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Stories of accidents

//Stories of accidents

What happened?

Goiânia (Brazil )

On September 13, 1987, scrap dealers broke into the old hospital in the city of Goiânia (State of Goiás) which was no longer in use. In a room, they discovered a metallic device, which they hoped contained steel and lead that they could sell. After a quick removal of the device (in a wheelbarrow!), the two scrap dealers started to dismantle the metal structure. A few hours later, they experienced violent vomiting: the first symptom of acute radiation poisoning.

With makeshift means, they had dismantled what turned out to be a radioactive source. Containing a hundred grams of caesium-137, the source was one of the key pieces of the old hospital’s radiotherapy machinery. According to the International Atomic Energy Agency (IAEA), the source’s total activity was around 50 terabecquerels (Tbq), or 50 trillion becquerels. At a distance of one meter, it was delivering a lethal dose of 4.5 Gy/hour.

Despite their symptoms, our two scrap dealers continued to dismantle the source. Using screwdrivers, they opened the steel container and removed the caesium. With their deep blue colour, the chloride salts of radioactive caesium were fascinating.  . Everybody wanted a piece for makeup, to create jewelry, to play marbles.

Within weeks, 4 people died as a result of irradiation. In total, 271 people were contaminated, 28 of whom were severely burned by radiation.

More than 100,000 people had to be examined. A whole section of the city (85 houses and buildings) had to be fully decontaminated and 200 people evacuated. The decontamination operation involved more than 500 people and generated 3,500 cubic meters of radioactive waste. This waste had to be stored at a purpose-built site 20 kilometres from the city.


The safety culture breach

Failure to comply with many rules of safety and security caused the accident in Goiânia. A series of breaches of safety culture:

The source was not listed by the former operators of the hospital nor by the Brazilian authorities, which explains why the source was abandoned. The building in disrepair of the former hospital was not under surveillance on the day the source was stolen. Of American origin, the source had no reference number, preventing rescue forces from identifying the contents and adapting their response.


How this accident changed the world

The Goiânia accident was rated (after the fact) at level 5 on the INES scale, whose maximum level is 7. International and national standards now require that all radioactive sources must be registered by public authorities. They may be used only for legally permitted uses and by licensed specialists. The sources should be stored in confined areas. Transportation must also be entrusted to specialists.

Sources :

The Radiological accident in Goiânia, IAEA, 1988

Chernobyl (Ukraine)

Cheynobyl Power plant reactor, Wikipedia

In the early hours of 25 April 1986, excitement was at its peak among the shift crew of the 4th reactor of Chernobyl Nuclear Power Plant (NPP). Engineers were engaged in a new experiment. The small team piloted one of the atomic jewels of the USSR: the newest of 4 RBMK units at Chernobyl, a modern PP in Ukraine near the border with Belarus. The RBMK (reaktor bolshoy moshchnosty kanalny, or high-power channel reactor) was considered by the Soviet scientific and technical community as a masterful technical achievement.

Source: OECD Nuclear Energy Agency

For its designers, the RBMK was ideal. The operator controls the activity of each fuel assembly (increasing flexibility to pilot the reactor). Unlike pressurised water reactors (PWR) and boiling water reactors (BWR), the reactor could be refuelled while operating. The RBMK was also simpler to build, since no large forged metal components were necessary. And designers considered that the RBMK did not require a special overarching structure to contain radionuclides that might be released during a severe accident – a concrete or steel structure that in western PWRs and BWRs was known as the containment building.


Well, almost ideal…

On April 25th, a special team was charged with testing at Chernobyl unit 4 an hypothesis formulated by Soviet physicists. As any other reactor, RBMKs must be cooled continuously. This implies electric pumps for constant circulation of the water. In case of an offsite power failure, stand-by generators start to power the pumps. However, emergency generators made in the USSR reached full power only after 75 seconds — too long to maintain the stability of the reactor. Hence the idea – to be tested – of using the residual kinetic energy of the two turbines to generate electricity for the pumps, leaving time for the stand-by generators to reach full power. Theoretically, this was possible. In real life, it wasn’t that simple. Three full-scale tests had already been carried out without success.

A fourth attempt was programmed. For safety reasons, it would not be conducted at full power 1,600 thermal megawatts (MWt), but at 700 MWt.

However, when operators had first reached the required power, the load dispatcher requested that they maintain the reactor at the intermediate power. They had to postpone the experiment for several hours, a situation not anticipated in the test protocol. Operators left the emergency core cooling system disconnected for that entire period, in anticipation of the test which called for that disconnection.

Around midnight, after being allowed to resume power decrease, the required power was reached. Then the troubles began.

The RBMK design, with its large core, is inherently unstable at low power.

