Zacatlilco disaster

Zacatlilco disaster

Reactor 2 of the Zacatlilco plant was enclosed within sarcophagus containment structure after the disaster.
Date	18 March 1989
Time	7:19 local time
Location	Zacatlilco, Zacaco Republic, Zacapican
Type	Nuclear accident
Casualties
194 deaths from radiation sickness Unknown number of deaths from radiogenic illnesses

The Zacatlilco disaster was a severe nuclear accident which occurred on the 18th of March, 1989, in the Zacatlilco atomaltepetl in the far south of the Zacaco Republic in Zacapican. It is one of the worst nuclear disasters in the history of the the world. The disaster was caused by the explosion of Reactor 2 of the Tequipanco Yocoyahuitecoc Atomtic Zacatlilco nuclear power plant, causing a massive release of dangerous radionuclides into the environment. The cleanup operations spearheaded by the CETZ emergency response forces as well as the military and reservist elements would mobilize some 400,000 people, primarily military servicemembers as well as volunteers from the civilian nuclear industry and medical sectors. In total, the cleanup would cost 93 billion Amatl or roughly 70 million Latin solidus, a figure which does not include the ongoing and perpetual mitigation operations conducted in and around the contaminated zone to this day. An area of roughly 2,000 square kilometers surrounding the Zacatlilco facility would be deemed uninhabitable by the Zacapine authorities and designated as a zone of alienation, permanently displacing some 164,000 people. The official death toll of the disaster is 194 persons, the number of known deaths attributed to the explosion of Reactor 2 as well as lethal cases of acute radiation syndrome caused by the radiological contamination of the surrounding area. The true human cost of the disaster is ultimately unknown, with anywhere from 1,200 to 100,000 people all across the southern Zacaco and Tlaximallico Republics believed to have died from various cancers and other health complications as a result of the disaster.

The disaster had a major impact on the society and culture of Zacapican reaching far beyond areas directly affected by the consequences of the accident. Investigations into the cause of the accident revealed critical design flaws in the graphite-moderated TCTN reactor type using at the Zacatlilco facility, flaws which were not common knowledge in the nuclear industry due to a lack of transparency by the TCTN's designers. While operator error contributed greatly to the disaster, revelations surrounding the flaws of the reactor and errors made by the designers became the focus of public anger in the aftermath of the meltdown. The events of the Zacatlilco incident became a symbol for the opacity and non-responsiveness of the increasingly technocratic Zacapine government and its state-backed engineering establishment. Although the huge mobilization of financial, technical and human resources to mitigate the long term effects of the disaster turned many low level officials into heroes in the public eye, in general the national government was accused of keeping secrets, failing to make public critical information that could have prevented or reduced the severity of the disaster, and endangering its citizens through negligence and an attitude of dangerous complacency towards the power of the atom. The Zacatlilco disaster is directly credited with ending the era of one party rule known as the Macehualtlatollo, although this only formally ended with the elections of 1991 which brought down the Macehualque party from government. In many aspects, the disaster was a watershed moment for the country which had a major cooling effect on the industrialization and optimism for the future, laying the foundations for the modern era of Zacapine society in which anxieties over an uncertain future and skepticism towards government are far more prevalent than in previous decades.

Background

The TCTN reactor type found at Zacatlilco was also built at two other power plants across the country, with the last reactor only being decommissioned in 2007

The reactor type found at the Zacatlilco power plant, the TCTN, was designed in the early phases of the nuclearization of Zacapican in the 1970s. Although nuclear science and nuclear power generation had been pioneered in Zacapican in the prior decades, with the Zacapine CAT-1 reactor being one of the first commercially viable nuclear reactors in the world, nuclear power generation represented only a minor part of Zacapican's energy mix until a concerted effort came about to revolutionize the Zacapine energy sector. The goal of the nuclearization effort was to totally eliminate fossil fuel use, especially coal-fired power plants common at the time, and replace them nuclear power plants emitting no chemical pollutants into the air. A major stumbling block with this initiative was the lack of any major supplies of uranium in Zacapican, making the country dependent on small deposits and foreign imports to supply its new nuclear reactors. The result of this was higher fuel costs. In response to these higher prices, Zacapine engineers endeavored to produce a type of breeder reactor which could generate power using low enrichment uranium fuel which would in turn cut fuel costs. This was the TCTN reactor which was developed using the CAT-1, itself a a graphite-moderated light water reactor, as a basis for the design. The TCTN reactor would end up being truly massive in size, built to fit inside a cubic concrete reactor pit 25 meters to a side, with the cylindrical reactor itself having an outer diameter of 20 meters. This massive size meant that the containment building which would contain the reactor in power plants such as the Zacatlilco facility could not be adequately reinforced with a hardened dome structure found in other Zacapine reactor buildings. The internal volume of the reactor also made controlling the nuclear reaction exceptionally difficult, as local conditions of over-moderation or under-moderation could occur simultaneously in different regions of the reactor.

