Radiation. The word conjures images of mushroom clouds and man-made hazards - but nuclear events can be a natural part of the landscape as well. Let's take a look at three bits of radioactive history, including...
- The Elephant's Foot: The remnants of Chernobyl's reactor 4 could be the deadliest substance on Earth
- The Demon Core: An unused bomb core that killed researchers... twice
- The Oklo Fission Reactor: The remains of a two billion year old natural nuclear reactor
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| Pripyat was abandoned during the Chernobyl disaster... (Anonymous) |
The Elephant's Foot
In 1986 the Chernobyl nuclear power plant of Ukraine entered meltdown. Ruptures in the reactor allowed coolant to escape, with the resulting steam explosions blowing radioactive contaminants into the surrounding environment.
Less obvious was what happened inside the ruins. The intense heat liquefied parts of the reactor and the nuclear fuel, producing radioactive lava slurry called "corium." This artificial substance contains melted control rods, fuel, cladding, concrete and fission byproducts, making it one of the most lethal substances to man. Not only does it emit massive amounts of radiation, the substance is incredibly toxic and remains so to this day.
The mass of corium was dubbed "the elephant's foot" due to the dark, wrinkled appearance of the once-liquid substance - much like the feet of the titular pachyderm. At around 3m in length, the foot is thought to have hit temperatures of around 2600oC during the meltdown and initially emitted enough radiation for a fatal dose in a matter of minutes. Even worse, the foot cracked as it cooled and made it possible to breath in alpha emitting dust. That's not something you want to do.
Despite the sheer danger of exposure, photographs have been taken of the foot. Radiation specialist Artur Korneyev visited the site of the disaster 10 years after the meltdown to document the extent of the damage - including the mass of lava. Photos taken by Korneyev using a timed camera show him inspecting the foot, with all kinds of visual distortions thanks to ambient radiation damaging the film.
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| A bold choice, to be sure... (Chris Mclaughlin) |
The Demon Core
As the source of two fatal lab accidents, this plutonium core lived up to its name.
Resembling a hemisphere of metal with a protruding sphere of plutonium, the core had been intended for a nuclear bomb. When the Japanese capitulated, the core was repurposed for research and remained at Los Alamos in New Mexico
Some cultures say that a drawn weapon should taste blood before being sheathed, lest it turn against the wielder. Less than a week after the surrender of Japan, Harry Daghlian was performing a criticality experiment on the core. These experiments aimed to reveal the "critical mass" of materials - a risky procedure that was termed "tickling the dragon's tail" by researchers.
Daghlian was building a neutron reflector around the core with tungsten carbide bricks. This would reduce the amount of plutonium required to reach criticality - just like a sealed room is easier to heat than an open one. He was on his fifth layer of bricks and was about to place a further brick directly over the core when his monitoring equipment lit up - placing the brick could drive the core supercritical. At that moment he slipped and dropped the brick directly onto the core.
Though Daghlian managed to knock the brick clear and prevent a potentially catastrophic reaction, he did so at the cost of a fatal dose of radiation. He was dead within a month.
Daghlian's colleague Louis Slotin had also worked on the first atomic weapons of the USA. In 1946 he ran a demonstration for his fellow researchers, bringing the core just shy of criticality by lowering a reflective beryllium tamper until it partially covered the plutonium. The procedure would reflect some of the neutrons back and produce a short-lived reaction that could be sampled for data.
This delicate and precise operation was performed by inserting a screwdriver between the core and the tamper to keep them apart - and on this occasion, the screwdriver slipped. The fully-covered core emitted a flash of blue light before Slotin could flip the tamper away - while he'd stopped a true chain reaction, a massive burst of radiation had blasted the room.
Now fully covered, the reaction surged and a flash of blue light accompanied a wave of heat. Slotin flipped the tamper back off before a chain-reaction could really take hold, but a potent burst of radiation had been released in the room.
Though Slotin appeared in good health, he vomited several times and complained of pain in his left hand (the one that had been closest to the core) which became bluish, painful and blistered overnight. After five days, his white blood cell count had plummeted, he experienced nausea, confusion and abdominal pain as radiation burns appeared across his body. He soon fell into a coma and died shortly afterwards.
Chillingly, Slotin had worked with Daghlian and comforted him in hospital - he knew what was in store for him the moment the tamper fell.
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| A natural nuclear meltdown occurred beneath Gabon - it seems to be doing ok now though (JEaLiFe Pictures) |
The Oklo Fission Reactor
Fair or not, few things seem as unnatural as a nuclear reactor. There's just something about them that seems like the apex of man-made technology. Which makes the discovery that a naturally occurring nuclear reactor sat below Oklo in Gabon, Africa two billion years ago really weird.
Researchers were tipped off by finds of high grade uranium ore with a slight (but noticeable) amount of fissile material missing. All natural uranium extracted today contains 0.720% uranium-235 (regardless of whether it came from the Earth's crust or rocks on the lunar surface) but this ore had only 0.717%.
Tests confirmed that not only was this ore completely natural, it also contained the products of a fission reaction. The only possible explanation was that the ore had gone through a natural fission reaction more than two billion years ago - plutonium, caesium, curium and americium just don't occur natively (if they can occur at all) in the concentrations found at the mine.
The conditions in Gabon would have needed to be ideal for this to happen - large concentrated deposits of uranium-235 infiltrated by water to slow down the neutrons and moderate the reaction. It's as though a man-made light-water reactor simply sprung from the rocks.
What makes this situation particularly interesting is that the are was not turned into an irradiated, toxic wasteland. Researchers found that most of the reactor products were encapsulated in "shells" of ruthenium that seemingly contain the dangerous materials indefinitely. This might actually be a useful technique for managing our own radioactive waste!