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When speaking about Chernobyl and Fukushima, people claim that they are still radiating energy, so my question is for how long?

For Chernobyl, it's been 30 years by now! I'm trying to imagine the whole process from the first law of thermodynamic's perspective. Shouldn't the energy from the rod be "consumed" (or converted to other forms of energy) by now? I know that the half-time of uranium, for example, is a couple of thousands of years and cesium a couple of hundreds, but isn't that related also to the mass? Shouldn't the mass decompose if it radiates continuously? Why are those power plants still radiating, and for how long? And one last question, are those rods still hot?

Can anyone help me understand this?

Thank you!

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  • $\begingroup$ en.wikipedia.org/wiki/Chernobyl_disaster gives the main details, including the new system for removing the waste. To sum up your question, without giving offence hopefully, they would not be doing all this if there was not still a major health risk $\endgroup$ – user140606 Jan 8 '17 at 18:16
  • $\begingroup$ Thank you for the answer. So would you say that according to this: "...the loss of uranium from the wrecked reactor is only 10 kg (22 lb) per year..." and this "about 95% of the fuel in Reactor 4 at the time of the accident (about 180 metric tons) remains inside the shelter" means that in theory, it would take 18000 years to decompose the entire rod? $\endgroup$ – Physther Jan 8 '17 at 19:38
  • $\begingroup$ I'm voting to close this question as off-topic because information about the current status of a nuclear accident is not the subject of physics. $\endgroup$ – sammy gerbil Jan 8 '17 at 23:14
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This is partially based on: Half Life Foundations of Chemistry and might show what the problem is, apologies if you know some or all of it already:

The half-life of any element is the time taken for half of it to decay to another lower number element (or elements).

For example, if one begins with a gram of carbon-10, 20 seconds later only half a gram will remain, after 40 seconds only a quarter gram will be left, after 60 seconds an eighth of a gram, after 80 seconds one sixteenth of a gram, and after 100 seconds have elapsed from the beginning of the experiment, only one thirty-second of the original carbon-10 will remain.

The Chernobyl problem stems from 3 (possibly 4) differences to the above decay pattern.

  1. As you say, instead of 1g, we have 180 metric tons of uranium dioxide fuel.

  2. The half life of the fuel is much longer than carbon, one component of the fuel has a half life of 4.5 billion years; that is, half the atoms in any sample will decay in that amount of time.

  3. As it decays, it releases gamma rays, which have more than enough energy to break the DNA strands in human body cells, and produce both long term and short term serious health "issues".

  4. Uranium-238 decays by alpha emission (a helium nucleus) into thorium-234, which itself decays by beta emission (where a neutron is converted into a proton, an electron, and an electron antineutrino) to protactinium-234, which decays by beta emission to uranium-234. So there is still radioactivity from the by-products.

The source for this picture: Half Life Diagram provides more information on the decay process:

enter image description here

Black humor though it might be, considering the immense bravery of the people involved in the cleanup, the story/urban myth goes that they have road signs near the plant that say :

Chernobyl Region: Close your windows and drive very fast.

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  • $\begingroup$ One final related question to make sure I understand it correctly. So basically a smaller mass of the rod would decrease the risk of long time radiation in the event of an accident, right? $\endgroup$ – Physther Jan 8 '17 at 23:35
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    $\begingroup$ Six of one, half dozen of the other.......The half life is so long, it is the main problem and if you require the same power output you still need the same total mass of fuel, however you divide up. Everything inside that reactor is melted together hot, concrete, metal, all hot. I saw a video last month that showed continously firing little flashes, that was the gamma rays hitting the film in the camera. If you can get BBC programs, there's a good documentary on their website and there is this: youtu.be/Oe_zzTQFV3o $\endgroup$ – user140606 Jan 8 '17 at 23:51
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    $\begingroup$ Note that the 4.5 billion year half-life is pretty innocent stuff, precisely because it's so long. In 4.5 billion years, 1800 kg of the 3600 kg of Uranium will decay, but that means less then a microgram decays each year. The shortlived isotopes are no longer a danger, either. $\endgroup$ – MSalters Jan 9 '17 at 0:51

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