Why Fukushima will not become a second Tschernobyl

I've been watching events in Japan unfold with, I think, much the same sense of horror anyone viewing the footage must be feeling. I knew something massive had to have happened when I checked my email on Friday morning and saw over 30 notifications sitting in the "USGS" folder. I'm signed up to the US Geological Service earthquake notifications list, which means I get an email notification of any earthquake of magnitude 4.5 or higher. 30+ notifications invariably means something absolutely massive has hit; and sure enough, it had.

In the first 24 hours, there were over 150 aftershocks of magnitude 5.0 and over, of which more than 20 were magnitude 6.0 or higher, with 2 at 6.8, a 6.9 and a 7.1.

The Japanese Geological Service has since upgraded the initial temblor to 9.0; Robert Geller - seismologist working at the University of Tokyo - has referred to it as a 9.1. It triggered a tsunami over 10m high that raced inland several kilometres - and that raced across the Pacific at 500kmh to hit the opposite side of the ocean. In most places it had diminished to only 0.6m high, but local geography caused it to reach 2m when it hit some US coastal areas, causing significant damage to the marina at Santa Cruz and to Crescent City.

Initial measurements showed that the force of the temblor had forced a 500km long section of the ocean floor 7m towards Japan; measurements at an observatory in northern Tokyo showed it had moved 8m west. Subsequent GPS from 1200 sensors mshowed that the whole island of Honshu had shifted 4m to the west, and the Earth's axis had been tilted by 4-6" due to the force.

All these statistics are staggering enough without adding in the statistics of those injured, displaced, missing or dead. I've found it easier to concentrate on facts and figures; it's not that I'm callous to the loss of life - there is simply nothing I can do for it other than express sympathy and condolences, and post links to suitable charitable organisations, which I've been doing on Facebook, Twitter and a few online communities.

Increasingly I've been reading people expressing most concern over the damaged nuclear power plants, specifically Fukushima Daiichi (Number 1).

Fukushima is not going to become a second Tschernobyl, which I think is what everyone is dreading; it can't, because the conditions are completely different. The reactors are completely different types, for a start; for another, at Tschernobyl the reactor was still running when the meltdown occurred - at Fukushima all three reactors shut down automatically and safely the moment the earthquake was detected.

At Tschernobyl, the reactor core was fully powered and operating when (against all safety protocols) they decided to take all the safety systems offline to simulate a power cut - and then couldn't bring them back online again. The core started to overheat but they were unable to scram the control rods to shut down the reaction. The core went into meltdown and exploded, blowing open the top of the containment chamber and the roof of the plant, spreading radioactive vapour and contamination over a wide area. The core continued to burn for three days before they were able to bring it under control and quench it.

The situation at Fukushima is very different. As I said, the emergency systems kicked in automatically the moment the earthquake hit and the control rods were scrammed safely, halting the reaction. The core pile still holds a lot of residual heat however - in excess of 250°C. At this point the cooling systems should kick in to circulate water around the pile to cool it, but the power was knocked out. Back-up generators should kick in at this point, but the power station was built to withstand up to a magnitude 8.6 earthquake and a tsunami of up to 5.7m in height - and the earthquake was actually a 9.0 (they revised it yesterday from the initial reports of 8.9) and the tsunami was in excess of 10m, so the back-up generators were knocked out as well.

As the water in the reactor core evaporated, the fuel rods were exposed to the steam which split into oxygen and hydrogen. This started happening first in Reactor no. 1; they tried venting off some of the hydrogen, but detected radioactive caesium and iodine, which meant enough of the rods had been exposed to cause a meltdown as the zincaloy casing of the fuel rods melted. This meant the control rods were no longer fully inhibiting the reaction so the temperature started to rise to the point where the hydrogen exploded, which happened on Saturday. This only damaged the outer building shell of the reactor however, not the inner containment core.

Once they'd ascertained that the inner containment shell was still secure, they started pumping in sea water laced with boric acid; the boron in the boric acid retards any further reaction and basically acts as one huge liquid control rod. However, the core in Reactor no.3 went the same way as no.1 resulting in a hydrogen explosion that blew off the concrete roof over the reactor - again, without damaging the inner containment shell. They are flooding no.3 with sea water and boric acid as well, and are trying to do the same with no.2 - however they're having problems pumping in the sea water as fast as it is evaporating, and it's believed the core was, briefly, completely exposed for a short while. They don't know yet whether any melting of no.2's core has taken place, but they've detected hydrogen and rising pressure inside the core which suggests at least partial meltdown. They are carefully trying to vent off the hydrogen and prevent another explosion.

However, even if all three reactors go into complete meltdown, it won't be a disaster; each reactor stands inside an outer steel containment shell filled at the bottom with several metres depth of inert boron-containing concrete designed in such a way that if meltdown occurs, breaching the base of the reactor core, the molten core will spread out on the floor of the containment chamber, increasing the surface area so it will cool faster. Once it has all cooled and hardened, engineers in protective clothing would then break up the core mass and it would be removed to a nuclear waste processing plant. There would be no massive explosion and no radioactive contamination beyond a local amount.

Currently there are members of the US atomic energy commission, 12 French engineers who are experts in nuclear accidents and a consultant from Tschernobyl on site as well as British scientists assisting the engineers at Fukushima. Yukiya Amano, head of the UN's nuclear watchdog, says Fukushima's reactor vessels "have held and radioactive release is limited" despite the effects of the earthquake and tsunami.Japan has asked the International Atomic Energy Agency (IAEA) to provide expert help at its damaged nuclear plant. IAEA will be giving daily briefings on the situation at Fukushima Daiichi, and is satisfied that TepCo have been given full, honest and detailed information on what has been going on - another way in which this situation is completely different to Tschernobyl, where the Russian authorities did not admit a major nuclear incident had taken place until other countries had started detecting elevated radiation levels. Thus far Russia has confirmed it is monitoring radiation levels at its nearest observatory on an hourly basis but detected no increase in radiation, which is in line with what TepCo has reported.