On other blog sites several folks are out there comparing Japan’s reactor crisis with Chernobyl and Three Mile Island (TMI). This got me to thinking. As a former reactor plant operator I thought I would offer my two cents…
There were a couple of big differences in Chernobyl. One is poor containment construction. The other is that the core itself suffered a direct explosion. As a result of that uncontained explosion the raw core material (the hottest of the radioactive material) went into the atmosphere to become fallout. It was a chemical explosion, but the explosion took place inside the core and was not contained.
TMI was a completely different matter altogether. It hardly compares. There was no explosion at TMI. There was a steam bubble in the core that the operators didn’t interpret correctly until damage was done. Still, it was contained and there was no explosion. There was some core damage, but the nasties never left the primary cooling system and little left the core.
It is too early to fully know the details of the situation in Japan. What follows is additional information and some thoughts of my own…
There are pros and cons to cooling a “melted” core. The pro is the actual cooling effect. Keeping the temperature down will prevent additional damage. Sea water is terrible for an operational reactor, but at this point it makes little difference because the entire system is down. The con to cooling is that water actually reflects the neutrons allowing additional fission activity. Cool water actually generates core activity resulting in more heat. When there is no water, the neutrons do not cause substantial fission and the loss of nuclear geometry causes an end to the heat generating activity. The metal remains hot, however, and may continue to deform and allow highly active contaminants into the water (or atmosphere if exposed). Between the two, it is still much better to attempt to cool the core in order to minimize the total amount of contaminants available to be released and minimize possible damage to the containment facility (thus keeping the nasties where they belong). Water is also a great natural barrier to the neutrons and photons produced in the core, even while shutdown.
The key is containment of the water/steam. If the bad stuff never makes it out of the building, there can only be a limited amount of radiation leakage outside the facility. There is a profound and vital difference between radiation and contamination. When particles from the damaged core are carried by water or steam out of the facility, those particles emit radiation to the outside environment. Chernobyl caused a direct release of particles high into the atmosphere in the flash-steam and debris particles blown high into the atmosphere. Chernobyl was not a “nuclear explosion” which is to say it was not a fission or fusion explosion, however the way contaminated particles went into the atmosphere mimics very closely the effects of a ground detonation of a fission bomb. Japan’s reactor plant explosions will only be similar if the core and its immediate containment are breached and there is a substantial release of contaminants into the atmosphere. Even if it does, most of the bad stuff will blow out to sea.
It is also worth noting that fallout, whether from a bomb or a Chernobyl type explosion, is only substantially dangerous for a matter of days. The big concern is from secondary fission products that continue to alpha and beta decay rapidly. Uranium contaminants will only offer low level radiation after the daughter products decay away. It takes a high concentration of uranium to present a health risk. Alpha decay is only a danger if it enters the lungs or bloodstream. Beta decay can be dangerous on the skin or close to the body. Only the fissioning uranium will emit neutrons – the most dangerous ionizing agent. Gammas are also released and the initial gamma burst is also part of the ionizing radiation that goes far and passes through solid objects. Water, metal, concrete, and other materials reduce the amount of radiation. Distance also reduces the amount by a square of the distance. The radiation from a nuclear explosion is most dangerous during the explosion due to the neutron release at the time of the explosion, not from the fallout. Fallout is only a problem when ingested, breathed in, or coating the skin. This is why spending a couple of weeks indoors, preferably below ground level, will greatly reduce potential health effects from fallout.
One of the most dangerous contaminants to us is a radioactive isotope of Iodine. The body can only absorb so much iodine. That’s why they give iodine for radiation sickness – high concentrations of the stable isotopes prevent absorption in the body of the radioactive isotopes.
And one last thought. As a past radiation worker, I know that it takes several orders of magnitude more than background radiation begin to present even minor long term health risks. It also depends on the duration as well as rate of exposure. A human body can be exposed to a million times the background level for a brief period before being in any immediate danger, or a thousand times background over a period of weeks or months. As of Tuesday (Mar 15) most news agencies were reporting levels of 10 to 20 times background near the Japanese plant. It is more dangerous to get an MRI once a year at that level. A full body MRI is equivalent to about 100 chest x-rays. Either the Japanese are being very over-careful to stave off future law suits or they are not telling us the real numbers.
By explaining all this I do not mean to pretend there is no danger. Buildings are exploding, after all. Seawater pumped into a metal box that’s a few hundred degrees (F) causes a variety of problems from dangerous chemicals not present with pure water to accelerated corrosion of the protective cladding layers over the uranium. The core itself will probably not be usable again nor can it easily be “cleaned up.” The facility is probably a permanent loss as a power source.
A few people will suffer from the radiation. I do not mean to belittle the tragedy of it, but all of us will suffer the economic loss associated with a nation that can no longer fund our debt. Like the concentric rings of the tsunami wave that eventually found our shores, the economic loss in Japan will surely result in damage to the world’s economies that could lead to even greater disasters. Like the Japanese reactor cores, it is too early to know the true extent of the damage, all we know for sure is that is bad.