File:Chernobyl reactor before the explosion, 1982.webp

Reactors 2 and 3 held within the West Reactor Building at the Zacatlilco atomaltepetl

The atomaltepetl of Zacatlilco began life in 1981 with construction beginning on Reactor 1, an earlier generation of TCTN reactor. The location of the generation calpolli near the border of the Zacaco Republic with neighboring Tlaximallico was chosen to provide power to the growing urban centers of the southern Zacaco valley while being separated from these major population centers by a significant distance due to safety concerns. Zacatlilco was purpose built to house the nuclear facility's workers and their families, with a site at the floor of a wide valley being chosen for the reactor due to its proximity to the Atliltic river whose waters could be used for feed cooling ponds for the nuclear reactors. Reactors 2 and 3 were added as twins in an adjascent facility in 1984 to increase the generation capacity in response to the rapidly expanding power demands from the steel industry of Tequitinitlan and the southern Zacaco valley as the steel industry transitioned to electric arc furnaces from the older coal-burning blast furnaces. By the time all three reactors were fully operational, Zacatlilco had a nominal capacity of 3,000 megawatts and had become integral to the supply of electric power to both the Zacaco steel industry and the sawmills and furniture factories of northern Tlaximallico. This placed significant pressure on the plant operators to generate at maximum capacity without interruptions, which is believed to have incentivized maintenance and routine safety tests of the reactors to be rushed in order not to risk underproducing and causing a local power shortage which affects the capability of factories to operate normally.

The TCTN reactor type had several advantages such as its function as a breeder reactor and its capability to use low enrichment uranium fuel. However, it also had a number of design flaws such as the dangerously high positive void coefficient due to its design as a graphite moderated, light water cooled reactor. This condition meant that a loss of cooling accident which would result in the boiling off of the water in the core would result in a runaway nuclear chain reaction as even bubbles of steam forming inside the core could increase reactivity. Another separate design flaw lay in the piping of the very same water cooling loops needed to prevent such a condition from taking place. These systems were made with Zirconium alloys favored because they would resist corrosion and also have a low neutron cross section so as not to interfere with the nuclear reaction in the core. However, flaws in the design an manufacture of the piping would cause a zirconium-steam oxidation reaction to take place in parts of the system, generating dangerous buildups of hydrogen in the control systems regulating the temperature of the core. To address this, as system was added to the water cooling loops and storage tanks to vent any build-up of flammable hydrogen gas from the system. However, these systems did not have any redundancy in case of a failure of the sensors or the vents, meaning that a failure of one component could leave the control room either unaware of a hydrogen buildup or unable to fix a hydrogen buildup, potentially leading to a hydrogen explosion.

Accident

Control system explosion

The exact causes which set off the chain of events leading to the Zacatlilco disaster are not known. It is generally understood that the terminal chain of events causing the catastrophic second explosion of the core of Reactor 2 was a preliminary hydrogen explosion which blew open a water tank that was part of the cooling system, seriously damaging many other components of the cooling and regulation systems of the reactor in the process. The cause of the hydrogen buildup, likely from the zirconium-steam reaction that was a known issue in the TCTN reactor type, is generally attributed to an equipment failure in the safety systems designed to remove such dangerous flammable buildups although given the damage of the explosion and the subsequent disaster it is impossible to determine which part of the mechanism was ultimately responsible for the failure. The most plausible and widely accepted chain of events suggests that a sensor meant to detect a hydrogen buildup had failed and was indicating safe levels within the water tank which would later explode, leaving the control room blind to the danger. Based on surviving records, no note is made regarding venting hydrogen or a detected fault in the system, suggesting that the nightly inspection was either rushed or otherwise failed to detect the the equipment failure, further suggesting a problem with the sensors rather than the venting system or other component related to removing byproduct hydrogen from the system.

The hydrogen explosion occurred at 7:01 am, roughly one hour after the shift change to the day shift. The immediate result of the explosion was that part of the Reactor 2 building was lit on fire, believed to be due to flammable material in the outer cladding of the turbine hall. The rupturing of the tank and damage to the surrounding equipment, however, also placed the core of Reactor 2 in immediate danger of a loss of cooling incident as it destroyed most of the secondary cooling loop. Without this loop flowing, the primary loop, the water flowing through the reactor itself, would continue to heat up from the nuclear reaction in the core, and turn to steam which would in turn could cause a meltdown of the core. In response to this, the plant operators called the Zacatlilco fire brigade, both to combat the fire and in an attempt to use the fire engines as an ad-hoc water pumping mechanism to cool the reactor.

