Michael Aranda: We’ve covered our fair share of natural disasters, from hurricanes to tornadoes to earthquakes, but us humans are also pretty good at mucking things up in our own horrendous ways. There are dozens of human-caused catastrophes that we could talk about, but three of the most prominent are worth learning from, because they weren’t just simple accidents.  In each case, there was no one broken piece of machinery or human error that was responsible. In a way, they’re each examples of science and engineering gone wrong.

But it’s not the science’s fault.

Disasters like these are the result of a series of bad decisions and unfortunate events that kept people from understanding or maybe even just looking at the data that could have prevented them. So, understanding the science behind these kinds of catastrophes, whether they took their toll in human lives, environmental damage or both, may help us make sure they don’t happen again.

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We begin with the deadliest industrial disaster of all time. It’s been nearly 30 years since 40 tons of poisonous gas enveloped the central Indian city of Bhopal, the result of an accident at the nearby Union Carbide pesticide plant. Throughout the early-morning hours of December 3rd, 1984, more than 8,000 people died, most living in a slum near the plant. Estimates vary, but the accident is believed to have ultimately killed more than 20,000 and injured up to 300,000 people exposed to the gas. The culprit was methyl isocyanate, a toxic gas used to produce a pesticide sold under the name Sevin.

Unfortunately, that’s all that’s really certain about this disaster.

The events that led to the uncontrollable chemical reaction that ultimately released the gas continues to be disputed today. Plant workers and many Indian officials blame the accident on design flaws, faulty maintenance and poor safety protocols. Union Carbide, which has since become a part of Dow Chemical, maintains that it was an act of sabotage.

According to accounts by employees working on the night of December 2nd, what began as routine maintenance quickly deteriorated. The plan was to clean out a large section of the plant’s pipe network with water in an attempt to flush out dirt, but the pipes became clogged and water began backing up into the main pipe system. In other sections, it began leaking into one of the three tanks containing the methyl isocyanate.

The problem is, methyl isocyanate is extremely volatile. When it reacts with water, it creates huge amounts of heat and carbon dioxide, as well as the toxic gas methylamine. And while methyl isocyanate is liquid at room temperature, it vaporizes when it reaches body temperature. When the water backed up into the tank, it combined with the methyl isocyanate to kick-start a violent exothermic reaction, and as the temperature inside the tank increased, so did the rate of reaction.

For its part, Union Carbide disputes this version of events and says the water must have been introduced into the MIC tank deliberately.

Four hours after the water began entering the tank, the growing heat and pressure inside could no longer be contained. A pressure-relief valve blew and released a plume of CO2, methylamine and methyl isocyanate, which had now turned into a gas because of the heat. The winds that night carried the gas away from the plant and into Bhopal. Because the plume was denser than air, it stayed close to the ground, putting children in particular at risk.

Methyl isocyanate affects the wet parts of the body first, including the eyes, mouth and throat. Victims experienced blindness and vomiting, with many dying from internal drowning as their lungs filled with fluid. And decades later, the disaster is still the subject of legal wrangling, and clean-up at the site, which is now controlled by the local government, still isn’t finished.

Let’s move on to cheerier things, like the worst nuclear accident of all time.

You’ve probably heard of the Ukrainian city of Chernobyl, which has become synonymous with the accident that happened there on April 26th, 1986, during a test of reactor number four at the Chernobyl nuclear power plant. At the time, Soviet officials wanted to know how long the huge steam-powered turbines that generate electricity would keep running if there was a loss of power, such as if there were an attack on the facility by the Americans.

In nuclear power plants, water must be continuously pumped around the reactor core to keep it cool, but in the case of Chernobyl, the electricity needed to run those water pumps came from the very turbines that were powered by the reactor. So the idea was to decrease the power by about 25% to see how well the turbine kept the pumps going until back-up diesel generators kicked in.

The test did not go smoothly.

The engineers decreased power by inserting graphite-tipped boron control rods into the core. These rods act as a kind of moderator in a nuclear reactor, slowing down the flow of neutrons in the ongoing fission reaction that powers the plant. But the engineers put them in a little too quickly, so when the power supply plummeted to less than 1%, the cooling system nearly failed and the water in the reactor core approached boiling point. Making things worse, engineers had disabled all the automatic shutdown and safety mechanisms before the test.

Steam pressure inside the reactor began to skyrocket, enough to lift the graphite caps (each weighing 350kg) that covered the fuel rods. With more fuel exposed and no coolant to absorb neutrons, the reactor suddenly experienced a dramatic increase in power. To quell this power surge, engineers began lowering all of the remaining control rods into the reactor at once. But, by this point, the reactor was just too unstable.

The graphite tips of the control rods momentarily increased the reaction instead of slowing it down, and that is all it took. In a split second, all of the water turned to steam, followed by an enormous explosion when the steam pressure could no longer be contained. And with the explosion came the release of radioactive fuel and debris. Official estimates put the amount of radiation released as equal to 10 Hiroshima bombs.

The problems, unfortunately, didn’t end there.

More than 45,000kg of nuclear fuel and 700,000kg of graphite caught fire following the explosion and burned for more than nine days. More than 30 Chernobyl workers and firefighters died in the explosion or in the days that followed from radiation sickness, and it’s nearly impossible to quantify the number of cancers and deaths caused by Chernobyl in the decades since. Scientists have linked increased rates of thyroid cancer in children in Russia and Ukraine to the disaster, but estimates regarding other cancer deaths range from a few thousand to more than a hundred thousand.

More than 150,000 residents were evacuated in the days following the accident and they never returned home. A zone of exclusion now exists around Chernobyl, a 2,600 square kilometer area where radiation levels remain unsafe decades later.

Finally, to find the most recent man-made disaster, look no further than the Gulf of Mexico. The 2010 Deepwater Horizon accident was the result of a blow-out, a rare event in which a combination of pressurized natural gas, oil, mud and water escapes from an oil well and quickly makes its way to the top of the drill pipe, where it expands and ignites.

In April of that year, the floating exploratory drilling rig was six weeks behind schedule and millions over budget on a project to drill a well nearly five kilometers beneath the ocean floor. With the drill pipe complete, engineers were in the process of temporarily sealing the well. The oil company BP was set to return with their own equipment shortly after to begin the process of extracting the oil.

The final steps involved cementing the outside of the well to protect the metal walls from gas pressure and also plugging the well on top. Shortly after the cement was poured, engineers tested it for leaks, and this may have been the first broken link in the chain of events.

Managers may have misinterpreted data from this test, during which 60 barrels of mud possibly leaked and would have been a major warning sign. Mud is vital to any deep sea drilling operation, because its heavy weight maintains well pressure so the oil doesn’t rise to the surface. On April 20th, for reasons that are still debated, the cement failed. As natural gas shot towards the surface, there wasn’t enough mud on top to hold it back. It exploded soon after reaching the platform of Deepwater Horizon.

The final fail-safe measure, an enormous device called a blow-out prevented, which is supposed to shut off the flow of gas and clamp the well in an event like this, also failed. Despite numerous attempts, it failed to clamp the well shut. The ensuing explosions and fire killed 11 workers. Deep Water Horizon sank two days later.

For 87 days, oil flowed continuously into the Gulf of Mexico from the ruptured well. In all, nearly 5 million barrels gushed into the water before BP was able to cap the well on July 15th.

So, sorry for ruining your day, but hopefully you learned something new about these events, and hopefully humanity as a whole will learn from these mistakes, as well.

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