Japan has 55 nuclear reactors at 19 sites.  Two more are under construction and another twelve are in the advanced planning stage.  Net Generating capacity is around 50 GW providing around 30% of the country's electricity (more here).  

As a result of Japan’s largest earthquake in history on March 11 and subsequent tsunami all reactors shut down automatically as they were designed to do but cooling systems associated with two sites had been damaged. 

Three reactor sites are adjacent to the earthquake epicentre and two were in the direct path of the tsunami.  The Fukushima-Daiichi plant belonging to Tokyo Electric Power Company was particularly hard hit.  It lost all grid connections, providing electricity, and its backup power plant was seriously damaged. 

But unlike a great deal of other infrastructure in the area, the reactors and their containment survived the destructive force of a wave that was significantly larger than the design expectations.

Cooling pumps were totally knocked out and alternative cooling methods have resulted in the production of hydrogen released from the reaction of water on hot metals.  This has subsequently exploded on several occasions destroying parts of the outer containment building.  There has been some success in pumping in water to cool the shutdown reactors and spent fuel rods but there are now exposed radioactive elements within the plant and steam and waste water may contain radioactive isotopes. 

This older-style plant dates from 1966.  Reactor 1 began production in 1970 and is the smallest and oldest while reactor 3 has recently undergone an upgrade to change its fuel type.  Reactor 4 was shutdown for maintenance and its unexpended fuel rods were in a holding pond.   The four older units have an early-style containment structure consisting of a rectangular steel-reinforced concrete building with an additional layer of steel-reinforced concrete surrounding; a steel-lined cylindrical drywell; and a steel-lined pressure suppression torus below.  

There are six reactors on site and nearby including number 6, a much larger more modern reactor with more modern containment and two more under construction.  All were hit by the tsunami.  The newer plant has not failed.

There were less critical cooling problems at Tokai nuclear power station, 120km from Tokyo, where one of two cooling systems, on one reactor, also stopped working. 

In addition, briefly elevated radiation levels led to a low-level emergency being declared at a Tohoku Electric nuclear plant, in Onagawa.  But all three reactors there are functioning properly and it is thought that the spike in the radiation sensors may have originated from the release of steam to atmosphere during emergency cooling at the Fukushima reactors (above). This plant is located 114km to the South of Fukushima across Ishinomaki Bay but is directly under the prevailing wind from Fukushima.

Compared to other damage, that to nuclear plants seems to have been minimal. 

Elsewhere in the same region at least 15,000 people have lost their lives.  Oil storages have been swept away with the oil covering large areas; a refinery exploded; and there have been numerous fires and deaths resulting.  Homes, factories, communications infrastructure, roads, rail lines and bridges have been destroyed.  

At the present time no one, not directly involved, has suffered any more additional radiation than they would get from an X-Ray but some workers have been heavily exposed.

As a precaution an exclusion zone of 20 kilometres has been set up but there remain significant numbers of people within this radius. Children have been given iodine against the possibility of radioactive iodine accumulation.  This was the most significant health risk from Chernobyl the World's worst nuclear accident. There, 203, mainly emergency and plant workers, were hospitalised with acute radiation sickness, 31 people died; and up to 9,000 people are believed to have been seriously injured by a higher than normal cancer risk. 

We can hope that the demonstrated Fukushima vulnerability to complete cooling failure will lead to the closure or modification of the remaining older style boiling water reactors that have the older-style containment structure.

But it is important to note that part of the issue at Fukushima is not the reactor vessels but fuel rods in holding tanks due to maintenance on reactor 4 and the lack of appropriate off-site storage for the spent fuel.  These are circumstances that might have been avoided by better administrative and management procedures or even different timing, that missed the tsunami.

At least two of the reactors at Fukushima are damaged beyond recovery and ongoing radiation may have consequences for the viability of other units on the site.  This will result in a reduction in electricity generating capacity in Japan of between 1.5 and 4.5 GW; or up to 3%.  

Until this situation is stabilised there will be ongoing concerns about possible longer term consequences in the region and for industry.