The Minor Scale Explosion in 1985 occurred when the United States detonated nearly 5,000 tons of ammonium nitrate to simulate the effects of a nuclear bomb explosion.
The Partial Nuclear Test Ban Treaty of 1963, signed by the three nuclear superpowers— the United States, the Soviet Union, and the United Kingdom—rendered atmospheric nuclear testing impossible. Consequently, the U.S. Department of Defense turned to using conventional explosives in massive quantities to replicate the impact of nuclear bombs, though these were often unreliable. In 1966, L. D. Sadwin and J. Petes, two scientists at the Naval Weapons Center, proposed the use of ammonium nitrate/fuel oil (ANFO), an explosive that had been utilized in the mining industry for many years, according to Amusing Planet.
Fireball from the Minor Scale explosion. (Image: Amusing Planet).
ANFO is extremely inert to impact and cannot be detonated with a small quantity of primary explosives. Instead, it requires a substantial amount of secondary explosives, known as boosters. ANFO is also cost-effective. Its primary component is ammonium nitrate, a widely used commercial fertilizer produced in millions of kilograms each day. The other ingredient is Number 2 fuel oil, a distillate of crude oil commonly used for heating. The combination of these two substances results in an extremely stable and less sensitive mixture, allowing for safe handling.
The first simulation of a powerful explosion using ANFO took place in 1976 during the Dice Throw experiment. This test involved several phases, with explosive quantities increasing from 0.5 kg to 500 tons. Over the next 12 years, the U.S. Department of Defense conducted several powerful explosion tests known as the Misty Castle Series. The first test in the Misty Castle series, called Mill Race, took place in 1981, just 5.6 km south of the Trinity test site at the White Sands Missile Range. To create the desired explosive effect, approximately 24,000 bags, each containing 23 kg of ANFO, were stacked in a cylindrical formation nearly 11.6 m tall and 9.1 m in diameter. Detonating this mass produced an impact equivalent to 500 tons of TNT, simulating a one-kiloton nuclear bomb explosion.
The most powerful test in the Misty Castle series, named Minor Scale, took place on June 27, 1985. A 26.8 m wide hemispherical structure made of fiberglass was constructed, containing 4,744 tons of ANFO. Due to the enormous quantity of ANFO needed, the project team had to build a mixing plant to combine diesel fuel with ammonium nitrate near the test site. The fuel oil was transported to the mixing plant by truck, and the finished product was then delivered to the dome at Ground Zero by truck. Even with the mixing plant, the team could only work with 100 tons of ammonium nitrate and 100 tons of diesel fuel at one time.
The detonation of 4,744 tons of ANFO produced an explosion equivalent to 4 kilotons of TNT. The shock wave from the explosion was comparable to an 8-kiloton nuclear weapon. For comparison, the yield of the atomic bomb dropped on Hiroshima during World War II was 16 kilotons. Nevertheless, Minor Scale remains the largest non-nuclear artificial explosion in history.