As it operated for several hours at low power, the reactor generated Xenon-135, a noble gas whose characteristic is to “kill” a nuclear reaction. The nuclear reaction declined until the reactor power dropped to only about 30 MWt .

Operators withdrew control rods fully to revive the chain reaction and raise the power to the required level for the test.

In the process, they left only 6-8 control rods inserted, versus the minimum of 30 rods specified in RBMK safety documentation. But it worked – they managed to stabilise the reactor at 200 MWt : The test could begin!

Around 1:23 a.m., the operators shut off the steam flow to the turbines, as stipulated in the test protocol. As expected, the rotor continued to turn, producing electrical power for the coolant pumps — but not enough coolant was circulated to stop the fuel channels from heating up ! In the pressure tubes, the flow of water dried up, causing a rapid rise in temperature and a rise in nuclear activity within the fuel.

At that point, someone triggered the emergency button to lower the shutdown rods.

Alas, the RBMK design had a fatal flaw: the shutdown rods, although made mostly of neutron-absorbing boron carbide, had tips made of graphite. That meant that when the rods were inserted into the core, they displaced water in the channels below them. The decrease in water actually fed the nuclear reaction for the first few seconds, instead of inhibiting it. That was enough to trigger an uncontrollable chain reaction.

Within seconds, the power of reactor 4 increased more than a hundredfold. Under the combined effect of pressure and heat, the pressure tubes exploded. The blast dislodged the reactor roof slab, several meters thick and weighing more than 1,000 tons. The reactor was open to the atmosphere. Three seconds later, a second explosion, more powerful, sent tons of uranium and burning graphite oxide up and out into the atmosphere. In the melting reactor, the graphite burned hot. Now open, the “atomic forge” spit radioactive particles several kilometres high. Bits of fuel were discovered later over 100 km from the plant.

Both the fuel and the reactor structures were melting under the extreme heat, combining into a magma that flowed down to the bottom of the reactor and beyond into the rooms below.

Emergency teams entered into action. With little if any protection, firemen from the local fire station fought the fires that were devastating roofs, the turbine hall and the reactor building. Attempting to smother the fire and stop the nuclear reaction that threatened to unravel the plant’s foundations, helicopter pilots discharged, in 2,000 rotations, 5,000 tons of sand, boron, lead, clay, and sodium phosphate onto the destroyed reactor. It would take three weeks to extinguish the burning graphite. Over 600,000 so-called liquidators were brought to the site to deal with the accident’s aftermath. The first 1,000 or so received high doses, but no comprehensive official assessment has been possible of the health impact of the accident on those emergency workers.

The most commonly accepted official death toll from the disaster points to 28 early deaths from radiation among the initial emergency workers, and 19 more over the subsequent 7 years.   Some 4,000 children developed thyroid cancers from exposure to radioactive iodine-131 due to the accident, nine of whom died.

Areas of Belarus, Ukraine and Russia close to the Chernobyl site received heavy contamination by radioactive fallout. 340,000 people were evacuated from the worst-affected areas, particularly in Belarus.

Because of the explosions that spewed the reactor’s contents high into the air, fallout also spread to many European countries, some far away. The accident was rated at Level 7 on the INES scale, the highest level, when the scale was developed.


The safety culture breach

At reactor 4 of Chernobyl NPP, the go-ahead for tests was in the hands of political authorities (in the Soviet Union, nuclear power plant managers were officials of the Communist Party). The latter did not take into account the intrinsic weaknesses of the RBMK.

The reactor design featured a positive void coefficient of reactivity, which meant that reactivity in the core increased as water inside the fuel channels turned to steam (leaving voids instead of liquid).

The effect of that feature, which did not exist on any nuclear power plant design in the West, was to enable a runaway power excursion in the Chernobyl 4 core that could not be stopped.

Unlike most western design reactors and the similar-size Soviet pressurised water reactor called VVER-1000, the RBMK also has no external structure specifically designed to contain radioactive materials inside the plant.


How this accident changed the world

Soviet authorities immediately ordered safety improvements to the other RBMKs operating in the USSR, including introduction of enriched uranium fuel that reduced the void coefficient of reactivity to a safe level and redesign of control rods, as well as addition of fast-operating shutdown rods. .

In fact, before the Chernobyl accident, the RBMK’s designers knew the design was unstable and even dangerous: a similar experiment to the one at Chernobyl, conducted at the Leningrad RMBK plant in 1982, had shown the risk, The information came to the designers in Moscow, but they did not circulate it to all the RBMK operators.

This showed that the Soviet nuclear power bureaucracy had not taken on board the lessons of the Three Mile Island (TMI) accident that had occurred in the US only seven years earlier: that information should be circulated among all operators so that all can learn from the others’ experience.

