 It is the 14th of December 1994, and the Iowa Morning Darkness is about to be interrupted. At approximately 6 minutes past 6 in the morning, the Terra International Incorporated Virtualizer Facility is consumed by an explosion. Today's video will look at how this turned into this. My name is John and welcome to Plainly Difficult. A brief history of Terra at Port Neill. Our story starts in 1965 here in Port Neill, Woodbury County in the US state of Iowa. A new fertilizer plant is under construction. It's planned to supply farms within a 200-mile radius with pheromonia and other nitrogen crop chemicals. Shipping of the products was mainly undertaken by road with only about a third by rail, understandable due to the local nature of the business. The plant had two major areas. Area one consisted of an ammonia plant, tank farm and utilities, with area two being home to a nitric acid plant, urea plant and ammonium nitrate plant. So the whole goal of the site was to create ammonium nitrate urea. To make this the plant needed all of the other parts of the site to produce their respective ingredients. Once enough nitrate, ammonia and urea are produced, ammonium nitrate can be made. So let's look at the process used on the Terra Industries site. So the whole goal of the site was to create ammonium nitrate urea. Ammonium nitrate for this was produced, not surprisingly, at the ammonium nitrate plant in area two. The process took 55 and 56% nitric acid and ammonia and reacted it inside a thing called a neutralizer. The nitric acid came from the plant in area one and the ammonia came from the urea plants on site, but could be assisted by any of the four tanks varying from 30,000 US gallons to 5 million US gallons. It was introduced at the bottom of the neutralizer. The flow of ammonia was determined by the site's production capacity. Nitric acid was dispersed a bit above the ammonia. The reaction is very exothermic, which allows water to be evaporated out of the neutralizer, creating the required 83% ammonium nitrate. Temperature is monitored via probe in the upper part of the unit. The temperature maintained in the neutralizer was around 267 degrees Fahrenheit or roughly 130 degrees centigrade. The neutralizer was constructed of stainless steel. The juicy ammonium nitrate overflowed off the neutralizer into a rundown tank. This acted as a surge vessel for the neutralizer before the ammonium nitrate is pumped to the storage tank farm. The rundown tank was insulated with two inches of calcium silicate. Water vapor was vented to atmosphere via a scrubber, where any weak ammonium nitrate solution was returned to the neutralizer. In 1992, the site received an upgrade in the form of a new computer control distribution control system. This would increase production from 800 tons per day to over 1,000 tons per day. It was also able to monitor and record data. It can also alert operators to process variable deviations and even automatically control output. This leads us to 1994 and a disastrous series of events. The disaster. It is the 27th of November 1994 and the ammonium nitrate plant has encountered a problem. A pH probe on the neutralizer's rundown line was found to be defective. No replacement is available and the decision is made to continue working with the defective probe. Just a week later, on the 5th of December, a leak was discovered. A surface condenser in the nitric acid plant had been leaking and this required the shutdown of this part of the site. Although apparently getting fixed a couple of days later, the plant would be shut down once again. Meanwhile, at roughly 10pm on the 11th of December at the ammonium nitrate plant, the outside operator found the neutralizer to be operating in highly acidic conditions. The discovery was found during a grab sample and the pH was minus 1.5. The fumes were so strong that their operator's face became burnt. Another operator was called in and managed to monitor the neutralizer's acid levels reduce, eventually reaching normal levels at around 1am on the 12th of December. Just an hour later, a leak was discovered on the condensate line from the neutralizer to the scrubber. The north pump was shut down and the south pump was started, only for that one to also leak. The gaskets for the north pump were replaced and it was returned to work, but an unknown amount of ammonium nitrate had leaked. The nitric acid plant would for another time switch off acid supply with an emergency shutdown again on the 12th of December. The less than regular supply of acid over the first week of December had caused production issues and by early afternoon on the 12th, the ammonium nitrate plant would be forced to start shutting down production. Shutdown started at around 2pm with the neutralizer being switched off at 3pm. The nitric acid line was purged with air and the product pumps were shut down and blocked off. The air pushed the nitric acid into