April 14, 2011 / 10:49 AM / 8 years ago

Q+A-Risks at each reactor of Japan's stricken plant

TOKYO, April 14 (Reuters) - The Fukushima Daiichi nuclear power plant at the centre of Japan’s crisis has six reactors. The plant is operated by Tokyo Electric Power Co (TEPCO).

The following summarises the main risks as Japanese engineers scramble to deal with the worst nuclear crisis since the 1986 Chernobyl meltdown:

WHAT’S THE OVERALL SITUATION?

Two of the plant’s reactors are seen as safe, but the other four are still volatile. The nuclear safety agency has said the temperatures and pressure in all reactors are stable.

Workers have succeeded in halting the chain reactions at reactors No.1, No.2 and No.3 that resulted after the quake hit, but they were unable to cool the reactors in time before some of the fuel melted inside the reactor cores.

Engineers are now struggling to cool the reactors to the point where they can be safely shut down. Until the cooling pumps, or alternatives, are online to continually cool the reactors, workers are forced to periodically inject new water to keep the fuel cool, but that creates more contaminated water.

The immediate challenge is to transfer and store the contaminated water, while at the same time limit as much as possible the spread of radiation still leaking into the atmosphere and sea. At the moment, workers do not have access to the cooling systems in reactors No.1, No.2 and No.3.

So far, eight workers have sustained injuries from radiation at the Daiichi plant. All have been released from hospital.

WHAT ARE THE RISKS AT EACH REACTOR?

— REACTOR No 1: 460-MW (Manufacturer GE)

This is the reactor whose fuel may have melted the most, with TEPCO estimating that 70 percent of the fuel may have been damaged, judging from the radiation released. The detection of neutron beams in mid-March may also mean that flashes of nuclear reaction occurred when the fuel was at its hottest.

Pumps are now injecting fresh water into the reactor, and workers hope to install an independent power line for another pump, in case an aftershock knocks out the power again.

Workers are also injecting nitrogen into the reactor in hopes of preventing a repeat of a March 12 hydrogen blast, which could endanger lives of workers, block access within the plant and spread further radiation into the environment.

The oldest of the four damaged reactors, reactor No.1 may have weak spots that could make it more vulnerable to an explosion inside the reactor core.

— REACTOR No 2: 784-MW (Manufacturer: GE, Toshiba)

This reactor is flooded with the most deadly waters. A worker would exceed his annual quota of radiation after just 15 minutes in the basement of the turbine building.

Workers have transferred 660 tonnes of the radioactive water out of basement and reactor’s tunnel into its condensers, but the water level failed to go down by much, and workers now plan to pump water to tanks on the site’s central radioactive waste facility.

If engineers cannot pump out water faster than it leaks back in, TEPCO might have to set up an external cooling pump system outside the turbine building.

Nobody wants that water dumped into the sea. Japan has already dumped low-level contaminated water into the ocean to make space to store the more toxic water on site, drawing international criticism.

Damage to the reactor could be extensive. Judging from the volume of water, the reactor vessel as well as the valves and pipes leading from the reactor and the suppression pool at the bottom of the containment vessel may have been damaged after an explosive sound in mid-March was heard near the pressure suppression chamber.

The reactor’s fuel rods were exposed fully at one point. When fuel rods are not covered in coolant, they can heat up and start to melt, raising the risk of a radiation leak. TEPCO has estimated that 30 percent of the fuel may have been damaged.

— REACTOR No 3: 784-MW (Manufacturer Toshiba)

This reactor is the only one to use plutonium in its original fuel mix, meaning that there may be more of the highly toxic substance inside the reactor than in the others, which also create plutonium as a by-product.

The earthquake and a hydrogen explosion on March 14 may have damaged pipes or valves leading from the reactor.

TEPCO has estimated that 25 percent of the fuel may have been damaged before they succeeded in cooling down the reactor core through hosing operations.

A pump powered by an off-site transmission line is now injecting fresh water into the reactor.

— REACTOR No 4: 784-MW (Manufacturer Hitachi)

At the time of the earthquake and tsunami on March 11, the reactor was undergoing maintenance, and all fuel had been transferred to the spent fuel pool.

Four days after the quake, the spent fuel pool caught fire and caused an explosion. Japan’s nuclear safety agency said the blast punctured two holes around 8 metres square in the wall of the outer building of the reactor.

The pools contain racks that hold spent fuel taken from the reactor. Operators need to constantly add water to the pool to keep the fuel submerged and trap radiation.

TEPCO sampled the water this week and detected small increases in radioactivity in the water. Nuclear authorities are trying to gauge if the increase in radiation is airborne or from damage to the Unit 4 fuel.

— REACTOR No 5: 784-MW (Manufacturer Toshiba)

The No 5 reactor has been stable since it was safely stopped on March 20 , when the temperature of the water inside the reactor fell low enough for it to achieve a cold shutdown.

At the time of the earthquake and tsunami on March 11, the reactor was undergoing maintenance

— REACTOR No 6: 1,100-MW (Manufacturer GE, Toshiba)

The No 6 reactor has been stable since it was safely stopped on March 20, when the temperature of water inside the reactor fell low enough for it to achieve a so-called cold shutdown.

At the time of the earthquake and tsunami on March 11, the reactor was undergoing maintenance.

Editing by Alan Raybould and Edwina Gibbs

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