What’s going on? (Image: DigitalGlobe)

With muddled media reports of the ongoing crisis, we spell out exactly what has happened up to 15 March, and what might happen next

Which reactors have been hit hardest by the quake, and where are they?

Two major nuclear power plants are at the heart of the crisis, both of which were hit by the quake and the tsunami. They are on the coast halfway between Sendai, the city which bore the brunt of the tsunami, and Tokyo.


The first, called Fukushima Daiichi – literally, Fukushima Number 1 – has six units, each housing its own nuclear reactor. Only the first three units were working when the quake struck.

The second power plant involved, Fukushima Daini – Fukushima Number 2 – has four units, and all were working at the time of the quake. Both installations are run by the Tokyo Electric Power Company (TEPCO), and updates on the status of both, and all their individual reactors, are available here.

Did everything work as planned initially?

As with all nuclear power stations in Japan, the reactors are fitted with quake sensors that shut them down automatically at the first sign of potentially hazardous tremors. The main problems since then have arisen because the tsunami severely damaged back-up sources of power, which stopped pumping equipment designed to keep the dormant reactors cooled and safe even in emergencies from working.

What is happening at each of the two power plants?

According to a report today (15 March) from the Japanese authorities, all four reactor units at Fukushima Daini are in “cold shutdown”, which means that the temperature and operation of the reactors are fully under control. Residents within 10 kilometres of the site are still being evacuated until the situation is fully under control.

So the main problems are at Fukushima Daiichi?

Yes. Reactor units 5 and 6 are both OK for now. But the other four have all sustained damage of varying severity, and each has discharged at least some radioactivity.

What is the current status at each reactor?

With no fresh, cold water flowing through the system, the reactors began to behave like enormous kettles, boiling the water that was no longer flowing out of the system. But unlike kettles, there were no spouts to vent the escalating pressures of the steam, which was contaminated with vapours of radioactive elements such as iodine-131 and caesium-137.

The situation began to deteriorate soon after the quake on 11 March. By the time mobile electricity supplies arrived, pressure had built up to the point where engineers had to contemplate venting contaminated gases into the environment, potentially putting people at risk of exposure to radiation. Here, we summarise what has happened since the quake at each of the six reactors of Fukushima Daiichi.

Reactor Unit 1 The first signs of disaster were on 12 March, when engineers began venting the contaminated steam to relieve the pressure within the reactor. Later in the day, an explosion occurred at the reactor, as hydrogen produced from water vapour detonated. This destroyed the roof and outer shell of the reactor unit, but apparently left the reactor itself intact.

On 14 March, engineers decided they had to take further steps to cool the reactor vessel down, and so began pumping sea water into the vessel along with boron, an element that dampens nuclear reactions by soaking up the neutrons which drive them. Later that day, a residual heat removal system was revived, and work towards a “cold shutdown” of the reactor resumed.

Reactor Unit 2 On 14 March, engineers decided that the water level in the reactor was so low that it, too, needed sea water and boron injected into it. Early on 15 March, TEPCO reported an explosion in the unit.

Japan’s chief cabinet secretary Yukio Edano said that the explosion had damaged a doughnut-shaped supporting vessel called a suppression chamber which recycles cooling water. However, the International Atomic Energy Agency is concerned that the containment vessel itself may also have been damaged.

Reactor Unit 3 Venting of vapour to relieve pressure began on 13 March. The next day, engineers began pumping water and boron into the containment vessel to cool down the fuel. They couldn’t fill it up enough to cover the fuel rods completely, however, and so decided to relieve growing pressure by venting contaminated steam. Then, on 14 March, there was an explosion in unit 3. As in the other units, there was said to be no damage to the containment vessel.

Reactor Unit 4 Not working when the quake struck, so ostensibly more stable than the other three. But on 15 March a fire broke out in the pond where spent fuel rods are stored. This is potentially more worrying than the venting of contaminated gases from reactors 1, 2 and 3, as the fire released radioactive material straight into the atmosphere. Although the fire was rapidly brought under control, the scale of contamination remains unknown at this time.

Reactor Unit 5 Again, not working from the outset. But as of 15 March there are reports of temperatures rising in ponds where spent fuel rods stored.

Reactor Unit 6 Same as unit 5, also with spent fuel increasing in temperature.

So what might happen next?

The battle continues to bring units 1, 2 and 3 into cold storage, so the fuel in the reactors no longer heats up, and venting of contaminated steam is no longer necessary. This would also avoid the meltdown of the fuel rods.

If the fuel does melt, it won’t explode, but will need to be sealed forever, probably at the base of the containment vessel. Even if it penetrates through this vessel, it would settle on ground beneath and slowly decay.

The fire in the spent fuel store of reactor unit 4 has probably released the worst of the radioactive materials so far. Plant workers need to work hard to prevent further fires from breaking out, especially at units 5 and 6.