by Michael Graham Richard, Ottawa, Canada 
on 03.14.11
Photo: GFDL
Latest Update: Japan's Nuclear Crisis: 2 Weeks After the Mega-Quake & Tsunami(March 25, 2011)
How Bad is Japan's Nuclear Problem?
There is a lot that is being said and written about Japan's earthquake-damaged nuclear power plants right now. Sadly, unless there's a big catastrophe, few people care to learn about nuclear power, so when things go wrong, people aren't sure what is going on and this can lead to panic. I'm no nuclear expert myself, but in this post I will share what I've learned about this situation and answer the most pressing questions that people might have ("Can it blow up? Is it another Chernobyl") to the best of my knowledge.
The bottom image shows an explosion that was probably caused by the venting of gases to reduce pressure. At high temperature, water splits into hydrogen and oxygen, so it can cause these types of explosions. Photo: Wikipedia
Can Japan's nuclear power plants explode like a nuclear bomb?
Thankfully, it is physically impossible for a nuclear power plant to explode like a nuclear bomb. It simply doesn't have the right kinds of materials: A fission bomb uses highly enriched uranium or plutonium (90%+ of U-235 or Pu-239), while a nuclear power station generally uses Uranium that is only enriched to around 5% (sometimes up to 20% in smaller research reactors). A nuclear power station also lacks all the other mechanisms that are necessary to create a nuclear explosion (like for example the implosion or gun-type assembly configurations that allow supercritical mass to be reached).What is a 'meltdown'?
When you get right down to it, a nuclear power plant is very sophisticated way to boil water. Controlled fission in the core generates heat, which creates steam that spins turbines.When a big earthquake hits, nuclear power plants are programmed to automatically shut down. But even after the fission reaction in the core is stopped, it takes a certain time for the radioactive byproducts to decay and cool down. If something - in this case a huge earthquake and tsunami double hit - prevents the cooling system from pumping water to the core and the emergency cooling system is prevented from kicking in, it can get hot enough to melt. In the very worst cases, part of the containment building, which envelops the reactor vessel which itself contains the fuel rods (it's like a russian doll of buildings), can partially melt, but containment buildings are designed to withstand the high pressures and high temperatures that occur in a meltdown, so in principle they should hold and allow the fuel rods to cool down safely over the next days.
Is a repeat of what happened at Chernobyl likely?
It appears extremely unlikely because the Japanese nuclear power plants, as well as all nuclear power plants now in operation around the world, are designed very differently from the plant that exploded in Ukraine in 1986. The crucial difference is that the soviet reactor was not inside any kind of hard containment vessel. This means that when it failed, the high pressure couldn't be contained, which caused steam explosions that destroyed the reactor building and caused fires that sent a plume of radioactive fallout into the atmosphere. Chernobyl didn't explode like a nuclear bomb, it was closer to a "dirty bomb", which is a conventional bomb that spreads radioactive material around. The Japanese nukes have containment buildings and some seem to already have been vented to reduce the pressure inside.What kind of radiation exposure can be expected, and what are the potential health effects?
Matt just did a post about this very thing, so I suggest you go check it out. The good news is that potential exposure seems extremely low even for people who are close and likely to remain so.
See also: Timeline of the Fukushima nuclear accidents
If you like this article, you can follow me on Twitter (@Michael_GR) and Stumbleupon (THMike). Thanks.
More on Japan's Nuclear Power Plants
Important Questions About the Crisis at Japanese Nuclear Power Plants
How Much Radiation Exposure Do You Normally Get Every Year?
Nuclear Reactions To Japan's Crisis From Around The World
Nuclear Threat Worsens as Japan Response to Earthquake and Tsunami Damage Continues
Nuclear Blast From The Past: Tectonics of Science, Politics, War & Climate Change
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