Excursion

Instrument readings in the control room of Reactor 2 led the operators to erroneously assume the core was shutting down while the excursion was continuing

A runaway nuclear excursion was now taking place within the core thanks to the loss of cooling, resulting in the the water within the core rapidly converting to steam due from the rising heat given off by the nuclear reaction. As more water turned to steam, the positive void coefficient caused by the difference in neutron cross section of the liquid water compared to steam and water vapor was leading to an increase in reactivity and thereby feeding into the ongoing excursion. The control room was aware of this situation and initiated an emergency shutdown of the reactor by pressing the scram button, which released neutron absorbing boron rods. Ordinarily, these rods would then fall into channels within the reactor due to gravity and absorb the neutrons causing continued fission, breaking off the fission chain reaction and shutting down the reactor. However, under the conditions already unfolding within the core, runaway vaporization of water into steam had already built up such pressures that a large number of the control rod channels had ruptured, blocking many of the boron control rods from fully entering the reactor. To make matters worse, the control room had no way of detecting this failure, with their systems only confirming that the locks holding the rods out of the reactor had been released. The few control rods that did enter the core temporarily reduced the power excursion by curtailing the fission within limited regions of the massive TCTN reactor. Upon reading the drop in output, the operators assumed that the rods had lowered and the reaction was now being safely shut down.

In fact, the power excursion had only been partially curtailed and soon began building up again as reactivity in most of the core was still rising. The steam buildup was once again producing hydrogen in the core through the zirconium alloy oxidation reaction as had occurred in the cooling loop earlier, as well as continuing to build up extreme pressures. This hydrogen did not initially ignite because of the lack of oxygen within the reactor. The reactor pressure vessel contained the buildup of pressure for several minutes, but without a means to relieve the pressure or reduce the fission reaction driving the buildup, it could only serve to delay the inevitable failure and build even greater pressures within making the resulting explosion all the more powerful.

Core explosion and fire

The second explosion at the Zacatlilco plant, this time within the core of Reactor 2 itself, was far more powerful than the one that had burst the control system tank minutes earlier. It occurred within moments after the pressure within the core finally cracked open the multi-ton lid of the pressure vessel allowing air to rush in. The oxygen in the air, combined with the superheated metal, igniting the hydrogen built up inside the vessel. The explosion which breached the core had been a steam explosion which now combined with the force of this hydrogen explosion occurring fractions of a second later. The now combined explosions blasted the reactor's lid and upper shield, weighing a combined 2000 tons, through the roof of the building. The breach of the Reactor 2 building, which took place at 7:19 am, killed four plant workers and exposed the now burning fuel rods and reactor assembly to the outside environment. The control room at the Zacatlilco plant were not initially aware of the situation, assuming the second blast had been another explosion within the cooling system from the fire igniting another pocket of hydrogen. This was due in part to the erroneous assumption that the scram button had worked as designed and the boron control rods had been lowered into the core, which would have made shut off the fission reaction and made the scenario of a core explosion virtually impossible. The firefighters which arrived to fight this fire were therefore not informed of the reactor core breach, and rushed to Reactor 2 to put out the fire. According to later accounts from these firefighters, they were under the impression that quick and decisive action would be needed to stop the fire and prevent a serious accident with the reactor, and were unaware that such an accident had already taken place moments before they arrived at the scene. As a result, many of the firefighters and a number of plant workers attempting to divert cooling water into the core which had already exploded were fatally irradiated and would later die of acute radiation syndrome. However, the efforts of these first responders to the scene of the disaster did successfully contain and put out the fire consuming the structure of the Zacatlilco power plant, likely saving Reactor 3 from catastrophe which would have worsened the disaster. By 7:45, the firefighters had realized that the core had been breached, by which time most of the crews had already received lethal doses of radiation. While most evacuated upon hearing the devastating news of the reactor explosion, a number of firefighters continued to fight the blaze on the ground while a wider emergency response was mobilized, having been made aware that they may have already received a fatal exposure and so resigned themselves to die helping to contain the disaster. The fires in and around the building of Reactor 2 were extinguished by noon, with the surviving firefighters who remained being evacuated to the local hospital and later to a hastily established radiation syndrome treatment center in Tequitinitlan where the majority would die within the days and weeks following the disaster. Despite these heroic efforts, the remains of the reactor itself would continue to burn due to the ongoing fission reaction of the nuclear fuel within, burning the moderator graphite rods and other materials with a fire too hot to be put out with water.