The TMI accident led to the creation of the Institute of Nuclear Power Operations (INPO), a club of all US nuclear power plant operators, to facilitate exchange of experience and foster good practices.

Chernobyl led to the creation of the same kind of association on an international level, obviously a much more difficult enterprise: the World Association of Nuclear Operators (WANO), which polices its members through peer pressure and monitoring.

Nuclear plant operators from around the world, but especially from western Europe, as well as safety experts, provided assistance to the Soviet authorities, notably through the IAEA but also through bilateral channels, since no world safety or operators’ associations existed at the time of the accident.

The accident caused a rethink of severe accident response in many countries, even though the design of the RBMK was specific to the USSR.

Along with the Three Mile Island accident, Chernobyl led to the realisation of the importance – to mitigate the radioactive consequences of a potential severe accident – of the containment structure present over almost all nuclear power reactors in the western world.

Recently, reactor designs such as the French EPR have made provisions for recovering a molten core and preventing it from burning through the reactor base slab.

Reactivity accidents were re-assessed and measures taken if necessary to further lower the probability of them occurring.

In many countries, the organisation of post-accident management was revised – a process still under way 30 years after the Chernobyl accident.

Sources

The Chernobyl Project, IAEA

IAEA – The Chernobyl Project

NEA – Chernobyl: Assessment of Radiological and Health Impact 2002 Update of Chernobyl: Ten Years On

Fukushima (Japan)

Earthquake and Tsunami damage, Japan-March 16, 2011: This is a satellite image of Japan showing damage after an Earthquake and Tsunami at the Dai Ichi Power Plant, Japan. (credit: DigitalGlobe) www.digitalglobe.com

On March 11, 2015 at 14:46, a hundred kilometres off the eastern coast of Japan, an earthquake of unprecedented power shook the archipelago on its foundations. Over 500 km of sea floor was broken. The earthquake was terrifying. The main island of Honshu was moved … 2.4 meters eastward. The damage was considerable. And the aftershocks, sometimes very violent, continued for more than a month.

In Japan’s nearby nuclear power plants, safety systems were working perfectly. After the first shock passed, the 11 reactors at 4 nuclear power plants in areas hit by the earthquake stopped automatically.

The only incident to affect a nuclear plant involved the collapse of the power line connecting Fukushima-Daichi (Fukushima-I) to the grid.

Automatically, again, the stand-by generators started, ensuring power supply to the pumps in the cooling circuit

of the 6 reactors of Fukushima Dai-ichi, one of the country’s largest power plants, operated by TEPCO, the company supplying electricity to Tokyo and nearby areas.

Meanwhile, offshore, a monster rose. The gigantic landslide that upset the bottom of the Pacific Ocean raised two waves. In the open sea, no sailor noticed them. Hitting the coast, their height sometimes exceeded 23 meters. In 10 minutes, they reached the coast and penetrated up to 5 km inland.

More than 23,000 people were to be killed by the combined effect of the earthquake and tsunami – one of the worst tsunamis ever to hit the Land of the Rising Sun.

The 10-meter-high seawall built to protect the Fukushima Dai-ichi site from tsunamis was overwhelmed by the two giant waves, 14 meters high. They easily breached the concrete seawall, flooded the plant site platform, destroying equipment and vehicles near the shore . Emergency diesel generators designed to power the first 4 units, located at sea level, and their fuel reservoirs were quickly flooded. Emergency diesel generators and backup batteries in the basements of the turbine buildings were also drowned.

Quickly, the fatal spiral began. Deprived of cooling, because the electrical pumps were all blacked-out (no connection to the grid and no back-up available), the fuel in the reactors of the first three units heated up dangerously. Above the reactors, cooling ponds where spent fuel is stored were no longer supplied with cooling water. The heat released by the spent fuel was for a while feared to have evaporated the water in the unit 4 spent fuel pool, which was full at the time of the accident because the unit was in a maintenance outage.

In the core of the reactors, the fuel temperature reached 800 ° C. The zirconium fuel cladding reacted with water, decomposing into oxygen and hydrogen and producing heat. This exothermic process brought the temperature in the reactor buildings to over 1,200 ° C, causing melting of ducts and liquefaction of the fuel and producing ever greater volumes of hydrogen . Plunged into darkness (as the emergency batteries failed), technicians of the NPP struggled to understand the magnitude of the devastation and its consequences. They even tried to supply power by using batteries from cars parked on the plant site.

To reduce the risk of explosion and rupture of the containment which was by then under high pressure, some reactor buildings were depressurised, releasing steam, hydrogen and some radioactivity in the environment. That was not sufficient, however. Between the 12th and the 15th of March, three hydrogen explosions shook reactors 1, 2 and 3. On the 14th and 15th, an explosion ripped a hole in the wall of the spent fuel pool building at unit 4, and a fire was detected in that unit’s reactor building.

