Fukushima reactors how many

It has high-pressure and low-pressure elements. The high pressure coolant injection HPCI system in units had pumps powered by steam turbines which were designed to work over a wide pressure range. The HPCI drew water from the large torus suppression chamber beneath the reactor as well as a water storage tank, and required only DC battery power. For use below about kPa, there was also a low pressure coolant injection LPCI mode through the RHR system but using suppression pool water, and a core spray system, all electrically-driven.

All ECCS subsystems require some power to operate valves etc , and the battery backup to generators may provide this. Beyond these original systems, Tepco in s installed provision for water injection via the fire extinguisher system through the RHR system injecting via the jet-pump nozzles as part of its severe accident management SAM countermeasures. The Fukushima reactors had much of their switchgear on the ground floor in the turbine buildings rather than elevated, as at some similar US plants.

Also they had control rooms with analogue instrumentation typical of the period, so not only did many instruments fail, but data could not be downloaded and accessed remotely to assist diagnosis and remedial action.

Climate Change - The Science. Is radiation safe? Even the special cooling system known as the reactor core isolation cooling system that uses waste heat to run the critical systems could not provide the power needed to operate the control systems.

Unit 1 exploded on 12 March knocking down the external concrete building. However, the reactor and the steel containment structure remained intact. The radiation levels rose to microsievert, which is equivalent to the maximum permissible level for a year in a single day. This scale measures from deviation-no safety significance — major accident and is used to communicate the safety significance of events associated with radiation sources.

Unit 3 exploded due to hydrogen ignition on 14 March Pressure in the reactor was built-up to kiloPascals kPa even while sea water was being injected into the reactor to control the radiation. On the same day, the fifth floor of Unit 4 building was damaged. Fire was sighted in the north-west part of the fourth floor and efforts to put it out were initiated immediately. The reactor Core Isolation system of Unit 2 had also stopped functioning, resulting in a third explosion in the suppression chamber.

However, since they were leaking, the normal definition of 'cold shutdown' did not apply, and Tepco waited to bring radioactive releases under control before declaring 'cold shutdown condition' in mid-December, with NISA's approval. This, with the prime minister's announcement of it, formally brought to a close the 'accident' phase of events. The AC electricity supply from external source was connected to all units by 22 March.

Power was restored to instrumentation in all units except unit 3 by 25 March. However, radiation levels inside the plant were so high that normal access was impossible until June. Results of muon measurements in unit 2 in indicate that most of the fuel debris in unit 2 is in the bottom of the reactor vessel. Summary : Major fuel melting occurred early on in all three units, though the fuel remained essentially contained except for some volatile fission products vented early on, or released from unit 2 in mid-March, and some soluble ones which were leaking with the water, especially from unit 2, where the containment is evidently breached.

Cooling is provided from external sources, using treated recycled water, with a stable heat removal path from the actual reactors to external heat sinks. Access has been gained to all three reactor buildings, but dose rates remain high inside. Tepco declared 'cold shutdown condition' in mid-December when radioactive releases had reduced to minimal levels.

See also background on nuclear reactors at Fukushima Daiichi. Used fuel needs to be cooled and shielded. This is initially by water, in ponds. After about three years underwater, used fuel can be transferred to dry storage, with air ventilation simply by convection. Used fuel generates heat, so the water in ponds is circulated by electric pumps through external heat exchangers, so that the heat is dumped and a low temperature maintained.

There are fuel ponds near the top of all six reactor buildings at the Daiichi plant, adjacent to the top of each reactor so that the fuel can be unloaded underwater when the top is off the reactor pressure vessel and it is flooded.

There is some dry storage onsite to extend the plant's capacity. At the time of the accident, in addition to a large number of used fuel assemblies, unit 4's pond also held a full core load of fuel assemblies while the reactor was undergoing maintenance, these having been removed at the end of November, and were to be replaced in the core.