the neutralizer. This was roughly 220 US gallons in total. The purging of the lines took until 8.30pm, after this, 200 PSIG steam was applied to the nitric acid line. This was to stop freezing in the December night, but what the operators didn't know was that the neutralizer, now 14 years old, was in a highly acidic and contaminated condition caused by the leaks over the previous days. But little did the operators know that injecting the high pressure steam into the neutralizer would result in a potentially runaway thermal reaction. The ammonium nitrate was now sensitized and was a ticking time bomb. The scrubber was still working and with the recirculating pumps on, was blowing down ammonium nitrate back into the neutralizer. By the early hours of the morning of the 13th of December, the plant was shut down and upon vision inspection by one of the operators, apparently no issues could be seen. Startup was planned for later on in the morning and as such, at 6am, the nitric acid air compressor was being started up. But just 6 minutes later, two explosions erupted from the neutralizer. The explosion destroyed the neutralizer, replacing it with a crater. Four would lose their lives in the disaster almost instantly, with another 18 being seriously injured. Debris was littered over a 500 yard radius, with a blast being heard nearly 30 miles away. Some of the debris had damaged the on-site nitric acid tanks, spilling liquid onto the ground. No building on site escaped damage and windows on nearby houses were broken. The explosion damaged ammonia gas storage tanks on site, causing a release of the chemical into the air. As the gas spread, some 3000 residents would be required to temporarily evacuate. Contaminants floated around the plant for 6 days, causing horrendous local health scares. To add a cherry to the feces pie, a power station across the Missouri River also took damage, reducing power to the local community. The total cost of the disaster would be estimated at $300 million, but as the human and societal cost became apparent, the cause needed to be investigated. Enter the EPA. The investigation So almost immediately after one of the country's biggest fertilizer plants self-immolated, the EPA got stuck into investigation. They looked over the disaster site and interviewed any witnesses they could find. As they constructed the timeline, the cause became ever so apparent. No surprises, it involved incompetence and improper plant operation. They found the training of operators to be lackluster to say the least. There was little in the way of safe operating procedures and to make things worse, communications between the various plants on site was pretty much useless. The human factors created the environment for the disaster, but how did the explosion start? Well, the neutralizer was allowed to reach a dangerously acidic level, which was caused by the purging of the acid line after the acid plant couldn't supply the ammonium nitrate plant. This isn't the end of the world, so long as it's properly monitored and has its pH level adjusted accordingly, but this wasn't done. Steam was applied to neutralizers to keep it from freezing. This in combination with the compressed air used to purge the nitric acid created low density zones, i.e. bubbles. These zones load the amount of energy required to initiate thermal decomposition of the ammonium nitrate. This occurred for 15 hours before the explosion, creating localized areas of heating, which couldn't be diffused through the rest of the ammonium nitrate. The ammonium nitrate had also been contaminated with chlorides during various leaks. This also reduced the energy needed for activation. All of this destabilized the ammonium nitrate in the neutralizer and the slow roll of pumps during the planned startup added more super heated steam and basically became the ignition point. The EPA concluded that the main cause was sheer lack of safe operating procedures. Any procedures that were actually used were rather worryingly never written down, so obviously this was a recipe for disaster. So I'm going to rate this disaster as a bad day at the office and a six on the legacy scale. Do you agree? Let me know in the comments below. This is a plainly difficult production. All videos on the channel are creative commons at tuition share like licensed. Plainly difficult videos are produced by me John in the currently quite nice sunny corner of southern London UK. I have instagram and twitter so check them out if you fancy looking at some random pictures. And I'd also like to make a very warm thank you to my patreons and youtube members as well as the rest of you who tune in every week to see my videos. If you're enjoying this outro song then please go over to my second channel made by John where you can listen to and watch the video in full. And all that's left to say is thank you for watching and Mr Music Man, play us out please.