salt domes offered the advantage of being impervious to groundwater, thus preventing the leakage of any radioactive ground water around the test site and making the project theoretically safe for the surrounding area.)

By this time, most American nuclear testing was being done in Nevada, not all that far from Las Vegas. But there were no suitable salt domes at the Nevada Test Site, so a different location had to be found. After considering nineteen potential sites, the Army decided on the Tatum Salt Dome, in an undeveloped area around 20 miles away from the town of Hattiesburg MS. The US Department of Energy, which was legally responsible for all US nuclear weapons and their testing, leased a 1500-acre patch of land in the middle of a pine forest and pecan farm, and began drilling. By September 1964, after some technical delays, they were ready: the test borehole reached the top of the salt dome at 1200 feet, and continued a further 1500 feet into the salt layers.

The first nuclear explosion, codenamed “Salmon”, was scheduled for September 29, but safety regulations prohibited any action if the wind could potentially carry any radioactive fallout towards an inhabited area, and there were several postponements. Then Hurricane Hilda came barreling through. It wasn’t until October 22 that the weather was finally favorable. In the meantime, all of the local inhabitants within 20 miles (including the 400 residents of Hattiesburg) were evacuated. (For their trouble, the temporarily-displaced civilians were paid $10 each by the military.)

The Salmon device was a gun-type uranium configuration similar to Little Boy, but with a yield of only 5.3 kilotons–about one third the size of the Hiroshima bomb. It was carefully lowered into the borehole to a depth of 2720 feet, and then detonated.

The effect was much greater than anyone had thought. As the Pentagon later noted: “Instrumentation at selected points showed that the observed ground motion was generally consistent with the pre-event predictions; however, almost immediately after the detonation, damage complaints were received from much greater distances than anticipated, including large numbers of complaints and claims from communities such as Purvis (10 miles), Lumberton (12 miles), Hattiesburg (20 to 23 miles), and a few claims from Columbia (17 miles). Prompt inspection of the involved structures revealed a high incidence of new cracks in plastered walls and ceilings, stucco, concrete blocks and masonry construction. Eyewitness reports substantiated that much of this damage had occurred at about the time of the detonation.”

Although the explosion did not break the surface, the ground above the test site was pushed up over four inches by the force. Underground, the bomb blasted out a spherical hollow 110 feet in diameter, coated on the inside by a glassy sheet of melted salt and sand. When the Pentagon drilled a test hole down into the chamber three months later, they found the temperature to still be over four hundred degrees. (The test hole also released a cloud of radioactive vapor which contaminated some of the surface.)

It was thus discovered that the underground shock waves were magnified and became more powerful as they moved through the surrounding earth. Later, this principle would be used for earth-penetrating warheads designed to take out underground bunkers and command centers, but in 1964 the important lesson was that the seismic waves from an underground nuclear test could indeed be detected by seismographs located at considerable distances. With verification now possible, negotiations with the Soviets could begin again, to result later in a new treaty banning underground tests. (Such a treaty was signed in 1996 but was not ratified by the US and has not gone into effect.)

In 1966, after the Salmon detonation chamber had cooled sufficiently, plans were made for the second test shot, to be called “Sterling”. The Sterling device was another gun-type uranium mechanism, but with a yield of just 380 tons, or a little more than a third of a kiloton. It was lowered into the center of the cavity and detonated on December 3. Seismic sensors determined that the shock waves were indeed buffered by the air void and were greatly reduced, but still detectable. For further testing, it was decided that nuclear devices were no longer needed, and the original plans for a third atomic explosion were dropped. The Pentagon instead used an explosive mixture of methane gas and oxygen. Two of these charges were set off inside the chamber: the “Diode Tube” test in February 1969 and the “Humid Water” test in April 1970. These detonations each had an explosive yield of around 300-315 tons of TNT.

With that, Project Dribble came to an end. The Defense Department, having learned that they could indeed detect any Soviet cheating, cleaned up the radioactive mess it had made at the surface, carrying away contaminated soil and pumping radioactive water back into the chamber. (In 2015, the US paid out $16.8 million as compensation for people who had filed claims for damages due to radiation at the site during and after the testing.)

The Department of Energy continued to lease the Tatum Dome site for twenty years, then bought it outright in 1992. In 2010 the site was given to the State of Mississippi, who turned it into a timber area and a wildlife refuge. The Department of Energy still owns the subsurface, however, and continues to monitor the site with a number of test wells. Though there is a marker stone with a plaque, put up by the Department of Energy’s Office of Legacy Management, the actual site is closed to the public.

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