A separate set of problems arose as the fuel ponds, holding fresh and used fuel in the upper part of the reactor structures, were found to be depleted in water. The primary cause of the low water levels was loss of cooling circulation to external heat exchangers, leading to elevated temperatures and probably boiling, especially in the heavily-loaded unit 4 fuel pond.

Here the fuel would have been uncovered in about 7 days due to water boiling off. However, the fact that unit 4 was unloaded meant that there was a large inventory of water at the top of the structure, and enough of this replenished the fuel pond to prevent the fuel becoming uncovered — the minimum level reached was about 1. After the hydrogen explosion in unit 4 early on Tuesday 15 March, Tepco was told to implement injection of water to unit 4 pond which had a particularly high heat load 3 MW from used fuel assemblies in it, so it was the main focus of concern.

Initially this was attempted with fire pumps but from 22 March a concrete pump with metre boom enabled more precise targeting of water through the damaged walls of the service floors. There was some use of built-in plumbing for unit 2. Analysis of radionuclides in water from the used fuel ponds suggested that some of the fuel assemblies might have been damaged, but the majority were intact.

There was concern about the structural strength of unit 4 building, so support for the pond was reinforced by the end of July. Each has a primary circuit within the reactor and waste treatment buildings and a secondary circuit dumping heat through a small dry cooling tower outside the building. The next task was to remove the salt from those ponds which had seawater added, to reduce the potential for corrosion.

In July two of the fresh fuel assemblies were removed from the unit 4 pool and transferred to the central spent fuel pool for detailed inspection to check damage, particularly corrosion.

They were found to have no deformation or corrosion. Unloading the spent fuel assemblies in pond 4 and transferring them to the central spent fuel pool commenced in mid-November and was completed 13 months later. These comprised spent fuel plus the full fuel load of The next focus of attention was the unit 3 pool. In the damaged fuel handling equipment and other wreckage was removed from the destroyed upper level of the reactor building. Toshiba built a tonne fuel handling machine for transferring the fuel assemblies into casks and to remove debris in the pool, and a crane for lifting the fuel transfer casks.

Installation of a cover over the fuel handling machine was completed in February Removal and transferral of the fuel to the central spent fuel pool began in mid-April and was completed at the end of February In June , Tepco announced it would transfer some of the fuel assemblies stored in the central spent fuel pool to an onsite temporary dry storage facility to clear sufficient space for the fuel assemblies from unit 3's pool.

The dry storage facility has a capacity of at least assemblies in 65 casks — each dry cask holds 50 fuel assemblies. Summary: The spent fuel storage pools survived the earthquake, tsunami and hydrogen explosions without significant damage to the fuel, significant radiological release, or threat to public safety.

The new cooling circuits with external heat exchangers for the four ponds are working well and temperatures are normal. Analysis of water has confirmed that most fuel rods are intact. See also background on Fukushima Fuel Ponds and Decommissioning section below.

Regarding releases to air and also water leakage from Fukushima Daiichi, the main radionuclide from among the many kinds of fission products in the fuel was volatile iodine, which has a half-life of 8 days.

The other main radionuclide is caesium, which has a year half-life, is easily carried in a plume, and when it lands it may contaminate land for some time. It is a strong gamma-emitter in its decay.

Cs is also produced and dispersed; it has a two-year half-life. Caesium is soluble and can be taken into the body, but does not concentrate in any particular organs, and has a biological half-life of about 70 days. In assessing the significance of atmospheric releases, the Cs figure is multiplied by 40 and added to the I number to give an 'iodine equivalent' figure.

As cooling failed on the first day, evacuations were progressively ordered, due to uncertainty about what was happening inside the reactors and the possible effects.

By the evening of Saturday 12 March the evacuation zone had been extended to 20 km from the plant. See later section on Public health and return of evacuees.

