The Largest Man-Made, Non-Nuclear Explosion in History

After the end of World War II in 1945, until the dissolution of the Soviet Union in 1991, there was intense geopolitical tension between the United States and the Soviet Union—a period known as the Cold War. This era was marked by a pervasive fear of nuclear warfare, which cast a long shadow over global politics and daily life. The development and stockpiling of nuclear weapons by both superpowers led to a precarious balance of power, where any direct conflict risked annihilating much of the world's population and environment. This constant threat of nuclear catastrophe influenced international relations, domestic policies, and even popular culture, embedding a sense of existential dread and uncertainty in the global consciousness.

The detonation of 4 kiloton of conventional explosives, constituting the Minor Scale test, at White Sands Missile Range.

A lot of military hardware that were produced during this period, including protective personal equipment, armored vehicles and shelters were hardened to withstand the enormous pressure, intense heat and radiation generated in a nuclear explosion. In order to ensure that these hardware actually worked as intended, rigorous testing was required.

In the first few decades of the Cold War, nuclear effects testing was typically conducted during nuclear detonation events. Some notable tests conducted by the United States include Operation Ivy (1952, which featured the first-ever hydrogen bomb test), Castle Bravo (1954), Operation Plumbbob (1957), Operations Hardtack I and II (1958), Operation Nougat (1961-1962), and Operation Sunbeam (1962).

However, the adoption of the 1963 Partial Test Ban Treaty by the world's three nuclear superpowers—the United States, the Soviet Union, and the United Kingdom—made conducting atmospheric tests of nuclear weapons impossible. In response, the United States Department of Defense turned to using regular explosives in enormous quantities to simulate the effects of a nuclear bomb.

---

More like this:
The 100 Ton TNT Test
Operation Sailor Hat

---

The main problem with using explosives such as TNT and similar materials revolved around safety and scalability. Primary explosives like nitroglycerin are highly sensitive in large quantities, as they can be set off by relatively small amounts of electricity, impacts, friction, or heat. Explosive gases proved just as unreliable; an attempt to use 20 tons of an oxygen-propane mixture as a large-scale explosive ended in failure after the hemispherical balloon holding the mixture experienced structural failure.

In 1966, L. D. Sadwin and J. Petes, scientists with the Naval Surface Weapons Center, proposed using Ammonium Nitrate/Fuel Oil (ANFO)—a tertiary explosive that had been used in the mining industry for years. ANFO is highly insensitive to shock and cannot be detonated with regular blasting caps containing small amounts of primary explosives. Instead, it requires a larger quantity of secondary explosive, known as a primer or a booster. In mining operations, one or two sticks of dynamite are typically used to trigger ANFO explosives.

ANFO is also cost-effective. Its primary ingredient, ammonium nitrate, is a common commercial fertilizer produced in millions of pounds per day. The other ingredient is Number 2 fuel oil, a crude oil distillate commonly used in furnaces or boilers for heating homes and buildings. Mixing the two results in an extremely stable and insensitive mixture that is safe to handle.

The first high-explosive simulation using ANFO took place in 1976 as a part of the Dice Throw test. The test consisted of several phases where progressively larger amounts were detonated, beginning with a paltry 1-pound test and concluding with a 500-ton test.

For the next 12 years, the Department of Defense would conduct several high explosive tests under the Misty Castle Series. The objective of these tests was to determine the vulnerability of different types of shelters made of steel, wood, brick and concrete, as well as the survivability and vulnerability of various type of military hardware, including tank and armored vehicles. The tests also simulated airblast effects on mannequins in vehicles and shelters and in the open. Other experiments included measurements of ground motion, airblast and airflow, perturbations in the earth’s magnetic field, dust density and particle size, and the degradation of optical visibility, and so on.

The first Misty Castle test was Mill Race conducted in 1981, just 3.5 miles south of Trinity Site in the White Sands Missile Range. In order to create the desired detonation effects, approximately 24,000 fifty-pound bags of ANFO, with loose ANFO filling the spaces between the bags, were stacked in a cylindrical shape nearly 38 foot high and 30 foot in diameter. The detonation of this stack produced the equivalent of approximately 500 tons of TNT, and simulated the blast and shock of a one-kiloton nuclear bomb.

A dome of ANFO bags for the Mill Race test. In the background are various types of equipment to be used on the testbed during the detonation.

The most powerful test in the Misty Castle Series, named Minor Scale, took place on 27 June 1985. A dome-shaped hemisphere of fiberglass, 88 feet across, was constructed and filled with 4,744 tons of ANFO. Due to the sheer volume of ANFO required, the project team had to build a mixing plant, which added diesel oil to the ammonium nitrate, near the site. Fuel oil was delivered to the mixing plant in trucks and the mixed product was transported by trucks again to the dome at Ground Zero. Even with a dedicated mixing plant, the team could only work with 100 tons of ammonium nitrate and 100 tons of diesel fuel oil at a time.

The detonation of 4,744 tons of ANFO generated an explosion equivalent to 4 kilotons of TNT. The airblast was calculated to be the equivalent of an 8-kiloton nuclear weapon. For comparison, the yield of the nuclear bomb detonated over Hiroshima during World War 2 was 16 kiloton. Minor Scale remains the largest man-made, non-nuclear explosion in history.

The detonation of Minor Scale caused a massive explosion that kicked up an enormous cloud of dust and debris.

Prior to Minor Scale, there were two contenders for the largest known man-made, non-nuclear explosion in history. The first was the 1917 Halifax Explosion in Nova Scotia, Canada, estimated to have the yield of approximately 2.9 kilotons of TNT. The second was Operation Big Bang, a British operation to destroy the bunkers and military installations on the island of Heligoland, located off the German coast in the North Sea. The Heligoland operation resulted in an explosion with an approximate yield of 3.2 kilotons of TNT.

Another high-explosive test of similar magnitude was Misty Picture that was detonated on 14 May 1987. The explosive charge consisted of 4,685 tons of ANFO poured in bulk into 88-foot diameter, fiberglass hemisphere, similar to the one used for Minor Scale. The test was originally planned for twice the yield, but was reduced to 4 kilotons.

The final test of the Misty Castle Series was Minor Uncle that took place in 1993.

References:
# Misty Castle: High-Explosive Nuclear Effects Simulations at White Sands Missile Range, White Sands Missile Range Museum