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During the March 11 earthquake Units 1, 2 and 3 of the Fukushima I Nuclear Power Plant shut down automatically. Diesel generators kicked in to run the backup cooling system and extract excess heat from the core. Unfortunately, these generators were damaged by the subsequent tsunami and failed about an hour after the ‘quake struck. Moving in new generators, or giant batteries would be very difficult in the best of situations, but with the damage to infrastructure caused by the earthquake and tsunami it was impossible.
The Fukushima I Power Plant is a 4.7 gigawatt 6-unit Boiling Water Reactor (BWR) complex. Although fission reactions stopped when the reactor shut down (when it was “scrammed“) the residual heat from radioactive decay within the nuclear fuel presents a problem. Unlike other reactor designs (e.g. the Advanced Gas-cooled Reactors used in most of the UK’s nuclear power stations) the BWR design cannot cool itself passively and requires power to run the coolant systems. With no supply from the electrical grid, and no diesel generators, no power was available.
Without a functioning cooling system the temperature, and therefore the pressure, inside the reactor vessels continued to increase. The pressure inside the Unit 1 and Unit 2 containment structures increased beyond their design limits. In order to reduce this pressure some radioactive steam was released from the primary cooling circuit, though most radiation will have been removed by filters before the steam reached the atmosphere.
The steam is radioactive because neutrons from the reactor bombarding the coolant water transmute the water’s hydrogen atoms into tritium (hydrogen-3) atoms to create radioactive tritiated water. Tritium is a lightly radioactive beta emitter and beta particles cannot travel very far in air and do not usually penetrate the skin. The steam would also have contained some nitrogen-16, formed in an ‘np’ reaction when neutrons bombarded the water’s oxygen atoms, but as nitrogen-16 has a half-life of only 7.1 seconds it does not present a significant risk.
Even with the release of steam, the pressure and temperature inside the reactors continued to increase. The high temperatures inside the reactor caused the protective zirconium cladding on the uranium fuel rods to react with steam inside the reactor to form hydrogen. This hydrogen leaked into the buildings that surround the reactors and ignited, due to an aftershock in the case of Unit 1 and for unknown reasons in Unit 3. The fact that the zirconium cladding was damaged also indicates that the nuclear fuel was exposed to the air inside the reactor, not covered by coolant and this lends weight to the idea that the fuel itself has melted to some degree. Vent holes have been made in the rooves of Unit 5 and Unit 6 in case hydrogen begins to build up there.
Initially it seemed that neither the Unit 1 or Unit 2 reactor vessels had been significantly compromised. (The roof and walls of the buildings surrounding the reactors are deliberately designed to blow away, to prevent the force from being focused inwards on the reactor.) However, white smoke has now been seen coming from Unit 3 which suggests that there may be damage to the reactor or the containment structure; this would seem possible as the explosion at that site was noticeably larger than the explosion at Unit 1.
Alterations were made to the walls of the Unit 2 building to vent any hydrogen produced there, but nonetheless there was an explosion at the site. This explosion seems more serious than the first two, damaging part of the reactor’s pressure supression system. An “abnormal noise … emanating from nearby the pressure suppression chamber” caused workers to be evacuated from the area. A 20cm crack in the maintenance pit underneath the reactor is allowing radioactive contamination to seep into the sea surrounding the plant.
The problems at Unit 3 may be more dangerous than at Units 1 and 2 because Unit 3 uses mixed oxide (MOX) fuel which contains a small percentage of plutonium oxide as well as uranium oxide; this “burns” at a higher temperature and contains more radio-toxic fission fragments than pure uranium oxide fuel.
Unit 4 was shut down at the time of the quake and the reactor’s core, along with spent fuel rods, was stored in the spent fuel pond towards the top of the building. The failure of the coolant system again led to the formation of hydrogen and an explosion damaged the roof and walls of Unit 4 and caused a fire. This released radioactive material into the air, as the spent fuel ponds are not surrounded by containment vessels in the same way as the reactors are. There have been sporadic fires, and firefighting efforts are being hampered by releases of radiation from Units 1, 2 and 3, but it seems that the fires are currently extinguished.
Seawater is being pumped into the reactor core of Units 1, 2 and 3 and the containment buildings of Units 1 and 3. Workers from the fantastically named Hyper Rescue Squad and the Japanese Self Defense forces have been spraying water onto the reactors to cool them, both from the ground using water cannons and from the air using a modified lead-lined Chinook helicopter. Replacement power lines have been laid and connected to the plant but it is not clear how well the cooling systems (the ECCS and RCIC) will operate considering all the damage that has occured.
The level of radioactivity at the site is a little unclear and will vary across the site according to the location of any leaked material. The most recent reliable figure I could find reported that the border of the site is at about 265 microsieverts (µSv) per hour, significantly above the normal background but not hugely dangerous. The average exposure due to natural background in the UK is 2600 µSv/year; in Cornwall, where there is a lot of radioactive granite the background dose is 7000 µSv/year; and the residents of Ramsar in Iran receive the highest natural background radiation dose in the world at 260000 µSv/year. For a single dose received in a single event, mild radiation sickness will set in at about a million microsieverts.
There does not currently seem to be a wide dispersion of radioactive material outside the border of the plant. Workers have been moved out from, and back to, the plant according to surges in the levels of radioactivity, but current exposure levels for workers are unknown. It is known that one worker did receive a dose of 106000 µSv, enough to cause mild bone marrow supression but not enough to be fatal; and two workers received radiation burns after walking through radioactive water in Unit 3.
The roof of the Unit 1 building was blown off by the hydrogen explosion.
Fukushima will not be “another Chernobyl” or “Japan’s Chernobyl”. The Fukushima reactor does not have a combustible core made of graphite like RBMK-type (Chernobyl-type) reactors do and the Japanese Nuclear Safety Agency has said that damage to the reactor vessel is minimal. The authorities have now combined the incidents at Units 1, 2 and 3 – previously rated individually as Level 5 accidents – into one Level 7 incident.