2.4 History of Bangladesh in Nuclear

2.3 History Accident’s in Nuclear Power

2.3.1 The Fukushima Daiichi nuclear disaster

The Fukushima Daiichi nuclear disaster was a series of equipment failures, nuclear meltdowns and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March

2011.[5][6] It is the largest nuclear disaster since the Chernobyl disaster of 1986 and only the second disaster (along with Chernobyl) to measure Level 7 on the International Nuclear Event Scale.[7]

Fig 2.4: Simplified cross-section sketch of a typical light water, boiling water reactors

The plant comprises six separate boiling water reactors originally designed by General Electric (GE) and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the quake, Reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance.[8]

Immediately after the earthquake, the remaining reactors 1–3 shut down automatically and emergency generators came online to power electronics and coolant systems. However, the tsunami following the earthquake quickly flooded the low-lying rooms in which the emergency generators were housed. The flooded generators failed, cutting power to the critical pumps that must continuously circulate coolant water through a nuclear reactor for several days in order to keep it from melting down after being shut down.

As the pumps stopped, the reactors overheated due to the normal high radioactive decay heat produced in the first few days after nuclear reactor shutdown (smaller amounts of this heat normally continue to be released for years, but are not enough to cause fuel melting).

At this point, only prompt flooding of the reactors with seawater could have cooled the reactors quickly enough to prevent meltdown. Salt water flooding was delayed because it would ruin the costly reactors permanently. Flooding with seawater was finally commenced only after the government ordered that seawater be used and at this point it was already too late to prevent meltdown.[9]

As the water boiled away in the reactors and the water levels in the fuel rod pools dropped, the reactor fuel rods began to overheat severely and melt down. In the hours and days that followed, Reactors 1, 2 and 3 experienced full meltdown.[10][11]

In the high heat and pressure of the reactors, a reaction between the nuclear fuel metal cladding, and the water surrounding them, produced explosive hydrogen gas. As workers struggled to cool and shut down the reactors, several hydrogen-air chemical explosions occurred.[12][13] It is estimated that the hot cladding-water reaction in each reactor produced 800 to 1000 kilograms of hydrogen gas, which was vented out of the reactor pressure vessel, and mixed with the ambient air, eventually reaching explosive

concentration limits in units 1 and 3, and due to piping connections between unit 3 and 4, unit 4 also filled with hydrogen, with the hydrogen-air explosions occurring at the top of each unit, that is in their upper secondary containment building.[14][15]

Concerns about the atmospheric venting of radioactive gasses led to a 20 km (12 mi)-radius evacuation around the plant. During the early days of the accident, workers were temporarily evacuated at various times for radiation safety reasons. The earthquake damage and flooding in the wake of the tsunami hindered external assistance. Electrical power was slowly restored for some of the reactors, allowing for automated cooling.[16]

Measurements taken by the Japanese government 30–50 km from the plant showed caesium-137 levels high enough to cause concern,[17] leading the government to ban the sale of food grown in the area. Tokyo officials temporarily recommended that tap water should not be used to prepare food for infants.[18][19] In May 2012, TEPCO reported that in the first three weeks of the 2011 incident at least 900 PBq had been released into the atmosphere, which is an amount roughly equivalent a sixth (17%) of the total release following Chernobyl.[20][21] However, as of April 2013 radiation is still leaking from the facility.[22]

2.3.2Chernobyl disaster

The Chernobyl disaster was a catastrophic nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine (then officially Ukrainian SSR), which was under the direct jurisdiction of the central authorities of the Soviet Union. An explosion and fire released large quantities of radioactive particles into the atmosphere, which spread over much of Western USSR and Europe.

The Chernobyl disaster is widely considered to have been the worst nuclear power plant accident in history, and is one of only two classified as a level 7 event on the International Nuclear Event Scale (the other being the Fukushima Daiichi nuclear disaster in 2011).[23] The battle to contain the contamination and avert a greater catastrophe ultimately involved over 500,000 workers and cost an estimated 18 billion rubles.[24]