Contamination

The breach of Reactor 2's containment building and the ongoing fire within the core released a massive amount of radioactive contamination into the air. Many of the radionuclides released by the explosion and the fire and carried on the wind had a short half-life of a few hours or days and would no longer be dangerously radioactive within a matter of a few half-lives. A number of other radionuclides, however, would have far longer half-lives and would continue to release life-threatening levels of ionizing radiation for decades, even centuries to come. These elements, still present within the Zacatlitlco zone of alienation, are primarily cesium-137, strontium-90 and several isotopes of plutonium. Relatively short lived radionuclides, such as Iodine-131 with a half-life of 8 days, would nevertheless pose a significant health risk due to being commonly absorbed by the human body where it could stay for a long period of time, irradiating a person from within and potentially killing them as a result. All of these substances were spread by the wind, which was blowing primarily south and east during the period of time that the plume of contaminants was emanating from the breached Reactor 2. Additionally, the ongoing fissioning reaction and fire within the reactor pit could continue to release these radionuclides for days and weeks following the disaster.

Immediate response

The Secretariat of Energy, an organ of the Zacapine government responsible for supervising nuclear power plants, was informed of a serious industrial accident at the Zacatlilco atomaltepetl at 7:29 in the morning, a few minutes after the core had exploded but before this fact had been discovered by the plant operators. While resources were being quickly mobilized to deal with the unfolding disaster, this was being done under the false assumption that the core was still intact and that personnel would primarily be dealing with any damage at the plant and securing the nuclear material to prevent any accidental contamination. A new mobilization order for the emergency response was issued at 8 in the morning, once the Secretariat had been informed of the core breach, but this second response would be significantly delayed due to confusion within the chain of command caused by the contradictory instructions only a half hour apart. This would seriously delay the evacuation of Zacatlilco and other affected areas, leading to the widespread irradiation of many of the local people. In part, this failure was due to a lack of an emergency plan at the Secretariat, at the level of the local grid, or indeed at any level of the nuclear industry. Emergency response forces were generally unprepared for the scenario of a nuclear reactor exploding, as such a thing had been previously considered an impossibility.

Once the top levels of the Zacapine government fully realized the threat and began to organize, however, the scale of the response immense. By noon on the 18th, the Emergency Commission for the Liquidation of the Zacatlilco Accident was created as a special board at the level of the national cabinet to deal with the disaster. The first priority of the Commission in the hours following the disaster was to eliminate the fire still burning in the reactor pit and the radioactive smoke it was emanating into the wind. On the advice of the Tepachoani's scientific advisors, it was understood that conventional firefighting would not be effective in quenching the reactor fire as it was not a conventional fire but rather a runaway fissioning reaction burning and melting the components of the reactor core. In order to extinguish the fire and stop further contamination from being released, it was determined that a mixture of sand to smother the burning components and boron to capture neutrons and break off the runaway fission reaction would need to be dropped onto the open pit of Reactor 2. In order to carry out this directive, the available fleet of CETZ civilian firefighting aircraft were mobilized to the task. Many of their aircraft were ill suited to carrying the dry mixture of sand and boron in the built in tanks normally used for water or other liquid fire retardants but were pressed into service regardless. As the CETZ firefighting air fleet proved insufficient for the scale of the operation the Commission demanded, military helicopters were brought in to deploy the mixture over Reactor 2.

Simultaneously, other resources of the CETZ were mobilized to carry out the Commission's second priority, the immediate evacuation of the region to minimize the loss of life from radiological exposure. Zacatlilco and the surrounding villages in the valley were ordered to be immediately evacuated, an order which was carried out six hours after the disaster. The following day, a mandatory evacuation zone of 75 kilometers from Reactor 2 would be established, with as municipalities as far as 300 kilometers from Zacatlilco receiving voluntary evacuation notices. The Zacaco Republican Guard, Tlaximallico Republican Guard, as well as units of the National Army were mobilized to carry out the evacuation orders of the region. The vast majority of the affected populations cooperated with the evacuation plan, although in some cases the evacuation troops resorted to coercion and threats of force to enforce mandatory orders. Their civilian counterparts, the CETZ emergency workers, were engaged in preparing evacuee centers across the Zacaco Republic and Tequitinitlan in particular where Zacatlilco evacuees would be housed in prepared emergency shelters as well as public buildings, schools and gymnasiums until more long term housing could be made available.