Earthquake and Tsunami damage at Fukushima, Japan -March 14, 2011

Unless you’ve been under a rock for the last month you’ll have read all about the magnitude 9 earthquake that struck just off the northern coast of Japan and the resulting devastating tsunami that followed.

At time of writing, the estimated number of fatalities stands at around 18,000 (though only 9,000 are confirmed) which itself is a testament to Japanese engineering and organisation (compare this figure to the estimated 230,000 fatalities due to the boxing day tsunami in 2004 which was of a similar strength). However, this article is about the complications that developed at the Fukushima nuclear power plant.

 

Fukushima

The plant was built between 1967 and 1971 and has a total of six reactors that began operation between 1970 and 1979.  It is an old construction and was likely due to be decommissioned within the decade in favour of modern facilities that are currently being built.

When the earthquake struck, all the reactors “scrammed” (the control rods were injected to cease all fission) as a standard safety precaution and this would’ve been the end of it were it not for the tsunami that arrived shortly after.  The plant had been built to withstand a magnitude 8.2 earthquake and a five metre tsunami.  Though the plant survived the magnitude 9 earthquake, which is 6 times more powerful than a magnitude 8.2, the tsunami was estimated to be about 10 metres high which breached the flood defences. This resulted in the diesel generators that are used to pump the cooling water around the fuel rods becoming disabled.

Due to the nature of the nuclear fuel rods, even though all fission of the uranium had ceased, the rods will continue to generate a lot of heat.  If this heat is not actively removed by cooling then the fuel rod will heat up and could possibly melt, which is called a ‘meltdown’.  On its own, the melting of fuel rods is not particularly hazardous, merely very expensive to clean up.  However, the melting can allow the fuel to pool together in masses large enough to reignite the fission reactions (re-criticality) and this has the potential to become a run-away nuclear reaction that could explode, destroying the reactor.  Even then, this would not pose an immediate threat if the containment shield (a wall of concrete many metres thick designed to withstand the impact of a jumbo jet without rupturing) holds preventing the blast from escaping into the environment.

In two of the reactors there was partial melting of the fuel rods allowing some of the fuel to fall to the bottom but not enough to pose any hazard of reigniting the reaction.  Due to the design of the reactor floor, significant amounts of fuel cannot come together to reignite.  The explosions you read about in the news were due to the hot fuel rods oxidising and releasing hydrogen gas.  This gas built up in quantities sufficient enough to explode which destroyed the outer building but did not harm the containment shield.  In short, it was impossible for the Fukushima reactors to ever ‘Chernobyl’.

 

Radiation leaked

There were deliberate releases of radioactive water to relieve the pressure from the reactor housing.  Had this been the ordinarily pure water they used this would’ve been nothing of concern, however the seawater they used contains a lot of impurities that can pick up neutrons from the reactor core and become mildly radioactive for a short time.  This vented steam would pose a health risk to anyone standing next to the outlet but would disperse quickly and remain radioactive for only a few days.

The single most dangerous event at Fukushima was the release of radioactive Iodine-131. This was most likely released with the vented steam after a partial melting of a fuel rod had occurred allowing this waste to mix with the water. Though it is not particularly radioactive, what makes Iodine-131 so dangerous is that it can accumulate in the human thyroid in quantities enough to cause cancer (it has been estimated that 4,000 additional cases of thyroid cancer was caused by the Chernobyl disaster).  Safety officials take the release of this seriously and the procedure is to distribute potassium iodide tablets to anyone who may come into contact with this material.  These tablets have the effect of stopping the radioactive iodine from accumulating in your thyroid and so remove the risk.  It is worth knowing that thyroid cancer is a very treatable form of cancer with a 96% survival rate.

 

How bad was it really?

Because of the release of these small, but detectable, amounts of radioactive material it is no surprise that safety officials are detecting small amounts of radioactive material in the local wildlife and water supplies.  The amounts being detected are not high enough to cause any threat to human health though.  The maximum amount of radioactive iodine that is permitted in drinking water is 0.04 millisieverts (mSv) per year and amounts like this are being detected in the Tokyo drinking water.  For context, this is equal to 1/75th of your annual background dose (3 mSv) from natural sources (like the radioactive potassium and carbon in your body).  0.04 mSv is the same dose you would get by taking a flight between New York and Los Angeles or eating 400 bananas or sleeping next to another person every night for 3 years.

The lowest amount of radiation that has ever been scientifically linked to an increase in cancer is around 100 mSv (equivalent to about 17 CT scans or 34 mammograms) which will increase your risk of getting cancer by 0.8%.  It is likely that some of the nuclear plant operators received a dose of this scale and their health will need to be monitored in the coming years.

Only one person died at the reactor and that was because a crane fell on him.  No one will die as a result of any fault with the reactor or from any radiation released.  How many people died at the Ichihara and Sendai oil refinery fires after the earthquake?

With climate change set to increase this century, can we really afford to overlook one of the most carbon neutral energy sources available to us right now?  Must we increase our dependency on foreign oil and gas all because some of us didn’t pass A-level physics?

 

David Carnegie