The Chernobyl event was caused by a number of undesirable factors occurring at the same time .  This included a poor design and quite a few poor operational practices at the time of the event.  The reactor core itself was largely made of a flammable material (effectively charcoal briquettes) so that a meltdown resulted in the core catching fire and releasing very large amounts of radioactivity. The operators were conducting unapproved tests under conditions which would have failed standard safety reviews had they been utilized.  The Chernobyl reactor was operated in such a way that the water coolant was caused to flash into steam which in turn burst the reactor vessel preventing the water coolant from passing through the core. This left the core without any forced heat removal capabilities so that once the core caught fire, things got substantially worse.  Dozens of first responders were exposed to enough radiation to cause them to succumb to acute radiation syndrome and eventually death.  There was also a measurable increase in childhood thyroid cancers due to the initial government cover up and lack of radioprotectant iodine pills being distributed to the public (these only protect people from ingesting radioactive iodine).  No other cancers have been identified which could be scientifically identified as being (with statistical significance) caused by the Chernobyl accident.

The Three Mile Island event did result in some core damage from melting and released a relatively small amount of radioactivity.  This event was a combined effect of poor training accompanying design issues allowing operator misinterpretation of readings.  The event resulted in major changes to the safety requirements for all US nuclear power plants along with substantial improvements to safety analysis of nuclear facilities as well as to the federal radiological emergency response system.

The Fukushima event also had design shortcomings, it was built to withstand earthquakes and tsunamis (tidal waves) but only held up to the earthquake.  The tsunami was substantially bigger than what was predicted and so took out far more infrastructure than the facility could withstand.  The explosions at the Fukushima plant were due to elemental hydrogen created through a catalytic reaction with the fuel cladding under extreme heat. The eventual radioactivity releases were predominantly directed out toward the ocean with the portion depositing on the island not being sufficient to cause any measurable health effects.

One common theme in all of these accidents is that in general, health consequences are not global and unless you are up close and personal with the reactor core, health effects are not noticeable in any measurable way.  The thyroid cancers from Chernobyl are the only exception to this and did produce some measurable offsite consequences.  These particular cancers can be attributed to gross negligence in the emergency response efforts from the Soviet government as they were only focused on the reactor and sadly, these were preventable.  All the Soviet government had to do was to evacuate and/or distribute iodine pills to the public and so block uptake of the radioactive iodine released from the accident.

Another common theme in these accidents was the inability to remove heat from the reactor core.  The reactor in a nuclear power plant is the main functional difference between it and any fossil fuel power plant.  The reactor in a fossil fuel power plant burns hydrocarbons (oil, coal or natural gas) to produce heat sufficient to vaporize water which then turns a turbine to produce electricity.  With a nuclear power plant, the heat is no longer produced by traditional fire but rather by splitting atoms.  The functional difference in this context is that fossil fuels don’t stay thermally hot long after they are burned, nuclear fuel on the other hand does.  Nuclear fuel is full of fission products which give off energy as they undergo radioactive decay.  This energy is quickly converted to heat resulting in rather long term heat production from the fuel even after it is no longer effectively useful as fuel.  This means that even when a nuclear reactor is turned off (i.e., the fission chain reaction is no longer taking place), large amounts of heat are still being generated in the fuel and must be removed if the fuel is to remain below its melting point temperature.  These 3 events, Three Mile Island, Chernobyl and Fukushima all underwent this type of catastrophic failure of reactor core damage resulting in environmental releases of radioactivity.

Worldwide, the amount of radioactivity these have caused are actually only a small fraction of that which is present in the environment due to historic atmospheric nuclear weapons tests carried out by ours and other countries.  The typical dose the average American receives from having all of the man made radioactivity in the environment is really only a very small fraction of that received due to natural radioactivity such as potassium, thorium and radon.

When comparing these to industrial failures at other factories or production plants, do any of these rise to the occasion of an actual disaster?

On March 11, 2011, a tsunami over 40 feet high breached a protecting barrier on the coast of Japan causing a nuclear meltdown at the Fukushima nuclear power plant.  This happened as a result of one of the most powerful earthquakes ever recorded.  The tsunami cut off all electricity in the area and flooded the nuclear stations backup diesel engines preventing them from starting.  As a result of the lack of any continual electricity generation capability, the station only had limited battery power.  Once the stations batteries had run out, the radioactive decay of the fission products in the nuclear fuel continually dumped its heat into the reactor core with no means to remove that heat.  With the heat being produced in the fuel faster than it could passively be removed, the core continued to increase in temperature until it reached the melting point.  Chemical reactions from the resulting intense heat generated hydrogen gas which later ignited causing multiple explosions.  A very bad day for public relations in the nuclear industry.

Besides this effect, the tsunami caused the death of around 20,000 people and caused approximately 500 billion dollars in damage.  As a result of the nuclear power plant event, radioactive gasses were released into the environment which has left measurable traces around the world.  With substantial measurement and evaluation having already been done, scientific evaluations of the health effects from the release are now being published by various international expert consensus committees.

The international committee UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) has evaluated the evidence for harm to all humans from the Fukushima event.  The following is a direct quote from their recent report (GAOR, 67th sess., Suppl.No.46), “To date, there have been no health effects attributed to radiation exposure observed among workers, the people with the highest radiation exposures. To date, no health effects attributable to radiation exposure have been observed among children or any other member of the population“.  This is profoundly significant in a way that might not seem so obvious (considering the measurable damage done by the tsunami).  The reason is this, a common cold, a mild allergy or even spending an hour in the sun (if you are pasty white like me) all have observable health effects.   Given that such small things can cause observable health effects, the fact that all the radiation emitted from the Fukushima event has not been able to produce a single observable health effect is worth repetition.  The worst nuclear power plant accident of any kind in many decades has been unable to produce a single observable radiological health effect to date and may continue to do so indefinitely.

These findings are consistent with those found by another expert international committee, the World Health Organization (WHO).  The WHO in their publication, “Preliminary dose estimation from the nuclear accident after the 2011 Great East Japan Earthquake and Tsunami”.  The worst case bounding potential dose was for radioiodine intake by year old infants where the dose could theoretically produce a very small yet statistically significant increase (1%) in thyroid cancer (with this being a bounding worst case possibility).  In general, the worst case doses applied to any member of the public in the Fukushima prefecture were bounded by the dose range of 1 to 5 rem.  Currently 5 rem is the legal dose to a radiation worker in the United States.  All other members of the public in Japan received doses less than 0.1 rem which is a legal dose in the United States for a member of the public in a nuclear facility.  Natural background radiation is around 0.3 rem per year.  In other words, most members of the public in Japan received doses which were small fractions of natural background from the Fukushima event.

Another conclusion of the UNSCEAR report was that evaluating small doses to large populations should not simply be used to predict cancer probabilities to populations.  One possible simplified analogy would be like saying that taking the minimum amount of salmonella to get one person sick and distributing 1/10th of this to 10 people will get at least one of them sick.  Although these are oversimplifications of the whole physics underlying the meaning here, the full interpretation requires a detailed understanding of uncertainties involved in these kinds of measurements but UNSCEAR clearly cautioned against these approaches which are often used when evaluating radiation effects to populations.