A significant problem in tracking radioactive release was that 23 out of the 24 radiation monitoring stations on the plant site were disabled by the tsunami. There is some uncertainty about the amount and exact sources of radioactive releases to air see also background on Radiation Exposure. Most of the release was by the end of March Tepco sprayed a dust-suppressing polymer resin around the plant to ensure that fallout from mid-March was not mobilized by wind or rain.

In addition it removed a lot of rubble with remote control front-end loaders, and this further reduced ambient radiation levels, halving them near unit 1. In mid-May work started towards constructing a cover over unit 1 to reduce airborne radioactive releases from the site, to keep out the rain, and to enable measurement of radioactive releases within the structure through its ventilation system. The frame was assembled over the reactor, enclosing an area 42 x 47 m, and 54 m high.

The sections of the steel frame fitted together remotely without the use of screws and bolts. All the wall panels had a flameproof coating, and the structure had a filtered ventilation system capable of handling 40, cubic metres of air per hour through six lines, including two backup lines.

The cover structure was fitted with internal monitoring cameras, radiation and hydrogen detectors, thermometers and a pipe for water injection. The cover was completed with ventilation systems working by the end of October It was expected to be needed for two years.

In May Tepco announced its more permanent replacement, to be built over four years. It started demolishing the cover in and finished in In December it decided to install the replacement cover before removing debris from the top floor of the building. A crane and other equipment for fuel removal will be installed under the cover, similar to that over unit 4.

A cantilevered structure was built over unit 4 from April to July to enable recovery of the contents of the spent fuel pond. This is a 69 x 31 m cover 53 m high and it was fully equipped by the end of to enable unloading of used fuel from the storage pond into casks, each holding 22 fuel assemblies, and removal of the casks.

This operation was accomplished under water, using the new fuel handling machine replacing the one destroyed by the hydrogen explosion so that the used fuel could be transferred to the central storage onsite.

Transfer was completed in December A video of the process is available on Tepco's website. A different design of cover was built over unit 3, and foundation work began in Large rubble removal took place from to , including the damaged fuel handling machine.

An arched cover was prefabricated, 57 m long and 19 m wide, and supported by the turbine building on one side and the ground on the other. A crane removed the fuel assemblies from the pool and some remaining rubble.

Spent fuel removal from unit 3 pool began in April and was completed in February Maps from the Ministry of Education, Culture, Sports, Science and Tehcnology MEXT aerial surveys carried out approximately one year apart show the reduction in contamination from late to late Areas with colour changes in showed approximately half the contamination as surveyed in , the difference coming from decay of caesium two-year half-life and natural processes like wind and rain.

Tests on radioactivity in rice have been made and caesium was found in a few of them. Summary : Major releases of radionuclides, including long-lived caesium, occurred to air, mainly in mid-March.

The population within a 20km radius had been evacuated three days earlier. Considerable work was done to reduce the amount of radioactive debris onsite and to stabilize dust. The main source of radioactive releases was the apparent hydrogen explosion in the suppression chamber of unit 2 on 15 March.

A cover building for unit 1 reactor was built and the unit is now being dismantled, a more substantial one for unit 4 was built to enable fuel removal during By the end of , Tepco had checked the radiation exposure of 19, people who had worked on the site since 11 March. For many of these both external dose and internal doses measured with whole-body counters were considered. It reported that workers had received doses over mSv.

Of these had received to mSv, twenty-three mSv, three more mSv, and six had received over mSv to mSv apparently due to inhaling iodine fumes early on. There were up to workers onsite each day.

Recovery workers wear personal monitors, with breathing apparatus and protective clothing which protect against alpha and beta radiation. The level of mSv was the allowable maximum short-term dose for Fukushima Daiichi accident clean-up workers through to December , mSv is the international allowable short-term dose "for emergency workers taking life-saving actions".

No radiation casualties acute radiation syndrome occurred, and few other injuries, though higher than normal doses, were being accumulated by several hundred workers onsite. High radiation levels in the three reactor buildings hindered access there. Monitoring of seawater, soil and atmosphere is at 25 locations on the plant site, 12 locations on the boundary, and others further afield.