It took all the skill and courage of the Japanese helicopter pilots to keep the reactors cooled by dumping large quantities of water from above. But that strategy proved insufficient.

On the ground, firemen and operators were equally heroic. After making their way through rubble and carcasses of trucks crushed by the tsunami, they managed to approach the reactor buildings with fire trucks and fire pumps. Thus began a crucial operation: injecting massive amounts of water (mainly seawater brought by dedicated new piping) to cool the damaged reactors and fuel elements. Until the beginning of April, most of the water contaminated by this makeshift cooling procedure would be discharged into the Pacific Ocean.

The three reactors operating on March 11, units 1, 2 and 3 (unit 4 was in a refueling shutdown) did not resist. Under the effects of the cooling loss, their cores melted, producing a kind of lava (known as corium, a mixture of molten fuel and reactor structures) that escaped at several points through the steel reactor vessels.

The lids of these large pressure cookers lost seal, releasing large amounts of radionuclides: noble gases, iodine, cesium. According to estimates by the French Institute for Radiological Protection and Nuclear Safety (IRSN), gaseous releases from Fukushima Dai-Ichi could exceed 13,000 petabecquerels (PBq) — two-thirds of the radioactivity released into the environment during the Chernobyl disaster.

With the wind, most of the gases and radioactive particles were driven out to the Pacific Ocean. Most, but not all… A total of 24,000 km2 of territory (often mountains and forests) were contaminated by cesium-137, usually within a radius of 80 kilometres around the damaged plant.

But, like Chernobyl a quarter century earlier, many ‘leopard spots’ of radioactive contamination were formed, sometimes hundreds of kilometres from Fukushima, due to rainfall and wind.

Dramatic, yes, but also to be put into perspective. Scientists estimate at 9,000 km2 the surface of land heavily contaminated (at a level of 1 curie or 37,000 Bq/m2) by the fallout from Fukushima, 5% of the total area affected by radioactive releases around Chernobyl.

Shocked by the earthquake and tsunami, the local population was not always aware that a nuclear accident had occurred. . Many people in Fukushima prefecture had no electricity, limiting access to media and telecommunication networks. A number of roads were also cut off, preventing movement.

But one must act quickly in the event of a nuclear accident. Less than 5 hours after the earthquake, a nuclear emergency was declared. The governor of Fukushima prefecture ordered evacuation of people living within 2 km of the damaged plant. An hour later, the Prime Minister set a 3 km radius for the “forbidden zone”; he extended it to 7 km the next day. On April 21, the Japanese nuclear safety regulator expanded the exclusion zone to a radius of 20 km around the plant. Between March 12 and April 22, more than 145,000 people were evacuated with great difficulty.

The accident was rated at Level 7, the highest level on the INES scale, The safety culture breach

The tsunami protection measures at Fukushima Dai-ichi were clearly undersized for a country that had experienced 10 historical tsunamis between 1605 and 1933, including near the site. Similarly, the authorities – and the nuclear industry – had not imagined that a succession of natural disasters (earthquake plus tsunami) could lead to such a catastrophe. The emergency devices were not protected against flooding..

The governance of the plant was in the hands of Tepco’s management. The nuclear industry held the belief that their units were safe. Tepco management, especially, had resisted the conclusion of studies showing a large tsunami was possible at Fukushima-I, successfully postponing measures such as moving the diesel generators to higher ground or increasing the height of the tsunami wall in front of the site.

These problems were made worse by the lack of independence of the Japanese nuclear safety authority, at that time an agency (Nisa) within the powerful industry and economy ministry, METI, which also promoted nuclear energy.

Nowhere in Japan were reactor buildings equipped with devices to recombine hydrogen that might be generated in the reactor and fuel pool buildings during a severe accident.

Yet it is the hydrogen explosions that blew the walls of the units 1, 3 and 4, threatening the integrity of the last unit’s spent fuel pool.


How this accident changed the world

Following the Fukushima accident, regulators in Europe required operators there to conduct “stress tests” at their facilities, postulating the simultaneous occurrence of multiple external aggressions and seeing how well the installations could resist. In France, the operator, on the basis of the results of the stress test analyses, was required to propose a large number of measures to protect vital systems of its nuclear power plants. It also proposed the creation of a “rapid reaction force” that could be deployed quickly to rescue a plant threatened by loss of cooling capability and/or total loss of power.

Sources:

International expert fact finding Mission of the Fukushima Dai-Ichi NPP accident Following the Great East Japan earthquake and tsunami, IAEA, 2011.

www-pub.iaea.org/…/cn200_Final-Fukushima-Mission_Report.pdf

INES SCALE