After the terrible devastation that occurred in Japan, they are now dealing with a growing crisis at one of their nuclear facilities located near the earthquake-affected region. I’ve seen a fair amount of disinformation and confusion on the web and I though it might be prudent to answer some questions about what is going on. I’ll address the big questions first and then go into some  background details.

The Big Questions

I’ll address some large questions here:

  • Were the explosions at the reactors nuclear explosions? Absolutely not. The explosions were caused by pressurized hydrogen gas within the building igniting when coming into contact with oxygen and a heat source. The hydrogen was generated as the seawater or possibly superheated steam generated in the reactor came into contact with the hot zirconium alloy that encased the fuel rods. The heat and the zirconium can act as a catalyst to split water and produce hydrogen. In addition the fuel mix used in nuclear power plants is not as concentrated as the fuel used for a nuclear bomb, so it could never cause a nuclear explosion.
  • If there were a meltdown, would the radiation reach the USA? Again, absolutely not. There is a fake map circulating on the internet showing radiation reaching the west coast. I have no idea who would make such a fear-mongering graphic but it is totally false. You can read more about it on Snopes. Some people may remember what happened with the meltdown and explosion of the Chernobyl plant, but that reactor design was radically different. Even if a meltdown did occur in Japan the radiation would not reach the USA.
  • What is a meltdown? Did a meltdown occur in Japan? When the cooling systems fail in a nuclear reactor and the radioactive fuel rods are exposed to air they can continue to heat up to extreme temperatures, in excess of 3000° C. This causes the fuel rods and their metal casings to melt, releasing the dangerous radioactive products. A full meltdown occurs when all of the fuel is liquefied and burns through the bottom of the reactor housing. A partial meltdown occurs when only a small “hot spot” in the fuel rods melt. A full meltdown did not occur in Japan, but a partial meltdown may have occurred in two reactors when the pumps bringing seawater to the reactor failed. Information is hard to come by and we won’t know for sure until later.

General Timeline

Here’s a broad overview from what I have read, mostly summarized from this report:

  1. Earthquake – all nuclear reactors automatically and safely shut down.  “Shut down” means that the control rods are inserted into the fuel rods and the main nuclear reaction involving the splitting of Uranium is stopped. However the secondary decay of radioactive Iodine and Cesium still occurs, which produces heat. This means that cool water still needs to be pumped over the fuel rods.
  2. With the power plant shut down, it is not producing any power to run the water pumps that put fresh cold water onto the fuel rods. The pumps then started to run on diesel backup generators as planned.
  3. The diesel backup generators worked as planned until the Tsunami hit and wiped out all of them. As a backup to this occurring, the cooling pumps then switched over to battery power.
  4. Unfortunately the batteries only run the pumps for 8 hours. After this time was up there was no more power available to keep the pumps running.
  5. There are still a few emergency cooling options built into the design of the reactor, and it is assumed that these were implemented but we don’t know exactly what was done at this point. As time went on with heat building up, some of the water currently in the reaction chamber boiled into steam. Due to the heat and pressure, some of this steam was likely converted to oxygen and hydrogen gas.
  6. As more and more water turned into steam, the pressure inside the reaction chamber built up. Plant operators likely mitigated this pressure by venting some of the slightly radioactive steam out of the reaction chamber and into the outer containment building.
  7. The volatile hydrogen gas that built up inside the building, but outside of the reaction chamber, is likely what caused the explosions. While these destroyed the upper parts of the building, they left the reaction chamber intact.
  8. Power was restored as mobile generators were brought onsite. However as the water inside the system was being turned into steam and vented, there was not enough water left to properly cool the fuel rods. Thus seawater was used to add more water into the cooling system.
  9. The reactors that have had seawater pumped inside (#1 and #3 I believe) are now stable. Reactor number #2 is currently (as of this morning on 3/15/11) seeing higher radiation spikes and temperatures because the pumps bringing seawater inside failed. The situation is changing rapidly, but this seems to be where the most concern currently lies. If the reactor gets too hot, the fuel can melt, and if the reactor containment is cracked then larger amounts of radiation can be released.

That seems to be the current state of affairs, but again things are changing extremely rapidly and up-to-date information is hard to come by.

More Reading

The NYT graphic is especially informative. Source: NYTimes.

Quite a bit has been written about this topic over the last few days. There’s a good video (along with a crazy haired professor) over at Periodic Table of Videos, a great animated graphic from the New York Times about what happens during a meltdown and what happened in Japan, and finally an incredibly detailed explanation in laymens terms covering everything you’d ever want to know about nuclear reactor design and the timeline of what occurred recently. Also, here’s a good write-up of why the government distributes Iodine tablets in potentially affected areas. Let me know if you find any other good explanations – or any other questions as to what is going on. I’ll do my best to answer.

6 responses »

  1. Thanks for the clear explanation, Paul!

  2. Dave says:

    Have you taken into account whether news reports have been downplayed given the ‘face factor’ in dealing with Japanese culture?

    • Paul Vallett says:

      Not sure what you mean by the “face factor” but it’s certainly true (especially in the article linked to above) that some are attempting to downplay the risk.

      • Hickstein says:

        I think Dave is referring to the Japanese culture of politeness that sometimes interferes with communication with westerners. Wikipedia puts it nicely: “They present disagreeable facts in a gentle and indirect fashion.” I have not been to Japan, but apparently it is commonplace for the Japanese to smile and nod even when you are saying something that they disagree with. So, if the Japanese are telling you that something is going wrong, it’s probably going really wrong.

  3. Hickstein says:

    I found the mitnse article pretty convincing that the reactors themselves weren’t a threat outside of Japan, but what about the storage pools that don’t have much containment?

    • Paul Vallett says:

      Good point. I can’t get good info on the containment pools. There are conflicting reports that low water levels in the containment pools caused the fire that was reported this afternoon, but other sources say that it’s from an oil leak, so I am not sure about the threat posed by them. The real problem is that these rods are not in the reactor and as such they don’t have the pressurized reactor vessel available to contain any release of radioactive materials.

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