Government and IAEA monitoring of air and seawater is ongoing. Some high but not health-threatening levels of iodine were found in March, but with an eight-day half-life, most I had gone by the end of April A radiation survey map of the site made in March revealed substantial progress: the highest dose rate anywhere on the site was 0. The majority of the power plant area was at less than 0. These reduced levels are reflected in worker doses: during January , the workers at the site received an average of 0.

Media reports have referred to 'nuclear gypsies' — casual workers employed by subcontractors on a short-term basis, and allegedly prone to receiving higher and unsupervised radiation doses. This transient workforce has been part of the nuclear scene for at least four decades, and at Fukushima their doses are very rigorously monitored. If they reach certain levels, e. Tepco figures submitted to the NRA for the period to end January showed workers had received more than mSv six more than two years earlier and had received 50 to mSv.

Early in there were about onsite each weekday. Summary : Six workers received radiation doses apparently over the mSv level set by NISA, but at levels below those which would cause radiation sickness. On 4 April , radiation levels of 0. Monitoring beyond the 20 km evacuation radius to 13 April showed one location — around Iitate — with up to 0. At the end of July the highest level measured within 30km radius was 0. The safety limit set by the central government in mid-April for public recreation areas was 3.

In June , analysis from Japan's Nuclear Regulation Authority NRA showed that the most contaminated areas in the Fukushima evacuation zone had reduced in size by three-quarters over the previous two years. In August The Act on Special Measures Concerning the Handling of Radioactive Pollution was enacted and it took full effect from January as the main legal instrument to deal with all remediation activities in the affected areas, as well as the management of materials removed as a result of those activities.

It specified two categories of land: Special Decontamination Areas consisting of the 'restricted areas' located within a 20 km radius from the Fukushima Daiichi plant, and 'deliberate evacuation areas' where the annual cumulative dose for individuals was anticipated to exceed 20 mSv. The national government promotes decontamination in these areas.

Intensive Contamination Survey Areas including the so-called Decontamination Implementation Areas, where an additional annual cumulative dose between 1 mSv and 20 mSv was estimated for individuals. Municipalities implement decontamination activities in these areas. The doses to the general public, both those incurred during the first year and estimated for their lifetimes, are generally low or very low. No discernible increased incidence of radiation-related health effects are expected among exposed members of the public or their descendants.

However, the report noted: "More than additional workers received effective doses currently estimated to be over mSv, predominantly from external exposures. Among this group, an increased risk of cancer would be expected in the future. However, any increased incidence of cancer in this group is expected to be indiscernible because of the difficulty of confirming such a small incidence against the normal statistical fluctuations in cancer incidence.

These workers are individually monitored annually for potential late radiation-related health effects. Tens of thousands of workers will be needed over the next 30 to 40 years to safely remove nuclear waste, fuel rods and more than one million tons of radioactive water still kept at the site.

But some residents have decided never to return because they fear radiation, have built new lives elsewhere or don't want to go back to where the disaster hit. Media reports in said the government could start to release the water - filtered to reduce radioactivity - into the Pacific Ocean as early as next year.

Some scientists believe the huge ocean would dilute the water and that it would pose a low risk to human and animal health. Environmental group Greenpeace however said that the water contains materials that could potentially damage human DNA.

Officials have said no final decision has been taken about what to do with the liquid. Japan nuclear disaster residents return. Fukushima water could damage DNA, Greenpeace says. Five remarkable stories of Japan's tsunami debris. Diving into the world of the dead. Fukushima: Is fear of radiation the real killer? Image source, Reuters. The earthquake was the most powerful ever recorded in Japan.

Where is the plant? What happened at Fukushima? Image source, Getty Images. The tsunami overcame the sea wall and hit the plant. The damage led to nuclear meltdowns and a number of hydrogen explosions. How many people were hurt?